A coupled elasto-plasto-damaged formulation for ordinary concrete at the meso-scale with explicit modelling of entrapped air macropores

Modeling the post-peak behaviour of brittle materials like concrete is stilla challenge from the point of view of computational mechanics, due to thestrong nonlinearities arising in the material behaviour during softening andthe complexity of the yield criterion that may describe their deformation capacityin generic triaxial stress states. A numerical model for plain concrete in compression is formulated withinthe framework of the coupled elasto-plasto-damage theory.The aim is tosimulate via the Finite Element (FE) method the stress-strain behaviour ofconcrete at the meso-scale, where local confinement effects are generally produced by the action exerted by the aggerates on the cement paste. Specifically, the meso-scale analyses involve the study of the influence of entrapped air macropores in damage propagation during uniaxial compression tests. The elasto-plasto-damaged model is formulated in terms of the invariants of the deviatoricstress tensor in compliance with the hardening, non-associated plasticitymodel by Mentrey and Willam. The influence of multiaxial stressstates on the deformation capacity is taken into account by using the volumetric partof the plastic strain tensor as strain-hardening parameter in the potentialfunction, as proposed by Grassl et al. Damage is assumed a scalar isotropic variable. Specifically, the damage variableis controlled by two distinct internal variables associated to the unilateralbehaviour of the material in tension and in compression (related to openingand closure of cracks), in agreement with. These variables are supposedto characterize the extreme loading state reached in the tensile part and compressivepart, respectively, of the strain space.Therefore, they are assigned aspecific threshold surface. To avoid mesh-dependency issues related to softening,a non-local regularization technique is adopted.