The effect of grain size in rock-like materials on the strength and cracking process under uniaxial compressive stress

Abstract

Considering the importance of using tunnels and caverns created in rock masses under high stresses, it is necessary to study the crack development which leads to failure. There are various methods to study the mechanism of crack development in rock and rock-like materials. In recent years, the use of compressional wave velocity due to its unique and non-destructive features has been considered. In this research, concrete samples used as rock-like material were made with different mix designs, which included a discontinuous aggregate gradation ranges of 1.18-2.38 mm, 2.38-4.75 mm, 4.75-9.5 mm, 9.5-12.5 mm and a continuous gradation range with the maximum aggregate size of 12.5 mm and by loading in four stress levels of 40%, 60%, 80% and 90% of the strength of the samples, the effect of loading, and the growth and development trend of the crack in samples using the compressional wave velocity have been studied. The results indicated that by loading the samples up to 40% of peak stress, due to the closure of small cracks in the samples and the increase in the density of the samples, the compressional wave velocity increased. By increasing the loads up to 90% of strength, the compressional wave velocity decreases by about 10%. Then, by conducting experiments on samples with different ranges of gradation, the effect of aggregate dimensions on the growth and development of the crack using the compressional velocity was analyzed. The results show that by increasing the grain size, due to the consistency of the water-to-cement ratio and the aggregate volume to the volume of cement mortar, the porosity of the samples with larger granis increased more than the smaller ones. This factor has led to a decrease in the uniaxial compressive strength and also an increase in the specific weight by increasing the grain size which indicates that the specific weight of the coarse aggregates is more than fine ones. This issue causes that the increasing of specific weight as a result of grain size increase negates the porosity and compressional wave velocity tends to increase.