Inflence of the cooling process on the residual mechanical properties of ordinary concrete

The concrete structures have usually good behavior in fire. However, when the fire is severe and of long duration the concrete elements may lose partial or totally their load bearing capacity. This phenomenon can be aggravated by the extinction process of the fire used that causes quite always high thermal stresses and cracking in concrete. With the aim to clarify the effect of the cooling process on the mechanical properties of the concrete a research program was carried out in the University of Coimbra. The concrete specimens were heated up to different temperature levels (300, 500 and 700ºC) and after a period of stabilization they were cooled down. Two processes of cooling were tested, cooling on the air (intending to simulate the natural extinction of the fire) and cooling by water jet (intending to simulate the action of the firemen in fire combat). During the heating / cooling process the specimens were subjected to a constant compressive load (0.3fcd and 0.7fcd) in way to simulate the real situation of the concrete when in a structural member. The specimens after the heating / cooling process and at room temperature were then tested in compression, allowing determining the residual compression strength of the concrete