Fire resistance of concrete refers to its ability to withstand fire and provide protection against it. This capability allows concrete structural elements to maintain their structural function or confine fire, or both. The fire rating, defined in ASTM E 119, specifies the duration for which elements like beams, columns, walls, floors, or roofs can withstand fire. Fire resistance is influenced by the physical and thermal properties of the structural element. The structural performance is determined by factors such as stress levels in the concrete and steel, concrete cover, potential for aggregate and moisture-induced spalling, and lateral restraint conditions. Thermal performance, on the other hand, is controlled by parameters such as aggregate type, moisture content in the concrete (both absorbed and capillary), and the volume of concrete per square meter of exposed area.
Mechanism of Concrete Fire Resistance
Concrete has excellent fire resistance properties due to its composition. The main components of concrete, namely cement and aggregate materials, are chemically inert and mostly non-combustible. This means that they do not readily catch fire. Additionally, concrete has a slow rate of heat transfer, which is crucial in its ability to act as an effective fire shield. The slow rate of conductivity allows concrete to protect adjacent spaces by containing flames and preventing their spread. Furthermore, concrete can safeguard its own structural integrity when exposed to intense heat. In residential settings, walls made of concrete serve as reliable fire shields, ensuring the safety of adjoining rooms and maintaining the overall stability of the structure.
How Fire Influence Concrete Structures?
Aggregate used in concrete can be categorized into three classes: carbonate, siliceous, and lightweight. Carbonate aggregates, such as limestone, dolomite, and lime rock, are composed of calcium or magnesium carbonate, or a combination of both. When subjected to fire, these aggregates undergo calcination, where carbon dioxide is released and calcium (or magnesium) oxide remains.
Calcination, being a heat-dependent process, absorbs some of the fire’s heat. The reaction initiates at the surface exposed to the fire and gradually progresses towards the opposite side. As a result, carbonate aggregates exhibit relatively better fire performance compared to other conventional aggregates with normal weight.
Factors Influencing Concrete Fire Resistance
1. Aggregate Type
At high temperatures during fires, hydrated cement in concrete loses water and reverts to steam and cement, leading to decreased strength and stiffness of the concrete. Some fires can cause concrete spalling, where fragments break off forcefully. Building codes determine the necessary fire-resistance ratings based on building type and occupancy. These ratings are measured in hours, with high-rise hospitals typically requiring more stringent ratings for columns (e.g., four hours) compared to single-story buildings used for noncombustible storage (e.g., one hour for exterior walls).
Siliceous aggregates, such as granite and sandstone, contain silica. Lightweight aggregates are typically made by heating shale, slate, or clay. When lightweight or carbonate aggregates are used in concrete, it maintains most of its compressive strength up to approximately 650°C.
Compared to normal weight concrete of the same thickness, lightweight concrete exhibits insulation properties and a slower rate of heat transmission. As a result, it generally offers improved fire resistance.
2. Moisture Content
Moisture content significantly affects the behavior of concrete in fire. Insufficient drying of concrete can lead to spalling, especially when the concrete is highly impermeable (e.g., silica fume or latex-based concretes) or has an extremely low water-cement ratio.
3. Density
Concrete with lower unit weights (densities) generally exhibits superior fire performance compared to normal-weight concrete. Specifically, dried lightweight concrete demonstrates enhanced fire resistance when compared to its heavier counterpart.
4. Permeability
More permeable concretes perform well, especially when they are somewhat dry.
5. Thickness
Thicker or more massive concrete exhibits improved fire resistance.
What does Fire Rating Mean?
According to the 2000 edition of the International Building Code (IBC-2000), the term “fire resistance rating” refers to the duration for which a building or its components can effectively contain a fire or continue to perform their intended structural functions. This rating is determined through tests outlined in Section 703 of the code. For walls, floors, roofs, columns, and beams, the applicable test is the ASTM E119 standard fire test, titled “Fire Tests of Building Construction Materials.” This standard specifies that the test specimen should meet certain size requirements, unless the actual size is smaller than the specified minimum.
How are fire ratings achieved?
IBC-2000 offers multiple approaches to achieve fire-resistance ratings. One common method is conducting fire tests on building components. Another option is following the prescriptive designs outlined in the code. Additionally, calculations can be performed using the code’s specified procedures.
The code’s “calculations” section contains some formulas, but it predominantly provides tabulated data that is user-friendly. This data is derived from standard fire tests (ASTM E119) and is presented in an easily understandable format.
For instance, Table 1 showcases the information extracted from Table 720.2.1.1 of IBC-2000, which specifies the minimum thickness of cast-in-place or precast walls for various fire resistance ratings. The values in this table coincide with the minimum thickness of floor slabs found in Table 720.2.2.1 because both tables utilize 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 |
Carbonate aggregates consist of limestone, dolomite, or lime rock, which are primarily composed of calcium or magnesium carbonate. On the other hand, siliceous aggregates encompass most other normal-weight aggregates. Sand-lightweight concrete is a type of concrete that incorporates normal-weight sand along with lightweight coarse aggregate, resulting in a weight range of 1682 to 1922 kilograms per cubic meter. In contrast, lightweight concrete is produced using lightweight coarse and fine aggregates, leading to a weight range of 1361 to 1842 kilograms per cubic meter.