Bench-scale Fire Stability Assessment of Cross-Laminated Timber

Abstract

As mass timber construction gains momentum worldwide, cross-laminated timber (CLT) has emerged as an important material in the built environment. However, the combustible nature of wood presents challenges and fire safety concerns, and a key interest with respect to CLT is the adhesive bond line. CLT is commonly used in ceiling configuration, and current fire stability evaluations rely predominantly on resource-intensive large-scale fire resistance tests, creating a barrier to adhesive development. A part of this research addresses industry needs by optimising a proposed bench-scale screening test to assess adhesive formulations prior to large-scale testing.

The first research component used cone calorimeter testing to study how the adhesive bond line changes the burning behaviour through varying parameters such as bond line position (2-6 mm depths), lamella thickness (4-24 mm), and wood species (spruce/beech). The results demonstrated that the adhesive bond line created a separation between otherwise continuous burning processes and manifested a peak in the heat release rate profiles. Increasing lamella thickness reduced heat emission intensity, suggesting that a thicker lamella improves fire performance. Comparison of wood species also revealed fundamental differences in burning and char residues.

The second research component optimised a bench-scale adhesive screening test and evaluated multiple variations in the construction of test sample. An advancement in test strategy was achieved through the development of a cascading method in result analysis, where data was grouped by two different failure mechanisms of the adhesive observed. This approach has improved the performance ranking capabilities of the screening test for the evaluation of different adhesive formulations.

This research has provided insight into the burning behaviour of laminated wood and contributed to the delivery of practical tools for the industry partner in the adhesive formulation screening test, with the potential to accelerate adhesive development and enhance fire stability of CLT products.

Popular Abstract

Bench-scale Fire Stability Assessment of Cross Laminated Timber

Context

CLT consists of layers of timber boards bonded together, using structural adhesive, in a cross-wise arrangement. This creates panels that are strong yet lightweight, making it a good material as a structural element. Mass timber offers a range of benefits in construction engineering, in addition to cost and time saving, it is less energy-intensive and beneficial to health and wellbeing of occupants.


Fire Safety Challenge

Wood burns. While the developed char layer has an insulating and protective effect, the adhesive bond between wood layer presents a unique challenge. When exposed to heat, this adhesive can degrade at a lower temperature than the charring temperature of wood, causing layers to separate and fall off, also known as bond line integrity failure. This exposes fresh wood to high temperature and can potentially intensity the fire.
Currently, testing whether CLT building elements will perform safely in a fire requires large and expensive experiments, i.e. large-scale fire resistance test. These tests are resource-intensive and are not practical for industry developing new, safer adhesive for CLT, especially in testing a wide range of promising formulations. A bench-scale screening test can address this challenge.


Burning Behaviour of CLT

This research was conducted in partnership with Bundesanstalt für Materialforschung und -prüfung (BAM), using two bench-scale tests to research on this topic. The first test targeted on the burning behaviour of CLT, used a standard fire test apparatus, Cone Calorimeter, to characterise the key fire performance difference in CLT compared to solid wood without bond line. By systematically varying multiple parameters such as the bond line depth, the layer thickness and the wood species, several insights were revealed. The bond line creates a separation between otherwise continuous burning processes, manifests additional peak in the heat release rate measured consistent across thickness and wood species variation. Increased thickness in lamella reduces the heat emission intensity over time, improving the fire performance. Characteristics specific to species were also discovered.


Adhesive Screening Test

The second research component used a proposed bench-scale test setup to optimise test strategy in collaboration with industry partner. The experiment included five variations in sample construction to identify the most reproducible results, followed by developing reliable data analysis approach. A viable option was proposed along with a method developed to provide details on fire performance of adhesive formulations that enables ranking for screening application. Statistical methods were also applied and proposed to improve the resolution of the analysis capabilities for broader application.


Conclusion

This research has touched on scientific knowledge as well as practical engineering practice in the two bench-scale tests that were carried out. The fundamental understanding in burning behaviour of CLT was provided, covering the differences in bond line configurations, layer thickness and species. A bench-scale screening test strategies were refined the application addresses industry needs in adhesive formulations screening and facilitates innovations in creating the next generation of CLT.

• author: Loh, Wei Le
• supervisor: Patrick Van Hees
• organization: Division of Fire Safety Engineering
• course: VBRM05 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Cross-laminated timber, fire stability, bond line integrity failure, burning behaviour, bench-scale test, cone calorimeter, heat release rate profile, adhesive screening
• publication/series: LUTVDG/TVBB
• report number: 5739
• other publication id: LUTVDG/TVBB—5739--SE
• language: English
• id: 9192205
• date added to LUP: 2025-06-05 12:51:38
• date last changed: 2025-06-05 12:51:38