This week’s highway structure collapse in Atlanta should highlight the concept that concrete and masonry may be “non-combustible”, but it cannot be considered completely fire-proof. Often, I run into individuals who believe that because their building is of steel or masonry construction, this means it is not particularly susceptible to heavy damage from a fire. Certainly there is an element of fire-resistance in these cases, simply from the inability of the material to burn and thus the overall reduction of fire load. However, just because the structural components themselves do not specifically contribute to the fire load, this does not mean that exposure to extreme heat from a fire will not still cause serious or even catastrophic damage.
Concrete’s core materials (i.e. cement and aggregates) chemically combine to form a material that is essentially inert. As noted in the publication “Concrete and Fire” produced by The Concrete Centre, “Concrete does not burn – it cannot be ‘set on fire’ like other materials in a building and it does not emit any toxic fumes when affected by fire. It will also not produce smoke or drip molten particles, unlike some plastics and metals, so it does not add to the fire load.”
More importantly for fire safety design, concrete has a relatively poor thermal conductivity. This slow rate of heat transfer enables concrete to act as an effective fire barrier between adjacent spaces.
“The rate of increase of temperature through the cross section of a concrete element is relatively slow and so internal zones do not reach the same high temperatures as a surface exposed to flames,” the publication continues.
When a fire exposes concrete to high heat, extensive damage can occur because of the temperature shock to the material. Basically, like most any other material, concrete expands as it’s heated. When extreme heat is applied, the outer layers will expand much more quickly than the inner sections. This “differential expansion” is not easily absorbed by the concrete material, which can cause the layers to separate and eventually break away. The same effect can occur in reverse when a fire is doused by hose streams or automatic sprinkler systems. In this case, as the hot concrete cools suddenly, the outer layer may shrink at a different rate and break away.
Additionally, if extreme heat penetrates concrete enough to weaken the steel reinforcement components within the concrete, the structural integrity of the entire structure could be adversely affected.
Graphic by The Concrete Centre
While concrete structures like the Atlanta highway bridge are certainly resistive to fire damage, you must always consider the effects of any fire where concrete or masonry is exposed to high heat. In this case, the effects of heat during an ongoing incident could have caused serious injuries or death to responders or civilians who had not maintained adequate distance from the area of collapse.
If at any point during a fire situation you observe severe spalling, formation of obvious cracks, exposed structural steel, or deflection of the structure, you must take immediate precautions to remove all personnel from the collapse area.