Experimental and modelling study of the alkali-silica-reaction in concrete

The alkali-silica reaction (ASR) is a durability issue of concrete. The amorphous silica of aggregates reacts with the alkalies present in the cement paste pore solution to form a hydrophilic gel which swells in the presence of moisture. Many mass concrete structures are affected and understanding of the reaction and its development is crucial, notably for dam owners and managers. Although some parameters affecting the reaction are well understood, such as temperature, others which depend on the concrete mix design, such as aggregate sizes and particle size distribution (PSD) and external parameters such as the applied load have an effect on the development of the reaction which is not as well understood. To advance the understanding of ASR an experimental programme was put into place to explore some of these factors. In parallel, a modelling platform was designed and implemented to allow the simulation of the reaction at the material microstructure level. The expansion of affected mortars and concretes had been linked to the damage state of the aggregates by Ben Haha. We could model this effect and reproduce the effect of changing the aggregate sizes. Simple kinetics were implemented in the model with two factors were required to account for changes in the cure conditions and sample sizes. The expansion due to the reaction has been reported to be anisotropic in the literature with respect to the direction of casting. We could demonstrate this effect in two independent set of experiments. The overall shape of the expansion curve was found to be related to the fracture of the aggregates and the interactions between them rather than changes in the rate of the chemical reaction. The effect of restraining stress was found to more complex than previously reported in the literature, as it notably affects the direction of propagation of microcracks in the aggregates and paste. This leads to an acceleration of the damage and expansion for loads above about 5MPa threshold.