National Academies Press: OpenBook

Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete (2008)

Chapter: Chapter 3 - Experimental Program and Results

« Previous: Chapter 2 - Literature Review
Page 26
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 26
Page 27
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 27
Page 28
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 28
Page 29
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 29
Page 30
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 30
Page 31
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 31
Page 32
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 32
Page 33
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 33
Page 34
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 34
Page 35
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 35
Page 36
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 36
Page 37
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 37
Page 38
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 38
Page 39
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 39
Page 40
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 40
Page 41
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 41
Page 42
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 42
Page 43
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 43
Page 44
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 44
Page 45
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 45
Page 46
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 46
Page 47
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 47
Page 48
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 48
Page 49
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 49
Page 50
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 50
Page 51
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 51
Page 52
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 52
Page 53
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 53
Page 54
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 54
Page 55
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 55
Page 56
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 56
Page 57
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 57
Page 58
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 58
Page 59
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 59
Page 60
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 60
Page 61
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 61
Page 62
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 62
Page 63
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 63
Page 64
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 64
Page 65
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 65
Page 66
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 66
Page 67
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 67
Page 68
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 68
Page 69
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 69
Page 70
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 70
Page 71
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 71
Page 72
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 72
Page 73
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 73
Page 74
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 74
Page 75
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 75
Page 76
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 76
Page 77
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 77
Page 78
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 78
Page 79
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 79
Page 80
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 80
Page 81
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 81
Page 82
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 82
Page 83
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 83
Page 84
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 84
Page 85
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 85
Page 86
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 86
Page 87
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 87
Page 88
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 88
Page 89
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 89
Page 90
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 90
Page 91
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 91
Page 92
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 92
Page 93
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 93
Page 94
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 94
Page 95
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 95
Page 96
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 96
Page 97
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 97
Page 98
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 98
Page 99
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 99
Page 100
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 100
Page 101
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 101
Page 102
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 102
Page 103
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 103
Page 104
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 104
Page 105
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 105
Page 106
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 106
Page 107
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 107
Page 108
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 108
Page 109
Suggested Citation:"Chapter 3 - Experimental Program and Results." National Academies of Sciences, Engineering, and Medicine. 2008. Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete. Washington, DC: The National Academies Press. doi: 10.17226/13916.
×
Page 109

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

26 3.1 Introduction to the Experimental Program Each section of this chapter focuses on a phase of the re- search program. Each section begins with a discussion of test- ing procedures for the phase of the research program covered and then discusses the test results. Prestressed sections are discussed in Sections 3.2 through 3.7. The mild steel phase is discussed in Section 3.8. The experimental thrust areas con- sist of the following: • Refinement of the NASP Bond Test, culminating in the re- sults from round robin testing by OSU and Purdue. Based on its repeatability and the reproducibility, the NASP Bond Test is presented as the Standard Test Method for the Bond of Prestressing Strands (Standard Test for Strand Bond). The Standard Test Method for the Bond of Prestressing Strands is recommended for adoption by AASHTO. The Standard Test Method for the Bond of Prestressing Strands was modified by testing strand in concrete of varying strengths. The tests demonstrate that the bond strength be- tween strand and concrete is improved by increases in con- crete strength. The results indicate that bond performance improves in proportion to the square root of the concrete strength. This relationship is subsequently used in the rec- ommended code expressions for transfer length and devel- opment length. • Transfer length measurements made on pretensioned con- crete beams, both rectangular beams and I-shaped beams. The results show a direct correlation between decreasing transfer length and increasing concrete strength. Based on the results, a design expression for transfer length is rec- ommended that includes a factor for concrete strength. • Development length tests on the pretensioned concrete beams. The results show that strand development length requirements shorten with increased concrete strength. Additionally, the development length tests provide the data to support the recommendation for minimum threshold values from the Standard Test Method for the Bond of Prestressing Strands. Based on the results, a design expression for development length is recommended that includes a factor for concrete strengths up to 15 ksi. • The object of the mild steel phase of the experimental pro- gram was to evaluate the bond strength under monotonic loading of lap-spliced and hooked uncoated and coated bars in tension embedded in normal weight higher strength concrete. The 318 Code (ACI 2005) places an upper limit on the of 100 psi in the calculation of splice length/development length of bars as well as on the development length of standard hooks in tension in higher strength concretes. This limitation was first introduced in the 1989 edition of 318 Code (ACI 1989). Section 5.4.2.1 of the 2004 edition of the AASHTO LRFD Bridge Design Specifications states that design concrete strengths above 10 ksi shall be used only when allowed by specific articles or when physical tests are made to establish the relationships between the concrete strength and other properties. The object of this experimental program was to provide the information necessary to determine whether these limita- tions can be removed for concrete compressive strengths up to 15 ksi. 3.2 The Standard Test Method for the Bond of Prestressing Strands In the past, testing programs intended to measure transfer and/or development length have instead highlighted the vari- ation in bond-ability that resulted from the varying bonding properties of prestressing strands. So, rather than addressing the primary research focus, which was often to develop code equations for strand transfer and development lengths, the results of these testing programs were muddled and confus- ′fc C H A P T E R 3 Experimental Program and Results

27 Figure 3.1. NASP specimen on the loading frame at OSU. ing to transportation agencies and others. Therefore, as a first step in this research program, a Standard Test Method for the Bond of Prestressing Strands was refined from prior testing. The research reported herein continues and expands research begun by NASP. The focus of NASP’s research has been to de- velop a standardized test for bond that would be repeatable at a testing site, reproducible among sites, and provide a reliable prediction of the performance of a pretensioned concrete product. With the development of a repeatable, reproducible standard test, design expressions for transfer and develop- ment length can be developed. Figure 3.1 shows a NASP specimen mounted in the load- ing frame at OSU. Each test specimen is prepared by casting a single prestressing strand in a sand-cement mortar within a cylindrical steel casing. The sand-cement mortar is pro- portioned to produce a strength of 4750 ± 250 psi at 24 hr, after standard curing. Additionally, the sand-cement mortar is required to produce a flow in the range of 100 to 125 as measured by ASTM C 1437. The strand is pulled from the concrete mortar at a displacement rate of 0.10 in./min, 24 hr after casting. Pull-out force is measured in relation to the movement of the free end of the strand to the hardened mortar. The NASP Bond Test records the pull-out force that corresponds to 0.10 in. of free strand end slip. One single NASP Bond Test consists of six or more individual test spec- imens; the average value from the set of six becomes the “NASP Bond Test Value.” The appendices to this report contain three separate bond test protocols; each protocol represents a different stage in the development and refine- ment of the NASP Bond Test. 3.2.1 Refinement of the NASP Bond Test The NASP Bond Test was originally developed in Round II and Round III research sponsored by NASP. The NASP research investigated the repeatability and reproducibility of the test method together and also compared the NASP Bond Test with other test methods. In Rounds II and III, the research showed that the NASP Bond Test was a better pre- dictor for bond than the Moustafa Test or the PTI Bond Test. The NASP Bond Test also showed convincing results when compared with transfer lengths measured on prestressed con- crete beams. Additionally, Round III testing showed evidence that the NASP Bond Test could be used to ensure adequate strand development. The early versions of the NASP Bond Test protocols are included in Appendix I. Appendix I con- tains two versions of the NASP Bond Test, the first dated August 2001 and the second dated May 2004. The earliest ver- sion of the NASP Bond Test was employed for Rounds II and III of the NASP-sponsored research. The May 2004 protocols were used for NCHRP Project 12-60 for the purpose of further refining the NASP Bond Test. Some refinements in protocol were made to develop the final version found in Appendix H and titled, “Standard Test Method for the Bond of Prestressing Strands.” For this research, minor changes were made to the NASP Test procedures that were used in NASP Round III research. Although the underlying methodology in the procedure was not changed significantly, changes in the sample preparation were made and test procedures were refined. The NASP pro- tocols in 2001 specified a sample preparation in which the cement mortar had a sand-cement-water ratio of 2:1:0.45 and a target 1-day mortar cube strength of 3,500 to 5,000 psi. The wide range in the mortar cube strength proved to adversely affect the NASP Bond Test values. Weaker mortar produced lower pull-out strengths, whereas stronger mortar produced higher pull-out strengths. The May 2004 protocols used in the NCHRP research targeted a smaller range (4,750 ± 250 psi) for mortar cube strength. Later, through refinement, the mortar proportions were not specified so that consistent mortar strengths could be produced despite possible varia- tions in the constituent materials from site to site. Therefore, the August 2006 protocol for the Standard Test Method for the Bond of Prestressing Strands required mortar strength in the range of 4,500 and 5,000 psi, but did not specify the mix- ture proportions. Additionally, the test methodology adopted a mortar flow requirement in the range of 100 to 125, whereas flow mea- surements were not made during the NASP Round III tests of the August 2001 protocols. The standardized flow rates help

28 NASP Test Results Ba tc h # Water- to- Cement Ratio Mortar Strength (psi) N A SP IV ST R A N D ID N C H R P ID St ra nd D ia m et er (i n.) Pull-Out Force at 0.1" slip (lbs.) N S(lbs.) LC/DC 8N 0.45 4765 C D 0.5 6,870 12 861 DC 11N 0.45 4730 G A 0.5 20,710 11 1604 DC 14N 0.45 4953 G A 0.5 20,010 12 3088 LC 15N 0.45 4815 G A 0.5 21,930 6 1106 LC 15N 0.45 4815 G A 0.5 21,190 6 1333 DC 17N 0.45 4484 C D 0.5 8,710 5 432 LC 17N 0.45 4484 C D 0.5 6,910 5 338 DC 21N 0.5 4043 G A 0.5 20,060 12 1129 LC 22N 0.5 4117 C D 0.5 6,110 12 421 DC 23N 0.5 3981 G A 0.5 16,360 12 1629 DC 24N 0.4 5763 C D 0.5 8,420 12 415 DC 27N 0.45 4933 K6 0.6 19,010 5 4311 DC 27N 0.45 4933 L6 A 0.6 17,960 6 1292 DC 28N 0.45 4843 K6 0.6 22,420 5 1964 DC 28N 0.45 4843 L6 A 0.6 18,610 6 717 DC 29N 0.45 4723 A C 0.5 14,130 6 1144 DC 29N 0.45 4723 E 0.5 15,950 6 1266 DC 30N 0.45 4723 J B 0.5 19,330 5 808 DC 30N 0.45 4723 E 0.5 17,210 6 823 DC 31N 0.45 4927 J B 0.5 21,090 6 733 DC 31N 0.45 4927 A C 0.5 13,300 6 1763 DC 34N 0.45 4659 H 0.5 15,940 6 1153 DC 34N 0.45 4659 F 0.5 13,570 6 968 DC 35N 0.45 4659 H 0.5 18,080 6 1202 DC 35N 0.45 4659 F 0.5 16,540 6 684 DC 36N 0.45 4451 I 0.5 12,100 6 1455 DC 36N 0.45 4451 B 0.5 13,440 6 1243 DC 37N 0.45 4724 I 0.5 14,710 6 1181 DC 37N 0.45 4724 B 0.5 15,600 6 1044 DC 38N 0.45 4153 K6 0.6 19,510 12 2079 DC 39N 0.45 4303 D E 0.5 5,240 6 635 DC cif Table 3.1. Results of NASP Bond Tests at OSU. ensure workability of the mortar and consistent consolida- tion of the mortar. The strand is centered in a steel casing with an outer diameter of 5 in. and a bond length of 16 in. The cement mortar is cast and consolidated in the steel casing. The NASP Bond Test protocols in 2001 did not specify the frame used for loading the NASP specimen. The loading frames used in the Round III trials were more “flexible” when compared with the frame used in the current NCHRP re- search, which is more “rigid.” Because the NASP Bond Test protocols require a displacement rate, the rigidity of the test apparatus affects the loading rate. Therefore, the Standard Test Method for the Bond of Prestressing Strands limits the loading rate to 8,000 lb/min for 0.5 in. diameter strands and 9,600 lb/min for 0.6 in. diameter strands. In its recommended form, the Standard Test Method for the Bond of Prestressing Strands requires a loading rate of 0.1 in./min, as before, and the NASP value is reported as the load at which the free strand end slip is 0.1 in. The average of six or more NASP specimens is reported as the NASP value for the strand. Studies con- ducted earlier in the NASP Round II concluded that the least variation in the NASP values is exhibited for the 0.1 in. of strand end slip. The largest variation in the NASP values was reported in the 0.01 in. of free strand end slip. The Moustafa Test and the PTI Bond Test, which are used by some to identify the bonding properties of prestressing strands with concrete, were neither repeatable nor repro- ducible. The NASP Bond Test was convincingly superior to the others in its ability to reproduce results among sites. Table 3.1 provides the results of NASP Bond Tests that were performed at OSU. Ten different 0.5 in. diameter

29 strands were tested along with two different 0.6 in. strands. In Table 3.1, LC/DC refers to whether the test was conducted using load control (LC) or displacement control (DC). These tests were critical to refining the test protocols and also to determining which strand samples would provide high and low targets for NCHRP Project 12-60 transfer length and de- velopment length tests. Testing also included variations in water-to-cement ratio (w/c), which resulted in variations in mortar strength. W/c ratios of 0.40, 0.45, and 0.50 were tested. Additionally, some tests were performed using load- controlled protocols instead of displacement control. From these tests, it was determined that displacement control pro- vides more data that can be valuable in evaluating strand bond performance. Therefore, the recommended Standard Test Method for the Bond of Prestressing Strands requires displacement control instead of load control. The Standard Test Method for the Bond of Prestressing Strands (see Appendix H) includes specific dimensions for the test specimens and the procedures for the test. Figure 3.2 shows a schematic of the Standard Test Method for the Bond of Prestressing Strands. Additional details for the NASP Bond Test are shown in Figure 3.3. Figure 3.4 shows detail for the methodology employed to measure the strand end slip on its “free” end, i.e., the end of the strand that is not loaded in ten- sion. The photograph in Figure 3.5 shows the strand end slip measurement device. Finally, in Figure 3.6, the photograph shows an entire Strand Bond Test specimen placed within the loading frame and ready for testing. V er tic al A xi s l in e LOAD TRANSDUCER WITH 22,000 LB CAPACITY MTS CONSOLE 1" BOLT 2 14" x 8" CHANNEL SECTION 9 12 " x 9" x 1 14 " THK STEEL PLATE WITH 34" WIDE SLOT AT THE CENTER THROUGH 4 12" 12 12 " x 9" x 1 14 " THK STEEL PLATE WITH 34" WIDE SLOT AT THE CENTER THROUGH 4 12" 2 14" x 8" CHANNEL SECTION 1" BOLT 12 12 " x 9" x 1 14 " THK STEEL PLATE 7" 2'-8" 2.5"4.5" 8" 2'-6" 8" CLEAR DISTANCE 5 14" CLEAR DISTANCE 5.5" Figure 3.2. Schematic diagram of NASP Test setup.

30 Figure 3.5. LVDT on NASP Test specimen. Base plate 6" x 6" x 14" with 58" diameter hole welded to the specimen cylinder Neoprene pad 6" x6" Base plate 6"x6"x 34" steel plate Steel plate 3 14" x 3 14" x 12" with 58" diameter hole 1 2" chuck Consolidated mortar mix 1 2" or 6 10" diameter strand 2" long styrofoam bond breaker firmly attached to the strand Figure 3.3. Details of the NASP Bond Test specimen. 13.0" 7.50" 1 34 " steel block bolted to the aluminum plate for weight Aluminum plate 13" x 1" x 34 in 3" clear spring loaded LVDT (DCT 1000A) Magnetic base with control switch 9.0" NASP specimen casing 1 2" or 6 10" diameter strand Mortar mix cured for 24 hours 2" long styrofoam bond breaker firmly attached to the strand Figure 3.4. NASP Test specimen strand end slip measurement. 3.2.2 Reproducibility of the NASP Bond Test Between Sites The NASP Bond Test was performed on specific strand samples at Purdue and OSU. Round robin trials were per- formed on five 0.5 in. diameter and two 0.6 in. diameter, Grade 270, low-relaxation strands. The strands included in the round robin trials are shown in Table 3.2. Some of the Strand Bond Test data from OSU was developed from tests with 12 samples. At Purdue, all of the tests had a sample size of six. Table 3.2 lists both the NASP identifiers (Round III and Round IV) and the NCHRP strand ID. Purdue performed the bond tests as completely blind trials—even the Purdue strand identifiers were changed from those used at OSU. The results from the round robin testing are reported in Table 3.3. The five 0.5 in. diameter strand samples are Strand A, Strand B, Strand C, Strand D and Strand E. The two 0.6 in.

31 Figure 3.6. NASP Test specimen inside loading frame. diameter samples are labeled Strand A6 and Strand B6. For example, Strand E had the lowest reported results at both test- ing sites, 5240 lb at OSU and 6070 lb at Purdue. For Strand C, OSU reported an average of 13,715 lb, whereas Purdue reported an average of 14,710 lb. Note that the table reports results of testing with both 0.5 in. and 0.6 in. strands. The results in Table 3.3 are illustrated in Figure 3.7. Figure 3.7 plots the average NASP values from OSU against the av- erage NASP values from Purdue. A linear regression line and a “perfect fit” line are plotted in the figure. The test results match the “perfect fit” line very closely, with an R2 value of 0.92. 3.2.3 Recommendation for the Standard Test Method for the Bond of Prestressing Strands The NASP Bond Test performed in this research program was conducted on ten 0.5 in. diameter and two 0.6 in. diam- eter strands. In the NCHRP testing, round robin tests were performed at Purdue and OSU. As shown in Figure 3.7, the results from the two testing sites closely match. This research builds upon earlier work done by NASP to develop a standard test for bond. The NCHRP research fur- ther refined the testing protocols to the point where the test results are now demonstrably reproducible between testing sites. The refined test is recommended as a standard test method to evaluate the ability of a prestressing strand to bond with concrete. 3.3 The NASP Bond Test in Concrete The NASP Bond Test protocol was modified to test the strand in concrete in place of mortar. This is important to the overall research because the NASP Bond Test modified for concrete demonstrates the relationship between bond strength and concrete strength. The overarching conclusion from this segment of the testing was that bond strength improves in proportion to the square root of the concrete strength. This conclusion stems from an examination of data from three of the 0.5 in. diameter strands and one of the 0.6 in. diameter strands. The concrete used for the modified NASP Bond Test had 1-day strengths varying from 4 ksi and to 10 ksi. STRAND DIAMETER (IN) NASP ROUND III ID NASP ROUND IV ID NCHRP OSU ID OSU Purdue 0.5 A C x 0.5 B x 0.5 FF C D x x 0.5 II D E x x 0.5 E x 0.5 F x 0.5 AA G A x x 0.5 H x 0.5 I x 0.5 J B x x 0.6 K6 B6 x x 0.6 L6 A6 x x Table 3.2. Round-robin testing at OSU and Purdue.

32 NASP Test Results at OSU NASP Test Results at Purdue N C H R P ST R A N D ID St ra nd D ia m et er (in ) Mortar Strength cif (psi) Pull-Out Force at 0.1" slip (lb) Mortar Strength cif (psi) Pull-Out Force at 0.1" slip (lb) C 0.50 4,723 14,130 4,498 14,270 C 0.50 4,927 13,300 4,810 15,150 Avg. 13,715 14,710 D 0.50 4,765 6,870 4,665 7,280 D 0.50 4,484 6,910 4,365 9,770 1 D 0.50 4,767 9,970 Avg. 6,890 8,625 E 0.50 4,303 5,240 4,000 6,070 A 0.50 4,730 20,710 4,847 2,0880 A 0.50 4,815 21,190 4,318 16,470 1 A 0.50 4,638 18,880 Avg. 20,950 19,880 B 0.50 4,723 19,330 4,893 22,700 B 0.50 4,927 21,090 4,798 22,280 Avg. 20,210 22,490 B6 0.60 4,843 22,420 4,356 19,130 B6 0.60 4,933 19,010 B6 0.60 4,153 19,510 1 Avg. 20,715 19,130 A6 0.60 4,933 17,960 4,628 15,450 A6 0.60 4,843 18,610 Avg. 18,285 15,450 1Value omitted from average because the mortar strength was out of range. Table 3.3. Results from round-robin testing—Standard Test for the Bond of Prestressing Strands in Concrete. 0 5000 10000 15000 20000 25000 0 5,000 10,000 15,000 20,000 25,000 Average NASP Pull-out Values (lb) Oklahoma State University A ve ra ge N A SP P ul l-o ut V al ue s ( lb) Pu rd ue U ni ve rs ity "Perfect" Test Linear Regression R2 = 0.92 E D C A6 B B6 A Figure 3.7. Comparison of NASP Bond Tests at OSU and Purdue. The modified NASP Bond Test was conducted in con- crete to understand the effects of varying concrete strengths on the bond of prestressing strands. The test procedure was identical to the NASP Bond Test protocols discussed in Section 3.2 except that concrete with varying strengths was used instead of the standard cement-sand mortar. The NASP tests in concrete were conducted on three 0.5 in. di- ameter strands with NCHRP strand designations A, B, and D and on one 0.6 in. diameter strand with an NCHRP strand designation of A6. The number of NASP tests con- ducted on concrete for varying concrete strengths is re- ported in Table 3.4. Each test listed in Table 3.4 contains six

33 or more NASP specimens. The target concrete strengths for each of the tests were 4, 6, 8, and 10 ksi. The concrete mixtures used for making the NASP specimens in concrete included Type III cement from Lafarge North America, coarse and fine aggregate from Dolese Bros. Co., cement slag from Lafarge North America, and admixtures from De- gussa Admixtures, Inc. Admixtures used included HRWRs, normal range water reducers (NRWRs), and air entraining admixtures (AEAs). Table 3.5 gives the mix proportions and the target fresh and hardened properties for the concrete cast in the modified NASP specimens. The mix proportions were named based on the target 1-day strength. The mix C-0 targets a concrete strength of 4 ksi at release. Similarly, C-I, C-II, and C-III target strengths of 6, 8, and 10 ksi at release, respectively. The concrete mix C-IA has a target release strength of 6 ksi with AEA. Detailed trial batching was performed (Tessema 2006) to arrive at the concrete mix proportions and the target fresh and hardened properties. The results and discussion on the concrete mix proportions are beyond the scope of this report. The mixture propor- tions reported in Table 3.5 were also employed to make the transfer length and development length beams. The NASP Bond Test, modified to be tested in concrete, conforms to the same protocols for NASP Bond Testing that are found in Appendix I. The only variation is that concrete is used in place of the sand-cement mortar. Also, concrete slumps of 2 to 3 in. were achieved instead of the mortar flow rates of 100 to 125. The handling and preparation of the strands, the steel casing, and the bond breakers were identi- cal to the NASP Bond Tests conducted in sand-cement mor- tar. The mixing procedures used for the NASP Bond Test conformed to ASTM C 192. The fresh concrete is placed in two layers; each layer is consolidated using a handheld elec- tric vibrator. The slump, unit weight, and air content are measured per ASTM C 143, ASTM C 138, and ASTM C 231, respectively. The NASP specimens and the test cylinders were cured in conformance with ASTM C 192. The compressive strength testing was conducted during the time of the NASP Bond Test in concrete, in conformance with ASTM C 39. The NASP specimens are then kept in a laboratory curing room for 22 to 24 hr from the time of hydration. Curing conditions near 73.4 °F and 100-percent relative humidity were main- tained. The modified NASP Bond Test is performed at 24 ± 2 hr after the hydration of the cement. The NASP specimen in Target 1d Concrete Strengths (ksi) NASP Round IV ID NCHRP ID Strand Diameter (inches) 4 6 8 10 G A 0.5 1 1 1 1 J B 0.5 1 1 1 1 C D 0.5 2 4 2 1 L A6 0.6 1 1 1 1 Table 3.4. Number of NASP Bond Tests modified for concrete with varying concrete target strengths. Concrete Mixture Designations C-0 C-I C-IA C-II C-III Cement (PCY) 650 800 800 800 900 Cement Slag (PCY) 100 Coarse Aggregates (PCY) 1,800 1,703 1,800 1,805 1,747 Fine Aggregates (PCY) 1,243 1,203 922 1,219 1,183 Water (PCY) 298 303 272 277 251 Glenium 3200 (fl oz/cm. wt) 10 14 7 Glenium 3400 (fl oz/cm. wt) 8 5 5.5 Polyheed 997 (fl oz/cm. wt) 3 MB-AE 90 (fl oz/cm. wt) 1.88 Target Properties for Fresh and Hardened Concrete 1-Day Strength (ksi) 4 6 6 8 10 28-Day Strength (ksi) 6 8 8 10 14 56-Day Strength (ksi) n/a 10 10 14 15 Slump (in) 8 8 8 8 9 Unit Weight (pcf) 145 148 148 150 157 Air Content (%) 2 2 6 2 2 Table 3.5. Concrete mixture proportions for transfer and devel- opment length testing and for the NASP Bond Test in concrete.

34 Strand ID Concrete NASP Test Results N C H R P ID N A SP IV ST R A N D ID St ra nd D ia m et er (in .) w/cm ci f )ksi( ci f ksi N A SP in M or ta r (k ips ) NASPValue (kips) N S (ksi) A G 0.5 0.425 4.52 2.13 23.58 6 0.66 A G 0.5 0.38 7.02 2.65 26.35 6 1.44 A G 0.5 0.36 8.05 2.84 30.68 6 1.77 A G 0.5 0.235 11.79 3.43 20 .9 5 35.29 6 2.33 B J 0.5 0.46 3.56 1.89 22.55 6 5.57 B J 0.5 0.4 5.58 2.36 30.8 6 1.04 B J 0.5 0.32 7.11 2.67 28.78 6 4.55 B J 0.5 0.24 10.06 3.17 20 .2 1 34.33 6 4.17 D C 0.5 0.45 4.71 2.17 7.48 6 2.76 D C 0.5 0.46 4.56 2.13 6.66 6 2.52 D C 0.5 0.36 6.99 2.64 8.96 6 2.23 D C 0.5 0.38 7.34 2.71 9.51 6 2.64 D C 0.5 0.4 6.13 2.48 6.74 6 0.25 D C 0.5 0.3 8.67 2.94 10.26 6 0.26 D C 0.5 0.32 8.34 2.89 9.97 6 1.06 D C 0.5 0.26 9.95 3.15 6. 89 11.56 6 0.84 A6 L 0.6 0.46 2.23 1.49 11.6 6 0.61 A6 L 0.6 0.38 5.02 2.24 23.13 6 1.24 A6 L 0.6 0.28 8.79 2.96 24.84 6 0.82 A6 L 0.6 0.235 10.42 3.23 18 .2 9 28.74 6 1.39 Table 3.6. Results of NASP Bond Tests in concrete. concrete is mounted on a rigid steel frame in the same man- ner described for the NASP Bond Test (in mortar). 3.3.1 Results from the NASP Bond Tests in Concrete The NASP Bond Test standardized in mortar was con- ducted in concrete to understand the effect of concrete strengths on the NASP Bond Test. The results from this ex- perimental testing are reported in Table 3.6. Table 3.6 reports the NCHRP Strand ID, the NASP Strand ID (for comparison purposes), the 1-day concrete strength (f ′ci), the NASP Bond Test result (from the Standard Test for Strand Bond in mor- tar), and the NASP Bond Test when modified and performed in concrete. The table reports the w/cm (water–cementitious materials) ratio because there were pozzolanic materials added for some of the concrete mixtures reported in Table 3.5. The concrete strengths reported in Table 3.6 are averages of three or more concrete specimens tested during the NASP test. The number of NASP Bond Test specimens (N) that were included as part of the test and the standard deviation (S) for each set of tests are reported for the modified NASP Bond Test in concrete. 3.3.2 Discussion of the Results from the NASP Bond Tests in Concrete Figure 3.8 shows the pull-out values from the Modified NASP Bond Test for Strands A and B plotted versus the concrete strength. There are a total of 8 data points, also re- ported in Table 3.6, found in Figure 3.8. Both linear regres- sion and the power regression curves are plotted on the figure. The coefficient of determination (R2) value for both the regressions is 0.82. The linear and the power best-fit equations are reported in Figure 3.8. Figure 3.8 clearly shows that increases in concrete strength result in a higher NASP pull-out value for NCHRP Strands A and B. Note that the NASP Bond Test pull-out value for the standardized test in mortar is 20.95 kips for NCHRP Strand A and 20.21 kips for Strand B. Also, note that the regression plots cross the 4 ksi concrete strength at a corresponding NASP Bond Test (modified) value of about 23 kips.

35 y = 1.5293x + 18.02 R2 = 0.8153 y = 14.078x0.3734 R2 = 0.8222 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) N A SP P ul l-o ut V al ue in C on cr et e ( kip ) Figure 3.8. Pull-out values from the modified NASP Bond Test for Strands A/B versus concrete strength. Figure 3.9 illustrates the pull-out values from the Modified NASP Bond Test for NCHRP Strand D plotted against con- crete strength. NCHRP Strand D had a NASP Bond Test value of 6.89 kips in the standardized test, which was lower than the standardized NASP Bond Test values of Strands A and B. There are a total of 8 data points for Strand D, and the data shown in Figure 3.9 correspond to data reported in Table 3.6. Linear regression and the power regression curves are plotted on Figure 3.9 for Strand D. The coefficient of deter- mination (R2) values are 0.89 for the linear regression and 0.84 for the power regression. The linear and the power best- fit equations are also reported in the figure. Figure 3.9 clearly shows that increases in concrete strength result in a higher NASP pull-out value for NCHRP Strand D. Please note that the NASP Bond Test pull-out value for the standardized test in mortar is 6.89 kips for the NCHRP Strand D. Also, note that the regression plots cross the 4 ksi concrete strength at a corresponding NASP Bond Test (modified) value of about 6 kips. Figure 3.10 makes the same comparison as Figures 3.8 and 3.9, but for 0.6 in. diameter strand, NCHRP A6. The data shown in Figure 3.10 are also reported in Table 3.6. Both linear regression and the power regression curves are plotted on Figure 3.10 for NCHRP Strand A6. Please note that the ex- ponent in the best-fit power curve is approximately 0.56. As in Figures 3.8 and 3.9, Figure 3.10 clearly shows that increases in concrete strength result in higher NASP pull-out values for NCHRP Strand A6. Please note that the NASP Bond Test pull-out value for the standardized test in mortar is 18.29 kips for the NCHRP Strand A6, and that the regression plots cross the 4 ksi concrete strength at a corresponding NASP Bond Test (modified) value of about 17.5 kips. y = 2.4161x0.6677 R2 = 0.8431 y = 0.8833x + 2.6335 R2 = 0.8917 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) N A SP P ul l-o ut V al ue in C on cr et e ( kip ) Figure 3.9. Pull-out values from the modified NASP Bond Test for Strand D versus concrete strength.

36 y = 1.8439x + 9.8805 R2 = 0.8566 y = 7.9751x0.5565 R2 = 0.9203 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 0 2 4 6 8 10 12 14 Concrete Strength (ksi) N A SP P ul l-o ut V al ue in C on cr et e ( kip s) Figure 3.10. Pull-out values from the modified NASP Bond Test for Strand A6 versus concrete strength. 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 0.00 0.50 1.00 2.50 1.50 3.00 2.00 3.50 4.00 4.50 5.00 (ksi)0.5 N A SP P ul l-o ut V al ue (k ip) Strand D R 2 = 0.816 6 Strand A6 R 2 = 0.9053 Strand A R 2 = 0.93 51 Strand B R 2 = 0.7003 fci′ Figure 3.11. NASP pull-out values versus for all strands.′fci Table 3.6 also reports values for the square of the 1-day concrete strength. The NASP pull-out values and the square root of the concrete strength are presented for all the strands tested in concrete in Figure 3.11. The linear best fit-lines are plotted in the figure with the corresponding R2 values for the four strands tested in the modified NASP test in concrete. In Figure 3.11, the best-fit curves tend to have a steeper slope for strands with higher NASP values in the same range of con- crete strengths. The NASP value increases with increases in concrete strength, and the high-performing strands have a steeper best-fit line. Thus, for a given change in the concrete strength, the NASP results can have a higher variation for the high-performing strands (strands with higher NASP values) when compared with the moderately performing strands (strands with lower NASP values). The data presented in Table 3.6 are normalized and pre- sented all together in Figure 3.12. The NASP Bond Test val- ues were normalized by dividing by the NASP Bond Values in concrete by the Standard NASP Bond Test (in mortar) values. Figure 3.12 includes data from all three 0.5 in. diameter strands and the one 0.6 in. diameter strand. Concrete strength at 24 hr from the modified NASP Bond Test is plot- ted against normalized NASP values. The data are plotted against a best-fit power regression curve, also shown in Fig- ure 3.12. The R2 value for the test data is 0.80, indicating that the power regression equation closely agrees with the test data. The best-fit equation is given in Equation 3.1. (3.1) NASP NASP fci concrete( ) = ′0 49139 0 51702. .

37 y = 0.49139x0.51702 R2 = 0.79826 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 Concrete Strength at 24 hours (ksi) N A SP in C on cr et e N or m al iz ed b y N A SP in M or ta r ( kip s/k ips ) NCHRP A NCHRP B NCHRP D NCHRP A6 (0.6") 0 2 4 6 8 10 12 14 Figure 3.12. Normalized NASP pull-out values versus concrete strength for all strands. where NASPconcrete = the value obtained from the NASP Bond Test in concrete, and NASP = the value obtained from the Standard NASP Bond Test (NASP Bond Test in mortar). The equation is modified to fit the NASP values as a func- tion of the square root of concrete strengths. In Figure 3.13, the normalized NASP pull-out values are plotted against the square root of the concrete strength. The linear regression results in the following equation: (3.2) NASP NASP fci concrete( ) = ′0 51. The result of the regression is remarkable for two reasons. One, the data’s best fit regression demonstrates a coefficient of determination of 0.79, illustrating that the data set is fairly well predicted by the regression; two, the data demonstrate that bond improvements are directly proportional to the con- crete strength at 1 day of age. Furthermore, the normalized value of 1.0 is achieved at an f ′ci of 4 ksi. These significant re- sults are used later in the recommendation for transfer and development length code expressions. Also note that the modified NASP Bond Test in concrete nearly matches the Standard NASP Bond Test if the concrete strength is only 4 ksi, as compared to the requirement for mortar strength of 4,500 to 5,000 psi. y = 0.5096x R2 = 0.7889 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 0 1 2 3 4 5 Square Root of Concrete Strength at 24 hours ( ksi) N A SP in C on cr et e N or m al iz ed b y N A SP in M or ta r ( kip s/k ips ) NCHRP A NCHRP B NCHRP D NCHRP A6 (0.6") Linear (all) cify = 0.51 Figure 3.13. Normalized NASP pull-out values versus .′fci

38 0.5 in Diameter Strands 0.6 in Diameter Strands Target Release Strength (ksi) Concrete Design Strength (ksi) Target Air Content (%) Strand A Strand B Strand D Strand A6 Two-Strand Rectangular Beams 4 6 2 0 2 2 2 6 10 2 2 0 2 3 6 10 6 2 0 2 0 8 14 2 2 0 2 3 10 15 2 2 0 2 3 Four-Strand Rectangular Beams 6 10 2 2 0 2 0 8 14 2 2 0 2 0 10 16 2 2 0 2 0 I-Shaped Beams 6 10 2 1 0 1 2 10 15 2 1 0 1 2 Table 3.7. Number of transfer length beams and research variables employed. 3.4 Measured Transfer Lengths versus Varying Concrete Strengths and Varying NASP Bond Test Values The research aims at assessing the effects that varying con- crete strength can have on strand bond. This section deals primarily with transfer lengths measured on pretensioned beams. Variables included strand with varying bond quality and concrete strengths varying between 4 ksi at release and 10 ksi at release. Beams were either rectangular in shape or I-shaped. A total of 43 rectangular-shaped beams and 8 I-shaped beams were cast using 4 different strand sources. The number of beams made and the corresponding research variables are reported in Table 3.7. Two-strand rectangular beams included two strands placed near the bottom of the cross section. The four-strand rectangular beams had two strands placed near the bottom and two strands placed near the top. Beams were cast using both 0.5 in. and 0.6 in. diam- eter strands. Figure 3.14 illustrates the beam numbering system that describes the variables that are contained within each beam specimen. Cross section details for the rectangu- lar beams are found in Figures 3.15 and 3.16. Figure 3.17 depicts some of the rectangular beams during fabrication, prior to release of the prestressing strands. Beams were made with a target 1-day concrete strength of 4,000, 6,000, 8,000 and 10,000 psi. The 6,000-psi release strength concrete beams were made using both air-entrained and non- air-entrained concrete to study the effects of air entrainment on transfer lengths. Three different sources for 0.5 in. diam- eter strand and one source for 0.6 in. diameter strand were employed in this research program. Figure 3.18 shows the details of the I-shaped cross section. In the I-shaped beams made with 0.5 in. diameter strands, four strands were located within the bottom bulb of the cross section with a fifth strand located 2 in. from the top of the cross section. In the I-beams made with 0.6 in. diameter strands, three strands were located within the bottom bulb of the cross section with a fourth strand located 2 in. from the top of the cross section. 3.4.1 Fabrication of Beams Transfer lengths were measured at release on all the beams using strand end slips. On some of the beams, transfer lengths were measured using a detachable mechanical strain gage (DEMEC gage), which effectively measures changes in con- crete surface strains. The transfer lengths measured from strand end slips are compared with those measured using the DEMEC gage. The rectangular beams were 17 ft in length with a cross section that was 6.5 in. wide by 12 in. high. Two #6 bars were placed within 1 in. of the top of the cross section in all rectangular beams to ensure ductile flexural failures. The cross section for the I-shaped beams is shown in Figure 3.18. The beams were fabricated 24 ft in length. The beams cast had 0.5 in. diameter strands and 0.6 in. diameter strands. All of the I-beams contained horizontal web rein-

39 RA6-5-1-T Beam Shape R: Rectangular 6.5” x 12” I: “I” Shaped beams Strand Source A, B, or D for 0.5 in. strand A for 0.6 in. strand Nominal Concrete Strength at Release 4, 6, 8, or 10 ksi A6: for 6 ksi with Air Entrainment Strand Size 5 for 0.5 in. diameter 6 for 0.6 in. diameter Specimen Number 1, 2, or 3 is the number in a series of companion beams Top Strand If the rectangular beam contains top strands, T is used. Not applicable for “I” shaped beams Figure 3.14. Beam number identification. 2- #6 Bars 16'-8" #3 Tie at 6" c/c 2 – ½ in. Ø Strands 12 6 ½ 2- ½ in. Ø Strands 17' 2 2 Figure 3.15. Details of four-strand beams. forcement consisting of four or two #4 bars, 96 in. long, lo- cated near the ends of the beams and anchored with stan- dard hooks. Two horizontal #4 bars were placed at the south end of every beam, and four horizontal #4 bars were placed at the north end. The deck slab contained two #3 straight bars in the longitudinal direction in the deck slab. Internal hoop reinforcements were placed in the form of triangular cages at both ends of the beam. Strands were tensioned to 75 percent of fpu (the guaran- teed breaking strength) or 202.5 ksi. The expected elonga- tions were calculated and compared with the measured elongations to ensure proper stressing. The strands were stressed to an initial level of 2,000 lb. Once the force on the strand reached 2,000 lb, the strand was marked with a per- manent marker coinciding with the datum level marking on the prestressing bed. The strand was then stressed to 202.5 ksi. The elongation was then measured as the distance the mark on the strand moved from the datum marking on the prestressing bed. Concrete was batched onsite at Coreslab’s batch plant. Fresh properties of concrete, slump, unit weight, and air con- tent were checked before casting the concrete. Extensive trial batching was performed (Tessema 2006) to determine the fresh and hardened properties of the concrete mix designs. If the fresh properties of unit weight, slump, or air content did not meet with the design expectations, the concrete was not used. Concrete cylinders were made at the site and placed in the same prestressing bed as the test beams until transfer. Steam curing was used if the ambient temperatures were low. The test beams together with the concrete cylinders were kept under cover if steam curing was used. 3.4.2 Measuring Transfer Lengths Transfer lengths were measured on all strands by measur- ing the distance each strand slipped into the concrete after prestress release. A depth micrometer was used in combina- tion with specially made clamps to measure the strand end slip. Figure 3.19 shows the depth micrometer measuring strand end slips immediately after prestress release. Strand end slips are directly related to measured transfer lengths, as shown in Figure 3.20. In Figure 3.20, stresses are used to indicate the loss of prestress caused by elastic short- ening (ES). After release, ES is the primary prestress loss. The transfer length of the strand is directly related to the area of

40 #3 Tie at 6" c/c 2 – #6 Bars 16'-8" 12 6 ½ 2- ½ in. Ø Strands 17' 2 2 Figure 3.16. Details of two-strand beams. Figure 3.17. Fabrication of rectangular beams. 23 1.5 2 10 20.5 3 6.5 20 2324 3 # 3 stirrups at 7" c/c # 4 bars with standard hooks 2" c/c for 96" from ends 4 bars at north end and 2 bars at south end # 3 bars 4" c/c shape for internal hoop reinforcement for 72" from end # 3 bars on deck at 9" c/c and 2 bars throughout the length Prestressing strand Mild steel reinforcement Figure 3.18. Details of I-shaped beams. the shaded triangle shown in Figure 3.20. The shaded area di- vided by the elastic modulus of the strand gives the strand end slip measurement. Thus, by measuring the strand end slip, the transfer length can be calculated directly. Over time, the beam experiences additional losses and a lengthening of the transfer length. The transfer length over time is illustrated in Figure 3.20 by the larger, unshaded triangle. In Figure 3.20, fsi is the stress in the prestressing strand just prior to release, and fse is the strand stress after all losses. ES is the elastic shorten- ing loss that occurs immediately upon release of the pre- stressing force. Changes in concrete surface strains were measured on some of the specimens using a DEMEC gage. The DEMEC gage is pictured in Figure 3.21. DEMEC target points were set at 100-mm spacings. The DEMEC gage spans 200 mm, so read- ings were taken over a 200-mm gage length. The procedure

41 Figure 3.19. Strand end slip measurement using a micrometer. Figure 3.21. Concrete surface strain measurements with DEMEC gage. Lt (fsi – ES) Initial Losses fsi fse Figure 3.20. Variation in strand stress variations with length and relation to strand end slip measurements. 0 50 100 150 200 250 300 350 400 450 0 2 4 6 8 10 12 14 16 Length of the Beam (north to south) Concrete Strains (10-6 in/in) Unsmoothed Profile Smoothed Profile Figure 3.22. Concrete strain profile highlighting strand transfer lengths. requires initial readings to be made prior to strand cutting. After release, the measurements are repeated, and the differ- ences can be plotted as a strain profile, such as the one shown in Figure 3.22. As shown, concrete strains on the north end and the south end are plotted along the length of the beam. The strain profile is “smoothed” by averaging three measure- ment points. The Average Mean Strain (AMS) is found out by averaging the points on the strain plateau on the north and the south sides independently. The measured transfer length obtained from the DEMEC readings is the location where the 95-percent AMS line intersects the Smoothed Strain profile. 3.4.3 Results of the Transfer Length Measurements Results of the transfer length measurements are reported in several tables, generally organized by strand type. Table 3.8 reports the transfer lengths computed from measured strand end slips on Strands A and B. Table 3.8 reports trans- fer lengths only on strands located at the bottom of the cross sections. Table 3.8 reports a transfer length for each

42 Beam Number Location North South X(kips) S (kips) cif (psi) )56( dfc (psi) RB4-5-1 East 17.06 18.31 West 19.78 18.66 RB4-5-2 East 24.13 22.47 West 18.1 22.45 20 .1 2 2. 56 4,033 7,050 RA6-5-1 East 20.66 20.24 West 17.68 16.16 RA6-5-2 East 15.94 11.78 West 17.12 18.23 RA6-5-1T East 19.39 18.7 West 20.62 18.93 RA6-5-2T East 18.7 18.84 West 19.07 16.27 18 .0 2 2. 23 6,183 8,500 RA8-5-1 East 12.01 13.09 West 14.58 13.9 RA8-5-2 East 13.9 11.74 West 15.93 12.42 RA8-5-1T East (a) 12.51 West (b) 14.71 RA8-5-2T East 14.52 15.6 West 12.55 13.36 13 .6 3 1. 32 8,570 13,490 RA10-5-1 East (c) (d) West (e) 13.57 RA10-5-2 East 12.75 15.25 West 12.75 14.8 RA10-5-1T East 17.74 12.06 West 18.16 11.32 RA10-5-2T East 12.2 11.78 West 11.46 14.53 13 .7 2 2. 27 9,711 14,470 (a) Lt of 1.48 in. not included (c) Lt of 25.65 in. not included (b) Lt of 5.26 in. not included (d) Lt of 5.82 in. not included (e) Lt of 22.89 in. not included Table 3.8. Summary of transfer lengths at release for bottom Strands A/B. strand, two at each end of the beam, with each end of the beam designated as either north or south; thus, all together, four transfer length measurements are reported for each beam. Table 3.8 also reports the average transfer length for all of the transfer length measurements on beams for a particular concrete strength, . The standard deviation, S is reported in inches for the data set. The release strength, (psi), is the average of at least three 4 in. by 8 in. cylinders. The 56-day strength, (56d), is the average of three cylinders placed in laboratory curing conditions. Table 3.9 reports the measured transfer lengths on beams that contained air-entrained concrete. Only Strand A was used for this set of beams. While the transfer lengths measured in air-entrained concrete appear to be longer than the transfer lengths measured in the companion beams without air en- trainment, no clear pattern emerges with the limited data. ′fci ′fci X Table 3.10 reports the measured transfer lengths for Strand A, placed near the tops of cross sections in the respective beams. Again, no clear pattern emerges of the top strands having longer transfer lengths than the bottom strands. Table 3.11 reports the measured transfer lengths for 0.5 in diameter Strand D. Table 3.11 includes the beam number, the measured transfer length for each strand, the average transfer length for Strand D by concrete strength, the standard devia- tion of the transfer lengths, and 1-day and 56-day concrete strengths. Table 3.12 reports transfer lengths measured on Strand D placed in top locations of four-strand beams. Again, there is no clear pattern of top strands having longer transfer lengths than bottom strands. Table 3.13 reports transfer lengths measured on I-shaped beams, including data from both 0.5 in. diameter strands— Strand A and Strand D—and data from the 0.6 in. diameter

43 Beam Number Location North South X (kips) S (kips) cif (psi) )56( dfc (psi) RA6A-5-1 East 19.26 17.47 West 16.22 17.88 RA6A-5-2 East 26.41 22.63 West 22.6 21.42 20 .4 9 3. 39 7,960 11,420 RD6A-5-1 East 36.25 30.04 West 34.55 28.15 RD6A-5-2 East 21.16 21.79 West 19.79 18.36 26 .2 6 6. 93 7,960 11,420 Table 3.9. Summary of transfer lengths at release of bottom Strands A/B in air-entrained concrete. Beam Number Location North South X(kips) S (kips) cif (psi) )56( dfc (psi) RA6-5-1T East 21.03 19.11 West 19.47 20.58 RA6-5-2T East 17.07 16.52 West 21.82 16.71 19 .0 4 2. 07 6,183 8,500 RA8-5-1T East 13.38 14.88 West 10.74 14.42 RA8-5-2T East 17.61 15.7 West 15.12 14.56 14 .5 5 1. 96 8,570 13,490 RA10-5-1T East 14.93 13.5 West 14.53 11.32 RA10-5-2T East 10.63 11.2 West 14.11 12.99 12 .9 0 1. 65 9,711 14,470 Table 3.10. Summary of transfer lengths at release of Strand A in top locations. strand, Strand A6. As in Tables 3.8 through 3.12, measured transfer lengths are reported for each strand, the average and standard deviation are reported for each beam, along with concrete strengths at release and at 56 days. Table 3.13 includes data collected from strands located in the bottom bulbs on the I-shaped beams only. Table 3.14 reports the measured transfer lengths on top strands from the I-shaped beams. The data are erratic, so no conclusions can be drawn from these measurements. All of the transfer lengths reported in Tables 3.8 through 3.14 report transfer lengths measured immediately after release. Tables 3.15, 3.16 and 3.17 all include both transfer length measurements made with the DEMEC gage and transfer length measurements made from strand end slips for com- parison. Approximately 43 percent of the beam ends had transfer length measured using both methods. Figure 3.23 presents the data from Tables 3.15 through 3.17 graphically and shows that generally the transfer lengths measured by the DEMEC gage are approximately the same as the transfer lengths obtained from strand end slip measurements. Tables 3.18 through 3.22 provide the transfer length meas- urements over time, from release through 240 days after release. Strand end slips can be measured individually for each strand. In the tables reporting measured transfer lengths from strand end slips, the east strand is represented in the column headed by “E” whereas the west strand is re- ported in the columns headed by “W.” Transfer lengths were not measured beyond release for beams RB4-5-1 and RB4-5-2. As the data indicate, transfer lengths grow over time, and the 240-day transfer lengths are considerably longer than the transfer lengths measured at release. All of the transfer

44 Beam Number Location North South X(kips) S (kips) cif (psi) )56( dfc (psi) RD4-5-1 East 31.69 32.11 West 33.88 29.93 RD4-5-2 East 36.9 (a) West (b) (c) 32 .9 0 2. 64 4,033 7,050 RD6-5-1 East 29.88 30.42 West 30.6 25.71 RD6-5-2 East 25.35 30.15 West 25.84 28.29 RD6-5-1T East 23.89 25.12 West 23.43 26.59 RD6-5-2T East 25.53 19.93 West 24.67 23.71 26 .1 9 2. 99 6,183 8,500 RD8-5-1 East 21.16 20.89 West 19.13 19.41 RD8-5-2 East 16.79 21.43 West 10.54 13.17 RD8-5-1T East 35.63 29.78 West 15.94 26.34 RD8-5-2T East 20.87 21.99 West 18.99 23.01 20 .9 4 6. 05 8,570 13,490 RD10-5-1 East 23.48 16.16 West 28.59 17.54 RD10-5-2 East 13.95 19.33 West 15.74 17.12 RD10-5-1T East 21.76 16.22 West 21.1 17.4 RD10-5-2T East 16.36 15.25 West 17.13 16.58 18 .3 6 3. 72 9,711 14,470 (a) Excessive movement of the beams during flame cutting, Lt observed as 50.43 in. (b) Excessive movement of the beams during flame cutting, Lt observed as 47.48 in. (c) Excessive movement of the beams during flame cutting, Lt observed as 48.98 in. Table 3.11. Summary of transfer lengths at release for bottom Strand D. length measurements over time were made using the strand end slip method. 3.4.4 Discussion of Transfer Length Measurements The discussion on transfer lengths focuses on two essential elements: (1) what effects, if any, concrete strength has on transfer length and (2) whether the NASP Bond Test provides an indicator regarding transfer length. Another objective of this discussion is to present to the industry a reasonable code equation to adequately predict the transfer lengths of preten- sioned strands. Figure 3.24 illustrates the transfer length measurements at release plotted against the concrete strengths at 1 day of age for Strands A/B. (Although Strand A and Strand B represent two different sources of strand, their NASP Bond Test values were very similar; therefore, the data from the two strands are treated as part of one data set.) Two regression curves are shown in Figure 3.24; one shows the best fit for data derived from the DEMEC gage, and the other shows the best fit for the data derived from strand end slip measurements. Both re- gression curves in Figure 3.24 show that transfer lengths shorten as concrete strengths increase. Figure 3.25 shows the transfer length measurements at re- lease plotted against the concrete strengths at 1-day of age for

45 Beam Number Location North South X (kips) S (kips) cif (psi) )56( dfc (psi) RD6-5-1T East 27.91 21.46 West 27.52 20.26 RD6-5-2T East 23.7 20.2 West 23.61 25.43 23 .7 6 3. 03 6,183 8,500 RD8-5-1T East 22.06 16.45 West 17.61 18.84 RD8-5-2T East 27.82 23.71 West 28.67 25.94 22 .6 4 4. 68 8,570 13,490 RD10-5-1T East 16.79 15.56 West 17.27 16.32 RD10-5-2T East 18.98 15.02 West 16.19 11.29 15 .9 3 2. 22 9,711 14,470 Table 3.12. Summary of transfer lengths at release for Strand D in top locations. Beam Number Location North South X(kips) S (kips) cif (psi) )56( dfc (psi) IB6-5-1 East 16.12 6.42 West 17.82 2.9 Cent. 10.93 9.45 Midd. 16 6.48 10 .7 7 5. 43 5,810 9,350 IB10-5-1 East 11.14 12.45 West 10.03 5.8 Cent. 11.6 12.45 Midd. 11.31 9.9 10 .5 9 2. 15 7,615 13,490 ID6-5-1 East 24.47 12.23 West 23.47 2.56 Cent. 26.69 (a) Midd. 28.96 11.04 18 .4 9 9. 88 5,492 9,840 ID10-5-1 East 19.03 19.03 West 20.34 23.61 Cent. 15.99 21.13 Midd 23.51 23.94 20 .8 2 2. 8 8,225 14,160 IA6-6-1 East 18.36 16.33 West 29.83 22.21 Cent. 20.15 20.15 21 .1 7 4. 68 IA6-6-2 East 9.62 14.18 West 15.48 19.47 Cent. 22.58 14.92 16 .0 4 4. 49 4,381 8,990 IA10-6-1 East 9.4 21.15 West 14.35 5.81 Cent. 10.19 18.85 13 .2 9 5. 91 10,480 14,990 IA10-6-2 East 17.94 10.64 West 13.85 10.76 Cent. 17.83 17.32 14 .7 2 3. 46 10,590 14,930 (a) Spalling of concrete surface during flame cutting Table 3.13. Summary of transfer lengths at release for I-shaped beams—bottom Strands B and D (0.5 in.) and Strand A6 (0.6 in.).

46 Beam Number Location Nort h South X (kips) S (kips) ci f (psi ) ) 56 ( d f c (psi ) IA6-6-1 Top 22.84 9.36 IA6-6-2 Top 20.22 21.84 18.57 6.23 4,381 8,990 IA10-6-1 Top 3.82 1.91 2.87 1.35 10,480 14,990 IA10-6-2 Top 9.3 9.04 9.17 0.18 10,590 14,930 IB6-5-1 Top 21.43 6.16 13.80 10.80 5,810 9,350 ID6-5-1 Top 36.25 29.99 33.12 4.4 5,492 9,840 ID10-5-1 Top (a) 16.86 16.86 - 8,225 14,160 (a) End clamp loosened during detensioning Table 3.14. Summary of transfer lengths at release for top strands in I-shaped beams. Strand End Slips DEMEC Beam North (in.) South (in.) North (in.) South (in.) RB4-5-1 18.4 18.5 24.2 27.1 RB4-5-2 21.1 22.5 – – RA6A-5-1 17.7 17.7 16.0 17.5 RA6A-5-2 24.5 22.0 – – RA6-5-1 19.2 18.2 – – RA6-5-2 16.5 15.0 – – RA6-5-1-T 20.3 19.8 – – RA6-5-2-T 19.4 16.6 – – RA8-5-1 13.3 13.5 14.3 12.0 RA8-5-2 14.9 12.1 – – RA8-5-1-T 12.1 14.7 12.0 15.6 RA8-5-2-T 16.4 15.1 – – RA10-5-1 24.3 9.7 24.3 14.4 RA10-5-2 12.8 15.0 – – RA10-5-1-T 14.7 12.4 12.5 11.7 RA10-5-2-T 12.4 12.1 – – IB6-5-1 12.2 Not available 15.2 – IB10-5-1 11.1 Not available 11.0 – – measurements were not taken. Table 3.15. Transfer length at release measured by DEMEC gage and strand end slip for 0.5-in. Strands A/B.

47 Strand End Slips DEMEC Beam North (in.) South (in.) North (in.) South (in.) RD4-5-1 32.8 31.0 25.6 24.8 RD4-5-2 36.9 Not available – – RD6A-5-1 35.4 29.1 39.0 26.4 RD6A-5-2 20.5 20.1 – – RD6-5-1 30.2 28.1 – – RD6-5-2 25.6 29.2 – – RD6-5-1-T 27.7 20.9 – – RD6-5-2-T 23.7 22.8 – – RD8-5-1 20.2 20.2 11.3 18.5 RD8-5-2 13.7 17.3 – – RD8-5-1-T 19.8 17.6 12.4 12.0 RD8-5-2-T 28.2 24.8 – – RD10-5-1 26.0 16.9 23.4 19.4 RD10-5-2 14.8 18.2 – – RD10-5-1-T 17.0 15.9 16.1 15.7 RD10-5-2-T 17.6 13.2 – – ID6-5-1 25.2 Not available 25.9 – ID10-5-1 17.5 Not available 19.7 – – measurements were not taken. Strand End Slips DEMEC Beam North (in.) South (in.) North (in.) South (in.) RA4-6-1 33.4 25.0 31.4 30.3 RA4-6-2 30.2 29.3 – – RA6-6-1 29.7 28.2 22.4 21.1 RA6-6-2 31.7 30.1 – – RA6-6-3 25.8 33.6 – – RA8-6-1 28.2 29.2 19.5 22.0 RA8-6-2 28.2 25.7 – – RA8-6-3 22.8 28.3 – – RA10-6-1 20.0 21.9 16.6 15.0 RA10-6-2 15.6 21.8 – – RA10-6-3 16.3 22.7 – – IA6-6-2 24.3 26.1 15.9 16.2 IA10-6-1 18.0 Not available 11.3 – IA10-6-2 16.0 Not available 16.5 – – measurements were not taken. Table 3.16. Transfer length at release measured by DEMEC gage and strand end slip for 0.5-in. Strand D. Table 3.17. Transfer Length at release measured by DEMEC gage and strand end slip for 0.6-in. Strand A6.

48 R2 = 0.5768 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 Transfer Length from End Slips (in) Tr an sf er L en gt h fr om D EM EC (in ) 0.5" A Strand 0.5" B Strand 0.5" D Strand 0.6" A6 Strand Perfect Fit Linear (All Strands) Figure 3.23. Transfer lengths measured by DEMEC gage versus transfer lengths measured by strand end slip. Transfer Length at Release from Strand End Slips (in.) Transfer Length after 60 Days from Strand End Slips (in.) Transfer Length after 90 Days from Strand End Slips (in.) Transfer Length from 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number Average W & E Average W & E Average W & E Average W & E RB4-5-1 18.42 18.48 RB4-5-2 21.11 22.46 RA6-5-1 19.17 18.20 33.07 28.86 33.07 29.55 33.69 30.03 RA6-5-2 16.53 15.01 26.57 20.82 26.56 23.38 27.95 23.52 RA6A-5-1 17.74 17.68 25.23 26.62 26.33 28.14 26.54 28.55 RA6A-5-2 24.50 22.02 28.92 27.72 31.41 29.03 31.75 29.38 RA8-5-1 13.30 13.50 15.59 21.13 17.68 21.46 24.91 22.54 RA8-5-2 14.92 12.08 22.07 19.24 23.69 19.71 35.23 19.98 RA10-5-1 24.27 9.69 23.92 9.83 24.13 12.11 24.34 13.14 RA10-5-2 12.75 15.02 16.47 16.67 18.05 17.23 19.15 17.30 Table 3.18. Change in transfer lengths over time for bottom 0.5 in. diameter Strands A/B in two-strand rectangular beams.

49 Transfer Length at Release from Strand End Slips (in.) Transfer Length after 60 Days from Strand End Slips (in.) Transfer Length after 90 Days from Strand End Slips (in.) Transfer Length from 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number Average W & E Average W & E Average W & E Average W & E RD4-5-1 32.78 31.02 RD4-5-2 42.19 49.70 RD6-5-1 30.24 28.07 43.00 38.57 46.82 44.37 49.75 45.26 RD6-5-2 25.60 29.22 36.79 39.87 41.99 44.72 44.24 48.27 RD6A-5-1 35.40 29.10 37.39 34.47 39.40 36.41 39.94 37.16 RD6A-5-2 20.48 20.08 26.26 35.24 30.73 39.37 32.39 40.07 RD8-5-1 20.15 20.15 28.34 26.55 32.66 30.33 39.08 34.54 RD8-5-2 13.66 17.30 34.14 46.08 36.82 47.73 37.38 50.41 RD10-5-1 26.03 16.85 26.31 25.27 26.45 26.51 30.24 27.14 RD10-5-2 14.85 18.23 17.47 20.16 18.71 22.30 22.30 22.03 Table 3.19. Change in transfer lengths over time for bottom 0.5-in. diameter Strand D in two-strand rectangular beams. Transfer Length at Release from Strand End Slips (in.) Transfer Length after 60 Days from Strand End Slips (in.) Transfer Length after 90 Days from Strand End Slips (in.) Transfer Length from 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number Average W & E Average W & E Average W & E Average W & E RA4-6-1 33.42 24.98 RA4-6-2 30.24 29.35 RA6-6-1 29.73 28.19 36.87 41.73 39.00 44.45 40.85 55.13 RA6-6-2 31.65 30.10 47.03 46.36 49.24 48.20 52.18 49.37 RA6-6-3 25.83 33.63 39.73 44.60 44.08 44.82 44.96 45.93 RA8-6-1 28.21 29.17 42.46 41.87 43.87 43.26 45.48 43.41 RA8-6-2 28.20 25.70 42.68 38.55 46.28 42.35 46.35 42.37 RA8-6-3 22.80 28.26 36.85 44.00 41.17 46.93 43.00 49.22 RA10-6-1 20.03 21.92 25.69 25.77 28.08 28.82 29.98 32.15 RA10-6-2 15.62 21.78 20.99 25.99 26.14 29.47 26.79 30.70 RA10-6-3 16.34 22.73 24.46 28.82 26.13 32.30 27.73 33.32 Table 3.20. Change in transfer lengths over time for 0.6-in. Strand A6 in two-strand rectangular beams.

50 Transfer Length at Release from Strand End Slips (in.) Transfer Length after 60 Days from Strand End Slips (in.) Transfer Length after 90 Days from Strand End Slips (in.) Transfer Length after 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number and Location Average W & E Average W & E Average W & E Average W & E RA8-5-1-T (Top) 12.06 14.65 24.27 24.67 24.96 26.18 25.16 27.21 (Bottom) 3.37 13.61 11.59 21.29 11.66 25.11 12.80 27.27 RA8-5-2-T (Top) 16.37 15.13 27.30 27.30 28.20 28.68 28.96 29.44 (Bottom) 13.54 14.48 22.03 24.25 23.31 25.40 24.05 25.33 RA6-5-1-T (Top) 20.25 19.84 33.01 32.69 34.46 34.96 34.60 34.89 (Bottom) 20.00 18.82 31.92 26.29 34.06 28.36 34.06 28.56 RA6-5-2-T (Top) 19.44 16.61 37.07 35.15 39.33 37.28 40.64 37.49 (Bottom) 18.89 17.55 45.33 35.77 47.82 42.71 49.55 43.34 RA10-5-1-T (Top) 14.73 12.41 21.59 18.79 22.16 19.43 22.16 19.70 (Bottom) 17.95 11.69 19.00 14.40 19.07 15.30 19.62 15.93 RA10-5-2-T (Top) 12.37 12.10 14.29 22.15 15.42 22.22 15.63 22.36 (Bottom) 11.83 13.16 16.28 16.01 16.56 16.29 17.33 16.43 Table 3.21. Change in transfer length over time for 0.5-in. Strand A in four-strand rectangular beams. Transfer Length at Release from Strand End Slips (in.) Transfer Length after 60 Days from Strand End Slips (in.) Transfer Length after 90 Days from Strand End Slips (in.) Transfer Length from 240 Days from Strand End Slips (in.) North South North South North South North South Beam Number and Location Average W & E Average W & E Average W & E Average W & E RD8-5-1-T (Top) 19.84 17.64 35.57 35.90 40.15 39.59 41.18 42.47 (Bottom) 25.78 28.06 23.98 38.41 30.63 41.05 27.73 44.59 RD8-5-2-T (Top) 28.25 24.82 65.51 67.04 67.56 68.62 68.52 68.62 (Bottom) 19.93 22.50 49.52 32.36 50.91 33.26 52.86 35.00 RD6-5-1-T (Top) 27.71 20.86 53.89 56.65 57.32 59.03 58.79 60.29 (Bottom) 23.66 25.85 38.10 38.09 40.76 42.20 42.89 45.27 RD6-5-2-T (Top) 23.66 22.81 49.07 48.64 57.90 53.33 63.27 54.49 (Bottom) 25.10 21.82 65.45 39.67 69.56 44.05 RD10-5-1-T (Top) 17.03 15.94 26.10 24.12 27.87 26.36 30.19 27.11 (Bottom) 21.43 16.81 23.51 19.77 23.51 21.63 23.77 RD10-5-2-T (Top) 17.58 13.15 24.81 23.58 26.30 24.95 26.58 26.99 (Bottom) 16.74 15.92 24.05 23.01 25.98 23.01 28.18 23.01 Table 3.22. Change in transfer length over time for 0.5-in. Strand D in four-strand rectangular beams.

51 From Strand End Slips R2 = 0.6429 From DEMEC R2 = 0.4215 0 5 10 15 20 25 30 35 40 0 40002000 6000 8000 10000 12000 Concrete Strengths (psi) Tr an sf er L en gt hs (in .) Strand End Slip DEMEC Omitted Data Figure 3.24. Transfer length versus for Strands A/B in rectangular beams.′fci From DEMEC R2 = 0.3195 0 5 10 15 20 25 30 35 40 0 40002000 6000 8000 10000 12000 Concrete Strengths (psi) Tr an sf er L en gt hs (in .) Strand End Slip DEMEC Omitted Data From Strand End Slips R2 = 0.6556 Figure 3.25. Transfer length versus for Strand D in rectangular beams.′fci Strand D. Again, it is clear that the transfer length decreases with increasing concrete strength. Finally, Figure 3.26 illustrates the transfer length measure- ments taken on beams made with the 0.6 in. diameter strand, Strand A6. Again, the data clearly show the inverse relation- ship between transfer lengths and concrete strength. The data from all three of the strand sources are illustrated in Figure 3.27, where the transfer lengths for each strand are plotted against the concrete strengths at release. Figures 3.24 through 3.27 show the relation between trans- fer length data and linear regression models. Linear regression is often used because the methodology is less abstract than others and perhaps more easily understood. However, there is a direct relationship between the NASP Bond Test values in concrete and the square root of concrete strengths. Figures 3.12 and 3.13 show a strong correlation between the NASP Bond Test value and the square root of concrete strength. The coefficient of determination in those comparisons is a very robust 0.8. If the NASP Bond Test value, which is a direct measure of bond between the strand and concrete, varies with the square root of concrete strength, then it is logical that the transfer length would also vary with the square root of concrete strength. Figure 3.28 plots the same data as Figure 3.27, but does a best-fit curve from power regressions. The coefficients of determination for these power curves are nearly as good as the coefficients of determination for the linear regressions. Furthermore, the best-fit regressions provide an exponent in the equation of −0.56, −0.83 and −0.56. As a reminder, the inverse of the square root would be an exponent of −0.50. Figure 3.29 plots the transfer lengths for Strands A/B at re- lease and at 240 days after release. The data are fitted to a

52 From Strand End Slips R2 = 0.6399 From DEMEC R2 = 0.687 0 5 10 15 20 25 30 35 40 0 2000 4000 6000 8000 10000 12000 Concrete Strengths (psi) Tr an sf er L en gt hs (in .) Strand End Slip DEMEC Figure 3.26. Transfer length versus for Strand A6 (0.6 in) in rectangular beams. ′fci Strands A/B R2 = 0.6429 Strand D R2 = 0.6556 Strand A6 (0.6 in.) R2 = 0.6399 0 5 10 15 20 25 30 35 40 45 0 2 4 6 8 10 12 14 Concrete Strength (ksi) Tr an sf er L en gt h (in .) Strands A/B Strand D Strand A6 (0.6 in.) Figure 3.27. Linear regression for transfer lengths and f′ci. Strand A/B R2 = 0.6112 Strand D R2 = 0.5815 Strand A6 (0.6 in.) R2 = 0.6526 0 10 20 30 40 50 60 70 0 2 4 6 8 10 12 14 Concrete Strength (ksi) Tr an sf er L en gt h (in .) Strand A/B Strand D Strand A6 (0.6 in.) Figure 3.28. Power regression for transfer lengths and f′ci.

53 power regression curve. The best-fit equations are also shown in Figure 3.29. In Figures 3.29 through 3.32, transfer length data obtained immediately after release are represented by diamond-shaped data points and the solid regression curve. Transfer lengths measured at 240 days are represented by triangular-shaped data points and the dashed regression curve. Figure 3.30 plots the transfer lengths for Strand D at both release and at 240 days after release. Again, these data are fit- ted to a power regression curve. Note that the transfer lengths for Strand D are considerably longer than those for Strands A/B. Recall that Strand D had a NASP Bond Test value of 6,890 lb, whereas both Strands A and B had NASP Bond Test values in excess of 20,000 lb (see Table 3.3). These data would support the idea that higher NASP Bond Test values will result in shorter transfer lengths. Figure 3.31 plots the same data but for the 0.6 in. diameter strand, Strand A6. Again, the data clearly show that transfer lengths decrease with increasing concrete strength. y = 44.79x-0.56 R2= 0.72 y = 149.93x-0.8913 R2 = 0.3886 0 10 20 30 40 50 60 0 2 4 6 8 10 12 14 Concrete Strength at Release (ksi) Tr an sf er L en gt h at R el ea se (in .) Figure 3.29. Transfer lengths versus concrete strengths for 0.5-in. Strands A/B at release and at 240 days. y = 115.53x-0.83 R2 = 0.71 y = 432.02x-1.19 R2 = 0.63 0 10 20 30 40 50 60 0 2 4 6 8 10 12 14 Concrete Strength at Release (ksi) Tr an sf er L en gt h at R el ea se (in .) Figure 3.30. Transfer lengths versus concrete strengths for 0.5-in. Strand D at release and at 240 days. y = 68.78x-0.56 R2 = 0.68 y = 156.73x-0.75 R2 = 0.87 0 10 20 30 40 50 60 0 2 4 6 8 10 12 Concrete Strength at Release (ksi) Tr an sf er L en gt h at R el ea se (in .) Figure 3.31. Transfer lengths versus concrete strengths for 0.6-in. Strand A6 at release and at 240 days.

54 y = 67.78x-0.46 R2 = 0.58 y = 103.17x-0.45 R2 = 0.49 y = 97.2x-0.5 0 10 20 30 40 50 60 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 Normalized NASP Value (kips) Tr an sf er L en gt h at R el ea se (in .) Transfer Lengths at Release vs. Normalized NASP Test Transfer Lengths at 240 d Design Curve for Transfer Length Power (Transfer Lengths at Release vs. Normalized NASP Test) Power (Transfer Lengths at 240 d) Power (Design Curve for Transfer Length) Figure 3.32. Transfer lengths versus normalized NASP bond values for Strands A/B and plotted together at release and at 240 days. Finally, in Figure 3.32, the transfer lengths at release and at 240 days after release are plotted against the normalized NASP Bond Test value. The normalized NASP Bond Test value is obtained from Equation 3.1. In Equation 3.1, the nor- malized value can be obtained because the ratio of the NASP Bond Test Value in concrete to the standard NASP Bond Test value (in mortar) is essentially equal to one-half of the square root of the concrete strength at 1 day. In this manner, data from strands with widely dissimilar NASP Bond Test values can be plotted on the same chart and the results compared. In Figure 3.32, we see that the power regression curve fits through both sets of data. The data set shown with lower NASP Bond Test values, toward the left side of the chart, are the data derived from Strand D; data with higher NASP Bond Test values, toward the right side of the chart, are obtained from Strands A/B. The power regression curve shows a best fit with an exponent of −0.46. Also plotted on Figure 3.32 is the curve that corresponds to the proposed equation for transfer length. The normalized NASP Bond Test value is obtained from Equation 3.1. For ex- ample, for a concrete strength of 4 ksi, the transfer length should be 60 strand diameters. For 0.5 in. diameter strand, the transfer length would be 30 in. The data illustrated in Figure 3.32 show that transfer lengths are shortened with increasing NASP Bond Test values. However, it is not pro- posed to shorten the transfer length equation as a function of NASP Bond Test values. It is worth noting, however, that the data illustrated in Figures 3.12, 3.13, and 3.32 clearly show that increases in concrete strength have a similar effect in improving bond, as do the increasing NASP Bond Test val- ues. The proposed design equation shown indicates that transfer length can be obtained by dividing 97.2 in. by the square root of the normalized NASP Bond Test value. The normalized NASP Bond Test value factors in the same factor for the square root of the concrete strength. The proposed design equation will provide a transfer length of 60 strand diameters for concrete strength of 4 ksi. Increasing concrete strengths will reduce the proposed transfer length in propor- tion to the square root of the concrete strength. 3.5 Development Length Tests Measured transfer and development lengths of prestress- ing strands are indications of the quality of bond between the strand and concrete. The research conducted as part of NCHRP Project 12-60 and described earlier resulted in five overarching conclusions: 1. The Standard NASP Bond Test method provides a reliable and repeatable method to test for the bond performance of prestressing strands. Results were found to be repeat- able at different testing sites. 2. The Standard NASP Bond Test is able to determine ac- ceptable quality levels for bond of prestressing strands. This ability is demonstrated by the correlation of NASP Bond Test results with measured transfer lengths. In- creases in measured transfer lengths correlate directly with decreases in bond performance, as measured by the NASP

55 Bond Test. The development length tests reported in this chapter supplement the findings from the NASP Bond Tests and transfer length measurements. 3. The NASP Bond Test can be modified by testing strand bond performance in concrete instead of mortar. The modified NASP Bond Tests in concrete demonstrate that increases in concrete strength result in improving bond performance. The results develop a strong statistical cor- relation, and the best fit indicates that bond strength improves in proportion to the square root of concrete strength. 4. Concrete strength influences the bond of prestressing steel with concrete. In the NASP Bond Tests (modified), in- creasing concrete strengths resulted in increasing bond strength between strand and concrete. In beams where transfer lengths were measured, increasing concrete strength correlated to shortening transfer lengths. The measured pull-out forces from the NASP Bond Tests established that bond strength improves in proportion to the square root of concrete strength at release. 5. The transfer length data further establish that measured transfer lengths decrease in inverse proportion (approxi- mately) to the square root of concrete strength. The influ- ence of concrete strength and NASP Bond Test value correlates with the inverse of strand end slip measure- ment, which is a direct indicator of transfer lengths. The development length tests are necessary to determine the following: • Whether the NASP Bond Test can be used as a predictor of strand bond performance in flexural applications, • The minimum acceptable level of bond performance as measured by the NASP Bond Test, and • What modifications are necessary to the LRFD develop- ment length equation to account for variations in concrete strength. 3.5.1 Testing Program The experimental program consisted of the flexural tests on two types of beam specimens: • Rectangular beam specimens. In all, 43 rectangular beam specimens were fabricated with target release concrete strengths varying from 4 ksi to 10 ksi. Rectangular beams were cast with two prestressing strands at a depth of 10 in. in a beam 12-in. deep. Both 0.5 in and 0.6 in diameter strands were used. The rectangular beams were 7 ft in length and designed to be tested independently at each end to assess the development length of embedded strands. Prior to development length testing, transfer lengths were measured on each beam end, either directly by measuring concrete surface strain or indirectly by measuring strand end slip before release. • I-shaped beam specimens. In all, eight I-shaped beam specimens were fabricated with target concrete release strengths of 6 ksi and 10 ksi. These beams were 24 ft in length and designed to be tested at each end. Transfer lengths were also measured on these beams prior to devel- opment length testing. 3.5.1.1 Terminology The testing program terminology was as follows. Embedment length, le. For the purposes of this research and generally in the broader literature, the embedment length is the length of bond provided from the beginning of bond (usually at the end of the beam) to the critical section of the beam. The critical section in these tests is generally under- stood to be the section where maximum moment occurs. In this testing program, the embedment length is the distance from the end of the beam to the point of loading, which corresponds to the point of maximum moment. Development length, ld. Development length of prestress- ing strands is the minimum distance from the free end of the strand over which the strand should be bonded to concrete so that the section under consideration achieves its full nominal capacity. Flexural bond length. The flexural bond length is meas- ured from the section where the prestressed force is fully effective (at the end of the transfer length) to the critical section. In these tests, the flexural bond length is equivalent to the embedment length minus the transfer length. Often, the flexural bond length is used in conjunction with the development length, ld . In that case, the embedment length is the development length minus the transfer length. 3.5.1.2 Beam Identification System and Section Properties Each beam carries a unique identifying name. The system for identification is described in Figure 3.14. The identifica- tion system indicates the following beam characteristics: shape (rectangular [R] or I-shaped [I]), strand source, strand size, nominal concrete strength at release, and specimen number in a series. The section properties and materials are described in the sections under transfer length. Rectangular beams 17 ft in length were fabricated with two strands in each beam. Longitudinal top steel was in- cluded in the cross section to provide additional compres- sion reinforcement and to ensure under-reinforced flexural

56 Figure 3.33. Typical loading geometry for rectangular beams. conditions at capacity. As shown in Figures 3.15 and 3.16, #3 stirrups, or “ties,” were provided on 6-in. centers. The nominal flexural capacity, Mn, of the rectangular beams var- ied from about 700 k-in. for the lower strength concrete (nominal 4 ksi at release) to approximately 754 k-in. for the 10 ksi (release) concrete. Four-strand beams were cast for transfer length measure- ments with two strands in the bottom of the cross section and two strands at the top of the cross section. Four-strand beams were not tested for development length and are not discussed in this chapter. Figure 3.18 shows the cross-section of the I-shaped beam with the reinforcement details. Each I-shaped beam was cast with a length of 24 ft. Top flanges were reinforced longitudi- nally with two #3 bars that ran the length of the beam. Trans- verse reinforcement in the top flange consisted of #3 bars at 9-in. centers over the beam length. Stirrups were made from #3 bars with standard 90° hooks and spaced at 7-in. centers. Stirrups were arranged so that the legs alternated directions. Horizontal reinforcement was placed in the webs of each end of each I-shaped beam. 3.5.1.3 Loading Geometry Both rectangular and I-shaped beams were designed to be tested on both ends, enabling a distinct development length test at each beam end. The loading geometry varied from end to end so that a different embedment length was tested at each end. Embedment lengths varied for each test and were cho- sen depending upon results from prior tests. The typical loading geometries for rectangular beams with 0.5-in. strands are shown in Figure 3.33. The geometry shown for the south end corresponds with an embedment length of 58 in., which is approximately 80 percent of the computed devel- opment length requirement. The geometry shown for the north end corresponds with an embedment length of 73 in., which is approximately equivalent to the AASHTO LRFD and ACI re- quirements for development length. Rectangular beams with 0.6-in. strands required longer embedment lengths than those shown in Figure 3.33. The two testing lengths, 73 in. and 58 in., were established through testing programs conducted by Rose and Russell (1997) and Logan (1997). The typical loading geometries for the I-shaped beams are shown in Figure 3.34. The geometry illustrated is typical for

57 beams made with 0.5 in. strands. The test on the south end shows an embedment length of 58 in. The north end shows development length test geometry for an embedment length approximately equal to the LRFD and ACI requirements, 72 in. As with the rectangular beam series, tests on I-shaped beams with 0.6 in. diameter strands required longer embed- ment lengths than those shown in Figure 3.34. 3.5.1.4 Test Frame The test frame was designed to perform flexural tests on both rectangular beams and I-beams. The photograph in Figure 3.35 shows the test frame with a beam in position for testing. The test frame has four sides that form a rectangular “frame.” Load is applied through a hydraulic actuator (attached to the top hori- zontal beam in the frame) to a spreader beam (attached to the bottom of the actuator). The spreader beam distributes loading to the pretensioned concrete test beam. The loading geometry was arranged so that constant bending moment is applied between the two load points. In this picture, the beam that is being tested is a rectangular beam. It is supported by a pin on the near end and a roller at the far end. 3.5.1.5 Instrumentation The following instrumentation was used. Electronic data acquisition. Load, hydraulic pressure, beam deflection, and strand end slips were measured and recorded by an electronic data acquisition system. Data were sampled and recorded at regular intervals without manual prompting. The rate of sampling was fixed at 1.0 Hz, which provided smooth transition of load, displacement, and strand end slip values. The data were stored on a laptop computer Figure 3.34. Typical loading geometry for I-shaped beams.

58 Figure 3.35. Test frame with a rectangular beam readied for testing rupture. Figure 3.36. Wire transducers (foreground) and a dial gage. and were then available for analysis. During each development length test, data were also recorded manually in the event that electronic data were corrupted by unforeseen circumstance. Load. Load was measured electronically with a load cell placed between a spherical head under the hydraulic actuator and the spreader beam. The load cell can be seen in Figure 3.35 just above the steel loading beam. Load was applied hydraulically, and the hydraulic pressure was also monitored electronically by a pressure transducer. The pressure trans- ducer also sent electronic signals to the data acquisition sys- tem for monitoring and recording. A hydraulic pressure gage was employed during the test for visual observations and manual recording. Deflection. Wire transducers with a range of 30 in. and accuracy ±0.005 in. were used to determine the vertical de- flection. Deflection was measured below the center of the loading point. Two wire transducers were used to measure de- flection, one on each side of the beam, so that any twisting of the beam would be taken out when computing the average be- tween the two sides. Data from the wire transducers were recorded and stored electronically. In addition to the elec- tronic data, a dial gage with a precision of one one-thousandth of an inch was used to manually record deflection readings. The dial gage was also used to monitor displacements when the testing switched from load-controlled testing to displace- ment controls. The wire transducers and the dial gage are shown in Figure 3.36. Strand end slip. Linear voltage displacement transducers (LVDTs) were used to measure strand movement relative to the concrete. The LVDTs had a stroke limit of 1.0 in. and recorded strand end slips to one one-thousandth of an inch. Clamps were attached to the strands, and LVDTs were mounted on these clamps at a location providing an initial reading of approxi- mately 0.9 in. with an error of (±0.003 in. Strand end slips were measured and recorded for each strand on the “test” end. The photograph in Figure 3.37 shows the LVDTs clamped to strands to measure strand end slip relative to concrete. At the far end of the beam, or at the end of the beam opposite the end being tested, strand end slips were measured by a mechanical deflection gage with an electronic readout. The device and arrangement are shown in Figure 3.38. Measurements with a precision of ±0.005 in. were possible using this technique. 3.5.1.6 Testing Procedure For each test, the instrument readings were initialized prior to the application of external load. Load was then applied to beams in regular load increments. Load was applied manually by an hydraulic pump. At all load increments, values of load, displacement, and strand end slips, as well as DEMEC read- ings (wherever applicable) were noted and recorded manually. In addition to electronic data being stored at the 1-Hz refresh rate on the data acquisition system, data were recorded man- ually. Once cracking began, cracks were marked with perma-

59 Figure 3.37. LVDTs clamped to strands to measure strand end slip relative to concrete. Figure 3.38. Mechanical deflection gage arrangement for measuring strand end slips at the beam end opposite to the test end. nent markers as soon as they were observed. The loads at which the cracks first appeared were noted alongside of the cracks. Photographs were taken at regular intervals to record cracking patterns. As displacements became larger with smaller increments in load, the manual system of loading switched from regular load increments to regular displacement increments. This was done arbitrarily by the researchers conducting the test. At each load or displacement increment, manual readings of hydraulic pressure and beam displacements were made. Additionally, manual and electronic instruments were checked to determine whether strand end slip had occurred during the prior loading increment. Loading was continued until failure. Failure was defined by the beam’s inability to sustain or maintain load with increasing deflections or by abrupt failures of the concrete or strand. Throughout the test, manual readings at every load increment were noted along with any significant develop- ment such as first flexural crack, first shear crack, appear- ance of flexure-shear crack, first strand end slip, concrete spalling, concrete crushing, and any audible developments. Written summaries of each development length test appear in the appendices. Detailed progress of each test was docu- mented and is included along with significant photographs and data plots in Appendices C through G. Also, plots of moment versus deflection, strand end slip versus deflec- tion, and shear versus average shear strain were plotted from the acquired data. Shear strains were measured from DEMEC target points attached to the webs of I-shaped beams. Shear stress was determined by dividing the shear force applied by the product of the web width and the beam depth. 3.5.2 Experimental Results from Development Length Testing All together, 50 flexural tests were performed on rectangu- lar beams and 14 tests on I-shaped beams. All of these tests were carried out at the Civil Engineering Laboratory at OSU. Most of the beams were tested on both ends. For each beam test, the embedment length was determined on the basis of various factors, including the AASHTO development length equation with changes to account for prior results, concrete strength, or strand bond strength. In this section, Tables 3.23 through 3.27 report the results from development length test- ing. These tables report on the following parameters: • Concrete strength at release; • Concrete strength at 56 days; • Average NASP Bond Test value for the strands contained in the beams; • Embedment length for each individual test; • Test span; • Failure Moment, which is the maximum applied moment measured during the test; • Percentage of the Failure Moment to the nominal flexural capacity, Mn, as determined by strain compatibility. The calculation for Mn assumes that the strands are fully devel- oped; no reduction in flexural capacity was assumed for embedment lengths provided that are less than the calcu- lated development length; • Maximum beam deflection; • Maximum strand end slip; and • Classification for each type of failure.

60 Beam End cf @ Release (psi) cf 56 Days (psi) Avg. NASP Pull-Out Value (lb) Avg. Lt @ Release (in) Avg. Lt (56-Day or @ Test) (in) Actual Le (in) Span (in) Failure Moment (kip-in) %Mn Deflection @ Failure (in) Max. End Slip (in) Failure Mode RD-4-5-1-N 4,033 7,050 6,890 32.79 38.54 73 162 804 115 3.4 0.00 Flexure RD-4-5-1-S 4,033 7,050 6,890 31.02 42.28 58 132 759 108 1.6 0.35 Bond RD-4-5-2-N 4,033 7050 6,890 42.19 63.05 73 162 831 119 2.7 0.40 Flexure RD-4-5-2-S 4,033 7,050 6,890 49.71 51.81 58 132 513 73 2.5 0.57 Bond RD-6-5-1-N 6,183 8,500 6,890 30.24 49.75 73 162 797 111 2.5 0.06 Flexure RD-6-5-1-S 6,183 8,500 6,890 28.07 45.26 58 132 788 109 2.0 0.18 Flexure RD-6-5-2-N 6,183 8,500 6,890 25.60 44.24 73 162 735 102 2.0 0.01 Flexure RD-6-5-2-S 6,183 8,500 6,890 29.22 48.27 58 132 724 100 2.0 0.25 Bond RD-6A-5-1-N 7,960 11,420 6,890 35.4 39.94 73 162 794 106 2.3 0.00 Flexure RD-6A-5-1-S 7,960 11,420 6,890 29.1 37.16 58 132 805 108 2.5 0.08 Flexure RD-6A-5-2-S 7,960 11,420 6,890 20.08 40.07 58 132 778 104 1.9 0.02 Flexure RD-8-5-1-N 8,570 13,490 6,890 20.15 39.08 73 162 811 107 2.6 0.00 Flexure RD-8-5-1-S 8,570 13,490 6,890 20.15 34.54 58 132 805 106 2.6 0.08 Flexure RD-8-5-2-N 8,570 13,490 6,890 13.67 37.38 58 132 775 102 2.2 0.08 Flexure RD-8-5-2-S 8,570 13,490 6,890 17.30 50.41 58 132 813 107 2.0 0.00 Flexure RD-10-5-1-N 9,711 14,470 6,890 26 30.24 58 132 821 108 2.1 0.00 Flexure RD-10-5-1-S 9,711 14,470 6,890 13.57 27.14 46 120 819 107 2.6 0.00 Flexure RD-10-5-2-N 9,711 14,470 6,890 14.85 22.30 58 132 788 103 1.9 0.00 Flexure RD-10-5-2-S 9,711 14,470 6,890 18.23 22.03 46 120 794 104 1.9 0.01 Flexure Table 3.23. Development length test results on rectangular beams containing Strand D. 3.5.2.1 Tabulated Beam Test Results—Rectangular Beams Table 3.23 reports the results from development length tests on rectangular beams made with Strand D. Strand D was the 0.5 in. strand with the lower NASP pull-out value, 6,890 lb. Concrete strengths at release varied from a target of 4 ksi to a target of 10 ksi. 56-day concrete strengths ranged from 7.05 ksi to 14.47 ksi. Table 3.23 reports only three bond failures, all occurring with lower strength concretes. Also, all of the bond failures occurred at an embedment length of only 58 in., which is approximately 80 percent of the ACI- and AASHTO- prescribed development lengths. Of the three bond failures, two occurred at an applied moment that matched or exceeded Mn, the nominal flexural capacity for the beams. Table 3.23 also shows that at higher strengths, in general, flexural failures were observed in all tests. For example, two ends of the beams with 14.47 ksi concrete were tested with an embedment length of only 46 in., or approximately 63 percent of ld. In these cases, the development length test resulted in flexural failures with- out bond slip (beams RD-10-5-1-S and RD-10-5-2-S). Table 3.23 also reports the maximum strand end slip that occurred during testing, which corresponds to the maximum strand end slip measured at the time the beam failed, whether a flexural failure or a bond failure. Note that it is not uncom- mon for strand end slips to be measured even though a beam fails in flexure. For example, RD-4-5-2-N failed in flexural at a load that exceeded its nominal capacity by 19 percent. Further, the beam achieved adequate ductility as demon- strated by 2.7 in. of overall deflection while sustaining capac- ity. However, the measured strand end slip was 0.40 in. This finding is consistent with other research that has been performed to date. More notably, the results in Table 3.23 demonstrate that the measured strand end slips decrease measurably with increasing concrete strengths. At higher concrete strengths, strand end slips did not occur. Overall, the results support a conclusion that higher concrete strengths result in increasing bond strength and reducing the required development lengths. Detailed testing summaries on each development length test are found in Appendix C. Table 3.24 reports the results from development length tests on rectangular beams made with Strands A/B. Strands A

61 Beam End cf @ Release (psi) cf 56 Days (psi) Avg. NASP Pull-Out Value (lb) Avg. Lt @ Release (in) Avg. Lt (56-Day or @ Test) (in) Actual Le (in) Span (in) Failure Moment (kip-in) %Mn Deflection @ Failure (in) Max. End- Slip (in) Failure Mode RA-6-5-1-N 6,183 8,500 20,950 19.2 33.70 73 162 790 110 2.1 0.00 Flexure RA-6-5-1-S 6,183 8,500 20,950 18.2 30.03 58 132 800 111 2.1 0.00 Flexure RA-6-5-2-N 6,183 8,500 20,950 16.5 28.00 58 120 772 107 1.5 0.00 Flexure RA-6-5-2-S 6,183 8,500 20,950 15.01 23.50 46 120 777 108 1.5 0.00 Flexure RA-6A-5-1-N 7960 11,420 20,950 17.74 26.54 73 162 769 103 2.4 0.00 Flexure RA-6A-5-1-S 7,960 11,420 20,950 17.68 28.55 58 132 770 103 1.7 0.00 Flexure RA-6A-5-2-N 7,960 11,420 20,950 24.51 31.75 58 132 788 105 1.9 0.00 Flexure RA-6A-5-2-S 7,960 11,420 20,950 22.03 29.38 46 120 788 105 1.7 0.01 Flexure RA-8-5-1-N 8,570 13,490 20,950 13.3 24.91 58 132 829 109 1.7 0.01 Flexure RA-8-5-1-S 8,570 13,490 20,950 13.5 22.54 46 120 832 110 1.9 0.00 Flexure RA-10-5-1-N 9,711 14,470 20,950 24.27 24.34 58 132 788 103 1.7 0.00 Flexure RA-10-5-1-S 9,711 14,470 20,950 9.69 13.14 46 120 796 104 1.7 0.00 Flexure RB-4-5-1-N 4,033 7,050 20,210 18.42 22.10 73 162 776 111 1.9 0.00 Flexure RB-4-5-1-S 4,033 7,050 20,210 18.49 20.51 58 132 802 114 2.0 0.00 Flexure RB-4-5-2-N 4,033 7,050 20,210 21.12 22.52 73 162 721 103 2.4 0.00 Flexure RB-4-5-2-S 4,033 7,050 20,210 22.46 23.75 58 132 748 107 1.7 0.00 Flexure Table 3.24. Development length test results on rectangular beams containing Strands A/B. Beam End c f @ Release (psi ) c f 56 Days (psi ) Avg. NASP Pull-Out Valu e (lb) Avg. L t @ Release (in) Avg. L t (56-Day or @ Test) (in) Actual L e (in) Span (in) Failure Moment (kip-in) % M n Deflection @ Failure (in) Max. End- Slip (in) Failure Mode RA-4-6-1-N 4,033 7,050 18,290 33.42 41.82 88 192 1084 114 3.0 0.00 Flexure RA-4-6-1-S 4,033 7,050 18,290 24.96 28.87 70 156 964 102 2.7 0.00 Flexure RA-4-6-2-N 4,033 7,050 18,290 30.24 37.66 73 162 1011 107 2.4 0.13 Flexure RA-4-6-2-S 4,033 7,050 18,290 29.35 33.19 58 148 921 97 3.0 0.33 Bond RA-6-6-1-N 4,855 8,040 18,290 29.73 40.85 88 192 1012 104 2.5 0.00 Flexure RA-6-6-2-N 4,855 8,040 18,290 31.65 52.18 73 162 1001 103 2.1 0.02 Flexure RA-6-6-2-S 4,855 8,040 18,290 30.1 49.37 58 148 913 94 2.7 0.41 Bond RA-6-6-3-N 4,855 8,040 18,290 25.83 44.96 88 192 1046 108 2.6 0.00 Flexure RA-8-6-1-N 5,413 8,220 18,290 28.21 45.48 88 192 1008 103 2.4 0.00 Flexure RA-8-6-2-N 5,413 8,220 18,290 28.2 46.35 73 162 1007 103 2.0 0.01 Flexure RA-8-6-2-S 5,413 8,220 18290 25.7 42.37 58 132 988 ~101 2.5 0.14 Bond RA-10-6-1-N 9,150 14,610 18,290 20.03 29.98 88 192 1084 102 2.8 0.00 Flexure RA-10-6-2-N 9,150 14,610 18290 15.62 26.79 73 162 1070 101 2.5 0.00 Flexure RA-10-6-2-S 9,150 14,610 18,290 21.78 30.70 58 148 1083 102 2.4 0.00 Flexure Table 3.25. Development length test results on rectangular beams containing Strand A6 (0.6-in. diameter).

62 Beam End Measured Overall Depth (h) (in) cf @ Release (psi) cf 56 Days (psi) Avg. NASP Pull-Out Value (lb) Le (in) Span (in) Maximum Moment (kip-in) %Mn Maximum Deflection (in) Max. End- Slip (in) Failure Mode IB-6-5-1-N 24 5,810 9,350 20,210 58 166 3,526 82 1.1 0.04 Shear IB-6-5-1-S 24 5,810 9,350 20,210 72 222 3,980 98 3.1 0.03 Flexure IB-10-5-1-N 24 7,615 13,490 20,210 54 168 4,282 102 2.0 0.03 Flexure IB-10-5-1-S 24 7,615 13,490 20,210 58 180 4,196 100 1.6 0.02 Flexure ID-6-5-1-N 24 5,492 9,840 6,890 72 222 3,538 82 2.5 0.80 Bond ID-6-5-1-S 24 5,492 9,840 6,890 88 270 3,280 81 3.5 0.75 Bond ID-10-5-1-N 24 8,225 14,160 6,890 88 270 4,026 92 5.2 0.08 Flexure ID-10-5-1-S 24 8,225 14,160 6,890 72 222 4,039 92 3.7 0.75 Bond Table 3.26. Development length test results on I-shaped beams containing 0.5 in. diameter strands. Beam End Measured Overall Depth (h) (in) cf @ Release (psi) cf 56 Days (psi) Avg. NASP Pull-Out Value (lb) Le (in) Span (in) Maximum Moment (kip-in) %Mn Maximum Deflection (in) Max. End- Slip (in) Failure Mode IA-6-6-1-N 24.125 4,381 8,990 18,290 75 156 3,267 81 1.7 0.05 Shear @ opposite end IA-6-6-1-S 24.125 4,381 8,990 18,290 91 188 4,387 109 2.8 0.12 Flexure IA-6-6-2-N 24.125 4,381 8,990 18,290 88 270 4,125 102 3.2 0.13 Shear IA-10-6-1-N 24.25 10,480 14,990 18,290 58 166 4,243 103 1.2 0.05 Shear @ opposite end IA-10-6-1-S 24.25 10,480 14,990 18,290 72 222 4,620 112 2.5 0.03 Flexure w/ Strand Rupture IA-10-6-2-N 24.375 10,590 14,930 18,290 72 222 2,983 73 0.9 0.00 Shear @ opposite end IA-10-6-2-S 24.375 10,590 14,930 18,290 88 270 4,559 111 5.7 0.00 Flexure Table 3.27. Development length test results on I-shaped beams containing 0.6 in. diameter Strand A6. and B were used interchangeably in this beam series as the two strand samples tested with approximately the same NASP Bond Test value. Concrete strengths at release varied from a target of 4 ksi to a target of 10 ksi. 56-day concrete strengths ranged from 7.05 ksi to 14.47 ksi. Table 3.24 reports no bond failures. These results demonstrate that the NASP Bond Test is a good predictor of the ability of strands to perform in pre- tensioned applications. At concrete strengths above 4 ksi, em- bedment lengths as short as 46 in. were tested. All of these tests also resulted in flexural failures without any strand end slip. All of the flexural failures occurred at an applied moment that matched or exceeded Mn, the nominal flexural capacity for the beams. Detailed testing summaries on each develop- ment length test are found in Appendix D, for Rectangular Beams Made with Strands A and B. Both Tables 3.23 and 3.24 report results on beams made with air-entrained concrete. The development length test results on the air-entrained beams closely match from beams made with 6-ksi concrete without air entrainment. In other words, all of the ends tested with air-entrained concrete failed in flexure, with strand end slip in only a few cases. These results mirrored the results of the development length tests without air entrainment. Table 3.25 reports the results from development length tests on rectangular beams made with 0.6 in. diameter strand. The strand is called Strand A6. Strand A6 had an NASP Bond

63 Test value of 18,290 lb, which is interesting as it falls between the higher and lower NASP Bond Test values for the 0.5 in. strands tested. Concrete strengths at release varied from a tar- get of 4 ksi to a target of 10 ksi. The range for 56-day concrete strengths was 7.05 ksi to 14.61 ksi. For 0.6 in. strands, the ACI and AASHTO development length provision would require a development length of ap- proximately 88 in. From Table 3.25, it can be seen that several of the tests were performed at an embedment length of 88 in., which roughly corresponds to 100 percent of the AASHTO re- quired development length. At the embedment length equal to the required development length of 88 in., all of the beam specimens failed in flexure, regardless of concrete strength. This would indicate that the strand performance was adequate and suitable for making pretensioned concrete beams. Other tests on beams made with Strand A6 were conducted at an embedment length of 72 in., which roughly corresponds to 80 percent of ld. This was done intentionally to mirror the 80 percent of ld that was tested for 0.5 in. strands. Addition- ally, note that some tests were conducted at an embedment length of 58 in., which is about 55 percent of ld. Three bond failures occurred in the tests on rectangular beams made with Strand A6. Notably, all three bond failures occurred at embedment lengths of 58 in., which is consider- ably shorter than the required development length. The three bond failures occurred in beams made with the three lower concrete strengths, with nominal release strengths of 4 ksi, 6 ksi, and 8 ksi. In contrast, the fourth beam, made from con- crete with a nominal release strength of 10 ksi, failed in flex- ure when tested at an embedment length of 58 in. The results of these tests would support the conclusion that increasing concrete strength improves the bond performance of pre- stressing strands. Detailed testing summaries on each devel- opment length test are found in Appendix N, for Rectangu- lar Beams Made with 0.6 in. Strands A, or Strand A6. 3.5.2.2 Tabulated Beam Test Results—I-Shaped Beams Table 3.26 reports the results from development length tests on I-shaped beams made with 0.5 in. strands. Strand D was the 0.5 in. strand with the lower NASP pull-out value, 6,890 lb, and Strand B possessed the higher NASP Test value of 20,210 lb. Two different concrete strengths were employed, concrete with a target release strength of 6 ksi and concrete with a target release strength of 10 ksi. The beams were made in pairs, and the release strength of 10 ksi was not achieved. 56-day concrete strengths ranged from 9.35 ksi to 14.16 ksi, which is very near the target design strengths of 10 and 15 ksi. Detailed testing summaries on each development length test are found in Appendix F, for I-Shaped Beams Made with 0.5 in. strands, including both Strand B and Strand D. Strand D. Table 3.26 reports three bond failures out of four tests on I-shaped beams made with the lower bond per- former, Strand D. The fourth flexural test resulted in a flexural failure; this beam was made with the higher strength concrete. Bond failures occurred at both the lower concrete strength and the higher concrete strength. Unlike the rectan- gular beams, bond failures of Strand D occurred at lengths equal to and exceeding the ACI and AASHTO development length design equation. At the lower concrete strength, 9.48 ksi, bond failures occurred at embedment lengths of 72 and 88 in. At the higher concrete strength, 14.16 ksi, one bond failure occurred at an embedment length of 72 in. The flex- ural failure had an embedment length of 88 in. These results support two primary conclusions: 1. The strand with an NASP Bond Test value of 6,890 lb is inadequate to develop the tension necessary to support flexural failures as intended, and 2. Higher concrete strength can improve the bond between prestressing steel and concrete. Strand B. Table 3.26 reports results of four tests done on beams made with Strand B. In the four tests, none of the beams failed in bond. The highest strand end slip measured was 0.04 in. Of the four failures, one was a shear failure and the other three were flexural failures. Three of the four tests were conducted with embedment lengths of 52, 54, and 58 in., lengths which are significantly less than the development length prescribed by ACI and AASHTO. These results sup- port one of the primary conclusions, i.e., that strand with a high NASP Bond Test value, in this case 20,210 lb, will provide bond that exceeds the implicit requirement of the development length design equations. Strand A6. Table 3.27 reports the results from develop- ment length tests on I-shaped beams made with 0.6 in. strands. Strand A6 was the only 0.6 in. strand cast in beams. It has an NASP pull-out value of 18,290 lb. Four beams were made, two with a target release strength of 6 ksi and two with a target release strength of 10 ksi. These casts achieved the tar- get release strength of 10 ksi, and 1-day strengths measured 10,590 lb. The range for 56-day strengths was 8,990 and 14,910 lb. Of the seven beam ends tested, three ends failed in shear at the end opposite the “test” end. The larger diameter strands required longer testing spans, and the beams were not able to overcome the damage sustained during tests on the south end when tests were performed on the north end. Of the four tests that would qualify as development length tests, one resulted in shear failure whereas the other three tests resulted in a flexural failure. None of the failures resulted from bond failure. At the lower strength, some strand end slips were measured and observed; however, these strand end

64 slips were consistent with behavior that was noted in previ- ous testing and did not prevent the strands from develop- ment tension adequate to support flexural failures at, or exceeding, the nominal flexural capacity. Detailed testing summaries on each development length test are found in Appendix G, for I-Shaped Beams Made with 0.6 in. strand, Strand A6. 3.5.3 Discussion of Development Length Test Results Development length tests must be conducted to failure, and the type of failure observed determines whether the em- bedment length provided was adequate to ensure proper strand development. Three distinct types of beam failures were observed in the conduct of the development length tests: (1) flexural failure, (2) bond failure, or (3) shear failure. 3.5.3.1 Types of Failure—Flexure Flexural failures are characterized by two primary criteria: 1. The beam is able to resist a flexural moment that ap- proaches and often exceeds the nominal flexural capacity (strength), and 2. The beam is able to undergo large deformations while sus- taining its capacity for resistance (ductility). Flexural failures of the beam specimens were typically characterized by the crushing of concrete at the top of the cross section where the compression zone exists. The beams were designed to be under-reinforced, which ensures that the strands themselves will experience large strains at flexural failure. Even so, crushing of the concrete is the most common failure mode. In one or two specimens of this test series, strands ruptured in tension. It should be noted that some strand end slip can be observed even during a flexural failure. The strands consistently exhibit an ability to develop strand tension even with small amounts of slip. However, when larger amounts of slip are observed, often the result is a bond failure. When strand end slips are observed, the determina- tion of whether the failure is a flexural failure with adequate strand bond or a bond failure is based on whether the beam meets the two criteria listed above for a flexural failure. Beams that failed in flexure also showed considerable duc- tility, with deflection increasing dramatically with sustained loads or with some incremental load increases. In some fl- exural failures, strand fractures occurred. Typically, strand fractures occurred in beams made with higher strength concrete. In these cases, failures did not cause crushing of concrete at the top surface; rather, the applied moments were large enough to cause the strands to rupture in tension. Flexural failures of rectangular beams. A typical flexural failure is observed from the test on the south end of Beam RB- 4-5-1. The rectangular beam contained two 0.5 in. strands, with a Strand B designation and a 56-day concrete strength of 7.05 ksi. The embedment length for this test was 58 in., or about 80 percent of the AASHTO design requirement for 0.5 in. strands. Strand B had a relatively high NASP Bond Test value of 20,120 lb. The moment versus deflection curve is found in Figure 3.39. Note that the beam achieves its nominal flexural ca- pacity, Mn, and that it also displays the ability to sustain the moment under large deflections. Additionally, for this beam, strand end slips remained small or the strand did not slip at all. The beam failed in flexure as the concrete in the compression zone crushed. A photograph of the beam at failure is shown in Figure 3.40. Flexural failures of I-shaped beams. The test on the south end of I-shaped beam IA-10-6-1 provides a good example of a flexural failure. In this test, one of the strands ruptured in tension, an obvious indicator that the strand was able to fully develop the tension necessary to resist the flexural capacity. The embedment length for this test was 72 in., which is approximately 80 percent of the ACI and AASHTO required development length for 0.6 in. strands. The NASP Bond Test value for Strand A6 was 18,290 lb. The moment versus deflection curve is found in Figure 3.41. Note that the beam achieves its nominal flexural capac- ity, Mn, and that it also displays the ability to sustain the load under large deflections. The beam failed at a moment of 4,620 kip-in., which exceeded the calculated Mn by about 12 per- cent. In this beam, the strands slipped a small amount as loads increased to capacity; the maximum strand end slip measured was 0.03 in. This small amount of strand end slip is also consistent with many of the flexural failures that occur during development length testing. The beam failed when one of the strands ruptured in ten- sion. Strand rupture was accompanied by a loud noise. The cracking pattern at failure is shown in the photograph shown in Figure 3.42. The cracking pattern is typical for I-shaped beams. There are two distinct regions of cracking. Flexural cracking is predominant in the regions of maximum mo- ment. These cracks are distinguished by a vertical propaga- tion near the bottom fibers of the beam. Web shear cracking occurs in the webs within the shear span of the tested end. These cracks are distinguished by their diagonal nature. It was uniformly observed that web crack propagation was limited to the webs of the I-shaped beams until loads approaching flexural capacity were applied. As loading increased, the web cracks would propagate into the bottom “bulb” of the I-shaped beam. Additionally, the photograph in Figure 3.42 shows inclined flexural cracks that propagate vertically from

65 RB-4-5-1-S Figure 3.40. Concrete crushing in the compression zone of Beam RB-4-5-1-S. 0 200 400 600 800 1000 1200 0 0.5 1 1.5 2 2.5 3 3.5 4 Deflection (in.) M om en t ( kip -in ) 0.00 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50 En d- sl ip (in .) Moment vs Deflection Strand End Slip vs Deflection Mn = 705.3 kip-in Figure 3.39. Moment versus deflection and strand end slip for Beam RB-4-5-1-S. the bottom of the beam, but then incline as the crack approaches and enters the webs. 3.5.3.2 Types of Failure—Bond Failures of pretensioned bond are characterized by the following two primary markers: (1) an inability to develop resistance to meet its design capacity and (2) excessive strand end slip. Oftentimes, although not always, bond failures can be abrupt and occur without warning. However, it is generally noted that test beams failing in bond demonstrate some measure of gradual failure; that is, they possess an ability to sustain some load through large deformations. However, bond failures nearly always occur at loads less than the calcu- lated nominal flexural capacity, Mn. Bond failures in rectangular beams. A typical bond failure is observed from the test on the south end of Beam RD-4-5-2. The rectangular beam contained two 0.5 in. strands. The con- crete strength at 56 days was 7.05 ksi. The embedment length for this test was 58 in., or about 80 percent of the AASHTO design requirement for 0.5 in. strands. The beam contained strands from the sample Strand D, which possessed a rela- tively low NASP Bond Test value of 6,890 lb. The moment versus deflection curve is shown in Figure 3.43. The moment versus deflection curve illustrates that the beam was unable to reach its nominal flexural capacity, Mn. Mn for this beam was 705 kip-in., and the beam’s actual ca- pacity was 513 kip-in., as measured during the test. In re- viewing the load versus deflection curve and the strand end slip curve, it is apparent that the strand started slipping very soon after flexural cracking first occurred. The beam was unable to resist loads that were much larger than the cracking moment, and strand end slips continued to increase with additional beam deflections. At a total deflection of about 3 in., the compression block at the top of the beam exhibited

66 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 0.5 1 1.5 2 2.5 3 3.5 4 Deflection (in.) M om en t ( kip -in ) 0 0.1 0.2 0.3 0.4 0.5 St ra nd E nd S lip (in .) 0.6 0.7 0.8 0.9 1 Moment vs Deflection Strand End Slip vs Deflection Mn = 4110 kip-in Figure 3.41. Load versus deflection and strand end slip for IA-10-6-1 South. crushing failure. The cracking pattern and the crushing fail- ure of the beam can be viewed in Figure 3.44. Note the one wide flexural crack, which is often a characteristic of bond failures. Because the beam was unable to achieve its nominal flexural capacity and because the beam exhibited excessive strand end slips, this test was classified as a bond failure. It should be noted that two rectangular beams were con- structed with Strand D and a targeted release strength of 4 ksi. These beams are the RD-4-5-1 and RD-4-5-2 beams. Of the four ends tested, bond failures occurred on the beams where the embedment length was only 58 in. The companion beam to Beam RD-4-5-2 (south end), described above, was Beam RD-4-5-1 (south end). It also failed in bond but at a load equal to the nominal flexural capacity. Still, the beam exhib- ited excessive strand end slip during the test, and the failure was not particularly ductile in that the beam was unable to sustain its resistance through large deformations. A descrip- tion of that test and all other development length tests can be found in the appendices to this report. Bond failures in I-shaped beams. In development length tests on I-shaped beams made with 0.5 in. strands, three bond failures occurred. All of the bond failures occurred in beams made with Strand D, the strand with the lower NASP Bond Test value of 6,890 lb. Of the three tests that failed in bond, two ends failed at embedment lengths of 72 in. and 88 in. These were two ends of the same beam that had a release strength of 5,490 psi and a 56-day strength of 9,840 psi. On the higher strength beam, with a 56-day concrete strength of 14.16 ksi, a bond failure occurred at an embedment length of 72 in., and a flexural failure occurred at an embedment length of 88 in. These tests demonstrated that Strand D, with an NASP Bond Test value of 6,890 lb, was inadequate in its ability to bond with concrete and satisfy the design requirements implied in the ACI and AASHTO expressions for development length. Beam ID-6-5-1 (south end) shows a typical bond failure. This I-shaped beam contained five 0.5 in. strands; the con- Figure 3.42. Cracking pattern for Beam Test IA-10-6-1 South, at strand.

67 Figure 3.44. Cracking pattern of bond failure for Beam RD-4-5-2 (South). 0 200 400 600 800 1000 1200 0 0.5 1 1.5 2 2.5 3 3.5 4 Deflection (in.) M om en t ( kip -in ) 0.00 0.15 0.30 0.45 0.60 0.75 0.90 1.05 1.20 1.35 1.50 St ra nd E nd S lip (in .) Moment vs Deflection Strand End Slip vs Deflection Mn = 705.3 kip-in Figure 3.43. Applied moment versus deflection and strand end slip for Beam RD-4-5-2-South. crete strength at 56 days was 9.84 ksi. The embedment length for this test was 88 in., or about 120 percent of the AASHTO design requirement for 0.5 in. strands. The beam contained strands from the sample Strand D, which possessed a rela- tively low NASP Bond Test value of 6,890 lb. The moment versus deflection curve is found in Figure 3.45. The moment versus deflection curve illustrates that the beam was unable to reach its nominal flexural capacity, Mn. The results indicate that the beam’s flexural capacity of 3,280 kip-in. was only about 81 percent of its calculated nominal flexural capacity. In reviewing the results from the test, it is apparent that the incidence of web shear cracking coincided with the initial strand end slips. Strand end slips continued to increase with increased beam loadings and increased beam deflections. The test was concluded at a total deflection of about 3.5 in., when it was apparent that deflections were in- creasing without further increase in beam capacity. The cracking pattern and the crushing failure of the beam can be viewed in Figure 3.46. The photograph shows one flexural crack under the loading point that became very wide under load. The excessive width of the crack is further evidence of bond failure in the prestressing strand. Because the beam was unable to achieve its nominal flexural capacity and because the beam exhibited excessive strand end slips, this test was classified as a bond failure. 3.5.3.3 Types of Failure—Shear Failure Two shear failures occurred in I-shaped beams; no shear failures occurred in the rectangular beams. Prior research has shown that significant interaction can exist between shear and bond behaviors, especially in I-shaped beams with nar- row webs (Kaufman and Ramirez 1988). In these beams, shear behavior is improved considerably by the inclusion of horizontal mild reinforcement within the webs and extend- ing for the first 96 in. from each end of the I-shaped beam.

68 Figure 3.46. Cracking patterns at the maximum load (failure) of Beam ID-6-5-1 (South). 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 M om en t ( kip -in ) St ra nd E nd S lip (in .) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Moment vs Deflection Strand End Slip vs Deflection Mn = 4051.6 kip-in 0 0.5 1 1.5 2 2.5 3 3.5 4 Deflection (in.) Figure 3.45. Applied moment versus deflection and strand end slip for Beam ID-6-5-1 (South). An example shear failure is observed from the test on Beam IA-6-6-2 (north end). This I-shaped beam contained four 0.6 in. strands; the concrete strength at 56 days was 8.99 ksi. The embedment length for this test was 88 in., or approximately equal to the AASHTO design requirement development length for 0.6 in. strands. The beam contained strands from the sample Strand A6, which possessed a NASP Bond Test value of 18,290 lb. Also, this beam was dropped and damaged during handling at the prestressing plant. Several cracks re- sulted from the dropping of the beam. The moment versus deflection curve and the strand end slip versus deflection curve are shown in Figure 3.47. The moment versus deflection curve follows a pattern indicative of a flexural failure. The curve also shows that the beam was unloaded and then reloaded a second time. Web shear cracking and flexural cracking occurred at the same load increment, corresponding to a moment of about 2,400 kip- in. Strand end slips did not occur with the initial web crack, but soon followed. One of the interesting things about this test is that the shear failure occurred as the beam had reached its nominal flexural capacity. The large deformations also suggest that strand yielding was probably occurring, and, as the test on the beam was being conducted, a flexural failure was indicated. How- ever, as one can view in the photograph shown in Figure 3.48, the beam failed suddenly and violently with a diagonal com- pression failure of the web. The shear failure shows that even though the beam is failing in shear, the strand possesses bond adequate to develop the beam’s capacity. 3.5.3.4 Summary of Development Length Tests There are three key issues: 1. Whether the NASP Bond Test can be used as a predictor of strand bond performance in flexural applications, 2. What the minimum acceptable level of bond performance is as measured by the NASP Bond Test, and 3. What modifications are necessary to the LRFD develop- ment length equation to account for variations in concrete strength.

69 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 M om en t ( kip -in ) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Moment vs Deflection Strand End Slip vs Deflection Mn = 4040 k-in Shear Failure 0 0.5 1 1.5 2 2.5 3 3.5 4 Deflection (in.) St ra nd E nd S lip (in .) Figure 3.47. Applied moment versus deflection and strand end slip for Beam Test IA-6-6-2 (North). Figure 3.48. IA-6-6-2 (North) at shear failure. The NASP Bond Test as a predictor of strand bond per- formance in flexural applications. Two 0.5 in. strands were tested in beams. Strand D had an NASP Bond Test value of 6,890 lb and Strands A and B had an NASP Bond Test value exceeding 20,000 lb. In the rectangular beams made with Strands A or B, no bond failures were experienced, even at rel- atively short embedment lengths. In I-shaped beams made with Strand B, no bond failures were experienced, even at em- bedment lengths shorter than the AASHTO design require- ment for development length. In contrast, both rectangular beams and I-shaped beams made with Strand D experienced bond failures at shorter embedment lengths. In I-shaped beams made with Strand D, the strand failed in bond even at lengths in excess of the ACI and AASHTO design require- ments for development length. In other words, Strands A and B, which have relatively high NASP Bond Test values, demon- strated excellent bond characteristics. In contrast, Strand D, with a relatively low NASP Bond Test value, demonstrated poor bond characteristics. The results clearly show that the NASP Bond Test can distinguish between strands with good bonding behavior and strands with poor bonding behavior. The minimum acceptable level of bond performance as measured by the NASP Bond Test. To determine a mini- mum level of bond performance as measured by the NASP Bond Test, results from the testing program conducted in the NASP Round III testing program are required. However, the results from the testing described in this chapter clearly indi- cate that the minimum value for the NASP Bond Test should be greater than the value measured on Strand D, 6,890 lb, but need not be as strong as the bond value measured on Strands A and B, which exceeded 20,000 lb. Modifications necessary to the LRFD development length equation to account for variations in concrete strength. The results clearly show that increases in concrete strength bring about improvements in strand development. Strand D, which failed in bond at lower concrete strengths, was still able to fully develop adequate tension at the higher concrete strengths.

70 Embedment Length (in) Beam No. cf @ Release (psi) cf 56 Days (psi) 46 58 73 RD-4-5-1 4,033 7,050 B F RD-4-5-2 4,033 7,050 B F RD-6-5-1 6,183 8,500 F F RD-6-5-2 6,183 8,500 B F RD-6A-5-1 7,960 11,420 F F RD-6A-5-2 7,960 11,420 F F RD-8-5-1 8,570 13,490 F F RD-8-5-2 8,570 13,490 F, F* - RD-10-5-1 9,711 14,470 F F - RD-10-5-2 9,711 14,470 F F - F = Flexural failures B = Bond failures * Both ends were tested at an embedment length of 58 in. Both ends failed in flexure. Table 3.28. Development length tests on rectangular beams with 0.5-in. Strand D (average NASP pull-out value = 6,870 lb). 3.5.4 Discussion of Test Results This section includes analysis in three primary areas: 1. What influence does concrete strength have on the devel- opment length for pretensioned prestressing strands? 2. What is the proper expression for development length? 3. What should be the minimum NASP Bond Test Value of the prestressing strand for achieving adequate anchorage? The NASP Bond Tests in concrete clearly demonstrate that concrete strength can exert great influence over the bond of strand with concrete. This trend was also demonstrated in measured transfer lengths as the transfer length for a given strand was shortened as concrete strength increased. In this section, the results from development length tests are ana- lyzed to determine the influence of concrete strength. Based on the analysis, certain modifications to the current AASHTO equation for development length are recom- mended. Comparisons among flexural test results are used to assess the validity of such recommendations. 3.5.4.1 Evaluating Development Length from the Flexural Tests The development length is the length for which the strand must be fully bonded to ensure strand anchorage adequate to develop the tension stress necessary to support the nominal flexural capacity of the cross section. The development length is distinguished from the embedment length, which is the length of bond that is actually provided. In the course of test- ing, a specific embedment length may be longer or shorter than the strand’s development length. If a beam test results in a bond failure, then one must conclude that the embedment length provided was shorter than the required development length. Conversely, if a beam test results in a flexural failure, then one can conclude that the embedment length provided was longer than the required development length. Each inde- pendent beam test therefore becomes a single data point that can indicate whether the embedment was sufficient. In most cases, it is difficult to discern from a single test what the “true” development length must be. Ideally, the “true” value of development would be when the flexural test results in simultaneous flexural, shear, and bond failures (Meyer 2002). Research that varies the embedment length between the values corresponding to complete flexural failure and the values corresponding to complete bond failure can get closer to identifying the “true” development length. Based on prior test results, the embedment length can be systematically lengthened or shortened for the purpose of brack- eting the test results. In this manner, an accurate picture for de- velopment length may be obtained through multiple beam tests. The variables for development length tests in this research were embedment length, concrete strength, and the type of strand. These parameters were changed for flexural tests on both rectangular and I-shaped beam specimens. The current ACI/AASHTO equation does not include the concrete strength parameter for calculating transfer and de- velopment length. However, results obtained during the flex- ural tests strongly suggest that the anchorage ability of the strands is improved as concrete strength increases. The next section reports on the effects of increasing concrete strength on the results obtained during the flexural tests. 3.5.4.2 Direct Tabular Method Table 3.28 summarizes the results from development length tests performed on Strand D cast in rectangular beams. In the Tables 3.28 through Table 3.30, the letter “F”’ denotes a flexural failure, and the letter “B” denotes a bond failure. In Table 3.28, the results indicate that for embedment lengths of 73 in. and concrete release strengths of about 4 ksi (56-day strength of 7 ksi), Strand D was able to develop the necessary tension to achieve a flexural failure in the beam. However, at an embedment length of 58 in. and tested at the opposite ends of the same beams, Strand D failed in bond. The embedment length of 73 in. corresponds to 100 per- cent of the development length prescribed in the AASHTO

71 Embedment Length (in) Beam No. cf RLS (psi) cf 56 Days (psi) 46 58 73 RB-4-5-1 4,033 7,050 F F RB-4-5-2 4,033 7,050 F F RA-6-5-1 6,183 8,500 F F RA-6-5-2 6,183 8,500 F F RA-6A-5-1 7,960 11,420 F F RA-6A-5-2 7,960 11,420 F F RA-8-5-1 8,570 13,490 F F RA-10-5-1 9,711 14,470 F F F = Flexural failures B = Bond failures Table 3.29. Development length tests on rectangular beams with 0.5-in.Strands A/B (average NASP pull-out value for A = 20,210 lb and for B = 20,950 lb). LRFD Bridge Design Specifications, while the embedment length of 58 in. corresponds to 80 percent of the code- specified value. Important to the purposes of this research, the bond of Strand D demonstrates marked improvement as concrete strengths increase. At a concrete strength of 11 ksi, Strand D was able to develop the necessary tension at embedment lengths of either 58 in. or 73 in. The test results indicate that for Strand D, cast in 11 ksi concrete, the devel- opment length required is equal to or less than 58 in. Further, in Beams RD-10-5-1 and RD-10-5-2, Strand D was able to develop its tensile force in only 46 in. of bonded length. These tests indicate that for Strand D cast in 14 ksi concrete, the development length required is equal to or less than 46 in. The tests demonstrate that, had the concrete strength been 7 ksi, the development length required for Stand D would be less than 73 in. but greater than 58 in. The dark line in the table separates the zone of bond failures from the zone of flex- ural failures. The test results clearly show that the strand bond improves in development length applications with increases in concrete strength. Table 3.29 shows the results from development length tests performed on beams made with Strands A/B. The re- sults show that (1) Strands A/B bonded better with concrete than Strand D, and (2) the bond of Strands A/B improved as concrete strength increased. The dark line in the table separates the zone of bond failures from the zone of flexural failures. Table 3.30 summarizes the results of beam tests on rectan- gular beams made with 0.6 in. strands. The current AASHTO expression gives a development length requirement equal to 88 in. for 0.6 inch diameter strands. Test results show that flexural failures occurred at lengths of 88 in. and 73 in. for all concrete strengths. The results also show that bond failures occurred for the three concrete strengths when an embed- ment length of 58 in. was tested. However, when Strand A6 was cast in concrete with a release strength of 10 ksi and a 56-day strength of over 14 ksi, the strand was able to develop the required tension force at an embedment length of 58 in. The dark line in the table separates the zone of bond failures from the zone of flexural failures. These results show clear improvements in strand bond behavior with increasing con- crete strength. The current ACI/AASHTO equation does not include the concrete strength parameter for calculating transfer and development length. However, results obtained during the Embedment Length (in) Beam End cf RLS (psi) cf 56 Days (psi) 58 70 73 88 RA-4-6-1 4,033 7050 F F RA-4-6-2 4,033 7,050 B F RA-6-6-1 4,855 8,040 F RA-6-6-2 4,855 8,040 B F RA-6-6-3 4,855 8,040 F RA-8-6-1 5,413 8,220 F RA-8-6-2 5,413 8,220 B F RA-10-6-1 9,150 14,610 F RA-10-6-2 9,150 14,610 F F F = Flexural failures B = Bond failures Table 3-30. Development length tests on rectangular beams with 0.6-in. Strand A6 (average NASP pull-out value = 18,920).

72 flexural tests demonstrate that the anchorage ability of the strands is improved as concrete strength increases. 3.6 Discussion of Design Recommendations The current AASHTO code provisions do not include the effects of concrete strength when calculating the required development length of prestressing strands. As a result, the development length for strands is the same regardless of concrete strength. However, the results of this research clearly demonstrate that the required transfer and development lengths are shortened as concrete strength increases. The approach develops from the findings of the research: 1. The current AASHTO transfer length of 60db is adequate to predict the transfer length of prestressing strands in “normal strength concrete” (4-ksi release strength). 2. The data support modification of the AASHTO transfer length to account for variations in concrete release strength and in recognition of the finding that bond strength improves in proportion to the square root of the concrete strength. 3. The current AASHTO development length equation can be used to adequately predict required development lengths for “normal strength concrete” with a release strength in the range of 4 ksi and a design strength of 6 ksi. 4. The data demonstrate that shorter development lengths are required as concrete strength increases. 3.6.1 Discussion of Transfer Length Recommendations The standard NASP Bond Test is a test where a prestress- ing strand is pulled from sand-cement mortar. The mortar is made from sand, cement, and water and possesses a 1-day compressive strength of 4,500 to 5,000 psi. The NASP Bond Test can be modified to perform the test in concretes with varying concrete strengths. However, the NASP Bond Test values used in the discussions regarding minimum Bond Val- ues are pull-out strengths obtained from the standardized NASP Bond Test performed in mortar. The results from NASP pull-out tests in concrete are pre- sented and compared in this section. Figure 3.12 presents normalized NASP values (obtained by dividing the NASP pull-out values in concrete by the NASP standardized test val- ues [from tests conducted in mortar]) versus the concrete strengths for the NASP tests in concrete. The tests demon- strate remarkable correlation between the bond-ability of prestressing strand and the concrete strength. Compared with a power regression, the chart in Figure 3.12 shows the following relationship between NASP values in concrete and NASP values in mortar (standard NASP values): (3.3) The equation was further modified to fit the NASP values as a function of square root of concrete strengths. Figure 3.13 is a plot of normalized NASP values against the square roots of corresponding concrete strengths. Following is the rela- tionship shown in Figure 3.13: (3.4) With the help of this relationship, it was possible to use the Standardized NASP Bond Test, conducted in mortar, to estimate the bond strength as if the test were conducted in con- crete with various strengths. The graphs in Figures 3.9 through 3.13 demonstrate that the NASP Bond Test pull-out value in concrete is inversely proportional to the square root of the con- crete strength. From these data, one can further assert that the average bond stress, taken as the pull-out force divided by the bonded length, is also inversely proportional to the concrete strength at release. Further evidence for this same relationship between bond strength and pull-out force is found in Figures 3.24 through 3.32, which chart measured transfer lengths ver- sus concrete strengths. The transfer length data demonstrate that transfer lengths change inversely with concrete release strength. Figure 3.32, which charts transfer length measured on three different strand samples, shows that transfer lengths are approximately inversely proportionate to the square root of the concrete strength. The best fit power regression indicates an ex- ponent of −0.46 for measured concrete strengths at release. This is approximately equal to the inverse of the square root. It can therefore be concluded that transfer length is inversely pro- portional to the square root of concrete strength. Therefore, a transfer length expression is recommended that is equivalent to the current design expression of 60 strand diameters at a release strength of 4 ksi, but that shortens in proportion to the square root of the concrete strength at release. The recom- mended code provision also provides a minimum transfer length of 40 db. The 40 db value corresponds to 10-ksi concrete, which was the highest 1-day strength tested. The transfer length equation is modified by the square root of the concrete release strength, as follows: (3.5) where lt = transfer length (in.), f ′ci = release concrete strength (ksi), and db = diameter or prestressing strand (in.). l d f t b ci = ′ 120 NASP NASP fci concrete( ) = ′0 51. NASP NASP fci concrete( ) = ′0 49139 0 51702. .

73 Using concrete release strength of 4 ksi, this equation re- sults in a transfer length equal to 60 db. The recommendation for transfer length is only modified so that a minimum length for transfer length is used, regardless of concrete strength. The recommendation effectively limits improvements in transfer length based on a concrete release strength of 9 ksi, which is less than the maximum release strength obtained in the beams cast for this research (9.7 ksi on rectangular beams). Therefore, the final recommended expression for transfer length is the following: (3.6) 3.6.2 Development Length Recommendations Since the inception of the pretensioned, prestressed concrete industry in the United States, the development length equation has been made from the sum of two components: (1) transfer length and (2) “flexural bond length,” which is the additional length of bond beyond the transfer length required for devel- opment. This approach has been utilized in the industry for decades. Research continues to demonstrate that the approach is adequate to explain observed behavior and predict results. Thus, the same approach is followed, but with modifications to include the effects of varying concrete strengths: • The results demonstrate that for all types of 0.5 in. strands—Strands A/B and Strand D—flexural failures oc- curred at embedment lengths of 73 in. The embedment length of 73 in. corresponds to 100 percent of the current code provision for development length for these speci- mens. The results included tests on beams made with concrete strength of approximately 4 ksi at release and approximately 6 ksi at the time of the beam test. • The results uniformly indicate that the development length requirements diminish with increasing concrete strength. • The required development length calculated from the cur- rent code provisions is approximately 150 db, although some variations will exist due to variations in strand stress- ing, beam geometry and subsequent variations in com- puted prestress losses. • If the transfer length is approximately 60 db, and the devel- opment length is approximately 150 db, then the flexural bond length must be approximately 90 db. The development length expression can then be written as follows: (3.7)l l d f d t b c = + ′ 225 l f d dt ci b b= ′ ≥ 120 40 where ld = development length, lt = transfer length, db = diameter of the prestressing strand, and f ′c = design concrete strength. Using concrete design strength of 6 ksi, which roughly cor- responds to a “normal” concrete strength within the industry and forms the base case from the experimental results, the co- efficient of 225 corresponds to flexural bond length of 90 strand diameters. Like the transfer length expression, the development length expression is limited by a minimum value. The rec- ommended expression for development length, therefore, is based on a limiting concrete strength of approximately 14 ksi, which is slightly less than the maximum concrete strength attained in beams tested in the research program (14.9 ksi). Thus, the recommended development length equation is as follows: (3.8) 3.6.3 Distribution of Failure Types in Beams Tested This section presents the development length test results in graphical fashion. The result of each beam test, whether flex- ural failure or bond failure, is plotted on a chart showing con- crete strength versus embedment length. The recommended design equation for development length is also shown on each of the charts. Note that the development length varies with concrete strength. For the purpose of plotting the values while using the equation, release strength is taken as 66.7 per- cent of the design strength. This is a reasonable ratio of release strength to design strength, borne out by years of experience in prestressed concrete. Figure 3.49 shows the results of development length tests on Strand D. Strand D demonstrated below average to poor bond performance with a relatively low NASP Bond Test result (6,890 lb), longer transfer lengths, and longer devel- opment length requirements than Strands A/B. Figure 3.49 shows that bond failures occurred in rectangular beams with embedment lengths of 58 in. at the lower concrete strengths. More importantly, the figure shows improve- ment in strand bond behavior as concrete strengths in- creased. Note, however, that bond failures occurred in I-shaped beams cast with Strand D. Results of the tests demonstrate that the Strand D, with an NASP Bond Test value of only 6,890 lb, does not provide adequate bond-ability with concrete. Figure 3.50 shows the results of development length tests on Strands l f f d dd ci c b b= ′ + ′ ⎡ ⎣⎢ ⎤ ⎦⎥ ≥ 120 225 100

74 Flexural Failures = R-beams Flexural Failures = I-beams Proposed Equation Curve Shear Failures = I-beams 0 2000 4000 6000 8000 10000 12000 14000 16000 0 20 40 60 80 100 Embedment length (in.) C on cr et e S tr en gt h, f' c (p si) Proposed Design Equation Figure 3.50. Distribution of bond and flexural failures for Strands A/B (0.5 in.). Flexural Failures - R-Beams Bond Failures - R-Beams Flexural Failures - I-Beams Bond Failures - I-Beams 0 2000 4000 6000 8000 10000 12000 14000 16000 0 20 40 60 80 100 Embedment length (in.) C on cr et e S tr en gt h, f' c (p si) Proposed Design Equation Figure 3.49. Distribution of bond and flexural failures for Strand D (0.5 in.). A and B. Both of these strands can be considered “high bond- ing,” since the NASP Bond Test value was so high. Strand B was cast in the 4 ksi rectangular beams and I-shaped beams, and Strand A was used in the higher strength rectangular beams. The chart shows that the high-bonding strand was de- veloped in all concrete strengths, even in embedment lengths as short as 46 in. The proposed design equation is shown on the chart along with the beam test results. Figure 3.51 shows the distribution of bond and flexural failures for 0.6 in. strand, Strand A6, with respect to concrete strength and embedment lengths. As in Figure 3.50, the pro- posed design equation is shown in Figure 3.51 along with the beam test results. There are no bond failures occurring in the region where embedment length exceeds the calculated development length using the proposed equation. The tests support the proposed equation for development length.

75 Bond Failures = R-beams Flexural Failures = R-beams Flexural Failures = I-beams Proposed Equation Curve 0 2000 4000 6000 8000 10000 12000 14000 16000 0 20 40 60 80 100 120 Embedment length (in.) f' c (p si) Proposed Design Equation Figure 3.51. Distribution of bond and flexural failures for Strand A6 (0.6 in.). Embedment Length (in) Beam No. cf 56 Days (psi) Average NASP Pull-Out Value (lb) 58 73 II11 6,290 4,140 B F II12 6,280 4,140 B B FF11 6,260 7,300 V F FF12 6,070 7,300 B F HH11 6,330 10,700 F F HH12 6,300 10,700 B F AA11 6,220 14,950 F F AA12 6,160 14,950 F F F = Flexural Failure V = Shear Failure B = Bond Failure Table 3.31. Failure modes on single-strand beams (Russell and Brown 2004). 3.6.4 NASP Value and Bond Performance Along with the recommendation for the development length design expression, it is important to recommend a minimum value from the NASP Bond Test. First of all, how- ever, it was important to establish a correlation between the NASP pull-out test values and the bond performance of the same strands in transfer and in development length tests. Russell and Brown (2004) measured transfer lengths and per- formed flexural tests on rectangular-shaped beams. Table 3.31 and Table 3.32 summarize the test results and the failure modes obtained from flexural tests performed by Russell and Brown (2004).The NASP pull-out test values are also given. Strand II had the lowest NASP Bond Test value, only 4,140 lb. Strand II is the same strand as that labeled Strand E in the NCHRP research. One can see also that Strand II was the worst performer of the four strands in both single strand and double strand beams, with bond failures at the AASHTO development length of 73 in. Strand FF from Russell and Brown’s research (2004) is the same strand labeled Strand D in the NCHRP research. As seen in Tables 3.31 and 3.32, Russell and Brown reported a NASP Bond Test value of 7,300 lb for Strand FF. This compares with a NASP Bond Test value of 6,890 lb in the NCHRP testing. Strand FF demonstrated the ability to develop adequate ten- sion in an embedment length of 73 in. in the rectangular beams. However, if one looks at the results of the I-shaped beams in Table 3.27, one can see that Strand D or Strand FF was unable to develop adequate strand tension at the devel- opment length of 73 in. Embedment Length (in) Beam No. cf 56 Days (psi) Average NASP Pull-Out Value (lb) 58 73 II21 6,290 4,140 B F II22 6,280 4,140 B B FF21 6,260 7,300 F F FF22 6,070 7,300 F F HH21 6,330 10,700 F F HH22 6,300 10,700 F F AA21 6,220 14,950 F F AA22 6,160 14,950 F F F = Flexural Failure V = Shear Failure B = Bond Failure Table 3.32. Failure mode on beams made with two strands (Russell and Brown 2004).

76 0 2000 4000 6000 8000 10000 12000 14000 16000 0 20 40 60 80 100 Embedment length (in.) C on cr et e S tr en gt h, f' c (p si) Flexural Failures - Round III Single Strand Beams Bond Failures - Round III Single Strand Beams Flexural Failures - Round III Double Strand Beams Proposed Design Equation Figure 3.52. Distribution of bond and flexural failures for Strand HH (Russell and Brown 2004). Number of Bond Failures Strand Name NASP Value (lb) 58-in Embedment Length 73-in Embedment Length II 4,140 4 2 D 6,590 3 2* FF 7,300 1 0 HH 10,700 1 0 AA 14,950 0 0 B 20,210 0 0 A 20,950 0 0 * Embedment lengths were 72 in. instead of 73 in. Table 3.33. Bond failures at 58 in. and 73 in. for all 0.5 in. strands—I-shaped beams and rectangular beams. Also, in Russell and Brown’s research (2004), NASP Round IV testing, Strand HH demonstrated the ability to develop ad- equate strand tension at the development length of 73 in. The NASP Bond Test value was 10,700 lb. One bond failure oc- curred at an embedment length of 58 in. This occurred in a single strand beam. The results from NASP Round IV testing reported by Russell and Brown (2004) indicate that the bond performance of Strand HH was adequate. Figure 3.52 shows the distribution of bond and flexural failures for Strand HH (0.5 in.) with respect to the concrete strength and the provided embedment lengths. There are no bond failures occurring in the region where provided em- bedment length exceeds the calculated development length using the proposed equation. The tests support the proposed equation for development length and also indicate that bond performance of strand HH was adequate. No bond failure was recorded on the beams with Strand AA. Comparing the NASP values of these strands, the follow- ing observation can be made: as the NASP value increases, chances of bond failure at provided embedment length de- crease. In other words, Strand II had the lowest NASP value and the highest number of bond failures, Strand FF and Strand HH had NASP values lying between those of Strand II and Strand AA, and bond failures were noted on fewer occa- sions for Stand FF and Strand HH than for Strand II. Strand AA had the highest NASP value and no bond failures, sug- gesting that it was capable of developing enough anchorage to achieve flexural failures. A higher NASP value seems to in- dicate better bonding qualities for the strand. Table 3.33 presents the number of failures obtained for all types of strands (0.5 in.) including NASP Round III Strands. In Table 3.33, strands are arranged in the order of increasing NASP pull-out values. The number of bond failures obtained at 58-in. and 73-in. embedment lengths is shown. The number of bond failures is lower for strands with higher NASP pull-out values. Strand HH, with NASP pull- out value of 10,700 lb, lies at a critical position (boldfaced in Table 3.33): strands with NASP pull-out values lower than Strand HH’s pull-out value sustained bond failures, but no strands with NASP pull-out values higher than Strand HH’s pull-out value suffered bond failure. Embedment lengths of 58 in. and 73 in. correspond to 80 percent and 100 percent, respectively, of the code provision for development length. Strand HH suffered a bond failure at an embedment length of 58 in., but none at 73 in. These data show that a NASP pull-out value of 10,700 lb is adequate to develop enough an- chorage for achieving flexural failures at the code-specified development length.

77 3.7 The Effect of Concrete Strength on Bond Performance— Summary, Conclusions, and Recommendations The research program involved development length tests on two types of beam specimens. Four types of strands were employed to cast 43 rectangular-shaped beams and 8 I-shaped beams. Both 0.5 in. and 0.6 in. diameter strands were included in the testing program. The beam specimens had concrete re- lease strengths varying between 4 ksi and 10 ksi for both types of beams. Transfer lengths were measured on all beam speci- mens using the strand end slip of the strands with the help of clamps attached to the strands. Transfer lengths were also measured using the concrete surface strain measurements. Fifty flexural tests were carried out on the rectangular beams, and 14 flexural tests were carried out on the I-shaped beams. Values of load, deflection, and strand end slip were recorded electronically and manually along with photographic records of failure stages and crack patterns. I-shaped beam specimen concrete surface strains were measured at 36 in. from the end of the beam and vertically at the center of the web. Prestressing strand anchorage requirements were assessed using the data collected from the development length tests. Results from the development length tests were compared with the NASP pull-out values of corresponding strands. Based on the failure modes during the development length tests, the effect of concrete strength on bond performance was analyzed. The current AASHTO code requirements for de- velopment length of prestressing strands were assessed for their effectiveness in predicting accurate anchorage require- ments. The conclusions from this research are the following: • Development length tests can be used to assess the bond performance of prestressing strands. • The ability of a prestressing strand to bond with concrete is affected by concrete strength. Increasing concrete strength improves the bond-ability of a given prestressing strand. • The development length requirement for a particular strand is reduced if cast in higher strength concrete. • The NASP Bond Test provides a good indicator of strand bond performance in a pretensioned concrete beam. • The required development length shows a clear relation- ship with the NASP Bond Test values of the prestressing strand. Higher NASP Bond Test values result in shorter de- velopment lengths. • Rectangular beams with all types of strands were able to achieve flexural failures at embedment lengths less than or equal to the AASHTO-specified development length. • With increased concrete strength, it is possible to achieve flexural failures at an embedment length less than the AASHTO-specified value. • Current AASHTO code provisions may overestimate the required development length of prestressing strands in higher strength concretes. • I-shaped beams were more susceptible to bond failures than rectangular beams because of the higher incidence of web shear cracks developing in I-shaped beams. Finally, on the basis of the study findings, the following recommendations are made: • AASHTO code equations for transfer length should in- clude a parameter reflecting the reduced transfer length with increasing concrete release strength. The recom- mended equation for transfer length, lt (in.), is (3.9) where f ′ci = release concrete strength in ksi, and db = diameter of prestressing strands in inches. • AASHTO code equations for development length should include a parameter reflecting the reduced transfer length with increasing concrete release strength. Further, the flex- ural bond length is reduced by higher strength concrete as well. The recommended equation for development length is the following: (3.10) where ld = development length (in.), f ′ci = release concrete strength in ksi, f ′c = design concrete strength in ksi, and db = diameter of prestressing strands in inches. • A relatively large database has been collected during the course of this research project. The data include crack pat- terns, crack spacing, and surface strain measurements on I-shaped beams. A more detailed analysis should be made using the information embedded in the summary reports for a better understanding of the failure mechanisms. It is recommended that the Standardized Test for Strand Bond be adopted into the AASHTO LRFD Bridge Design Specifica- tions. The Standard Test for Strand Bond, formerly known as the NASP Bond Test, requires an average pull-out value of 10,500 lb with no single test out of a sample of six tests falling below 9,000 lb. These values are established from the review of the data obtained from the testing reported herein. Supporting data is found in the NASP Round III test report, which is incorporated into this report via discussion in pre- vious sections. The Standard Test Method for the Bond of Prestressing Strands is recommended to ensure adequate l f f d dd ci c b b= ′ + ′ ⎡ ⎣⎢ ⎤ ⎦⎥ ≥ 120 225 100 l d f dt b ci b= ′ ⎡ ⎣⎢ ⎤ ⎦⎥ ≤ 120 40

78 (1) Specimen (2) Bar Size (3) Cover (in.) (4) Beam Size (B x H) (in.) (5) Effective Depth (in.) (6) Number of Spliced Bars (7) Splice Length (in.) (8) 318-05 Cal. Stress (ksi) (9) Test Date Compressive Strength ( cf , ksi) I-1 #6 0.75 9 x 18 16.88 3 16 52.10 16.2 I-2 #8 1.00 12 x 18 16.50 3 24 44.68 14.6 I-3 #11 1.50 18 x 18 15.75 3 36 49.89 16.2 I-4* #6 0.75 9 x 18 16.88 3 16 60 (66.91) 15.1 I-5* #8 1.00 12 x 18 16.50 3 24 60 (64.54) 14.6 I-6* #11 1.50 18 x 18 15.75 3 36 60 (63.55) 15.1 1 in. = 25.4 mm; 1 ksi = 6.89 MPa. B = specimen width. H= specimen height. (* shows specimens with transverse reinforcement in the splice region) Table 3.34. Specimen dimensions and variables. anchorage at embedment lengths equal to or higher than AASHTO code development length provision for normal- strength concretes. • The effect of admixtures on the transfer and development length tests should be studied, with more development length tests carried out while changing the proportions of different admixtures in the concrete. 3.8 Experimental Program—Mild Steel Anchorage of Uncoated Bars in Tension An extensive literature review of test data was conducted, and the results were reported in Chapter 2. The findings of the literature review indicated the need to supplement the data with six additional tests of top cast uncoated bar splices in order to extend the use of the AASHTO LRFD Bridge Design Specifications for development and splice length of uncoated bars to higher strength concretes. The variables considered were bar size (#6, #8, and #11) and amount of transverse reinforcement over the splice length. All six speci- mens tested had clear concrete cover of db. 3.8.1 Specimen Design Six beam splice specimens were tested. The specimen dimensions and variables are shown in Table 3.34. The test variables were bar size and the presence of transverse rein- forcements in the splice region in higher strength concretes. The cover value given in Column 3 of Table 3.34 is for both top and side clear cover to the bar being developed or spliced. Details of typical specimens are shown in Table 3.34 and Fig- ure 3.53. In Specimens I-4, I-5, and I-6, transverse reinforce- ment was used in the splice region to confine the concrete as shown in Figure 3.54(b). The splice length shown in Column 7 of Table 3.34 was selected to provide a direct link with com- panion test specimens containing epoxy-coated bars, which were reported on in a separate paper and other tests in the lit- erature. The same splice lengths were used for the specimens with transverse reinforcement so that the confining effect of this reinforcement could be evaluated. Rearranging Equation 12-1 of the 318 Code (ACI 2005) with appropriate modifica- tion factors and with a splice class factor of 1.0, it was possi- ble to estimate a design stress and force in the bars for various anchorage conditions, as shown in Equation 3.11. To deter- mine the calculated stress, fy (specified yield strength of rein- forcing bars [psi]) is replaced with fs and ld is replaced by the splice length provided, 16, 24, and 36 in. for specimens with #6 (#19M) bars, specimens with #8 bars, and specimens with #11 bars, respectively. Note that all the specimens had more than 12 in. of concrete cast below the splice. As shown in Table 3.34, all the bars in the specimens with transverse rein- forcement had a calculated stress over the design stress of 60 ksi. These values are shown in Column 8 of Table 3.34 next to the yield design value. (3.11)f l d f c K d s d b c b tr b e s = ′ +⎛⎝⎜ ⎞⎠⎟⎡ ⎣ ⎢⎢⎢⎢ ⎤ 40 3 1 3( . )ψ ψ ⎦ ⎥⎥⎥⎥

79 Note. Ls = length of splice. 18” (12”, 9”) #11 Bars: #4@4.5” #8 Bars: #3+#4@8” #6 Bars: #3@8” #4@8” #4@8” #3@8” #4@4.5” #3+#4@8” #3@8” Ls = 36” (24”, 16”) 54” 48” 54” Splice Region: only I-4, I-5, and I-6 have stirrups in the splice region Figure 3.53. Specimen details (1 in. = 25.4 mm). (a) Specimen I-1 (b) Specimens I-4 & I-6 Figure 3.54. Specimen fabrication. The factor representing the contribution of confining re- inforcement across potential splitting planes is Ktr. The vari- able cb represents the spacing or cover dimension, calculated using either the distance from the center of the bar (or wire) to the nearest concrete surface or one-half the distance of the center-to-center spacing of the bars being developed. ψe is a coating factor of 1.5 for cases with cover less than 3db, or clear spacing less than 6db, and 1.2 for all other cases. The param- eter ψs is a reinforcement size factor: 0.8 for #6 bars and smaller and 1.0 for all other cases. The specimens were checked and reinforced in the over- hang region to prevent premature shear failures outside of the test region. To prevent shear failure, a stress of 1.25 times the yield strength of the bar was assumed in the overhang for pur- poses of estimating the required shear reinforcement to resist the maximum shear associated with the moment capacity of the section at the support. The shear reinforcement in the overhang region consisted of #3 @8 in., #3 + #4 @8 in., and #4 @4.5 in., in Specimens I-1 and I-4, I-2 and I-5, and I-3 and I-6, respectively. The shear reinforcement in the splice region consisted of #3 @8 in. on centers in I-4 and #4 @8 in. on cen- ters in both I-5 and I-6. Figure 3.54 shows the specimen rein- forcing cages. 3.8.2 Test Set-Up The beam splice setup used in this investigation is shown in Figure 3.55. In all specimens, the distance between the loading points and the support was 48 in. The constant mo- ment region was also 48 in. Splices were located within the constant moment region. To investigate the characteristics of spliced beams, the applied loads, the resulting deflections at each beam end and midspan, and strains developed in longi- tudinal bars and stirrups were monitored using load cells, lin- ear variable differential transducers (LVDTs) anchored to a reference frame, and electrical resistance strain gages attached to the bars, as shown in Figure 3.55 (b) and (c). 3.8.3 Materials Concrete and reinforcing steel were the materials used. Table 3.35 shows a typical concrete mix for the specimens. This mix was designed for a compressive strength of at least

80 Contents 15-ksi Mix Cement (lb) 900 Silica fume (lb) 200 Water (lb) 220 Coarse aggregate (lb) 1800 (1/2” crushed limestone) Fine aggregate (lb) 1000 High-range water reducer (oz) 520 Normal-range water reducer (oz) 38 1 lb = 0.454 kg; 1 oz = 28.35 gr; 1 yd3 = 0.765 m3; 1 ksi = 6.89 MPa Table 3.35. Typical concrete mix ratio (per 1 cubic yard). (a) Loading & Supporting (b) Measuring by LVDT (c) Location of gages and support (Specimen I-3) Note: G1 = Gage 1. G2 = Gage 2. Specimen Bar Size Ls (in.) G1 (in.) G2 (in.) I-1, I-4 #6 (#19M) 16 19 13 I-2, I-5 #8 (#25M) 24 27 9 I-3, I-6 #11 (#35M) 36 39 15 6” 48” 48” 48” 6” (Side view for Supporting) G2=15” G1=39” (Plan view for gage location) Ls = 36” Figure 3.55. Test setup (1 in.  25.4 mm). 15 ksi. The water to cement ratio was 0.20. The average mod- ulus of rupture was 834 psi at 28 days. Typical maximum compressive stress versus age data are shown in Figure 3.56. The concrete strength continued to increase after 28 days and achieved a strength of 17 ksi at 56 days. The specimens began to be tested after they reached a 15-ksi uniaxial compressive strength. The reinforcing bars were ASTM A615 Grade 60 steel and had a yield strength based on tests of samples of the reinforc- ing bars of 78.3 ksi, 70.3 ksi, and 66 ksi for the #6, #8, and #11 bars, respectively. Stress versus strain curves for #6, #8 and #11 bars are shown in Figure 3.57. 3.8.4 Cracking and Failure Mode In nearly all tests, the cracking sequence was similar. First, a flexural crack appeared in the constant moment region.

81 0 2 4 6 8 10 12 14 16 18 20 0 20 40 60 80 100 Age (days) Co m pr es si ve S tre ng th (k si) Figure 3.56. Concrete stress versus age relationship (1 ksi  6.89 MPa). #6 Black Bars 0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 Strain (x10^-6) St re ss (k si) 0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 Strain (x10^-6) St re ss (k si) #8 Black Bars 0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 Strain (x10^-6) St re ss (k si) #11 Black Bars Figure 3.57. Tensile stress versus strain relationship (1 ksi  6.89 MPa). With the increase of beam end loads, a shear crack appeared in the overhang region and was arrested by the presence of the shear reinforcement. Near the peak load, horizontal cracks appeared along the longitudinal bars within the splice region. Finally, the deformations pushed the concrete away from the bar by wedge action. Failure crack patterns of all the speci- mens are shown in Figure 3.58. All the specimens failed in splitting mode following yielding of the spliced bars in the constant moment region. 3.8.5 Beam End Displacement The applied load versus deflection at the tip of the overhang response for Specimens I-1 to I-6 is shown in Figure 3.59. Load represents the average of the two values from the actua- tors. Deflections were calculated by averaging displacements at both ends of the beam. The test results are summarized in Table 3.36. In the specimens without transverse reinforcement in the splice region (Specimens I-1, I-2, and I-3), the end

82 (a) Specimen I-1 (c) Specimen I-3 (e) Specimen I-5 (b) Specimen I-2 (d) Specimen I-4 (f) Specimen I-6 Figure 3.58. Failure crack patterns for all the specimens for the #6, #8, and #11 bars.

83 displacements at the peak load were 0.5 to 0.7 in. In the spec- imens with transverse reinforcement over the splice region (Specimens I-4, I-5, and I-6), the end displacements at the peak load were 0.8, 1.6 and 0.8 in., respectively. 3.8.6 Bar Strains In Figure 3.60, the typical end concentrated load versus measured longitudinal bar and transverse bar strains in the constant moment region of Specimen I-6 are shown. Yield strain in the longitudinal reinforcement was first recorded at around one-third of the peak load. Table 3.37 shows the measured maximum strains on all of the specimens. All the gages on the longitudinal reinforcement showed strains in ex- cess of the bar yield strain before reaching peak load. In the gages placed on the stirrups in the constant moment region, the measured maximum strain was around half of the bar yield strain in Specimens I-4 and I-5 and almost equal to the I-1 0 10 20 30 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) Lo ad (k ips ) (a) Specimen I-1 I-2 10 20 30 40 50 0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) Lo ad (k ips ) (b) Specimen I-2 (c) Specimen I-3 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) I-3 0 10 20 30 40 50 60 70 80 90 100 Lo ad (k ips ) (d) Specimen I-4 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) I-4 0 10 20 30 Lo ad (k ips ) (e) Specimen I-5 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) 0 Lo ad (k ips ) I-5 10 20 30 40 50 60 70 (f) Specimen I-6 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Displ. (in.) 0 10 20 30 40 50 60 70 80 90 100 Lo ad (k ips ) I-6 Figure 3.59. End load—end displacement curves for Specimens I-1 through I-6 (1 in.  25.4 mm; 1 kip  4.448 kN).

(1) Spec. (2) Max. Load (kips) (3) Displ. at Peak (in) (4) 318-05 Cal. Stress (ksi) (5) 318-05* Cal. Stress (ksi) (6) AASHTO Cal. Stress (ksi) (7) Test Max. Stress (ksi) (8) (7)/(4) (9) (7)/(5) (10) (7)/(6) I-1 28.2 0.506 52.10 41.68 38.10 78.55 1.51 1.88 2.06 I-2 39.6 0.429 44.68 44.68 42.86 70.93 1.59 1.59 1.65 I-3 88.6 0.654 49.89 49.89 45.59 67.65 1.36 1.36 1.48 I-4** 29.5 0.805 60 (66.91) 53.54 38.10 81.24 1.21 1.52 2.13 I-5** 59.4 1.572 60 (64.54) 60 (64.54) 42.86 91.88 1.42 1.42 2.14 I-6** 96.4 0.800 60 (63.55) 60 (63.55) 45.59 71.94 1.13 1.13 1.58 1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 ksi = 6.89 MPa * Shows stress calculated by removing bar size factor ** Shows specimens with transverse reinforcement in the splice region Table 3.36. Summary of test results for uncoated bar specimens. I-6 (longitudinal reinforcement gage) 0 10 20 30 40 50 60 70 80 90 100 0 1000 2000 3000 4000 5000 6000 7000 Strain (µ) Lo ad (k ips ) (a) Longitudinal gage I-6 (transverse reinforcement gage) 0 10 20 30 40 50 60 70 80 90 100 0 1000 2000 3000 4000 5000 6000 7000 Strain (µ) Lo ad (k ips ) (b) Transverse gage Figure 3.60. Beam end load versus measured strain relationship in Specimen I-6 (1 kip  4.448 kN). bar yield strain in Specimen I-6. The use of stirrups in the splice region of Specimens I-4, I-5, and I-6 resulted in an in- crease in the displacement capacity when compared with companion specimens I-1, I-2, and I-3, respectively. 3.8.7 U.S. Design Specifications 3.8.7.1 318 Code (ACI 2005) Orangun, Jirsa, and Breen (1977) evaluated the results of a large number of bond and splice tests. The evaluation high- lighted the importance of parameters such as bar diameter, stress in the bar to be developed ( ), cover or bar spacing, and the amount of transverse reinforcement. The Orangun, ′fc 84 Jirsa, and Breen study—together with contributions on bond of reinforcement from ACI Committee 318 and ACI Com- mittee 408 that were meant to simplify the provisions for calculating development length of straight bars in tension— led to Equation 12-1 in the 318 Code (ACI 2005), which is Equation 3.12 herein: (3.12) In Equation 3.12, fy is the specified yield strength of rein- forcing bars (psi), ψt is the reinforcement location factor of l f f c K d d y t e s c b tr b = ′ +⎛⎝⎜ ⎞⎠⎟ ⎡ ⎣ ⎢⎢⎢⎢ ⎤ ⎦ ⎥⎥⎥ 3 40 ψ ψ ψ λ ⎥ db

85 1.3 to reflect the adverse effects on top casting position on the bond strength of the reinforcement. The parameter ψe is a coating factor of 1.5 for cases with cover less than 3db, or clear spacing less than 6db, and 1.2 for all other cases. These factors are consistent with a ratio of bond strength of coated bars to bond strength of uncoated bars observed in the literature of 1/1.5 = 0.67 and 1/1.2 = 0.82. However, the product of ψt and ψe need not be taken greater than 1.7. The parameter ψs is a reinforcement size factor: 0.8 for #6 bars and smaller and 1.0 for all other cases. The factor reflecting the lower tensile strength of lightweight concrete is λ. Bar diameter is db. The factor representing the contribution of confining reinforce- ment across potential splitting planes is Ktr. The variable cb represents the spacing or cover dimension, calculated using either the distance from the center of the bar (or wire) to the nearest concrete surface or one-half the distance of the cen- ter-to-center spacing of the bars being developed. The ratio of (cb+ Ktr)/db should not be taken greater than 2.5. However, the development length, ld, so calculated, cannot be less than 12 in. In addition, when calculating anchorage length requirements for tension lap splices, these should be as required for a Class A or B splice, but not less than 12 in., where Class A splice..................1.0 ld Class B splice..................1.3 ld It must be noted that this factor is associated with the potential mode of failure when multiple bars are spliced at the same location and does not speak to the actual strength of the spliced bar. 3.8.7.2 2004 AASHTO Specifications (Section 5.11: Development and Splices of Reinforcement) The bond provisions for mild reinforcement in the AASHTO LRFD Bridge Design Specifications mirrored the 318 Code provisions first introduced in the 1963 edition of the ACI Standard (ACI 1963). At the end of the last decade, the ACI 318 provisions for development and splices of reinforce- ment were extensively modified; however, the AASHTO provisions for development and splices of reinforcement continued to mirror the ACI provisions first introduced in 1963. A brief description of the background of the 1963 ACI specifications is provided below. The 1963 edition of the 318 Code provisions for bond and anchorage for ultimate strength design were stated on the basis of the ultimate flexural bond stress at the sections of interest (ACI 1963), μu (3.13) Critical sections were stated to occur at the face of support, at each point of inflection, and at each point where tension bars were terminated within a span. Vu was the factored shear at the section, ∑o, which represented the sum of bar perime- ter(s) at the same section, and jd was the flexural lever arm. To prevent bond failure or splitting, the calculated tension or compression force in any bar at any section had to be de- veloped on each side of that section by proper embedment length or end anchorage, or, for tension only, by hooks. Anchorage, or development bond stress (μu), was to be de- termined as the bar force, computed from M (moment at the section due to factored loads) /ϕ, divided by the product of ∑o times the embedment length. The two values so calcu- lated—ultimate flexural bond stress and anchorage bond stress—were not to exceed the limits given below, except that flexural bond stress did not have to be considered in com- pression or in those cases of tension where anchorage bond was less than 0.8 of the permissible stress given below. For tension, there were two equations given for each of the two types of steel included: ASTM A 305 and ASTM A 408. For instance, for ASTM A 408, the permissible values were the following: • Top bars (more than 12 in. of concrete below the bar)— 4.2 ; • Bars other than top bars—6 ; and • For all deformed bars in compression—13 or 800 psi. In 1971, there was a complete revamping of the bond spec- ifications in ACI’s 318 Code. In the new format, a basic devel- opment length, ldb, was determined and then modified by ap- propriate factors to obtain the required anchorage length, ld. ′fc ′fc ′fc μ ϕu u o d V j = ∑ Gage Location I-1 I-2 I-3 I-4 I-5 I-6 Longitudinal Bar 3,405 2,370 3,100 10,300 10,750 5,960 Transverse Reinforcement N/A N/A N/A 1100 875 1,910 Table 3.37. Measured maximum strains () in specimens I-1 through I-6.

(3.14) The development length concept replaced the dual system contained in the 1963 ACI Code. It was no longer necessary to use the flexural bond concept, which placed an emphasis on the computation of nominal peak bond stresses. The av- erage bond resistance over the full development length of the bar is more meaningful in part because of the highly empiri- cal nature of the design provisions and because bond tests in- volve averaging of bond resistance. The current minimum development length for bars in tension and in compression is based on the attainable average bond stress over this length. The various ld lengths in the 1971 ACI Code were based di- rectly on the 1963 ACI Code permissible bond stresses. Slightly modified versions of the 1971 provisions in ACI’s 318 Code (due to the fact that fy and f ′c are stated in terms of ksi) are the current provisions for these design situations in the AASHTO LRFD Bridge Design Specifications. The basic tension development length, ldb (in.), for #11 bar and smaller bars shall be taken as Equation 3.15: ldb = 1.25 Abfy/ but not less than . . . 0.4 dbfy For #14 bars: ldb = 2.7 fy/ (3.15) For #18 bars: ldb = 3.5 fy/ and for deformed wire: ldb = 0.95 dbfy/ In Equation 3.15, Ab is the area of bar or wire (in.2), fy is the specified yield strength of reinforcing bars (ksi), f ′c is the spec- ified compressive strength at 28 days unless another age is specified (ksi), and db is the diameter of bar or wire (in.). The tension development length, ld, shall not be less than the product of the basic tension development length, ldb, and modification factor specified in Article 5.11.2.1.2 (for epoxy- coated bars with cover less than 3db or with clear spacing be- tween bars less than 6db . . . 1.5, For epoxy-coated bars not covered above . . . 1.2 ). The tension development length shall not be less than 12.0 in., except for lap splices specified in Article 5.11.5.3.1 (Class A splice . . . 1.0 ld, Class B splice . . . 1.3 ld, Class C splice . . . 1.7 ld). In the 1989 ACI Code, major changes were made in the procedures for calculating development lengths for deformed bars and deformed wire in tension. This represented a major departure in approach between the ACI Code and the current AASHTO LRFD Bridge Design Specifications. These changes resulted in an increase in the development lengths for closely spaced bars and bars with small covers. The basic develop- ment length was modified to reflect the influence of cover, spacing, transverse reinforcement, casting position, type of aggregate, and epoxy coating. The basic development lengths remained essentially the same as in the 1971 edition of the ACI Code and the current AASHTO LRFD Bridge Design Specifications with the exception of the equation for #18 bars, ′fc ′fc ′fc ′fc l l f fd db= * *1 2.... 86 which was revised on the basis of a review of available test re- sults on large bars. The revised version for #18 bars was the following: (3.16) with fy and f ′c in psi. If put in ksi units, (3.17) This is an increase of 12 percent over the values given by the current AASHTO LRFD Bridge Design Specifications for the same size bars. Another important change introduced in the 1989 ACI Code was the limitation that cannot be taken greater than 100 psi. This limitation meant that devel- opment lengths would no longer decrease with concrete strengths greater than 10,000 psi. It was noted that research on development of bars in high-strength concretes was not sufficient to substantiate a reduction beyond the limit imposed. While these provisions were based on extensive research and professional judgment, many found them overly com- plex in application. In 1999, Committee 318 of the ACI re- examined these procedures with the goal of formulating a more user-friendly format while maintaining general agree- ment with the research results and professional judgment that produced the changed provisions. The revision was based on the same general equation for development length that served as the basis for the 1989 provisions. This equation was Equation 12-1 in the 2005 version of the 318 Code (ACI 2005) and Equation 3.11 in this report. In 1977, provisions for tension lap splices of deformed bars and deformed wire encouraged the location of splices away from regions of high tensile stresses to locations where the area of steel provided at the splice location is at least twice that required by analysis. A lap splice of any portion of the total area of steel in regions where (As provided/As required) was less than 2.0 had to be at least 1.3 times the development length of the individual bar in tension (Class B splice) in length. If more than one-half of the reinforcement was spliced in such regions, lap splices had to be at least 1.7 times the development length of the individual bar (Class C splice) in length. Class A splices where the length of bar was equal to the development length of the individual bar were only per- mitted in regions where (As provided/As required) was less than 2.0 and no more than 25 percent of the total area was spliced within one lap length. These same provisions are in the current AASHTO LRFD Bridge Design Specifications. When the changes in development in tension that eliminated many concerns regarding tension splice due to closely spaced bars were introduced in the 1989 version of the 318 Code (ACI 1989), Class C splices were eliminated. In summary, there are a few major differences between the ACI Code and the AASHTO LRFD Bridge Design Specifica- ′fc l f fdb y c= ′3 95. * / l f fdb y c= ′0 125. * /

87 tions with respect to development and splice length of tension reinforcement: • The AASHTO LRFD Bridge Design Specifications don’t have bar size factor for smaller bars. • The AASHTO LRFD Bridge Design Specifications don’t consider the role of confining reinforcement over the splice region; however, in the ACI Code, the Ktr factor represents the contribution of confining reinforcement across poten- tial splitting planes in the case of closely spaced bars with small covers. • The AASHTO LRFD Bridge Design Specifications still con- tain Class C splices. The second and third differences are, of course, related. This parameter is especially important because bars are being developed in higher strength concretes. 3.8.8 Bond Strength Comparisons Table 3.36 shows the comparison of calculated stress in the bar using Equations 3.12 and 3.15 and test results. In the spec- imens with transverse reinforcement (Specimens I-4 through I-6), the 318 Code (ACI 2005) calculated stress was higher than the calculated stress in specimens without transverse re- inforcement (Specimens I-1 to I-3). Also, the use of transverse reinforcement over the splice region increased deflection at failure. The ratio of test maximum stress to ACI-calculated stress in the bar ranged from 1.13 to 1.59. The ratio of test maximum stress to AASHTO-calculated stress in the bar ranged from 1.48 to 2.14. It should be noted that the second part of Equation 3.15 controlled the basic development length in the entire specimen, and the calculated flexural capacity was greater than the moment at failure. The failure moment ranged between 60 and 98 percent of the flexural capacity. Column 5 in Table 3.36 shows the calculated stress with the bar size factor removed. Even though the test results of NCHRP Project 12-60 do not result in ratios of test maxi- mum stress to calculated stress less than 1.0, on the basis of the analysis of the entire database, it is proposed that the 0.8 bar size factor not be used for smaller bars. In Figure 3.61(a), the comparison of test maximum stress to the stress calcu- lated using 318 Code (ACI 2005) for uncoated bottom bars (reported by ACI Committee 408 [2003] and discussed in Chapter 2) is shown. It can be seen that many of the speci- mens had ratios less than 1. Figure 3.61(b) and (c) show the comparison of test maxi- mum stress to calculated stress in the bar using Equation 3.11 without bar size factor for test results on uncoated bars reported by ACI Committee 408 (2003). In these figures, the specimens are divided by casting position. The bond efficiency (the ratio of test maximum stress to stress calcu- lated using 318 Code [ACI 2005] without bar size factor) of specimens with bottom bars (478 specimens) was 0.51 to 3.02, and some specimens had a ratio of less than 1, which means the test bond strength was lower than the strength cal- culated using the 318 Code (ACI 2005) without bar size factor. The bond strength of specimens with top bars (111 specimens) was 1.04 to 3.27. In tests for this study, the ratio of test result to calculated result was 1.13 to 1.88. The design equation without the bar size factor conservatively estimated bar stress for the specimens with top bars. However, it over- estimated the bar stress in many specimens with bottom bars, especially for specimens with concrete compressive strength higher than 10 ksi. These are tests with values greater than 100 psi along the horizontal axis. Figure 3.61(d) shows the com- parison of test maximum stress to calculated stress in the bar using Equation 3.15 (AASHTO LRFD Bridge Design Specifi- cations) on uncoated bars for the specimens reported by ACI Committee 408 (2003). The bond efficiency (the ratio of test stress to calculated stress using AASHTO LRFD Bridge Design Specifications) of specimens with bottom bars (478 speci- mens) was 0.50 to 2.63, and 85 specimens had less than 1, which means the test bond strength was lower than the strength calculated by Equation 3.15. ACI Committee 408 (ACI 408R-03) proposed a new design equation for the bond and development of straight reinforc- ing bars in tension based on research by Zuo and Darwin (2000). Figure 3.61(e) shows the comparison of stress in the bar calculated using Equation 3.18 and the previous test results reported by ACI Committee 408 (2003). The result shows that the ratio of bond efficiency of specimens with bot- tom bars was 0.79 to 2.26, and only 12 specimens showed a ratio of less than 1. (3.18) In Equation 3.18, α is a factor reflecting the lower tensile strength of lightweight concrete, β is 1.2 for all epoxy-coated bars, λ is a factor reflecting the lower tensile strength of light- weight concrete, and c, ω, and Ktr are defined as follows: c = cmin + 0.5db (3.19) where c = spacing or cover dimension = cmin + db/2; cmin = minimum concrete cover or one-half of the clear spacing between bars, whichever is smaller, = minimum (cb, cs); cb = bottom concrete cover for reinforcing bar being de- veloped or spliced; cs = minimum [cso , csi + 0.25 in.]; cso = side concrete cover for reinforcing bar; l f f c K d d y c tr b = ′ − ⎛ ⎝⎜ ⎞ ⎠⎟ +⎛⎝⎜ ⎞⎠⎟ ⎡ 1 4 2200 70 / ω αβλ ω ⎣ ⎢⎢⎢⎢ ⎤ ⎦ ⎥⎥⎥⎥ db

88 0 0.5 1 1.5 2 2.5 3 0 20 40 60 80 100 120 140 Concrete Strength (√fc', psi) Bo nd E ffi ci en cy (T est /A CI -05 ) (a) Specimens with Bottom Bars (ACI-05) (b) Specimens with Top Bars (ACI-05*) 0 20 40 60 80 100 120 140 Concrete Strength (√fc', psi) 0 0.5 1 1.5 2 2.5 3 3.5 Bo nd E ffi ci en cy (T est /A CI -05 *) (c) Specimens with Bottom Bars (ACI-05*) 0 20 40 60 80 100 120 140 Concrete Strength (√fc', psi) 0 0.5 1 1.5 2 2.5 3 3.5 Bo nd E ffi ci en cy (T est /A CI -05 *) (d) Specimens with Bottom Bars (AASHTO) 0 20 40 60 80 100 120 140 Concrete Strength (√fc', psi) 0 0.5 1 1.5 2 2.5 3 Bo nd E ffi ci en cy (T est /A AS HT O) (e) Specimens with Bottom Bars (ACI-408) 0 20 40 60 80 100 120 140 Concrete Strength (√fc', psi) 0 0.5 1 1.5 2 2.5 Bo nd E ffi ci en cy (T est /A CI -40 8) (*shows stress calculated by removing bar size factor, 1 psi = 6.89 kPa) Figure 3.61. Comparison of bond efficiency with concrete strength.

89 csi = one-half of the bar clear spacing; and db = diameter of bar. (3.20) where cmax = maximum (cb, cs) Ktr = (0.52trtdAtr/sn)f ′c1/2 (3.21) where tr = 9.6 Rr + 0.28 ″ 1.72; Rr = relative rib area of the reinforcement; td = 0.78db + 0.22; Atr = area of each stirrup or tie crossing the potential plane of splitting adjacent to the reinforcement being de- veloped, spliced, or anchored; n = number of bars being developed or spliced; and s = spacing of transverse reinforcement. 3.8.9 Summary and Conclusions On the basis of the analysis of results from the tests of six beam specimens with lap-spliced uncoated bars embedded in higher strength concretes conducted as part of NCHRP Project 12-60 and the evaluation of an extensive database of test results compiled by ACI Committee 408, the following conclusions can be drawn: • The ratios of test maximum stress on the top spliced bars to the stress calculated from the design equation in the 318 Code (ACI 2005) ranged from 1.13 to 1.59. A similar ratio of test maximum stress to stress calculated from the AASHTO specifications ranged from 1.48 to 2.14. Thus, the procedure in the 318 Code (ACI 2005) and the AASHTO specifications for top bar uncoated splice and development length in tension can be extended to normal-weight con- crete with uniaxial cylinder strength up to 16 ksi. • The design equation in the 318 Code (ACI 2005) and the design equation in the AASHTO LRFD Bridge Design Spec- ifications, Equations 3.11 and 3.15, respectively, overesti- mated the bar stress in several of the bottom cast specimens in the ACI 408 Committee Database, especially for speci- mens with concrete compressive strength higher than 10 ksi. However, the calculated result proposed by ACI 408 Committee, Equation 3.18, resulted in fewer cases where the ratio of test to calculated stress was less than 1.0. It also resulted in more conservative estimates of the bond strength defined by the stress of spliced bars embedded in higher strength concrete beams. • Based on the maximum bar stress and beam end displace- ment at peak load in all the specimens, the use of stirrups in the amount of Ktr from 0.37 to 0.67 in the splice region resulted in increases in both maximum bar stress and max- ω = + ≤0 1 0 9 1 25. . .max min c c imum displacement capacity of the beam end at failure for higher strength concretes. 3.9 Anchorage of Epoxy-Coated Bars in Tension The object of this phase of NCHRP Project 12-60 was to evaluate the bond strength of epoxy-coated bar lap splices in concrete with strengths up to 15 ksi. An extensive literature review of test data was supplemented with 12 additional tests of top cast epoxy-coated bar splices. The variables considered in the experimental program included bar size (#6 and #11), concrete strength (12 to 17 ksi), and the amount of transverse reinforcement over the splice length. 3.9.1 Literature Review Epoxy-coated bars have been used as an economical method of protection against deterioration of reinforced con- crete structures associated with corrosion of steel reinforce- ment. Treece and Jirsa (1989) tested 21 beams in 9 series. The variables were bar size (#6 and #11), concrete strength (4, 8, and 12 ksi), casting position, and coating thickness (5 and 12 mils). The splice lengths were selected so that the bars would fail in bond before reaching yield, and no transverse rein- forcement was provided in the splice region. Test results showed that epoxy-coated bars with an average coating thick- ness above 5 mils developed 67 percent of the bond strength of black bars. DeVries, Moehle, and Hester (1991) reported the test re- sults of 36 beams. The variables were casting position, bar size (#6 and #9), and the presence of an antibleeding agent in the concrete. The range of concrete strengths was 8 to 15 ksi. Test results indicated that the ratio of bond strength of epoxy- coated bars to black bars was 0.84. Based on the test results, De Vries and Moehle indicated that the effects of casting po- sition and epoxy coating were not cumulative and that the modification for top cast epoxy-coated bars relative to bot- tom cast epoxy-coated bars was not needed. Also, the results showed that the presence of an antibleeding agent in the con- crete did not significantly alter the bond stress of the splice for either top cast or bottom cast bars. Choi et al. (1991) reported on the tests of 15 beams. The variables were bar size (#5, #6, #8, and #11), average coating thickness (3 to 17 mils), and deformation patterns (three pat- terns designated S, C, and N). The concrete strength was around 6 ksi. Test results indicated that the ratio of the bond strength of epoxy-coated bar splices to that of black bar splices varied from 0.71 to 0.94 with an average value of 0.82. They reported that all splice specimens exhibited extensive longitudinal and transverse cracking in the region of the splices at failure. The salient conclusion was that differences

90 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 50 100 150 Concrete Strength (√f'c) Bo nd E ffi ci en cy (T est / A CI -05 ) Choi et al. (1991) Hamad and Jirsa (1993) Treece and Jirsa (1989) DeVries, Moehle, and Hester (1991) Grundhoffer et al. (1998) Figure 3.62. Bond efficiency of the spliced bars with concrete strength relationship (1 psi-6.89 kPa). in coating thickness have little effect on the amount of the bond strength reduction for #6 bars and larger with coating thicknesses between 5 and 12 mils. Hamad and Jirsa (1993) reported on an experimental study in which 12 beams were tested. The main variables were bar size, bar spacing, and the amount of transverse reinforce- ment in the splice region. The concrete strength was around 4 ksi. Failure of all beams was governed by splitting of the concrete cover in the splice region. Test results indicated that the presence of transverse reinforcement in the splice region increased the deformation capacity of the beams and im- proved anchorage strength of epoxy-coated bar splices rela- tive to black bar splices more than 10 percent. Cleary and Ramirez (1993) reported on an experimental study in which 23 beam splice tests were subjected to repeated loadings and then tested to failure to compare the service and ultimate load behavior of beams with coated and uncoated reinforcement. The range of concrete strengths was 4 to 7 ksi. They reported that the differences in crack widths, deflec- tions, and reinforcement stresses in beams with coated and uncoated reinforcement were reduced with repeated loading. The ratio of the average bond stress at failure for a beam containing epoxy-coated bars to its companion specimen containing uncoated reinforcement ranged from 0.82 to 0.96, with an average of 0.88. Hester et al. (1993) tested 65 beam and slab splice specimens containing #6 and #8 bars. The average coating thickness ranged from 6 to 11 mils, and concrete strength ranged from 5 to 6.5 ksi. The Hester et al. study concluded that transverse re- inforcement improved the strength of splices containing both coated and uncoated bars, and the percentage increase in strength was approximately the same for both coated and un- coated bars with an equal amount of transverse reinforcement. A maximum development length modification factor of 1.35 was proposed for design with epoxy-coated reinforcement. Grundhoffer et al. (1998) reported on a series of 94 in- verted half-beam specimens. The variables were bar size (#6, #8, and #11), bar surface (epoxy and uncoated), concrete strength (6, 10, 12, and 14 ksi), and the addition of micro- silica to concrete. A comprehensive review of the effect of epoxy-coating on bond strength was conducted using the results of this study and 151 test results from seven other research studies. They concluded that ACI’s 1989 318 Code was more conservative than the 1995 318 Code for all the test results based on the comparison between experimental re- sults and the values of design bond strength calculated using ACI’s 1989 and 1995 318 Code equations. The review of past work shows that only two specimens of Treece and Jirsa (1989), eight specimens of DeVries, Moehle, and Hester (1991) and some specimens of two groups out of eight groups in Grundhoffer et al. (1998) used concrete strengths greater than 10 ksi. Other researchers’ concrete strengths vary from 4 to 10 ksi. The relationship between the bond efficiency (the ratio of test stress to stress calculated using 318 Code [ACI 2005]) of the spliced bars and the square root of concrete compressive strength in this literature review is shown in Figure 3.62. Note that the upper limit on the of 100 psi was removed in this calculation. Generally, the calculated stress was conservative in the range of higher strength concretes. 3.9.2 U.S. Design Specifications 3.9.2.1 318 Code (ACI 2005) 318 Equation 12-1 (ACI 2005) for estimating tension splice and development length requirements, Equation 3.12 in this report, contains several factors. One of these is ψt, the tradi- tional reinforcement location factor of 1.3 to reflect the adverse effects of the top reinforcement casting position. Parameter ψe is the specific coating factor to deal with epoxy- coated bars. It is 1.5 with cover less than 3db or clear spacing less than 6db, and it is 1.2 for all other cases. These factors are consistent with the ratio of bond strength of coated bars to bond strength of uncoated bars reported in the literature of 1/1.5 = 0.67 and 1/1.2 = 0.82. However, the product of ψt and ψe need not be taken greater than 1.7. All other factors are the same as for uncoated bars. In addition, as for uncoated bars, when calculating anchorage length requirements for tension lap splices, these should be as required for Class A or B splice but not less than 12 in., where Class A splice..................1.0 ld Class B splice..................1.3 ld 3.9.2.2 2004 AASHTO Specifications (Section 5.11 Development and Splices of Reinforcement) In 1989, on the basis of several test programs that showed that the bond strength of epoxy-coated bars is reduced ′fc

91 because coating prevents adhesion between the bar and the concrete, two factors—1.5 and 1.2 (function of the amount of concrete cover or bar spacing)—were introduced in the 318 Code provisions for development length of bars in ten- sion. No factors were stated for similar bars in compression or epoxy-coated bars terminated by means of standard hooks anchored to resist tension. Similar factors are currently em- ployed in the AASHTO LRFD Bridge Design Specifications. The rest of the approach is the same as for uncoated bars. No factors were stated for similar bars in compression or epoxy- coated bars terminated by means of standard hooks anchored to resist tension. The tension development length, ld , shall not be less than the product of the basic tension development length, ldb (see Equation 3.15), and the modification factor specified in Arti- cle 5.11.2.1.2 (1.5 for epoxy-coated bars with cover less than 3db or with clear spacing between bars less than 6d and 1.2 for epoxy-coated bars not covered above). The tension develop- ment length shall not be less than 12.0 in., except for lap splices specified in Article 5.11.5.3.1 (Class A splice . . . 1.0 ld, Class B splice . . . 1.3 ld, Class C splice . . . 1.7 ld). When the changes that eliminated many concerns regarding develop- ment length of tension lap splices due to closely spaced bars were introduced in the 1989 version of the 318 Code, Class C splices were eliminated. In summary, although the factors are the same in both the ACI Code and AASHTO LRFD Bridge Design Specifications with respect to development and splice length of tension of epoxy-coated reinforcement, the same differences observed in the case of uncoated bars for the calculation of tension development length remain. 3.9.3 Experimental Program 3.9.3.1 Test Specimens The experimental program covers the testing of 12 beam splice specimens reinforced with epoxy-coated bars. The spec- imen dimensions and variables are shown in Table 3.38. The test variables are bar size, concrete cover, concrete strength, and transverse reinforcements in the splice region in higher strength concretes. The cover value given in Column 3 is both top and side clear cover to the bar being developed or spliced. Details of typical specimen are shown in Figure 3.63. In Spec- imens II-15 through II-18, transverse reinforcement was used in the splice region to confine the concrete as shown in Figure 3.64(b). In the splice region, the transverse reinforcement consisted of #3 @8 in. for Specimens II-15 and II-17 and #4 @8 in. for Specimens II-16 and II-18, respectively. The splice length shown in Column 7 of Table 3.38 was selected to provide a direct link with previous tests in order to (1) Specimen (2) Bar Size (3) Cover (in.) (4) Beam Size (B x H) (in.) (5) Effective Depth (in.) (6) Number of Spliced Bars (7) Splice Length (in.) (8) 318-05 Cal. Stress (ksi) (9) Compressive Strength ( cf , ksi) II-7 #6 0.75 9 x 18 16.875 3 16 34.82 12.4 II-8 #11 1.50 18 x 18 15.750 3 36 33.34 12.3 II-9 #6 2.25 18 x 18 15.375 3 16 66.67 13.6 II-10 #11 4.50 24 x 18 12.750 2 36 63.83 13.6 II-11 #6 0.75 9 x 18 16.875 3 16 40.78 16.8 II-12 #11 1.50 18 x 18 15.750 3 36 39.05 16.8 II-13 #6 2.25 18 x 18 15.375 3 16 73.50 16.6 II-14 #11 4.50 24 x 18 12.750 2 36 70.38 16.6 II-15* #6 0.75 9 x 18 16.875 3 16 54.51 17.2 II-16* #11 1.50 18 x 18 15.750 3 36 51.76 17.2 II-17* #6 2.25 18 x 18 15.375 3 16 72.93 16.4 II-18* #11 4.50 24 x 18 12.750 2 36 69.83 16.4 1 in. = 25.4 mm; 1 ksi = 6.89 MPa (* denotes specimens with transverse reinforcement in the splice region). B = specimen width. H = specimen height. Table 3.38. Specimen dimensions and variables.

92 (a) Specimens II-7 & II-8 (b) Specimens II-17 & II-18 Figure 3.64. Construction of beam-splice specimen with epoxy-coated bars. Specimens with #11 Bars: #4@4.5” Splice Region #4@4.5” Ls = 36” (16” ) Specimens with #6 Bars: #3@8” 54” 48” 54” Specimens with transverse reinforcement in the splice region: #4@8” with #11 Bars (#3@8” with #6 Bars) 18” (9”) Figure 3.63. Typical beam-splice specimen reinforced with epoxy-coated bars (1 in.  25.4 mm). extend the specifications to higher strength concretes for epoxy-coated bars and to permit a more straightforward cover effect evaluation among specimens. The splice lengths have been selected to get a yielding stress in the basic speci- mens with 3db concrete cover (II-9 and II-10) as shown in Column 8 of Table 3.38. Using Equation 3.11 with appropri- ate modification factors, including the epoxy-coated bar factor, and with a splice class factor of 1.0, it was possible to calculate stress and force in the bar for various anchorage conditions. To determine the calculated stress, fs, ld is replaced by the splice length provided, 16 and 36 in. Note that all the specimens were cast with more than 12 in. below the splice. As shown in Table 3.38, all the bars in specimens with 3db concrete cover had a calculated stress greater than 60 ksi. The specimens were reinforced in the overhang region to prevent premature shear failures outside of the test region. For safety against shear failure, a stress of 1.25 times the yield strength of the longitudinal bar was assumed in the overhang for purposes of estimating the required shear reinforcement to resist the maximum shear associated with reaching the mo- ment capacity of the section at the support. In the overhang region, the spacing of shear reinforcement was #3 @8 in. on centers and #4 @4.5 in. on centers for specimens with #6 bars and specimens with #11 bars, respectively. Figure 3.64 depicts the construction of the specimens. 3.9.3.2 Test Setup and Loading Protocol The test setup is shown in Figure 3.65(a). In all speci- mens, the distance between the loading points and the sup- port was 48 in., and the distance between supports was also 48 in. To investigate the characteristics of spliced beams, the applied loads, resulting deflections at each beam end and midspan, and strains developed in longitudinal bars and stirrups were monitored using load cells, LVDTs attached to an external reference frame, and electrical

93 (a) Loading & Supporting (b) Measuring by LVDT (Plan for gage location and bars) 6” 48” 48” 48” 6” (Elevation for Supporting) G2=15” G1=39” Ls = 36” #6 Bar Specimens (Ls = 16”, G1=19”, G2=13”) (c) Location of gages and support (Specimen II-8) Figure 3.65. Test setup for beam-splice specimens reinforced with epoxy-coated bars (1 in.  25.4 mm). resistance strain gages affixed to the bars as shown in Fig- ure 3.65 (b) and (c). 3.9.3.3 Materials Table 3.39 shows the design concrete mixes. The water-to- cement ratio was 0.32 for the 10-ksi Mix I and 0.20 for the 14-ksi Mix II. A sample of the uniaxial stress versus strain re- lationship for the concrete is shown in Figure 3.66(a). The average modulus of rupture was 566 psi and 834 psi at 28 days for Mix I and Mix II, respectively. Also, typical data for uniaxial compressive stress by age are shown in Figure 3.66(b). As shown, the strength of Mix II continued to increase after 28 days and achieved a strength of 17 ksi at 56 days. ASTM A615 Grade 60 reinforcing bars were used for both longitudinal and transverse reinforcement. The yield strength, calculated by a 0.2-percent offset from tensile tests of samples of the reinforcing bars, was 70.3 ksi and 74 ksi for the #6 and #11 bars, respectively. The average thickness of epoxy coating was 12.5 mils and 11.5 mils for the #6 and #11 bars, respectively. The relative rib area was 0.091 and 0.135 for the #6 and #11 bars, respectively. The measured tensile Contents Mix I: 10 ksi Mix II: 14 ksi Cement (lb) 780 900 Silica fume (lb) 50 200 Water (lb) 265 220 Coarse aggregate (lb) 1,600(3/8” pea gravel) 1,800 (1/2” crushed limestone) Fine aggregate (lb) 1,240 1,000 High-range water reducer (oz) 190 520 Normal-range water reducer (oz) 35 38 1 lb = 0.454 kg; 1 oz = 28.35 gr; 1 yd3 = 0.765 m3; 1 ksi = 6.89 MPa Table 3.39. Concrete mix (per cubic yard).

94 Concrete Stress vs. Strain Relationship 0 5 10 15 20 0.0000 0.0005 0.0010 0.0015 0.0020 Strain, in/in Co m pr es si ve S tre ng th , k si 10 ksi Mix 14 ksi Mix (a) Concrete Stress vs. Strain Relationship 0 2 4 6 8 10 12 14 16 18 20 0 20 40 60 80 100 Age (days) Co m pr es si ve S tre ng th (k si) 10 ksi 14 ksi (b) Concrete Strength vs. Age Relationship 0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 10000 Strain (x10^-6) St re ss (k si) (c) # 6 (#19M) Bars 0 10 20 30 40 50 60 70 80 90 100 0 2000 4000 6000 8000 10000 Strain (x10^-6) St re ss (k si) (d) #11 (#35M) Bars Figure 3.66. Material properties for beam-splice specimens reinforced with epoxy-coated bars (1 in.  25.4 mm; 1 ksi  6.89 MPa). stress versus strain curves for #6 and #11 bars are shown in Figure 3.66(c) and (d). 3.9.4 Experiment Results 3.9.4.1 Cracking Pattern and Mode of Failure In nearly all tests, the cracking sequence was similar. First, a flexural crack appeared in the constant moment region. With the increase of beam end loads, a shear crack appeared in the overhang region. Near the peak load, splitting hori- zontal cracks appeared along the longitudinal bars in the splice region. Finally, the deformations pushed the concrete away from the bar by wedge action. Typical failure crack pat- terns are shown in Figure 3.67. All the specimens failed in splitting mode after yielding of the spliced bars in the con- stant moment region.

95 (a) Specimen II-7 (c) Specimen II-9 (e) Specimen II-17 (b) Specimen II-8 (d) Specimen II-10 (f) Specimen II-18 Figure 3.67. Typical failure crack pattern for beam-splice specimens reinforced with epoxy-coated bars.

96 II-8 0 10 20 30 40 50 60 70 80 Lo ad (k ips ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Displ. (in) (b) Specimen II-8 0 10 20 30 40 50 60 70 80 Lo ad (k ips ) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Displ. (in) (d) Specimen II-10 II-10 Lo ad (k ips ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displ. (in) (e) Specimen II-17 II-17 0 5 10 15 20 25 30 35 II-7 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Displ. (in) (a) Specimen II-7 Lo ad (k ips ) 0 5 10 15 20 25 30 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Displ. (in) (c) Specimen II-9 II-9 Lo ad (k ips ) 0 5 10 15 20 25 30 35 0 10 20 30 40 50 60 70 80 Lo ad (k ips ) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Displ. (in) (f) Specimen II-18 II-18 Figure 3.68. Applied load versus deflection at the tip of the overhang response (Specimens II-7 through II-10 and II-17 through II-18). 3.9.4.2 Load versus End Displacement Characteristics The applied load versus deflection at the tip of the over- hang response for Specimens II-7 to II-10 and II-17 and II-18 is shown in Figure 3.68. Load was calculated by averaging the two values from the actuators, and deflection was obtained averaging displacements at both ends of the beam. 3.9.4.3 Summary of Test Results The test results are summarized in Table 3.40 and findings from these results are presented on the basis of three main parameters. Concrete Cover (db). Comparison of Specimens II-7 and II-9 (#6 bars) and comparison of Specimens II-11 and II-13

97 (1) Speci- men (2) Max. Load (kips) (3) Displ. at Peak Load (in) (4) 318-05 Cal. Stress (ksi) (5) 318-05* Cal. Stress (ksi) (6) AASHTO Cal. Stress (ksi) (7) Test Max. Stress (ksi) (8) (7)/(4) (9) (7)/(5) (10) (7)/(6) II-7(db) 20.7 0.311 34.82 27.86 31.37 63.81 1.83 2.29 2.03 II-8(db) 61.5 0.423 33.34 33.34 37.55 65.50 1.96 1.96 1.74 II-9 (3db) 29.0 0.687 66.67 48.94 31.75 78.39 1.18 1.60 2.47 II-10 (3db) 49.4 0.701 63.83 58.57 37.99 66.19 1.04 1.13 1.74 II-11 (db) 21.0 0.315 40.78 32.63 31.37 65.50 1.61 2.01 2.09 II-12 (db) 64.5 0.395 39.05 39.05 37.55 65.00 1.66 1.66 1.73 II-13 (3db) 32.1 1.161 73.50 53.96 31.75 83.45 1.14 1.55 2.63 II-14 (3db) 52.9 0.793 70.38 64.58 37.99 69.31 0.98 1.07 1.82 II-15** (db) 28.8 0.602 54.51 43.60 31.37 65.34 1.20 1.50 2.08 II-16** (db) 92.0 0.662 51.76 51.76 37.55 65.96 1.27 1.27 1.76 II-17** (3db) 32.4 1.185 72.93 53.54 31.75 84.80 1.16 1.58 2.67 II-18** (3db) 67.4 1.924 69.83 64.08 37.99 86.41 1.24 1.35 2.27 1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 ksi = 6.89 MPa * shows stress calculated by removing bar size factor and using one epoxy-coated bar factor of 1.5. ** shows specimens with transverse reinforcement in the splice region. Table 3.40. Summary of test results. (#6 bars) show that increasing the concrete cover increased both maximum stress and deflection at failure. This result can also be seen in comparison of Specimens II-8 and II-10 (#11 bars) and comparison of Specimens II-12 and II-14 (#11 bars). However, the increase in maximum stress for #11 bar specimens was less than the increase in maximum stress for #6 bar specimens. Effect of Concrete Strength. For Specimens II-7 and II-11 (#6 bars) with small cover (equal to db), increasing the con- crete strength led to an increase in maximum stress, but did not significantly increase the maximum deflection at failure. When larger cover (3db) was used, increasing the concrete strength increased both maximum stress and deflection at failure as can be seen by comparing Specimens II-9 and II-13. For Specimens II-8 and II-12 (#11 bars) with small cover (db), an increase in concrete strength did not increase the maximum stress or deflection at failure. In Specimens II-10 and II-14 with larger cover (3db), increasing the concrete strength resulted in increases in both maximum stress and deflection at failure. Effect of Minimum Amount of Transverse Reinforce- ment in Higher Strength Concretes. A comparison of Specimens II-11 and II-15 (#6 bars) shows that the use of transverse reinforcement over the splice region did not re- sult in an increase in the maximum stress but more than doubled the deflection at failure when the small cover (db) was used. When the large cover (3db) was used, it resulted in increases to both maximum stress and deflection at fail- ure, as can be seen by comparing Specimens II-13 and II-17. Comparison of Specimens II-12 and II-16 (#11 bars) and comparison of Specimens II-14 and II-18 (#11 bars) show that the use of transverse reinforcement over the splice

98 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 0 20 40 60 80 100 120 140 Concrete Strength (√f'c) *Stress calculated by removing bar size factor and the epoxy-coated bar factor of 1.5 Bo nd E ffi ci en cy (T est / A CI -05 *) Purdue No Stirrups With Stirrups Purdue(S) Figure 3.69. Comparison of bond efficiency with concrete strength. region resulted in increases to both maximum stress and deflection at failure. 3.9.5 Comparison of Calculated Stress and Test Results The comparison of stress in the bar calculated using Equa- tions 3.11 and 3.15 with appropriate modification factors and test maximum stress is also shown in Table 3.40. The ratios of test maximum stress to ACI-calculated stress in the bar ranged from 0.98 to 1.96, as shown in Column 8. The ratios of test maximum stress to AASHTO-calculated stress in the bar ranged from 1.73 to 2.67, as given in Column 10. It should be noted that the second part of Equation 3.15 controlled the basic development length. The findings from these compar- isons are discussed on the basis of three main parameters studied: effect of concrete cover, effect of concrete strength, and effect of minimum amount of transverse reinforcement in higher strength concretes. Effect of Concrete Cover. In the specimens without trans- verse reinforcement, the ratio of test maximum stress to ACI-calculated stress (see Column 8 in Table 3.40) in the specimens with 3db concrete cover was 0.98 to 1.18. The ratio of test maximum stress to ACI-calculated stress in the speci- mens with db concrete cover was 1.61 to 1.96, much higher than in the specimens with 3db concrete cover. This tendency was consistent regardless of other parameters, such as con- crete strength and bar size. Effect of Concrete Strength. In higher strength concrete specimens without transverse reinforcement, the average ratio of test maximum stress to ACI-calculated stress for the specimens with db and 3db concrete cover was near 1.64 and 1.06, respectively (see Column 8 in Table 3.40). These obser- vations point to the possibility that the current cover contri- bution in the code may be overestimated in the case of higher strength concrete specimens for larger covers. Effect of Minimum Amount of Transverse Reinforce- ment in Higher Strength Concretes. The ratios of test max- imum stress to ACI-calculated stress for Specimens II-11 and II-14 are 1.61 and 0.98, respectively; the ratios of test maxi- mum stress to ACI-calculated stress for Specimens II-15 and II-18 are 1.20 and 1.24, respectively. These data show that for specimens with transverse reinforcement over the splice re- gion the difference between ratios of test maximum stress to ACI-calculated stress was smaller than the difference between ratios for specimens without transverse reinforcement (see Column 8, Table 3.40). The lower ratios in Specimens II-15 and II-16, with small covers, came from higher calculated stress, considering the contribution factor of confining rein- forcement. However, the similarity of the ratios for Speci- mens II-17 and II-18 (with larger covers and transverse rein- forcement) to the ratios for Specimens II-13 and II-14 (with larger covers but no transverse reinforcement) is due to the requirement that the ratio of (cb+ Ktr)/db in Equation 3.11 should not be taken greater than 2.5. 3.9.6 Design Recommendation When the spliced bar stress was calculated using 318 Code (ACI 2005), without a limitation on the square root of the compressive concrete strength, only Specimen II-14 with 3db concrete cover had a ratio of test maximum stress to calculated stress of less than 1. However, the average ratio of test maxi- mum stress to 318 Code (ACI 2005) calculated stress for the specimens with 3dbcover was less than average of the same ratio for the specimens with db cover. Therefore, it is possible to con- clude that the current cover contribution may be overestimated in the case of higher strength concrete specimens with large cov- ers. Using only one coating factor of 1.5 may be the simplest way to handle the possible overestimation of cover contribution. Regarding bar size factor, no stress ratios less than 1 were obtained when the calculated stress included the 0.8 bar size fac- tor within the range of specimens covered in this study. The three specimens shown in Figure 3.62 with a ratio of test maxi- mum stress to calculated stress (defined in Figure 3.62 as “bond efficiency”) of less than 1 were specimens reinforced with #11 (#35M) bars. However, following the position of ACI Commit- tee 408, the authors of this report also suggest not using the 0.8 bar size factor. Column 5 of Table 3.40 shows the calculated stress without the bar size modification factor. Figure 3.69 shows a comparison of bond efficiency (defined as the ratio of

99 Research Study Without Stirrups With Stirrups Hamad and Jirsa (1993) (Black) 1.37 to 3.11 1.14 to 1.83 Hamad and Jirsa (1993) (Epoxy) 1.31 to 2.73 1.20 to 1.77 Treece and Jirsa(1989) (Black) 1.07 to 1.82 – Treece and Jirsa (1989) (Epoxy) 0.86 to 1.71 – Choi et al. (1991) (Black) 1.05 to 1.61 – Choi et al. (1991) (Epoxy) 1.19 to 2.01 – DeVries, Moehle, and Hester (1991) (Black ) – 1.07 to 2.21 DeVries, Moehle, and Hester (1991) (Epoxy) – 1.22 to 2.20 –no data available. Table 3.41. Comparison of test results to calculated results. test maximum stress to calculated stress) using 318 Code (ACI 2005) without a limit on the square root of the concrete com- pressive strength, without a bar size factor, and with a single epoxy-coated bar factor of 1.5 with concrete strength (defined as the square root of the concrete compressive strength). In Figure 3.69, the specimens of Hamad, Jirsa, and D’Abreu de Paulo (1993), Treece and Jirsa (1989), Choi et al. (1991), De- Vries, Moehle, and Hester (1991), and the tests on epoxy-coated bar splice specimens carried out under NCHRP Project 12-60 (Purdue [S]) were separated into two groups: test results from specimens with stirrups and test results from specimens with- out stirrups over the splice region. As can be seen from Figure 3.69, the bond efficiency values ranged from 0.86 to 3.11 for the specimens without stirrups and from 1.07 to 2.20 for the spec- imens with stirrups. The ranges for each of the studies, except for NCHRP Project 12-60, are listed in Table 3.41. The use of transverse reinforcement over the splice region increased the ACI-calculated stress, causing a decrease in the ratio of test max- imum stress to ACI-calculated stress, and this tendency was consistent with the tendency of the tests conducted under NCHRP Project 12-60. It is interesting to note as well that for the studies in the literature, the range of stress ratios in the spec- imens with epoxy-coated bars and companion specimens with uncoated bars was similar, as shown in Table 3.41. Thus, on the basis of the maximum concrete compressive strength included in the experimental evaluation and the evaluation of the data in the literature, the procedure in Chapter 12 of the 318 Code (ACI 2005) for splice and development length of epoxy-coated bars in tension could potentially be extended up to 17 ksi without a limit of 100 psi to the square root of the concrete compressive strength and with these two modifications—removal of the bar size factor and use of a single epoxy-coated bar factor of 1.5. 3.9.7 Summary and Conclusions From the test results of 12 beam splice specimens rein- forced with epoxy-coated bars, the following conclusions can be drawn: • The ratios of measured maximum stress on the spliced bars to stress calculated using 318 Code (ACI 2005) ranged from 0.98 to 1.96. Ratios calculated using 318 Code (ACI 2005) with these two proposed modifications—no bar size factor and a single epoxy-coated bar factor of 1.5—ranged from 1.07 to 2.29. Thus, the procedure in Chapter 12 of the 318 Code (ACI 2005) for splice and development length of epoxy-coated bars in tension can be extended up to 17 ksi with the modifications suggested in this section. • The ratios of measured maximum stress on the spliced bars to the stress calculated using the AASHTO specification ranged from 1.73 to 2.67. • The use of transverse reinforcement over the splice region resulted in increases both in the test maximum stress and deflection at failure. • The current contribution of the cover in the 318 Code (ACI 2005) can be overestimated in the case of higher strength concrete specimens with large covers. 3.10 Anchorage of Bars Terminated with Standard Hooks in Tension This section deals with the tensile strength of black and epoxy-coated reinforcing bars terminated in 90-deg hooks with and without transverse reinforcement under monotonic loading in normal-weight concrete with uniaxial compressive strength up to 16 ksi. As part of this examination, in addition to 43 previous tests, the test results of 21 beam-column joint type specimens are reported. Variables in the tests conducted under NCHRP Project 12-60 included bar size (#6 and #11), concrete strength (10, 14, and 16 ksi), and amount of trans- verse reinforcement in the anchorage region. Codes and spec- ifications have limits to their applicability to higher strength concretes (ACI 2005, AASHTO 2004). These limits are justi- fied on the basis of the empirical nature of code and specifi- cation requirements. The requirements for bars in tension

100 anchored by means of a standard hook are an example of such specifications. In 1975, Marques and Jirsa reported a series of tests to de- termine capacities of uncoated hooked bars. Twenty-two specimens simulating exterior beam-column joints were tested to evaluate the capacity of uncoated anchorage beam reinforcements subjected to varying degrees of confinement at the joint. The types of confinement included vertical col- umn reinforcement, lateral reinforcement through the joint, side concrete cover, and column axial load. To simulate beam moment acting on the column, tension was applied to an- chored bars and a reaction assembly transferred compression load to the specimen. Failure in most tests was sudden and resulted in the entire side cover of the column spalling away to the level of the hooked anchorage. The maximum concrete compressive strength in these tests, which served as the basis for the current anchorage requirements, was 5.1 ksi. Anchorage of epoxy-coated hooked bars was evaluated by Hamad, Jirsa, and D’Abreu de Paulo (1993) in a series of tests. Twenty-four hooked-bar specimens simulating exterior beam-column joints were tested. It was reported that #11 hooked bars (coated or uncoated) were consistently less stiff than #7 hooked bars. Epoxy-coated hooked bars consistently developed lower anchorage capacities and load-slip stiffness than companion uncoated hooked bars. The companion hooked-bar specimens that had ties in the beam-column joint region improved both the anchorage capacity and load- slip behavior of both coated and uncoated bars. To date, there has been little work on the anchorage performance of hooked bars, black and epoxy-coated, in high-strength concrete. In the 2005 ACI Building Code, the equation for the basic development length (lhb) of a hooked bar is limited to concrete strength of 10 ksi. Therefore, fur- ther investigation on anchorage strength of hooked bars in high-strength concrete is needed. 3.10.1 U.S. Design Specifications 3.10.1.1 318 Code (ACI 2005) Development length for deformed bars in tension termi- nating in a standard hook, ldh, is determined using Section 12.5.2 and applicable modification factors of 12.5.3, as shown in Equation 3.22. However, ldh shall not be less than the larger of 8db and 6 in. as indicated in Section 12.5.1 of the 318 Code (ACI 2005). (3.22) In Equation 3.22, ψe is the coating factor, taken as 1.2 for epoxy-coated reinforcement; λ is the factor reflecting the lower tensile strength of lightweight concrete, which is 1.3. In l f f ddh e y c b= ′ ⎛ ⎝⎜ ⎞ ⎠⎟ 0 02. ψ λ other cases, these two factors are taken equal to 1.0. Other pa- rameters are db, which is the bar diameter of the hooked bar; f ′c, which is the concrete compressive strength in psi; and the square root of the concrete compressive strength, which shall not exceed 100 psi as per Section 12.1.2 of the 318 Code (ACI 2005). The modification factors of Section 12.5.3 of the 318 Code (ACI 2005) are all less than 1.0 and thus reduce the cal- culated length on the basis of cover, presence of ties where the first tie encloses the bent portion of the hook within 2db of the outside of the bend, and where anchorage or development for specified minimum yield strength, fy, is not specifically required. These modification factors are the following. For #11 bar and smaller hooks with side cover (normal to the plane of the hook) not less than 2.5 in. and for 90-deg hooks with cover on the bar extension beyond the hooks that are not less than 2 in.: 0.7 For 90-deg hooks of #11 and smaller bars that are enclosed within ties or stirrups perpendicular to the bar being developed, spaced not greater than 3db along ldh; or enclosed within ties or stirrups parallel to the bar being developed, spaced not greater than 3db along the length of the tail extension of the hook plus bend: 0.8 For 180-deg hooks of #11 and smaller bars that are enclosed within ties or stirrups perpendicular to the bar being developed, spaced not greater than 3db along ldh: 0.8 Where anchorage or development for fy is not specifically required, reinforcement in (As required / excess of that required by analysis: As provided) The factor As required/As provided, also referred to as the factor for excess reinforcement, applies only where anchor- age for full fy is not specifically required because the area of steel required to resist the factored flexural moment at the section, As required, is less than the area of steel provided, As provided, at the same section. 3.10.1.2 2004 AASHTO Specifications (Section 5.11.2.4 Standard Hooks in Tension) The 1995 318 Code provisions for anchorage of bars termi- nated in a standard hook in tension are the current procedure in the AASHTO LRFD Bridge Design Specifications (ACI 1995). The 1983 provisions for development of standard hooks in tension in the 318 Code were a major departure from the 1977 318 Code in that they uncoupled hooked bar anchorages from

101 straight bar development provisions and measured the hooked bar embedment length from the critical section to the outside end or edge of the hook. The development length of the hooked bar is represented by the product of a basic devel- opment length and appropriate modification factors. In the 1995 edition of the 318 Code, a factor of 1.2 was introduced in the calculation of development lengths of epoxy-coated bars terminated in a standard hook (ACI 1995). The development length, ldh (in.), for deformed bars in ten- sion terminating in a standard hook specified in Article 5.10.2.1 shall not be less than the following: • The product of the basic development length and the applicable modification factor or factors, as specified in Article 5.11.2.4.2; • 8.0 bar diameters; or • 6.0 in. Basic development length, lhb, for a hooked-bar with yield strength, fy, not exceeding 60.0 ksi shall be taken as: (3.23) where db = diameter of the hooked bar (in.) and f ′c = specified compressive strength of concrete at 28 days, unless another age is specified (ksi). Below, cases in which basic hook development length, lhb, should be multiplied by a factor are given, as well as the ap- plicable factor. l d f hb b c = ′ 38 0. 3.10.2 Experimental Program 3.10.2.1 Test Specimens The experimental program reported in this research con- sisted of the monotonic loading in tension only (see Figures 3.70 and 3.71) of 20 specimens with two bars terminated in 90-deg standard hooks (see Figure 3.72). Key test parameters are given in Table 3.42. The test specimens were cast using normal-weight concrete (see 17.5” 15” 17.5” 11” Strong Column (W14x99) Strong Girder (2-MC10x33.6) Hydraulic Ram Concrete Column (15”x15”) Load Cell Bar Lock Loading Plate Stiffener Anchorage Plate Compression Plate Figure 3.70. Test setup for beam-column-type specimens (1 in.  25.4 mm). Reinforcement has a yield strength exceeding 60 ksi: fy/60 Side cover for #11 bar and smaller, normal to the plane of the hook, is not less than 2.5 in., and cover on bar extension beyond 90-deg hooks is not less than 2 in.: 0.7 Hooks for #11 bar and smaller that are enclosed vertically within ties or stirrup ties spaced along the full development length, ldh, at a spacing not exceeding 3db: 0.8 Anchorage or development of full yield strength is not required, or reinforcement is provided in excess (As required / of that required by analysis: As provided) Lightweight concrete is used: 1.3 Epoxy-coated reinforcement is used: 1.2

102 Anchorage Plate 3” 9” 3” 3” 3” Column Section (9”x15”) 6.5” Ldh=6.5” 9” 3” 9” 3” 3” 3” Column Section (15”x15”) Anchorage Plate 6.5” Ldh=12.5” 15” Figure 3.71. Specimen details for anchorage tests of bars terminated with standard hooks (1 in.  25.4 mm). Figure 3.72. Detail of Specimen I-2. Table 3.43). Specimens I-1 to I-6 contained black hooked bars. Specimens II-7 to II-12 had epoxy-coated hooked bars. In Specimens III-13 to III-20, transverse reinforce- ment was provided in the joint area to confine the concrete along the anchorage length of the hooked bars. The speci- mens were provided with an anchorage length, ldh, as per 318 Code (ACI 2005) (see Table 3.42). Test specimens contained two #6 hooked bars or two #11 hooked bars. The concrete column size of the specimens with the #6 hooked bars was 9 by 15 in. The column cross section of the specimens reinforced with the #11 bars (such as I-2) was 15 by 15 in. The width of the column was kept the same in all specimens, but the depth was changed to ac- commodate the different development lengths. In both types of specimens, concrete cover was 2.5 in. Each concrete column was reinforced with five or seven #8 main vertical bars and 4 stirrups spaced at 6 in.—two at the top and two at the bottom of the column as shown in Figure 3.72. The 318 Code (ACI 2005) anchorage requirements for uncoated bars anchored by a combination of standard hook and straight embedment length were based on the test results of Marques and Jirsa (1975). These provisions were later extended by Hamad, Jirsa, and D’Abreu de Paulo (1993) to epoxy-coated bars. In NCHRP Project 12-60, a similar test setup was used in the evaluation of these provisions in higher strength concretes. 3.10.2.2 Test Setup and Procedure The test setup used in this investigation is shown in Figures 3.70 and 3.73. A force couple consisting of a tensile force in the test bars (applied by two center-hole hydraulic rams) and a compressive force concentrated at a distance of 15 in. below the centerline of the bars was applied. The compression force at the face of test specimen was applied through two plates (3 in. and 3/4 in. thick) attached to the reaction column sim- ulating a 6 in. deep compression zone of the assumed beam. The reaction column consisted of a W14x99 column welded to a base plate 1 in. thick and bolted to the strong girder on the floor. Pull-out load was applied in 3.5-kip in- crements to the #6 bar specimens and in 10-kip increments to the #11 bar specimens until failure occurred. Two strain gages were affixed to each bar, and the slip of the anchored reinforcing bar relative to the concrete surface was measured using LVDTs. 3.10.2.3 Materials The two concrete mixes ordered from a concrete ready- mix company were proportioned to yield a concrete com- pressive strength of 10 ksi (Mix I) and at least 14 ksi (Mix II). Table 3.43 shows a typical concrete mix. The water-to- cement ratio was 0.32 for Mix I and 0.20 for Mix II. A stress versus age relationship is shown in Figure 3.74. The modulus of rupture was 566 psi and 834 psi at 28 days for Mix I and Mix II, respectively. Each size of reinforcing bar was from the same heat of steel, and all bars had the same deformation pattern. The relative rib area of #6 and #11 bars was 0.091 and 0.135, respectively. Grade 60 steel was used for both black and epoxy-coated bars. The yield strength obtained from tensile tests was 81.9 ksi and 63.1 ksi for the #6 and #11 black bars, respectively. For the epoxy-coated bars, the yield strength calculated by 0.2-

103 Name Bar Size Concrete Strength (psi) Bar Type ldh (in.) Concrete Cover (in.) Stirrup Spacing I-1 #6 8,905 Black 6.5 2.5 None I-2 #11 8,905 Black 12.5 2.5 None I-2’ #11 9,535 Black 15.5 2.5 None I-3 #6 12,455 Black 6.5 2.5 None I-4 #11 12,455 Black 12.5 2.5 None I-5 #6 12,845 Black 6.5 2.5 None I-6 #11 12,845 Black 12.5 2.5 None II-7 #6 9,535 Epoxy-coated 6.5 2.5 None II-8 #11 9,535 Epoxy-coated 12.5 2.5 None II-9 #6 13,670 Epoxy-coated 6.5 2.5 None II-10 #11 13,670 Epoxy-coated 12.5 2.5 None II-11 #6 14,800 Epoxy-coated 6.5 2.5 None II-12 #11 14,800 Epoxy-coated 12.5 2.5 None III-13 #6 13,980 Black 6.5 db 3db III-14 #11 13,980 Black 12.5 db 3db III-15 #6 16,350 Black 6.5 db 3db III-16 #11 16,500 Black 12.5 db 3db III-17 #6 13,670 Epoxy-coated 6.5 db 3db III-18 #11 13,670 Epoxy-coated 12.5 db 3db III-19 #6 16,350 Epoxy-coated 6.5 db 3db III-20 #11 16,500 Epoxy-coated 12.5 db 3db 1 in. = 25.4 mm; 1 psi = 6.89 kPa Table 3.42. Description of key test parameters. percent offset from tensile tests was 72.5 ksi and 74.7 ksi for the #6 and #11 bars, respectively. The average coating thick- ness measured with a dry film thickness gage for all epoxy- coated bars was around 12 mils. 3.10.3 Experimental Results 3.10.3.1 Load versus Slip Behavior and Cracking Pattern Pull-out load versus slip responses for Specimens II-9, II-10, III-17, and III-18 are shown in Figure 3.75. The pull-out load versus slip responses for Specimen III-19 and III-20 are given in Figure 3.76. Load was measured using a load cell attached to each bar terminated with a 90- deg standard hook. In almost all the specimens, the gages placed on the bar at a distance of 2 in. from the column surface showed yielding before reaching the maximum pull-out load, with less than 0.05-in. slip between hooked bar and concrete surface on the loaded side. As can be seen from Figure 3.75(a) and (b), spec- imens without shear reinforcement in the test region, II-9 and II-10, had a significant decrease in load at a 0.2-in. relative slip between hooked bar and concrete surface. However, in the specimens with shear reinforcement in the test region, III-17 and III-18 (see Figure 3.75[c] and [d]), the load decrease (20 Contents 10-ksi Mix 14-ksi Mix Cement (lb) 780 900 Silica fume (lb) 50 200 Water (lb) 265 220 Coarse aggregate (lb) 1,600 (3/8” pea gravel) 1,800 (1/2” crushed limestone) Fine aggregate (lb) 1,240 1,000 High-range water reducer (oz) 190 520 Normal-range water reducer (oz) 35 38 1 lb = 0.454 kg. 1 oz = 28.35 gr. 1 yd3 = 0.765 m3. 1 ksi = 6.89 MPa. Table 3.43. Typical concrete mix ratio (per 1 cubic yard).

104 Figure 3.73. Beam-column-type specimen test setup and instrumentation. 0 2 4 6 8 10 12 14 16 18 20 0 20 40 60 80 100 Age (days) Co m pr es si ve S tre ng th (k si) 10 ksi 14 ksi Figure 3.74. Concrete compressive strength versus age relationship in standard hook tests (1 ksi  6.89 MPa). percent of the peak load) was not as severe as the load decrease observed in the specimens without shear reinforce- ment (almost 50 percent of the peak load). In Specimens III-17 and III-18, at 0.2-in. slip, the sustained anchorage force was more than 80 percent of the maximum pull-out force. It can be concluded that the #6 epoxy-coated bar specimen with shear reinforcement in the test region and smaller cover was able to reach a higher peak load than its companion specimen without shear reinforcement but with a larger cover (see Fig- ure 3.75[a] and [c]). This was not the case for specimens with #11 epoxy-coated bars anchored by standard hooks (Figure 3.75[b] and [d]). However, the specimens with shear rein- forcement (Figure 3.75[c] and [d]) were able to sustain almost 80 percent of the peak load at a slip of 0.2 in. regard- less of the bar size. The load versus slip behavior of Specimens III-19 and III-20 are shown in Figure 3.76(a) and (b), respectively. Comparing the behavior for the #6 bar specimens, III-17 and III-19, recorded in Figure 3.75(c) and Figure 3.76(a), respectively, it can be observed that the increase in concrete compressive strength from about 13.5 ksi to 16.5 ksi resulted in an increase in pull-out strength. The same increase in concrete strength in the case of the specimens anchoring #11 bars, III-18 and III-20, also resulted in an increase in pull-out strength (see Figure 3.75[d] and Figure 3.76[b]).The same type of finding was ob- served for the specimens anchoring uncoated bars. In almost all of the tests, the cracking sequence was simi- lar. The first flexural (horizontal) crack occurred on the back face of the specimens at a load of 20 kips for the #6 bar spec- imens and 60 kips for the #11 bar specimens. The crack ap- peared near the tail end of the hook. After the initial flexural crack, a shear crack appeared on the side of the specimen as shown in Figure 3.77. With the increase in the pull-out load, the gage near the hook showed signs of yielding. At 90 per- cent of the peak load, the vertical cracks appeared along the column main bar. Finally, the concrete block near the hooked bar pushed out in Type I and II specimens that had no stir- rups in the joint (see Figure 3.78). In the Type III specimens containing stirrups over the anchorage length, with the fail- ure of the concrete near the hook, some of the side concrete cover spalled off (see Figure 3.79). Within the range of these tests, there was no significant difference on the pull-out char- acteristics of the hooked bars with different concrete com- pressive strength up to 16 ksi. It must be noted that with large slips and with the tendency of the bar to straighten under ten- sion, the tail end of the hook tended to kick out, thus splitting the concrete behind the hook. However, these cracks were very small, implying that a cover of 2.5 in. over the tail end of the hook used was sufficient for design purposes in the range of concrete strengths considered in this study. 3.10.3.2 Maximum Pull-Out Stress and Failure Mode Table 3.44 shows a comparison of maximum pull-out stress and the calculated stress on the basis of Equation 3.22. This equation gives the straight embedment length calculated in accordance with the 318 Code (ACI 2005) and measured from the critical section to the outside portion of the hook. In this equation, fy is the yield strength of hooked bar, ψe is the coating factor (epoxy-coated reinforcement = 1.2, uncoated reinforcement = 1.0), λ is the lightweight aggregate concrete factor (for lightweight concrete = 1.3, for normal concrete =1.0), and f ′c represents the concrete compressive strength. Substituting fs in place of fy, stress in the bar for a given anchorage length, and solving for s with a given design an- chorage length, as in Equation 3.24, it is possible to obtain the calculated stress shown in Table 3.44. The maximum stress corresponding to the peak pull-out load is obtained by

105 Load- Slip of II-9 (Right) 0 5000 10000 15000 20000 25000 30000 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Lo ad (lb ) (a) II-9 (#6, fc' = 13.5 ksi, cover = 2.5”) Load - Slip of II-10 (Right) 0 20000 40000 60000 80000 100000 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Lo ad (lb ) (b) II-10 (#11, fc' = 13.5 ksi, cover = 2.5”) (c) III-17 (#6, fc' = 13.5 ksi, cover = db) 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Load- Slip of III-17 (Right) 0 5000 10000 15000 20000 25000 30000 35000 40000 Lo ad (lb ) (d) III-18 (#11, fc' = 13.5 ksi, cover = db) 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Load - Slip of III-18 (Right) 0 10000 20000 30000 40000 50000 60000 70000 80000 Lo ad (lb ) Figure 3.75. Effect of transverse reinforcement in the anchorage region on the pull-out load versus slip response (1 in.  25.4 mm; 1 kip  4.448 kN). Load- Slip of III-19 (Left) 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Lo ad (lb ) (a) III-19 (#6, fc' = 16.5 ksi, cover = db) 0 0.1 0.2 0.3 0.4 0.5 Slip (in.) Load - Slip of III-20 (Right) 0 20000 40000 60000 80000 100000 120000 140000 Lo ad (lb ) (b) III-20 (#11, fc' = 16.5 ksi, cover = db) Figure 3.76. Effect of high-strength concrete in the anchorage region on the pull-out load versus slip response (1 in.  25.4 mm; 1 kip  4.448 kN). averaging the values from the two load cells attached to each hooked bar divided by the area of the bar. (3.24) Specimens with #6 bars, except Specimens II-9 and III-15, at failure reached a stress equal to or greater than the calcu- lated stress (ratio of test result to calculated value of 0.99 to f l d f s dh b c e = ( ) ′⎡⎣⎢ ⎤⎦⎥* .0 02ψ 1.23). However, most of the specimens anchoring # 11 bars at failure reached a stress less than or equal to the calculated stress (ratio of test result to calculated result of 0.83 to 1.02). In the case of specimens anchoring epoxy-coated bars (Series II), the tendency was the same as in Series I specimens rein- forced with black bars. In Series III, the specimens anchoring #11 bars reached failure stress levels less than or equal to the calculated stress values, yielding a ratio of test to calcu- lated stress ranging from 0.83 to 0.98 while the specimens

106 (a) Specimen I-1 (c) Specimen III-13 (e) Specimen III-19 (b) Specimen I-2 (d) Specimen III-14 (f) Specimen III-20 Figure 3.77. Crack patterns.

107 Figure 3.78. Concrete block push off in specimens without stirrups in the anchored hooked bar specimens. Figure 3.79. Failure region in the case of Specimen III-13 with stirrups. anchoring #6 bars reached failure stresses greater than the cal- culated values, i.e., ratios ranging from 1.01 to 1.14, except Specimen III-15, which failed at a lower level. A similar com- parison was conducted in terms of force developed in the bar. Comparison of the results of Specimens II-9 and II-10 with Specimens III-17 and III-18 indicated that the use of ties over the joint region developed 56 percent more force in the bar in the case of #6 bars, and 15 percent more force in the case of #11 bars. Each pair of specimens had the same dimensions and material properties, but had different details, such as hav- ing ties, having no ties, or having different concrete cover. Taking into account these similarities and differences in the specimens, it can be concluded that the confinement (with ties) of the anchorage region produced stronger bond char- Specimen db (in.) f c (psi) ldh (in.) Calculated Test Ratio (T/C) Force (kips) I-1-9 0.75 8905 6.5 58.4 68.2 1.17 30.0 I-2-9 1.41 8905 12.5 59.8 56.4 0.94 88.0 I-2'-10 1.41 9535 15.5 76.7 67.3 0.88 05.0 I-3-12 0.75 12455 6.5 69.1 68.2 0.99 30.0 I-4-12 1.41 12455 12.5 70.7 63.5 0.90 9.1 I-5-13 0.75 12845 6.5 70.2 69.3 0.99 30.5 I-6-13 1.41 12845 12.5 71.8 73.1 1.02 14.0 II-7-10 0.75 9535 9.5 73.6 90.9 1.23 40.0 II-8-10 1.41 9535 15.5 63.9 56.4 0.88 8.0 II-9-14 0.75 13670 6.5 60.3 56.1 0.93 4.7 II-10-14 1.41 13670 12.5 61.7 53.5 0.87 3.5 II-11-15 0.75 14800 6.5 62.8 64.8 1.03 28.5 II-12-15 1.41 14800 12.5 64.2 54.5 0.85 5.0 III-13-14 0.75 13980 8.3 92.9 93.8 1.01 41.3 III-14-14 1.41 13980 13.5 80.9 67.3 0.83 05.0 III-15-16 0.75 16350 8.3 100.5 87.5 0.87 38.5 III-16-16 1.41 16500 13.5 87.8 76.9 0.88 20.0 III-17-14 0.75 13670 8.3 76.6 87.5 1.14 38.5 III-18-14 1.41 13670 13.5 66.6 61.5 0.92 9 8 2 8 8 95.9 III-19-16 0.75 16350 8.3 83.7 89.8 1.07 39.5 III-20-16 1.41 16500 13.5 73.2 71.8 0.98 1 1 1 1 112.0 1 in. = 25.4 mm; 1 kip = 4.448 kN; 1 psi = 6.89 kPa; 1 KSI = 6.89 MPa Table 3.44. Comparison of maximum pull-out bar stress compared with calculated stress using the 318 Code (ACI 2005) method with a modification factor of 0.7 (ksi).

108 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Calculated Stress (ksi) M ax im um E xp er im en ta l S tr es s ( ks i) Marques & Jirsa (1975) Kim Hamad, Jirsa & D’Abreu de Paulo (1993) Figure 3.80. Maximum experimental stress versus 318 Code calculated stress (1 ksi  6.89 MPa). Figure 3.81. Test to calculated stress ratio versus concrete compressive strength (1 psi  6.89 KPa). acteristics in hooked bars than no ties with a 2.5 in. cover. This tendency is observed with both the epoxy-coated bars and black bars. 3.10.3.3 Comparison with Other Tests and Recommendations The comparison of the stress calculated using Equation 3.24 for NCHRP Project 12-60 tests (Kim) and test results reported by Hamad, Jirsa, and D’Abreu de Paulo (1993) and Marques and Jirsa (1975) are plotted against the experimen- tal values in Figure 3.80. The comparison shows that NCHRP Project 12-60 test results follow in general the same trend as those of previous researchers. Thus, it is plausible to propose to extend the current ACI procedure for hooked bars up to 16 ksi without a limit on the term. In Figure 3.81, the ratio of test to calculated stress for hooked bars is shown versus the concrete compressive strength of the specimen. It can be seen that the ratio de- creases as the concrete compressive strength is increased in both black and epoxy-coated bars terminated with a standard hook and subjected to direct tension. To increase the values of the ratio of test to calculated stress in specimens with higher concrete strengths, it is proposed that a 0.8 modifica- tion factor be used instead of the current factor of 0.7 [for hooks with side cover not less than 2-1/2 in. and for 90-deg hooks with cover on bar extension beyond hook not less than 2 in. in ACI Code 12.5.3(a) in concrete strengths above 10 ksi]. The calculated results using the proposed modification factor and current factor are shown in Figure 3.82. 3.10.4 Summary and Conclusions Based on the review of over 40 specimens in the literature and the results from 21 tests of hooked bar anchorages in beam-column specimens with normal-weight concrete strengths up to 16 ksi, the following conclusions can be drawn: • The approach in the 318 Code (ACI 2005) provision for anchorage of bars terminated in standard hooks in tension, black and epoxy-coated, can be extended to concrete com- pressive strengths up to 15 ksi. However, a minimum transverse reinforcement (3db spacing) should be provided in higher strength concretes to improve the bond charac- teristics of both epoxy-coated and black #11 hooked bars. fc 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 0 5000 10000 15000 20000 Concrete Compressive Strength (psi) Te st / C al cu la te d Marques & Jirsa (1975) Kim Hamad, Jirsa & D’Abreu de Paulo (1993) (a) Black Bar Specimens 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Te st / C al cu la te d 0 5000 10000 15000 20000 Concrete Compressive Strength (psi) (b) Epoxy-Coated Bar Specimens

109 Figure 3.82. Maximum test stress versus calculated stress using factors of 0.7 and 0.8 for Marques and Jirsa (1975), Hamad et al. (1993), and those tested in NCHRP Project 12-60 (1 ksi  6.89 MPa). • The epoxy-coated hooked bars developed lower anchorage capacities than uncoated hooked bars. In the #11 hooked bar specimens, the ratios of measured stress to calculated stress were 0.85 to 0.88. • Transverse reinforcement in the anchorage length of a bar terminated with a standard hook improves the max- imum pull-out strength and load versus slip behavior. In the #11 epoxy-coated hooked bar specimens, the ratios of measured stress to calculated stress increased up to 0.98. • While the minimum concrete cover of 2.5 in. at the end of the hook appeared to be adequate to prevent kicking out of the tail end of the hooked bar, it is proposed that a modi- fication factor of 0.8 be used instead of 0.7. The use of a 0.8 modification factor eliminated almost the entire test to cal- culated stress ratios less than 1.0. This value of minimum concrete cover can be reduced to db if transverse reinforce- ment is used in the anchorage length of a bar terminated with a standard hook. 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Calculated Stress (ksi) M ax im um T es t S tr es s ( ks i) Cal.-0.7 Cal.-0.8

Next: Chapter 4 - Design Recommendations »
Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete Get This Book
×
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

TRB's National Cooperative Highway Research Program (NCHRP) Report 603: Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete explores recommended revisions to the American Association of State Highway and Transportation Officials Load and Resistance Factor Design (LRFD) Bridge Design Specifications, which are designed to extend the applicability of the transfer, development, and splice length provisions for prestressed and non-prestressed concrete members to concrete strengths greater than 10 ksi.

Appendices A and B are published as part of NCHRP Report 603. Appendices C through I are available online via the links below:

* Appendix C: Rectangular Beam Summaries-Strand D

* Appendix D: Rectangular Beam Summaries-Strands A&B

* Appendix E: Rectangular Beam Summaries-Strand A (0.6 in.)

* Appendix F: I-Beam Summaries-0.5-in. Strand

* Appendix G: I-Beam Summaries-0.6-in. Strand

* Appendix H: AASHTO Mxxx-Standard Test Method for the Bond of Prestressing Strands

* Appendix I: NASP Test Protocols

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  6. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  7. ×

    View our suggested citation for this chapter.

    « Back Next »
  8. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!