THE NATURE OF CONCRETE
When concrete construction was first developed it was regarded by some as the answer to all fire protection problems—it was truly “fireproof.” After a series of disastrous fires, it became evident that concrete, like any other noncombustible material, can be destroyed by fire given sufficient fire load and fire duration. Unfortunately, many people, including some who should know better, confuse noncombustibility with fire resistance.
Concrete is inherently noncombustible. It may have been fabricated to meet a fire resistance standard.
Concrete is made in the field and the only test of its fire resistance is a fire. An experienced concrete foreman built a concrete fire demonstration tank for the Navy Fire School in Panama. It survived hundreds of fires beautifully. His associate, using the same specifications, built an open pit of similar reinforced concrete conStruction to be used for spill-fire training It spalled explosively during every fire. Uniform performance can’t be expected with regard to concrete structures. Concrete is like a cake — the mix and the instructions are the same but the results are always different.
“Reinforced concrete” is not an accurate term. The steel does not actually reinforce the concrete. The concrete is strong only in compression. Steel provides the tensile strength that concrete lacks. The resultant product is reinforced concrete, a composite material. By definition, a composite is a structural element in which both components must react together under the load. If the bond between the steel and concrete fails, the element fails. This is the most important fact to understand about the strength and integrity of reinforced concrete.
Concrete construction can be built to deliver any required level of fire resistance. Unlike steel, which is fireproofed* by protecting it with an insulator, the fireproofing of concrete is integral. Some of the concrete, particularly that poured below the reinforcing rods, is necessary not for structural strength but for insulation.
*“Fireproof” is a term still used in the construction trade to describe the insulation of steel assemblies against fire.
In evaluating the potential for fire in a concrete building, we must first determine whether the building was even intended to be rated as fire resistive. If it was not, then the concrete is merely noncombustible and nothing should be assumed as to its integrity in a fire.
Fire-resistive concrete and noncombustible buildings are not designed to perform alike in fire conditions. A concrete building defined by code as noncombustible may have a lightweight truss roof whose structural members are connected by gusset plates; in such a circumstance, the potential for roof collapse is always there—there’s nothing fire resistive about it.
While the only true test of any fireresistive building is a severe fire, there are signs of possible building distress that should be noted in prefire plans. Deteriorated concrete, spalling that has exposed reinforcing rods, and cracks that can admit corrosive moisture to the reinforcing rods are among these signs.
Also note improperly protected openings that will provide paths for fire extension and negate the positive effects of compartmentation.
Consider these examples of fires in steel-reinforced concrete structures:
- Fire-weakened concrete can generate “missiles” when struck by heavy hose streams. The Mid Hudson Warehouse in Jersey City, New Jersey was a large, heavily reinforced concrete building. It was ignited by a massive exposure fire in May 1941. On the second day of the fire, I watched the New York City fireboat Firefighter attack the upper-floor fires with a stream from its 5-inch tipped monitor. Very shortly, sizable pieces of the building were flying to the street on the other side. The report of the fire notes that several structural members had “disappeared.” They hadn’t, of course—they were rubble in the street.
- The Military Records Center of the United States General Services Administration near St. Louis, Missouri was severely damaged in a fire in 1973The six-story building was of steel-reinforced concrete construction. The top floor, where the fire occurred, was more than 200,000 square feet of undivided floor space. The incredible fire load was more than
- 21 million military personnel files in cardboard boxes on metal shelves.
The fire destroyed the top floor. Thermal expansion of the steel members caused the roof to elongate several feet around the perimeter. The elongation caused top-floor columns to shift and fail at either the top or the bottom. It should be noted that the portion of the roof that collapsed held together for about 22 hours after the start of the fire. The building did all that could be expected of it and more, considering the huge fuel load.
- Computer installations in the Pentagon were destroyed when a 100watt light bulb ignited a suspended acoustical ceiling of combustible, lowdensity fiberboard. The fire then burned its way through the concrete floor of the concourse above. The guard on duty had been provided with a CO2 extinguisher rather than a water unit. CO2 is not effective on fires in fibrous Class A materials. This is another example of the reverse thinking in which more worry is placed on damage by the extinguishing agent than on damage by the fire.
- In the 1940s, a Kansas City, Kansas multistory warehouse, constructed of steel-reinforced concrete and filled with rubber tires, was destroyed by fire. The investigators were astonished that concrete buildings could be destroyed by fire in its contents.
- Concrete by itself provides no guarantee of life safety. The reinforced concrete Joelma Building in Sao Paolo, Brazil burned in February 1974 with a loss of 179 lives. The structure sustained relatively minor damage. Factors other than the structure itself, such as interior finish and the ability or inability of the occupants to reach a nontoxic environment, speedily may be far more important.
The article accompanied by this sidebar describes a fire in a monolithic, conventionally reinforced concrete building. These comments are limited to steel-reinforced concrete buildings and do not refer to any other type of concrete structure.
For further information, see Chapter 8 of Building Construction for the Fire Service, Second Edition, by Francis L Brannigan, published by the NFPA