Optimising the Bond Quality in Hybrid Maple–Poplar Cross-Laminated Timber (CLT): Influence of Specimen Size and Manufacturing Pressure

With the rapidly increasing population and the impact of climate change, green construction materials are evolving into sustainable alternatives. Cross-laminated timber (CLT) is gaining attention as a sustainable alternative to traditional construction materials due to its numerous advantages, such as high prefabrication potential, reduced construction time, seismic safety, and a favourable strength-to-weight ratio, as well as being a potential carbon sink. Even though hardwood CLT has better mechanical performance, it is still underutilised compared to softwood CLT. This is mainly because of the bonding challenges due to their variations in densities and anatomical structures. Three-layer CLT panels were produced utilising maple for the outer layers and poplar for the core, employing one-component polyurethane (PUR) adhesive without edge gluing with two different pressure levels—0.6 MPa and 1.0 MPa—using a hydraulic press. Delamination tests were performed on specimens of two dimensions, 70 × 70 × 60 mm and 100 × 100 × 60 mm (length × width × height), following EN 16351:2015. Forty specimens were evaluated for percentage wood failure. Results demonstrated that both smaller specimen size and bonding pressure significantly affected delamination, with 1.0 MPa achieving the most consistent bond integrity. The lower specimen size reduces the amount of delamination by reducing the exposed surface area to the vacuum-pressure delamination method. The findings highlight that optimal bonding pressure is critical for hardwood-based CLT production, with implications for improving manufacturing protocols and expanding the commercial viability of hardwood CLT in load-bearing applications.