Neutron Imaging of Hygroscopic Moisture Transport in Wood Exposed to High Temperature

2012

Neutron imaging of hygroscopic moisture transport in wood exposed to high temperature

M. Sedighi-Gilani1, E. Hugi1, S. Carl1, D. Derome2 and J. Carmeliet 3

1EMPA (Swiss Federal Laboratories for Materials Science and Technology), Laboratory for Building Science and Technology, Zurich, Switzerland

2EMPA (Swiss Federal Laboratories for Materials Science and Technology), Fluid and Porous Materials Group, Zurich, Switzerland

3ETHZ, Chair of Building Physics, Zurich, Switzerland

ABSTRACT:

Wood is a cellular, hierarchical, hygroscopic, porous material. Mechanical properties of the wood cell material are strongly dependent to moisture and its structure leads to anisotropy of heat and moisture transport properties. In wood exposed to fire, high temperature gradient leads to migration of moisture and shrinkage in front of the progressing char layer. We document the moisture migration in wood specimens exposed to a surrogate fire, with high accuracy by means of neutron radiography.

Hardwood and softwood samples, conditioned at 80%RH, are mounted in a custom-made device keeping the sample in contact with a heating copper plate. A step change in temperature to 250°C is maintained for 10 minutes. Thermal neutron radiography documents the spatial- and time-resolved moisture content over the area of samples (40 mm by 40 mm), with resolution of 100 microns that allows distinguishing moisture content with more than 100g/m3 resolution in seasonal growth layers. Temperature is measured in few points along the specimen height, with thin thermocouples.

This study provides evidence of the combined contribution of diffusion and convection phenomena during the process of moisture migration in moist wood exposed to high temperatures. The drying front in wood pushes away moisture which leads to a zone with increased moisture content ahead of the drying front. The temporal evolution of moisture redistribution depends on the wood species and orthotropic directions. These results are used to validate our thermo-hygro-mechanical model of wood at high temperature.