Study On The Damage Constitutive Relationship Of Concrete Under The Coupled Conditions Of Freeze-thaw Cycle Fatigue Load

In China’s mountainous regions,concrete bridges,tunnels,dams,and other structures are often exposed to the combined impact of freeze-thaw cycles and fatigue loads,which can cause considerable harm to their durability.Consequently,numerous academics have conducted studies on the destruction of concrete caused by freeze-thaw cycles and fatigue loads.This paper will explore the effects of freeze-thaw cycle frequency and maximum stress ratio of fatigue load on single-factor,fatigue loading,and coupled loading tests,as well as the alterations in the mechanical properties of concrete.A constitutive equation for concrete damage,based on the coupled damage-related theory,is formulated by combining the research group’s existing data and the experimental data presented herein,taking into account freezethaw cycles and fatigue loads.The main results are as follows:1.An investigation of the mechanical characteristics of concrete under single-factor freeze-thaw cycles was conducted,exploring the effect of varying freeze-thaw cycle frequencies,ranging from 0 to 75 cycles.The mechanical properties of concrete,including mass loss rate,relative dynamic elastic modulus,compressive strength,and peak strain,were assessed using ultrasonic and uniaxial compression tests.The findings indicated that the concrete samples’ quality displayed an initial rise and then decline as the freeze-thaw cycle frequency increased.As the frequency of the freeze-thaw cycle augmented,the relative dynamic elastic modulus and peak strain of the concrete specimens augmented;however,the compressive strength diminished in tandem.2.The mechanical properties of concrete under single-factor fatigue load were studied.Scanning Electron Microscopy was employed to observe the microstructure of specimens post-load,and ultrasonic and uniaxial compression tests were conducted to evaluate the influence of the upper-stress ratio of fatigue load(0.4,0.5,0.6,and 0.7)on concrete mechanical properties.The test results showed that the relative dynamic elastic modulus,compressive strength,and peak strain decreased with a rise in the upper-stress ratio of fatigue load.As fatigue load’s upper-stress ratio augmented,the peak strain attenuation rate rose concurrently,yet the compressive strength attenuation rate was disparate.When the upperstress ratio was 0.5,the compressive strength attenuation rate was the lowest among the four upper-stress ratios,and the attenuation amplitude of compressive strength only increased by0.39% for every 0.1 increase in the upper-stress ratio.With the increase of the upper-stress ratio of fatigue load,the number and size of micro-cracks in concrete materials increased,and the degree of deterioration gradually increased.3.An examination of the mechanical characteristics of concrete when exposed to the combination of freeze-thaw cycle and fatigue load was conducted.Ultrasonic and uniaxial compression tests were administered to the specimens after every 25 freeze-thaw cycles or2500 fatigue loadings.The results indicated that the changes in mechanical properties were consistent with those under single-factor action.The coupling effect weakened the compressive strength and relative dynamic elastic modulus of concrete,leading to a decrease of up to 11.89% compared to the undamaged state.Notably,the peak strain value of concrete specimens under freeze-thaw cycles was higher than those under fatigue loading only after the coupling effect.Additionally,the microstructure of the specimens was observed by scanning electron microscopy.4.A model for concrete damage under the coupling of two factors was established based on existing concrete damage theories.The model was fitted using existing data from the research group and experimental data from this paper.A constitutive equation for concrete damage,coupled with freeze-thaw cycles and fatigue load,was established by the inclusion of influence coefficient β and random coefficient γ.The theoretical values calculated according to the equation were compared with the experimental data.This model’s accuracy in predicting mechanical behavior alterations in concrete samples due to the combined effect of freeze-thaw cycles and fatigue loading was demonstrated by its relative error of compressive strength being less than 15%.