Effect of aggregate size on the residual strength of rapidly and normally cooled concrete after fire exposure

Nowadays fire accidents are very common and occur recurrently almost everywhere in Bangladesh. Burnt structures are reused, demolished and replaced by new ones after the fire events. In Bangladesh people reuse the burnt structures without knowing the residual capacity of the structures. This experimental study aims to know about the residual strength of fire burnt structures which is very much crucial for the reusability of the burnt structures. In this study, tests were done on cylindrical concrete specimens after heating them in the furnace for couple of hours with an elevated temperature of about 600°C. Cylindrical specimens were cooled down in two processes – i) rapid cooling with water spray and ii) normal cooling without water spray. Cylindrical concrete specimens (100 mm diameter × 200 mm height) were prepared with 20 mm, 12.5 mm, 10 mm uniform aggregate sizes to achieve a target design strength of 20 MPa at 28 days. Afterwards the compressive strength and splitting tensile strength tests of these burnt specimens were performed and compared with the 90 days strength of the unburnt reference specimens. For rapidly cooled specimens, it was found that for the uniform gradation of 20 mm, 12.5 mm & 10 mm sizes of aggregate, concrete specimens retained 32.3%, 53.8% & 43.2% residual compressive strength and 52.5%, 51.8.2% & 53.6% residual splitting tensile strength respectively. On the other hand, the normally cooled specimens retained 47%, 64% and 53% residual compressive strength and 50%, 52% and 53% residual splitting tensile strength respectively. From this experiment, it was also observed that the normal cooling process could retain more residual strength than rapid cooling. Moreover, the 12.5 mm uniform graded concrete specimens showed better results than the other two sizes. In conclusion it can be said that reusability of fire burnt structures may be possible to some extent after considering the type of fire, duration, process of cooling and gradation of aggregates.