Mechanical and physical properties of CLT from hardwoods

Sumanta Das

Doctoral thesis

Abstract:

Hardwood species have recently been the focus of a growing number of studies for use in manufacturing Cross-laminated timber (CLT). This research evaluates the viability of using lower-grade, fast-growing poplar and aspen lumber for both homogenous CLT and hybrid configuration with outer Norway maple layer with core poplar layer by examining the bonding properties (delamination and shear) and bending and rolling shear properties. Two types of adhesives, namely one-component polyurethane (1C-PUR) and melamine adhesive (ME), were utilized in the process. Samples were prepared from spruce for comparison. The wood species andmanufacturing pressure significantly affect the CLT panels' water absorption (WA) and thickness swelling (TS). The observed WA and TS values in aspen, poplar, and hybrid CLT were comparatively greater than spruce CLT. Both homogenous and hybrid maple-poplar CLT bonded with 1C-PUR adhesive with 1 MPa pressure passed the delamination test, demonstrating excellent bonding. However, ME adhesive exhibited satisfactory performance in homogenous CLT panels, while only 50% of the hybrid maple-poplar CLT samples met the delamination test criteria. The CLT panels prepared with 0.6 MPa pressure experienced significant failure due to insufficient adhesive penetration. Both homogeneous and hybrid CLT panels met the minimum glue line shear strength. Nevertheless, it is worth noting that hybrid maple-poplar CLT exhibits significantly greater shear strength when compared to its homogeneous CLT counterparts. Compared to the delamination testing method, the performance of ME bonding indicates a slight advantage over 1C-PUR bonding in all CLT panels, regardless of the species and composition. Moreover, the primary determinant contributing to shear failure in shear testing was predominantly attributed to wood failure, accounting for more than 80%. The adhesive type did have any significant impact on global bending stiffness (EImg), bending strength (fm) and rolling shear strength (fr). The wood species employed in manufacturing significantly influence these outcomes. According to theoretical methods, the bending stiffness of CLT panels was underestimated by 5% for polar CLT and 10% for hybrid CLT. The Modified Gamma (MG) hypothesis demonstrated the highest accuracy in predicting bending stiffness values by incorporating the connection efficiency factor to account for shear effects, while both Shear Analogy (SA) and Timoshenko Beam Theory (TBT) exhibited a high degree of similarity. Furthermore, the strength predictions of SA were found to be more accurate regarding bending and shear compared to other theoretical methodologies. The finite element technique can be utilized to accurately forecast the bending stiffness, with a minimal variance of 7%. Both aspen and poplar CLT panels demonstrated comparable results to spruce CLT panels. Incorporating maple as the outer layer of the hybrid CLT panels led to improved flexural and shear performance.

Keywords

Aspen, Bending stiffness and strength, Cross-laminated timber, Delamination, Hybrid CLT, Rolling shear strength, Shear strength, Water absorption

Language used: English

Date on which the thesis was submitted / produced: 23. 11. 2023

Thesis defence

• Supervisor: doc. Ing. Miroslav Gašparík, PhD.
• Reader: Jitka Beránková, externi, Richard Hrčka, externi, Petr Ptáček, externi

Czech University of Life Sciences Prague

Faculty of Forestry and Wood Sciences

Doctoral programme / field:
Forestry Engineering / Wood Processing and Forest Machinery