A Chemoplastic Model for Alkali-Silica Expansion: Effect of Stress-Induced Anisotropy

The alkali-silica reaction (ASR) is an expansive chemical reaction that occurs between certain reactive aggregates and the alkaline pore solution surrounding them. In the presence of water, the volume of ASR products greatly exceeds that of the reactants, resulting in the generation of internal pressures that can cause substantial damage to civil engineering structures. This thesis aims to develop a model that employs principles of chemistry and mechanics to accurately predict the severity of ASR expansion with regard to material parameters and other relevant variables. In this first chapter, the mechanisms of ASR expansion are first addressed and elucidated. Then, in Chapter 2, a literature review of past modeling attempts is presented. Finally, in Chapter 3, a new chemoplastic model of these mechanisms is formulated. Starting from energy considerations, this model includes a description of ASR kinetics, that is, an account of how the chemical reaction proceeds through time. Additionally, the model provides a link between reaction kinetics and material mechanics that finally can be extended to a structural level. This aspect of the modeling requires an understanding of the interaction between internal and external stresses that drive the expansive strain, by which the magnitude of ASR is measured. The chemoplastic model also incorporates another phenomenon observable in laboratory tests but not addressed in past literature, that of stress-induced anisotropy, which essentially states that the uniaxial restraint of ASR gel growth induces directionally preferential expansion of the gel. In other words, if the gel is restrained in one direction, it expands primarily in the stress-free directions. Based on a chemomechanics approach, using the chemoplastic model and available experimental data, it is shown that the micromechanisms that lead to ASR swelling are the same as the ones activated during concrete fracture under macroscopic load application. This combined experimental-theoretical identification confirms both the model basis (chemoplasticity) and the capacity of the model to actually predict deleterious effects induced by the alkali-silica reaction in concrete structures. Finally, by way of conclusion, the model is calibrated with respect to test data obtained in the LCPC ASR test campaign by Larive, and the calibration is subsequently verified by a comparison of predicted and experimentally measured expansion curves.