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41.1 Problem Statement and Research Objective This final report documents research findings regarding the transfer length and development length of prestressing strand and the development length and splice length of rein- forcement in normal-weight concrete with compressive strengths up to 15 ksi. Recommended revisions to the 4th edi- tion of the AASHTO LRFD Bridge Design Specifications were also developed in the research (AASHTO 2007). Structural engineers have continually tried to optimize building materials by improving their durability and effec- tiveness. An example of such efforts is the use of high-strength concrete in bridge members. High-strength concrete, defined for this report as concrete having a compressive strength in ex- cess of 10 ksi, is more brittle than normal-strength concrete; consequently, the designer has to be cautious in extending em- pirically based rules to this new material. The 2004 AASHTO LRFD Bridge Design Specifications (with 2005 and 2006 in- terim revisions) states that concrete with compressive strengths above 10 ksi should be used only when physical tests are made to establish the relationships between concrete strength and other properties (AASHTO 2004). This require- ment reflects the lack of data to demonstrate that the provi- sions were applicable to concrete strengths above 10 ksi at the time the specification was written. The objective of this study was to develop recommended revisions as appropriate to the 4th edition of the AASHTO LRFD Bridge Design Specifications for normal-weight concrete having compressive strengths up to 15 ksi, relating to the following (AASHTO 2007): ⢠Transfer and development length of prestressing strand with diameters up to 0.62 inches; and ⢠Development and splice length in tension and compres- sion of individual bars, bundled bars, and welded wire reinforcement and development length of standard hooks. The knowledge gained and the resulting improvements to the 4th edition of the AASHTO LRFD Bridge Design Specifi- cations will assist engineers in the safe design of high-strength concrete bridge members (AASHTO 2007). The 4th edition of the AASHTO LRFD Bridge Design Spec- ifications provides a good starting point for higher strength concrete bridge members (AASHTO 2007). However, the existing provisions were optimized for concrete strengths be- tween 5 and 10 ksi and require modifications if they are to be extended to higher strength concretes. Because of its brittle nature, high-strength concrete must be properly utilized in both design and construction. The more brittle nature of high-strength concrete means that if cracks form, they may propagate more extensively than they would in lower strength concretes. This may result in the loss of effectiveness of the concrete cover and raise safety concerns regarding the bond strength of strand and deformed bars embedded in high-strength concrete. 1.2 Research Approach Chapter 2 of this report provides a review of relevant prac- tice, performance data, research findings, and other relevant information related to the transfer and development length of prestressing strand and the development and splice length of mild reinforcement. This chapter presents an information summary on the factors that affect the behavior of transfer and development length of prestressing strand and the devel- opment and splice length of mild reinforcement. Chapter 2 includes a critical review of existing testing protocols for determining bond characteristics and presents the testing protocols used in NCHRP Project 12-60. Article 5.4.2.1 of the 3rd edition of the AASHTO LRFD Bridge Design Specifications limits the applicability of the spec- ifications to 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 C H A P T E R 1 Introduction and Research Approach
2004). A comprehensive, article-by-article review of Section 5 of the 2nd edition of the AASHTO LRFD Bridge Design Speci- fications (with Update 2000) was performed under NCHRP Project 12-56 to identify all the provisions that directly or indirectly have the potential to prevent the extension of the specifications in their current form to high-strength concrete (AASHTO 1998; Hawkins and Kuchma 2002). In addition, a comprehensive, article-by-article review of Section 5 of the 2nd edition of the AASHTO LRFD Bridge Design Specifications with the 1999, 2000, and 2001 interim revisions (AASHTO 1998) was performed as part of the FHWA study, Compilation and Evaluation of Results from High Performance Concrete Bridge Projects (Russell et al. 2006). The provisions identified by both of these reports as having the potential for preventing the extension of development and splice length specifications to high-strength concrete were reviewed and considered dur- ing the work conducted under NCHRP Project 12-60. The results of the expanded plan of research are included in Chapter 3. The findings of the extensive literature review and the experimental program conducted under NCHRP Project 12-60 were used to recommend revisions as appropriate to the AASHTO LRFD Bridge Design Specifications for transfer, development, and splice lengths for strand/reinforcement in normal-weight concrete having compressive strengths up to 15 ksi. Chapter 4 contains design recommendations supported by the research conducted under NCHRP Project 12-60. 5
