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Spalling is an umbrella term, covering different damage phenomena that may occur
to a concrete structure during fire. These phenomena are caused by different
mechanisms: |
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• Pore pressure rises due to evaporating water when the temperature rises;
• Compression of the heated surface due to a thermal gradient in the cross section;
• Internal cracking due to difference in thermal expansion between aggregate and
cement paste;
• Cracking due to difference in thermal expansion/deformation between concrete
and reinforcement bars;
• Strength loss due to chemical transitions during heating. |
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In different combinations of these mechanisms, possible spalling phenomena
include: |
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• Violent Spalling,
• Progressive Gradual Spalling,
• Corner Spalling
• Explosive Spalling,
• Post-Cooling Spalling. |
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Spalling of concrete during fire causes serious damage to concrete structures, with
significant economic costs and risk to human life.
New developments in concrete
technology like improved grain size distributions and the application of extra fine
particles have resulted in concrete types with improved durability, strength and
workability. However, these high performance concrete types have shown to be more
susceptible to spalling during fire than ordinary concrete types. |
The problem of
spalling in buildings has been known for decades, but also has been highlighted in
recent intense tunnel fires in Europe. As a consequence of severe damage due to
spalling and the non-operational time of tunnels after a fire, the fire resistance of
newly developed concrete types has been questioned. |
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During fire tests the observations of spalling of concrete cover a wide range, in
random order: observation of spalling with slow (1ºC/min.) or fast (250ºC/min.)
heating, from gradual to explosive spalling, cracking along or through aggregate
grains, spalling in the beginning of the fire or after some time, stopping after some
time or progressing, stopping at the reinforcement level or continuing far beyond it,
and so on.
In the paragraphs below the different observed spalling phenomena are
described and related to these mechanisms, see also Breunese and Fellinger (2003). |
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A summary of these relations is given in table 1. |
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Pore pressure
due to
evaporation of
moisture |
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Compression
due to thermal
gradient |
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Internal cracking due
to different thermal
expansion aggregate -
cement paste |
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Cracking due to
different thermal
deformation
concrete - steel |
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Strength loss
due to
chemical
transitions |
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Violent Spalling
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x |
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x |
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x |
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| Sloughing Off |
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x |
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x |
| Corner Spalling |
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x |
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| Explosive Spalling |
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x |
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x |
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| Post-Cooling Spalling |
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x |
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x |
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