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1 SUMMARY Design of Concrete Structures Using High-Strength Steel Reinforcement Recent revisions to 9.2 of the AASHTO LRFD Bridge Construction Specifications and to AASHTO MP 18 Standard Specification for Uncoated, Corrosion-Resistant, Deformed and Plain Alloy, Billet-Steel Bars for Concrete Reinforcement and Dowels permit the specification of ASTM A1035 reinforcing steel. A1035 reinforcing bars are low carbon, chromium steel bars characterized by a high tensile strength (minimum yield strength of 100 or 120 ksi determined using the 0.2% offset method) and a stress-strain relationship having no yield plateau. Because of their high chromium content, A1035 bars are reported to have superior corrosion resistance when compared to conventional reinforcing steel grades. For this rea- son, designers have specified A1035 as a direct, one-to-one, replacement for conventional reinforcing steel as an alternative to stainless steel or epoxy-coated bars. The specifications, however, limit the yield strength of reinforcing steel to 75 ksi for most applications. There- fore, although A1035 steel is being specified for its corrosion resistance, the benefits of its higher yield strength cannot be utilized. A number of types and grades of steel reinforcement with yield strengths exceeding 80 ksi are commercially available in the United States. If allowed, using steel with this higher capac- ity could provide various benefits to the concrete construction industry by reducing mem- ber cross sections and reinforcement quantities, leading to savings in material, shipping, and placement costs. Reducing reinforcement quantities also would reduce congestion problems leading to better quality of construction. Finally, coupling high-strength steel reinforcement with high-performance concrete should result in a much more efficient use of both materials. This report provides an evaluation of existing AASHTO LRFD Bridge Design Specifications relevant to the use of high-strength reinforcing steel and other grades of reinforcing steel having no discernable yield plateau. The report identifies aspects of reinforced-concrete design and of the specifications that may be affected by the use of high-strength reinforcing steel. An integrated experimental and analytical program intended to develop the data required to permit the integration of high-strength reinforcement into the LRFD specifica- tion is presented. In addition, a number of "proof tests" intended to validate existing spec- ifications provisions applied to higher strength reinforcing steel are presented. The focus of the experimental phase of this study is the use of ASTM A1035 reinforcing steel since it cap- tures both behavioral aspects of interest (i.e., it has a very high strength and has no discern- able yield plateau). In addition, this study specifically considers the use of higher strength concrete. The experimental and analytical studies include concrete having compressive strengths of 5, 10, and 15 ksi. The primary deliverable of NCHRP Project 12-77 was to provide recommendations for changes to the specifications necessary for the use of high-strength reinforcing steel. This report provides the background and engineering basis, in the form of experimental and ana-

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2 lytical studies, supporting these recommendations. Although summarized in Chapter 3, the recommendations forwarded to the Project Panel, and eventually to the AASHTO Techni- cal Committee for Concrete Design (T-10), are not presented in this document. In all cases, recommended language was proposed that specifically permits the use of high-strength rein- forcing steel with specified yield strengths not greater than 100 ksi when the specific article permits it. This methodology is consistent with the manner by which the specifications handle high-strength concrete, allowing its use only when a specific article permits it. Specifications Sections 3, 5, and 9 were identified as having articles potentially requiring changes. Although considered in its entirety, no changes were identified in the AASHTO LRFD Bridge Construc- tion Specifications. The 2009 revisions to 9.2 of the Construction Specifications permit the use of A1035 reinforcing steel. Yield Strength A critical objective of the present work was to identify an appropriate steel strength and/or behavior model to adequately capture the behavior of high-strength reinforcing steel while respecting the tenets of design and the needs of the designer. A value of yield strength, fy, not exceeding 100 ksi was found to be permissible without requiring significant changes to the specifications. Flexure The current specifications design methodology for flexure, that is, a simple plane sections analysis using stress block factors to model concrete behavior and an elastic-perfectly plastic steel behavior (having E s = 29,000 ksi), is shown to be appropriate for values of fy 100 ksi. To ensure ductility, steel strains corresponding to tension- and compression-controlled lim- its (defined in of the specifications) are recommended as follows: Current; No Recommended Limits for Recommended Changes High-Strength Reinforcement fy 60 ksi fy = 100 ksi Tension-Controlled Section t 0.005 t 0.008 Compression-Controlled Section t 0.002 t 0.004 Values may be interpolated between limits These strain limits were developed through a rigorous analytical study of 286 cases, which included seven different grades of reinforcing steel, three concrete strengths, and multiple section geometries. Six large-scale beam specimens reinforced with A1035 reinforcing steel confirmed the appropriateness of the proposed tension- and compression-controlled lim- its. All beam specimens met and exceeded their designed-for strength and ductility criteria and exhibited predictable behavior and performance similar to beams having conventional reinforcing steel. Fatigue Two large-scale proof tests conducted as part of this study and a review of available pub- lished data demonstrate that presently accepted values for the fatigue or "endurance" limit for reinforcing steel are applicable--and likely conservative--when applied to higher strength bars. Additionally, it is shown that fatigue considerations will rarely affect the design of typical reinforced-concrete members having fy 100 ksi.

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3 Shear Five large-scale reinforced-concrete beams and four AASHTO Type I prestressed girders were tested to evaluate the performance of high-strength A1035 steel as shear reinforcement in comparison to that of the commonly used A615 steel. Test specimens were designed using the specifications approach of summing concrete and steel contributions to shear resistance (i.e., Vc + Vs). All beams exhibited good performance with little difference noted between the behavior of spans reinforced with A1035 or A615 transverse steel. The use of current specifications procedures for calculating shear capacity were found to be acceptable for val- ues of shear reinforcement yield fy 100 ksi. Shear Friction A series of eight push-off (direct shear) proof tests of "cold construction joint" interfaces rein- forced with either A1035 or A615 bars demonstrated that current specifications requirements for such joints are adequate. Significantly, the restriction that fy be limited to 60 ksi when calcu- lating shear friction capacity must be maintained regardless of the reinforcing steel used. This limit is, in fact, calibrated to limit strain (and therefore interface crack opening) to ensure ade- quate aggregate interlock capacity across the interface and is, hence, a function of steel modu- lus rather than strength. It is noted that steel modulus does not vary with reinforcing bar grade. Compression Analytical parametric studies were performed to examine behavior of columns reinforced with A1035 longitudinal and transverse reinforcement. Results indicate the current specifi- cations requirements for both longitudinal and transverse reinforcement design in compres- sion members are applicable for fy 100 ksi. Bond and Development The applicability of current specifications requirements for straight bar and hooked bar development lengths was confirmed through a series of spliced-bar beam tests and pull-out tests, respectively. "Proof test" spliced-bar beam specimens, having development lengths that were shorter than those required by the present specifications equations (with all appro- priate reduction factors applied), were tested. All developed bar stresses exceeding fy and approaching the ultimate bar capacity, fu, prior to the splice slipping, and in one case bar fracture. Tests of hooked bar anchorage resulted in bar rupture outside of the anchorage region with very little slip, clearly indicating the efficacy of the hooked bar development requirements in the specifications. Significantly, it is recommended that development, splice, and anchorage regions be provided with cover and confining reinforcement--based on current design requirements--when high-strength bars are used. Existing equations for development where no confinement is present are demonstrated to be unconservative. The presence of confining reinforcement effectively mitigates potential splitting failures and results in suitably conservative development, splice, and anchorage capacities. Serviceability--Deflections and Crack Widths A fundamental issue in using A1035, or any other high-strength reinforcing steel, is that the stress at service load (fs; assumed to be on the order of 0.6fy) is expected to be greater than when conventional Grade 60 steel is used. Consequently, the service-load reinforcing strains

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4 (i.e., s = fs/Es) are greater than those for conventional Grade 60 steel. The large strains affect deflection and crack widths at service loads. Based on the results of the flexural tests con- ducted in this study, deflections and crack widths at service load levels were evaluated. Both metrics of serviceability were found to be within presently accepted limits and were pre- dictable using current specifications provisions. A limitation on service-level stresses of fs 60 ksi is recommended; this is consistent with the recommendation that fy 100 ksi. Summary The extension of present AASHTO LRFD Bridge Design Specifications to permit reinforc- ing bar yield strengths not exceeding 100 ksi was investigated and, for the most part, vali- dated for concrete strengths up to 10 ksi, and, in some instances, 15 ksi. This study did not address seismic applications, and no such increase in permitted yield strength is addressed for Seismic Zones 2 through 4. Other limitations to the use of high-strength reinforcing steel also are identified. Recommended specifications language was proposed to the Project Panel that specifically permits the use of high-strength reinforcing steel with specified yield strengths not greater than 100 ksi when the specific article permits it. This report provides the necessary background and engineering basis, in the form of experimental and analytical studies, supporting these recommendations.