Degradation mechanism and evaluation of the carbonation resistance of concrete after high-temperature exposure

Abstract

High-temperature exposure during fire decreases not only the mechanical properties of concrete structures but also their durability, especially their concrete carbonation resistance. However, the degradation of concrete carbonation resistance after fire exposure remains unclear. Concrete specimens with water/cement (w/c) ratios of 0.4, 0.5, and 0.6 were fabricated to solve such problems, and high-temperature experiments were performed at 400 °C, 500 °C, and 600 °C and sustained for one hour. Then, mechanical and accelerated carbonation experiments were conducted, and the carbonation depths and pH values of concrete were tested. X-ray diffraction, thermogravimetry analysis, and mercury intrusion porosimetry experiments were also performed on concrete with and without high-temperature treatment. Results showed that high-temperature exposure caused severe degradation in concrete compressive strength, relative dynamic modulus of elasticity, and carbonation resistance, and the carbonation resistance loss of concrete was higher than its mechanical property loss. Concrete with lower w/c ratio suffered more losses in mechanical properties, whereas w/c ratio had no obvious effect on the carbonation resistance degradation of fire-damaged concrete. The key mechanism of the decline in the carbonation resistance of concrete after fire was not mainly the alkalinity drop in concrete but the considerable increase in concrete porosity and harmful pores. Instead of the traditional phenolphthalein method, a new method of residual OH- concentration of superficial concrete after accelerated carbonation was proposed to evaluate the carbonation resistance of concrete after fire. The average carbonation resistance of the concrete with the three w/c ratios was reduced by 66.3 %, 84.2 % and 95.1 % after exposure to 400 °C, 500 °C, and 600 °C, respectively.