Understanding the fire resistance of concrete

Concrete is one of the most fire-resistant materials available to the construction industry. Under European Standards (EN 13501-1:2007-A1:2009), it’s classified as an A1 material – the highest grade of fire resistance. In this post, we consider why concrete has such a high fire resistance rating and what factors influence it.

Why is concrete resistant to fire?

There are three main reasons why concrete has been categorised as one of the most fire-resistant materials; concrete is non-combustible, it’s non-toxic and it has low thermal conductivity. This means that it does not easily transfer thermal energy, and does not react easily with other substances (meaning that in the event of a fire there are no noxious gases released). This makes concrete one of the safest and most effective materials for structural fire protection.

What affects concrete’s fire resistance?

The precise fire resistance of a particular type of concrete depends on a few different factors. Here are the main ones:

Aggregate type

The type of aggregate used in concrete can affect how effective it is preventing fire damage. There are generally three different types of aggregate used in the production of concrete, namely: carbonate, siliceous, and lightweight. Whilst all types of aggregate are effective against fire, research has shown carbonate aggregates to be most efficient; in experiments, dolomite (a calcium carbonate) provided the highest compressive and tensile strength when subjected to high temperatures.

Moisture content

The moisture content of concrete affects how it behaves in a fire. Research has shown that spalling can occur on the surface of concrete that has not dried sufficiently or has a very low water-cement ratio.

Density

The greater the density of concrete, the higher its fire resistance. A study by the University of Auckland, New Zealand, found that the fire resistance of ultra-lightweight concrete with a density of 400kgm3 was more than three times that of other concrete samples with a density of 150 kgm3.

Thickness

Generally speaking, thicker concrete performs better when exposed to fire. The American Society of Civil Engineers has laid out the minimum thickness of different types of concrete (based on aggregates used) for varying levels of fire-resistance; a siliceous aggregate of 7.0 inches thickness, for instance, can withstand fire for up to 4 hours, while a lightweight aggregate (clay, shale, or slate) only needs to be 5.1 inches in thickness to endure the same length of time.

Improving the fire resistance of concrete

Despite being one of the most fire-resistant materials, it’s still possible to improve the effectiveness of concrete in a fire by:

• Applying a heat coated in intumescent paint.
• Applying a layer of lightweight mortar to exposed concrete surfaces.
• Protecting concrete surfaces with board systems.

CLM Fireproofing is the UK’s leading passive fireproofing specialist. Using the latest technological developments in the fireproofing industry, our experts survey, install and monitor fireproofing measures that ensure 100% compliance with current regulations and minimise the risk of fire damage to buildings. For more information on our specific services, please get in contact with us.