Experimental Study And Damage Evolution Analysis On Acoustic Characteristics Of Concrete Under Split-pull Cyclic Loading

The building such as water storage dams and the bridge foundations for vehicles are prone to damage under cyclic loading.In this dissertation,the laboratory mechanical tests and numerical simulations were used to study the damage characteristics of concrete under the action of split-pull cyclic loading.Firstly,the ultrasonic testing and the acoustic emission testing were carried out on the loaded concrete,and then the correlation between acoustic parameters(such as sound velocity and ring count)and stress were discussed,the relationship between acoustic parameters and damage was established,and finally the numerical simulation software RFPA2D was used to simulate the cracking process of the concrete split-pull cycle.The conclusions are as follows:(1)Under the action of split-pull cyclic loading,the peak strength values of samples with different water-cement ratios are different.When the water-cement ratio decreased from 0.55 to 0.42,the peak strength value increased from 1.82MPa to 2.72MPa,with an increase of 49.5%,and the saturation state strength reduced.In addition,The ultrasonic signal monitoring of the concrete sample under tension shows that the increase and decrease of the sound velocity in the first three cycles were equal to the stress increase and decrease.Since the fourth cycle,the sample cracked and the sound velocity began to decrease.The speed of sound drops to the lowest value when the crack penetrated in the sixth cycle.It indicates that the speed of sound is related to the propagation of the splitting crack.By monitoring the acoustic emission activity of the concrete sample under loading,it can be obtained that the ring count started to rise after the fourth cycle,and the peak value was reached when the sample was destroyed,which is consistent with the relationship between ultrasound and stress.And the speed of sound decreases and the cumulative ring count increases,with increasing of the concrete strength.(2)According to the correlation between sound speed,cumulative ring count and damage,it can be concluded that the damage variable of the first few cycles of concrete will not increase,there is a damage threshold,which is about 60%～70%of the peak value of the damage.When the damage threshold is exceeded,the sample begins to break rapidly,and the peak value of damage variable is between 0.7～0.9,indicating that the sample has a certain residual strength.In addition,the higher the concrete strength,the more cycles to reach the peak value of damage variable.During the water-cement ratio decreasing from 0.55 to 0.42,the growth rate of damage variables are 0.51,0.41 and 0.34,respectively,indicating that the higher the concrete strength,the slower the growth of damage and the stronger the ability to resist external loads.(3)Through the RFPA2D simulation of split-pull cyclic loading,it can be found that there are almost no new cracks in the concrete sample during the first three cycles.Since the third cycle,there are some tensile micro-fractures and compression microfracture appearing in samples.The tensile failure and shear failure continue to increase in the following several cycles until the model breaks.Through the RFPA2D simulation of acoustic emission signals,it can be concluded that in the early stage,the acoustic emission signal was inactive.Since the fourth cycle,the acoustic emission signal surge,indicating that the acoustic emission signal of concrete loading and unloading test under RFPA2D simulation is consistent with the indoor test.