Building Back Better: Concrete and Fire

There are many ways to build a concrete home. The most popular methods today include removable form, precast concrete, grout-filled concrete block, and insulating concrete forms (ICF). Concrete homes can be very expensive modern constructions, built to look like any other house on the street, or mass-constructed to be the most cost-efficient.

In some ways, it’s easier to discuss how concrete houses can resist hurricanes, tornadoes, floods, and earthquakes, than it is to relate how they can perform in fire situations. However, for people in Los Angeles, the concern is about fires and knowing how to rebuild lost homes with better fire resistance. Certainly no one wants this to happen again in a climate that will likely produce conditions ever more favorable to out-of-control fires.

What can be said about concrete without reservation, is that it is absolutely a non-combustible material and has good resistance to fire. But, it is only fire-resistant, not fire-proof. No material is fire-proof, because at some point, intense heat changes the characteristics of all materials, including concrete. The two factors that change concrete’s resistance to fire are the intensity of heat it is exposed to and the length of time of that exposure. These two conditions are what designers and engineers work with to provide safe structures.

Fire testing

The National Bureau of Standards (now, NIST) instituted building fire research in the early 20th century, testing several different building materials. The focus on concrete accelerated in the early 1950s, when the Portland Cement Association (PCA), located in Skokie, Illinois, built a large testing facility for fire testing concrete. Much of this testing was directed toward building elements such as columns, beams, and floors. They worked to increase its performance and provide needed guidelines and specifications for building with concrete. They also heated concrete to failure to better understand the nature of failures.

Kevin Mueller, a senior associate in the Protective Design Group at Thornton Tomasetti in Chicago, Illinois, says he once tested a concrete wall in a fire testing laboratory for 14 hours. The heat was directed at only one side of the wall and it ultimately reached 2000 degrees Fahrenheit. He said that at the nine-hour mark, there was little observable damage. After 14 hours, there was surface damage, but the wall remained structurally intact. Maged Youssef, a professor of engineering at Western University in London, Ontario, says the water in the concrete (there is always some water in concrete) can be turned to steam with sustained heating, causing spalling on surfaces. Thomas Gernay, an Assistant Professor in structural fire engineering at Johns Hopkins University in Baltimore, Maryland, cautions that the load-bearing level of concrete also changes when it is heated. “At 1100 degrees Fahrenheit it loses about half its compressive strength, and this remains the case after the concrete cools afterwards. But it takes a high fuel load to do this,” he says.

Mueller adds that the first signs of concrete deterioration due to heat occur when water begins to leave the concrete. “At 1100 degrees the water-of-hydration has been completely driven off, leading to a permanent and irreversible breakdown in the chemical bonds of the concrete.”

The relationship between concrete and reinforcing steel

When engineers design slabs with concrete and rebar together, the result is structural concrete that can support high loads. Concrete is very strong in compression and steel is strong in tensile strength, so the two materials complement each other. Gernay, Mueller, and Youssef (each members of ACI 216 - Fire Resistance and Fire Protection of Structures) all agree that when steel temperatures reach 800 degrees it becomes pliable and bends in the direction of forces applied to it. Concrete, on the other hand, is a poor conductor of heat and it takes considerable time for heat to move through it, protecting steel reinforcement (rebar) from reaching critical 800-degree temperatures. There is general agreement in the industry that an inch of concrete (referred to as “cover”) over rebar protects it from reaching 800 degrees for approximately one hour—two-inch cover protects it for two hours. Building code authorities sometimes state the thickness of cover they want for buildings in their jurisdictions.

Protecting steel reinforcement from heat is the highest priority for concrete in a fire setting. Mueller says that an intense local fire can do some damage to part of a concrete home, whereas a long exposure to temperatures of 1000 degrees Fahrenheit over a large area can cause the concrete to fail.

Wildfires

The 2025 fires in Los Angeles were driven by winds as high as 100 mph, burning brush and things that produced high temperatures. Burning embers driven by the wind at high speeds preceded the fires, igniting anything that would burn. The embers found their way inside houses and under roofs, starting very hot whole-house fires that burned them to the ground. In the best of all worlds, a wildfire would pass around a concrete house designed to be fire-resistant and continue on its way, with little time to develop much heat in the concrete. Afterwards, homeowners could clean up the debris, move back in, and life would go on.

Youssef says the goal for all homes should be to keep combustible materials away from the outside of the home to ensure low heat development. But, for example, knowing how to protect propane tanks in regions where they are needed is a problem requiring expert help.

Preventing fire from getting inside

A major objective for all types of house construction should be to prevent outside fires from getting inside. For concrete houses, the primary concern is the fuel load of the furnishings. It’s possible that the furnishings could elevate temperatures enough in some rooms to cause damage to the structure although that is perhaps unlikely. But, homeowners should consider their furnishings and the amount of fuel they represent.

Regardless of the material used to construct a house, builders and owners should prevent fires from getting to the inside.

Youssef says the two most vulnerable locations for fire entry are roofs and windows. Flammable materials such as asphalt or wood shingles on roofs shouldn’t be used. Most roofs have vents to allow for air circulation inside the roof cavity. These vents should be constructed to prevent sparks and embers from entering the building envelope. The same is true for washer, dryer, and furnace vents.

Some concrete home systems have concrete walls, but use wood to construct pitched roofs. Flat roofs constructed with engineered concrete slabs are a better option for fire protection and they can be designed to handle whatever load is needed.

Many concrete home constructions use other materials to construct interior walls to help reduce construction costs and make it easier to install plumbing, electrical, and allow for the redesign of inside spaces in the future. Instead of using wood studs for this, consider using steel studs as a better way to reduce the inside fuel load; this is a standard approach for commercial construction.

Youssef says that fire doesn’t typically gain entrance to the inside of a house by melting the glass in windows, the uneven temperature gradient developed in the glass panels causes them to break. Surprisingly, some national scope window manufacturers don’t offer fire-rated windows or fire-rated glass at all, so people who live in fire-prone areas like Los Angeles should be careful to buy windows with fire ratings. They might also consider installing fire-resistant shutters to protect windows.

Sprinkler systems are also an option, but can add significant cost to construction and would require an adequate water supply.

Concrete and fire

No building material is perfect, but concrete is probably the best all-around choice for fire resistance. It minimizes the fuel load in a home and ensures that fire temperatures won’t get too high. When you build a concrete house, you are also being kind to your neighbors because you are helping reduce the fire load in your neighborhood. Also, it’s not hard to engineer your home for seismic resistance at the same time you are designing for fire resistance.

In the next article in this series, structural engineering and designing homes to resist seismic activity will be discussed.

RELATED:
Part 1: The Case for Fire-Resistant Concrete Homes
Part 2: Concrete and Fire
Part 3: Reinforcement for Concrete Homes
Part 4: Resiliency