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This paper describes fundamental characteristics of the compressive stress distribution in the compression zone of flexural members with concrete compressive strengths up to 18 ksi (124 MPa). The proposed model is based on testing of 21 plain concrete specimens subjected to combined flexure and axial compression up to failure. The main variable considered was the strength of concrete that ranged from 10.4 to 16 ksi (71.7 to 110.3 MPa). Each specimen was subjected to two independent loads with a specific configuration to induce maximum compressive strain at one face and zero strain at the opposite. The measured stress-strain curves and stress block parameters were compiled with the data found in the literature. The results were used to develop recommended revisions for the LRFD specifications to extend their current limitation of 10 ksi (69 MPa) for concrete compressive strength up to 18 ksi (124 MPa).
Keywords: compression; flexure; high-strength concrete; loading; rectangular stress block; strain; stress.
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INTRODUCTION
Flexural failure of a reinforced concrete member occurs when its extreme fiber reaches the ultimate compressive strain of concrete. Concrete in the compression zone is subjected to a stress distribution, referred to as the stress block, that follows the stress-strain relationship of a concrete cylinder tested in axial compression. This paper focuses on the evaluation of the stress block in the compression zone of high-strength concrete (HSC) flexural members. The strength considered in this investigation ranged from 10 to 18 ksi (69 to 124 MPa). The authors used a similar test setup developed by Hognestad et al.1 in which an eccentric bracket specimen was subjected to an axial compression and a moment to simulate the stress profile in the compression zone of a rectangular flexural member.
The load and resistance factor design (LRFD) specifications,2 first published in 1994, include an article (5.4.2.1) limiting its applicability to a maximum concrete strength of 10 ksi (69 MPa), unless physical tests are made to establish the relationship between concrete strength and its other properties. This limitation was imposed due to the lack of sufficient research data on HSC at the time when the specifications were developed. Many design provisions stipulated in the LRFD specifications2 are still based on test results obtained from specimens with compressive strengths...