Evaluation of fly ash and Metakaolin concrete at elevated temperatures through stiffness damage test

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 temperature through Stiffness Damage Test (SDT). Variables of the test program include partial replacement of cement with MK (10% and 20%) and FA (20% and 40%) and temperature from 27 to 400 °C. To quantify the damage, chord loading modulus, unloading stiffness, plastic strain, damage index and non-linearity index were evaluated from SDT. Correlation among SDT parameters and in between SDT parameters and elevated temperature were also studied. According to the test results, the SDT parameters showed that the stiffness and elasticity decreased and damage increased with the increase in temperature. The stiffness changes were evident from chord modulus and unloading stiffness, elasticity changes from plastic strain and damage to concrete from damage index. For all mixes, significant change in the SDT parameters occurred at 300 °C. Therefore, 300 °C can be considered as critical temperature. At elevated temperatures (300 °C and 400 °C), FA mixes showed lower values of plastic strain and damage index than MK mixes. Thus, it may feasible to increase the allowable working temperature for FA mixes. The SDT parameters were found to be sensitive to elevated temperatures caused by changes in the microstructure of concrete. Test results also revealed that plastic strain and damage index correlate well with loading elastic modulus with values of coefficient of correlation equal to 0.9657 and 0.9835 respectively. Therefore, plastic strain and damage index measured from SDT can be used to estimate the percentage residual chord loading modulus of concrete affected by fire. Strong correlation also exists between PS and DI with coefficient of correlation equal to 0.985.