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5 CHAPTER 1 Background 1.1 Introduction ment in compression members where the use of yield strength up to 100 ksi is permitted. The AASHTO LRFD Bridge Design TheAASHTO LRFDBridgeConstruction Specifications (2004) Specifications (2007) similarly limit the use of reinforcing steel permit the use of uncoated reinforcing steel conforming to any yield strength in design to no less than 60 ksi and no greater of the specifications given in Table 1. Since all grades of rein- than 75 ksi (exceptions are permitted with owner approval). forcing steel (bars and wires) have an ASTM designation, Both ACI and AASHTO limits have been written and inter- ASTM designations will be used throughout this report. preted to not exclude the use of higher strength grades of Recent revisions to 9.2 of the construction specifications steel, but only to limit the value of yield strength that may be and to MP 18 Standard Specification for Uncoated, Corrosion- used in design. Resistant, Deformed and Plain Alloy, Billet-Steel Bars for Con- The limits on yield strength are primarily related to the crete Reinforcement and Dowels (AASHTO 2009) permit the prescribed limit on concrete compressive strain of 0.003 and specification of ASTM A1035 (2009) reinforcing steel. A1035 to the control of crack widths at service loads. Crack width is reinforcing bars are low-carbon, chromium steel bars charac- a function of steel strain and consequently steel stress (Nawy terized by a high tensile strength and a stress-strain relationship 1968). Therefore, the stress in the steel reinforcement will having no yield plateau. Yield strength is determined using the always need to be limited to some extent to prevent cracking 0.2% offset method and is specified to exceed 100 ksi or 120 ksi. from affecting serviceability of the structure. However, with Because of their high chromium content (specified to be recent improvements to the properties of concrete, the ACI between 811%; slightly less than stainless steel), A1035 bars 318 limit of 80 ksi and AASHTO limit of 75 ksi on the steel are reported to have superior corrosion resistance when com- reinforcement yield strength are believed to be unnecessarily pared to conventional reinforcing steel grades. For this reason, conservative for new designs. Additionally, an argument can designers have specified A1035 as a direct, one-to-one replace- be made that if a higher strength reinforcing steel is used but ment for conventional reinforcing steel as an alternative to not fully accounted for in design, there may be an inherent stainless steel or epoxy-coated bars. The AASHTO LRFD overstrength in the member that has not been properly taken Bridge Design Specifications (2007), however, limit the yield into account. This concern is most critical in seismic applica- strength of reinforcing steel to 75 ksi for most applications. tions or when considering progressive collapse states. Therefore, although A1035 steel is being specified for its corro- Steel reinforcement with yield strength exceeding 80 ksi is sion resistance, its higher yield strength cannot be utilized. commercially available in the United States. If allowed, using For many years, the design of reinforced-concrete structures steel with this higher capacity could provide various benefits to in the United States was dominated by the use of steel rein- the concrete construction industry by reducing member cross forcement with yield strength, fy, equal to 40 ksi and, since sections and reinforcement quantities, which would lead to sav- about 1970, 60 ksi. Design with steel having higher yield ings in material, shipping, and placement costs. Reducing rein- strength values has been permitted, but since the 1971 Ameri- forcement quantities would also reduce congestion problems can Concrete Institute (ACI) edition of ACI 318, yield strength leading to better quality of construction. Finally, coupling high- values have been limited to 80 ksi. Currently, ACI 318 (2008) strength steel reinforcement with high-performance concrete permits design using steel reinforcement with yield strength, should result in a much more efficient use of both materials. defined as the stress corresponding to a strain of 0.0035, not Additionally, much of the interest in higher strength rein- exceeding 80 ksi. The exception is spiral transverse reinforce- forcement stems from the fact that many of the higher strength