Fire Resistance of Concrete

Understanding the Fire Resistance of Concrete

Understanding the Fire Resistance of Concrete:

The capacity of concrete to resist fire or to provide protection against fire is referred to as its fire resistance. This refers to a concrete structural element’s ability to continue performing the required structural function or to contain fire, or to do both.

The time period during which the concrete element (for example a beam, column, or floor) can withstand fire, as stated in ASTM E 119, is referred to as the fire rating.

The resistance to Fire is determined by the structural element’s physical and thermal characteristics. The level of stress in the concrete and steel, the concrete cover, and the aggregate’s tendency to spall all affect structural performance.

However, thermal performance is determined by the aggregate type, the amount of free moisture in the concrete, and the volume of concrete per m2 of exposed area.

Fire Resistance of Concrete
Concrete Exposed to Fire

How Concrete Resists Fire?

Concrete’s fire resistance properties are self-evident. Concrete’s constituents, such as cement and aggregate materials, are chemically inert and thus largely incombustible, and concrete has a low heat transfer rate.

This low heat conductivity allows concrete to be an efficient fire barrier not only between adjacent areas, but to also protect itself against fire damage. Thus, certain concrete structural components in a home, such as walls, performs as a fire shield, shielding adjacent rooms from flames and ensuring the structure’s structural integrity even when exposed to extreme heat.

What Effect Does Fire Have on Concrete?

When exposed to the extreme temperatures of a fire, hydrated cement in concrete eventually dehydrates, reverting to water vapor and cement. As a consequence, the strength ability and modulus of elasticity of concrete are reduced.

Concrete spalling happens in some fires – pieces of concrete separate violently from the rest of the concrete. The majority of standards associated with rating against fire-resistance are determined by building codes, which vary according to the type of structure and its intended use.

Hours are used to rate fires. For instance, the fire resistance ratings needed of columns in high-rise hospitals are significantly higher than those required of single-story buildings used to store non-combustible goods or materials.

Columns in a multi-storey hospital may require a 4 h rating, while exterior walls in a single-storey building may only require a1 h rating.

Factors Affecting Concrete Resistance to Fire

1. Type of used aggregate

Carbonate aggregates, siliceous aggregates, and lightweight aggregates are all types of aggregates used in concrete. Because limestone or dolomite are composed of calcium or magnesium carbonate or a combination of the two, they are referred to as carbonate aggregates. These aggregates calcine when exposed to fire; carbon dioxide is removed and only calcium (or magnesium) oxide remains.

Due to the fact that calcining requires heat, it absorbs some of the heat from the fire. The reaction begins on the fire-exposed surface and gradually spreads to the opposite face. As a result, carbonate aggregates perform slightly better in a fire than other aggregates of similar weight.

Siliceous aggregates are composed of silica-based materials, such as granite and sandstone. The most common method of producing lightweight aggregates is to heat shale/slate/or clay. Up to approximately 650°C, concrete made with lightweight aggregates and carbonate aggregates retains the majority of its compressive strength.

Due to the fact that lightweight concrete is insulating and conducts heat more slowly than normal weight concrete of the same thickness, it generally give enhanced fire resistance.

2. Amount of Water in Concrete (Moisture)

The moisture content of concrete has a complex effect on its behavior during a fire. Concrete which has not been allowed to dry may spall, especially if the concrete has very low  permeability, such as silica fume concrete, or if the water-cement ratio is extremely low.

3. Unit weight

Concretes with lower densities would perform better in case of fire. This means lightweight concrete operates better than normal-weight concrete in resisting fires.

4. Penetrability

Permeable concretes perform better in general, especially when partially dry.

5. Thickness of the concrete element

The thicker the concrete element is, the more stable it is in the presence of fire.

What is Fire Rating?

As described in the International Building Code (IBC-2000), “fire resistance rating” refers to “the time period during which a building or component of a building retains its ability to contain a fire or continues to perform a specified structural function, or both”

Satisfying Fire Ratings

As previously stated, IBC-2000 permits a variety of methods for determining a building’s fire resistance rating. A straightforward method is to conduct a fire test on a specific building component. Alternatively, prescriptive designs as defined in the code may be used, as are calculations performed in accordance with the code’s procedures.

As an example, the IBC-2000 specifies a minimum thickness of concrete slabs depending on the required duration of fire resistance and type of used aggregate in concrete.

Read Also: Measuring the Temperature of Fresh Concrete

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