Heating rate effects on the air-void network in mortars exposed to high temperatures

Abstract

This paper examines the effect of heating rate on the microstructure of cement-based systems exposed to sustained elevated temperatures. Besides a conventional mortar designed to attain a compressive strength of 30 MPa at 28 d, a high-strength mortar was also designed to achieve a corresponding strength of 90 MPa. Samples were extracted for testing under three different heating rates, ranging from 1°C/min to 10°C/min, to a sustained elevated temperature up to 400°C. This study finds that the microstructure of high-strength mortar is more sensitive to the heating rate in comparison with that of the normal-strength mortar. The porosity of the mortar mixtures uniformly increases with an increase in the temperature of exposure. However, there was a decrease in the porosity, at any sustained elevated temperature, with an increase in the heating rate. It is seen that the fractal dimension of the pores increases with an increase in the sustained temperature. Additionally, this fractal dimension was seen to increase with an increase in the heating rate. However, this trend was more pronounced at lower temperatures of exposure, so that the rate of heating appears to become less conspicuous at higher temperatures of soaking.