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Abstract
The research carried out so far 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 before the tensile strength of the FRP material was achieved. It should be emphasized that al-though the load-bearing capacity of RC members strengthened with passive EB FPR laminates increases, it has a small effect on the serviceability limit state (i.e. cracking moment and deflections). Prestressing the EB FRP was proposed as a method of increasing the utilization of the FRP tensile strength and improving the strengthening efficiency in terms of serviceability limit state.
An experimental research program consisted of three series of RC beams with variations in: the longitudinal steel rein-forcement ratio (4#12, ρs=0.49% and 4#16, ρs=0.87%), concrete strength, preloading level before strengthening, and adhe-sion between the CFRP laminate and concrete as well as the anchorage used.
A practical and unique aspect of this research program focuses on the effect of preloading on the strengthening effi-ciency of RC beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) laminates. In the engineering practice, one can often come across structural elements preloaded before strengthening, in which serviceability limit state was exceeded ( cracking moment or deflections). The influence of preloading of existing RC structures strengthened with prestressed composites has been analyzed very rarely.
Two levels of the beams preloading were considered: the beam self-weight acting alone and the self-weight plus addi-tional external load. The self-weight preloading level corresponded to 25 and 14% of the yield strength of a non-strengthened beam in Series I and III, respectively. The higher preloading level, equal to 76% of the yield strength of a non-strengthened beam was chosen to estimate the efficiency of strengthening the element in which reinforcement was ap-proaching the elastic limit of the beam behaviour.
Experimental tests yielded promising results for the ultimate and serviceability limit states, i.e. ULS and SLS, of the strengthened beams. The strengthening ratio, defined as the ratio of the difference between the ultimate load of a strength-ened and a non-strengthened beam to the ultimate load of the non-strengthened beam, reached values within the range of 0.64–1.19. The authors discuss the influence of all the parameters, i.e ordinary reinforcement ratio, adhesion between the prestressed CFRP laminate and concrete, the level of preloading during strengthening and the level of pretensioning of a composite.
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