Bulk thermostabilization and surface UV activation as a wood modification method for glued beams in long-span structures

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Abstract

This paper presents an innovative two-stage physicochemical modification approach for softwood species, aimed at enhancing the operational reliability of glued laminated timber beams used in long-span building structures. In the first stage, bulk thermal modification (TM) is carried out in the exhaust gas atmosphere of a waste-heat boiler at 180–240 °C, resulting in reduced hygroscopicity, improved dimensional stability, and enhanced biological resistance. In the second stage, the surface layer of the thermally modified wood undergoes ultraviolet (UV) irradiation (wavelength: 253 nm; dose up to 7.4 kJ/m²) to restore hydrophilicity and improve adhesive bonding performance. Experimental results confirm that the contact angle of the surface decreases from 82° (TM only) to 8° at a UV dose of 7.4 kJ/m² – corresponding to a 90 % increase in wettability. Shear strength of the adhesive joint increases by 22.4 % compared to untreated thermally modified wood and approaches the level observed for joints made from untreated pine wood (deviation < 9 %). After two-stage modification, the mechanical performance of glued laminated beams – specifically, the modulus of rupture in static bending – reaches 58.3 ± 2.1 MPa, fully complying with the requirements of GOST 20850-2014 for glued laminated timber structures of strength class C24 and above. The proposed technology successfully combines the high moisture and biological resistance of thermally modified wood with reliable adhesive bonding-an essential requirement for load-bearing structural elements exposed to cyclic variations in temperature and humidity.

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