Timber Adhesives in Fire
Holding it all together: The role of wood adhesives in the fire performance of mass timber.
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How can we design safe timber buildings?
Burnout. And why it is difficult to achieve where exposed timber is involved.
How can we still achieve burnout in a room with exposed timber?
What if timber layers delaminate?
What is the role of adhesives in the fire performance of mass timber?
What do we know about wood adhesives?
How do adhesives react to heat?
Some illustrations from recent research & What was observed in these tests?
How does the bond between wood and adhesives work?
What does this mean for adhesives used in mass timber?
Conclusion & References
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Timber is becoming increasingly popular as a sustainable construction material that offers a fast and quiet assembly on site and one that allows a range of architectural expression. Not least, timber is recognized for its positive impact on occupant wellbeing. Certainly, in the past months, lockdown measures and social distancing have sharpened our focus on the quality of our indoor lives. Our indoor environments and the structures surrounding us have an immense impact on our wellbeing. Building certificates such as WELL or BREEAM have long been giving credit for architecture which fosters occupant wellbeing through incorporating biophilic design elements such as timber materials. Biophilic architecture like the interior use of exposed timber can benefit human physiology, memory, and even creativity [1] [2].
Holding it all together: The role of wood adhesives in the fire performance of mass timber
Nonetheless, with ever more complex timber structures emerging around the world including towers and even an all-timber stadium [3], we cannot overlook the uncertainties surrounding the obvious issue with timber buildings: their fire safety. To inform the design of timber structures despite the associated fire safety concerns, this article aims to introduce the reader to key mechanisms of timber compartments in fire with a focus on one of the main factors [4] that is influencing the fire performance of mass timber. That is, the adhesives which bond layers of wood together in engineered wood products such as cross-laminated timber (CLT). Afterall, daring timber structures, be it ‘the tallest’, ‘the most complex’, or ‘the quickest to build’ are not unbuildable, nor inherently unsafe, as long as designers understand their unique fire safety risks and account for them [5].
To successfully design timber structures using codes and guidance, it is crucial to understand the underlying principles and the fundamental aims of these prescriptions. As an example, the upcoming 2020 NBCC includes a section for Encapsulated Mass Timber Construction, prescribing limits to the maximum permitted area of exposed timber in residential suites. These limits are based on experimental tests carried out by the National Research Council on full-scale compartments with varying percentages of exposed timber on their interiors [6]. Designers wishing to expose mass timber in a building should be aware of these underlying tests that informed the code provisions and what their limitations are. The primary aim for such code limits on the area of exposed timber is to contain a fire in its room of origin, and to ensure that the fire will eventually decay if not supressed by sprinklers, occupants, or firefighters. This means that walls and ceilings must contain the fire for a reasonable duration to ensure the safe egress of occupants and access for firefighters, and they must retain their structural stability.
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Without this fundamental requirement, fire safety strategies which anticipate people to remain in the building for a long time after the fire has started, such as phased evacuation, stay-put, or internal firefighting, are unsuitable.
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The above aim relies on the fact that a fire will eventually decay and go out once it has consumed all the fuel available to it. A phenomenon called ‘burnout’, or ‘self-extinction’. Commonly, ‘fuel’ means furniture and other moveable items present in a room, not the structure of the room itself. However, in compartments where timber is exposed, the combustible walls and ceilings become fuel to the fire. At the material level, small-scale tests have shown that mass timber requires a critical heat flux to maintain combustion [7]. Put simply, mass timber needs heat applied to it in order to keep burning. Although a single piece of mass timber may not continue to burn once the heat source is removed, a group of timber elements will. This is because a burning timber wall or ceiling will re-radiate energy onto neighbouring surfaces. Essentially, the burning wall or ceiling becomes the heat source and continues to deliver energy even after the contents of the room are consumed [8]. Therefore, in rooms with exposed timber surfaces, the additional heat generated from burning walls and ceilings can keep neighboring timber elements exposed to heat fluxes above the critical level required for sustained burning, and as a result, inhibit the timber from auto-extinguishing [8]. In this scenario the walls or ceiling do not stop burning until the structure has failed, and burnout of the compartment cannot be achieved.
Illustrations by Hana Plathner
There are two ways to achieve burnout even in timber compartments. Firstly, codes are beginning to prescribe allowable arrangements and percentages of exposed mass timber areas that have been shown to minimize re-radiation and thus ensure auto-extinction of the structure. Alternatively, engineers can design for burnout by careful consideration of the building compartment material and geometry, showing the heat flux on the timber surfaces drops below a critical value as discussed above [5]. In these calculations, the wall material as well as the configuration and size of doors and windows govern the amount of heat that can leave the compartment, therefore controlling the amount of energy available in the compartment and indirectly influencing the burning behaviour of timber surfaces. [5]. Designers thus have two options and need to understand the implications of both: they can either follow prescriptive guidelines and stay within the realm of what has been tested, or solve the energy balance of a compartment to prove any timber surfaces can auto-extinguish.
The carefully crafted energy balance in a compartment breaks down if at any point during the fire, fresh fuel is introduced. This can, in fact, happen with glued timber products when the adhesive that bonds layers of wood together weakens prematurely [8] and causes pieces of char to ‘fall’ off prematurely. This is called ‘delamination’ or sometimes ‘char fall-off’. Normally, the burning of timber leaves behind a layer of insulating char which slows the rate of burning. However, when fresh, uncharred timber is exposed to the fire through delamination, this can lead to fire regrowth [4] and potentially re-ignition even after auto-extinction has occurred [8]. The effects of localized re-ignition and global increase in burning rate caused by delamination may inhibit burnout completely [7] until the structure fails. This, of course, would have unacceptable consequences. For this reason, it is critical to appreciate the role that adhesives play in the overall mass timber fire performance.
Delamination is unique to engineered wood products where layers of timber are glued together and depend strongly on the type of adhesive used. As a consequence, the fire performance of the chosen adhesive has a major influence on the fire performance of the whole structure. As a response, the North American CLT product standard ANSI/APA PRG 320 was updated to evaluate and exclude adhesives that would delaminate at elevated temperature. Along with this standard, so-called ‘second generation adhesives’ or ‘heat resistant adhesives’ are being developed which promise no delamination. Ultimately however, relatively little is known on the practical performance of the varying adhesive types in real fires, particularly since their formulations are continuously being developed and can vary between different manufacturers.
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Two of the most common adhesive types used to produce CLT panels are melamine urea formaldehyde, short MUF, and polyurethane, PUR. Both, originally used in naval architecture and the aviation industry, were gradually developed throughout the 20th century specifically for buildings. Between the two adhesive types, MUF is comparably stiff and fails in a brittle manner. In comparison, PUR adhesives are more flexible. The benefit of a tough but ductile glue lies in its capacity to redistribute stress concentrations and thereby avert premature failure [9]. A major advantage of PUR over MUF adhesives is the lack of formaldehyde emissions, which have adverse effects on human health. Although MUF adhesives contain formaldehyde, stringent test standards have been developed to keep these emissions at safe levels.
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At high temperatures, adhesive strengths generally decrease [11] [9]. Most adhesives are formulated for temperatures up to 60 °C, whereas highly cross-linked polymers can resist temperatures up to 150 °C [12]. For example, PUR begins to degrade around 90 to 110 °C [12]. A blanket value for a critical temperature for PUR adhesives can hardly be quantified, as the adhesive’s response to heat varies greatly between different formulations. To illustrate this, a 2011 study tested three different PUR adhesives and found that some lost significant strength from as low as 50 °C and 70 °C, whereas others retained their initial, ambient shear strengths up to 150 °C. In the same study, MUF adhesives retained their ambient shear strength up to 70 °C before dropping slightly at 110 °C [13]. Despite the similar temperature ranges in which both adhesive types degrade, it is repeatedly acknowledged in literature that MUF exhibits a higher thermal resistance than PUR [9] [14]. Perhaps most crucially for designers, formaldehyde-based adhesives exhibit much less, if any, delamination compared to PUR adhesives [15] although this is subject to change as new formulations continue to emerge. This demonstrates the importance of testing the adhesives for their performance under elevated temperature, using standards such as the updated ANSI/APA PRG-320.
Experiments quantifying the temperatures at which wood adhesives in CLT elements weaken when simultaneously subject to a load have been rare. Adhesive strength reduction is directly related to their mechanical properties which affect phenomena such as delamination. To understand the behavior of wood adhesives subject to heat, the author recently undertook an undergraduate research project at the University of Edinburgh investigating the residual shear response of MUF and PUR adhesive bondlines in heated small-scale CLT elements. For this, a novel test set-up subjected bondlines in a CLT member to shear forces and heat simultaneously.
Some illustrations from recent research
Initial tests at ambient temperature showed clearly the two different failure modes between MUF and PUR bonded CLT samples. The brittleness of the thermoset MUF adhesive could be seen in the sudden and complete separation of bond lines. Whereas PUR bondlines split in two stages, showing that the flexible PUR adhesive was potentially absorbing further deformation even during shear failure. This more ‘ductile’ failure mode may be favourable over the sudden, brittle failure of MUF bonded CLT elements. Unfortunately, a similar difference between brittle and ductile failure was not observed in the load response of specimens that were subject to an elevated temperature.
From the limited number of tests, data indicated that overall, PUR bonded specimens had a lower stiffness than MUF bonded specimens which agrees with literature. However, PUR specimens also underwent slightly higher applied displacements before failing at marginally higher shearing loads than their MUF counterparts. This is surprising as MUF adhesives are frequently reported to have a higher load-bearing capacity and shear strength [16] [17]. Yet under certain conditions, an adhesive with higher elongation and lower strength may allow a more even stress distribution, resulting in a higher load-bearing capacity overall [18].
Overall, in heated tests the amount of displacement reached prior to failure reduced to 47% and 45% of their ambient temperature displacements for MUF and PUR bond lines respectively. Similarly in heated conditions, the shear strengths fell to about 58% of the ambient temperature value. It remains unclear whether these shear strengths at elevated temperature are those of the adhesives or the timber.
Finally, a higher number of wood failures was observed in heated tests compared to identical tests carried out at ambient temperature. Previously, experiments on similar CLT specimens had shown a move away from wood failure, and towards an increased number of adhesive failure with increasing temperatures [19]. The high number of wood failures was attributed to the following two causes. Firstly, the relative movement of separate timber plies along weakened bondlines may have induced additional stresses within each ply, causing the timber to fracture. Secondly, the elevated temperature may have increased the composite action between the adhesive and the timber, thus rerouting the stresses away from the adhesive bondline and into the timber, where failure occurred.
What was observed in these tests?
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The interactive processes between the glue and the wood in CLT elements may also impact bondline performance and thereby delamination. One such process is the seeping of adhesive into wood, which occurs where glue is applied onto a wood panel, forming a so-called ‘interphase’. This phase is one of three material phases present at every bondline in a CLT element and describes the zone in which the adhesive has penetrated the wood [12]. You could imagine this as a sort of ‘reinforcement’ of the wood. There are two levels of wood penetration. Firstly, the adhesive can either simply fill the wood cell lumens (little cavities in the porous structure of wood), or the adhesive can penetrate the wood cell wall. Only certain adhesives can penetrate the cell wall and thereby interlock mechanically and chemically with the wood [20]. From the two types mentioned earlier, only MUF adhesive is capable of cell-wall penetration [16].
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Since impregnation with MUF can significantly increase the strength of the wood but also embrittle it [21]; [10], [22], adhesive penetration may significantly affect bondline performance. Past investigations argue that no correlation exists between penetration depth and mechanical performance of bondlines in shear [10] and indicate that chemical bonding may have a larger impact [16]. Nonetheless, it is known that exposure to heat can lead to additional interlocking of polymers [23]. It is hence possible that such further cross-linking of adhesives stiffens not only the interphase, but also increases the composite action between the adhesive and the timber. This would improve stress redistributions between the two materials and explain the high number of wood failures described above.
Conclusion
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This article explored the main factors preventing burnout – a crucial requirement for fire safety strategies in many tall and complex structures – in rooms with exposed timber surfaces. Namely, re-radiation and delamination. The role of wood adhesives commonly used in the fabrication of engineered timber such as cross-laminated timber on the fire behavior of such elements was discussed, along with illustrative examples from recent research. Awareness of mechanisms informing the fire performance of timber structures, and the underlying science which informs current codes are essential for anyone involved in the conception of timber structures. This is especially true in tall or complex mass timber structures which may go ‘beyond the code’ and require a performance-based fire safety solution. On a case-by-case basis, code limits such as the maximum exposed timber area may be expanded cautiously to achieve certain design aspirations if, and only if, designers are aware of the experimental origin of such limits and the implications that such a deviation would have on the remaining holistic fire safety strategy.
References
[1] J. Allen and J. Macomber, Healthy Buildings: How Indoor Space Drive Performance and Productivity, Cambridge, Massachusetts: Harvard University Press, 2020. [2] Arup, Rethinking Timber Buildings, 2019. [3] ezeen, "Zaha Hadid Architects wins approval for world's first all-timber stadium," 27 December 2019. [Online]. Available: https://www.dezeen.com/2019/12/27/worlds-first-timber-stadium-zaha-hadid-architects/. [Accessed 14 August 2020]. [4] S. Zelinka, K. Sullivan, N. Ottum, N. Bechle, D. Rammer and L. Hasburgh, "Small scale tests on the performance of adhesives used in cross laminated timber (CLT) at elevated temperatures," International Journal of Adhesion and Adhesives, vol. 95, 2019. [5] A. Law and R. Hadden, "We need to talk about timber: fire safety design in tall buildings," The Structural Engineer, vol. 98, no. 3, pp. 10 - 15, 2020. [6] J. Su, P. Leroux, P. Lafrance, R. Berzins, K. Gratton, E. Gibbs and M. Weinfurter, "Fire Testing of Rooms with Exposed Wood Surfaces in Encapsulated Mass Timber Construction," National Research Council Canada, 2018. [7] A. Bartlett, R. Hadden, J. Hidalgo, S. Santamatia, F. Wiesner, L. Bisby, S. Deeny and B. Lane, "Auto-extinction of engineered timber: Application to compartment fires with exposed timber surfaces," Fire Safety Journal, vol. 91, pp. 407-413, 2017. [8] A. Bartlett, R. Hadden, L. Bisby and B. Lane, "Auto-extinction of engineered timber as a design methodology," in 2016 World Conference on Timber Engineering, Vienna, 2016. [9] F. Stoeckel, J. Konnerth and W. Gindl-Altmutter, "Mechanical properties of adhesives for bonding wood: A review," International Journal of Adhesion and Adhesives, vol. 45, pp. 32 - 41, 2013. [10] P. Hass, Doctoral Thesis: Penetration behaviour of adhesives into solid wood and michromechanics of the bondline, Zurich: ETH Zurich, 2012. [11] R. Iwata and N. Inagaki, "Durable adhesives for large laminated timber," Journal of Adhesion Science and Technology, vol. 20, no. 7, pp. 633 - 646, 2006. [12] G. Habenicht, Kleben; Grundlagen, Technologien, Anwendungen, Berlin: Springer, 2009. [13] S. Clauss, M. Joscak and P. Niemz, "Thermal stability of glued wood joints measured by shear tests," Eur. J. Wood Prod., vol. 69, pp. 101-111, 2011. [14] E. Schaffer, A simplified test for adhesive behavior in wood sections exposed to fire USDA Forest Service Research Note FPL-0175, Madison, WI: Department of Agriculture, 1968. [15] L. Hasburgh, K. Bourne, P. Peralta, P. Mitchell, S. Schiff and W. Pang, "Effect of adhesives and ply confirguration on the fire performance of Southern pine cross-laminated timber," in 2016 World Conference on Timber Engineering, Vienna, 2016. [16] F. Kamke and F. Lee, "Adhesive penetration in wood: a review," Wood and Fibre Science, vol. 39, no. 2, pp. 205 - 220, 2007. [17] F. Wiesner, Doctoral Thesis: Structural behaviour of cross-laminated timber elements in fire, Edinburgh: The University of Edinburgh, 2019. [18] B. Burchardt, "Advances in polyurethane structural adhesives," in Advances in Structural Adhesive Bonding, Woodhead Publishing, 2010, pp. 35 - 65. [19] F. Wiesner, D. Bell, L. Chaumont, L. Bisby and S. Deeny, "Rolling shear capacity of CLT at elevated temperature," in 2018 World Conference on Timber Engineering, Seoul, 2018. [20] C. Frihart, "Adhesive interactions with wood," in Fundamentals of Composite Processing, Madison, WI, 2004. [21] J. Konnerth, A. Jaeger, J. Eberhardsteiner, U. Mueller and W. Gindl, "Elastic properties of adhesive polymers. II. Polymer films and bond lines by means of nanoindentation," Journal of Applied Polymer Science, vol. 102, no. 2, pp. 1234 - 1239, 2006. [22] W. Gindl, T. Schoeberl and G. Jeronimidis, "The interphase in phenol-formaldehyde and polymeric methylene di-phenyl-di-isocyanate glue lines in wood," International Journal of Adhesion and Adhesives, vol. 24, pp. 279 - 286, 2004. [23] S. Oezdemir, Master Thesis: Alterungsmechanismen von Klebstoffen fuer tragende Holzbauteile, Vienna: University of Natural Resources and Applied Sciences, 2009. Bibliography [1] H. Plathner, Master Thesis: Residual shear response of MUF and PUR adhesive bond lines in cross-laminated timber elements subject to elevated temperatures, Edinburgh: The University of Edinburgh, 2020.
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Taylor Institute for Teaching and Learning at the University of Calgary CALGARY, CANADA
Hana Plathner MEng.
Fire Safety and Structural Engineering
Authors
Matthew Smith M.A.Sc., M.Eng., P.Eng.
Associate
