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Concrete Spalling During Tunnel Fires

27/01/2025

Concrete spalling is a critical issue in tunnel fires, posing significant risks to structural integrity and safety. It's essential to understand the mechanisms behind spalling, the chemical transformations involved, and the measures to mitigate its effects. This article delves into the processes and temperature ranges at which these transformations occur.

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Concrete Spalling Mechanisms

When a fire occurs, it induces mechanical stress and chemical changes within the structural concrete. These changes can degrade mechanical properties such as strength and elasticity. The concrete heats up from the outer edges, leading to drainage and evaporation processes. This results in steam pressure buildup, causing explosive cracking of the concrete, known as spalling or temperature shock.
 

Chemical transformations in concrete during a tunnel fire:

/concrete-spalling-tunnel-fire-infographic

 

The infographic on the top left outlines several key chemical transformations
that occur in concrete during a fire, each associated with specific temperature ranges:



  1. Loss of Chemically and Physically Bound Water at 1 atm:
  • Temperature Range: Around 100°C (212°F).
  • Process: physically bound water is lost, leading to initial weakening of the concrete structure.
     

       2. Hydrothermal Reactions and Loss of Chemically Bound Water:

  • Temperature Range: Begins at lower temperatures and continues up to around 300°C (572°F).
  • Process: As the temperature rises, chemically bound water within the concrete starts to evaporate. This increases the permeability of the concrete, making it more susceptible to further damage.
 

      3. Dehydration of Some Flint Stones: 

  • Temperature Range: Approximately 300°C (572°F).
  • Process: Flint stones within the concrete begin to dehydrate, contributing to the overall weakening of the material.


      4. Portlandite Dehydration:

  • Temperature Range: Around 400°C (752°F).
  • Process: Portlandite (calcium hydroxide) dehydrates, resulting in a significant loss of strength in the concrete.


      5. β-Quartz Transition in Aggregates:

  • Temperature Range: Around 573°C (1063.4°F).
  • Process: Quartz aggregates within the concrete undergo a phase transition, contributing to internal stresses and potential spalling.


      6. Marked Increase in Thermal Creep:

  • Temperature Range: Around 600°C (1112°F).
  • Process: Thermal creep increases significantly, causing the concrete to deform under sustained high temperatures.
 

       7. Decomposition of Carbonates:

  • Temperature Range: Around 700°C (1292°F).
  • Process: Carbonates within the concrete decompose, releasing carbon dioxide and further weakening the material.
 

      8. Ceramic Bond Formation:

  • Temperature Range: Around 800°C (1472°F).
  • Process: At this stage, a ceramic bond forms within the concrete, which can provide some temporary stability but is ultimately brittle.
 

      9. Total Loss of Hydration Water:

  • Temperature Range: Around 800°C (1472°F).
  • Process: All hydration water is lost, leading to a complete breakdown of the concrete's internal structure.
 

     10. Concrete in the Melting Phase:

  • Temperature Range: Begins around 1200°C (2192°F) and continues upwards.
  • Process: The concrete starts to enter a melting phase, where its structural integrity is severely compromised.
 

Factors Influencing Concrete Spalling in tunnel fire

The tendency for spalling increases with the strength of the concrete. High-strength concretes have reduced pore volumes, which lowers their permeability and makes them more susceptible to spalling. High-performance concretes are particularly prone to this phenomenon. Additionally, smoke can infiltrate the concrete through hairline cracks formed during a fire, accelerating carbonation or introducing chlorides, which can corrode the reinforcing steel.

 

Consequences of concrete spalling behaviour in tunnel fire

The damage caused by spalling and subsequent fire damage in underground transport systems can have severe financial implications for tunnel operators. Repair and downtime can lead to significant costs for tunnel operators, and the impact on surrounding infrastructure, such as vital transport routes, must also be considered. The structural integrity of the tunnel is compromised, which can lead to long-term safety concerns and potential legal liabilities for the operators. Together with STUVA (German tunnel authority), Aestuver studied in detail the costs of passive fire protection in tunnels. You can download the research paper here
 

How to prevent concrete spalling in tunnel fires?

Implementing passive structural fire-protection measures is crucial to prevent spalling and protect the load-bearing reinforcement.

Aestuver® fire-protection boards, for example, can shield concrete structural elements from harmful temperatures, reducing the duration and cost of repairs and minimizing tunnel closure times. These boards act as a barrier, preventing the heat from reaching the concrete and thus maintaining its structural integrity during a fire.


Besides preventing concrete spalling, Aestuver fire protection boards have numerous unique properties. You can find out more why you should choose Aestuver fire protection panels here.

Easy installation process of tunnel fire protection sytem is one of the major benefits in choosing a method to prevent concrete spalling. 

At which temperature does concrete spalling occur?

It is extremely difficult to predict the exact temperature at which spalling may occur. Different concrete mixes behave differently. What we can see in the fire protection specifications is usually a requirement to prevent concrete surface from reaching maximum temperatures of 380°C. However, Aestuver fire protection systems were applied in tunnels where the designed maximum concrete surface temperature was much lower.
Based on our experience, the only method to correctly assess the spalling temperature of concrete is via real life fire test with the actual concrete mixture. An example of such a test can be found in "Fire testing procedure for concrete tunnel linings and other tunnel components" by Efectis.
 

Conclusion

Understanding and mitigating concrete spalling during tunnel fires is vital for maintaining the safety and integrity of underground transport systems. Utilising relevant tunnel fire curves for design is one of the starting points. By employing effective fire-protection measures, we can significantly reduce the risks and financial impacts associated with tunnel fires. The detailed analysis of chemical transformations and temperature ranges provided in the infographic underscores the complexity of the spalling process and the importance of comprehensive fire protection strategies. Through a combination of passive and active measures, along with regular maintenance and emergency preparedness, we can enhance the resilience of tunnels and ensure the safety of their users.

Reach out to our Aestuver team for further support in tunnel fire protection matters.

Do you want to learn more about the passive fire protection? Head over to our knowledge centre for more documents, data sheets, white papers, brochures. 

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