By Claude Goguen, P.E., LEED AP
On Sept. 13, a devastating fire engulfed dozens of businesses in Seaside Heights, reducing the buildings and boardwalk to ash. Some of the businesses had just reopened in July. Many of these structures were landmarks.
Also prominently in the news in recent weeks are the forest fires in western states. Already this year, 35,440 reported fires have burned a total of 3.9 million acres, including a significant portion of the iconic Yosemite National Park.
Fire, of course, isn’t isolated to rural areas. According to the National Fire Protection Association, U.S. fire departments responded to an estimated 1,375,000 fires in 2012. These fires resulted in 2,855 civilian fire fatalities, 16,500 civilian fire injuries and an estimated $12.4 billion in direct property loss. There was a civilian fire death every 3 hours and 4 minutes and a civilian fire injury every 32 minutes in 2012. Home fires caused 2,380, or 83%, of the civilian fire deaths.
Fire has been our foe for as long as man has walked this earth. After Rome burned in 64 AD, Emperor Nero had regulations drawn up requiring fireproof materials to be used for external walls in rebuilding the city. This was perhaps the first recorded example of using the science and engineering of the day in the practice of fire protection engineering.
With all the advances in fire protection and detection technology, building with a fire-resistant material is still the best way to protect homes or businesses (or Roman cities). Current building codes require that resistance to fire be considered in the design of buildings. That is why precast concrete is often chosen for wall panels or floor assemblies to provide fire resistance and containment solutions.
You’ve probably heard of a fire rating, which is a measurement of a material’s ability to withstand fire or to give protection from fire. As defined in the International Building Code, “fire resistance rating” means “the period of time a building or building component maintains the ability to confine a fire or continues to perform a given structural function or both.”
Fire ratings are given in hours. For example, the required fire-resistance ratings for columns in high-rise hospitals may be four hours, which is more stringent than those for single-story building exterior walls, which may only need a one-hour rating. Some fire ratings are stipulated by codes, and some others by the owner. For example, a utility company may require that its vaults have two-hour ratings even though the building code may be silent on requirements for such structures.
In building design, materials are often classified as fire resistive, noncombustible or combustible. Fire resistive means something is hard to burn, but not impossible to burn. A prime example is the World Trade Center towers. The planes did not cause the towers to collapse, but rather it was the resulting jet-fueled fire. The flames were so intense that the structure lost strength and buckled.
Those materials in the tower were fire resistive. Other materials may be labeled as noncombustible, which means they are unable to combust or burn. Precast concrete does not burn. It also resists fire very well.
Materials and assemblies of materials of construction are tested in accordance with the requirements set forth in ASTM E119, “Standard Test Methods for Fire Tests of Building Construction and Materials.” Typical fire ratings of a precast concrete slab can be found in the table at left, taken from ACI 216.1, “Determining Fire Resistance of Concrete and Masonry Construction Assemblies.”
What characteristics influence fire resistance?
Fire resistance of concrete is influenced by aggregate type, moisture content, density, permeability and thickness. Aggregate used in concrete can be classified into three types: carbonate, siliceous and lightweight. Carbonate aggregates include limestone and dolomite. Siliceous aggregates include materials consisting of silica, granite and sandstone. Lightweight aggregates are usually manufactured by heating shale, slate or clay.
Carbonate aggregates consist of calcium or magnesium carbonate or combinations of the two. During exposure to fire, these aggregates calcine – in other words, carbon dioxide is driven off and calcium (or magnesium) oxide remains. Since calcining requires heat, the reaction absorbs some of the fire’s heat. The reaction begins at the fire-exposed surface and slowly progresses toward the opposite face. The result is that carbonate aggregates behave somewhat better than other normal-weight aggregates in a fire.
Moisture content has a complex influence on concrete’s behavior in fire. Concrete that has not been allowed to dry may spall, particularly if the concrete is highly impermeable, such as concretes made with silica fume or latex, or if it has an extremely low water-cement ratio. Concretes that are more permeable will generally perform satisfactorily, particularly if they are partially dry.
In general, concretes with lower unit weights (densities) will behave better in fire; dried lightweight concrete performs better in fire than normal-weight concrete.
Rebuilding with precast concrete
Whether we’re talking about homes in eastern California or commercial structures on the Atlantic shore, damage from fires is a real risk, and that risk can be dramatically reduced by considering precast concrete as a building material. Precast concrete is used all over the world to provide a resilient and sustainable structure that can resist the effects of many types of natural and man-made disasters.
After Hurricane Sandy, officials in Long Beach, N.Y., decided to rebuild portions of the boardwalk with durable precast concrete. Recently, US Concrete Precast began constructing a portion of the 2,500 precast pieces that will serve as the base of the Long Beach Island boardwalk in Long Beach, N.Y. The boardwalk, destroyed by Hurricane Sandy in October of last year, is an overall rebuild estimated at nearly $44.2 million.
The precast concrete is less susceptible to storm damage than the traditional wood planks used in the past and is built to withstand the high-velocity winds of hurricanes that commonly occur on East Coast beaches. The nostalgic feel of the boardwalk will be preserved by inlaying tropical wood planks on top of the precast, on which visitors will then be able to walk.
Precast will not only provide long-lasting strength and resistance to hurricane-type storms, but also a resistance to the harsh saltwater environment. It will also provide important fire resistance, which one does not often think about when building on the ocean; however, this attribute may gain importance in light of recent events in Seaside Heights.
Claude Goguen, P.E., LEED AP, is NPCA’s director of Technical Services and Sustainability. For more information on fire resistivity of precast concrete or other sustainable attributes, contact him at [email protected] or (317) 571-9500.