BOND AND SERVICEABILITY CHARACTERIZATION OF CONCRETE REINFORCED WITH HIGH STRENGTH STEEL

Recent revisions to the AASHTO Construction Specifications permit the specification of ASTM A1035 reinforcing steel. A1035 reinforcing bars are low carbon, chromium steel bars characterized by a high tensile strength (100 or 120 ksi) 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 reason, 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 AASHTO LRFD Design Specifications, however, limit the yield strength of reinforcing steel to 75 ksi for most applications. Therefore, although A1035 steel is being specified for its corrosion resistance, its higher yield strength cannot be utilized.The objective of this research is to evaluate existing AASHTO LRFD Design Specifications to determine their applicability when using high strength ASTM A1035 reinforcing bars in reinforced concrete structures. The study encompasses material testing and characterization, detailed analytical studies, component tests, and full-scale member testing. The analytical and experimental research program investigates ultimate strength and service behavior and detailing of members designed with high-strength reinforcement. Specifically, the topics addressed are: a) experimental evaluation of hooked bar development length of high-strength reinforcement; b) experimental evaluation of the fatigue performance of members reinforced with high-strength reinforcement; and c) parametric evaluation of serviceability and crack opening in flexural members, comparing the results with available experimental results.The applicability of current Specification requirements for hooked bar development lengths was confirmed through a series of pull-out tests having development lengths that were shorter than those required by present Specifications equations. Tests resulted in bar rupture outside of the anchorage region with very little slip clearly indicating the efficacy of the hooked bar development requirements in Specifications. It is recommended that such anchorage regions be provided with cover and confining reinforcement ¨C based on current design requirements ¨C when high-strength bars are used. The presence of confining reinforcement effectively mitigates potential splitting failures and results in suitably conservative anchorage capacities. Two large-scale proof tests conducted as part of this study and a review of available published data demonstrate that presently accepted values for the fatigue or i®endurancei¯ 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 iU 100 ksi.The extension of present AASHTO LRFD Bridge Design Specifications for hooked bar anchorage and fatigue to permit reinforcing bar yield strengths not exceeding 100 ksi was validated for concrete strengths up to 10 ksi.A fundamental issue in using A1035 or any other high-strength reinforcing steel is that the stress at service load is expected to be greater than when conventional steel is used. Consequently, the service-load reinforcing strains are greater, affecting deflection and crack widths. Based on the results of available flexural test, deflections and crack widths at service load levels were evaluated. Both metrics of serviceability were found to be within presently accepted limits, and were predictable using current Specifications provisions. A limitation on service-level stresses of fs iU 60 ksi is recommended.