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The performance under applied and environmental loads and the proportions selected for reinforced concrete members are determined primarily by two factors: the properties of the concrete used in those members and the mathematical models used to design those members. Standard tests such as cylinder compression, modulus of rupture, and splitting tension are now used to characterize the properties of the concrete for design and for quality control purposes. Yet those tests do not provide the information necessary to distinguish between the likely characteristics in practice of different high-performance concretes. Post-cracking behavior, which controls stiffness and durability at service loads especially for unidirectionally and lightly reinforced members, is closely tied to concrete properties (fracture softening) best established through displacement controlled fracture type tests.
Fracture mechanics concepts will not influence building code provisions for reinforced concrete until practitioners start using those concepts to improve their designs. To facilitate that action, efforts need to be concentrated on developing data that correlate the relative contributions of the fracture softening properties of different concretes to building code provisions that control designs in which only unidirectional reinforcement is desired. When that correlation has been established, and when appropriate fracture softening characteristics of different typical concretes have been determined through a comprehensive test program, then designers will need an ASTM Standard that they can use to check that, for the actual concrete placed in their structure, the fracture softening characteristics used in their design are being achieved.
Professor of Civil Engineering, University of Illinois at Urbana-Champaign, Newmark CE Laboratory, Urbana, IL
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