Modelling of Alkali-Silica Reaction under Multi-Axial Load

Alkali-Silica Reaction (ASR) is a deleterious expansion phenomenon which affects the long-term behaviour of concrete. Its origin is a chemical reaction between amorphous silica present in the aggregates and alkali ions from the concrete pore solution. The silica gel produced is highly hydrophilic and swells by absorbing surrounding water. The induced pressure causes a macroscopic expansion and internal damage in the material microstructure. In ASR-affected structures the overall expansion depends notably on the service load of the structure. Previous studies have shown that application of an uni-axial load reduces or eliminates the expansion in the direction of the load, but can increase the expansion in the lateral directions in a non-linear way. The influence of multi-axial stress states have rarely been studied, and experimental data are still needed. In this study, an experimental apparatus based on tri-axial cells is developed for the study of ASR-reactive concrete under multi-axial loads. The lateral and vertical deformations are measured in-situ with fibre-optic sensors cast in the concrete. The setup is designed to withstand the aggressive experimental conditions in terms of temperature, pressure, and alkali concentration for an extended period of time. Numerical models are required to characterise and understand the different components of the measured strains. An existing microstructural model is extended to account simultaneously for the damage process and the stress relaxation which occurs in the visco-elastic cement paste. A numerical method based on finite elements in space and time was developed and implemented to represent the continuous growth of the ASR products in time. This method is complemented with a novel, thermodynamic compliant, continuum damage algorithm which can be applied concurrently with the viscous relaxation of the material This model is applied to the analysis of the stress distribution in the microstructure of ASR-affected concrete. It is shown that both the internal damage and the stress relaxation must be accounted for in order to model the influence of applied stress on ASR. The model allows a reinterpretation of the accelerated tests commonly used in ASR testing and should allow these test results to be better related to field performance.