Flexural Behavior of Preloaded RC Slabs Strengthened with Prestressed CFRP Laminates

Many studies performed on reinforced concrete (RC) members strengthened in flexure with externally bonded (EB) fiber-reinforced polymers (FRPs) have indicated quite low strengthening efficiency caused by debonding of the FRP from the concrete surface prior to the capacity of the FRP material being achieved. It should be emphasized that although flexural strengthening with FPR increases the load-bearing capacity of RC members, it has little effect on the serviceability limit state (i.e., cracking moment and deflections). Prestressing the EB FRP has been proposed as a method of increasing utilization of the FRP tensile strength and of improving the efficiency of strengthening in terms of serviceability limit states. An experimental research program consisting of three series of RC slabs with variations in the longitudinal steel reinforcement ratio, concrete strength, preloading level before strengthening, and adhesion between the CFRP laminates and the concrete is described. A practical and unique aspect of the program focuses on an analysis of the effect of preloading on the strengthening efficiency of RC slabs strengthened with prestressed carbon fiber-reinforced polymer (CFRP) laminates. Although the preloading is one of the most important parameters to be accounted for in the design of strengthening existing RC structures, this aspect has been investigated only rarely. Two levels of slabs preloading were considered: the slab self-weight acting alone and the self-weight plus an additional external load. The self-weight preloading level corresponded to 25 and 14% of the yield strength of nonstrengthened slabs in Series I and III, respectively. The higher preloading level, equal to 76% of the yield strength of the nonstrengthened slab, was chosen to approach the elastic limit of the slab behavior. Experimental tests yielded promising results for the ultimate and serviceability limit states of the strengthened slabs. The strengthening ratio, defined as the ratio of the difference between the ultimate load of the strengthened and nonstrengthened slabs to the ultimate load of the nonstrengthened slab, reached values in the range of 0.64-1.19. The influence of the tensile steel reinforcement ratio, adhesion between the prestressed CFRP laminate and concrete, and preloading level on the ultimate load carrying capacity following strengthening is discussed. (C) 2013 American Society of Civil Engineers.