Concrete spalling assessment methodologies and polypropylene fibre toxicity analysis in tunnel fires

Concrete is by far the largest component of tunnels. Given the high relative humidity in tunnels (e.g. 75%) when compared with buildings in general (e.g. 50%), there is a higher risk of the occurrence of explosive spalling in tunnels during a fire, which increases with increase of the level of pore filling with water in the concrete. Tunnel fires described by hydrocarbon-type fire scenarios are also more severe than building fires described by cellulose fire scenarios (e.g. ISO 834 fire scenario) owing to their confined nature. Passive fire protection in tunnels involves the use of thermal barriers and/or polypropylene fibres in the concrete mix. The latter operates on the pore pressure mechanism of explosive spalling. This paper presents the concept and methodology of the separation of pore pressure spalling from thermal stress spalling for the first time in large-scale experiments as part of the NewCon international research project, by the use of thermally stable lightweight aggregate of negligible thermal expansion. This paper also presents the concept of the pressure induced tangential space (PITS) as a mechanism for increased permeability during fire even before the fibre is melted. The prediction of explosive spalling is still not a fully developed science. Prediction methods include large-scale testing, use of nomograms, theoretical models and numerical models. Numerical modelling, in addition to costly large-scale testing, offers a promising way forward. This paper also introduces for the first time the concept of the expert assessment of spalling in tunnels with a tentative example following a risk-based approach for a given concrete, different traffic conditions and initial pre-fire stress in an example separating tunnel wall. Finally, definitive conclusive calculations for a tunnel example in a severe fire indicates negligible toxicity from the combustion of polypropylene fibres used in tunnel concretes to combat explosive spalling. This work was carried out as part of the NewCon international research project.