Effect of Mild Temperature Exposure on the Compressive Strength of Grade 20 Concrete: An Experimental Investigation for Tropical Construction Applications

Concrete elements in tropical environments are frequently subjected to sustained mild temperatures arising from solar radiation, proximity to heat-generating equipment, and bush fires, yet the thermal response of the low-grade concretes most used in such regions remains poorly documented. This study investigated the residual compressive strength of Grade 20 concrete after exposure to mild temperatures of 40, 60, 80, 100 and 120 °C. Seventy-two 100 mm cube specimens were produced from a 1:2:4 mix with a water/cement ratio of 0.6, water-cured for 7, 14, 21 or 28 days, heated for one hour at the target temperature, air-cooled for 24 hours, and tested in uniaxial compression against unheated control specimens. At early ages (7 and 14 days), exposure up to 100 °C increased strength by as much as 16.4 %, an effect attributed to accelerated hydration and free-water loss. This benefit disappeared with maturity: at 21- and 28-days strength decreased monotonically with temperature, the largest loss (23.3 %) occurring at 21 days and 120 °C. At the design age of 28 days, reductions were 1.6, 7.9, 10.1, 11.1 and 16.4 % at 40–120 °C respectively, and the residual strength at 120 °C (26.3 N/mm²) still exceeded the characteristic strength of 20 N/mm². Grade 20 concrete therefore exhibits acceptable thermal tolerance within the mild-temperature range, supporting its continued use in warm climates and providing baseline data for assessing low-grade concrete after mild thermal events.