Abstract
A multi-scale poromechanical model of damage induced by Delayed Ettringite Formation (DEF) as a consequence of progression of micro-cracks at the fine aggregate scale is developed. The aim is to link the DEF-induced expansion at both the microscopic and macroscopic scales to the loss of stiffness of the mortar and the increase of its diffusion coefficient. At the microscopic scale, mortar is assumed to be constituted of three phases: cement paste, sand and micro-cracks. Damage is assumed to be driven by a free expansion of cement paste due to ettringite crystallization pressures in small capillary pores, at a lower scale. The corresponding homogenised poroelastic properties are estimated along with the diffusion coefficient by resorting either to a Mori-Tanaka scheme or to a self-consistent scheme, as a function of paste and aggregate properties as well as on the density of micro-cracks. The latter is assumed to be an evolving internal variable in order to model DEF-induced damage in the mortar. As the DEF-induced expansive free strain in the cement paste is restrained by the sand particles, internal stresses arise in the mortar. The corresponding free energy can be partially released by an increase in the micro-cracks density by analogy with the energy restitution rate of linear elastic fracture mechanics. The role of the damage criterion adopted on the thermodynamic force associated with micro-cracks density increase is investigated.
Highlights
Delayed ettringite formation (DEF) is a concrete pathology that may occur in concrete structures subjected to temperature above 65-70°C at early age, left in contact with water
We instead focus on one elementary brick of these coupled models: damage induced by a possible free expansion of cement paste restrained by aggregates, and its consequences on both mechanical and transport properties
The homogenized constitutive laws may formally be obtained by resolution of the mechanical and diffusive homogenization problems defined by: To draw an analogy with poroelasticity in what follows, we introduce the notation p = 3kp εf which corresponds to the pressure generated in the cement paste if its free expansion εf where prevented
Summary
Delayed ettringite formation (DEF) is a concrete pathology that may occur in concrete structures subjected to temperature above 65-70°C at early age, left in contact with water. According to [3], these pressures may lead to a uniform expansion of cement paste, which is restrained by aggregates, and produces internal stresses The latter are eventually released by diffuse microcracking responsible for the apparent damage. We instead focus on one elementary brick of these coupled models: damage induced by a possible free expansion of cement paste restrained by aggregates, and its consequences on both mechanical and transport properties. 2 presents a micromechanical model of mortar subjected to a free expansion of cement paste and the homogenization of both poro-elastic and diffusive properties, as a function of micro-crack density, for two types of homogenization schemes. The role of the different damage evolution laws suggested by previous investigators is illustrated and compared to experimental results on Young’s modulus drop as a function of the macroscopic expansion
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