Alkali-Silica Reaction (ASR) affects concrete and decreases its mechanical characteristics. However, for some reinforced concrete structures, the global mechanical behaviour can appear to be improved by ASR: for similar reinforcement, the first flexural cracking of reactive beams usually occurs for higher loading than in non-reactive beams. The flexural failures of two reinforced beams, one reactive and one non-reactive, are numerically simulated here in order to discuss the origins of the delay in cracking observed for the reactive beam. The poromechanical model used considers the swelling anisotropy, and is able to differentiate ASR diffuse cracking from structural macrocracks and the coupling of both crack types with creep. According to the model results, the cracking delay is due to the chemical prestress in the concrete induced by the ASR swelling being restrained in the direction of the reinforcements; the concrete has to be decompressed before cracking. The mechanical modelling presented in the paper is able to reproduce the differences between the reactive beam and the reference one. The cracking delay obtained for the ASR beam in the flexural test seems interesting from the mechanical point of view. However, this performance could be counterbalanced by durability problems due to ASR diffuse cracking that is induced parallel to the reinforcements. In fact, these cracks, also evaluated by the present modelling, are privileged paths for the ingress of deleterious agents such as carbonates or chlorides.