Observations and impact of char layer formation and loss for engineered timber

The char layer plays a critical role in the fire behaviour of engineered timber. Several chemical and physical processes can reduce char layer thickness and integrity. A series of experiments studied 12 cross-laminated timber (CLT) columns (130 × 790 × 125 mm WxHxD) exposed to combined thermal (20 kW/m2 or 50 kW/m2) and mechanical loading (39 kN, eccentric). Char loss from the surface lamella was observed and the impact of this on the thermal response of the timber studied. In all cases, unprotected CLT exhibited fall-off of charred pieces. Cracking, shrinkage and movement of char contributed significantly to the exposure of underlying timber sections to external heating. There was no direct correlation between char fall-off and the measured glue line temperature in this configuration. To enable comparison between CLT with and without char fall-off, a thin layer of glass fibre-reinforced polymer was added to the exposed surface of six samples which prevented all char fall-off. Retention of the char layer significantly decreased temperatures beneath the char layer, loss of section and burning duration compared to samples with char fall-off. [Display omitted] •The effect of char loss on the burning behaviour of engineered timber was quantified.•Glass fibre mat prevented all char fall-off in a subset of samples.•Char fall-off increased mass loss, charring depths and in-depth heat transfer.•The results indicate that char loss may impact load-bearing capacity and fire dynamics.•Knowledge gaps around char loss critical for fire safety of timber buildings.