The behaviour of timber in fire including the decay phase - charring rates, char recession and smouldering

For the assessment of the fire resistance of timber members, the depth of the virgin wood has been documented as crucial parameter in numerous scientific experiments in the past; results have been used primarily for the load-bearing resistance verification. An excessive number of tests with timber members have been performed with various products (e.g. glulam, cross-laminated timber) in furnace environments showing a consistent one-dimensional charring rate of 0.65 mm/min ± 0.1 for standard fire exposure. More severe fire exposure (e.g. hydrocarbon fire) showed a moderate increase of the charring rates to about 1.4 mm/min. The interest for performance-based design (PBD) moved the focus away from the fully developed fire phase and included other conditions, where further parameters are relevant. In furnaces, mainly the charring rate has been observed while in oxygen rich environments further a reduction of the char layer can be observed. This paper presents results from a novel charring investigation apparatus where exposure levels up to 120 kW/m2 and gas velocities up to 8 m/s (heated gas) in moderately and highly turbulent conditions were investigated. Typically, the prediction of the charring rate is done using the external heat flux, which resulted in poor fit for the observed charring rates up to 2.4 mm/min. The predictions could be significantly improved if further characteristics were considered. The most significant characteristic was the smouldering combustion of the char layer. Findings of the experimental campaign can describe the contribution of the timber to the fire dynamics and are expected to be used in future calculation models.