Experimental Investigation on the Influence of Post-Heating Cooling Regimes on the Mechanical and Durability Characteristics of Fly Ash Concrete Subjected to Sustained Elevated Temperatures

1. The base concrete proportion of 1:2.5:4.0 with a water-cement ratio of 0.45 produced a reported slump of 60 mm. 2. Fly ash replacement influenced mechanical performance at ambient and elevated temperatures, with 25% replacement providing the most favourable overall response within the investigated range. 3. Sustained elevated temperature caused progressive deterioration in compressive, split tensile and flexural strength, becoming pronounced at approximately 600°C and severe at 800°C and 1000°C. 4. The 25% fly ash mixture retained comparatively high residual compressive strength at 200°C and 400°C under gradual cooling. 5. Gradual cooling generally produced the highest residual strength, intermittent cooling produced intermediate performance and sudden cooling produced the greatest deterioration. 6. The stronger deterioration under sudden cooling is associated with thermal shock and severe thermal gradients that can induce additional cracking and spalling. 7. Post-heating cooling history should be considered during residual assessment of fire-exposed concrete; maximum temperature alone does not completely define final material condition.

[1] Krzemien K., Hager I., “Post-fire assessment of mechanical properties of concrete with the use of the impact-echo method.,” Construction and Building Materials, 2015;96:155–163. [2] Akcaozoglu K., “Microstructural examination of concrete exposed to elevated temperature by using plane-polarized transmitted light method.,” Construction and Building Materials, 2013;48:772–779. [3] Kodur V.K.R., Naser M.Z., “Structural fire engineering.,” McGraw-Hill Professional, 2020. [4] Sarshar R., Khoury G.A., “Material and environmental factors influencing the compressive strength of unsealed cement paste and concrete at high temperatures.,” Magazine of Concrete Research, 1993;45:51–61. [5] Poon C., Azhar S., Anson M., Wong Y.L., “Comparison of the strength and durability performance of normal-and high-strength pozzolanic concretes at elevated temperatures.,” Cement and Concrete Research, 2001;31:1291–1300. [6] Annerel E., “Assessment of the residual strength of concrete structures after fire exposure.,” PhD thesis, Ghent University, 2010. [7] Czoboly O., Lublóy É., Hlavička V., et al., “Fibers and fiber cocktails to improve fire resistance of concrete.,” Journal of Thermal Analysis and Calorimetry, 2017;128:1453–1461. [8] Peng G.F., Bian S.H., Guo Z.Q., Zhao J., Peng X.L., Jiang Y.C., “Effect of thermal shock due to rapid cooling on residual mechanical properties of fiber concrete exposed to high temperatures.,” Construction and Building Materials, 2008;22:948–955. [9] Yermak N., Pliya P., Beaucour A.L., Simon A., Noumowé A., “Influence of steel and/or polypropylene fibres on the behaviour of concrete at high temperature: spalling, transfer and mechanical properties.,” Construction and Building Materials, 2017;132:240–250. [10] Ergun A., Kurklu G., Serhat B.M., Mansour M.Y., “The effect of cement dosage on mechanical properties of concrete exposed to high temperatures.,” Fire Safety Journal, 2013;55:160–167.