Influence of cooling methods on the mechanical properties of OPC and OPC/slag concretes after exposure to elevated temperatures

The present work investigates the residual mechanical properties of OPC and OPC/slag concretes, exposed to either 400 or 800ºC followed by furnace or water cooling. The heat transfer from surface to centre of the specimens maintained similar trend for all blends for both 400 and 800ºC. After 400ºC, OPC and OPC/slag water cooled specimens presented a further 20% loss in the residual strength when compared with furnace cooled specimens. This additional strength decrease of 20% is possibly due to thermal stresses caused by the thermal shock of the concrete with water at 20ºC. After 800ºC, all OPC/slag water cooled specimens presented further strength loss of 5% when compared with furnace cooled specimens. However, water cooled OPC specimens presented strength loss of 14% when compared with furnace cooled specimens, and therefore, thermal stresses cannot solely be the cause of this strength loss. At 800ºC dehydration of Ca(OH)2 into CaO would have taken place, followed by rehydration of CaO after cooling. Infrared spectroscopy showed that the peak intensity of Ca(OH)2 is much higher for OPC cooled with water than for furnace cooled, suggesting rehydration of CaO is accelerated with water cooling, resulting in greater strength loss for OPC concrete exposed to 800ºC.