Influence of initial damage on degradation and deterioration of concrete under sulfate attack

The variation of properties of concrete with initial damage under sulfate and wet-dry-cycle environments was experimentally investigated. With increased corrosion time, changes in the parameters of concrete with initial damage were analyzed, including mass, ultrasonic velocity, compressive strength, stress-strain curves, and the activities of acoustic emission under uniaxial compression. A quantitative evaluation and parameter fitting for corrosion damage was made based on damage mechanical theory. Based on the acoustic emission characteristics, a damage model of corroded concrete with initial damage was established and its damage evolution was analyzed. Using environmental scanning electron microscopy (ESEM) and energy-dispersive X-ray analysis (EDX), the damage mechanisms were revealed in observations of the microstructures and element compositions of concretes with initial damage induced by sulfate corrosion. The research results show that with increased erosion time, the mass, ultrasonic velocity, and compressive strength of concrete with different degrees of initial damage first increase and then decrease. Increases in the degree of initial damage may accelerate the degradation of physical and mechanical properties, but a threshold effect exists. Compressive strength and ultrasonic velocity can be regarded as damage variables, and corrosion damage evolution equations and the functional relationship between different damage formulas were established. With increased erosion time, the stress-strain curve of concrete with initial damage is obviously concave-upward and the elastic-stage time period can be comparatively shorter whereas the yield-stage time period increases. An increase in the degree of initial damage results in an obvious attenuation of the acoustic emission activity, which lags behind the time of the obvious acoustic emission response. Based on the acoustic emission characteristics, the damage evolution process of concrete with initial damage can be divided into three stages, including the compaction, stable damage evolution and development, and accelerating damage development stages. With an increase in the degree of initial damage, stronger corrosion reactions and composition changes occur inside the concrete. Compared to undamaged concrete, the expansion and extension of denser and deeper micro-cracks occur in concrete with initial damage under sulfate attack, which accelerates the degradation of the physical and mechanical properties of corroded concrete.