Effect of microstructure evolution on mechanical behaviour of concrete after high temperatures

This paper examines the mechanical behaviour of concrete following exposure to elevated temperature up to 800 degrees C through experimental investigation, with emphasis on the effect of microstructure evolution on the deterioration of concrete compressive strength. First, a specially built electrical furnace is employed for the application of high temperature to 21 concrete cylinders before static compression tests. The static stress-strain relationships of the concrete cylinders are presented and compared. Furthermore, a series of experimental techniques are employed to examine the development of concrete microstructure, including thermogravimetric analysis, scanning electron microscope test and mercury intrusion porosimetry investigation. The inherent reason for concrete strength deterioration following elevated temperatures is revealed to be the combined action of decomposition of concrete hydration products, deterioration of the cement paste-aggregate bond, crack development and an increase in porosity. Meanwhile, based on comparison with existing data, the effect of water/cement (w/c) ratio on the mass loss and porosity increase of hydrated cement paste following high temperatures is preliminarily examined. It is found that higher w/c ratio resulted in larger total porosity, and the increase of total porosity as well as the large proportion of harmful pores generally led to the degradation of the concrete's compressive strength.