Chemically Assisted Damage of Concrete: A Model of Expansion Under External Sulfate Attack

Abstract

A micromechanical model is proposed to simulate the deformation of cementitious composites exposed to external sulfate attack. The model involves coupled physico-chemical processes of nonsteady diffusion with reaction, topochemical reaction of ettringite formation, expansion of ettringite inclusions, microcracking of hardened cement paste and percolation of sulfates through heavily deteriorated mortar. The Fick's second law with reaction term is assumed to govern the transport of the sulfate ions. The Eshelby solution and the equivalent inclusion method are used to determine the eigenstrain of expanding ettringite crystals in microcracked hardened cement paste. The degradation of transport properties is studied in the effective medium and the percolation regime. An initial boundary value problem (2D) of expansion of a mortar specimen immersed in a sodium sulfate solution is solved and compared with available test data.

Alternative methodology to consider damage and expansions in external sulfate attack modeling

“…Traditionally, the increase in diffusivity due to cracking was related to a damage parameter (w) defined by an isotropic damage model based on continuum damage mechanics [3,11,15]. In this work, a more direct and intuitive approach is presented, based on the idea that the loss of concrete strength is directly related to the cracking state of concrete, and hence, to the increase in diffusivity.…”

“…Gypsum formation is considered as an intermediate phase that later may react with the aluminate phases to form secondary ettringite ( 6 3 ̅ 32 ) [11,15,16,17]. Three calcium aluminates phases are considered as described in Eqs.…”

The Conditions of Ettringite Formation by the Reaction of a Blast Furnace Slag with Aqueous Alkaline Solutions

“…In the liquid phase reaction, 17) it is assumed that nucleation and crystal growth occur randomly in the liquid phase, far from the solid phase, when the crystallization rate of ettringite is lower than the dissolution rates of Ca 2 + , Al(OH) 4 − , and SO 4 2 − , and harmful expansion does not occur. On the other hand, in the topochemical reaction, 18) it is assumed that crystal growth occurs at the surface of the solid phase when the crystallization rate of ettringite is higher than the dissolution rates of Ca 2 + , Al(OH) 4 − , and SO 4 2 − , and harmful expansion occurs, since the oriented growth exerts pressure on the surroundings.…”

Coupled Mechanical and Chemo-Transport Model for the Simulation of Cementitious Materials Subjected to External Sulfate Attack

“…The reason is that the calculated crystallization pressures are not sufficient to generate strains values comparable to experimental ones with a purely elastic approach. We then suspect that the observed expansions may also result at least partly from other phenomena such as microcracking and creep (see, e.g., [11]). This suggests that an alternative approach has to be developed for more accurately describing the free expansions.…”