Explosive Spalling of Concrete Structural Elements during Fire
When structural concrete elements are exposed to fire, it is possible that explosive spalling of concrete takes place. The explosive spalling of concrete is occurred when free water (water that is not employed for hydration reaction and remained unused in concrete) in concrete is changed to steam because of concrete exposure to fire, and if the steam is not released in concrete then it causes explosive spalling.
Concrete cover which protects reinforcement will be removed due to explosive spalling and thus steel bars will be subjected to fire. This weakens reinforcement bars and led to decrease ultimate load carrying capacity of the reinforced concrete element.
Therefore, it is considerably important to understand nature of explosive spalling of concrete and factors that influence it. These factors may be material, geometry, structurally or environmentally based and will be discussed in the following sections.
Fig.1: Concrete Structure Exposed to Fire
Fig.2: Explosive Spalling of Concrete Followed by Buckling
Factors Affecting Explosive Spalling of Concrete Structural Elements Exposed to Fire
- Heating rate
- Heating exposure
- Size of the section of reinforced concrete element
- Shape of reinforced concrete element section
- Moisture content of reinforced concrete member
- Permeability of reinforced concrete element
- Age of reinforced concrete member
- Strength of concrete used
- Compressive stress and restrain
- Type of aggregate used in reinforced concrete element
- The size of aggregate used in the reinforced concrete member
- Cracking of concrete member
- Reinforcement utilized in concrete element
- Concrete cover to reinforcement
- Supplementary reinforcement
Heating Rate of Concrete
This factor substantially influences the explosive spalling of concrete occurrence. As the heating rate increases both likelihood and severity of explosive spalling is increased.
Heating Exposure of Concrete
The possibility of explosive spalling of concrete occurrence rises as the number of reinforced concrete element subjected to fire is increased.
For example, the response of reinforced concrete slab to fire is better in comparison to reinforced concrete beams. This is because only one side of the former is subjected to fire whereas there are three faces in the latter that exposed to fire.
This is evident that simple external shapes that have no pronounced feather are a better option to construct when fire exposure is considered.
Size of the Section of Reinforced Concrete Element
Spalling explosion is less likely to occur in extremely thin wall, this is believed to be due to the ease of free water escape out of concrete and decrease in pore water pressure in the member.
It is reported that, explosions have low likelihood in thick sections such as nuclear containment walls.
Shape of Reinforced Concrete Element Section
Explosive spalling of concrete is more frequent in cross sections that changes rapidly for instance level surfaces and round corner outperform corners specifically in extreme angles when exposed to fire.
Moisture Content of Reinforced Concrete Member
Explosive spalling of concrete is likely to occur in normal strength concrete with moisture content of greater than 2% by weight and rarely occurred when moisture content is smaller than 2%. This amount of moisture content is difficult to obtain practically.
With a given set of conditions, explosive spalling is less likely to present in concrete with moisture content of less than 3% by weight whereas it is reported that considerably dense high strength concrete with moisture content of 2.3-3% suffered explosive spalling.
This is because of low permeability and porosity which lead to impede the escape of water and consequently pore water pressure is large hence the risk of explosive spalling is great even though tensile strength is large in high strength concrete.
Moisture content decreases with time which means the possibility of explosive spalling is reduced as well.
Moisture content depends hugely on the environment that the structure is constructed in, for example, moisture content of elements inside buildings is smaller than those of outside.
Permeability of Reinforced Concrete Element
It influences vapor release rate greatly, and it is demonstrated experimentally that the occurrence of explosive spalling is rare if concrete permeability is smaller than (5 x 10–11 cm2).
Age of Reinforced Concrete Member
It is claimed that, the likelihood of explosive spalling is induced with the increase of concrete element age and this might be due to decline in moisture content.
Strength of Concrete
Interestingly, the occurrence of explosive spalling of concrete is rarely expected in low strength concrete. In contrary, there is great possibility of explosive spalling occurrence in high strength concrete.
Compressive Stress and Restrain
Possibility of explosive spalling increases as compressive stress and restrain on the element is increased. The compressive stress can be increased by reducing section size or increasing the applied load.
Type of Aggregates used in Reinforced Concrete Element
It is observed that, if aggregate with low thermal expansion is employed, the likelihood of explosive spalling of concrete is decreased. The possibility of explosive spalling decreases if siliceous, limestone, basalt, and lightweight aggregate are utilized.
The above statement is applied when dry aggregate is used because explosive spalling is more likely to occur when the aggregate is saturated.
The Size of Aggregate used in Reinforced Concrete Member
It is demonstrated by test that, the larger the aggregate size the less likely that explosive spalling of concrete occur.
Cracking of Concrete Member
Cracks help the escape of moisture in concrete. On the other hand, it is a source of the crack propagation.
Reinforcement Utilized in Concrete Element
Explosive spalling of concrete occurs in unreinforced portion of concrete element and it is not extended beyond reinforcement layers like cage of stirrups and longitudinal bars in columns and beams.
Concrete Cover to Reinforcement
It is experimentally noted that, as concrete cover increases the possibility of concrete element to explosive spalling is increased. It is reported that, explosive spalling of concrete should be expected if concrete cover is larger than 40 mm for dense concrete and greater than 50 mm for lightweight concrete.
Sever explosive spalling is not anticipated if concrete cover is equal or less than 15 mm. this might be due to small mass of unsupported concrete.
Supplementary reinforcement does not prevent the occurrence of explosive spalling but is restrict its effect. Occasionally, mesh reinforcement is provided to limit the effect of explosive spalling when concrete cover is larger than 40 mm. Supplementary reinforcement ease the repair of concrete after the fire.
There has been continuous attempt to enhance fire resistance of concrete by adding steel fiber, polypropylene fiber, or by air entrainment.