Commercial Construction Considerations: Concrete Under Fire

March 27, 2008
Firefighters think that concrete will be unaffected by fire, and in most cases, that is far from the truth

Concrete is a material that is widely used in the construction industry for an expansive variety of purposes. It is found in everything from roads and sidewalks, columns and beams, to fireproofing.

Concrete is all too often misunderstood. Many firefighters think that concrete will be unaffected by fire, and in most cases, that is far from the truth. Firefighters most often see concrete as a component of the building's structure.

As we discussed in the first article, the structural component of a building is the component that keeps it standing. In my opinion, knowing how the building structure will react to fire is the most important piece of knowledge that any firefighter can possess.

Concrete has been used for thousands of years due to its high compressive strength. Most of us will see concrete used as the foundation of buildings because it has the ability to withstand the large loads imposed upon it by the weight of the building.

What is Cement?
Concrete is often confused with cement, a component of concrete. Concrete is a mixture of cement - with Portland cement being the most popular - water and aggregates. The aggregates are generally sand, crushed stone, and gravel, however recycled construction materials, and fly ash are commonly used too. These components are the basic parts of concrete and are mixed in different ratios based on the usage.

In addition to the three basic components, several other chemicals can be added to concrete depending on the type of pour and the weather. The most common chemicals are accelerators, retarders, and plasticizers. These are used to speed up the cure time, slow down the cure time and to keep the concrete "workable" for a pour that will take a long time or may be difficult.

Now that we know what concrete is made of, we need to know a little more about how it is used before we discuss how it is impacted by fire. Concrete is used in many ways and is also constructed differently depending on the use. You will most likely see concrete in one of two applications -- cast-in-place concrete and pre-cast concrete.

Cast-in-place concrete is formed and cured on the construction site. It is used in foundations, columns, walls and beams. Curing time can be anywhere from several hours to a month depending on the mix. It is important to note that the materials used to cast the perimeter of a concrete pour, also known as forms, can be of great concern when fighting a fire in a building under construction.

The forms are typically made of combustible materials and can add a large amount of fire load. In some cases, the forms are still in place because the concrete has not yet cured to reach its maximum strength, which makes the concrete more susceptible to damage under fire conditions.

Pre-cast concrete is made off-site and ready for use. Floors, walls, and roof decking are areas where you will commonly see pre-cast concrete used. This style is commonly engineered to reduce the amount of concrete to save on cost and weight. Some concrete floor systems have holes bored through them and some are in a waffle or "T" configuration. You will often see pre-cast concrete used in parking garages.

Increasing the Strength of Concrete
As we discussed earlier, concrete has a very high compressive strength, and is very weak in tension. The tensile strength of concrete is about 10 percent of its compressive strength. Due to its low tensile strength, concrete is often combined with a material with high tensile strength - steel. Steel will be the subject of next month's article and will be discussed in more detail. In photo 1, steel is used in conjunction with this concrete column to add tensile strength.

There are many ways to use steel to increase the tensile strength of concrete. The first is reinforcing, which uses steel reinforcing bars commonly referred to a "rebar" to provide tensile strength. Two other methods are the pre-tensioning and post-tensioning of concrete. Both methods use steel tendons (high-strength steel cables) to provide tensile strength to the concrete.

In the pre-tensioning method, the concrete is poured over the stretched tendons, when the concrete has cured to a pre-determined strength the tendons are released and impose a stress on the concrete. This process usually occurs in a factory and is used largely in pre-cast construction.

When concrete is post-tensioned, steel tendons are placed in sleeves in the concrete and the tendons are tensioned. They are not in contact with the concrete. They may be tightened over the duration of the construction project.>

Concrete and Fire
I know that the reason many of you are reading this article is to find out how concrete will respond to fire. Concrete is a non-combustible material, however many fires that have resulted in the loss of life have occurred in concrete buildings.

There are several key indicators you need to look for when pre-planning for a fire in a concrete structure. It is important that you determine some of the major structural components before you have a situation in a concrete facility. You should know what type of reinforcing is used in the concrete.

If pre- or post-tension walls or floors are cut and if the tendons are exposed or damaged, the likelihood of collapse is great. If the concrete has spalled or otherwise been damaged and exposes the reinforcement to fire, you no longer have to worry only about the concrete, but you need to worry about the steel used for reinforcing as well.

An additional problem that many firefighters may run across is steel reinforcement that has been previously damaged not by fire, but by water. If concrete is damaged and the reinforcement is exposed to water, the water will react with the steel weakening the tensile strength of the concrete, and increasing the likelihood of collapse.

Next month we will cover steel construction in more detail and discuss some of the problems steel poses when exposed to fire.

It is important to remember that concrete is often used as fire proofing. The rating of concrete as fire proofing is directly related to the thickness of the concrete and the type of concrete. For example, lightweight concrete can provide a four-hour fire rating if it is 5.1 inches thick. See chapter 7 of the International Building Code for a table that outlines these thicknesses.

To reiterate, it is important for you to know the structures in your area. Know what they are made of before you respond. Additionally, remember that the construction of the building is usually determined on cost and available materials, not on reaction to fire.

MATTHEW STIENE, a Contributing Editor, is a project manger for the Mecklenburg County Real Estate Services Department, and a firefighter with Robinson Volunteer Fire and Rescue, in Charlotte, NC. He is a licensed professional engineer in North Carolina, New York and Pennsylvania, and is a certified facility management professional. To read Matthew's complete biography and his archived articles, click here. You can reach Matthew by e-mail at [email protected].

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