Mechanical properties and pore structure of basalt–polypropylene fiber fly ash concrete exposed to high temperatures

The deterioration of concrete structures after fires has been widely concerned. The purpose of this paper is to study the influence of high temperature on the mechanical properties and pore structure of hybrid basalt–polypropylene fiber fly ash concrete. The mechanical properties and damage amount of hybrid basalt–polypropylene fiber concrete with different content of fly ash (FA0, FA10, FA20, and FA30) at different temperatures (20, 200, 400, 600, and 800 °C) were investigated. Additionally, the pore structure test was conducted, and the relationship between pore structure parameters and macromechanical properties was analyzed. The microstructure at different temperatures was observed by scanning electron microscopy (SEM) images. Finally, the main pore structure parameters affecting compressive strength, tensile strength, and damage amount were determined by using the gray correlation entropy method. From the results, after natural cooling at 200 °C, the compressive and tensile strength of concrete was slightly higher than that at room temperature, and it should be noticed that the FA10 concrete specimen showed better mechanical properties at different temperatures compared with other specimens. The increase in air content, spacing factor, and average chord length weakened the strength of concrete, while the increase in specific surface area had a positive impact on the compressive and tensile strength of concrete. The microstructure of FA10 at high temperature was better than that of FA0. The specific surface area was the main factor affecting the compressive and tensile strength, while the main factor affecting the damage amount was the spacing factor, followed by the average chord length.