Red Oak and Red Maple show potential for next-generation hardwood CLT

Researchers at West Virginia University are examining whether American hardwood species could support the next generation of cross-laminated timber (CLT).

New testing has identified American Red oak and Red maple as promising candidates after both species demonstrated stronger glue-line performance than Yellow poplar.

The research involved 96 glued hardwood joints. Each was subjected to bending until the adhesive bond cracked.

The results provide engineers with data needed to model how hardwood CLT assemblies could behave under stress.

Hardwood bonding comes under scrutiny

The West Virginia University team focused on the strength and behaviour of glued connections between hardwood boards.

Red oak and Red maple performed strongly in the testing. Yellow poplar produced significantly lower results.

Yellow poplar was the lightest of the three species examined.

The testing process was direct.

Each joint was bent until a crack developed along the glue line. This allowed the researchers to identify the point at which the adhesive connection began to fail.

Such information is important for engineers developing computer models of mass-timber systems.

The research was conducted by Levente Denes, Balázs Bencsik and Eduardo Sosa of West Virginia University.

Their paper was published online ahead of the 17 October issue of Construction and Building Materials.

Research could improve CLT modelling

Until now, many computer models used for hardwood CLT have assumed that the individual layers are perfectly bonded.

That assumption limits the ability of those models to represent what happens when an adhesive connection begins to fail.

The latest findings provide experimental data that can be incorporated into those models.

“With our latest results, we’ve laid the groundwork for finite element modelling of hardwood CLT bonding strength and elastic behaviour,” Levente Denes, an associate professor at West Virginia University, wrote this week.

The researchers developed their own model using the test results.

According to the study, the model identified the point at which individual joints began to crack with “reasonable accuracy”.

That represents an important step in understanding adhesive behaviour within hardwood laminated systems.

Species performance differs

The three hardwoods did not respond identically during testing.

Red oak and Red maple demonstrated the stronger glue performance, while Yellow poplar was found to be significantly lower.

The oak samples were bonded using a different adhesive from the one used for the maple and poplar samples.

That difference is relevant when interpreting the test results and considering how adhesive selection may affect future CLT development.

The testing focused on the glue line rather than the performance of full-size laminated panels.

The boards used in the experiments were 10 millimetres thick.

They were not full-size CLT panels.

As a result, additional research will be needed before the findings can be directly applied to larger commercial assemblies.

Interest in hardwood mass timber grows

The research arrives as more US researchers investigate hardwoods as an alternative to conventional softwood-based mass timber.

A study from the University of Tennessee and the USDA Forest Service, published late last year, found that hardwood CLT can be up to three times stronger in compression than southern yellow pine.

The potential structural performance of hardwoods is attracting attention.

Cost remains a major consideration, however.

Hardwoods typically cost two to three times more than softwoods. Hardwood lumber is also mainly graded for appearance rather than strength.

Those market conditions could influence how quickly hardwood CLT moves towards wider commercial use.

Material availability, grading practices, processing requirements and adhesive performance will all remain relevant to future development.

From laboratory testing to full-size panels

The West Virginia University findings provide additional data for engineers investigating hardwood-based laminated timber systems.

The results are particularly relevant to modelling because they capture the point at which glued joints begin to crack.

That information can help move simulations beyond the assumption of permanently perfect bonds.

The next stage will involve testing at a larger scale.

Denes said the next step is “upscaling the model to full-size samples and validating it against real-world performance.”

Full-size testing will be important for determining whether the laboratory findings translate effectively to larger CLT components.

It will also provide an opportunity to assess the behaviour of complete assemblies rather than individual 10-millimetre boards.

For the mass-timber industry, the research adds to a growing body of work examining hardwood species for structural applications.

Red oak and Red maple have now demonstrated stronger adhesive-joint performance than Yellow poplar under the reported test conditions.

The findings do not establish that either species is ready to replace softwoods in commercial CLT.

Instead, they provide experimental data for the next phase of research.

The combination of laboratory testing and finite element modelling could help researchers better understand hardwood bonding performance.

Further validation will determine how those results translate into full-size CLT construction and real-world structural performance.

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