Concrete is a composite construction material composed of aggregate, such as sand and gravel, bound together by a cement paste that hardens over time. The question of a “melting point” is scientifically misleading because concrete is not a pure substance. Unlike metals that transition cleanly from a solid to a liquid state, concrete undergoes a complex process of thermal degradation and decomposition. Instead of melting, the material experiences a sequence of chemical and physical changes that cause a catastrophic loss of strength and structural integrity. The material’s reaction to fire is better described by its thermal failure characteristics.
Defining Thermal Failure
Concrete fails under high heat through a series of internal decomposition processes rather than a single melting event. The first major step is the loss of physically and chemically bound water within the cement paste. Below \(100^{\circ}\mathrm{C}\), free water evaporates, but between \(100^{\circ}\mathrm{C}\) and \(400^{\circ}\mathrm{C}\), the chemically bound water in the calcium-silicate-hydrate (C-S-H) gel and calcium hydroxide (portlandite) begins to escape in a process known as dehydration. This moisture loss significantly increases the material’s porosity, leading to internal micro-cracking and a reduction in strength.
As temperatures rise above \(400^{\circ}\mathrm{C}\), the chemical breakdown of the cement paste accelerates. The calcium hydroxide dehydroxylates into calcium oxide (lime) and water vapor, a reaction that causes irreversible shrinkage in the cement matrix. The C-S-H gel, which provides the concrete its strength, continues to decompose up to \(700^{\circ}\mathrm{C}\). Above \(800^{\circ}\mathrm{C}\), the calcium carbonate in limestone aggregate or carbonated cement paste undergoes calcination, converting into calcium oxide and releasing carbon dioxide, which further compromises the material’s structure.
Temperature Thresholds for Structural Change
The gradual chemical changes translate directly into measurable losses of load-bearing capacity at specific temperature milestones. Significant strength reduction in concrete begins to occur around \(300^{\circ}\mathrm{C}\), where the material can lose up to \(40\%\) of its original compressive strength. This initial weakening is primarily due to the loss of bound water and the resulting micro-cracking within the cement paste matrix.
Between \(500^{\circ}\mathrm{C}\) and \(600^{\circ}\mathrm{C}\), the material suffers major structural failure due to a combination of factors. The decomposition of calcium hydroxide is nearly complete, and the differential thermal expansion between the aggregate and the cement paste causes substantial internal stress and cracking. Silica-based aggregates, such as quartz, undergo a sudden volume expansion at \(573^{\circ}\mathrm{C}\) that is particularly damaging to the surrounding cement paste.
At temperatures exceeding \(1,000^{\circ}\mathrm{C}\), the concrete is near-totally compromised, having lost between \(70\%\) and \(90\%\) of its original strength. The cement paste and aggregate components may begin to soften or even partially fuse, with some components melting in the range of \(1,150^{\circ}\mathrm{C}\) to \(1,200^{\circ}\mathrm{C}\). At this point, the concrete has largely transformed into an inert, granular ash, and any remaining structural integrity is negligible.
Factors Influencing Heat Resistance
Heat resistance varies based on the specific components used in the mix. The most influential factor is the type of coarse aggregate, which typically makes up \(60\%\) to \(80\%\) of the concrete volume. Concrete made with calcareous (limestone) aggregate generally exhibits higher fire resistance than that made with siliceous (quartz-rich) aggregate. Limestone aggregate does not experience the damaging volume change that occurs in quartz at \(573^{\circ}\mathrm{C}\).
The water-to-cement ratio (w/c) also affects performance; a lower ratio results in denser, less permeable concrete. While a low w/c ratio increases strength, it can paradoxically increase the risk of a physical failure mode under rapid heating. The use of supplementary cementitious materials (SCMs), such as fly ash or slag, can alter the concrete’s thermal properties. SCMs can reduce thermal conductivity, slowing the internal temperature rise, but their impact on high-temperature strength is complex and mix-dependent.
The Danger of Spalling
Spalling is a physical failure mode that occurs when concrete surfaces chip, flake, or explosively fragment under fire conditions. This phenomenon is caused by the rapid buildup of internal pressure from trapped moisture. When the concrete is exposed to intense heat, the free moisture within its pores turns to steam near the heated surface.
If the concrete has low permeability, the steam cannot escape quickly enough, and the resulting pressure exceeds the material’s limited tensile strength. The physical expulsion of chunks of concrete can rapidly expose underlying steel reinforcement to the fire, accelerating structural collapse. High-strength concrete, which is typically denser with fewer internal pathways for steam to escape, is especially susceptible to this explosive failure. The addition of small polypropylene fibers to the concrete mix is a common strategy to mitigate this risk, as the fibers melt under heat, creating channels for the steam pressure to vent.