| Carbon fiber reinforced polymer（CFRP）bar is expected to be a substitute for steel bar in civil engineering structures under harsh service conditions due to its light weight,high strength,excellent corrosion resistance and fatigue resistance.In order to maximize the high tensile strength of CFRP,it should be used as cable s（including prestressed external cables,cables of cable-stayed bridges,suspension bridges and suspenders of through arch bridges）.The bonding anchorage system with reactive powder concrete（RPC）as bonding medium is more suitable for anisotropic CFRP cables,and has been gradually applied in civil engineering.However,CFRP cable and its bonded anchorage system may be threatened by fire during construction and operation,so the fire resistance performance of CFRP Cable and its bonded anchorage system needs to be studied.Based on the National Natural Science Foundation of China（NSFC）,"Research on fire resistance performance of bond-type anchorage system for CFRP prestressed cable grouted by reactive powder concrete"（No.51908206）,this paper studies the fire resistance of CFRP cables and mechanical properties of CFRP bar and anchorage system after high temperature,in order to provide a theoretical basis for the compilation of fire resistance design code for CFRP cable structure.The main research contents are as follows:（1）In order to obtain the tensile constitutive model and fire resistance limit of CFRP cable under fire,tensile tests were carried out on 10 groups of CFRP cables under constant load constant temperature and constant load temperature rise constant laod.The main research parameters include temperature（25℃,100℃,210℃,300℃,400℃ and 600℃）and initial tension ratio（0.2,0.4,0.6 and 0.7）.The results show that the tensile strength and elastic modulus of the CFRP bars decrease by 3.9% and18.4%,respectively,compared with the normal temperature specimens,and the axial tensile properties of the CFRP bars deteriorate slightly.When the temperature reaches300℃,due to the softening of resin matrix,the tensile strength and elastic modulus of the reinforcement are only 55.6% and 36.0% of the normal temperature specimens.When the temperature is 600℃,the CFRP bars almost lose their static properties due to the decomposition of resin matrix and the significant attenuation of the mechanical properties of carbon fiber.For the specimens with constant load and temperature rise,the temperature and fire resistance of CFRP cables decrease with the increase of initial tension ratio;when the initial tension ratio R increases from 0.2 to 0.7,the temperature of CFRP cables decreases from 761 ℃ to 428℃,which decreases by43.8%;the fire resistance decreases from 17.35 min to 5.49 min,which decreases by68.4%;the temperature of four groups of specimens is more than 400℃.Based on the test results,the degradation mechanism of axial tensile prop erties of CFRP cables under high temperature and the influence of temperature load path on the axial tensile properties of CFRP cables are clarified.（2）In order to study the creep and relaxation behavior of CFRP cables at high temperature,48 groups of CFRP cables were tested under the coupled action of high temperature and thermal stress.The main parameters inclu de temperature（25℃,210℃,300℃ and 500℃）and initial tension ratio（0.2,0.4 and 0.6）.The stress-strain time history curve and residual strength after creep and relaxation of CFRP Cable at high temperature were measured.The results show that for the cre ep specimen,the creep rate of CFRP tendon is large in a short time（30min）when the temperature is ≤300℃,and then the creep growth is relatively gentle;when the temperature is 500℃,the creep rate increases exponentially when the strain reaches a certa in value until the specimen breaks The results show that the creep strain of the specimen increases with the increase of temperature and initial tension ratio;the creep value of specimens under constant load temperature rise constant load is greater than that under constant load constant temperature creep;compared with the CFRP reinforcement without creep at high temperature,the ultimate strength of the reinforcement increases after creep at high temperature.For the relaxed specimens,the variation of s tress loss with time is similar to that of creep test,and the stress loss increases with the increase of temperature and initial tension ratio.According to the test results,the influence of temperature load path on the residual bearing capacity,creep strain and relaxation stress of CFRP cables is clarified,and the prediction formulas of creep strain and relaxation stress of CFRP cables under different temperatures are proposed.（3）In order to study the axial tensile properties of CFRP bars after high temperature,the axial tensile tests of 12 specimens were completed with the treatment temperatures（25℃,100℃,200℃ and 300℃）as test parameters.The results show that the static properties of the steel bars do not change significantly when the treatment temperature is 100℃.The results show that the tensile strength,elastic modulus and ultimate tensile strain of the steel bars are decreased by 6.4%,8.2% and3.8% and 16.6%,18.3% and 8.3% respectively compared with those of the normal temperature specimens.Based on the test results,a practical formula for analyzing the axial tensile performance of CFRP bars after high temperature was established.（4）In order to study the mechanical properties of CFRP bar bond anchorage system after high temperature,the anchor age performance test of 36 specimens was completed with the treatment temperature（25℃,100℃,200℃ and 300℃）and the bond length（5d,10 d and 15d）as the test parameters.The results show that the load slip curves of all specimens can be divided into linear section,nonlinear rising section,falling section and residual section;the bond strength of the specimens decreases with the increase of elevated temperature and bond length.Compared with the specimen at room temperature,the bond strengths of the specimens exposed to the elevated temperature of 200℃ and 300℃ decrease by 31.5%-36.3% and 44.2%-47.4%,respectively,while the bonding length is fixed.The calculation formulas with high accuracy for determining the bond strength and critical anchorage length of the bond-type anchorage system after elevated temperature exposure were developed. |
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