Ultrasonic testing and microscopic analysis on concrete under sulfate attack and cyclic environment

Abstract

The damage process of concrete exposed to sodium sulfate attack and drying-wetting cycles was investigated. The water to binder (W/B) ratio and the concentration of sulfate solution were taken as variable parameters. Through the experiment, visual change, relative dynamic modulus of elasticity (RDME) and the surface damage layer thickness of concrete were measured. Furthermore, SEM and thermal analysis were used to investigate the changing of microstructure and corrosion products of concrete. The test results show that the ultrasonic velocity is related to the damage layer of concrete. It approves that an increase in damage layer thickness reduces the compactness and the ultrasonic velocity. The deterioration degree of concrete could be estimated effectively by measuring the surface damage layer and the RDME of concrete. It is also found that the content of gypsum in concrete is less than that of ettringite in test, and some gypsum is checked only after a certain corrosion extent. When the concrete is with high W/B ratio or exposed to high concentration of sulfate solution, the content of ettringite first increases and then decreases with corrosion time. However, the content of gypsum increases at a steady rate. The content of corrosion products does not correspond well with the observations of RDME change, and extensive amount of corrosion products can be formed before obvious damage occurs.

Keywords

sulfate attack / drying-wetting cycles / damage layer thickness / thermal analysis

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