Abstract
The relevance of this study stems from the need for the rational use of forest resources and the construction industry’s transition to the principles of the circular economy. Small-diameter Scots pine (Pinus sylvestris) timber, which is often regarded as waste or firewood, has significant potential for the creation of high-load structural elements. The aim of this study was to provide a mechanical and technological justification for the use of natural, debarked surfaces to form the internal cavity of the timber and to determine the optimal type of adhesive. To achieve this aim, methods of experimental mechanics were employed: specimens were subjected to four-point static bending in accordance with the requirements of the DSTU EN 408:2007 standard. The influence of polyvinyl acetate (PVA) dispersion and polyurethane (PUR) adhesive on the strength of built-up beams was evaluated. The principal findings demonstrate that a cross-section incorporating a star-shaped cavity reduces the weight of the product by 14% while decreasing the section modulus by only 0.77%. Experimental results confirmed that the application of a PVA adhesive eliminates stress concentrations at the sharp corners of the cavity formed by the debarked surfaces of the laminations. This resulted in an 85.8% increase in ultimate bending strength compared with a solid timber beam. It was established that the failure of such specimens was exclusively cohesive within the wood substrate, indicating the high reliability of the reinforcing polymer framework. In contrast, the use of an elastic PUR adhesive proved ineffective owing to the initiation of adhesive delamination, with a strength increase of only 2.0%. Fractographic analysis showed that the microporous structure of PUR cannot withstand peak stresses. The developed resource-efficient technology for manufacturing hollow timber beams makes it possible to produce an element whose structural performance (specific strength) exceeds that of solid timber by 116%. The implementation of such innovative approaches minimises energy consumption during machining and contributes to long-term carbon sequestration in building structures