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

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S U M M A R Y Article 5.4.2.1 of the 3rd edition of the AASHTO LRFD Bridge Design Specifications limits the applicability of the specifications for concrete compressive strengths of 10,000 psi or less unless physical tests are made to establish the relationships between concrete strength and other properties (AASHTO 2004). A comprehensive, article-by-article review of Section 5 of the AASHTO LRFD Bridge Design Specifications pertaining to transfer, development, and splice length for strand/reinforcement was performed under NCHRP Project 12-60 to identify all the provisions that have to be revised to extend their use to high-strength, normal-weight concrete up to 15 ksi. Upon completion of the experimental work under NCHRP Project 12-60, draft specifications and accompanying commentary for provisions to extend the application of the LRFD bridge design specifications to high-strength concrete were developed. The provisions cover the transfer and development length of prestressing strand and the development and splice length of reinforcement in normal-weight concrete with compressive strengths up to 15 ksi. Researchers from Purdue University and Oklahoma State University have jointly pre- pared this report. Transfer Length and Development Length for Strand Recommendations include new transfer length and development length equations for incorporation into Articles 5.11.4.1 and 5.11.4.2 of the AASHTO LRFD Bridge Design Specifications. Also, a new requirement is introduced for addition to Article 5.4.4.1 for the purpose of qualifying the basic bonding properties of prestressing strand. Article 5.4.4.1 addresses the material properties of prestressing strand. Heretofore, Article 5.4.4.1 addressed the mechanical properties of strand only, i.e., breaking strength, yield strength, and strand size. Based on research described in this report, a “Standard Test Method for the Bond of Prestressing Strands” (also called the “Standard Test for Strand Bond”) is recommended for inclusion by reference in Article 5.4.4.1. Details for testing procedures and material acceptance are included in Appendix H. The Standard Test Method for the Bond of Prestressing Strand requires that prestressing strands obtain an average minimum pull-out value of 10,500 lb for 0.5-in. strands and 12,600 lb for 0.6-in. strands. Further, the research supports, and this report recommends, that transfer length and development length equations include a parameter for concrete strength. The research shows a clear correlation between shortening of transfer and development lengths and in- creasing concrete strength. Therefore, a new transfer length expression is recommended for inclusion into Article 5.11.4.2 of the AASHTO LRFD Bridge Design Specifications: l d f' dt b ci b= ⎡ ⎣⎢ ⎤ ⎦⎥ ≤ 120 40 Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete 1

2where db = strand diameter, f ′ci = concrete strength at release, and lt = transfer length. At a concrete release strength of 4 ksi, the recommended expression provides for a transfer length of 60 strand diameters, which matches historic design procedures. The recommended expression also provides for a transfer length of at least 40 strand diameters, effectively lim- iting the benefits from release strength to about 9 ksi. Increases in concrete strength also result in shorter development lengths. Therefore a new development length expression is recommended for inclusion in Article 5.11.4.3 of the AASHTO LRFD Bridge Design Specifications: This expression provides a development length of about 150 strand diameters for con- crete with release strength of 4 ksi and design strength of 6 ksi. The expression is different in form than the current expression, but more “user friendly” to the designer. In the develop- ment length equation, ld is the development length (in.), db is the strand diameter (in.), f ′ci is the concrete strength at release (ksi), and f ′ci is the concrete design strength (ksi). The expression provides for a development length of at least 100 strand diameters. Recommendations are also made to revise the part of Article 5.11.4.3 of the AASHTO LRFD Bridge Design Specifications dealing with debonded, or shielded, strands. In brief, rec- ommendations contained in this report would remove the 2.0 multiplier applied to debonded strands, but add some restrictions to the use of debonded strands. Development Length and Splice Length for Reinforcement The proposed recommendations stemming from the work conducted under NCHRP Project 12-60 cover two aspects for mild steel: 1. Development length of black and epoxy-coated reinforcing bars anchored by means of straight embedment length and splices and 2. Development length of black and epoxy-coated bars terminated with a standard hook. Based on observations from tests conducted during NCHRP Project 12-60 on 18 top cast beam-splice specimens and the examination of an extensive database of previous tests compiled by ACI Committee 408, it is proposed that extension of the AASHTO LRFD Bridge Design Specifications to concrete strengths up to 15 ksi follow a format similar to the one used in ACI: 318-05: Building Code Requirements for Structural Concrete and Commentary (ACI 2005), with the following exceptions: • Removal of the bar size factor for #6 bars and smaller bars (thus γ = 1.0 in all cases). • Use of a single factor for epoxy-coated bars of 1.5 regardless of the ratio of cover to bar diameter. • Exclusion of evaluations of beam splice specimens with bottom cast bars in this study. ACI Committee 408 has indicated that the current approach in the 318 Code (ACI 2005) overes- timates the bar force at failure in many specimens with bottom bars available in the ACI Committee 408 database, especially for specimens with concrete compressive strengths greater that 10 ksi (ACI Committee 408 2003). ACI Committee 408 proposed a modified expression l f f d dd ci c b b= ′ + ′ ⎡ ⎣⎢ ⎤ ⎦⎥ ≥ 120 225 100

3for development and splice length in addition to removal of the bar size factor to address this issue. In the evaluation of test data conducted under NCHRP Project 12-60, the researchers found that the use of a bottom cast modification factor of 1.2 for uncoated bars anchored in concrete with compressive strengths greater than 10 ksi appeared to address the safety concerns raised by ACI Committee 408. This factor would not be needed for bottom cast epoxy-coated bars (because of the single modification factor of 1.5) or for uncoated top bars. This approach could be used as an alternative to the approach suggested by ACI Committee 408. The researchers note that additional testing of bottom cast uncoated splices is justified with higher strength concretes. Article 5.11.2.4 of the AASHTO LRFD Bridge Design Specifications (AASHTO 2004) was verified for high-strength concrete in the experimental work plan for NCHRP Project 12-60 with the exception of the lightweight aggregate factor. Based on the analysis of tests conducted during NCHRP Project 12-60 (21 full-scale tests of hooked bar anchorages) and the analysis of tests of additional specimens in the literature, it is possible to support the extension of the approach in the 318 Code (ACI 2005) provision for anchorage of bars terminated with standard hooks, black and epoxy-coated, to normal-weight concrete with concrete compressive strength of up to 15 ksi, with these two modifications: 1. A minimum amount of transverse reinforcement (at least #3 U bars at 3db spacing) needs to be provided to improve the bond strength of both epoxy-coated and black #11 bars and larger bars in tension anchored by means of standard hooks. 2. A modification factor of 0.8 instead of the current factor of 0.7 for #11 and smaller hooks with side cover (normal to plane of hook) not less than 2.5 in. and for 90-deg hooks with cover on bar extension beyond hook not less than 2 in. Tension lap splices were also evaluated under NCHRP Project 12-60. Splices of bars in compression were not part of the experimental program. Class C splices were eliminated based on the modifications to development length provisions. The proposed modifications to Article 5.11.2.1 of the AASHTO LRFD Bridge Design Specifications contain several changes that eliminated many of the concerns regarding tension splices due to closely spaced bars with minimal cover; however, development lengths, on which splice lengths are based, have in some cases increased. A two-level splice length was retained primarily to encourage designers to splice bars at points of minimum stress and to stagger splices to improve behavior of critical details; however, such provisions are not intended to reflect the strength of the splice.