Strengthening effects of cyclic load on rock and concrete based on experimental study

Abstract Cyclic load affects the mechanic behavior and stability of rock mass. Although cyclic load effect has been extensively studied for yielding stage, it is somewhat unclear in early stage of rock stress-strain curves. This paper describes an experimental study, carried out cyclic load test in elastic deformation phase, on sandstone, limestone, and concrete specimens, to research the mechanical behavior. The result showed strengthening effect of cyclic loads in elastic deformation stage within the first 100 cycles of the cyclic loads. The irreversible deformation under cyclic loads displayed similarity to creep curves, and it increased along with cyclic unloading stress level. Further analysis on cyclic times and unloading stress level demonstrated that there's threshold of strengthening effects under cyclic loads in elastic deformation stage, which could be developed in two aspects: (1) Strength increased with cyclic times until it reached an optimal cycling time then it decreased, and (2) the strength initially increased with unloading stress level then it decreased when the optimal unloading stress level was attained. The optimal cyclic times and optimal unloading stress were calculated according to quadratic function. The result showed that the optimal cyclic times increased with the decrease of unloading stress level. The optimal cyclic unloading stress level is about 30% of elastic limitation stress under same cycling times. Different specimens showed different strengthening potential under uniaxial cycling load since the initial porosity of specimens are different. The conclusion offered by this work would be beneficial to decisions of geoengineering and constructions.