There seems to be a hierarchy among architectural materials. Wood and granite are the most popular, glass and aluminum are the trendiest, and marble is the highest of high-end. And then there's concrete...poor, unlovable concrete. We walk on it, drive on it, and even spit gum on it. Some respect. It's just so hard to love a material that's given us block housing, parking decks, and most of the world's prisons. But things may be changing for the ugly duckling of architecture. There is a new trend toward embracing concrete for home construction, mainly because it's one of the strongest and most durable of building materials. It's also incredibly resistant to fire, which makes it a safe bet for homes.
The fire-resistant properties of concrete are fairly easy to understand. The components of concrete – cement (limestone, clay and gypsum) and aggregate materials – are chemically inert and therefore virtually non-combustible. Concrete also has a slow rate of heat transfer, which means that concrete walls in a home act as a fire shield, protecting adjacent rooms from flames and maintaining its structural integrity despite exposure to intense heat [source: The Concrete Center].
How do the properties of concrete make it so impervious to flames? Fire resistance refers to a material's ability to stand firm in a fire while everything around it burns. Determining a building material's fire resistance takes into account the rate of heat transfer and combustibility of that material under variable conditions such as the temperature of the fire, ventilation, and fuel sources within the building [source: Portland Cement Association]. While concrete walls can generally withstand up to four hours of extreme fire pressure, most wood-framed walls would fall in less than an hour [source: International Association of Certified Home Inspectors]. It's also important to note that when concrete does burn, it doesn't emit toxic fumes, produce smoke, or drip molten particles.
The fire resistant qualities of a material are very important in residential construction, as they determine how well the structural integrity of the home will be maintained when exposed to intense heat and flames. This is called fire performance, and we consider a few examples in the next section, including a comparison of concrete and wood.
Framing Your Home: How does Concrete Hold Up?
If you're fortunate enough to have something to say about the choice of framing material in your home, you might consider concrete over wood. Homebuilders have been turning to concrete as a framing material in recent years for a variety of reasons, not the least of which is its fire resistant properties.
Concrete is generally considered to be a good value in home building when it comes to fire performance, as it holds up much better than wood in intense heat [source: The Concrete Center]. This is partly because concrete allows for compartmentalization (i.e. containment) of fire. For an example of compartmentalization, let's take a look at the burning of the World Trade Center (WTC) Towers on Sept. 11, 2001.
Despite the intense heat of burning jet fuel when the planes hit the WTC buildings on that fateful September morning, the 4-inch (10-centimeter) concrete slabs between each floor of the towers were able to limit the spread of the fire, at least for a while. However, the tower walls were framed in steel and other materials, not concrete. As the heat of the fire topped 800 degrees Fahrenheit (427 degrees Celsius), the tower's supporting steel structures weakened and began to collapse [source: Scientific American]. Still, most engineers agree that the fire performance of the WTC buildings on Sept. 11th was impressive, mainly because the compartmentalization of heat and flames gave thousands of people a chance to escape before the buildings fell.
As a building material, concrete also stands up well in the face of other hazards such as hurricanes, tornadoes, and floods, and provides better soundproofing and energy efficiency than wood [source: International Association of Certified Home Inspectors]. However, despite these qualities, it does have some drawbacks. One is its significant carbon footprint. Generally speaking, for every ton of concrete produced, there is a ton of CO2 released into the atmosphere [source: Chemistry World]. The carbon footprint of other materials (for instance, steel) may be the same or worse, but since there is much more use of concrete in the world, the footprint seems bigger. Concrete factories are becoming more efficient all the time, but there is no denying this material's enormous environmental impact.
Nevertheless, if concrete is so fire-resistant, why is it not used in all homes? We'll explore that in the next section.
The Cost of Concrete
All things considered, concrete holds a great deal of promise for homes of the future. Currently, approximately 17 percent of new homes built in the U.S. are framed in concrete [source: International Association of Certified Home Inspectors]. According to Omar Garcia, President of SOGA Construction in Washington, D.C., a home made of concrete is going to last much longer than a wood-framed home.
"There would probably be many more homes made of concrete in the U.S. if they weren't so expensive to build," he says. The Portland Cement Association, the national concrete industry's trade group, estimates that the cost of a new house built using insulating concrete forms (the most common type of cement construction) costs 4 to 7 percent more than a similar house with a wood frame.
"There is a substantial jump in price based on the cost of material when you frame in concrete," says Garcia. "But the real increase comes when you factor in the additional labor hours for the installation of steel reinforcement and forming and for pouring the concrete."
However, with a concrete house, you do experience savings in the form of lower heating and cooling bills and lower home insurance policies.
"Considering how vulnerable wood is to rot, fire, and termite infestation, it's surprising how long wood has persisted as the dominant structural material of homes," Garcia says. "If the cost of building a home in concrete continues to decline, it makes sense that consumers would embrace it as the material of choice."
It seems likely that, in time, concrete will probably shed its image as a second-rate building material. There is no denying its durability, and few materials compare to its incredibly fire resistance. Perhaps concrete just needs a good PR campaign to bring it on par with much-loved materials such as wood, stone, and marble.
Author's Note: Why is concrete fire resistant?
When I received the assignment to write about the fire resistance of concrete, I immediately recalled the many conversations I've had on the subject with my husband, who is both an engineer and builder. Being from Mexico, where most homes are made of concrete, he often comments on Americans' fascination with wood as a building material. As we've pointed out in the article, concrete is much stronger and more hazard-resistant than wood. But the fact is that many Americans cling to wood in an emotional way, perhaps because it was once a living thing...a tree standing proudly in a forest somewhere. It's also quite beautiful, and sometimes that trumps practicality.
- Chemistry World. "The concrete conundrum." (March 29, 2012) http://www.rsc.org/images/Construction_tcm18-114530.pdf
- Garcia, O. Soga Construction. Personal interview. (March 29, 2012)
- International Association of Certified Home Builders. "Concrete for Exterior and Structural Walls." (March29, 2012) http://www.nachi.org/concrete-exterior-walls.htm
- Portland Cement Association. "High Strength Concrete and Fire." (March 29, 2012) http://www.cement.org/buildings/high_strength_fire.asp
- Portland Cement Association. "The Quality of Concrete Cost a Little More." (April 5, 2012). http://www.cement.org/homes/brief05.asp
- Scientific American. "When the Twin Towers Fell." Oct 9, 2001 (March 29, 2012) http://www.public-action.com/911/jmcm/sciam/
- The Concrete Center. "Fire Resistance." (March 29, 2012) http://www.concretecentre.com/technical_information/performance_and_benefits/fire_resistance.aspx