Tunnel fires of the past have demonstrated that the load-bearing capacity of the supporting structure is reduced by temperature-induced degradation of stiffness and strength of concrete and reinforcing steel as well as by spalling of near-surface concrete layers, eventually causing collapse of the tunnel. In this work, the following questions are addressed:
1. What are the main processes responsible for spalling and what is their individual influence? High-speed camera images allow determination of the size, shape, and velocity of the spalled-off pieces. With this information at hand, the released energy associated with spalling is computed and compared to the energies associated with pore-pressure and thermal-stress spalling. This comparison provides new insight into the impact of various thermal, mechanical, and hydral processes as well as the main influencing parameters controlling explosive spalling of concrete. In addition, permeability experiments are presented, where specimens made of in-situ as well as laboratory-cast concrete with or without additional polypropylene (PP) fibers after cooling from high temperature were tested. The obtained permeability values are related to the pore structure, accessible via mercury-intrusion-porosimetry (MIP) tests, highlighting the effect of the PP-fibers as well as of additives and the production process on transport properties of concrete under fire attack.
2. Which phenomena shall be considered for realistic predictions of temperature distributions within heated concrete? Results from finite-element analyses, taking the coupling between heat and mass transport into account, are compared to results from analyses considering heat transport only regarding their agreement with experimental values. In addition, the obtained gas-pressure distributions within the tunnel lining provide insight into the risk of spalling of concrete for varying amount of PP-fibers, agreeing well with experimental observations.
3. How is the structural behavior of concrete tunnel linings affected by fire loading? A structural analysis tool, considering spalling of near-surface concrete layers as well as the effect of temperature and gas pressure on the mechanical properties of the heated lining concrete, is employed to investigate the structural performance of a cross-section of the Lainzer tunnel (Austria) characterized by low overburden (shallow tunnel).