Study On Fatigue Properties Of RC Beams Strengthened With CFL Under Overload Spectrum

The fatigue property and durability of the reinforced concrete （RC） structuresstrengthened with Fiber Reinforced Polymer（FRP）is an advanced topic in civil engineering.For the strengthened RC bridges on highway, fatigue fracture is easier to happen because ofthe cyclic vehicle random load. Though some measures have been taken to limit theoverloaded vehicles, it is still a serious problem to harm the transportation safety by overloadin our country. The overload, especially "heavy overload" among the vehicle load has adverseimpact on the behavior of the bridges, but there is very limited related research at present.Therefore, it is necessary to carry out study on the fatigue damage mechanism under overloadfor the RC structure strengthened with FRP.According to the statistical analysis of the actual vehicle flowrate on a highway bridge,the method to determine the overload was put forward in this paper. The fatigue property ofRC beams strengthened by Carbon Fiber Laminate （CFL） which invented by the authors’research group under overload was studied through test and theory analysis. The mainresearch contents and conclusions in the paper are as follows:（1） The concept of overload was defined firstly in this paper combining with theoverload phenomena in current highway bridges. The method to determine the overload wasput forward in detail, that was, the moment corresponding to the maximum tensile stress ofthe concrete on the control section, width of the crack, and the tensile stress of thereinforcement or steel strand caused by vehicle load just exceeded the specification limits. Asimply supported prestressed concrete hollow plate beam with20meters span was taken as anexample, how to determine the overload was described in detail in this paper.（2） The simply supported concrete beam was loaded according to the collecting datain a highway such as the axle load data, the actual vehicle. The bending moment whichexceeded the critical moment caused by overload were divided into three ladder based on theinfluence line of the maximum bending moment caused by vehicle load. Then the ladderoverload spectrum was generated. The experimental ladder overload spectrum was formed byusing back calculation in accordance with the ratio of the most unfavorable stress on thecontrol section and the ultimate bearing capacity （load level）, which provided anexperimental method to study the fatigue damage mechanism of bridge structure underoverload.（3） The variable amplitude fatigue experiments of RC beams strengthened by CFL were carried out successfully by using the ladder overload spectrum with different stressamplitude and stress ratio. The relationship between stress ratio and the fatigue life ofstrengthened beams was explored, and a semi-empirical formula was given based on theexperimental results. The predicted fatigue life was further compared with that under constantamplitude, random loading.（4） A neural network model to predict the fatigue life of RC beam strengthened withCFL was built based on the statistical analysis of the experimental data for variable amplitudefatigue life and the previous relevant results offered by our group. Combined with the geneticalgorithm, the model used to predict the fatigue life of the strengthened beams with differentstress amplitude and stress ratio. The average relative error between the predicted life and theactual life was39.6%which proved the feasibility of the neural network model.（5） A series failure analysis model was built based on analyzing the failure mode,failure mechanism of the main composed material. Through analyzing the fatigue propertiesfor subunit and the relevance of each material on control section, a series failure model wasestablished to predict the fatigue life of the strengthened beam. By comparing the predictedresults with the experimental data, the average relative error was35%which showed that themodel was basically reasonable and feasible（6） A semi-empirical formula was put forward according to analysing the deflectionand stiffness degradation of the RC beams strengthened with CFL under ladder overloadspectrum. Experimental results showed that increase of the stress amplitude had greaterinfluence on the stiffness degradation rate than load stress. It also showed that the increase ofthe live load （such as overload） had greater effect on stiffness degradation of strengthenedbeam than dead load.