Comparative study on damage process of concrete subjected to uniaxial tensile and compression loads based on CT test and improved differential box counting method

Abstract Concrete is prone to be damaged due to external forces. Traditional concrete failure analysis and research mostly focus on the characterization of macro – mechanical properties, which cannot describe the damage of internal structure and the process of concrete failure after being subjected to a load on a mesoscopic level. Thus, a series of CT tests were carried out on the damage process of concrete under tensile and compression loads, and CT images of the whole process of concrete failure were obtained. On this basis, the fractal dimension calculated by the Improved Differential Box Counting (IDBC) method was used to quantitatively study the change of the internal structure of concrete during the loading process. Then the mean values of the CT number of concrete CT images were statistically analyzed. The results revealed that the fractal dimension of different stress stages in each section increased with the increase of scan times under tensile load, while the fractal dimension increasing first, then decreasing and finally increasing with the increase of scan times under compression load. Correspondingly, with the increase of scan times, the mean values of CT number under tensile load showed a trend of decreasing, while a trend of decreasing first, then increasing and finally decreasing of the mean values of CT number was exhibited when the concrete specimen was subjected to compression load, which indicated that the concrete specimen undergone the process of expansion, crack propagation, and failure under tensile load, whereas the concrete experienced the process of compaction, crack propagation, and failure under a compression load. Furthermore, research results have shown that the fractal dimension and mean value of CT number can be used to quantitatively describe the damage process of concrete, whether it is subjected to tensile or compression load.