Two methods for testing bending strength have been used،one using defect-free material over a short span and the other utilising pieces having natural defects over a longer span.
The results show that substantial strength loss in pine starts at temperatures over 220 0 c.
The strength of heat-treated (2300c،5hours) spruce was studied with larger test pieces according to EN 408.
The test pieces were conditioned at 45% and 65% relative humidity.
The results are presented with timer containing knots،the strength values for heat-treated wood are lower than those of untreated wood.
This is due to، among other factors، the resins being extracted from the wood.
The bending strength was reduced by up to 40% compared with normal untreated wood.
This was due to weakening of areas around the defects.
However، with wood treated at lower temperatures of about 1900c for 4 hours the difference in bending strength was far less.
The majority of testing so far has been performed on small، defect-free pieces.
More testing is needed on full-size test pieces and with varying numbers of knots and different knot types.
In the absence of sufficient information، we recommend that thermowood not be employed in load-bearing structural usage for the time being.
