This study presents the efficiency of adding lower concrete layer reinforced by different materials to increase the flexural strength for two-way R.C slabs. Eleven half-scale two-way R.C slab specimens were prepared and tested under four point bending. One of these slabs was unstrengthened and considered as a control specimen. The other specimens were strengthened by using different lower concrete layers reinforced mainly by fiber reinforced polymer (FRP) bars. The parameters of this study included the material type (reinforcement steel, glass fiber and carbon fiber), the thickness of strengthening layer (30 & 50 mm), spacing between strengthening layer reinforcement bars (100 & 200 mm), cross sectional area of this reinforcement (A & 2A) and the type of the strengthening reinforcement (FRP bars & FRP strips). The experimental results included cracking load, ultimate load, load-deflection relationships, relative ductility, flexural stiffness. The experimental results showed an improvement in the flexural behavior of the strengthened specimens compared to control specimen. The flexural strength of the different strengthened specimens increased by 37% to 112% compared to the control specimen.