Experimental And Theoretical Study Of RC Beams Strengthened With CFRP Under Freeze-thaw Cycles And Sustained Loads

Offshore structures are usually exposed to aggressive environment,so a great number of them are faced with the problem of reinforcement.Recently,CFRP has been widely used in the field of strengthening RC structures.However,for offshore structures in the north,except burdening load,RC beams strengthened with CFRP are also affected by freeze-thaw cycles environment.That will lead to a decline of the durability of structures,including the durability of concrete and the durability of interfacial bond behavior between CFRP and high strength concrete.Therefore,it should be an important project for scholars to work on.The reality is that the method used in previous research on the durability of structures reinforced with FRP was mostly about single or double-lab shear tests.But there are no study can prove it at present,that those conclusions obtained through shear tests are also useful for beams.Based on the above background,the effects of freeze-thaw cycles and sustained loads on durability of high strength concrete beams strengthened with CFRP were studied supported by National Natural Science Foundation of China.The main contents are as follows: twelve RC beams reinforced with CFRP subjected to freeze-thaw cycles environment under sustained loads were studied with four-point bending test.The effects of freeze-thaw cycles and sustained loads were elaborated on the load-deflection response,flexural capacity and type of failure.The suitable strength model of CFRP-concrete interface is used to calculate ultimate flexural capacity of beams’ mid-span section.After comparing with the results of experiment and theory,in this paper the model has been modified to propose a valid computational formula.Experimental results show that,the ductility of strengthened beams decline with the increase of sustained loads acting on beams,when those beams were exposed to the same number of freeze-thaw cycles.Fewer freeze-thaw cycle has little effect on the ultimate flexural capacity of those strengthened beams.However,the ultimate flexural capacity declined obviously after 200 freeze-thaw cycles.The coupled action of sustained loads and freeze-thaw cycles have significant influence on overall performance of beams strengthened with CFRP,mainly due to degradation of bond between CFRP and concrete.In this paper,an analytical method is proposed to calculate ultimate flexural capacity of beams’ mid-span section,for beams which failed by separation of the plate from concrete that caused by the flexural shear cracks in middle section.And for strengthened beams under freeze-thaw cycles and sustained loads,the reduction coefficient of bond strength of CFRP-concrete interface is presented.