Chemoporoplasticity of Calcium Leaching in Concrete

This paper presents a macroscopic material model for calcium leaching in concrete, for the quantitative assessment, in time and space, of the aging kinetics and load bearing capacity of concrete structures subjected to severe chemical degradation (such as radioactive waste disposal applications). Set within the framework of chemically reactive porous continua, the model accounts explicitly for the leaching of calcium of portlandite crystals and C-S-H, and its cross-effects with the elastic deformation (chemical damage) and irreversible skeleton deformations (chemical softening) treated within the theory of chemoplasticity. In the first part of this paper the governing equations are derived focusing on the chemomechanical couplings between calcium dissolution, increase in porosity, and deformation and (micro-) cracking of concrete. Without any a priori assumption concerning local equilibrium between the solid calcium concentration s and the interstitial calcium concentration c the well-known calcium leaching state function s = s(c) is then derived using combined thermodynamic equilibrium and dimensional arguments relating to the structural dimension of containment structures. In the second part, this paper addresses the experimental determination of chemical damage and chemical softening of the calcium leaching. For chemical damage, a simple mixture rule involving different skeleton constituents suffices to capture the main chemoelastic features of leaching; in turn, microhardness measurements allow access to the chemical softening state function capturing chemoplastic cross-effects. The intrinsic nature of these functions, and of the proposed procedure, is validated by means of finite-element analysis of experimental compression tests of a degraded specimen with nonhomogeneous chemical degradation states.