The behavior of concrete in extreme fire is dependent on the composition and properties of the concrete structure. The type of aggregates used, the mix design, the moisture condition, and age are the factors that govern the behavior of concrete under extreme exposure of fire.
Here a brief explanation on the concrete behavior towards fire is provided.
Effects of Temperature on Concrete Properties
The behavior of concrete, when exposed to high temperatures, can be easily understood by finding out the effect of temperature on the concrete properties. They are:
- The fire affects the density of the concrete structure in a very small percentage. Hence there is no appreciable effect on thermal conductivity of concrete.
- A high-temperature exposure increases the specific heat by 20% which is relative to the room temperature.
- A fire temperature greater than 500-degree Celsius reduces the compressive strength of the concrete
Behavior of Concrete in High Temperature
The concrete is not expected to undergo any sort of melting under high temperatures. They are expected to bring changes in their material composition and respective properties. But under an environment of thermic lance or hydrocarbon fire exposures, the concrete has chances to melt. The behavior of concrete internal structure, reinforcement, and strength on fire is explained below.
1. Concrete Reinforcement Under Extreme Fire
The behaviour of reinforcement to fire exposure is dependent on whether it is hot rolled or cold rolled form. This is a great concern as the fire affects the tensile, yield strength and the modulus of elasticity of the reinforcement. This happens for a temperature exposure from 500-degree Celsius to 300 degrees.
This is the reason why the adequate cover is provided for concrete structures. The issue is only expected after the destruction of concrete cover. But in unexpected areas with inadequate cover, the reinforcement is affected resulting in damage and collapse.
Read More: Fire Proof Concrete
2.Moisture in Concrete Under Extreme Fire
The fire exposure initially results in the evaporation of water present in the concrete pores and structures. These are either free water or physically bound water. If the permeability is less, the heat of dissipation cannot be transferred which increases the vapor pressure at the pores.
As the vapor pressure increases, cracks start to occur resulting in explosive spalling. This explosive spalling can occur after a few minutes and rates reported ranges up to 3 mm/min for normal-weight aggregate concrete and up to 8 mm/min for lightweight aggregate concrete.
Other types of spalling such as local spalling and sloughing-off (gradual reduction of a cross-section) that have been observed in cellulosic fires are possible but explosive spalling the major one especially under hydrocarbon fire.
3. Strength of Concrete in Extreme Fire
As mentioned in the effect of fire on concrete, the strength value reduces under the rapid increase of fire. With the increase in temperature, the aggregates and the cement hydrates are adversely affected. Under extreme conditions like hydrocarbon fire, complete destruction of aggregates and cement hydrates occur.
Performance of Concrete Under Hydrocarbon Fire
The performance of concrete under hydrocarbon fire is influenced by the following factors:
- the rate of temperature rise in the concrete
- the moisture content of the concrete
- the permeability of the concrete
The hydrocarbon effect results in:
- High Amount of Spalling
- Loss of concrete section
- Loss of Strength
- Crack Formation