Mechanical Behavior and Microstructural Development of Low-Carbon Steel and Microcomposite Steel Reinforcement Bars Defo
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INTRODUCTION
AS a construction material, concrete is limited in its structural integrity by its poor tensile properties, despite its exceptional performance in compression. To address this deficiency, materials with good tensile strength and ductility are selected for use as reinforcement bars, frequently concatenated to ‘‘rebar,’’ within the concrete. The most common material used in rebar is low-carbon (LC) steel, which is an inexpensive, high-strength ferritic-pearlitic alloy that is easily shaped into the forms necessary to reinforce concrete structures and is easily deformed at the surface, to improve the bond between the concrete and the reinforcement. As defined by ASTM specifications, rebar is available in three grades that correspond to the three different yield strength values of 280, 420, and 520 MPa.[1] These L.M. DOUGHERTY, formerly a Postdoctoral Associate with MST-8, Los Alamos National Laboratory, Los Alamos, NM 87545, is an R&D Engineer with WCM-1, Los Alamos National Laboratory. Contact e-mail: [email protected] E.K. CERRETA and G.T. GRAY III, Staff Scientists, and C.P. TRUJILLO and M.F. LOPEZ, Technicians, are with MST-8, Los Alamos National Laboratory. K.S. VECCHIO, Chair, is with the Nanoengineering Department, University of California at San Diego, San Diego, CA 92093. G.J. KUSINSKI, formerly with the MMFX Technologies Corporation, Irvine, CA 92606, is with the Chevron Energy Technology Company, Richmond, CA 94802-0627. Manuscript submitted July 30, 2008. Article published online June 16, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
grades do not prescribe or regulate the physical characteristics (e.g., phase and microstructure) of the steel; therefore, such characteristics can vary substantially from one manufacturer to another, despite identical strength and ductility performance. Because carbon steels are susceptible to rusting, a corrosion process that expands the volume of reinforcements and leads to cracking of the concrete and eventual failure of the structures, materials that offer greater corrosion resistance than LC steel have been developed for rebar applications in corrosive environments. To account for these engineered materials, additional specifications that include corrosion tests and corrosion resistance requirements have been approved by ASTM. Stainless steel[2] and epoxy-coated steel[3] provide corrosion resistance without sacrificing strength, but these materials can be costly. Fiber-reinforced polymer composites are used on a limited basis in bridge decks and industrial applications, but are even more costly, exhibit inferior stiffness compared to steel reinforcements, and are susceptible to environmental degradation.[4] Recently, a less costly option for corrosion-resistant concrete reinforcement bars was approved by ASTM; in this option, corrosion resistance was achieved in a twophase steel by lowering the carbon content to 0.15 wt pct or less and introducing up to 1.5 wt pct manganese and 8.0 to 10.9 wt pct chromium (which is less than the minimum content of 12 wt pct chromium
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