Experimental Study And Analysis Of RC T-beams Strengthened With Prestressed CFRP Sheets

Strengthening RC (Reinforced Concrete) beams with CFRP (Carbon FiberReinforced Polymer) pasted on the tensile faces of the beams to improve theflexural capacities has been used widely in recent years. However, severaldrawbacks have been found for this strengthening method, including: thedefection and the width of flexural crack can not be effectively reduced withthis method, the CFRP sheets can not bearing the initial load in the beam torecover the initial deflection, and debonding always happens in theFRP-to-concrete interface, so that the strength of CFRP can not be fully used.On the contrary, if pre-stress is applied to the CFRP sheets, above problemscan be overcome.In this study, firstly, based on the real practices of strengthening, specialequipment for applying pre-stress to the CFRP sheets has been improved,which comes to be a mature product. And construction techniques forinstalling pre-stress on CFRP sheets with proposed equipment have beendeveloped. Then, 7 full size T shape RC beams have been tested thatstrengthened with pre-stressed CFRP sheets to study their flexural behaviors.The parameters studied in these specimens include: the quality of CFRPsheets, the pre-stress in CFRP sheets, the reinforcement ratio, the loadinghistory and the shapes of FRP U-jacketing. The test results show that the highstrength of CFRP can be fully used in pre-stressed CFRP strengthened beams.The failure modes of the specimens are changed, and the flexural strengths ofthe specimens are increased obviously. Furthermore, compared to thespecimens strengthened with normal (i.e. no pre-stress) CFRP sheets, thepre-stressed CFRP sheets can retard the cracking, reduce the deflection andthe crack width, and increase the yield strength, which results in thesatisfaction of service limit state.Based on the experimental research, 4 different methods are adopted tocompute the ultimate flexural capacity of RC beams strengthened withpre-stressed CFRP sheets. And by comparison with domestic test data, designformulas are proposed which can be used for engineering application.In order to study the contribution of FRP U-jacketing to the bond strength ofinterface between concrete and flexural strengthening CFRP sheets, RCbeams flexural strengthened with normal CFRP sheets in existing literaturesare simulated with finite element (FE) method. Different interfaceconstitutive models and model parameters are discussed to find a proper onefor the FRP-to-concrete interfaces with U-jacketing. The FE models illustratethe influence of U-jacketing to FRP-to-concrete the interfacial behavior andthe contribution of U-jacketing to the flexural strength. Then, anFRP-to-concrete interfacial constitutive model with U-jacketing has beenproposed and the specimens tested in this study have been simulated with thismodel. The numerical results show that with sufficient U-jacketing bond, thedebonding failure can be avoided for pre-stressed CFRP sheets. And thestrength of CFRP sheets can be fully used.