Experimental study of hybrid strengthening technique using carbon fiber laminates and steel plates for reinforced concrete slabs

External bonding (EB) technique has been used widely to strengthen reinforced concrete (RC) structures in civil engineering over several decades. Steel and fiber reinforced polymers (FRP) are the two most common materials used for strengthening RC structures. Although each material has its advantages for EB application, they also have their own weaknesses. Steel plates are heavy and poor in corrosion resistance, and the thickness of steel plates is also limited due to lack of shape flexibility for strengthening RC structures. The EB-FRP technique is restricted for broader applications because the bond interface is the weakest link in such retrofitted or strengthened concrete members, resulting in sudden and brittle debonding failure, and the strength of FRP has not been fully utilized. To overcome these weaknesses, an innovative strengthening method combining FRP and steel materials was explored in this research. Carbon fiber laminates (CFL) and thin steel plates (SP) were combined in different ways to determine the most effective hybrid strengthening configuration. A total of nineteen RC slabs strengthened by different configurations were tested under four-point bending. Failure modes, load versus deflection curves, strains of internal reinforcement bars, CFL and SP, and stiffness of the strengthened specimens were analyzed. The test results proved that the CFL-SP hybrid strengthening technique was effective. The average maximum increases in flexural capacity and stiffness were 204.2% and 91% over the control slab, respectively. In addition, the cracking and steel yielding loads of slabs strengthened with CFL-SP composite material were higher than those of slabs strengthened by FRP or steel plate only. By combining with steel plate, FRP material was used more efficiently and the stiffness of the strengthened RC slabs was improved. The overlap-type hybrid configuration of bonding CFL as inner plates while steel as outer plates was proved as the most effective hybrid strengthening method within all configurations tested in this research. (C) 2019 Elsevier Ltd. All rights reserved.