Abstract

Durability issues in timber—A brief review.—

Construction materials are considered durable if they remain reliable, and resist external attack throughout their predicted service life (Van Acker et al. 2003). However, human activities coupled with natural processes can reduce the service life of structural materials (Achal et al. 2011). The cellulosic components of wood render it inherently hygroscopic, resulting in sorption and desorption of moisture depending on the surrounding climatic conditions (Militz 1993, Reinprecht 2016, Dungani et al. 2019).

Humans have long explored methods for improving timber durability. The earliest efforts involved the use of naturally durable species. The heartwood of these species contain chemicals or extractives that are toxic to or inhibit the growth of wood degrading organisms (Kirker et al. 2013, Kutnik et al. 2017, Verbist et al. 2019). Preservatives are used to enhance the durability of less durable wood species. These preservatives are generally toxic to the biological organisms and are introduced into the wood by spraying, brushing, or pressure to deliver the chemicals into the wood. The amount of chemical delivered and the depth to which it penetrates the wood determine the efficacy of the treatment process. Preservative treatments tend to work well with the sapwood of most species.

An alternative to traditional preservative treatment is to modify wood properties to reduce susceptibility to degradation. This process involves covalently bonding a chemical group to some reactive part of the cell wall polymers (Rowell 2006, Mantanis 2017). The resulting bond can create improved physical, chemical, mechanical, and biological properties. Chemical modification is particularly attractive because it offers better aesthetics, a uniform finish, potential property enhancement, and reduced maintenance without the use of pesticides (Kumar 1994). A number of wood modification processes have been studied but furfurylation and acetylation have shown the most promise (Mantanis 2017).

Although all these methods markedly improve timber durability, their use in the construction of mass timber buildings has been limited. Availability, consistent performance, and cost are key issues with naturally durable species. Natural durability varies with species, geographic regions, and growing conditions (Taylor et al. 2002, Viitaniemi et al. 2001, Kutnik et al. 2011), and there is increasing evidence that plantation resources of some traditionally durable species are less durable than those from native forests (Scheffer and Cowling 1966, Scheffer and Morrell 1998, Kirker et al. 2013). Limited availability of durable species, as well as potential effects on resin performance have also limited the use of these materials in mass timber. Preservatives have raised issues with architects because of concerns about their potential effects on nontarget organisms (Edlich et al. 2005). Chemically modified wood eliminates the concerns with other methods; however, it remains more expensive. As a result, most timber structures are built with kiln dried wood that is otherwise unprotected against biological agents of degradation.

Physical, biological, and chemical processes can all affect the service life of a timber structure (Previati et al. 2012, Verbist et al. 2019). Mechanical abrasion and general handling are common physical activities that can affect wood properties but are generally not a major cause of degradation. The exception would be applications where timber is subjected to repeated heavy loads such as use in railways. Timber can also degrade when exposed to excessive heat and, ultimately fire. Exposure to acidic and alkaline chemicals in industrial environments can also degrade wood. UV radiation from sunlight slowly degrades the timber surface in exterior exposures and, while the depth of damage is slight, it is a major cause of wood replacement (Singh and White 1997). However, biological degradation is the most common type of damage experienced in wood (Reinprecht 2016). Fungi, bacteria, insects, and marine borers can all degrade wood, but fungi are the most important agents of biodegradation (Blanchette et al. 1989, Highley 1999).

Fungi are heterotrophic and require organic material to survive. Although wood is more resistant to degradation than other biological materials, but some fungi have evolved systems to utilize this resource. Insects can also cause damage in specific environments, with termites being the most destructive wood degrading insects (Highley 1999). Bacteria are almost always present in wood, but their effects are generally minor except when wood is immersed in water for long periods. Bacteria can then slowly decay and weaken the wood (Highley 1999, Reinprecht 2016). Marine borers degrade wood in saltwater habitats. For the purposes of mass timber, they would not be expected to have any impact on performance.

Environmental and climatic conditions such as moisture exposure, temperature, pH, and time can all contribute to the rate of biological degradation in wood. Studies (Viitaneu 1998, Carll and Highley 1999) have established a temperature range of 23°C to 30°C as optimum temperature for the growth of most fungi. However, moisture is the most critical of all the factors. Direct wetting from splashes, spills, or floods can create moisture conditions that support biodeterioration. A moisture content of 26% is suitable to initiate growth of some fungi, but most fungi require moisture levels above 30% and many have optimum levels between 40% and 60% moisture content (Singh and White 1997, Wang et al. 2018, Verbist et al. 2019). Constant humidity conditions alone will not create these moisture levels, but they can help maintain moisture levels and contribute to fungal attack.

Durability in mass timber.—

Mass timber elements create the opportunity to design larger structures capable of supporting higher loads; however, their basic building block is wood that remains hygroscopic. These materials may get wet during construction and may not dry sufficiently, creating conditions for the growth of biological agents. The issue of moisture intrusion during construction is often overlooked because there is an assumption that the materials will readily dry in service. There is increasing recognition that the rapid drying processes used in conventional timber frame structures can result in excessive cracking and deformation. Cappellazzi et al. (2020) noted that prevention will be far more cost-effective than attempting to repair large elements in a complicated building assembly.

Moisture intrusion.—

Moisture intrusion is possible throughout the service life of any mass timber structure, making it essential that moisture does not reach levels that are detrimental to the structural health of the elements. Moisture intrusion is common during building construction depending on climatic conditions and the time it takes to completely enclose a structure (Lepage 2012, Kordziel et al. 2019, Schmidt and Riggio 2019). Locations such as uncoated edges and connections are particularly at risk of moisture accumulation (Schmidt et al. 2019), potentially leading to losses in physical and mechanical properties of the panels and connections (Sinha et al. 2020). Nairn (2017) noted that differential moisture expansion between individual lamina in non-edge-glued panels can propagate existing precracks, while natural cracks caused by residual stresses due to changes in moisture and temperature will form in edge-glue panels. These changes have the potential to affect mechanical properties such as shear modulus and Poisson's ratio as well as thermal and moisture coefficients (Nairn 2016). Apart from the formation of cracks, interlaminar stresses caused by differential swelling and shrinkage often result in physical deterioration such as cupping, checking, interfacial shearing, and possibly delamination (Schmidt et al. 2019). The mechanical properties of connections used in such panels might also be adversely affected. Silva et al. (2016) showed that every 1% increase in moisture content in the range of 12% to 18% led to a 1.8% decrease in withdrawal resistance of self-tapping screws in CLT. Prolonged exposure of panels to moisture during construction resulting in moisture contents above 30% could also create conditions conducive to fungal decay (Zabel and Morrell 1992). Thus, the benefits of moisture exclusion are clear, but numerous studies show that it is almost impossible to avoid some elements becoming wet enough during construction for fungal attack (Lepage 2012, McClung et al. 2013, Schmidt and Riggio 2019). Construction of mass timber structures during periods with low chance of rainfall, the use of protective canopies during construction, and delivering panels just in time could all help reduce the risk of wetting during construction (Schmidt and Riggio 2019, Cappellazzi et al. 2020).

Moisture intrusion in service presents a much great challenge because of the difficulty in locating leaks in a timely manner and effectively removing the moisture without adversely affecting panel or even structural integrity. Plumbing leaks, membrane failures, construction defects, and even moisture trapped during the construction phase can all lead to postconstruction issues. Austigard and Mattsson (2020) studied five in-service mass timber buildings and observed that construction error was responsible for moisture intrusion in four of the structures while leakage was the cause in the last one, with fungal decay present in three and mold growth in the remaining two. They also observed that drying took between one week and several months depending on the extent of wetness in the structural elements, the season of the year, and the presence of insulation and other drying barriers. McClung et al. (2013) discovered that the type of water resistive barrier used to enclose panels can be responsible for the rate of removal of moisture in wet elements. High-permeance materials tended to facilitate removal of trapped moisture; however, the drying rate was slow in low-permeance materials leading to prolonged conditions that favored fungal attack. Detecting moisture intrusion in service can be very difficult and cumbersome as a result of design barriers or sheathing that obscures surfaces where moisture might accumulate.

Prior research.—

Biological durability studies of mass timber structures (Table 2) are limited, but a number highlight the potential for moisture intrusion, decay and loss in properties (Wang et al. 2018, Cappellazzi et al. 2020). Wang et al. (2018) documented the risks of biodeterioration in mass timber buildings and concluded that mass timber elements will most likely be degraded by either fungi or insects. The authors suggested that insects such as dry-wood termites, powder post-beetles, or old house borers that could tolerate low moisture levels or subterranean termites that could move up from the soil posed the greatest risk of aging mass timber. The authors concluded that the unique characteristics of wood in relation to durability must be considered to avoid issues in mass timber structures. Likewise, Cappellazzi et al. (2020) noted that moisture intrusion during construction of mass timber exposed it to the risk of fungal attack. Sinha et al. (2020) formulated a method to characterize the effects of biological decay on CLT connections, concluding that both the physical and mechanical properties of the connections could be affected but extensive research would be needed to ascertain the effects of time, wood species and fungi species on these properties.

Table 2. Findings from previous biological durability studies on mass timber.

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Franca et al. (2018) studied the resistance of CLT, parallel strand lumber (PSL), and laminated veneer lumber (LVL) using mass loss over a 4-week exposure to termites and showed that untreated CLT was susceptible. A parallel study conducted by Stokes et al. (2017) showed that mold and decay fungi were visible in addition to termite attack, indicating that the parent materials in these products remained susceptible to degradation. These studies were of limited duration and need to be evaluated in longer term tests under more realistic conditions.

Mankowski et al. (2018) reported the initial results evaluating the effects of different treatment regimens on termite resistance of Douglas-fir (Pseudotsuga menziesii) CLT in proximity with the ground. The samples were protected from direct moisture using ventilated waterproof covers. Although the conditions created in this study were somewhat harsh because it is unlikely that CLT will be used in such proximity to the ground, the exposure represented more severe conditions intended to accelerate attack. No termite attack was observed after 6 months, but average moisture content had increased from 11.4% to 23.7%, approaching moisture contents suitable for fungal growth. The results illustrate the risk of moisture uptake, even if direct wetting is inhibited.

Singh et al. (2019) evaluated the resistance of radiata pine (Pinus radiata) CLT, LVL, and oriented strand board (OSB) exposed to Oligoporus placenta, or Antrodia xantha. Some samples were wetted using overhead sprinklers to simulate 3 months of rainfall exposure in Rotorua, New Zealand (NZ) while others were soaked in water. The soaking regime was designed to raise the moisture content of the samples above 25%. The samples included NZ-made OSB, US manufactured OSB, LVL, and boron-treated CLT subjected to leaching and soaking regimes as well as untreated CLT samples that were only subjected to the soaking regime. They found no significant differences in fungal growth as a result of the leaching or soaking regime; however, fungal growth in LVL was lower compared with the other materials possibly as a result of the elevated pressing temperatures and the pH of adhesive. OSB and untreated CLT showed little resistance to O. placenta while treated CLT samples were resistant regardless of leaching, but the authors did express concerns about the aggressiveness of the fungi employed. In a more recent study, Austigard and Mattsson (2020) identified Gloeophyllum sepiarium, a fungus well-adapted to large variations in temperature and heavy wetting as one of the fungi responsible for decay in outdoor elements of Norwegian mass timber structures and Antrodia sp. associated with decay in elements not exposed to outdoor conditions. This report highlighted the importance of protecting CLT elements used in balconies and galleries by water-tight layer and fittings, quick drying response to wetted elements, and the need for more research to understand the consequences of moisture intrusion in CLT.

Potential methods of decay evaluation.—

Decay resistance in wood and wood products can be characterized using many different methods. Accelerated laboratory tests such as those described in American Wood Protection Association Standards E10 and E 30 (AWPA, 2020) as well as the European Normal Standard EN113 (CEN,1996)use small blocks of wood or wood-based materials that are conditioned and then exposed to standard decay fungi under controlled temperature and moisture conditions that favor active fungal attack. Decay resistance of the block is determined by the mass loss experienced over the exposure period. This method is simple, rapid, and accurate for small blocks of wood, but it is difficult to replicate with larger samples reflective of mass timber elements. Additionally, unlike solid lumber, CLT panels contain resin layers that may create barriers to fungal attack.

Field tests that expose specimens to outdoor conditions expected during the service life of a building have also been employed. These methods do not expose the wood to specific biological agents, but rather create conditions that are conducive to attack by fungi and insects (notably termites). Nondestructive methods are used to inspect the specimens and check for growth of any biological agents. The stake test (AWPA E7-15), decking test (AWPA E25-15), and horizontal lap-joint test (AWPA E16-16) found in (AWPA, 2020) book of standards are examples of these kinds of tests. Like the laboratory decay tests, evaluating decay using the field tests is simple and straight forward but the conditions in these tests in terms of moisture are far more severe than those likely to be present in a mass timber structure. There are several tests, which expose wood above the ground. The ground proximity test exposes specimens on concrete blocks, avoiding direct contact with the soil, and covered with a permeable shade cloth to give direct protection from sun but allow moisture intrusion. Although this method is simple, rapid, and efficient in evaluating decay in solid timber, the larger sizes and longer test times required for mass timber make replication difficult. In addition, visual ratings and mass loss in timber are major parameters used to evaluate decay, but these are inadequate for mass timber.

Although some aspects of the risk of degradation of mass timber elements can be inferred using small-scale testing, full-scale tests are still necessary. The difficulty with such tests is the large number of parameters (physical chemical, biological, and mechanical). The large sample dimensions required also limit the number of replications and tests generally take longer because the decay rate remains the same.