Flexural Behavior of One-way Concrete Slabs Reinforced with Combined Steel-CFRP bars

The use of steel- and carbon-fiber-reinforced polymer (CFRP) bars as combined reinforcement within concrete members promises strength enhancement and ductility improvement. To verify this and to assist the design of concrete members with combined reinforcement, experimental and theoretical investigations were conducted on 17 one-way concrete slabs, including six control slabs reinforced exclusively with steel or CFRP bars. Flexural strength, failure modes, cracking loads, load-deflection behavior, moment-curvature relationship, and ductility evaluation were investigated. Amount of reinforcement and combination ratio of steel to CFRP were the key parameters. Test results indicated the effectiveness of combined reinforcement in terms of increasing the flexural strength of steel RC slabs from one hand and ductility improvement of CFRP RC slabs from the other hand. Theoretical analysis successfully predicted the ultimate flexural capacity and modes of failure. Two terms were defined: effective reinforcement ratio and critical reinforcement ratio, which can be used to effectively select a favorable mode of failure when designing concrete slabs reinforced with combined reinforcement. It was found that increasing ratio of steel to CFRP in combined reinforced slabs resulted in stiffer load-deflection and moment-curvature curves. Ductility evaluation showed the critical role of deformability factor in comparison with conventional ductility index and curvature ductility in determining the ductility of combined reinforced slabs. It was found that a combination ratio of steel to CFRP amount in the range of 2.54-3.38 can achieve a satisfactory ductility index and curvature ductility, as well as a low deformability factor in under-reinforced concrete slabs with combined reinforcement.