Micro-crack propagation behavior of pavement concrete subjected to coupling effect of fatigue load and freezing-thawing cycles

In order to quantitatively characterize and analyze the evolution laws of pavement concrete internal cracks under the coupling effect of fatigue load and freezing-thawing cycles, the SEM and the morphological image segmentation method were used to extract the micro-crack patterns, the Image-Pro Plus was used to quantify the micro-crack structure, and the relationship between the crack parameters and the loss of flexural strength under the coupling effect was studied by using the grey correlation analysis method.Analysis result shows that there are original micro-cracks in pavement concrete, the average width is 13 μm, and the maximum length is 144 μm.Under the coupling effect, the evolution of pavement concrete cracks along the length direction is the alternative process of extension and fracture, while the crack evolution along thewidth direction is the alternative process of expansion and contraction.When the concrete is destroyed, the maximum length of crack reaches 352.64 μm, and the average width of crack reaches 15.4 μm.After 150 times freezing-thawing cycles, the crack area density increases significantly, and is 6.7 times larger than the density of original micro crack when concrete is destroyed.The flexural strength of pavement concrete is greatly related to the fractal dimension, average width and maximum length of the crack under the coupling effect, and the grey correlation coefficients are 0.957, 0.954 and 0.871 respectively.Based on the regression analysis, the equation between the flexural strength loss and crack structural parameters of pavement concrete is established, there is good linear correlation between the flexural strength loss and the fractal dimension, average width, maximum length of crack, and the correlation coefficients reach 0.97.