Damage Degradation And Life Prediction Of Different Saturation Concrete Subjected To Coupling Fatigue Load And Freeze/thaw Cycles

During their service lifes the highways,the railways and the bridges in cold region of high latitudes are subjected to coupling fatigue loads and freeze/thaw(FT)cycles.The interactions between fatigue loadings and environmental effects accelerate the damange of concrete structures designed based on research of durability under single damage effect.Meanwhile the tranditional durability researches focus on saturated concrete,which is very defferent from the real fact that most concrete structure are always in unsaturated condition for moistrure exchange from environment.And the saturation of concrete is most important to its durability.So it is theoretical and practical value to study the durability of concrete subjected to coupling fatigue loads and freeze/thaw cycles.The damage deterioration and life prediction of different saturation concrete subjected to coupling fatigue loads and FT cycles were studied by strain messurement and acoustic emission(AE).The coupling mecheism of both damage factors was firstly presented.And the analytical model of damage deterioration and service life predition of concrete structure were presented based on mechanism of materials and Loland damage model of concrete.Firstly,the mechanical tests of concrete specimen of three kinds of water content(dried,80%and satured)subjected to quiescent load and single damage factor were done to get parameters for coupling tests,damage model and service life prediction.The results of four-point bending test,fracture tests,splitting tensile strength tests and compression tests showed that temperature and saturation could change mechanical properties significantly.The comparison of fatigue tests at different temperature and of saturation showed that the fatigue resistance of dried concrete at low temperature was distinctly better than that of unsaturated concrete at room temperature.And the fatigue resistance of saturated concrete at room temperature was worst.Secondly,to make the coupling mechanism clearly the coupling test on different saturation concret subjected to coupling fatigue loads and FT cycles was done,which was fundamental to damage model and service life prediction.According to results of strain test the damage mainly took place when temperature changed especially when cooling.The strain separation was used in coupling tests according to Wheatstone bridge theory.The coupling strain was measured and meanwhile the fatigue strain and temperature strain generated by fatigue loads and FT cycles respectively were simultaneously separated from coupling strain and measured.The coupling strain was expressed by the function of fatigue strain and temperature strain by strain data fitting.The fitting showed that coupling strain can be expressed as linear equation of fatigue strain and temperatue strain,and the equation coefficients could be expressed as linear function of saturation.The nondestructive real-time tests were done by AE to reveal the damage process of concrete subjected to coupling effect.The quasi-Kaiser effect was founded that AE didn’t happened when the strain was less than previous maximum strain.Thirdly,the damage model with the index of relative position of neutral axis was presented based on the coupling mechanism above,Loland damage model and material mechanics.The relations between strain at dangerous cross-section(ε_m)and the relative positon of neutral axis(ζ_n),the relative positon of neutral axis(ζ_n)and the number of fatigue cycles(N)were presented according to plane cross-section assumption,the equality of the tensile stress and pressure stress at cross-section,and the balance of bending moment between internal force and external moment loads.Coupled with Loland damage model,the recurrence relation between the damage degree(D_m)and the number of fatigue cycles N was determined.Finally,the relative positon of neutral axis(ζ_n)and the maximum fatigue cycles(Nc)of the concrete when destroyed was determined according to the damage model and the recurrence relation above,which could be applied to service life prediction of concrete subjected to coupling action.Whenε_m was measured,the relative positon of neutral axis(ζ_n),the experienced fatigue cycles N and the damage degree D could be calculated and comparing to the maximum fatigue cycles(Nc),left service life could be determined...