Experimental Investigation on the Mechanical Properties of Early-Age Concrete after Heating Up to 550 °C

The early-age mechanical properties of ordinary concrete after a thermal cycle are investigated in this paper, to provide information on the residual safety of concrete structures after a fire occurred during the construction phase. The specimens are heated up to 150, 350 and 550 degrees C, from 3 to 28 days after casting, and tested in compression and in splitting tension after a rest period. The effects of curing period before heating, exposure temperature, cooling methods (in sprayed water or in air) and of the rest period prior to testing are investigated. The rest period after a thermal cycle performed at an early age brings in a marked recovery in terms of final residual compressive and tensile strengths. More than 50-60% of the loss may be recovered which is much lower than that documented in recent studies on fly-ash concrete. The lowest recovery is exhibited by the concretes exposed to high temperature at 7(th) day. Extending the rest period increases the strength recovery. For the concrete heated at 3(rd), 7(th) and 14(th) day, cooling the specimens in sprayed water lead to larger strength recoveries than in air, both the strengths immediately after cooling and that after rest periods.