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4 CHAPTER 1 Introduction and Research Approach 1.1 Problem Statement The knowledge gained and the resulting improvements to and Research Objective the 4th edition of the AASHTO LRFD Bridge Design Specifi- cations will assist engineers in the safe design of high-strength This final report documents research findings regarding concrete bridge members (AASHTO 2007). the transfer length and development length of prestressing The 4th edition of the AASHTO LRFD Bridge Design Spec- strand and the development length and splice length of rein- ifications provides a good starting point for higher strength forcement in normal-weight concrete with compressive concrete bridge members (AASHTO 2007). However, the strengths up to 15 ksi. Recommended revisions to the 4th edi- existing provisions were optimized for concrete strengths be- tion of the AASHTO LRFD Bridge Design Specifications were tween 5 and 10 ksi and require modifications if they are to be also developed in the research (AASHTO 2007). extended to higher strength concretes. Because of its brittle Structural engineers have continually tried to optimize nature, high-strength concrete must be properly utilized in building materials by improving their durability and effec- both design and construction. The more brittle nature of tiveness. An example of such efforts is the use of high-strength high-strength concrete means that if cracks form, they may concrete in bridge members. High-strength concrete, defined propagate more extensively than they would in lower for this report as concrete having a compressive strength in ex- strength concretes. This may result in the loss of effectiveness cess of 10 ksi, is more brittle than normal-strength concrete; of the concrete cover and raise safety concerns regarding the consequently, the designer has to be cautious in extending em- bond strength of strand and deformed bars embedded in pirically based rules to this new material. The 2004 AASHTO high-strength concrete. LRFD Bridge Design Specifications (with 2005 and 2006 in- terim revisions) states that concrete with compressive 1.2 Research Approach strengths above 10 ksi should be used only when physical tests are made to establish the relationships between concrete Chapter 2 of this report provides a review of relevant prac- strength and other properties (AASHTO 2004). This require- tice, performance data, research findings, and other relevant ment reflects the lack of data to demonstrate that the provi- information related to the transfer and development length sions were applicable to concrete strengths above 10 ksi at the of prestressing strand and the development and splice length time the specification was written. of mild reinforcement. This chapter presents an information The objective of this study was to develop recommended summary on the factors that affect the behavior of transfer revisions as appropriate to the 4th edition of the AASHTO and development length of prestressing strand and the devel- LRFD Bridge Design Specifications for normal-weight concrete opment and splice length of mild reinforcement. Chapter 2 having compressive strengths up to 15 ksi, relating to the includes a critical review of existing testing protocols for following (AASHTO 2007): determining bond characteristics and presents the testing protocols used in NCHRP Project 12-60. Transfer and development length of prestressing strand Article 5.4.2.1 of the 3rd edition of the AASHTO LRFD with diameters up to 0.62 inches; and Bridge Design Specifications limits the applicability of the spec- Development and splice length in tension and compres- ifications to concrete compressive strengths of 10,000 psi or sion of individual bars, bundled bars, and welded wire less unless physical tests are made to establish the relationships reinforcement and development length of standard hooks. between concrete strength and other properties (AASHTO

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5 2004). A comprehensive, article-by-article review of Section 5 by both of these reports as having the potential for preventing of the 2nd edition of the AASHTO LRFD Bridge Design Speci- the extension of development and splice length specifications fications (with Update 2000) was performed under NCHRP to high-strength concrete were reviewed and considered dur- Project 12-56 to identify all the provisions that directly or ing the work conducted under NCHRP Project 12-60. indirectly have the potential to prevent the extension of the The results of the expanded plan of research are included in specifications in their current form to high-strength concrete Chapter 3. The findings of the extensive literature review and (AASHTO 1998; Hawkins and Kuchma 2002). In addition, a the experimental program conducted under NCHRP Project comprehensive, article-by-article review of Section 5 of the 12-60 were used to recommend revisions as appropriate to 2nd edition of the AASHTO LRFD Bridge Design Specifications the AASHTO LRFD Bridge Design Specifications for transfer, with the 1999, 2000, and 2001 interim revisions (AASHTO development, and splice lengths for strand/reinforcement in 1998) was performed as part of the FHWA study, Compilation normal-weight concrete having compressive strengths up to and Evaluation of Results from High Performance Concrete 15 ksi. Chapter 4 contains design recommendations supported Bridge Projects (Russell et al. 2006). The provisions identified by the research conducted under NCHRP Project 12-60.