Tunnels are complex, expensive constructions with a significant impact on society, so it is crucial to ensure that in crisis situations in tunnels, like fire, the users present run the least possible risk. TNO research must ensure the safety and sustainability of concrete tunnels, e.g. in case of fire.
Every day cyclists, cars and trucks use tunnels that sometimes lie beneath the built environment. Authorities have a considerable burden of guaranteeing internal and external safety for tunnels. In crisis situations like fire it is not always clear how concrete constructions will react, and the existing fire safety measures are not always the best option in terms of performance or financial aspects. Quite simply, because tunnel constructions are complex, they comprise different components and demand knowledge from various areas of expertise. Together with the universities TU Delft and TU Eindhoven, among others, TNO is carrying out a study of the different components of concrete constructions with the aim of boosting insight and ultimately arriving at a validated model that allows account to be taken of the fire resistance of a tunnel already in the design phase.
In the event of a tunnel fire the concrete and the moisture in the concrete heat up. This moisture undergoes a phase change and becomes gaseous, which increases the pressure in the concrete. This can cause the concrete to spall. Currently, the solutions to prevent spalling, are both limited and expensive. One possibility is to apply fire-resistant lining to the tunnel walls. Another possibility is to add polypropylene fibers to the concrete. These fibres disintegrate at high temperatures and form little channels through which the gas can be gradually removed. Modelling and testing provides more information on how the spalling process really works and which areas of expertise are relevant in this matter.
At the technology universities of Delft and Eindhoven three PhD students are currently working on fundamental research into concrete spalling in a project funded by the Dutch Technology Foundation STW. They are investigating the moisture transport and microstructure of concrete through measurements and modelling. In addition, TNO undertook concrete spalling tests together with Efectis Nederland BV, the former TNO Centre for Fire Safety, to obtain more insight in the major variables that determine concrete spalling, such as additive materials and moisture content. The ultimate goal of the research is, among other things, to develop a model, which allows to judge fire resistance measures already in the design phase of tunnels. A fully validated model would be a breakthrough for the authorities, legislators, policymakers, suppliers and builders that are together responsible for safe tunnel constructions.