Actions during service 129
presence of water will result in additional strength loss due to the hydration
of free lime, with a volume increase as a result. On the other hand, some
chemical bonds might be restored due to rehydration of gel and unhydrated
cement particles, which could lead to the partial recovery of strength compared to the strength at high temperature.
It is, however, very difficult to predict in a general way the residual strength of fire-damaged concrete structures. A detailed damage
assessment will have to be performed case by case. This can consist of
non-destructive strength evaluation on site (e.g. rebound hammer) or laboratory tests on drilled cores. New diagnosis techniques based on colour
changes of the cement paste during fire have been proposed recently by
Annerel (2010).
5.2.5.3.6 Structural behaviour
When a structure undergoes temperature changes, deformations will be
induced. As long as the imposed deformations can occur freely, no stresses
will be induced. However, in general, the thermal deformations will typically be restrained in two different ways:
• Internal restraint: When a temperature gradient exists over a cross
section, strains have to remain compatible. This will lead to partial
restraint of the free deformation of adjacent fibres of a section called
internal restraint. This is similar to what happens in the case of thermal stresses in hardening massive elements due to the heat of hydration (see Section 4.4 of Chapter 4).
• External restraint: The thermal deformation of structural elements
undergoing a temperature change will be (partially) restrained by
adjacent elements or other structures which are not exposed to the
same change.
Due to the (partial) restraint of thermal deformation, either internal or
external restraints or a combination of both, stresses will be induced in the
structure. This is also valid in case of fire-exposed structures or structural
elements. Due to a high temperature increase, stresses can become very
high. Consider as an example a temperature increase of 100°C and a coefficient of thermal expansion of 10 −5 /°C. When a concrete element having a
Young’s modulus of 37000 N/mm² is fully restrained, a stress of 37 N/mm²
results. Although this stress level is high enough to cause damage, it has to
be well interpreted considering a few other phenomena:
• Upon heating a loaded structure, an additional strain called transient
strain, is introduced. This transient strain will help relax the stress
levels (Annerel 2010).
presence of water will result in additional strength loss due to the hydration
of free lime, with a volume increase as a result. On the other hand, some
chemical bonds might be restored due to rehydration of gel and unhydrated
cement particles, which could lead to the partial recovery of strength compared to the strength at high temperature.
It is, however, very difficult to predict in a general way the residual strength of fire-damaged concrete structures. A detailed damage
assessment will have to be performed case by case. This can consist of
non-destructive strength evaluation on site (e.g. rebound hammer) or laboratory tests on drilled cores. New diagnosis techniques based on colour
changes of the cement paste during fire have been proposed recently by
Annerel (2010).
5.2.5.3.6 Structural behaviour
When a structure undergoes temperature changes, deformations will be
induced. As long as the imposed deformations can occur freely, no stresses
will be induced. However, in general, the thermal deformations will typically be restrained in two different ways:
• Internal restraint: When a temperature gradient exists over a cross
section, strains have to remain compatible. This will lead to partial
restraint of the free deformation of adjacent fibres of a section called
internal restraint. This is similar to what happens in the case of thermal stresses in hardening massive elements due to the heat of hydration (see Section 4.4 of Chapter 4).
• External restraint: The thermal deformation of structural elements
undergoing a temperature change will be (partially) restrained by
adjacent elements or other structures which are not exposed to the
same change.
Due to the (partial) restraint of thermal deformation, either internal or
external restraints or a combination of both, stresses will be induced in the
structure. This is also valid in case of fire-exposed structures or structural
elements. Due to a high temperature increase, stresses can become very
high. Consider as an example a temperature increase of 100°C and a coefficient of thermal expansion of 10 −5 /°C. When a concrete element having a
Young’s modulus of 37000 N/mm² is fully restrained, a stress of 37 N/mm²
results. Although this stress level is high enough to cause damage, it has to
be well interpreted considering a few other phenomena:
• Upon heating a loaded structure, an additional strain called transient
strain, is introduced. This transient strain will help relax the stress
levels (Annerel 2010).