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6 CHAPTER 2 Literature Review 2.1 Introduction issues that may influence the transfer and development of pre- stressing strands include confining reinforcement and strand The comprehensive and critical literature review under- spacing. The research reported in the literature indicates that taken during NCHRP Project 12-60 is described in this chap- 0.6-in. strand can be spaced at 2.0 in. center to center (c/c) or ter. In this report, important findings from prior research are that 0.5-in. strands may be spaced at 1.75 in. without penalty reviewed, with particular attention to the impact of these to the transfer and development of strands. The reported findings on the work plan for NCHRP Project 12-60. research also indicates that confining reinforcement has little The objective of the work related to prestressing strand was or no effect on transfer length of strands, but it can be quite to gather and synthesize existing data and information on the beneficial to strand development. Standardized confinement transfer length and development length of strand with diam- details were employed in beam testing where warranted. eters up to 0.6 in. In the area of mild reinforcement, the effort In the area of bond of mild reinforcement, the single most concentrated on development and splice length in tension of important issue not currently accounted for in the AASHTO individual bars and development length of bars in tension an- LRFD Bridge Design Specifications is the effect of confining chored with standard hooks. The database constructed from reinforcement on the bond strength of tension reinforcement this effort includes 71 tension development and splice tests of in the case of splitting type failures. This parameter is espe- specimens with top cast uncoated reinforcing bars, 493 speci- cially important as bars are being developed in higher strength mens with bottom cast uncoated reinforcing bars, 27 speci- concretes. The review also revealed that for epoxy-coated bar mens with top cast epoxy-coated bars, and 48 with bottom development/splice length and development length of bars cast epoxy-coated bars. In addition, 33 specimens with un- terminated with standard hooks there is a paucity of data on coated bars terminated with standard hooks and 13 specimens concretes with cylinder strengths above 10 ksi. reinforced with epoxy-coated bars have been reviewed. A significant effort during the initial 6 months of the A comprehensive analysis of the data collected was con- NCHRP Project 12-60 study was focused on identifying and ducted to identify issues and needs related to bond of strand evaluating testing protocols related to the experimental work and mild steel in high-strength concrete. This analysis assisted to be conducted. In the area of bond in concrete of prestress- in the identification of several key variables that are likely to ing strand, particular attention was given to the surface char- affect the transfer and development of prestressing strands, acterization tests to evaluate strand "bond-ability." There are development/splice length of bars in tension, and develop- three tests that have been offered in recent years as possible ment length of bars in tension terminated with standard tests to standardize acceptance of strand based on its "bond- hooks. Some of these variables are currently included in the ability": (1) the Moustafa Test, where untensioned strands AASHTO LRFD Bridge Design Specifications while some are are pulled from large concrete blocks; (2) the PTI Bond Test, not. In the area of transfer length and development length of where untensioned strands are pulled from a neat cement prestressing strand, specifications do not account for variables mortar; and (3) the NASP Bond Test, where untensioned such as concrete strength, strand size, "top bar" effects, epoxy strands are pulled from a sand cement mortar. In testing per- coating, bond quality of individual strand samples, and struc- formed by the North American Strand Producers (NASP), tural behavior issues (e.g., the interaction of shear and bond). the NASP Bond Test has proven to be the most reliable test of The work plan for NCHRP Project 12-60 included procedures the three. It has produced test results from "blind trials" with and testing to evaluate some, but not all, of these effects. Other the best repeatability and reproducibility.