Explosive Spalling of Concrete Structural Elements During Fire: Definition & Factors

The phenomenon of explosive spalling in concrete structural elements during a fire represents a critical aspect of fire safety engineering. Explosive spalling occurs when concrete, subjected to high temperature, experiences rapid internal pressure build-up due to the release of steam from the evaporation of moisture within its pores. This sudden pressure can lead to the violen t expulsion of concrete fragments, compromising the structural integrity of the elements. Understanding the mechanisms triggering explosive spalling is paramount in designing resilient structures, as it aids in formulating strategies to minimize the risk and enhance fire safety measures.

This discussion encompasses the definition of Spalling in Concrete and the factors that contribute to explosive spalling of concrete structural elements during extreme fire.This topic is important for all the upcoming Civil Engineering Examinations, including SSC JE CE and RRB JE Civil.

What is Spalling of Concrete?

Exposure of structural concrete to fire may result in the occurrence of explosive spalling. This phenomenon occurs when the free water within the concrete, which has not participated in the hydration reaction and remains unused, transforms into steam due to the heat from the fire. If the steam is not adequately released, it can lead to explosive spalling. This process poses a risk as it removes the concrete cover protecting the reinforcement, exposing the steel bars to the fire. Consequently, this weakens the reinforcement bars and diminishes the ultimate load-carrying capacity of the reinforced concrete element. 

The different factors affecting Spalling of Concrete due to fire exposure are:

Heating Rate of Concrete

The rate of temperature rise significantly impacts the likelihood and severity of explosive spalling in concrete when subjected to fire.

Heating Exposure of Concrete

The probability of explosive spalling increases with the number of reinforced concrete elements exposed to fire, with certain shapes, like simple external forms, exhibiting better fire response.

Size of the Section of Reinforced Concrete

Extremely thin walls are less prone to spalling explosions, possibly due to the ease of water escape and reduced pore water pressure, while thick sections, such as nuclear containment walls, have a lower likelihood of explosions.

Shape of Reinforced Concrete Section

Cross sections that change rapidly, such as level surfaces and round corners, are more susceptible to explosive spalling, especially at extreme angles during fire exposure.

Moisture Content of Reinforced Concrete Member

Explosive spalling is likely in normal strength concrete with moisture content over 2%, while it rarely occurs with moisture content below 2%. The risk decreases with moisture content less than 3%, influenced by environmental factors.

Permeability of Reinforced Concrete Element

Concrete permeability significantly influences vapor release rate, with rare explosive spalling occurrences reported when permeability is smaller than (5 x 10 -11 ).

Age of Reinforced Concrete Member

The likelihood of explosive spalling increases with the age of concrete elements, potentially due to a decline in moisture content over time.

Strength of Concrete

Explosive spalling is rarely expected in low-strength concrete, while high-strength concrete carries a greater possibility of occurrence.

Compressive Stress and Restrain

Increasing compressive stress and restraint on the element raises the likelihood of explosive spalling, achievable by reducing section size or applying heavier loads.

Type of Aggregates

The likelihood of explosive spalling decreases with aggregates of low thermal expansion, such as siliceous, limestone, basalt, and lightweight aggregates, particularly when dry.

The Size of Aggregate

Larger aggregate sizes are associated with a lower likelihood of explosive spalling, as demonstrated by experimental tests.

Cracking of Concrete Member

Cracks facilitate moisture escape in concrete and also serve as a source for crack propagation during explosive spalling.

Reinforcement Utilized in Concrete

Explosive spalling occurs in unreinforced portions, not extending beyond reinforcement layers like cages of stirrups and longitudinal bars in different types of columns and beams.

Concrete Cover to Reinforcement

An increased concrete cover raises the possibility of explosive spalling, with larger covers linked to heightened risks, especially exceeding 40 mm for dense concrete and 50 mm for lightweight concrete.

Supplementary Reinforcement

Supplementary reinforcement doesn't prevent explosive spalling but limits its effects. Mesh reinforcement can be provided to mitigate spalling effects when the concrete cover exceeds 40 mm.

Additions

Continuous efforts have been made to enhance concrete's fire resistance by adding steel fiber, polypropylene fiber, or through air entrainment.

Advantages of Understanding Explosive Spalling in Concrete

While explosive spalling itself is a destructive phenomenon, understanding and studying it offers the following indirect advantages:

• Improved Fire-Resistant Design: Knowledge of spalling mechanisms allows engineers to design concrete mixes and structural forms that are more fire-resistant, incorporating fire retardants or fibers to mitigate risks.
• Enhanced Safety Protocols: Understanding how spalling occurs helps in planning evacuation strategies and construction site safety measures during fire emergencies.
• Better Material Selection: Helps in selecting appropriate materials (like polypropylene fibers, lightweight aggregates) that reduce internal vapor pressure buildup and spalling risk.
• Development of Fireproofing Techniques: Drives innovation in coatings, insulation layers, and fire-barrier technologies for buildings and tunnels.

Disadvantages of Explosive Spalling of Concrete Structural Elements During Fire

• Structural Instability: Rapid loss of surface layers in concrete weakens the load-bearing capacity, increasing the risk of partial or total structural collapse.
• Exposure of Reinforcement Bars: Spalling exposes steel reinforcement (rebars) to direct flame and high temperatures, significantly reducing their strength and leading to rapid structural degradation
• Sudden and Violent Nature: The explosive nature of spalling can pose direct risks to occupants, rescue personnel, and nearby equipment or infrastructure.
• Difficult to Predict: Spalling behavior is influenced by various parameters (moisture, heating rate, permeability), making it challenging to predict accurately in design models.
• Increased Maintenance and Repair Costs: Post-fire rehabilitation of spalled concrete is expensive and time-consuming, requiring complete re-assessment of the damaged structure.

This discussion explained the Explosive Spalling of Concrete Structural Elements during Fire. If you are preparing for State and Central level Civil examinations and other diploma-level exams, get enrolled in AE/JE Civil Coaching on the Testbook App.Also, attempt a wide range of SSC JE Civil Mock Tests and SSC JE Civil Previous Year Papers available on the Testbook App itself.