Concrete has an excellent intrinsic behaviour when exposed to fire, especially when compared to other building materials. However, its fire resistance should not be taken for granted and a proper structural fire design is certainly necessary.
This design is based on the understanding of both the material and the structural behaviour of concrete exposed to fire. A number of complex physicochemical reactions occur when concrete is heated, causing mechanical properties as strength and stiffness to deteriorate.
Furthermore, the phenomenon of spalling causes pieces of concrete to break off from the surface, reducing the cross-section of an element and possibly exposing the reinforcing to the high temperatures.
Spalling can be highly dangerous and is most common in high strength concrete. However, its mechanism is still not fully understood. The Eurocode provides a number of procedures in order to design concrete structures for the fire situation, both prescriptive as performance based.
However, of the latter, only the basic principles are given and several gaps still need to be filled through research. Thus in practical design, either tabulated data or a simplified calculation method is used. In many cases, these design methods fail to predict the true behaviour of concrete structures in real fires.
Firstly, the standard heating curve is not able to represent the wide variety of realistic fires. Furthermore, design should investigate the behaviour of the complete structure, including alternative failure modes, whereas member analysis ignores effects as incompatible thermal expansions which can cause high thermal stresses.
Although a lot of research has been performed already, more in-depth study is needed. Several elements of the behaviour of heated concrete still need to be researched. A systematic study of the effects of realistic thermal exposures is needed and a lot more work is required in order to unravel the mystery of spalling.
The study of the response of complete concrete structures presents another challenge, requiring large-scale fire tests. The goal is to develop a concrete model that reflects the true behaviour of concrete structures exposed to fire. This model should incorporate the fully coupled hygro-thermal-mechanical behaviour combined with a sophisticated structural analysis, including the effect of transient strain.
Author: De Wit, Annelies