At the high temperatures experienced in fires, hydrated cement in concrete gradually dehydrates, reverting back to water (actually steam) and cement. This results in a reduction of strength and modulus of elasticity (stiffness) of concrete. In some fires, spalling of concrete occurs – fragments of concrete break loose from the rest of the concrete, sometimes violently. This article addresses questions that manufacturers of precast concrete have regarding fire resistance and fire ratings of precast concrete products. Most fire-resistance rating requirements are dictated by building codes, depending on the type of building and its occupancy.
Ratings are given in hours. For example, the required fire-resistance ratings for columns in high-rise hospitals are much more stringent than those for single-story buildings used for storage of noncombustible products or materials. In the highrise hospital the columns might need a four-hour rating, whereas in the single story building exterior walls may need only a one-hour rating. In other cases, fire resistance ratings are specified by owners.
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.
What products need fire ratings?
A fire rating, or more correctly "fire resistance rating" as used in building codes, has a specific technical meaning (as discussed later). "Fire resistance" is a descriptive term meaning the ability to withstand fire or to give protection from fire.
Certain precast concrete products such as wall panels, floor slabs or complete small buildings may require fire ratings depending on the types of structures and their occupancies, and sometimes on their locations relative to other structures or property lines. Certain other precast products such as risers in stadiums or portions of prison buildings may also need fire resistance ratings. Most other products such as manholes, drainage structures, median barriers, septic tanks and park furniture need not be fire resistant but may be required to be noncombustible.
One of the advantages of concrete is that it is noncombustible – it neither burns nor supports combustion, so it can be used wherever noncombustible construction is permitted.
What products might not need fire ratings but should be fire resistive?
Precast concrete products that might be subjected to repeated fires or to very hot accidental fires should be made as fire resistive as possible. For example, some cities provide short manhole sections at beaches so that cooking fires or camp fires can be made within the manhole sections by users of the beach. Making such manholes highly fire resistant is a real challenge. Unless properly made and conditioned, they are liable to spall violently the first time they are used.
Fires sometime occur in electrical transformers. Such fires are extremely hot – sometimes hot enough to melt siliceous aggregates. Precast transformer vaults should be capable of withstanding such fires without collapsing.
What does fire rating mean?
As defined in the 2000 edition of the International Building Code (IBC-2000), "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, as determined by tests prescribed in Section 703." For walls, floors, roofs, columns and beams, tests referred to are the standard fire test, ASTM E119, "Fire Tests of Building Construction Materials." That standard requires that the specimen to be tested be at least a certain size, unless the actual size is smaller than the minimum specified.
During a fire test, the specimen must generally support its maximum superimposed load as permitted by national standards. The specimen is then exposed to a standard fire. Walls are exposed to fire on one side, floor or roof specimens from the underside, beams from the underside and two sides, and columns from all sides. The fire must be of a certain intensity as defined by a time-temperature relationship – 1,000 degrees F at five minutes, 1,550 F at 30 minutes, 1,700 F at one hour, 1,850 at two hours and 2,000 F at four hours.
To comply with ASTM E119, walls must be large enough so that at least 100 square feet is exposed to fire with neither dimension less than 9 feet; at least 180 square feet of a floor or roof must be exposed to fire and neither dimension can be less than 12 feet. Columns must have at least 9 feet in height exposed to fire and beams 12 feet in length.
The test is continued until an "end point" is reached. A "structural" end point occurs if the specimen collapses. A "flame passage" end point occurs in walls, floors or roofs if a hole or crack occurs that is large enough to allow hot gases from the fire to reach the other side. A "heat transmission" end point occurs for walls, floors or roofs if the average temperature of the surface not exposed to fire increases by 250 F or if the temperature at any one point rises to 325 F. For concrete specimens, this latter criterion almost always governs; that is, the structural or flame passage end points seldom occur. There are other criteria for end points, but those are not critical for precast concrete products unless they are prestressed.
Standard fire tests are quite expensive to perform, and few facilities are equipped to perform such tests. As an alternative to fire testing, IBC-2000 allows the use of tabulated data included in the code as discussed later in this article.
What characteristics influence fire resistance?
Fire resistance of concrete is influenced by aggregate type, moisture content, density, permeability and thickness. Limestone, dolomite and limerock are called "carbonate" aggregates because they consist of calcium or magnesium carbonate or combinations of the two. During exposure to fire, these aggregates calcine – 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. The thicker or more massive the concrete, the better its behavior when exposed to fire.
How are fire ratings achieved?
As previously indicated, IBC-2000 allows various methods for achieving fire-resistance ratings. A fire test of a particular building component is an obvious method. Alternatively, prescriptive designs as listed in the code may be used, or calculations done in accordance with the procedures given in the code are permitted. Although the "calculations" section in the code includes a few formulas, most of the data is tabulated in easy-to-use form and is based on results of standard (ASTM E119) fire tests.
As an example, Table 1 presents the data from Table 722.214.171.124 of IBC-2000 for the minimum thickness of cast-in-place or precast walls for various fire resistance ratings. The data are identical to the minimum thickness of floor slabs given in Table 7126.96.36.199 because the values are based on the heat transmission end-point criterion.
Table 1: Minimum slab thickness for fire resistance rating of:
As noted above, carbonate refers to coarse aggregates of limestone, dolomite or limerock – those consisting of calcium or magnesium carbonate. Siliceous refers to most other normal-weight aggregates. Sand-lightweight refers to concretes made with normal-weight sand and lightweight coarse aggregate and generally weighing between 105 and 120 pounds per cubic foot. Lightweight refers to concrete made with lightweight coarse and fine aggregates and weighing between 85 and 115 pcf.