Study on mechanical properties and damage mechanism of recycled concrete containing silica fume in freeze-thaw environment

In order to study the uniaxial compressive mechanical properties and damage evolution of recycled aggregate concrete (RAC) containing silica fume (SF) under freeze-thaw cycles, the RAC with SF replaced partial cement was prepared. The replacement rates were 0% and 9%. The RAC were subjected to 0, 50, 75, 100, 125 freeze-thaw cycles and uniaxial compression tests. The microstructure, micro-crack propagation process and reaction products were observed by acoustic emission (AE) and scanning electron microscope (SEM). The statistical damage theory was used to explore the mesoscopic damage evolution mechanism of RAC. The results showed that the peak stress and modulus of elasticity of the specimen tended to decrease, the peak strain and mass loss rate gradually increased with the increase of the number of freeze-thaw cycles(N). The compressive strength of RAC with S = 0% decreases from -41.52MPa to -11.23MPa when N increases from 0 to 125. The addition of SF produced additional volcanic ash reactions, resulting in an increase in ettringite (AFt), which reduces the porosity of RAC and is able to increase the compressive strength at different N significantly. Compressive strength increases by 21.84% and 12.06% at N = 0 and 125, respectively. Considering the existence of two mesoscopic damage modes of fracture and yield, the deformation failure mechanism of RAC was analyzed from the perspective of effective stress. The all five characteristic parameters characterizing the mesoscopic damage show significant regular change with increase of the number of cycles. The relationship between the macroscopic mechanical properties of RAC and the mesoscopic damage mechanism was established by analyzing the evolution law for the characteristic parameters.