Souvik Saha

Experimental and Numerical Investigation of Fire-Induced Concrete Spalling Under Biaxial Stresses

Unlike metals, brittle materials like concrete and rocks undergo fracture without significant plastic deformation prior to failure. Tensile fracturing of concrete can therefore be abrupt and violent, leading to shattering without any pre-warning. Such spontaneous fractures in the field are commonly known as stress spalling or bursting caused by buckling of the rock layers detached by the tensile fractures in underground rock structures or in concrete structures at high temperatures. Fire-induced spalling in concrete have jeopardised the safety of personnel, seriously damaged structures, and shut down operations from months to more than a year, or permanently. The fire-induced spalling was responsible for the structural collapse of the Mont Blanc tunnel in 1999, which resulted in 39 deaths and 34 injures and in the rehabilitation cost of over 300 million euros. However, there are yet limited engineering knowledge of governing mechanisms in concrete spalling due to the erratic nature of this phenomenon and the inhomogeneity of concrete mixes. Given the complex thermo-hydro-chemo-mechanical nature of brittle concrete fractures, many factors have been deemed to affect concrete bursting failure modes including the stress path, rate of mechanical loading/unloading, moisture condition and build-up of pore pressure during damage accumulation, heated area, heterogeneity, and certain concrete admixtures. Nevertheless, the presence of intermediate principal stress and its effect on the spalling failure in many of the available models has largely been ignored. This present study aims to develop new methods of monitoring, predicting and preventing dangerous failures in concrete surfaces under stress conditions in underground tunnels that have received very little attention in the literature. Both experimental and numerical studies have been proposed to be conducted on the small-scale as well as large-scale tunnel models under elevated temperature. The findings of these studies would be utilized in developing the prediction models and to propose new design guidelines for preventing fire-induced spalling in precast and shotcrete concrete.