Concrete is a composite material made from a binder, water, and aggregate, and it does not melt like ice or metal at a single, distinct temperature. The material is a heterogeneous mixture of cement paste, sand, and gravel, which means its components react differently to extreme heat. Instead of liquefying, concrete undergoes a process of chemical decomposition and physical degradation when exposed to fire. This breakdown leads to a progressive loss of structural integrity, eventually causing the material to fail and crumble long before any true melting point is reached. The focus for understanding concrete’s behavior under fire must shift from a “melting point” to a “failure temperature.”
The Dehydration Process
The initial stage of concrete degradation is marked by the loss of water held within its structure. At temperatures around 100°C (212°F), the free water and loosely bound capillary water trapped in the pores begin to evaporate and turn into steam. This process is followed by chemical breakdown, known as dehydration, which starts to occur in the cement paste at temperatures beginning near 200°C (392°F). The most significant component affected is the Calcium Silicate Hydrate (CSH) gel, the primary binder that gives concrete its strength. This gel holds chemically bound water, and as this water is driven off, the microstructure of the CSH begins to collapse, which directly weakens the material.
The progressive loss of this bound water increases the internal porosity of the material, a change that is irreversible. The strength loss associated with CSH decomposition is a main mechanism of fire-induced failure. While CSH breakdown occurs across a wide range, the process becomes much more significant above 600°C. The resulting material is chemically altered, possessing a different, weaker microstructure than the original hardened concrete.
Temperature Thresholds for Structural Integrity
Concrete’s ability to bear a load is severely diminished across specific temperature ranges due to internal chemical changes. A significant reduction in compressive strength often begins between 300°C (572°F) and 600°C (1112°F), with many concrete types losing up to half their strength in this range. This loss is attributable to the thermal decomposition of calcium hydroxide (portlandite), which occurs between 400°C and 550°C. The decomposition of portlandite yields lime and water vapor, which causes the cement paste to shrink and contributes to extensive internal cracking.
Once temperatures exceed 600°C (1112°F), the remaining CSH gel breaks down rapidly, and the material’s load-bearing capacity is greatly reduced. Above 800°C (1472°F), the concrete structure essentially crumbles, having lost most of its original strength due to the complete transformation of the binding paste. Concrete exposed to temperatures this high is generally irreparable and structurally unsound. The failure point for a concrete structure is dictated by these chemical decomposition temperatures, not by the much higher theoretical melting temperatures of its individual components.
How Concrete Mix Design Influences Fire Resistance
The specific composition of the concrete mix has a profound impact on how the material reacts to high temperatures. The type of aggregate used is a primary factor influencing fire resistance, as different stones react at different points of thermal stress. For instance, concrete made with siliceous aggregates, like quartz, is more prone to damage. Quartz undergoes a sudden volume change near 570°C (1058°F), creating severe internal stresses and cracking within the cement paste, accelerating the rate of strength loss.
Conversely, concrete containing limestone or calcareous aggregates tends to perform better at these temperatures because their thermal expansion is more gradual and they do not undergo a destructive phase change. The water-to-cement ratio also plays a role, as a lower ratio often results in a denser paste with fewer internal pathways for steam to escape. This density can increase the risk of explosive spalling, a physical degradation mechanism. Additionally, the inclusion of supplementary cementitious materials (SCMs) can alter the CSH gel composition, sometimes enhancing stability and improving the concrete’s residual strength after fire exposure.
Spalling and Collapse
The most visible physical outcome of concrete exposure to fire is spalling, which is the flaking, popping, or bursting of the concrete surface. Spalling is primarily caused by the buildup of high internal vapor pressure as the free and chemically released water is converted to steam. Because concrete has low permeability, especially high-strength mixes, this steam cannot escape quickly, leading to immense pressure that overcomes the concrete’s tensile strength. This violent expulsion of material exposes the deeper layers of the concrete and the embedded steel reinforcement (rebar).
When the rebar is exposed, it heats up much faster than the surrounding concrete, leading to a quick reduction in its yield strength. Since the steel provides the structural element with tensile strength, its failure accelerates the overall collapse of the concrete member. Furthermore, the combination of high internal pressure and the thermal stresses contributes to the physical disintegration. Spalling is a physical mechanism that dramatically hastens structural failure by removing the protective concrete layer and weakening the steel support.