Behaviour of Concrete Under Fire

The fire resistance of concrete is very high when compared with other building materials like wood and steel. Concrete is regarded as a fire proof material because of the property of incombustibility and its ability to resist collapse under high temperature.

 

The drastically high temperature to which a concrete structure exposed will bring tremendous change in the important mechanical and the structural properties of the concrete. A large failure of the structure due to fire deterioration has been observed.

Fire Resistance of Concrete

The fire-resistant property of concrete is due to the inherent properties it gets from the constituent materials, aggregates, cement, fibers etc. Their composition together will form an inert material called the "concrete", that is chemically inert in nature. This does not undergo any reaction under the fire nor result in the evolution of any toxic or poisonous gases. The concrete will burn but won;t bring any toxic gases as mentioned and to have higher structural deterioration a higher temperature susceptibility is required.

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Behavior of Concrete

 

As mentioned before, a higher temperature will bring some change in the properties of the concrete. The temperature and the constituent concrete particles are the governing factors behind the behavior of concrete structures under fire.

Until a temperature of 95-degree Celsius, there is no much change in properties of concrete. Above this mentioned threshold temperature, the cement will undergo a shrinkage or contraction. This shrinkage or contraction due to the high temperature is due to tot he dehydration. This process will make the aggregates to expand at higher temperatures which will, in turn, result in the overall expansion of the concrete. This brings strength reduction.
The variation in concrete with an increase in the temperature is shown in the Table-1 below.

Temperature Degree Celsius	Effect in Concrete
100	Simple Dilatation
100-150	Evaporable Water lost
150-500	Cement Paste Contract due to the evaporation of hydration water Above 300-degree Celsius large change in density is observed
400-500	Calcium Hydroxide is Decomposed Ca(OH)2 -------> CaO + H2O
500-1300	Calcium Silicate Hydrate is Decomposed 50% reduction in concrete strength at 500 degrees Celsius

  Table.1: Effect of Temperature in Concrete- Fire Resistance of Concrete

Factors Affecting Fire Resistance of Concrete

The fire resistance of concrete will depend on the following factors:

1. Mix Proportions
2. Constituent Materials
3. Reinforcement Cover

Effect of Water-Cement Ratio and Mix Proportion on Fire Resistance of Concrete

Studies and experiments conducted on concrete specimens say that the leaner mix will undergo a larger decrease in strength and properties under high temperatures compared to the rich mix. 

Effect of Aggregates on Fire Resistance of Concrete

The constituent materials mainly aggregate used in the manufacture of concrete have an important role in the fire resistance of concrete. The aggregates employed for concrete manufacture can either be a carbonate or siliceous type. The carbonate type of aggregate will include granite and sandstone. The siliceous aggregate material will include silica, granite, and sandstone.

At a temperature of 650-degree Celsius, the strength of concrete containing siliceous aggregate is reduced by 55%. But in the case of concrete that contains carbonate aggregates the strength is regained at 650-degree Celsius.

The concrete that consists of carbonate aggregate will undergo slower heat transfer when compared to normal weight concrete with the same thickness. This is the reason carbon aggregates in concrete gives an increased fire resistance for the concrete.

Effect of Reinforcement Cover

Most of the structures exposed to fire attack are reinforced concrete made. The steel used and their arrangement in concrete have an important role in the fire resistance of the concrete structure. The steel will undergo melting when subjected to a temperature between 1153 and 1538 degree Celsius. This will vary based on the carbon content in the steel.
When a reinforced concrete is designed, provision for adequate cover for fire resistance is recommended. This is to keep the temperature of the reinforcement below their critical values.
When a concrete structure is subjected to fire, the temperature at the surface of the concrete is greater than the temperature at a point that is 3cm inside the concrete mass (measured from the surface). This is the location where the reinforcement is placed. This lower value within concrete is because of the low thermal conductivity of the material. Hence it is recommended to have a larger cover while having a special design for fire resistance.

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