Composite materials are widely used in retrofitting bridge and building construction in order to improve the load-carrying capacity of understrength or deficient structural members. This paper presents experimental research conducted on full-scale one-way RC slabs strengthened using three different composites. The flexure performance of a new innovative composite, fiber-reinforced cementitious matrix (FRCM), is evaluated and compared with that of conventional fiber-reinforced polymers for some specimens under laboratory condition and for other specimens exposed to environmental conditioning before testing. The test results illustrate the impact of the composite materials on enhancing the flexural strength of RC slabs and their durability performance.