Crack shear in concrete: Crack band microplane model

The crack band model is applied to the problem of crack shear in concrete. The constitutive law for concrete within the crack band is provided by the microplane model, in which the microstrains on weak planes of various orientations (the microplanes) are assumed to conform to the same macroscopic strain tensor, and the microstresses from all the microplanes are superimposed. Due to the neglect of shear stiffness on individual microplanes, the material behavior is completely characterized by the relation between the normal stress and strain for each microplane. To simulate crack shear, the law for unloading contribution on the microplanes after previous tensile strain-softening is important, since the shear stresses resisting crack shear, as well as the normal confining stresses and crack dilatancy, result from compression along lines inclined with regard to the crack plane. A satisfactory agreement with the existing results from shearing tests of cracked concrete blocks (i.e., aggregate interlock tests) is achieved. Since the same type of model was previously shown capable of modeling strain-softening in direct tensile tests, fracture of notched specimens, and deflections of cracked reinforced beams, the present model appears to have a general applicability. It can be applied to the shearing of cracks only partially formed (a system of distinct discontinuous cracks still in the strain softening stage), to cracks that are being produced simultaneously with shearing, to crack shear when the direction of shearing within the crack plane rotates, and to shearing of concrete intersected by cracks of various directions. Thus, the model appears suitable for general finite element programs.