Click for next page ( 27

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement

Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

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

OCR for page 26
27 ing to transportation agencies and others. Therefore, as a first represents a different stage in the development and refine- step in this research program, a Standard Test Method for the ment of the NASP Bond Test. Bond of Prestressing Strands was refined from prior testing. The research reported herein continues and expands research 3.2.1 Refinement of the NASP Bond Test begun by NASP. The focus of NASP's research has been to de- velop a standardized test for bond that would be repeatable at The NASP Bond Test was originally developed in Round II a testing site, reproducible among sites, and provide a reliable and Round III research sponsored by NASP. The NASP prediction of the performance of a pretensioned concrete research investigated the repeatability and reproducibility of product. With the development of a repeatable, reproducible the test method together and also compared the NASP Bond standard test, design expressions for transfer and develop- Test with other test methods. In Rounds II and III, the ment length can be developed. research showed that the NASP Bond Test was a better pre- Figure 3.1 shows a NASP specimen mounted in the load- dictor for bond than the Moustafa Test or the PTI Bond Test. ing frame at OSU. Each test specimen is prepared by casting The NASP Bond Test also showed convincing results when a single prestressing strand in a sand-cement mortar within compared with transfer lengths measured on prestressed con- a cylindrical steel casing. The sand-cement mortar is pro- crete beams. Additionally, Round III testing showed evidence portioned to produce a strength of 4750 250 psi at 24 hr, that the NASP Bond Test could be used to ensure adequate after standard curing. Additionally, the sand-cement mortar strand development. The early versions of the NASP Bond is required to produce a flow in the range of 100 to 125 as Test protocols are included in Appendix I. Appendix I con- measured by ASTM C 1437. The strand is pulled from the tains two versions of the NASP Bond Test, the first dated concrete mortar at a displacement rate of 0.10 in./min, 24 hr August 2001 and the second dated May 2004. The earliest ver- after casting. Pull-out force is measured in relation to the sion of the NASP Bond Test was employed for Rounds II and movement of the free end of the strand to the hardened III of the NASP-sponsored research. The May 2004 protocols mortar. The NASP Bond Test records the pull-out force that were used for NCHRP Project 12-60 for the purpose of corresponds to 0.10 in. of free strand end slip. One single further refining the NASP Bond Test. Some refinements in NASP Bond Test consists of six or more individual test spec- protocol were made to develop the final version found in imens; the average value from the set of six becomes the Appendix H and titled, "Standard Test Method for the Bond "NASP Bond Test Value." The appendices to this report of Prestressing Strands." contain three separate bond test protocols; each protocol 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- Figure 3.1. NASP specimen on the surements were not made during the NASP Round III tests of loading frame at OSU. the August 2001 protocols. The standardized flow rates help

OCR for page 26
28 ensure workability of the mortar and consistent consolida- Strands requires a loading rate of 0.1 in./min, as before, and tion of the mortar. The strand is centered in a steel casing the NASP value is reported as the load at which the free strand with an outer diameter of 5 in. and a bond length of 16 in. The end slip is 0.1 in. The average of six or more NASP specimens cement mortar is cast and consolidated in the steel casing. is reported as the NASP value for the strand. Studies con- The NASP Bond Test protocols in 2001 did not specify the ducted earlier in the NASP Round II concluded that the least frame used for loading the NASP specimen. The loading variation in the NASP values is exhibited for the 0.1 in. of frames used in the Round III trials were more "flexible" when strand end slip. The largest variation in the NASP values was compared with the frame used in the current NCHRP re- reported in the 0.01 in. of free strand end slip. search, which is more "rigid." Because the NASP Bond Test The Moustafa Test and the PTI Bond Test, which are used protocols require a displacement rate, the rigidity of the test by some to identify the bonding properties of prestressing apparatus affects the loading rate. Therefore, the Standard strands with concrete, were neither repeatable nor repro- Test Method for the Bond of Prestressing Strands limits the ducible. The NASP Bond Test was convincingly superior to loading rate to 8,000 lb/min for 0.5 in. diameter strands and the others in its ability to reproduce results among sites. 9,600 lb/min for 0.6 in. diameter strands. In its recommended Table 3.1 provides the results of NASP Bond Tests that form, the Standard Test Method for the Bond of Prestressing were performed at OSU. Ten different 0.5 in. diameter Table 3.1. Results of NASP Bond Tests at OSU. Diameter (in.) Mortar NASP Test Results STRAND ID NCHRP ID Water- NASP IV Strength Batch # Strand to- Pull-Out f ci Force at S LC/DC Cement N Ratio 0.1" slip (lbs.) (psi) (lbs.) 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

OCR for page 26
29 strands were tested along with two different 0.6 in. strands. In Test Method for the Bond of Prestressing Strands requires Table 3.1, LC/DC refers to whether the test was conducted displacement control instead of load control. using load control (LC) or displacement control (DC). These The Standard Test Method for the Bond of Prestressing tests were critical to refining the test protocols and also to Strands (see Appendix H) includes specific dimensions for determining which strand samples would provide high and the test specimens and the procedures for the test. Figure 3.2 low targets for NCHRP Project 12-60 transfer length and de- shows a schematic of the Standard Test Method for the Bond velopment length tests. Testing also included variations in of Prestressing Strands. Additional details for the NASP Bond water-to-cement ratio (w/c), which resulted in variations in Test are shown in Figure 3.3. Figure 3.4 shows detail for the mortar strength. W/c ratios of 0.40, 0.45, and 0.50 were methodology employed to measure the strand end slip on its tested. Additionally, some tests were performed using load- "free" end, i.e., the end of the strand that is not loaded in ten- controlled protocols instead of displacement control. From sion. The photograph in Figure 3.5 shows the strand end slip these tests, it was determined that displacement control pro- measurement device. Finally, in Figure 3.6, the photograph vides more data that can be valuable in evaluating strand shows an entire Strand Bond Test specimen placed within the bond performance. Therefore, the recommended Standard loading frame and ready for testing. 2'-6" 8" 5.5" LOAD TRANSDUCER WITH 22,000 LB CAPACITY 4.5" 2.5" 12 1 1 2 " x 9" x 1 4 " THK STEEL PLATE 1" BOLT 21 4" x 8" CHANNEL SECTION 8" CLEAR DISTANCE 2'-8" Vertical Axis line 12 1 1 3 2 " x 9" x 1 4 " THK STEEL PLATE WITH 4" WIDE 1 SLOT AT THE CENTER THROUGH 4 2" 91 1 3 2 " x 9" x 1 4 " THK STEEL PLATE WITH 4" WIDE SLOT AT THE CENTER THROUGH 4 1 2" 51 4" CLEAR DISTANCE 21 4" x 8" CHANNEL SECTION 7" 1" BOLT MTS CONSOLE Figure 3.2. Schematic diagram of NASP Test setup.

OCR for page 26
30 Consolidated mortar mix 1 6 2" or 10 " diameter strand 2" long styrofoam bond breaker firmly attached to the strand Base plate 6" x 6" x 1 5 4" with 8" diameter hole welded to the specimen cylinder Neoprene pad 6" x6" 3 Base plate 6"x6"x 4" steel plate Steel plate 3 1 1 1 5 4" x 3 4" x 2" with 8" diameter hole 1 2" chuck Figure 3.3. Details of the NASP Bond Test specimen. 3.2.2 Reproducibility of the NASP Bond Test Strand Bond Test data from OSU was developed from tests Between Sites 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 The NASP Bond Test was performed on specific strand Round IV) and the NCHRP strand ID. Purdue performed the samples at Purdue and OSU. Round robin trials were per- bond tests as completely blind trials--even the Purdue strand formed on five 0.5 in. diameter and two 0.6 in. diameter, identifiers were changed from those used at OSU. Grade 270, low-relaxation strands. The strands included in The results from the round robin testing are reported in the round robin trials are shown in Table 3.2. Some of the 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. 13.0" 7.50" 3 14 " steel block bolted to the aluminum plate for weight 3 Aluminum plate 13" x 1" x 4 in 9.0" 3" clear spring loaded LVDT (DCT 1000A) Magnetic base with control switch NASP specimen casing Mortar mix cured for 24 hours 1 6 2" or 10 " diameter strand 2" long styrofoam bond breaker firmly attached to the strand Figure 3.4. NASP Test specimen strand end slip measurement. Figure 3.5. LVDT on NASP Test specimen.