Behaviour and structural design of concrete structures exposed to fire
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
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