Fatigue Performance Of RC Beams Strengthened With CFRP Under Coupling Action Of Hot-wet Environment And Vehicle Random Load

Taking reinforced concrete(RC)bridges in subtropical area as research background and considering the temperature and humidity changes in this area as well as the actual vehicle random loads on the load-bearing members of highway bridges,RC bridge structures strengthened with carbon fiber laminate(CFL)was used as research object in this paper,an accelerated fatigue test method was proposed for the strengthened structures under the coupling action of hot-wet environments and vehicle random loads.Three-point bending fatigue tests of RC beams strengthened with CFL under hot-wet environments and vehicle random loads were carried out,and fatigue mechanism of the strengthened beams was discussed.The fatigue life prediction methods of the strengthened beams under the coupling action of hot-wet environments and vehicle random loads were proposed.The main research contents and conclusions of this paper are as follows:(1)Fatigue test method of RC members strengthened with carbon fiber reinforced polymer(CFRP)under coupling action of hot-wet environments and vehicle random loads.The traffic flow of highway in Guangdong Province was collected,and a series of methods such as damage equivalent and statistical analysis,non-Gaussian random process simulation,and experimental spectrum of vehicle random loads compilation were used to successfully obtain the experimental spectrum of vehicle random loads.The experimental spectrum,which can be applied to random fatigue tests conveniently,had considered the response characteristics of the bridge structures and had the same probability characteristics of the actual vehicle flow(time series).An accelerated environmental fatigue test method of RC members strengthened with CFRP under coupling action of hot-wet environments and vehicle random loads were proposed based on the above experimental spectrum and the intelligent environmental fatigue experimental platform constructed by our research group.Three-point bending fatigue tests of 45 RC beams strengthened with CFL under coupling action of 5 groups of hot-wet environments and vehicle random loads with 3 loading levels were carried out successfully.(2)Effect of the interaction of environment and load on fatigue failure mechanism of the strengthened beams.Based on the above fatigue test results,the influence mechanism of environment and load on the fatigue failure mode,crack morphology and fatigue deformation of the strengthened beams were discussed.The results showed that:1)Different hot-wet environments will lead to different fatigue failure modes of RC beams strengthened with CFL.When the temperature and relative humidity were lower,the failure mode was mainly CFL debonding caused by fatigue fracture of the main reinforcements;As the temperature and relative humidity increased,its failure mode gradually changed to the single mode of the CFL debonding;2)As the temperature and relative humidity increased,the bond performance of the CFL-concrete interface became weaker,and the peeling position of the CFL-concrete interface transfered from the“concrete layer”to the“concrete-adhesive layer”and the“adhesive-CFL layer”;3)As the temperature and relative humidity increased,the number of fatigue cracks on the strengthened beam reduced,and the crack spacing increased;Compared with constant amplitude loads,the cracks on the strengthened beams under random loads were more concentrated and the number was less.4)As temperature and relative humidity increased,deflection of the strengthened beams was larger and the growth rate of the deflection was faster at the same loading level;In the same hot-wet environment,the higher the loading levels,the greater the deflection,but it had no significant effect on the increase rate of the deflection.(3)Environmental random fatigue equation for RC beams strengthened with CFRP.Considering the influence of hot-wet environment on the material parameter m in the classical fatigue equation,the environmental random fatigue equation of the strengthened beams under the coupling action of hot-wet environments and vehicle random loads was deduced.The environmental random fatigue equation was fitted by four groups of test data in this paper,and a semi-empirical expression of the environmental random fatigue equation was obtained.Then,the semi-empirical expression was validated by one set of test data at another hot-wet environment(50~oC,78%R?H).The results showed that the environmental random fatigue equation can predict the fatigue life and fatigue limit of RC beams strengthened with CFRP under coupling action of hot-wet environments and vehicle random loads with high accuracy and effectiveness,and provide theoretical basis and basic data for fatigue design of the strengthened bridge structures.(4)Modified Miner rule and fatigue life prediction.Considering the influence of hot-wet environment and random loads on the damage evolution of the strengthened beams,a semi-empirical relationship between critical damage of the strengthened beams and temperature as well as relative humidity was proposed.Then,taking this relation as an amendment,a new modified Miner rule was established.Then,three life prediction methods based on Miner rule,modified Miner rule and environmental random fatigue equation were compared and analyzed.The prediction results showed that the fatigue lives of the strengthened beams were overestimated by the prediction results based on Miner rule,and the prediction method based on modified Miner rule and environmental random fatigue equation were effective and feasible.Among them,the prediction method based on the environmental random fatigue equation was more accurate and convenient.(5)Numerical simulation of environmental random fatigue process of the strengthened beams.Considering degradation of material performance under hot-wet environment and vehicle random loads,a finite element model for fatigue analysis of the strengthened beams under coupling action of hot-wet environment and vehicle random loads was established.The fatigue failure mechanism of the strengthened beams was discussed,and their fatigue lives were predicted and verified by experimental data.The research results showed that:1)The higher the temperature and relative humidity,the greater the stresses of CFL and the main reinforcements under the same loading level;the higher the loading level,the greater the stresses of CFL and the main reinforcements in the same hot-wet environment;2)The hot-wet environment had little effect on the damage evolution of the main reinforcement,while the effect on the damage evolution of CFL-concrete interface was significant;3)A failure criterion based on the damages of main reinforcements and CFL-concrete interface was established,and the fatigue lives of the strengthened beams were predicted.The preliminary verification results showed that the method was effective and feasible to predict the fatigue lives of the strengthened beams under coupling action of hot-wet environments and random loads.