The fire resistance of concrete is well appreciable, whether it is in material form or in the structural form. Many issues due to fire risk can be reduced drastically by the application of concrete as a structural material.
This is due to the inherent properties inbuilt in concrete. This does not ask for an additional fire resistance for the concrete used.
Altogether the performance of concrete in fire is such a way that the structural integrity of the concrete is maintained, the fire resistance is not compromised and provide perfect shielding from the heat attack. This makes concrete a reliable material against fire.
Fire Resistance of Concrete Material
We cannot let the concrete set on fire or let it to get burned. This material does not have any reaction with fire to emit any dangerous or toxic fumes or gases. A higher degree of fire resistance is observed in concrete and it is a perfectly cost-effective fire resistant structural material that can be used in construction.
The reason behind this amazing property of concrete is due to the presence of main ingredients cement and aggregate combination. The combination of these materials along with water bring a mass called the “concrete” which is found to be an inert material and most importantly inert for fire and safety structural design.
In other words, the concrete material has a very lower value of thermal conductivity. This lower value makes the concrete to conduct heat very slowly behaving as a perfect shield to protect the adjacent space and the material itself from damage due to fire.
Fire Resistance of Concrete Structures
If the concrete material performs good in fire, then the concrete structures too will possess the same property. The concrete structures have a greater fire resistance mainly due to the:
- Inherent properties of the construction material i.e. the concrete
- Quality based structural design of each structural members against fire attack
- Overall design to ensure robustness
We define the fire resistance of a structural member as the ability of the member to perform as per the designed requirements for a function of time when exposed to fire attack.
Preparation of High Fire Resistance Concrete
As mentioned before, one of the main reason behind the inherent property of fire resistance for the concrete is due to the constituent materials present in it. A well-designed use of concrete constituent materials can help to bring concrete with better fire resistance.
One of the important factor concerning the concrete fire resistance is composition of aggregates. The use of special aggregates can help in increasing the fire resistance and strength of concrete.
Another means is through the use of special plastic fibers (PP). The use of plastic fibers have brought considerable increase in the fire resistance of concrete.
The resistance of the cement matrix can be increased by the use of special sands in concrete manufacture.
The production of concrete for fire resistance does not have any kind of difference from the standard concrete production. If fibers are used in concrete, the mixing has to be carried out carefully and monitored throughout.
Methods to Improve Fire Resistance of Concrete
The concrete constituent materials and their property have a greater influence in fire resistance. There were situations where the concrete temperature increased rapidly within minutes and resulted in spalling.
There are various methods by which we can improve the concrete fire resistance.
- When concrete made of ordinary Portland cement is subjected to fire greater than 300 degrees Celsius, it will lose most of its important properties. This concrete will lose its structural performance for a temperature greater than 600 degrees Celsius.
- The depth of the weakened zone of the concrete will range from small thickness from millimeters to many centimeters as the temperature of the concrete increases with fire.
- The refractory linings above the temperature of 1600 degree Celsius can be protected with the help of high alumina cement. This is observed to have higher performance in fire. An excellent resistance to fire is shown for a temperature greater than 1000 degree Celsius.
A better performance is given by aggregates that are of carbonate type like limestone, lime rock and dolomite. These tend to perform good in fire as these exposed to heat will heat up the calcine and will liberate carbon di oxide. In order to undergo such a reaction, heat is required.
Hence this reaction absorbs some heat from the exothermic energy. Those aggregates with silica have less performance in fire. Heat performance is related to the thermal conductivity of concrete. Hence light weight aggregates in concrete will bring best resistance in fire.
Spalling in concrete due to high temperatures can be reduced by the use of polymers or polypropylene monofilament fibers. This is hence a means to increase the fire resistance of the concrete.
At temperatures around 160 degrees Celsius, these polymers will melt and make channels that will let the resulting water vapors to escape. This process will help in decreasing the pore pressures and the risk of spalling is reduced.
Other alternatives used to protect the structural system from fire other than the ones used against structural failure will include:
- Using heat shields that is coated with intumescent paint
- Spray applied lightweight mortars
- The used of protective board systems
These are called as passive structural systems used for fire resistance of concrete structures.