Enhancing Rolling Shear Performance of Cross-Laminated Timber Through Densification of Transverse Layers

Cross-laminated timber (CLT) has gained significant popularity in the construction industry for its strength, structural stability, and sustainability. However, a key challenge to the widespread use of CLT is its inherent susceptibility to rolling shear failures, particularly in layers where the wood grain is oriented perpendicular to the applied shear stress. Previous research on both conventional and hybrid CLT has focused on addressing these weaknesses by adjusting key factors such as wood species, layer orientation, and glue types. Additionally, wood is a renewable resource, and its use in construction has been advocated as a way to reduce carbon footprints in the building sector. There is an urgent need to improve the performance of CLT, which will support the shift towards more eco-friendly building materials and practices. Moreover, enhanced performance of wood products can lead to broader acceptance and usage, potentially reducing reliance on more energy-intensive materials like steel and concrete. To this end, a recent study published in Construction and Building Materials, conducted by Suman Pradhan, Dr. Edward Entsminger, Dr. Mostafa Mohammadabadi, and Dr. Kevin Ragon from Mississippi State University, in collaboration with Dr. William Nguegang Nkeuwa from Henkel, explored ways to enhance the rolling shear strength of CLT. The research focused on densifying the transverse layers of CLT using a thermomechanical process, aiming to improve its shear performance, an essential factor for the structural application of CLT in construction.

The team used Loblolly pine, a common species within the Southern Yellow Pine category. To begin, they soaked lumber in boiling water to soften the wood by reducing the stiffness of lignin and hemicellulose (the amorphous polymers in wood). After soaking, the wood samples were compressed to three specific thicknesses, representing compression ratios of 16.67%, 33.33%, and 50.00%. Following compression, the specimens were hot-pressed at 140 degrees Celsius to achieve the target thickness. They were then allowed to cool under pressure to minimize spring back and ensure stable densification. This densification process leverages the viscoelastic and plasticization properties of wood polymers such as lignin and hemicellulose, which become more pliable when heated above their glass transition temperatures. This plasticization is critical for achieving densification while preventing cracking or compromising the wood’s structural integrity. To assess the impact of densification, the authors prepared and tested control specimens with the same dimensions but without undergoing the densification process. This approach enabled a direct comparison of mechanical properties, isolating the effects of the densification process.

The researchers used the modified planar shear testing method to evaluate the shear strength of CLT specimens. This test ensured a consistent application of shear stress across the transverse layer of the CLT while minimizing other stresses, such as bending or compression, that could affect the results.

The authors observed a significant increase in rolling shear strength with increased densification. Specifically, the rolling shear strength improved by about 108% at the highest compression level (50%). This finding was significant, indicating that densification substantially enhances the structural performance of CLT in resisting shear stresses. Moreover, the researchers observed a linear relationship between the percentage of increase in rolling shear strength and the compression ratio. This was an important outcome because it suggested the ability to predict performance improvements through controlled densification. Furthermore, the densified specimens consistently outperformed the non-densified control specimens in rolling shear strength. Even with a constant aspect ratio, the densified wood displayed superior mechanical properties, highlighting the effectiveness of the densification process beyond just changing physical dimensions. Additionally, although an increased aspect ratio (resulting from reduced thickness) also contributed to greater rolling shear strength, the study revealed that the enhancements from densification were more significant than those from the increased aspect ratio alone. This underscores that material modification through densification is a more influential factor than geometric changes.

The study provides robust evidence supporting densification as a viable method to substantially improve the mechanical properties of CLT, specifically in contexts where rolling shear strength is critical. This has potential implications for the design and construction of taller and more complex wooden structures, where enhanced material performance is imperative.  The findings suggest that through controlled densification, manufacturers can tailor wood properties to meet specific demands of strength and stiffness without significantly altering the material’s natural qualities. This capability could lead to innovations in the manufacturing processes of wood products, allowing for more precise and efficient use of resources. As the mechanical properties of CLT are enhanced, its range of applications can expand, potentially opening new markets and lowering costs associated with construction materials. Furthermore, by increasing the durability and lifespan of wooden structures, long-term maintenance and repair costs can be reduced, offering economic benefits to builders and occupants alike.

In a statement to Advances in Engineering, Dr. Mostafa Mohammadabadi, the corresponding author, said, “CLT is gaining increasing popularity in the construction industry for its sustainability and performance in terms of strength and stiffness. The findings of our study significantly improve the applicability of CLT in load-bearing applications, particularly in scenarios where shear strength is a critical performance factor. Enhanced shear performance of CLT can facilitate the design of safer, more durable structures that can withstand greater forces, thereby unlocking new possibilities in wooden construction technology”. Overall, the experiments conducted by the researchers provided clear evidence that thermomechanical densification of the transverse layers of CLT significantly enhances its rolling shear strength. This enhancement is due to changes in dimension (aspect ratio) and, more importantly, a result of the increased density and altered material properties of the wood. These findings are critical for developing more robust CLT for construction, potentially leading to broader applications and more reliable performance in structural uses.