Investigation On Long-term Performance Degradation Of High-strength Concrete Beams With FRP Bars Under Accelerated Erosion

Fiber Reinforced Polymer(FRP)bars,as a new type of environment-friendly composite material,have the advantages of light weight,high strength,fatigue resistance,erosion resistance and electromagnetic insulation.It has gradually become a potential reinforcement in construction engineering,water conservancy and traffic engineering.However,the application time of FRP bars in practical engineering structures is relatively short,and the cognition of performance degradation and degradation mechanism of FRP bars in real environment is not comprehensive enough.These factors have become the core problems hindering its mass application in engineering.In addition,to take further advantage of the high tensile strength of FRP bars,combining them with high-strength concrete to form high-strength concrete members with FRP bars can effectively improve the load-bearing capacity of the members while improving the durability of the members.Therefore,it is of practical significance and engineering value to investigate the long-term performance degradation of concrete beams with FRP bars.Above on,based on national natural science foundation(NO.51578267and NO.51878319)and the existing academic achievements,this paper carried out the durability tests of BFRP and GFRP bars placed in concrete and the long-term performance tests of high-strength concrete beams with FRP(BFRP and GFRP)bars,respectively.The degradation of long-term performance of FRP bars and high-strength concrete beams with FRP bars before and after conditioned are compared and analyzed,and the time-varying reliability analysis of high-strength concrete beams with FRP bars after accelerated erosion was carried out based on the test data.The specific contents and research results are as follows:1.From the perspective of the material level,experimental studies have been conducted on the durability of BFRP and GFRP bars in concrete under different environments.The FRP bars were embedded in concrete beams and then placed in the atmospheric environment and accelerated erosion environment(external electrodes accelerate Cl~-erosion),respectively.The period of two erosion is one year.The effects of environmental categories on the mechanical properties of FRP bars in concrete were investigated.The test results show that the erosion of FRP bars was a process from the outside to the inside;After atmospheric erosion and accelerated erosion,the bending,shear and tensile properties of the two types of bars have been degraded to different degrees;For BFRP bars in concrete,the bending,shear and tensile strength decreased by 13.1%,12.4%,12.5%after atmospheric erosion and 22.8%,21.6%,22.3%after accelerated erosion;For GFRP bars in concrete,the bending,shear and tensile strength decreased by 8.7%,7.9%,7.7%after atmospheric erosion and 12.7%,11.1%,9.9%after accelerated erosion;The degradation degree of GFRP bars in concrete after two kinds of erosion environment was lower than that of BFRP bars;Arrhenius model and the method provided by Fib bulletin(40)were used to predict the long-term performance of GFRP bars in concrete under atmospheric environment and the results show that the retention of tensile strength after 100 years of service was 51.0%.2.From the perspective of construction level,the experimental study was conducted on the long-term performance of high-strength concrete beams with BFRP and GFRP bars under accelerated erosion(the acceleration method and erosion cycle are consistent with the material level),and the effects of accelerated erosion,types and forms of reinforcement,and concrete strength grades(CF50 and C30)were analyzed.The test results show that for accelerated erosion,the concrete compressive strength of the test beam increased by 5%-10%,the cracking bending moment increased by 6-10%(the concrete strength is the main factor affecting the cracking bending moment)and the mid-span deflection did not exceed the specification limit value(under normal use load)after accelerated erosion;For types and forms of reinforcement,the degradation rate of the ultimate bending moment of the test beams from large to small after accelerated erosion is as follows:test beams with steel bars(HRB400)>test beams with BFRP bars>test beams with GFRP bars(12mm)>test GFRP beams>test beams with GFRP bars(16mm)and the double-layer reinforced beam also has the above change rule.For the level of concrete strength,under the action of accelerated erosion,the appropriate increase of the concrete strength level can make the tensile properties of the FRP bars in concrete beams fully utilized,so that its bending bearing capacity is improved,thereby the long-term performance of the concrete beam being improved;After accelerated erosion,the average retention rate of maximum deflection of CF50 test beams after erosion(92.7%)was greater than that of C30 test beams(88.0%)and the maximum crack width of the test beam decreased with the increase of concrete strength grade.3.From the perspective of numerical calculation,the reliability of the time-varying flexural capacity of high-strength concrete beams after accelerated erosion was evaluated.The results show that the time-varying reliability degradation rate of reinforced high-strength concrete beams is significantly faster than that of high-strength concrete beams with GFRP bars;When the reliability index is degraded to 3.2,reinforced high-strength concrete beams need 27 years,while high-strength concrete beams with GFRP bars are 47 years;For the concrete beam,when the concrete strength grade is increased from C30 to CF50,the time varying reliability index also increases,and this phenomenon is more obvious on the concrete beam with GFRP bars.