Simulation of Expansion in Cement Based Materials Subjected to External Sulfate Attack

Sulfate attack in concrete structures is considered to be among the major durability concerns in civil infrastructure systems. Proper modeling techniques can help us understand the influence of aggressive environments on the concrete performance more readily and accurately. Such an understanding improves the decision making process in every stage of construction and maintenance and will help in better administration of resources. Aspects of cement chemistry, concrete physics, and mechanics are applied to develop a model for predicting sulfate penetration, reaction, damage evolution, and expansion, leading to degradation of cement-based materials exposed to a sulfate solution. The model is refined to address the interaction effects of various parameters using calibration data available from experiments conducted at the National Institutes of Standards and Technology (NIST). Parameters of the model were refined through parametric analysis, consideration of specific boundary conditions, and calibration with experimental data. Introduction Portland cement-based materials subjected to attack from sulfates may suffer from two types of damage: loss of strength of the matrix due to degradation of calcium-silicate-hydrate (C-S-H), and volumetric expansion due to formation of gypsum or ettringite that leads to cracking. Loss of strength has been linked to decalcification of the cement paste hydrates upon sulfate ingress, especially C-S-H, while cracking and expansion is attributed to formation of expansive compounds. Efforts of modeling the durability due to external sulfate attack have received attention only in the past decade [1]. An empirical relationship between ettringite formation and expansion is the basis for many models where the expansive strain is linearly related to the concentration of ettringite [2]. This approach has been incorporated in the 4SIGHT program which predicts the durability of concrete structures [3], as well as in a model that calculates the service life of structures subjected to the ingress of sulfates by sorption [4] or mechanical and transport properties [5]. The general conservation-type equations involve diffusion, convection, chemical reaction and sorption, as the governing phenomena for the transfer of mass through concrete. In the case of sulfates, some authors [6] assume that the process is controlled by reaction rather than diffusion, based on an empirical linear equation that links the depth of deterioration at a given time to the tricalcium silicate (C3A) content and the concentration of magnesium and sulfate in the original solutions. A solution of the diffusion equation with a term for first order chemical reaction has been proposed to determine the sulfate concentration as a function of time and space [7, 8]. Similar