Abstract:
Studied were the uniaxial compressive mechanical properties and damage evolution of recycled aggregate concrete (RAC) containing metakaolin (MK) at different curing ages. Prepared were RAC specimens with MK content of 0% and 15%, and obtained were the stress-strain full curves of RAC specimens at different ages. Nuclear magnetic resonance (NMR) was used to analyze the variation of porosity as well as the pore size distribution inside the RAC; scanning electron microscopy (SEM) was used to analyze the microscopic morphological characteristics of the hydration products in order to explore the microscopic mechanism of MK incorporation on the improvement of the pore structure; and acoustic emission (AE) was used to monitor the development process of microcracks in the RAC. The results showed that: with the growth of age, the hydration reaction was further sufficient, the peak stress and elastic modulus were gradually increased, the peak strain tended to decrease, the connection between the ascending section and the descending part of the stress-strain curve changed from flat to steep; and the addition of MK improved the microstructure of the specimen, of the peak stress and, of the elastic modulus and peak strain. Combined with the statistical damage theory, considering the existence of fracture and yield mesoscopic damage modes, revealed were the effects of age and MK on the mesoscopic damage mechanism of RAC by analyzing the evolution law for five characteristic parameters
E0,
εa,
εh,
εb and
H; explained from the perspective of effective stress was the phenomenon that the peak signal of AE lagged behind the peak stress. The theory of "effective stress extremum" was put forward, and the internal relationship between the mesoscopic damage evolution mechanism of concrete under complex conditions was further discussed.