Qualitative and quantitative analysis and identification of flaws in the microstructure of fly ash and metakaolin blended high performance concrete after exposure to elevated temperatures

Abstract

The extensive use of eco-friendly materials in concrete has led to the demand to fully understand the effect of fire on concrete. This research was carried out to evaluate high performance concrete (HPC) made with fly ash and metakaolin with replacement level of 20 percent by weight of cement after elevated temperatures exposure (200°C, 400°C, 600°C and 800°C). The mechanical performance was assessed from compressive strength while the durability was assessed from chloride permeability and water sorptivity test. Qualitative analysis of the microstructure of heated and unheated concrete was performed by SEM while quantitative analysis was performed on SEM images using Image Pro-plus software. Based on the qualitative and quantitative analysis of SEM images, the distribution of the number, type and surface area fraction of flaws were identified and changes in the structure of Interfacial Transition Zone (ITZ) were classified into different temperature ranges.Test results show that for all mixes post-elevated temperature compressive strength decreased while charge passed and sorptivity values increased with the increase in temperature from 27°C to 800°C. For all mixes, major strength and durability loss occurred after 400°C. Therefore, 400°C can be considered as critical temperature from the standpoint of strength and durability loss. From the qualitative and quantitative analysis, different types of flaws were identified. These were texture flaws (T), orientation flaws (O) collectively called as textured and orientation flaws (TO) and local flaws (L). The post-elevated temperature surface area fraction of TO and local flaws in the ITZ of each concrete mix continuously increased with the increase of elevated temperature. The increase in surface area fraction of flaws resulted in gradual loss in the mechanical and durability properties of concrete. Major increase in surface area fraction occurred in between 400 and 600°C, resulting in major strength loss and sharp increase in charged passed and sorptivity values through concrete specimens. Therefore, 400°C can be regarded as critical for change in the properties of concrete. No specific relationship between the number of TO flaws and the type of binding material in concrete and the temperature was found. However for local flaws, in general, the number increased with the increase of temperature. Also, the changes in the structure of ITZ were classified into three temperature ranges namely; the low range temperatures (27–200°C), the medium range temperatures (200–400°C) and the high range temperatures (400–800°C). The physical character of ITZ in HPC changes gradually from a discrete or discontinuous flaw zone at normal or mildly elevated temperature to a continuous and highly porous flaw zone at elevated temperatures. Thus, the classification signifies the effect of the texture of coarse aggregate and the orientation of fine aggregate in concrete matrix coupled with the effect of elevated temperatures on ITZ of HPC.