Experimental Study On Dynamic Behaviors Of Concrete After Fatigue Load And Freeze-Thaw Cycles

Concrete structures such as tunnel,dam and bridges in the cold regions would suffer from the effects of loading histories and freeze-thaw cycles,and dynamic loads have an important impact on the security and stability of those structures.Uniaxial dynamic compression tests are carried out on 180 concrete prism specimens after four categories of damage mode,such as fatigue damage(Mode F),freeze-thaw damage(Mode T),first fatigue loading and then freezing-thawing cycle(Mode F-T),and first freezing-thawing and then fatigue loading(Mode T-F).The effects of single factor and double factors on the dynamic behaviors of concrete are investigated systematically,and the microstructural differences and damage mechanism of concrete under different sequence of fatigue load and freeze-thaw action are analyzed contrastively.The main conclusions are as follows:1.The dynamic behaviors of concrete subjected to single factor are investigated,including dynamic behaviors of concrete after Mode F and Mode T.First,the uniaxial dynamic compression experiments are conducted on concrete specimens subjected to Mode F,and the influences of fatigue load cycles and freeze-thaw cycles on dynamic strength and deformation are analyzed respectively.The test results show that both of the compressive strength and peak strain firstly increase and then descend with the increasing of fatigue load cycles;As the strain rate increases,the compressive strength increases,the peak stain decreases.Second,the uniaxial dynamic compression experiments are conducted on concrete specimens after Mode T,the relationship between the compressive strength,peak strain of concrete and freeze-thaw cycles,strain rate are studied respectively.Based on the experimental data,the dynamic compressive stress-strain curve equation of concrete considering the effects of strain rate and freeze-thaw cycles is proposed.The test results show that as the strain rate increases,the compressive strength increases,the peak stain decreases;as the increase of freeze-thaw cycles,the compressive strength decreases and the peak stain increases,and strength descent scope can be up to 7.46%.2.The dynamic behaviors of concrete subjected to double factors are investigated,including dynamic behaviors of concrete after Mode F-T and Mode T-F.First,the uniaxial dynamic compression experiments are conducted on concrete specimens after Mode F-T.The influence rules of fatigue load cycles and freeze-thaw cycles on dynamic behaviors are systematically discussed.The test results show that the strength loss under Mode F-T is greater than that under Mode F;The compressive strength of concrete after Mode F-T decreases with the increase of freeze-thaw cycles or initial fatigue loading,and increases with the increase of strain rate;The great reduction in strength is up to 10 MPa;Second,the uniaxial dynamic compression experiments are conducted on concrete specimens subjected to Mode T-F.Based on the experimental data,the influences of freeze-thaw cycles,fatigue load cycles and strain rate on dynamic compressive strength,peak strain and stress-strain curve of concrete are analyzed.The test results show that the compressive strength of concrete after Mode T-F increases visibly with the increase of strain rate,less fatigue times has the function of relieving to the freeze-thaw damage of concrete.3.The influences of fatigue load and freeze-thaw cycles in a different order on dynamic compressive strength and peak strain of concrete are analyzed.The combined effect variable Kc is applied to characterize the superimposed effect of fatigue load and freeze-thaw cycles.The results showed that the combined effect of fatigue load and freeze-thaw cycle was positive under Mode F-T.For the same strain rate,the Mode F-T results in a greater loss of strength than the Mode T-F.Moreover,the microstructural differences and damage mechanism of concrete under different sequence of fatigue load and freeze-thaw action are analyzed.Scanning electron microscopy(SEM)analysis is carried out to observe the microstructure characteristics of damaged concrete,and the damage mechanism of concrete under different sequence of fatigue load and freeze-thaw action is revealed from the microscopic view...