Relationship between char depth of wood and cumulative heat exposure for fire investigation

Abstract

Char depth can be a useful measurement to help fire investigators in their work, but the conclusions that can be drawn from it are still debated. While its use to highlight fire spread patterns is usually recognised, there is no consensus on its further interpretation to determine heat exposure or fire duration. The present paper aims at investigating the relationship between char depth and cumulative exposure (heat flux density integral over the exposure duration, in [MJ/m2]). A cone calorimeter was used to expose samples of four wood species (oak, beech, fir, and particleboard) to controlled heat fluxes for varying durations. This proved that, for a given cumulative exposure, the measured char depths are comparable, even if this cumulative exposure is obtained in very different ways. This quantitative analysis (e.g. predicting the cumulative exposure based on a char depth measurement) is an important addition to the typical qualitative use of char depth measurements (i.e. the estimation of fire spread direction). Moreover, the layer of wood that might have burned away, which is often difficult to estimate in the field, could be easily related to the char depth, making this type of measurement even more useable by fire investigators.