Why is Concrete Fire-Resistant?

Concrete is the most fire-resistant material available in the construction industry. They are non-combustible, non-toxic, and have low thermal conductivity. 

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The fire-resistant properties of concrete are due to its material composition and chemical structure it possesses.

Fire-Resistance Properties of Concrete

The non-combustible, and low-thermal conductance properties of concrete that make it a fire-resistant material are explained below.

Concrete is Non-Combustible

In most applications, concrete is considered fireproof. The excellent performance of non-combustibility is due to the composition of concrete.

Concrete is composed of inert materials- cement, and aggregates that when combined together again form an inert product with relatively low thermal conductivity. This inert behavior towards fire exposure allows concrete to hold fire within the surface, without harming the concrete structure.

Under EN 13501-1:2007 -A1:2009 (Source: The Concrete Centre), concrete has the highest fire resistance to work as an engineered structure and as a material itself. 

EN 13501-1 classifies materials into seven grades (A1, A2, B, C, D, E, and F) based on the reaction of building materials toward the fire. The highest possible designation is A1 (non-combustible materials). In the code, concrete is classified as combustibility class A1.

The UK also has a National classification system, which has ‘non-combustible’, ‘limited combustibility’, Class 0, 1, 2, 3, and 4 (with the lower number indicating lower combustibility, smoke emission, or flame droplets). Concrete is considered ‘non-combustible’ in the National system and does not need any further testing or additional fire treatments.

Low Thermal Conductivity of Concrete

The thermal conductivity of concrete is one of the important thermal properties that affect heat transfer by conduction through concrete. 

Thermal conductivity is a measure of heat flow through a specific material independent of its thickness. The higher the thermal conductivity, lesser is the thermal resistance or the higher the heat flow.

Note: Thermal Conductivity is represented as 'λ' and measured in (W/mK).

Concrete has low thermal conductivity means that the effect of fire is limited to the surface zones of the concrete, with the middle of the element often unaffected. 

Concrete with low thermal conductivity reduces heat transfer and thereby energy consumption in buildings. 

The rate of transfer of heat through a concrete mass is very slow, acting like a fire shield. This property helps to maintain structural integrity despite its exposure to intense heat.

Different Fire Resistance for Concrete

Concrete with high and low thermal conductivity is designed based on the application. Hence, concrete products with various thermal conductivities can be manufactured.

The value of thermal conductivity for different concrete products is enlisted in the table below.

Type of Concrete	Thermal Conductivity in W/mK
Concrete (aerated)	0.16
Concrete (cellular 400 kg/m3)	0.1
Concrete (cellular 1200 kg/m3)	0.4
Concrete (dense)	1.4

The thermal conductivity of the concrete is influenced by density, porosity, temperature, amount or volume of materials, type of materials, water-cement ratio, micro-environment, relative humidity, and mineralogical characteristics of aggregate.

An aerated concrete or light-weight concrete has less thermal conductivity, as the structural interaction within the structure is not so strong compared to a dense normal concrete, and the tendency to create a continuous flow of heat transfer is less.

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