Concrete is composed of cement, water and aggregates that undergo a chemical reaction called hydration to harden. When exposed to high temperatures in fires, concrete's properties change as it undergoes dehydration, shrinkage, and decomposition of its compounds. The temperature reached in different types of fires can vary greatly from over 1000°C for fuels like hydrogen to under 1000°C for materials like wood. The resistance of reinforced concrete in fire depends on the type of aggregates used and the concrete cover thickness over the reinforcing steel bars. Damage assessment after a fire considers the fire duration and temperatures reached, with repair methods including removing damaged layers or applying calcium silicate to strengthen the surface.

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Fire Effects on Concrete
Concrete is a building material composed of cement, water and aggregates. When mixed
together, water and cement form a paste that coats the surface of the aggregates, fills the
space among the particles and glues them together. This mixture undergoes a chemical
reaction called hydration through which the paste hardens and gains strength to form the rock-
like mass known as concrete.
During a hydration reaction calcium hydroxide Ca(OH)2 and calcium silicate hydroxide (known
also as CSH) are produced as shows the following equation:
2 Ca3SiO5 + 7 H2O → 3 CaO · 2 SiO2 · 4 H2O + 3 Ca(OH)2 + 173.6 kJ
Concrete behavior in fire
One of the advantages of concrete over other building materials (steel, wood….) is its fire-
resistive properties. It is regarded as a fireproof because of its incombustibility and its ability to
withstand high temperature without collapse. However, its properties can change dramatically
when exposed to high temperatures and many problems were experienced with concrete in
fire such as deterioration in mechanical properties.
Concrete temperatures up to 95°C have little effect on the strength and other properties of
concrete. Above this threshold cement paste undergoes shrinkage (contraction) due to
dehydration and aggregates expand due to temperature rise which results in overall expansion
of concrete and reduction in its strength.
The table below summarizes the change in concrete as function of temperature.
100 °C simple dilatation
100 - 150 °C Loss of evaporable water
150-500°C
-Contraction of cement paste after liberation of
hydration water
- above 300 °C, large reduction in density
400- 500 °C Decomposition of calcium hydroxide
Ca(OH)2 -----> CaO + H2O
500 -1300°C
Decomposition of calcium silicate hydrate (CSH)
At 500°C Reduction of 50 % of the concrete strength.
Fire Types and reached temperatures:
The temperatures reached in fires depend on the type of combustible materials.

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References available upon request
The table below shows the temperatures reached in different kinds of fires:
Fuel type Temperature reached in °C
Electrical spark 1316
Methanol 1910
Methane 1920
Butane 1977
Propane 1977
Wood or most organic material (textiles ,paper, leather) 1977
Hydrogen 2210
Acetylene 2632
TNT explosion 3400
Factors that affect the resistance of concrete in fire
The behavior of reinforced concrete (concrete with embedded steel bars) in fire depends on its
mix proportions, constituents (the type of aggregate used) and on the cover of reinforcement.
1-Aggregates used in concrete can be classified into two types: carbonate, siliceous. Carbonate
aggregates include limestone and dolomite. Siliceous aggregate include materials consisting of
silica and include granite and sandstone. The strength of concrete containing siliceous
aggregates is reduced to about 55% at 650°C .On the other hand, Concrete containing
carbonate aggregates retains most of its compressive strength up to about 650°C .
Concrete with carbonate aggregates transmits heat at a slower rate than normal weight
concrete with the same thickness, and therefore generally provides increased fire resistance.
2- Steel melts at temperatures between 1153 and 1538 °C, depending on its carbon content
.The Design for fire resistance aims to ensure an average concrete cover to the reinforcement
sufficient to keep the temperature of the reinforcement below these critical values.
During a fire, the temperature reached at the surface of concrete is greater than the
temperature at 3 cm inside the concrete mass where the reinforcement is embedded .This is
due to the slow rise of temperature in concrete .Therefore, to increase the reinforced concrete
resistance in fire , a larger cover is required .
Repair of fire damage
After a fire, an investigation of the condition of the structure must be made. Many factors are
used to assess the fire damage: The extent and the duration of the fire, the period when
failures of members are observed, and the temperature reached within the building.

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References available upon request
Determining the type of repair to undertake is based on the evaluation of the damage .The
possible ways are:
1-Removing damaged layers of concrete and replacing them with fresh concrete
2-Recurring then applying CS (Calcium Silicate). CS being a viscous liquid, is usually used on the
surface of concrete to strengthen it, repair cracks and lower the water permeability. The CS can
react with Ca(OH)2 in hardened concrete to form CSH when the water exists. The hydrate fills
the pores and cracks in the concrete. This method is efficient when the concrete was heated
below the temperature of 750°C.
Josepha Sebaaly
Civil engineer trainee