Flexural Behavior Of Fiber Reinforced Polymer (FRP) Bars Reinforced Concrete Beams Under Static And Fatigue Loads After Elevated Temperature Exposure

In this study,in order to investigate the flexural behavior of fiber reinforced polymer(FRP)bars reinforced concrete beam under static and fatigue loads after high temperature exposure,the temperature field of internal section of concrete beams under high temperatures was measured,the mechanical properties of concrete and FRP bars,the flexural properties of concrete beam reinforced with FRP bars under the static and fatigue loads after different high temperatures were also experimentally investigated.Through above mentioned tests,the variation of temperature field of concrete beam section with holding time under different high temperature and heating rate conditions were discussed,and the deterioration trend and mechanism of mechanical properties of concrete after different high temperatures were analyzed.Further,the change trend and calculation methods of maximum crack width,stiffness and residual capacity of concrete beams reinforced with different kinds of FRP bars after high temperature were also studied.Finally,the influence of fatigue load on the flexural behavior of FRP bars reinforced concrete beams after elevated temperature was analyzed,and the calculation model and method of fatigue life of concrete beams reinforced with FRP bars were proposed.The main research contents and conclusions of this paper are summarized as follows:(1)Through conducting the heating test of 14 concrete beams in the furnace,the effect of high temperature,heating rate,heating conduction distance and heating mode on temperature distribution in the section of concrete beams were investigated.The test results show that the change trend of the temperature field of concrete beams with the heating treatment time in different elevated temperature conditions are similar.Meanwhile,the heating rate has certain influence on the temperature field in the section of concrete beam under different heating time.As the increase of the holding time,the effect of heating rate on the temperature in the section of concrete beams decreases gradually.In all kinds of temperature conditions,for the same holding time,the temperature of each measuring point of concrete beams is different from each other,which shows that the temperature distribution is not uniform and there is an obvious temperature gradient along the section of concrete beams.(2)Through testing the cubic compressive strength,axial compressive strength and elastic modulus of 126 concrete specimens after high temperature exposure,the change trend and deterioration mechanism of mechanical properties of high strength concrete in different temperature gradients(i.e.,elevated temperature and temperature holding time)were investigated.The test results show that the deterioration degree of mechanical properties of concrete after high temperature is determined by both elevated temperature and holding temperature time;the higher the elevated temperature is,the greater the temperature gradient in concrete is,and the more different the material properties of different temperature units is,the more significant decreased the mechanical properties of concrete are.Meanwhile,based on combination of the test results of the mechanical properties of concrete after high temperatures,the calculation models for mechanical properties of high strength concrete after different exposure temperatures were proposed.(3)Through conducting the tensile test of 105 glass FRP(GFRP)bars after high temperature exposure,the effect of high temperature and holding time on the static mechanical and fatigue properties of FRP bars under were investigated.The test results show that the tensile strength and elastic modulus of GFRP bars decrease with the increase of elevated temperature and the extension of holding time,and the tensile strength and elastic modulus of GFRP bars decrease at most by 28.0%and 18.3%respectively after the exposure temperatures(25-350~oC);the cyclic load accelerates the degradation of GFRP bars after high temperature,and the coupling of elevated temperature and holding time enhance the degradation effect of cyclic load on GFRP bars;the tensile modulus of GFRP bars after also decrease obviously after the coupling of cyclic load and elevated temperature,which decreases at most by 17.6%compared with compared with that at room temperature.In addition,according to the present test results and previous literatures,the prediction model of tensile strength and elastic modulus of FRP bars after high temperature were proposed.(4)Through conducting the static flexural test of 7 FRP bar reinforced concrete beams after high temperature exposure,the effects of high temperature and types of FRP bars on the cracking load,crack development,deflection and ultimate capacity and failure of concrete beams were investigated.The test results show that the maximum crack width,deflection and ultimate bearing capacity of FRP concrete beams decrease obviously with the increase of high temperature and the extension of holding time;after the exposure of 400~oC,compared with the behavior of concrete beam at room temperature,the maximum crack width of GFRP bars and CFRP bars reinforced concrete beams increase by 42.9%and 41.7%,respectively,the deflection increase by 103.6%%and 22.0%,and the ultimate bearing capacity decrease by11.9%and 3.9%.Meanwhile,the calculation models for the maximum crack width,deflection and residual ultimate capacity of FRP bars reinforced concrete beams after exposure of high temperature were proposed.(5)Through testing the fatigue behavior of 12 FRP bars reinforced concrete beams after high temperature exposure,the effect of high temperature,holding time,and types of FRP bars on crack development,failure mode,maximum crack width,deflection and fatigue life of concrete beams were investigated.The test results show that the maximum crack width of concrete beams increases gradually with the increase of cycling number during fatigue test,and the change trends of maximum crack width are affected by the high temperature,holding time,stress ratio and so on.The midspan deflection of FRP bar reinforced concrete beams increases with the increase of cycling number,and the deflection increases obviously within 20,000cycles,while increases slowly and tends to be stable with the increase of cyclic loads after more than 20,000 cycles.With the increase of high temperature,the holding time and the reduction of stress ratio,the fatigue life of FRP reinforced concrete beams is obviously reduced,and the fatigue life of CFRP bars reinforced concrete beams is obviously better than that of GFRP bars reinforced concrete beams under the same high temperature conditions and loading characteristics.The stiffness attenuation coefficient of the beam under fatigue loading was introduced,and the calculation method of the stiffness of FRP reinforced concrete beam under room temperature and high temperature were proposed.Further,the influence factors of fatigue life of FRP bar reinforced concrete beams was analyzed.Finally,based on the theory of damage mechanics and the degradation of stiffness,a fatigue life prediction model of FRP bars reinforced concrete beams considering high temperature and the cycle times of fatigue load was established.