Study On Durability Experimental Method Of RC Beams Strengthened With FRP In Hygrothermal Environments

Durability of reinforced concrete (RC) structures strengthened with fiber reinforcedpolymer (FRP) is the forefront topic in field of civil engineering. In this paper, existing RCbridges in subtropical environments were taken as the application background. Using CarbonFiber Laminates (CFL) developed by this project team, RC beams strengthened with CFLwere constructed and were taken as the research objects. Considering the hot and humidenvironment in South China and coastal areas and actual vehicle loads on highway bridges inoperation, durability experimental method of RC beams strengthened with CFL underhygrothermal environment and vehicle load coupling was discussed. Experimental systemwhich achieved the above durability experiment method was integrated and developed. Theeffectiveness and feasibility of the experiment method were later verified by differentexperiments.1. Against the environment and load coupling problem which is the bottlenecks ofstructure durability experiments, durability acceleration experiment method of bridgestructure under hygrothermal and actual vehicle load coupling was proposed, which has thefollowing innovations: a) the hygrothermal environment and dynamic load coupling problemsin durability experiment were solved; b) synchronous cycles or non-synchronized cycles oftemperature, humidity and load were achieved, where, environment temperature range was5℃~50℃and environment humidity range was65%~98%RH; c) working conditions(temperature, humidity and load conditions) of bridges in operation in subtropicalenvironments were faithfully reproduced; d) vehicle load spectrum was established fromnational highway or freeway traffic flow through data acquisition, statistical analysis,numerical simulation and experimental spectra calculation; e) the accelerated way ofenvironment and load coupling reproduced the effects on the components.2. In order to establish durability experiment of bridge structure under hygrothermalenvironment and actual vehicle load coupling, corresponding experimental system wasintegrated and developed, which composed of two parts: a) bridge working environmentsimulation experiment system; b) random vehicle loads simulation experiment system. Thefeatures of environment simulation experiment system were: experimental environmentsimulations of thermal and humidity cycle, temperature-marine environment, freeze-thawenvironment, wet and dry cycle were achieved; All technical indicators were not less than theexisting similar products; the needs of the larger size of the bridge structure model testexperiment were meet; environment system was matched with the load and test systems. Load and test system were upgraded from MTS810. Random vehicle loads were simulated bysoftware and then inputted to the controller of the experimental system. After debugging andrunning for nearly a year, this system passed the calibration test from the Municipal Bureau ofWeights and Measures. Two kinds of durability test of FRP strengthened RC beam underthermal and humid environment and the actual vehicle load coupling were carried out.Feasibility, stability and reliability of the system were confirmed.3. Using the above experimental system, four kinds of durability experiments of RCbeams strengthened with FRP were carried out; a) constant environment and constantamplitude fatigue loading non-coupling experiment, in which specimens were environmenttreated first and then imposed with constant amplitude fatigue load; b) constant environmentand constant amplitude fatigue loading coupling experiment; c) actual environment andconstant amplitude fatigue loading coupling experiment; d) actual environment and randomvehicle loading coupling experiment. In the same stress level, experiment results of a) and b),b) and c), b) and d) were compared and failure mechanism of components was analyzed.Results showed that the experiment method had a significant impact on the durability ofcomponents. Durability of components in coupling condition was worse and the worst inrandom vehicle load and environment coupling condition. This indicated that the durability ofbridge structure components determined by non-coupling experiment method in specificationswas less conservative than in actual conditions, which lead durability design to be somewhatdangerous. At the same time, it also showed the importance of using the experimental methodof random vehicle load and environment coupling in a bridge structure durability experimentsand proved the effectiveness and feasibility of durability experiment method proposed in thispaper.4. Analysis results of fatigue failure modes, stiffness degradation, deflection curves andS~N curves of RC beams strengthened with FRP in hygrothermal environment indicated that:a) Destruction layer in interface was moving from concrete layer to adhesive layer, with theincrease affect of temperature and humidity, destruction was eventually occurred in theadhesive layer. Fatigue failure mode types were reduced by influence of hygrothermalenvironment. As the hygrothermal environment continued affecting, fatigue failure modebecame singularity, which FRP debonding failure became the only failure mode. b) Stiffnessdegradation of strengthened beams in coupling condition was faster than that in non-couplingcondition. This showed hygrothermal environment had a great impact on the stiffnessdegradation of strengthened beams. In actual environment and constant amplitude fatigue loadcoupling, stiffness degradation of strengthened beams had the corresponding trend curve with temperature and humidity changes. c) Deflection f versus relative fatigue lifelnn/lnNjmodels of specimens were built in stable hygrothermal environment. Influence factors ofhygrothermal environments were proposed and determined by experiments. Dynamic secantstiffness versus fatigue life equations in cyclic hygrothermal environment and fatigue loadcoupling were built. This laid a good foundation for fatigue life study in hygrothermalenvironment. d) Results of comparisons of S~N curves in different experiment conditions alsoshowed experiment method had a significant impact on mechanical properties of thestrengthened beams and experiment method proposed in this paper is necessary, effective andfeasible.