By Henry Quesada
*Articled published in the Virginia Loggers Association Newsletter in August 2018.
Cross laminated timber (CLT) has been in the market since 2000 when it was launched in Austria by a company called KHL. A CLT panel is composed of 3, 5, or 7 layers of lumber. Each layer is glued perpendicularly to each other. Today almost 100% of the CLT panels being produced are made from softwood species and it is estimated that the current CLT production in Europe is around 1 million cubic meters.
In the United States, production of CLT started about 5 years ago. There are currently three companies producing CLT panels in the USA: DR Johnson (OR), Smartlam (MT) and Sterling (IL). DR Johnson uses Douglas Fir (DF) as the main raw material while Smartlam uses Spruce-Pine-Fir (SPF) and Sterling uses Southern Yellow Pine (SYP). It has been announced that over the next two years the following 4 CLT production facilities will start production: Katerra in Washington, a second plant by Smartlam in Maine, LignaCLT Maine, and International Beams in Alabama. All of the upcoming facilities will be using softwoods as raw material.
All of the US CLT current and planned producers (except Sterling Lumber) are in compliant with the CLT standard, PRG-320. Sterling Lumber produces CLT matts for energy projects (non-structural application) so there is no need to follow the CLT standard.
The CLT standard, ANSI/APA PRG-320, does not admit hardwood lumber yet; a major hurdle for hardwood lumber to become an accepted CLT raw material. Any softwood species as described in the ALCS under PS 20 with specific gravity higher than 0.35 should be an acceptable raw material for CLT, according to ANSI/APA PRG-320. In most of the cases, hardwood species have higher specific gravity than softwood, so this should not be a problem. In addition, lumber for CLT should be dried to a moisture content (MC) of 12%+-3%. This is also not an issue for hardwood lumber as most of it is dried to 8% MC.
A key requirement for lumber going into CLT is that the minimum thickness in the PRG-320 is 5/8. As we know, most of hardwood lumber is produced in 4/4 thickness. In addition, the board width should exceed its thickness by 1.5 times (in the major strength direction of the CLT panel) and by 3.5 times in minor strength direction of the panel. Currently, most hardwood mills produce random widths that definitely need to be sorted out to comply with this requirement.
Glue-line performance should be considered too. Hardwood lumber has a more complicated cellular structure than softwood lumber that could present challenges with adhesion. For example, some hardwoods are stiffer than softwoods and this might require additional pressure or pressing time. Also, chemicals in the hardwood lumber could also prevent an optimal glue-line in- between the panels.
Machining hardwood lumber is different than softwood lumber. Because hardwoods have a different structure, there could be a need for different tooling and energy requirements. Some hardwoods present crystals and other hard structures that could wear tools faster than softwood lumber. These issues ultimately will impact cost and productivity of the planer, finger joint, and computer numerical control (CNC) equipment of the CLT production line.
There is also the question about the supply of hardwood lumber for CLT. A medium size CLT plant could process about 50,000 cubic meters per year which translate to roughly 21 million board feet. It is estimated that CLT demand in the US would be very similar to Europe or around 1 million cubic meters (424 million bf). The current structure of hardwood industry is fragmented so it would be very difficult for a major CLT plant to establish a steady and consistent supply of hardwood lumber under these market conditions.
Hardwood sawmills that wish to become suppliers of a CLT panel plant must adjust their production mix. Virginia Tech researchers conducted a mill study and determined that Yellow Poplar lumber that is NHLA graded 2 Common and lower could be sold as raw material for CLT as long as the specific species meet the technical requirements in the PRG 320 (specific gravity, Modulus of Elasticity, etc). Higher grades (1 Common and higher) should continue to be sold in the appearance market as mills can get more revenue in this market than selling it as CLT raw material. Ultimately, hardwood sawmills would need to train their personnel to grade hardwood lumber under structural grading rules.
Other issues that should be considered for hardwood CLT panels is the weight of panels. It has been estimated that hardwood CLT panels could weigh up to 30% more than softwood CLT panels. In terms of logistics and transportation arrangements, this could increase the overall cost and time of the projects as additional trips are required to move the completed hardwood CLT panels to the construction site. An alternative would be produced 3 or 5 ply softwood CLT panels and add a layer of hardwoods just to meet the weight requirement
Finally, there is also the question of sustainability. It has been confirmed by the US Forest Service that growth of hardwood forest doubles its harvesting rates. However, it should be considered that growing hardwoods might take as much as double the time of growing softwood timber. In addition, softwood timber is growing in plantations which increases the productivity of the timber.
As we just pointed out, it seems that there are opportunities for hardwood lumber to participate in the CLT market. However, there are some critical hurdles that need to be resolved before this could happen. At Virginia Tech and other universities, we continue to generate research in technical, manufacturing, and marketing aspects of the potential use of hardwood lumber in the CLT market. If you have questions, please let us know at your earliest convenience.