Burnt-out building
However, above this temperature, significant
decreases can be anticipated.
Carbonation of concrete by attack from
carbon dioxide (CO 2 ) and carbon monoxide (CO)
caused by the fire results in the reduction of the
alkalinity of the concrete and an increase in the
risk of reinforcement corrosion. In the presence of
moisture, CO 2 forms carbonic acid (H 2 CO 3 ), which
reacts with cement hydrates, mainly portlandite
(Ca(OH) 2 ), to form calcite, (CaCO 3 ). This calcite can
alter the mineralogy of hydrated cement paste:
Ca(OH) 2 + H 2 CO 3 → CaCO 3 + 2H 2 O
In general, the effects of a high-temperature
fire on concrete components include loss of bond
between concrete and steel and possible loss of
residual strength of the steel reinforcement. It can
also have a negative impact on the mechanical
properties of the reinforcing steel. Steel’s yield
strength and modulus of elasticity decrease
with increasing temperature. Reinforced steel’s
strength and stability are affected by high
temperatures as follows:
● ●
● ●
● ●
Significant loss of strength occurs at high
temperatures (50% of the original yield
stress is lost at 550°C).
Original yield stress is almost completely
recovered on cooling from temperatures of
500-600°C for all steel.
On cooling from 800°C, yield stress is
reduced by 30% for cold-worked steel and
5% for hot-rolled steel.
How to assess the Residual Strength of
fire-damaged Reinforced Concrete?
The assessment of a fire-damaged building
usually starts with visual inspection of the
affected building, followed by non-destructive
tests to estimate the strength of existing
structural elements. In a serious condition,
destructive tests may be necessary by removing
samples of concrete and reinforcement to
ascertain the actual state of the materials’
properties.
33