The incorporation of fibres in Reinforced Concrete (RC) beams controls the width and evolution of cracks leading to positive effects on the durability of the element. The study of damage processes in concrete and their effects on the residual properties represents a key point related to the service life of RC structures. The contribution of fibres on the bending behaviour of sound and damaged RC beams was investigated. In order to use alkali silica reaction as a damaging tool, RC beams with and without fibres and reactive aggregates were subjected to service loading conditions during eight months in an environment with high humidity. The evolution of deformations and the distribution and propagation of cracks were recorded. As reference, similar RC beams without reactive aggregates were also evaluated. After the treatment, all RC beams were loaded up to failure. The free expansion, the compressive strength and the bending residual capacity of plain and fibre concrete were measured on companion specimens for material characterization. The effect of both fibres and alkali silica reaction on bearing capacity and ductility of RC beams were analysed. Results showed that alkali silica reaction damage provokes a significant reduction of RC beam ductility, while the flexural strength is preserved.