Project contacts are Frederico França at Mississippi State University and Robert J. Ross at the Forest Products Laboratory
With the rapid development of CLT manufacturing capacity around the world and the increasing architectural acceptance and adoption, there is a current and pressing need regarding adhesive bond quality assurance in manufacturing. As with other engineered glued composites, adhesive bondline performance is critically important. Bondline assessment requires technology in the form of sensors, ultrasonics, load cells, or other means of reliable machine evaluation.
The objectives of this cooperative study are to develop quality assurance procedures for monitoring the quality of mass timber and CLT during and after manufacturing and to develop assessment techniques for CLT panels in-service.
Eucalyptus grandis is South Africa‘s most important commercial hardwood species. The availability of E. grandis and its fast growth rate creates the opportunity to explore its uses further. Cross-laminated timber (CLT) is a prefabricated multilayer engineered panel product made of at least three layers, with the grain direction of some or all of the consecutive layers orthogonally orientated. In order to add value to E. grandis, reduce the export of low-cost chips, increase the profit margins of local plantation owners and create jobs, the development of E. grandis CLT in South Africa may be an option. There is concern among some researchers that the bonding quality evaluation tests proposed by CLT standards have been developed for glulam and are too severe for CLT. These researchers proposed that further analysis and possibly even revision of the test methods should be considered. There is also a need to evaluate the mechanical properties of CLT panels made of E. grandis to completely understand the structural performance of these panels, including their bond quality and durability, and therefore be able to rely on E. grandis CLT as a construction material. The objectives of this study were: . To evaluate the face-bonding quality of CLT panels from E. grandis timber bonded with a one component polyurethane resin; . To determine the influence of material and processing parameters on the face-bonding quality of CLT manufactured from E. grandis timber bonded with a one component polyurethane resin; . To analyse different testing methods for evaluating the face-bonding quality of CLT. The design for this experiment consisted of eight groups with different combinations of parameters for density, grooves and pressure per group. Four different testing methods were used to evaluate the face-bonding quality of CLT panels from E. grandis and to determine the effect of parameters on face-bonding quality: A delamination test on 100 x 100 mm block specimens (Test A), a shear test on 40 x 40 mm specimens (Test B), a shear test on 40 x 40 mm specimens with grain direction 45° to load direction (Test C) and a combined delamination and shear test on 70 x 70 mm specimens with grain direction 45° to load direction (Test D). Results of the statistical analysis indicated that E. grandis CLT made with 1C-PUR adhesive can obtain excellent face-bonding quality using a clamping pressure of 0.7 MPa and with no stress relief grooves present. All samples passed the shear test (Test B) which is the reference test method proposed by EN 16351 (2015). It was found that a strength component and durability component will be an advantage when testing the bond quality of CLT. Shear tests at 45° to the load direction did not completely eliminate the rolling shear effect. The combined delamination and shear test (Test D), seems to have potential as a good test for bond quality since it is a combination of a durability and shear strength test. There are still questions about the relative advantages of specific test methods for bond quality, especially on the effect of rolling shear. Further work should focus on this aspect and the use of stress models might be a way of gaining further insights.