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114 Concrete tunnel structures can lose their load-bearing Corner spalling capacity through several failure mechanisms. The main mech- Explosive spalling anisms relevant for tunnel linings are (65): Post-cooling spalling. Bending It is important to understand the post-cooling spalling mech- Buckling anism because it leads to a better understanding of a fixed fire Shear suppression system application for structural fire protection. Spalling. Post-cooling spalling occurs after the fire is out, after cool- A loss of resistance against these mechanisms is caused ing down, or maybe even during extinguishing. This type of by a loss of strength of both concrete and reinforcement. spalling was observed with concrete types containing calcare- Buckling, shear, and spalling can also be the result of addi- ous aggregate. An explanation is the rehydration of CaO to tional internal stresses that arise during a fire. Ca(OH)2 after cooling, with an expansion of more than 40%. This occurs after cooling down, when moisture is again pres- When concrete is heated, the temperature increase will ent on the concrete surface (65). result in a loss of (compressive and tensile) strength. Although this effect is dependent on the composition of the The expansions resulting from rehydration cause severe concrete, as in the particular type of aggregate material, the internal cracking on the meso-level and, thus, complete best way to prevent strength loss of the concrete is by reduc- strength loss of the concrete. Pieces of concrete will keep ing the heat penetration. This can be obtained by applying a falling as long as there is water to rehydrate the CaO in the heat-isolating layer on the concrete surface. dehydrated zone. A realistic structural load can be determined by advanced nonlinear finite element simulations. For concrete construc- It appears that the application of a fixed fire suppression tion, a main failure mechanism is spalling. When concrete is system on the very early stage of a fire development can actu- heated, spalling can lead to extensive damage to the construc- ally help cool down the fire and surface and protect the struc- tion. Therefore it is of vital importance to consider the risk of ture, whereas a delay can initiate a post-cooling spalling. This spalling. It is important to note that spalling of concrete is not leads to an understanding of the importance of a reliable fire- directly dependent on the strength of the concrete, but more on detection system and activation of the fixed fire suppression compression, concrete mix design, permeability, and moisture system at the very early stage of fire development to cool down in the concrete. Spalling is a highly relevant failure mechanism the tunnel's walls. By limiting the development of a fire its in concrete tunnel structures, which is driven by the tempera- duration can be limited, resulting in the tunnel structure endur- ture increase rate and thermal gradient over the structure than ing less harsh conditions. by temperature alone. As a result, a conservative upper bound for the fire curve cannot be obtained just by modification of the The question that needs additional studies is: it is well known fire load. It includes other parameters as well, such as ventila- that protecting tunnel structures with heat-resistant coatings or tion conditions and wall properties. materials will reject the heat generated by the fire back into the tunnel environment. In other words, it will not allow for heat to Spalling is an umbrella term covering different damage dissipate through the walls. This could potentially increase the phenomena that may occur to a concrete structure during fire. tunnel heat in the range of 30% or more. This requires addi- These phenomena are caused by different mechanisms: tional studies to answer the question: What is happening to the tunnel environment and tunnel heat by protecting the tunnel Pore pressure rises as a result of evaporating water walls? Will the fire life safety systems, including tunnel fans when the temperature rises. withstand that additional heat component? Compression of the heated surface resulting from a thermal gradient in the cross section. The cooling down of the tunnel's walls could be accom- Internal cracking resulting from differences in thermal plished by using sprinklers on a very early stage of fire devel- expansion between aggregate and cement paste. opment. By limiting the development of the fire the duration of Cracking resulting from differences in thermal expansion/ the fire can be limited and the structure of the tunnel will be deformation between concrete and reinforcement bars. subjected to less harsh conditions. Strength loss owing to chemical transitions during heating. SUMMARY In different combinations of these mechanisms, possible spalling phenomena include: Ventilation has an influence on the fire development: Violent spalling Owing to increased ventilation, the fire development Progressive gradual spalling for a car can be slowed if the fire is ignited at the front