Spalling behavior of concrete
Fire can cause mechanical stress and chemical changes inside the structural concrete, which may decrease mechanical properties such as strength or the modulus of elasticity. The concrete heats up from the edge areas, causing drainage and evaporation processes to take place. This causes steam pressure, leading to explosive cracking of the concrete, known as spalling or temperature shock.
This behavior becomes more marked as the strength of the structural concrete increases: the pore volume in high-strength concretes are reduced, lowering their permeability. High-performance concretes have proven to be particularly prone to spalling. In addition, smoke can enter the concrete through hairline cracks that develop when the material is exposed to fire, accelerating the carbonatation process or introducing chlorides, for example, and corroding the reinforcing steel.
The damage to the structural concrete and subsequent fire damage in underground transport systems may have serious financial consequences for the tunnel operator, as the repair and downtime of the structure can lead to significant costs.
If the concrete is heated quickly and high thermal stresses occur, considerable damage may be caused to the concrete structural elements.The impact on the surrounding infrastructure must also be taken into consideration – for example, if vital transport routes are unusable for long periods of time.
Preventive structural fire-protection measures prevent spalling of the concrete surface and thus exposure of the load-bearing reinforcement. AESTUVER fire-protection boards keep harmful temperatures away from the concrete structural elements. This can eliminate or significantly shorten the duration of the expensive, (time- and cost-intensive) reconstruction and thus closure of the tunnel.