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What is Fire Resistance of Concrete? Mechanism and Factors

What is Fire Resistance of Concrete Mechanism, and influencing factors

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Fire resistance of concrete is the ability of concrete to withstand fire or to give protection against fire. This involves the ability of concrete structural element to continue perform a specific structural function or confine fire or both.

The duration of time that an element such as beam, column, wall, floor, or roof can endure the fire, which is defined in ASTM E 119, is termed fire rating.

Fire resistance is controlled by both the physical and thermal properties of the structural element. Factors governing the structural performance include stress level in the concrete and the steel, concrete cover, tendency of aggregate and free moisture to cause spalling, and lateral restraint conditions.

However, parameters that control the thermal performance involves type of aggregate, free moisture in the concrete (both absorbed and capillary), and volume of concrete per square meter of exposed area.

Mechanism of Concrete Fire Resistance

Fire resistance properties of concrete is easy to understand. The components of concrete such as cement and aggregate materials are chemically inert and hence mostly non-combustible, and concrete possess slow rate of heat transfer.

It is this slow rate of conductivity (heat transfer) that enables concrete to act as an effective fire shield not only between adjacent spaces, but also to protect itself from fire damage. So, certain concrete structural elements like walls in a home act as a fire shield, protecting adjacent rooms from flames and maintaining its structural integrity despite exposure to intense heat.

Fig. 1: Mechanism of Concrete Fire Resistance

How Fire Influence Concrete Structures?

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. Most fire-resistance rating requirements are dictated by building codes, depending on the type of building and its occupancy.

Fire 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 high-rise hospital the columns might need a four-hour rating, whereas in the single story building exterior walls may need only a one-hour rating.

Fig. 2: Influence of Fire on Concrete Structure

Factors Influencing Concrete Fire Resistance

1. Aggregate Type

Aggregate used in concrete can be classified into three classes namely:  carbonate, siliceous, and lightweight. Limestone, dolomite and lime rock 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.

Fig. 3: Limestone Aggregate

Siliceous aggregate includes materials consisting of silica and include granite and sandstone. Lightweight aggregates are usually manufactured by heating shale, slate, or clay. Concrete containing lightweight aggregates and carbonate aggregates retain most of their compressive strength up to about 650C.

Lightweight concrete has insulating properties, and transmits heat at a slower rate than normal weight concrete with the same thickness, and therefore generally provides increased fire resistance.

Fig. 4: Granite Aggregate
Fig. 5: Lightweight Aggregate

2. Moisture Content

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.

3. Density

In general, concretes with lower unit weights (densities) would behave better in fire;dried lightweight concrete performs better in fire than normal-weight concrete.

4. Permeability

Concretes that are more permeable would generally perform satisfactorily, particularly if they are partially dry.

5. Thickness

The thicker or more massive the concrete, the better its behavior when exposed to fire.

Read Also: Factors Affecting Performance of Concrete During Fire

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.

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 720.2.1.1 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 720.2.2.1 because the values are based on the heat transmission end-point criterion.

Table 1: Minimum slab thickness for fire resistance rating

Concrete Type 1 hour 1.5 hours 2 hours 3 hours 4 hours
Siliceous 3.5 4.3 5.0 6.2 7.0
Carbonate 3.2 4.0 4.5 5.7 6.6
Sand-Lightweight 2.7 3.3 3.8 4.6 5.4
Lightweight 2.5 3.1 3.6 4.4 5.1

As noted above, carbonate refers to coarse aggregates of limestone, dolomite or lime rock - 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 1682 and 1922 Kilogram per cubic meter.

Lightweight refers to concrete made with lightweight coarse and fine aggregates and weighing between 1361 and 1842 Kilogram per cubic meter.

Also Read: Fire Resistance of Concrete Structures and Materials

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