Study On The Fatigue Behavior Of Reinforced Concrete Continuous Beams Prestressed With External CFRP Tendons

Externally prestressed concrete has been widely used in the structural engineering,such as bridge engineering,strengthening engineering,and large span roof,etc.With the extension of design life and frequently adverse environment(such as acid,alkali,chloride,moisture)apply on the structures,the fatigue problem under alternating load,the corrosion problem in corrosive environment,and corrosion-fatigue problem of the traditional steel prestressed tendons has seriously affected the safety and durability of the externally prestressed concrete structure.The carbon fiber reinforced polymer(CFRP)tendons are considered to be ideal materials to replace the external steel prestressed tendons,due to many advantages of them such as high specific strength(strength-to-weight ratio),excellent fatigue resistance and non-corrosive characteristics.The present work experimentally and analytically investigated the flexural behavior of reinforced concrete continuous beams prestressed with external CFRP tendons under fatigue loading.The main contents and results are as follows:(1)Five reinforced concrete continuous beams prestressed with external CFRP tendons were tested under static and fatigue loadings.Each specimen had identical cross-section and reinforcement.Test parameter was the cyclic load level(the ratio between maximum cyclic load and ultimate capacity).The results showed that the fatigue failure of the tested beams began with the fatigue fracture of the internal steel reinforcing bars.With the increase of the cyclic load level,the fatigue lives of the tested beams were reduced rapidly.With the increase of the cycle number,the deflection of fatigue span was continuously developed,same as the stress of external CFRP tendons and steel reinforcing bars,also the width of the main cracks.Meanwhile all the aforementioned features presented the three-stage development rule which is "rapid growth-slow and steady growth-sharp growth".Compared with the moment distribution of the continuous beam at the first cyclic,the moment(positive moment)of controlled cross-section in fatigue span presented the three-stage development rule of "rapid decrease-steady slow decrease-sharp decrease",while the moment(negative moment)of middle support presented the three-stage development rule of "rapid growth-slow and steady growth-sharp growth".Continuous beam presented the phenomenon of fatigue internal force redistribution,which was caused by the uneven stiffness degradation of different sections under fatigue loading.When the fatigue failure occurred,the fatigue moment redistribution coefficient(?f)of the controlled cross-section in fatigue span was more than 20%,while ?f of the middle support was more than 80%.(2)An analysis method was developed and implemented into a computer program which can predict the non-linear flexural response of reinforced concrete continuous beams prestressed with external CFRP tendons during the whole process under static loading.Good agreement was obtained between the measured and the predicted monotonic results,such as deflection,stress of external CFRP tendons and steel reinforcing bars,and section moment.(3)The fatigue damage model of concrete,steel reinforcing bars and CFRP tendons was established.Based on piecewise linear principle,a nonlinear analysis method of reinforced concrete continuous beams prestressed with external CFRP tendons during the whole process under fatigue loading was developed and implemented into a computer program,which can simultaneously consider the section stress redistribution and the fatigue internal force redistribution.The comparison between the experimental results(including fatigue life,deflection,stress of external CFRP tendons and steel reinforcing bars,section moment and stiffness)and the simulated ones showed that this method can successfully predict the response of reinforced concrete continuous beams prestressed with external CFRP tendons under fatigue loadings.(4)The evolution rule of secondary moment(Ms)in middle support section of reinforced concrete continuous beams prestressed with external CFRP tendons was analyzed under fatigue loading based on the fatigue analysis program and structural mechanics method.The results showed that Ms presented the three-stage development rule of "rapid growth-slow growth-sharp growth"with the increase of the cycle number.Compared with Ms at first cyclic.Ms increased by more than 28%when the fatigue failure occurred.Ms and the increment of Ms(compared with Ms at first cyclic)when the fatigue failure occurred decreased as the level of fatigue loading increased,while the growth rate of Ms increased as the level of fatigue loading increased.The effect of the stress changes in external CFRP tendons on the secondary moment of continuous beams was relatively small under fatigue loading,while the effect of the section relative stiffness changes was dominant.(5)Quantitative analysis of the main parameters(including effective prestress,reinforcement ratio of external CFRP tendons and steel reinforcing bars,concrete strength,and type of external prestressing tendons)that affect the fatigue life of externally prestressed concrete continuous beams,was carried out by using of the fatigue nonlinear analysis program.The results could provide references for design of reinforced concrete continuous beams prestressed with external CFRP tendons under fatigue loadings.The comparison of the fatigue life and failure mode showed that the durability and safety of externally prestressed concrete structures will be effectively improved by using of CFRP tendons instead of steel strands as the externally prestressed tendons.(6)Two-stage fatigue life prediction method which combine of local stress-strain method and linear elastic fracture mechanics theory was developed in order to predict the fatigue life of reinforced concrete continuous beams prestressed with external CFRP tendons.In which,the local stress-strain method was used for predicting the crack initiation life of the steel reinforcing bars,while the linear elastic fracture mechanics theory was used for predicting the crack propagation life of the steel reinforcing bars.Good agreement between the experimental and predicted fatigue life was observed.