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Concrete in fire

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Contents
  1. 1Heat transfer and moisture
  2. 2Spalling
  3. 3Strength and stiffness
  4. 4Expansion and structural response
  5. 5Fire resistance and protection
  6. 6Assessment and uncertainty
  7. 7Limits and common misunderstandings
  8. 8Evidence and model review
  9. 9See also
  10. 10Notes
  11. 11References

Concrete in fire describes the thermal, physical and structural changes that occur when concrete elements are exposed to high and rapidly rising temperatures. These include heat penetration, moisture-driven pressure, loss of strength and stiffness, expansion, cracking and spalling. The response depends on concrete mix, moisture, aggregates, reinforcement, loading and fire exposure.[1][2]

Concrete is noncombustible but can change substantially during heating. Its microstructure, moisture and thermal properties affect how energy moves through the element and whether surface layers remain intact. Noncombustibility therefore does not guarantee fire resistance or continued load-bearing performance.[3][1]

Assessment must connect material behaviour with structural response. Reinforcement temperature, concrete cover, loading and restraint all influence stability, while loss of concrete section can expose deeper material and accelerate deterioration.[2][4]

Heat transfer and moisture

Thermal conductivity determines how readily heat moves through concrete, while density and specific heat determine energy storage. Thermal diffusivity combines these properties and influences how quickly temperature rises at a given depth. These properties can vary during heating.[5][1]

Concrete is heterogeneous, with cement paste, aggregates and pores. Water held in those pores affects heat capacity and can evaporate, migrate and condense elsewhere, changing the local energy balance. A dry constant-property model can therefore miss important transient effects.[6][5]

Moisture also affects pore pressure. Kodur and Harmathy describe a moisture clog near the heated surface, where low permeability restricts vapour escape and produces strong pressure gradients. The resulting internal pressure can contribute to spalling when it exceeds the concrete's tensile capacity.[1]

Heat drives moisture into a low-permeability concrete clog, raising pressure and cracking the surface.
Moisture clog and pressure can contribute to spalling. Based on Kodur and Harmathy (2002), p. 1-163.

Their example gives a vapour pressure of approximately 8 MPa at 300 °C, compared with tensile strength of approximately 5 MPa for the discussed high-strength mix. These are source-specific values illustrating a mechanism, not a universal spalling threshold for every concrete.[1]

Example vapour pressure 8 MPa versus tensile strength 5 MPa at 300 °C.
Example of internal pressure exceeding concrete tensile strength. Based on Kodur and Harmathy (2002), p. 1-163.

Spalling

Spalling is the breaking away of layers or pieces from a concrete surface during high and rapidly rising temperature exposure. It can occur soon after heating and may be explosive, or it can happen later when cracking and weakness allow pieces to fall away. Extensive spalling can expose reinforcement and penetrate partitions.[1]

High-strength concrete is considered particularly susceptible because low permeability and a low water-cement ratio hinder vapour escape. Kodur and Harmathy report full-scale column comparisons in which high-strength concrete showed substantial spalling. These observations support careful evaluation rather than a simple strength-based assumption of better fire performance.[1][4]

Pore-pressure buildup is one proposed mechanism, while other researchers explain spalling through fracture mechanics and restrained thermal dilation. Compressive stresses parallel to the heated surface can then release through brittle fracture. The appropriate explanation can vary with concrete and exposure.[1]

Spalling exposes deeper concrete to fire temperatures and increases heat transfer to inner layers and reinforcement. Loss of concrete section and direct reinforcement heating can then reduce structural capacity. The timing and extent of section loss must therefore be included in structural analysis.[4]

The main influencing factors include mix strength and porosity, aggregate type, density, load intensity, fire intensity and relative humidity. These variables prevent a single spalling prediction from being applied to every concrete element.[4]

Strength and stiffness

Strength reduction at elevated temperatures affects both concrete and reinforcement. Fleischmann and Buchanan show different strength-temperature relationships for carbonate, siliceous and sand-lightweight aggregate concretes. Aggregate selection therefore matters when evaluating high-temperature behaviour.[2]

Concrete section with cover, reinforcement and spalled region reducing protection and stiffness.
Concrete and reinforcement respond together to heat and section loss. Based on Fleischmann and Buchanan (2002), pp. 4-241–4-242 and Kodur and Harmathy (2002), p. 1-164.

Modulus of elasticity also changes, affecting stiffness and deformation. Their figures present temperature-dependent curves for hot-rolled steel and for different concretes, showing why ambient values alone may not represent structural response. Both stress and deformation must be evaluated at temperature.[2][7]

Reinforcing and prestressing steel have their own strength-temperature relationships and critical-temperature conventions. Kodur and Harmathy give 593 °C for reinforcing steel and 426 °C for prestressing steel in their review, while noting that failure depends on applied load. These values are source-specific criteria rather than universal limits.[1]

Concrete cover protects reinforcement by delaying heat penetration. Minimum dimensions and cover thickness can therefore be used to manage reinforcement temperature and fire resistance. Spalling or cracking that removes this cover defeats part of that protection.[1][4]

Expansion and structural response

Concrete and steel expand during heating, producing deformation and, where restrained, additional forces. Fleischmann and Buchanan note that thermal expansion can improve fire resistance in some arrangements by changing structural response. Its effect must therefore be evaluated within the actual system rather than assumed to be purely harmful.[7]

Connections and adjacent construction determine restraint. Thermal gradients also create differential expansion through the section, which can produce curvature and cracking even without external restraint. Load paths and end conditions should be represented explicitly.[7][8]

Tensile membrane action is one structural mechanism through which elements or systems can redistribute forces after local damage or deformation. Its relevance depends on reinforcement, connections and the ability of the surrounding system to develop the required tensile response. Material loss and elevated-temperature properties must be included when assessing it.[7][4]

Structural fire engineering connects these material and system responses with the fire exposure and heat-transfer calculation. A concrete temperature alone does not establish failure; stress, deformation, section loss and stability must be compared with the required criteria.[9][10]

Fire resistance and protection

Fire resistance requires structural adequacy, integrity and insulation as applicable. Concrete elements can fail through loss of strength or stability, through cracks and openings, or by transmitting too much heat. Each criterion needs its own evaluation.[11][8]

Protective concrete cover, boards, coatings and other membranes can delay heating and preserve capacity. Their thickness, continuity, attachment and behaviour under cracking affect performance. A complete assembly, rather than only the concrete mix, determines protection.[12][9]

Fire-resistant design should also address unexposed-side conditions. Intact concrete can conduct heat to adjacent rooms and combustibles, so insulation must be assessed alongside integrity and stability. A wall that remains standing can still allow harmful thermal exposure.[13][5]

Testing and analysis should include actual loading and restraint. ASTM E119 uses a standard curve, but natural fires can differ in severity and duration. Applying a rating to another exposure requires appropriate heat-transfer and structural models.[9][8][14]

Assessment and uncertainty

A concrete fire assessment should start with mix composition, moisture, aggregates, porosity and element geometry. It should then define the fire exposure, heat-transfer model and structural criteria. This makes material and system assumptions visible.[1][2]

Spalling introduces discrete section loss that a smooth temperature model may miss. Where it is credible, assessment should consider its onset, extent and effect on reinforcement exposure and section capacity. Test evidence at the relevant scale is especially valuable for this mechanism.[4][2]

Property data must be temperature dependent. Conductivity, diffusivity, strength and stiffness change during heating, and heating and cooling paths can differ. Using ambient values throughout can produce a misleading temperature and structural prediction.[1][5]

Uncertainty also comes from exposure, loading and restraint. Sensitivity to those conditions should be evaluated alongside material variation. A single critical temperature or spalling rule cannot capture all of them.[9][15]

Limits and common misunderstandings

Concrete is noncombustible, but that does not make it fireproof. Heat can cause spalling, strength loss, expansion, cracking and excessive temperature transmission. Fire resistance depends on the complete element and its exposure.[1][4]

High ambient strength does not guarantee better fire performance. Low-permeability high-strength mixes can be particularly vulnerable to moisture-driven pressure and explosive spalling. Mix design and moisture conditions must be included.[1][4]

Spalling is not only an aesthetic defect. It can expose reinforcement, accelerate internal heating and reduce structural capacity or partition integrity. Its structural consequences must be evaluated rather than treating all surface loss as equivalent.[4]

Finally, a reinforcement critical temperature is not a universal concrete failure temperature. Failure depends on concrete condition, reinforcement, cover, loading and structural criteria. The complete temperature and load history determines performance.[1][2]

Evidence and model review

Material evidence should distinguish bulk concrete properties from assembly behaviour. Cement paste, aggregates, pores and reinforcement each influence the result, and their condition changes during heating. A single concrete label can therefore hide important differences between otherwise similar elements.[6][2]

Spalling evidence should identify the concrete mix, heating rate, load and moisture condition. Explosive loss in one high-strength specimen does not establish the same onset in another element, but it does show that the mechanism must be considered. Reporting only a temperature without those conditions would be incomplete.[1][4]

Moisture modelling should include both energy and transport. Evaporation absorbs heat, while migration and condensation can move energy to other regions and change permeability. These processes affect pore pressure as well as temperature, so a purely conductive model may be insufficient.[5][1]

The structural model should include section loss where spalling is credible. Removing cover changes both the remaining section and the reinforcement's thermal boundary, accelerating subsequent deterioration. A model that keeps the original intact geometry throughout would miss that feedback.[4][2]

Aggregate and reinforcement properties should be selected for the actual materials. Their strength and stiffness curves differ, and the reinforcing steel can have a different critical-temperature basis from the surrounding concrete. Combining one generic curve with another material would not represent the composite correctly.[2][1]

Restraint and load redistribution require system-level evidence. Connections and adjacent elements can resist expansion, create new forces or provide alternative load paths, including membrane action where applicable. Evaluating only an isolated material coupon cannot establish that system response.[7][8]

Model comparison should use matching temperatures and failure definitions. A reinforcement temperature, an average concrete temperature and an unexposed-surface criterion are different quantities. Confusing them can produce an apparent pass or fail that does not correspond to the actual performance requirement.[11][8]

A final review should address uncertainty in fire exposure, mix properties, moisture, spalling and restraint. These can change both the timing and the mode of failure. Presenting alternatives makes the result more useful than one precise but unsupported concrete temperature or fire-resistance duration.[15][14]

A final assessment should preserve the difference between concrete as a material and concrete as part of a structure. Mix properties explain heat and moisture response, but reinforcement, connections and loading determine structural performance. Both levels of evidence are needed for a fire-resistance conclusion.[1][2]

Spalling should not be treated as a binary material property. Its extent can range from limited surface loss to deep section loss that exposes reinforcement and breaches a partition. The assessment should state which of those consequences is being evaluated and how it changes the remaining thermal and structural path.[1][4]

The unexposed side should also be reviewed. Concrete can remain in place while conducting enough energy to heat adjacent rooms or fuel, so insulation must be assessed separately from integrity and stability. A barrier that appears intact can therefore still allow harmful thermal exposure.[13][5]

Post-fire condition requires the same care. Temperature-dependent properties, cracking, spalling and connection damage can leave reduced capacity even after the environment cools. Residual strength and deformation should be assessed from the element's condition rather than from its original rating alone.[2][5]

A clear report therefore identifies the mix, moisture, reinforcement, protection, exposure and performance criteria. It also records whether spalling and other discrete losses were included. This allows the concrete's fire response to be evaluated without treating noncombustibility as proof of structural safety in every fire scenario.[11][1]

The fire's duration should be considered alongside its maximum temperature. Prolonged heating can drive moisture and heat deeper, change pore pressure and reduce strength even when the peak is modest. A short peak and a long lower exposure therefore require different assessments of spalling and structural response.[5][14]

Protection should also be checked for its effect on unexposed details. Cover, membranes and coatings can protect broad surfaces while leaving connections or edges exposed. Those local conditions can control cracking, reinforcement temperature and the final load path, and should not be lost in an average assembly description.[12][9]

A final check is whether the concrete's actual performance criteria are represented. Strength, stiffness, section integrity, insulation and stability can fail at different times and through different mechanisms. The assessment should identify which governs the required function instead of treating all concrete deterioration as one failure mode.[11][2]

The report should also separate observed spalling and damage from modelled exposure. A predicted temperature does not demonstrate a particular spalling extent, and an observed surface condition does not establish the whole fire history. Linking material evidence to a defined exposure is necessary for a defensible conclusion.[1][14]

See also

Notes

  1. ↑ 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 Kodur and Harmathy (2002), pp. 1-163–1-164.
  2. ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 2.12 Fleischmann and Buchanan (2002), pp. 4-241–4-242.
  3. ↑ Kodur and Harmathy (2002), p. 1-155.
  4. ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 Kodur and Harmathy (2002), p. 1-164.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 5.7 Rockett and Milke (2002), p. 1-29.
  6. ↑ 6.0 6.1 Kodur and Harmathy (2002), p. 1-156.
  7. ↑ 7.0 7.1 7.2 7.3 7.4 Fleischmann and Buchanan (2002), p. 4-242.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 Milke (2002), pp. 4-209–4-210.
  9. ↑ 9.0 9.1 9.2 9.3 9.4 Milke (2002), p. 4-209.
  10. ↑ Cadorin et al. (2003), p. 430.
  11. ↑ 11.0 11.1 11.2 11.3 Ministry of Business, Innovation and Employment (2025), p. 64.
  12. ↑ 12.0 12.1 White (2002), pp. 4-259–4-260.
  13. ↑ 13.0 13.1 Australian Building Codes Board et al. (2005), p. 1.6-7.
  14. ↑ 14.0 14.1 14.2 14.3 Lie (2002), pp. 4-201–4-203.
  15. ↑ 15.0 15.1 Van Rossum (2027), §1.7.

References

  • Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Cadorin, J.-F., Pintea, D., Dotreppe, J.-C., & Franssen, J.-M. (2003). A tool to design steel elements submitted to compartment fires: OZone V2. Part 2: Methodology and application. Fire Safety Journal, 38, 429–451.
  • Fleischmann, C., & Buchanan, A. (2002). Analytical methods for determining fire resistance of concrete members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Lie, T. T. (2002). Fire temperature-time relations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Milke, J. A. (2002). Analytical methods for determining fire resistance of steel members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.). New Zealand Government.
  • Rockett, J. A., & Milke, J. A. (2002). Conduction of heat in solids. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
  • White, R. H. (2002). Analytical methods for determining fire resistance of timber members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
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