Safety Assessment of Concrete Tunnel Linings under Fire Load

An assessment of the structural performance of concrete tunnel linings under fire load is presented for the Lainzer Tunnel, a shallow tunnel recently constructed in Austria. Prior to the installation of the concrete lining at this tunnel, large-scale temperature experiments were conducted, revealing almost no reduction of the lining thickness in consequence of spalling for the employed lining concrete, reinforced by polypropylene fibers. In the absence of spalling, dehydration of concrete under high temperatures is the safety-determining process. Within the thermochemomechanical material model employed for the tunnel-safety assessment presented in this paper, dehydration is described by the decrease of the hydration extent, resulting in the reduction of both stiffness (thermal damage) and strength (thermal decohesion). The distribution of the hydration extent is determined by means of a thermochemical analysis. The obtained results serve as input for the subsequent chemomechanical analysis, providing insight into the deformation state of the ground-lining compound structure and the stress state in the lining. As a function of the stress state, a level of loading $\mathcal{L}$ is introduced, with $\mathcal{L}=0$ for the unloaded material and $\mathcal{L}=100\%$ for stress states reaching the compressive strength of concrete. During fire exposure the size of $\mathcal{L}$ increases because of increased loading by thermal expansion, on the other hand, and because of thermal decohesion resulting in a reduction of the compressive strength, on the other hand. The difference between 100% and $\mathcal{L}$ represents the remaining load-carrying capacity of the lining, giving access to the safety of the tunnel for the chosen fire-accident scenario.