Comparison of newly proposed test methods to evaluate the bonding quality of Cross-Laminated Timber panels by means of experimental data and finite element analysis

Significance Statement

Cross-laminated timber is a timber panel nowadays used in most constructions. It is constituted by three or more board layers that are orthogonally glued with respect to the board’s grain direction. It has been used for several years and now standardized procedures to production evaluation are important for both the consumers and producers.

For nearly all glued structural components, face bonding quality and consistency must be proved in the preliminary testing and continually monitored throughout the factory production. This calls for dependable testing methods to sufficiently assess the glue line integrity. However, for CLT the test methodology has been mainly based on the procedures currently being adopted for glulam, not entirely suitable for CLT too. For instance, shear tests provide an easy-to-measure value, but in CLT specimens may generate problems of rolling shear. Also, delamination tests have been applied in glulam for research and production control, but result quite severe for CLT.

Research collaboration between scientists at University of Florence and scientists at Italian National Research Council, Trees and Timber Institute evaluated the face bonding integrity of cross-laminated timber panel. They proposed new test methods based on the conventional methodologies developed for glulam as well as solid wood panels, but adapted for CLT panels. Their work is now published in Construction and Building Materials.

The authors used two cross-laminated timber panels, which were made of spruce. The panels were obtained from manufacturers who dully followed the gluing stipulations for their production processes. The panels were produced with polyurethane adhesive making five layers with varying thicknesses.

In a bid to evaluate the sampling techniques, the authors cut the panels into several strips and applied a total of 5 test methods. For every strip, the authors obtained specimens such that all typologies were taken from adjacent zones of each panel. This enabled them to control the variability related to areas with different bonding integrity then the delamination and shear tests were performed. The authors compared the test methods both by means of experimental data and finite element analysis.

From the results obtained in the shear tests, the authors observed that all the methods used were effective in assessing the bonding integrity among the layers of the selected panels. However, considerations on the easiness and practicability of specimen preparation and test execution, as well as on the theoretical distribution of internal shear stress, were provided in order to discuss and to elect the most suitable approach for inclusion in the technical standards.

Delamination results and shear tests were not correlated. For the delamination test, specimen size was an important factor that dictated the outcomes of the testing.

When it came to the sampling techniques, the authors observed that diagonal sampling was almost similar to sampling along the parallel strips. This also indicated that sampling along the ends would be sufficient in characterizing the bonding of panels homogenously glued, thus reducing the amount of material used by the company for quality control.



About the author

Michele Betti, Ph.D. is currently Assistant Professor in the Department of Civil and Environmental Engineering (DICEA) at the University of Florence, Italy. His research interests include output-only systems identification and data analysis, design of experiment, structural health monitoring, seismic vulnerability assessment, evaluation and rehabilitation of existing masonry buildings (with specific reference to monumental heritage).

He has authored many scientific papers on the subject, and he often serves as a structural engineer expert witness. Betti has been involved in many European Union (EU) international projects regarding lifelong learning for engineering education and in the spread of the EU standards for the accreditation of higher education programs in engineering.

About the author

Michele Brunetti is researcher at CNR-IVALSA (National Research Council, Trees and Timber Institute) in Florence, Italy. He graduated with honours in Forest Science in 1992. From 1994, he dedicates to scientific and technological research on wood science and from 2003 he is the head of physical and mechanical testing laboratory of IVALSA in Florence. He played the role of Italian expert in many COST (European Cooperation in Science and Technology) Actions related to timber and timber products and coordinated several National and International Scientific Projects on the subjects. He lectures in many professional training courses and master degree at Bologna University (Timber constructions) and Bolzano University.

His research interest currently relates to the characterization of timber and glued timber products for structural uses, with particular reference to the strength grading, quality control and CE marking, diagnostics and monitoring of ancient wooden structures and newly built.

About the author

Marco Pio Lauriola is a self-employed engineer with 20 years of experience in structural design of wooden buildings. He received his degree in Civil Engineering in 1994. He participated in major studies on static behaviour, seismic performance and fire-resistance of wood-structure buildings. Since 2005, he works as founding partner in a firm, dealing with projects of structural design of new construction, structural condition assessment, seismic evaluation and retrofit design of existing wooden building, historic preservation and strengthening design.

He is the author of numerous books and publications, speaker at conferences and courses on wooden structures. Since 2008 he teaches the course “Timber construction” at the University of Florence.

About the author

Michela Nocetti, Ph.D., is researcher at CNR-IVALSA (National Research Council, Trees and Timber Institute) in Florence, Italy. Her research is in the field of wood science and technology, focusing on the strength grading of structural timber, the evaluation and development of structural timber products and the diagnosis of wooden structures. She is also involved in the promotion and use of local timber species and the evaluation of wood quality deriving from plantations.

She published several scientific papers and serves in the Editorial Board of a Scientific Journal in the field of wood science. She actively participates at the technical standardization work within the CEN (European Committee for Standardization) with regard to the structural wood products. She graduated with honours in Forest Science in 2001 and received her Ph.D. in wood science in 2001 and 2009, respectively.

About the author

Francesco Ravalli graduated in Civil Engineering in 2014 at Florence University and is currently working as structural engineer. He got experience in research during his training at CNR-IVALSA, studying the quality assessment of glued timber products used in construction.

About the author

Benedetto Pizzo is researcher at CNR-IVALSA, the Trees and Timber Institute of the National Research Council of Italy. Since 2002 he’s in charge of the Laboratory of Chemistry of Wood and Wood Products. He received his M.Sc. with honours in Chemical Engineering and his Ph.D. in Technology of new materials in 1999.

His research interests concern the study of the interactions involving wood, that is, between wood and the other materials to which it is often coupled (with main emphasis to adhesives), and between wood and the environment in which it is preserved. This activity develops through some more specific lines: wood bonding; the effects of wood extractives on relevant technological wood properties (such as durability, wettability, gluability etc.); wood consolidation.

He has been advisor of several students for their B.Sc., M.Sc. and Ph.D. thesis in various degree Courses, and he published over 150 works among scientific JCR journals, professional magazines, monographs and international or national proceedings of conferences.


Michele Betti1, Michele Brunetti2, Marco Pio Lauriola1, Michela Nocetti2, Francesco Ravalli1, and Benedetto Pizzo2. Comparison of newly proposed test methods to evaluate the bonding quality of Cross-Laminated Timber (CLT) panels by means of experimental data and finite element (FE) analysis. Construction and Building Materials, volume 125 (2016), pages 952–963.

Show Affiliations
  1. Department of Civil and Environmental Engineering (DICEA), University of Florence, Via di Santa Marta, 3-I-50139 Florence, Italy
  2. CNR-IVALSA, Istituto per la Valorizzazione del Legno e delle Specie Arboree, via Madonna del Piano 10, I-50019 Sesto Fiorentino (FI), Italy


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