This paper provides a new general stress-strain law for concrete confined by steel, fiber reinforced polymer (FRP), or fiber reinforced cementitious matrix (FRCM), obtained by a suitable modification of the well-known Sargin’s curve for steel confined concrete. The proposed law is able to reproduce stress-strain curve of any shape, having both hardening or softening behavior, by using a single closed-form simple algebraic expression with constant coefficients. The coefficients are defined on the basis of the stress and the tangent modulus of the confined concrete in three characteristic points of the curve, thus being related to physical meaningful parameters. It will be shown that if the values of the parameters of the law are deduced from experimental tests, the model is able to accurately reproduce the experimental curve. If they are evaluated on the basis of an analysis-oriented model, the proposed model provides a handy equivalent design model. 1. Introduction Upgrading of reinforced concrete structures in seismic areas is often required due to the need for a higher performance and safety level or material deterioration. Increment of deformation capacity of the structure in critical regions, where large plastic deformations are expected, is one of the most efficient strategies in this field. For reinforced concrete (RC) elements, the simple application of fiber reinforced polymer (FRP) embedded in epoxy resin and/or fiber reinforced cementitious matrix (FRCM) can dramatically enhance strength and ductility of concrete columns. Jacketing over the entire length of the element by FRP or FRCM wraps is often preferred, due to the favourable properties of this retrofitting methodology: extremely low weight-to-strength ratio, easy application, minimal change in the behaviour of the structure, and protection and prevention of corrosion. In order to evaluate the effects of such a retrofitting strategy on the seismic behavior of the structure, stress-strain relationship of confined concrete is needed to evaluate the moment-curvature response of the elements [1]. When steel transverse reinforcement as a confining system is utilized, as soon as the transversal stress on the concrete reaches the cracking limit, the lateral strain suddenly grows and the confining steel hoops yield. From this point, a nearly constant confining pressure is applied to the column concrete core, and the concrete behaves as an active confined material [2]. Therefore, the stress-strain relationship of a steel-confined concrete member is characterized by a steep increasing branch up to
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