Thermally modified timber (TMT) has gained market share in Europe as an environmentally friendly and durable building material. Unfortunately, TMT products are currently prohibited for use in structural applications as there is insufficient data to estimate the loss in strength due to thermal modification. This thesis work explored the fundamentals related to the static 4-point bending (4PB) behaviour of TMT needed to expand its use in the construction sector.

The effect of treatment on checks in and around knots, and their combined effect on the 4PB behaviour of TMT, was studied with digital image correlation on 9 Norway spruce boards. For 190 matched board pairs of Norway spruce, one board was modified according to the ThermoWood^® Thermo-D process, and the other remained untreated for comparison. One-hundred (100) board pairs were used to assess the effect of treatment on the 4PB properties and behaviour, and on indicating properties (IPs): density, longitudinal resonance and ultrasonic wave speed. The remaining 90 board pairs were weathered for 30 months to evaluate differences in the degree of checking, the IPs and the 4PB properties and behaviour. Fibre angle was measured on all board surfaces using a WoodEye 5 high-resolution laser scanner to study the location of failure in TMT, and to potentially improve bending strength predictions.

Thermal modification decreased the bending strength by approximately 40%; however, the presence of knots still determined the type and location of failure. Thermal modification and weathering play a critical role in the formation of severe checks in timber, but their presence had no significant influence on the bending properties. TMT could be graded using acoustic-type grading machines already available at most sawmills, and these principles could be applied to predict the presence of internal checks. Scanning the fibre direction improved strength predictions of TMT and enabled the failure location to be predicted. Grading timber before thermal modification could reduce the rejection rate of TMT if manufacturers select raw material that is resistant to internal checking and is of a suitable grade.