A MIXTURE THEORY WITH MICROSTRUCTURE APPLIED TO THE DEBONDING, CRACKING AND HYSTERETIC RESPONSE OF AXIALLY REINFORCED CONCRETE

The highly nonlinear response of axially reinforced concrete to monotonic and cyclic uniaxial extension is represented by means of a binary mixture-theory-with-microstructure which synthesizes the global properties of a steel-concrete composite from the properties of plain concrete, steel, the steel-concrete bond and the steel layout geometry. Mixture equilibrium and constitutive equations are developed for axially reinforced concrete which debonds according to a specified local bond-slip relation. Progressive transverse fracturing is represented by a sequence of mixture boundary value problems each of which reflects a given uniform crack pattern.; The resulting mixture formulation accurately predicts the several stages of actual composite response due to the onset and cessation of progressive fracturing and debonding. The validated model provides a means of studying the relative importance of several geometric and material parameters in a theoretical parameter study. A result of this study is a stiffness degradation function which for a given steel-volume fraction specifies a continuous composite strain-softening behavior due to progressive cracking and debonding. The function is implemented in a homogenized constitutive model for the uniaxial tensile states of axially reinforced concrete.; A local bond-slip relation is developed which, together with the binary mixture model, yeilds accurate simulations of both pull-out and tension tests involving progressive debonding and transverse cracking but not longitudinal cracking. This study suggests that in the absence of longitudinal cracking the dependence of the local bond-slip relation on concrete strength, f'(,c) and on embedment length is very weak. Externally applied transverse pressures, on the other hand, are shown to significantly alter the local bond-slip relation.; A simplified cyclic local bond-slip relation is postulated which reflects the results of several experimental studies on short embedment length test specimens. This cyclic bond-slip model, together with crack-opening and crack-closing algorithms is incorporated into the numerical solution of the mixture equations, and the response of a full-scale reinforced concrete masonary panel to uniaxial load-unload-reload cycles is simulated. The hysteretic material damping and ductility predicted theoretically is in excellent agreement with test results.