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 the...
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