126 Damage to concrete structures
to the much higher thermal conductivity. This can lead to another problem
of thermal incompatibility, now between the concrete and the steel cage.
Due to its higher temperature, the steel cage wants to expand more than the
concrete in the core. As a result, as illustrated in Figure 5.17 for the case of
a reinforced concrete column exposed to fire conditions, the concrete cover
can be pushed off over a larger area. Figure 5.18 shows the example of a
real structure suffering from this phenomenon.
5.2.5.3.4 Explosive spalling
A more important problem, especially in the case of high-strength concrete,
is the occurrence of explosive spalling when exposed to rapid heating. Due
to the very low permeability of high-strength concrete, water vapour cannot
easily escape from the pores. As a result, a pore pressure builds up in the
cement paste. Upon further heating, the pore pressure can reach high levels,
causing important internal stresses near the concrete cover. This can result
in sudden explosive spalling. According to Khoury (2000), explosive spalling generally occurs under the combined action of pore pressure, compression in the exposed surface region (induced by thermal stresses and external
loading), and internal cracking. Due to the mutual interaction between pore
pressure and internal cracking, ongoing research focuses on the coupling of
a fracture mechanics model and a pore pressure approach in order to better
understand and predict fire spalling behaviour (Lottman et al. 2011).
Spalling can be limited to the pushing off of small pieces of the concrete
cover or the formation of small craters in the surface. Nevertheless, in some
cases fire spalling can lead to structural failure because the loss of larger
concrete layers can severely reduce the structural load-bearing capacity.
Figure 5.17 Thermal incompatibility between concrete and steel cage due to faster heating of the steel.
to the much higher thermal conductivity. This can lead to another problem
of thermal incompatibility, now between the concrete and the steel cage.
Due to its higher temperature, the steel cage wants to expand more than the
concrete in the core. As a result, as illustrated in Figure 5.17 for the case of
a reinforced concrete column exposed to fire conditions, the concrete cover
can be pushed off over a larger area. Figure 5.18 shows the example of a
real structure suffering from this phenomenon.
5.2.5.3.4 Explosive spalling
A more important problem, especially in the case of high-strength concrete,
is the occurrence of explosive spalling when exposed to rapid heating. Due
to the very low permeability of high-strength concrete, water vapour cannot
easily escape from the pores. As a result, a pore pressure builds up in the
cement paste. Upon further heating, the pore pressure can reach high levels,
causing important internal stresses near the concrete cover. This can result
in sudden explosive spalling. According to Khoury (2000), explosive spalling generally occurs under the combined action of pore pressure, compression in the exposed surface region (induced by thermal stresses and external
loading), and internal cracking. Due to the mutual interaction between pore
pressure and internal cracking, ongoing research focuses on the coupling of
a fracture mechanics model and a pore pressure approach in order to better
understand and predict fire spalling behaviour (Lottman et al. 2011).
Spalling can be limited to the pushing off of small pieces of the concrete
cover or the formation of small craters in the surface. Nevertheless, in some
cases fire spalling can lead to structural failure because the loss of larger
concrete layers can severely reduce the structural load-bearing capacity.
Figure 5.17 Thermal incompatibility between concrete and steel cage due to faster heating of the steel.