Study On Fatigue Performance Of Concrete Flexural Members Strengthened With Prestressed CFRP Plates

The Carbon fiber reinforced plastic(CFRP)has been highly concerned by the academic community due to its high strength-weight ratio,corrosion resistance,fatigue resistanceand many other advantages.It has been widely used in the reinforcement of flexural members.In some applications,it even has a better effect than steel plates.However,because CFRP is pure elastic,it can not produce plastic deformation,and effective shear connection keys can not be set when CFRP laminate is externally bonded.At present,the research on the static behavior of concrete flexural members strengthened with CFRP laminates is relatively mature,the research on its fatigue performance is still worth further replenished and futher studied.In order to study the performance of concrete flexural members strengthened with prestressed CFRP plates under fatigue load and the fatigue debonding behavior of CFRP plates,experimental study and corresponding theoretical analysis are carried out for CFRP plate-concrete interface and prestressed CFRP plate reinforced concrete beams.The main research contents and results are as follows:(1)11 specimens of CFRP plate-concrete interface are tested.The ultimate load capacity and failure mode of the interface under static load are mainly studied when the bond length of CFRP plate exceeds the effective bond length Le;The fatigue test mainly studies the fatigue performance of the interface with different upper limit of the fatigue load Pmax and the lower limit of fatigue load Pmin.The main variables considered are fatigue load amplitude(difference between upper and lower limits of fatigue load)and fatigue load level(mean value of upper and lower limits of fatigue load).The results show that the failure modes of CFRP plate-concrete interface under static and fatigue loading are both debonding failure of CFRP plate,and the failure surface are both in shallow concrete.The main difference between the static and fatigue specimens is in the stage of debonding growth phase.Under static load,the CFRP plate debond rapidly and continuously when the interface reaches its ultimate load capacity,while under fatigue load,the debonding of the CFRP plate will gradually develop with the increase of fatigue loading cycles.Moreover,when the upper limit of fatigue load Pmax on the CFRP plate is less than 0.55 times of the interface static ultimate load capacity Pu,the interface shows no fatigue failure after 2 million times of fatigue loading,and the fatigue debonding of CFRP plate is also not carried out.Meanwhile,the residual interface load capacity after 2 million fatigue loading is basically consistent with the ultimate load capacity of static pecimens;(2)The fatigue test of CFRP plate-concrete interface shows that the fatigue debonding of CFRP plate presents the basic trend of "fast in the early stage,slow down in the middle stage,and sudden failure in the end".The fatigue life of the interface is directly determined by the rate of the CFRP debonding.The test results show that when the interface is subjected to the same fatigue load amplitude,the increase of fatigue load level will lead to the sharp decrease of interface fatigue life,and the specimens with the same upper limit of fatigue load Pmax but different lower limit of fatigue load Pmin show similar fatigue life.The results indicate that the CFRP plate-concrete interface is more sensitive to the upper limit of fatigue load Pmax.So the fatigue load amplitude and the fatigue load level must be considered in the fatigue analysis of the interface;(3)Based on the basic principles of fracture mechanics and energy method,the "FRP fatigue debonding growth rate prediction model" is proposed.The model uses the product S(S=AS·S)of the relative fatigue stress amplitude △S and the relative fatigue stress level S as a parameter,and the influence of the debonded FRP on its subsequent fatigue debonding groeth rate is also considered to modify the model.The comparison with the test data proves that the prediction of the model is reasonable and accurate,and can show the basic trend of FRP fatigue debonding.And the foundation for the subsequent analysis of the fatigue performance of concrete beams strengthened with CFRP slabs is also prepared;(4)This paper also carried out the experimental research on the static or fatigue performance of 6 prestressed/non-prestressed CFRP plate reinforced concrete beams.The experiment includes 2 static test beams and 4 fatigue test beams.Each of the static and fatigue test beams contains 1 non-prestressed CFRP plate reinforced concrete beam.The rest of them are prestressed strengthened(σpp=1000MPa).The static load test of the strengthened beams mainly studies the ultimate load capacity and failure mode of the prestressed/non-prestressed CFRP plate strengthened concrete beam;the fatigue test mainly studies the effect of the fatigue load level and the effective prestress on the fatigue performance of the strengthened beams.The static load test found that the failure mode of the strengthened beam is the crushing of the concrete at the top of the compression zone under the loading point.The test shows that the application of prestress can effectively increase the cracking load,yield load and ultimate load capacity of the beam,but it will sacrifice some ductility.The fatigue test found that the fatigue failure mode of the strengthened beam is the fatigue fracture of the steel reinforcement under the loading point.During the fatigue loading process,the stress and stress amplitudeof the reinforcement,and the mid-span deflection of the reinforced beam all show up the "fast-slow-fast" three-stage development.The debonding growth of the CFRP plate is found in both static and fatigue tests.The debonding startes at the loading point,and develops to the adjacent support.At the same time,the test also observes the redistribution of the cross-sectional stress caused by the debonding of the CFRP plate,indicating that the debonding is a key part that cannot be ignored;(5)Based on the strip method and piecewise linear principle,this paper has proposed a non-linear analysis program for the whole process of fatigue damage of prestressed CFRP plate strengthened concrete bending members.The program fully considers the fatigue damage model and corresponding failure criteria including steel reinforcement,concrete and CFRP plates,and also considers the cross-sectional stress redistribution caused by the debonding of CFRP plate.The validity of the analysis method is proved by comparison with the test results.Subsequently,this paper conducted a large number of parameter analysis to quantitatively study the effect of fatigue load level,effective prestress,concrete strength grade and CFRP palte debonding on the fatigue life of strengthened beams.At the same time,in order to produce a stronger engineering application value,this paper proposes the tensile stress limit[σc0db,2E6]of the converted section tensile edge of concrete beams strengthened with prestressed CFRP plates with different prestress levels.When the tensile stress is less than this limit,the fatigue debonding of the CFRP plate will not occur during the fatigue loading of 2 million times,and the CFRP plate will always maintain a good bond with the strengthened concrete beam.