The performance of Fly ash and Metakaolin concrete at elevated temperatures

Ordinary concrete is generally considered to have satisfactory fire resistance but when it comes to high strength concrete it shows extensive damage or even catastrophic failure at elevated temperatures. This research work was carried out to evaluate the performance of High Performance Concrete (HPC) made with Fly ash (FA) and Metakaolin (MK) at elevated temperatures. Variables of the test program include partial replacement of cement with MK from 5% to 20%, FA from 20% to 60%, temperatures from 27 °C to 800 °C and two types of cooling methods (in air and water). The mechanical performance was assessed from compressive strength while the durability was assessed from chloride permeability and water sorptivity tests. Mass loss at elevated temperatures was also determined. Moreover, quantitative analysis of the SEM images on selected concrete specimens was performed using Image Pro-plus software. Test results showed degradation in the mechanical and durability properties of HPC at elevated temperatures. Quick cooling produced greater loss in compressive strength than slow cooling. This is because of the effect of thermal shock which was more pronounced at 400 °C. From the standpoint of durability, all mixes showed major increase in charge pass and sorptivity values in between 400 °C and 600 °C. Therefore, 400 °C could be regarded as the critical temperature for change in the properties of HPC. Quantitative analysis of the SEM images of Interfacial Transition Zone (ITZ) indicated that pore area fraction increased with the increase in temperature. This resulted in the degradation of microstructure and affected the strength and durability of concrete. In general, at temperatures (400 °C and above) FA20 showed better performance while MK mixes (MK10 and MK20) showed higher degradation in terms of durability. This gives an indication that MK mixes should be used with care especially in structures which may be subjected to temperature of 400 °C and above.