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Fire & Water - Cleanup & Restoration

How Does Fire Damage Affect Concrete Structures?

10/7/2021 (Permalink)

The effects of fire damage on concrete structures will manifest itself in different forms. Understanding the manner in which fire and heat damage concrete will make it easier to assess costs and perform the necessary repairs.

What Occurs When Fire Comes into Contact with Concrete?

Concrete that comes into contact with fire will change in chemical and mechanical ways. One is called spalling, where concrete bits will be expelled from its surface layer, and external cracks which come from concrete dehydration and heat expansion.

Should the temperature of the fire exceed 500 degrees Fahrenheit (260 degrees Celsius) then the rebar may turn brittle. If the temperature increases further to 1300 degrees Fahrenheit, then the strength of the rebar will be compromised by as much as twenty percent. Unless the fire is put out quickly, the reinforced concrete may suffer structural failure due to the loss of steel tensile strength.

The chemical changes often involve the quartz aggregates growing in volume, while limestone aggregates will deteriorate. The concrete may appear discolored and will often display pink coloration due to the chemical compound changes which involve iron. Carbonation will also occur, where the cement paste will breakdown and the protective coating will fail, which leads to rebar corrosion.

Is Concrete Non-Combustible?

Concrete is often advertised as being non-combustible, and technically it is. However, being non-combustible isn’t the same as being fire proof. Many property owners are under the impression that because their building is made with masonry and steel construction, they don’t have to worry about significant damage from fire.

While these materials do offer a degree of flame resistance, fire can still cause serious and even devastating damage. The primary ingredients which are used to make concrete are aggregates and cement.  They do not light on fire like some materials nor do they produce toxic fumes.

However, concrete which is exposed to excessive heat will still sustain damage due to temperature shock along the concrete surface. Concrete will expand as its temperature rises and the external layers in particular will expand far more rapidly than the interior. This difference in expansion is not readily absorbed by concrete itself, which means that if the fire is not put out in time the layers will eventually separate and break away.

What Happens When Concrete Is Penetrated by Extreme Heat?

In the event that concrete is penetrated by extreme heat, its supporting steel will begin to weaken, which in turn will compromise structural integrity. The precise effects will depend on temperature, but at five hundred and fifty to six hundred degrees Fahrenheit, the cement will begin to lose its load bearing capacity.

This is especially dangerous on bridges and other structures because a collapse could occur which could seriously injure or kill anyone in the vicinity. This is why anyone working in a fire zone that notices cracks, spalling or exposed steel should vacate the premises immediately.

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