The project investigated the replacement of conventional steel reinforcement rods in timber construction with hardwood rods, specifically beech and oak, and the substitution of synthetic epoxy or polyurethane adhesives with bio‑based, renewable‑resource adhesives. The primary aim was to reduce the carbon footprint and improve the ecological performance of timber structures while maintaining or exceeding the mechanical performance of existing steel‑based systems.
In the experimental phase, hardwood rods were glued into pine timber using the commercial adhesive 400/115. Ten pairs of wood plates were bonded and subjected to tensile‑shear testing after conditioning in a 1 % Kalialaun solution at 40 °C for seven days in a saturated NaCl environment. The measured tensile‑shear strengths ranged from 5.3 to 5.8 N mm⁻², with an average of 5.5 N mm⁻². These values surpass the DIN EN 1995‑1‑1/NA:2013‑08 requirement of 4 N mm⁻² for glued steel rods in pine, indicating that the bio‑based adhesive can provide adequate bond strength. The failure mode was predominantly wood fracture, confirming that the adhesive bond itself was not the limiting factor.
Further investigations focused on the moisture resistance of the glued joints. Hardwood rods of 6 mm and 15 mm diameter were exposed for 14 days to six different climatic conditions, representing relative humidities of 65 % (construction class 1), 85 % (class 2), and above 85 % (class 3). Weight measurements revealed that saturation in pine occurs within approximately seven days at 65 % RH, while at 85 % RH the moisture content remained higher throughout the exposure period. The highest moisture uptake was observed in the >85 % RH condition, as expected for the most demanding environment. These results guided the selection of storage climates for the glued specimens and highlighted the importance of moisture control during curing.
To enhance the long‑term durability of the adhesive joints, the team explored the addition of neutralized fatty acids and surface coatings. Preliminary tests suggested that these modifications can improve the moisture resistance of the adhesive layer, potentially extending the service life of the glued connections in humid environments. Numerical modeling (work package 5) was planned to predict the mechanical behavior of the glued joints under various loading and environmental scenarios, although detailed results were not reported in the final summary.
The project was carried out from 1 May 2021 to 31 October 2023 under the funding code 2220HV050B, supported by a German federal research program. The lead partner, Fritz Häcker GmbH + Co. KG, is a manufacturer and distributor of adhesives, including glutinous glues. The consortium comprised experts from timber construction, wood processing, and adhesive manufacturing, ensuring that the research addressed practical industry needs. Regular biannual meetings facilitated continuous feedback from industry partners, allowing the research to remain aligned with real‑world requirements. The outcomes are intended for rapid transfer to the market, with plans for patent filings, technical guidelines, and academic publications to disseminate the findings.