The PreloadTimber research project is investigating the influence of load history on the tensile strength of structural timber. The focus is on the extent to which plastic deformations due to compressive stress and rheological deformations due to long-term stress lead to irreversible changes in the material structure and thus to an impairment of the mechanical properties.

The background to the investigation is the stress change from compressive to tensile stress, which is critical for the wood fibres and cannot be ruled out when wood from existing buildings is reused in construction. However, the extent to which stresses from the first utilisation phase influence the integrity of used wood has not yet been sufficiently clarified for a material-appropriate and safe return of used wood to the material cycle.

With regard to the desired high-quality reuse of materials and the lowest possible devaluation steps within the wood utilisation cascade, the question therefore arises as to whether and from which load levels pre-loads in the load history are relevant for the strength of structural timber and to what extent they should be taken into account in the strength classification.

In the project, timber components are specifically conditioned with different load levels and durations in order to investigate both plastic compression and rheological deformation in isolation. The resulting effects on the tensile strength are recorded experimentally and correlated with structural changes in the wood microstructure. The aim is to clarify whether pressure-induced preloads can lead to significant strength reductions and are therefore relevant for the constructive use of used wood in the context of reuse.

Keywords: compression pre-loading, tensile strength, wood compression, rheology, reuse of used wood