Explosive spalling, often called “exploding concrete,” occurs when the material is subjected to rapid, intense heat, typically during a fire. This violent fracturing is a physical failure driven by extreme internal forces that overwhelm the concrete’s intrinsic strength. Concrete does not combust, but its reaction to high temperatures can compromise structural integrity with little warning. The ejection of fragments results from immense internal pressure buildup within the pore structure, making it a major concern for fire safety and structural resilience.
How Trapped Moisture Generates Explosive Pressure
The primary cause of explosive spalling is the vaporization of free water held within the concrete matrix. Concrete, even when fully cured, retains a certain amount of moisture within its microscopic pores and capillary network. When a fire rapidly heats the surface, the temperature inside the concrete quickly exceeds 100°C (212°F), causing this trapped water to convert into steam.
The volume of water expands dramatically—up to 1,700 times—when it changes phase to steam. In ordinary concrete, this steam can often escape through the porous structure, but in dense, high-strength concrete, the low permeability traps the vapor. This confinement leads to a buildup of internal vapor pressure, which acts outward on the pore walls.
As the fire continues, the trapped steam pressure can quickly reach several megapascals (MPa), which is far greater than the material’s inherent tensile strength. When this vapor pressure exceeds the structural capacity of the concrete, the outer layers are violently expelled in the form of explosive spalling. This process is analogous to a pressure cooker rupturing when its internal pressure relief mechanism is blocked.
The Stress of Differential Thermal Expansion
While steam pressure is the main explosive driver, mechanical stress significantly weakens the concrete, making it susceptible to failure. Concrete is a composite material made of cement paste and aggregate particles, and these components expand at different rates when heated. This difference in thermal expansion between the paste and the aggregate creates internal stresses, known as thermal incompatibility, which generate micro-cracks before the steam pressure becomes fully destructive.
Rapid heating creates a steep thermal gradient, where the concrete surface is extremely hot and expanding rapidly, while the interior remains relatively cool and restrained. This difference leads to compressive stresses on the heated surface and tensile stresses in the cooler interior. These combined thermal stresses can exceed the material’s strength, leading to initial cracking that further compromises the concrete’s integrity.
The difference in expansion coefficients is particularly pronounced at the interface between the cement paste and the aggregate, where cracks often initiate. This mechanical weakening provides pathways for cracks to propagate, which then link up with the stress caused by the internal vapor pressure. Ultimately, this combined effect of thermal stress and hydraulic pressure leads to the catastrophic failure observed as explosive spalling.
Material Characteristics That Increase Vulnerability
Certain material properties of concrete structures dramatically increase their vulnerability to explosive spalling under fire conditions. The density and low permeability of the concrete are significant factors because they prevent the internal steam from escaping safely. Modern high-strength concrete, which is formulated to be highly dense and less porous for durability, is paradoxically more susceptible to spalling than older, more porous mixes.
The initial moisture content of the concrete is another factor; structures with higher moisture content naturally have more water available to vaporize and generate internal pressure. Spalling risk increases substantially when the concrete’s moisture content is above a few percent by mass. Furthermore, the type of aggregate used in the mix affects the concrete’s thermal performance.
Aggregates containing high amounts of siliceous rock, such as quartz, exhibit a higher and more irregular thermal expansion than aggregates like limestone. This greater differential expansion creates higher thermal stresses within the composite material, which contributes to early micro-cracking. The combined presence of high density, high moisture, and thermally incompatible aggregates creates the most vulnerable structures.
Designing Concrete to Resist High Heat
Engineers employ specific material modifications during the mixing phase to mitigate the risk of explosive spalling, especially in structures like tunnels and high-rise buildings. The most effective strategy is to increase the concrete’s permeability to provide a safe escape route for the steam pressure. This is commonly achieved through the incorporation of small polypropylene (PP) fibers into the concrete mix.
These microscopic plastic fibers, typically added in small volumes, melt when the concrete temperature reaches approximately 160°C to 170°C. As the fibers melt, they leave behind tiny, continuous channels or micro-ducts within the concrete structure. These voids act as microscopic vents that allow the internal water vapor to diffuse and relieve the buildup of hydraulic pressure, preventing it from reaching explosive levels.
Alternative mix designs also involve using specific aggregates, such as lightweight or carbonate aggregates like limestone, which are more thermally stable than siliceous types. For structures requiring extreme fire protection, engineers may specify specialized refractory concrete, which is designed with materials that maintain their strength and integrity at temperatures exceeding 1000°C. The strategic use of PP fibers, however, remains the most cost-effective and widely adopted measure for enhancing fire safety in conventional concrete construction.