Coupled diffusion-damage modelling and the implications on failure due to strain localisation

For underground cement-based structures, the leaching of pore solution ions by pure water is one of the reference scenario of degradation. The penetration of this chemical attack leads to a loss of mechanical properties of the material. Under mechanical loading, progressive damage can develop too. In order to evaluate the coupling between these chemical and mechanical mechanisms of degradation (increase of porosity and cracking) a model is developed. The chemical attack kinetics follows a non-linear diffusion equation based on the mass conservation of the calcium ions in concrete. The mechanical behaviour is modelled according to a scalar damage constitutive relation. After a brief presentation of the model, the influence of damage development on the chemical attack kinetics is analysed. It is shown that due to the chemical degradation (activated by diffusion mechanisms) localisation due to strain softening can be at the origin of the failure. In spite of the chemical coupling, the problem of equilibrium is ill posed at the inception of strain localisation. A non local damage model is implemented in order to alleviate this problem. A preliminary example of finite element computations illustrates the different regimes of damage on a bending beam subjected to a chemical attack and to a variable deflection. (C) 1998 Elsevier Science Ltd. All rights reserved.