Sina Jahedi | Oregon State University, Graduate Research Assistant
Last updated: June 25, 2021
In the past decade, cross-laminated timber (CLT) has become more mainstream thanks to its many advantages. The alternating layer-wise structure of CLT spreads the impact of defects over the whole panel and creates a more uniform material. Additionally, the perpendicular orientation of plies produces enhanced directional properties and increases dimensional stability .
CLT is made of at least three orthogonally oriented lamellas which are bonded together by glue. Poor manufacturing, or incorrect handling of CLT panels may cause delamination which can lead to catastrophic events. In this article we discuss:
The most common adhesive systems used in making CLT are polyurethane (PUR), melamine-urea-formaldehyde (MUR) and melamine-formaldehyde (MF). If precautionary measures are not implemented, delamination can happen in any CLT panel, regardless of species or adhesive. CLT lamellas (plies) are oriented perpendicular to each other and bonded by structural adhesive. As a rule of thumb, a good bonding shall be stronger than the wood itself. In other words, if we apply excessive amounts of force to a CLT panel, the wood shall break, not the bonding line (adhesive). This is because we want to use the full strength of wood.
Merriam-Webster dictionary defines the term delamination as “separation into constituent layers”. However, in the wood industry this term has a more specific meaning. In the wood industry only the separations in bonding area that are caused by adhesive failure are called delamination. As illustrated in the figure below, sometimes cracks grow very close to the bonding area that might seems like a delamination, but they are not! Wood fibers pulled off from both sides of the crack is an indication for wood failure, which shows the bonding was stronger than the wood itself, as intended. The term delamination is mostly used when the bonding fails, which appears as two clear surfaces with a gap in between.
Comparison of delamination vs. wood failure
Basically, any force applied to the bonding area can cause delamination if the adhesive is not strong enough. Delamination can be caused by service loads, residual loads, self-weight load, or (most commonly) the load induced due to shrinkage of wood. As the moisture content of wood changes, it starts to shrink or swell. Wood is an orthotropic material, meaning that the properties are directional. While wood shrinks significantly in radial and tangential directions, the shrinkage in longitudinal direction is negligible. Having the plies of CLT panel assembled perpendicular to each other, this difference in directional shrinkage can induce forces in the panel.
When CLT is drying or absorbing moisture, the bonding area tries to resist the difference in the shrinkage of various lamellas. If the induced load is high enough, it can break the bonding between lamellas and cause delamination. If the bonding is stronger than the wood (which it should be), we will not observe any delamination in the CLT even with excessive moisture change. Instead the induced load form shakes and checks in the body of CLT.
Lamella shrinkage causing delamination in cross-laminated timber (CLT)
Widespread delamination reduces the strength of CLT panels significantly and can lead to catastrophic events. The separated plies do not resist shear forces as much and the panel can buckle, even by its own weight. In 2018, a delaminated CLT floor of a mass timber building crashed onto the floor below during construction. Fortunately, there were no casualties, but needless to say how much worse the situation could be if the floors collapse when the building is operational.
Even the smallest cracks in the material can propagate into deep separations if the conditions are right. The cracks formed due to delamination typically have sharp tips. The local forces spike near the tip of cracks instead of evenly distributing in the area. This is called stress concentration, which can help the growth of crack and lead to total delamination of the plies. Additionally, delamination (or any crack in general) aids the penetration of moisture into the CLT panel which in turn leads to more and deeper delaminated areas.
Progression of delamination due to stress concentration at the tip of crack.
Delamination has a severe impact on fire development. The gap between layers acts as an oxygen supply channel and makes the underlying layer ignite instantaneously. This will increase the temperature enormously, prolong the fire duration, and make the extinguishing procedure more challenging.
The main causes of delamination are poor fabrication and errors in handling of CLT panels. As stated in ANSI/APA PRG-320 (Standard for Performance-Rated Cross-Laminated Timber), it is on the manufacturers to determine whether the adhesive is compatible with the wood species. In order to sell their products commercially, manufacturers are mandated to send samples of their CLT panels to certification agencies for performance tests. PRG-320 suggests conducting AITC T110 cyclic delamination test to check the compatibility of the adhesive and tuning the fabrication parameters if necessary. It is worth mentioning that AITC T110 test was originally developed for structural glued laminated timber (glulam), however due to similarities of CLT and glulam, this test is commonly used by CLT producers and researchers in North America.
As described in previous sections, moisture variations can lead to shrinkage of lamellas and cause delamination. It is recommended to keep the moisture content of lumber (during fabrication) as close as possible to equilibrium moisture content of the end-use environment. In this sense, a CLT panel which is going to be shipped to a hot and dry environment shall have less moisture content compared to the one shipped to a humid environment. Moreover, it is important to properly cover the CLT panels during transportation and installation to avoid moisture exposure.
Poor fabrication and mishandling of cross-laminated timber (CLT) can cause delamination, a separation in bonding lines due to adhesive failure. Delamination reduces the ultimate strength of CLT panel significantly which can lead to catastrophic events, e.g. collapse of the structure. Moisture content variations play a big role in initiation and growth of delamination. Even the smallest cracks can propagate into total delamination of plies. Delamination prevention shall start from early stages of fabrication by checking the compatibility of the adhesive and making sure that moisture content variations during transportation, installation, and service are as little as possible.