Computational models using the ®nite element method for nonlinear transient analysis of reinforced concrete (RC) two-way slabs subjected to blast loading are presented. Both as-built and retro®tted slabs with carbon ®ber reinforced polymer (CFRP) composite strips are analyzed. The models are used to investigate different parameters including (a) loading duration, and (b) effect of CFRP retro®t on damage accumulation. In this study, damage is globally quanti®ed by the amount of reduction of the ®rst two vibrational frequencies of the slabs. Local representation of damage in terms of reinforcing steel strains is also discussed. The computational models for both the as-built and the retro®tted slabs are veri®ed using experimental results. In these experiments, a slowly increasing uniform pressure is applied to the bottom surface of large-scale RC slab specimens using high-pressure water bag. Experimental results showed that an increase up to 200% in the load carrying capacity is achieved when using the CFRP composite retro®t system. Transient nonlinear analysis results proved the ef®ciency of the CFRP composite retro®t in improving the slab behavior under blast loading for different loading durations, i.e. for small, medium, and large charge weights at the same applied maximum pressure. In particular, less than 50% reduction of the fundamental frequency due to concrete damage is obtained for the retro®tted slab compared to more than 85% reduction for the as-built slab. Moreover, the maximum displacement is reduced by 40±70% with the CFRP retro®t compared to the as-built slab. As for reinforcing steel strains, the application of CFRP retro®t signi®cantly limited the spread of yielding in time and space. The improved slab behavior with CFRP is best when retro®tting is applied to both sides of the slab.