Cross-laminated timber is a popular engineered wood product and production volumes have consistently increased in the last few years. Cross-laminated timber is a panel-shaped structural member that is composed of a minimum of three layers of graded timber, which are oriented orthogonally to each other. The cross-lamination is responsible for homogenous properties and dimensional stability of the product.
However, adhesive bonds must be used in order to realize positive properties. These adhesives are responsible for transferring loads and providing durable bonds during the structure’s service life. Therefore, the timber layers need to be bonded with structural adhesives, such as polyurethane (1C-PUR) covered by EN 15425 and melamine-urea-formaldehyde (MUF), which satisfy the EN 301 requirements.
Although this product has been used for decades now, the CLT product standard EN 16351 product standard was introduced recently. For this reason, cross-laminated timber has been based on manufacturer and technical approvals. A range of products therefore varies concerning number and thickness of single layers and product dimensions from one manufacturer to the next. This also implies that methods adopted to characterize this product as well as bond integrity within factory production control also differ between manufacturers. Various factory control stipulations may entail various combinations for bending tests, wood failure assessment, delamination tests, and block-shear tests.
Markus Knorz (Technical University of Munich) in collaboration with Stefan Torno (Bavarian Forestry and Wood Cluster), and Jan-Willem van de Kuilen (Technical University of Munich and Delft University of Technology) examined the bond quality of industrially produced cross-laminated timber through delamination tests. The main aim of the study was to analyze the effect of specimen shape on resistance to delamination. They also considered additional variables such as bonding pressure, and the number and thickness of the layers. Their work is published Construction and Building Materials.
The authors submerged round and square specimens in water and applied vacuum 15-30 kPa absolute pressure for 30 minutes and 600-700 kPa absolute pressure for 120 minutes. The specimens were then dried at 65-75 °C and at 8-10 % relative humidity. Drying was stopped when the specimens reached a mass of 1.0-1.1x the initial mass.
The authors then measured glue line openings on the lateral surfaces. Cohesion failure in adhesive layer and adhesion failure in the wood-adhesive interface were assessed as delamination. After analyzing the delamination, the authors split all the bonds to investigate the surfaces of the split glued regions in view of wood failure.
The research team observed that the specimen shape had a significant influence on the delamination results considering that round samples indicated higher resistance to delamination compared to square samples. For this reason, the option of using round or square samples in delamination test should be eliminated from the standard. Timber layer thickness and bonding pressure did not have an impact on delamination results. However, resistance to delamination for 7-layer specimens was lower than for 3-layer specimens indicating that the bonding process was sensitive for cross-laminated timber production. Wood failure generally confirmed delamination findings.
Furthermore, the cross-laminated timber standard enables one to choose between shear testing and delamination within factory production control. Delamination tests led to considerable failure rates, and seemed more severe. On the other hand, specifications for shear tests are low.
The results of their study lead to a simple recommendation that the method of establishing bond quality in cross-laminated timber should be revised. This should be done either by adjusting limit values, test parameters, or by inventing new test procedures.
Markus Knorz, Stefan Torno, and Jan-Willem van de Kuilen. Bonding quality of industrially produced cross-laminated timber (CLT) as determined in delamination tests. Construction and Building Materials, volume 133 (2017), pages 219–225.Go To Construction and Building Materials