The macroscopic effects of ASR are linked to the damage state at the microstructure level. In this paper we used a combination of experiments and modelling to study the effect of aggregate size on the manifestation of ASR. There are two main ways in which the size of the aggregates can affect damage evolution: the propagation of cracks in aggregates of different sizes and the interactions between expanding and non-expanding aggregates in a densely packed microstructure. To assess these effects, concretes were cast with the same PSD but each with a different size class of reactive aggregates. Numerical simulations were used to model the mechanical interactions in single aggregates and in complete microstructures at the mesoscopic level. From the simulations a mechanism is proposed to explain the experimental observations. This suggests that: the expansion rate of ASR affected concrete depends on the fracture behaviour of individual aggregates in the early stage, and on the fracture behaviour of the paste in the later stages. (C) 2012 Elsevier Ltd. All rights reserved.