Research On Flexural Behavior Deterioration Of Concrete Beams Hybrid-reinforced With FRP Bars And Steel Bars Under Chloride Environment

Fiber Reinforced Polymer(FRP)bars have good corrosion resistance.As a new kind of steel bar substitute material,it has been widely used in civil engineering.Replacing steel bar with FRP bar can not only reduce the self-weight of concrete structures,but also effectively solve the durability problem of concrete structures caused by steel corrosion.Due to the short application time of FRP bars in engineering,the long-term mechanical properties of FRP bars in service are not fully studied,which limits the application and promotion of FRP bars.In recent years,with the widespread application of FRP bars in engineering,people have paid great attention to the hybrid reinforced concrete members.However,the study of its long-term durability in corrosive environment is still relatively rare.Based on the above background,two kinds of reinforcements,GFRP and BFRP,are selected as the research objects against the background of Chloride environment.The degradation of mechanical properties of two kinds of FRP bars embedded in concrete,the long-term durability and time-varying reliability indices of concrete beams reinforced with hybrid FRP-steel bars(hybrid reinforced concrete beams)were analyzed.The research work and conclusions are shown as follows.1.The mechanical properties degradation of GFRP/BFRP bars in concrete under chloride environment was studied.Two kinds of FRP bars are arranged in concrete beams as main bars to carry out natural aging and accelerated aging tests,in which natural aging test is to place the test beams outdoors for natural corrosion,and accelerated aging test is to carry out chloride solution dry-wet cycling test on the test beams.After a total of 60 dry-wet cycling tests in 366 days,FRP bars were taken out from concrete after crushing.The surface morphology and mechanical properties of the two kinds of FRP bars before and after corrosion were compared and analyzed.The results show that the surface of FRP tendons is rough,glossy and loosely distributed under chloride attack environment.The bond between epoxy resin and composite fibers is severely damaged.Tensile,bending and shear strength of FRP bars decreased under natural aging and accelerated aging conditions,while the influence of corrosion environment on elastic modulus of FRP tendons was not obvious.2.The flexural behavior of hybrid reinforced concrete beams was studied.Six concrete beams reinforced with hybrid GFRP-steel bars,two hybrid BFRP/steel reinforced concrete beams and three steel reinforced concrete beams were designed and fabricated.The effects of the types of FRP bars,reinforcement ratio and the area ratio of FRP bars to steel bars on crack distribution,crack width and deflection development of hybrid reinforced concrete beams were analyzed.The results show that the cross section of hybrid reinforced concrete beam still meets the plane section assumption.The mid-span deflection of concrete beams hybrid-reinforced with FRP bars and steel bars is about 1.15-1.61 times of steel reinforced concrete beams.The ductility of concrete beams hybrid reinforced with FRP bars and steel bars can meet the requirement if the area ratio of FRP bars to steel bars A_f/A_s is controlled.The maximum crack width equation of hybrid reinforced concrete beam with single-layer reinforcement is given.The mean value of the ratio of theoretical value to experimental value was 1.05,and the coefficient of variation was 0.165.The deflection calculation of hybrid reinforced concrete beam is suggested to use the formula in CSA-S806-12 code.3.The durability of concrete beams reinforced with a combination of GFRP/BFRP bars and steel bars under chloride environment was studied.With the diameter and reinforcement ratio of FRP bars as variables,the hybrid reinforced concrete beams under crack-free and multi-crack conditions were fabricated.A total of 60 dry-wet cyclic tests of chloride salt corrosion were carried out in 366 days.The tests show that the surface of stirrups and tensioned steel bars is corroded after 366 days of chloride salt corrosion,the surface color of FRP bars changes,and the mechanical properties of tensioned steel bars and FRP bars decrease in varying degrees.The ultimate flexural capacity of steel reinforced concrete beams decreases by 7-16%,while that of hybrid reinforced concrete beams decreases by only 4-8%.The failure mode of partially hybrid reinforced concrete beams has changed,but the cross section still meets the plane section assumption.After corrosion,the flexural moment-deflection curve of the test beam is basically the same as that of the non-corroded beam.The flexural moment-deflection curve of the tested beams after corrosion were the same as that of the non-corroded beams,but the deflection decreases under the same load.The maximum crack width of steel reinforced concrete beams after corrosion decreases,while the maximum crack width of hybrid reinforced concrete beams before and after corrosion does not change obviously.The existing ductility calculation model can not predict the ductility trend of hybrid reinforced concrete beams under long-term chloride corrosion.4.The time-dependent reliability index of hybrid reinforced with GFRP bars and steel bars under chloride environment was carried out.Considering the time-varying effects of concrete strength,reinforcement strength and FRP reinforcement strength on the flexural capacity of hybrid reinforced concrete beams,the time-varying resistance calculation models of hybrid reinforced concrete beams are given.The time-varying reliability index of flexural capacity of concrete beams reinforced with GFRP and steel bars under chloride environment is calculated.The result shows that the flexural capacity and reliability index of steel reinforced concrete beams decrease faster than that of hybrid reinforced concrete beams.