CONCRETE’S EXCEPTIONAL FIRE RESISTANCE BASED ON UNIQUE PROPERTIES

Written by Jan de Beer on behalf of the Cement & Concrete SA’s School of Concrete Technology.

In the design and construction of buildings and structures, the preservation of life and prevention of injury due to fire are the most critical considerations – apart from minimising damage to the structure itself, says John Roxburgh, Technical Specialist at Cement & Concrete SA’s School of Concrete Technology.

Roxburgh says approaches taken to mitigate fires in buildings are multi-faceted, and include active measures such as smoke detectors, fire alarms, sprinkler systems, smoke extraction and evacuation protocols.  Passive measures include fire containment through fire-resistant walls, roofs, floors, and the installation of fire doors, along with clear signage to show accessible fire escape routes.

“An important component of fire engineering is to prevent the spread of fire within a building.  By containing the fire, further damage to the building is prevented while minimising the heat build-up and deadly smoke emissions. Concrete is an excellent building material to use for the containment of fire,” Roxburgh explains.

He says concrete is:

•           Non-combustible and consequently will not add to the fire load. 

•           Does not produce toxic smoke or drip hot molten material. 

•           Retains its structural strength under most fire conditions.

“Concrete also does not need any fire-protective coating and can slow down the transfer of heat in a building.   For all these reasons, it is classified as an A1 building material which represents the highest grade of fire resistance under the stringent European Standards.

“Concrete’s fire resistance is based on its unique composition and physical properties. It is made from a mixture of cement, water, and aggregates. These components combine to make a material with low thermal conductivity which allows the concrete to shield other parts of the building from the heat. Even after prolonged exposure to fire, the internal concrete temperatures can remain low to both prevent the spreading of fire and protect the steel reinforcement embedded in the concrete.”

Roxburgh says at temperatures above 300 0C, concrete will start to lose its strength. But even then high temperatures are typically localised at the surface of the concrete with the core concrete retaining its structural integrity.  “This is important to prevent sudden and catastrophic collapse. At very high temperatures, the surface of the concrete can spall, and at times this can be explosive, depending on the rate of heat build-up.  Spalling is caused by the residual water in the concrete turning to expansive steam.   To control this in areas where spalling may occur – such as in traffic tunnels – polypropylene fibres can be included in the concrete mix.  The fibres melt under high temperatures to provide space for the steam to expand into.

“Most concrete buildings are not completely destroyed by fire and, in many cases, there is no adverse effect on the structure’s load-bearing capacity.  Not having to demolish and replace the building, it is possible for repairs and reoccupation to take place quickly.  This gives obvious economic benefits to the property-owner, including reduced insurance premiums.”

Roxburgh says it is not surprising that concrete is regarded as the ideal building material for factories, warehouses, and power plants, particularly those with flammable materials or machines operating at high temperatures.

“Concrete can be manufactured to an extensive range of specifications to suit a wide variety of requirements and applications by using different mix designs or adding different materials in the concrete mix.

“Concrete offers both aesthetic appeal and economy. Its strength, durability and natural thermal mass produce structures that require low maintenance while providing high durability with economical energy efficiency. It is by far the most economical building solution based on initial cost, long-term durability, energy efficiency, low maintenance and operational costs, as well as opportunities for future modification and re-use should the occupancy of the building change,” Roxburgh adds.