Material protection

The service life of a bio-based building material is the period of time after application during which the product meets its minimal performance requirements and no replacement is needed (Kutnik et al., 2020). This depends on the durability of the material and the environmental conditions (Brischke and Rapp, 2010).

Protection by (building) design

In general, moisture is responsible for more than 50% of all building damage, either directly or indirectly. Damage due to moisture can be avoided by implementing good building practice (Langmans et al., 2012; Van den Bossche et al., 2016; Van Linden et al., 2019). The presence of liquid water should be avoided, and water should be removed as fast and effectively as possible (Jones and Brischke, 2017). These principles can also be applied to increase the service life of wood and bio-based building materials in practice. However, the importance of design details and the role they play in enhancing service life, especially in the case of outdoor applications, is often

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neglected and should not be underestimated. For instance, in an experiment on different designs for outdoor exposure, non-durable wood was able to withstand decay over a period of 10 years in applications designed to avoid water trapping (Kutnik et al., 2020). Some examples of design rules that should be considered are the following (Figure 1-12):

• Avoid water accumulation by providing drainage

• Avoid ground contact

• Avoid direct contact with wet walls

• Provide end grain protection (liquid water is absorbed more easily from the end grain)

• Provide gutters to avoid splash

In indoor applications, water leakages and interstitial condensation can cause water accumulation in building elements as well and must be prevented (Finch et al., 2008;

Janssens and Hens, 2003).

Figure 1-12 a) Decking with space between boards to avoid water accumulation, b) load-bearing pillars of wooden pedestrian bridge separated from the ground by concrete fundaments, c) separation between wood beam and wet wall, d) metal covering of end grain of exterior beams and e) wetting of cladding due to splash water in absence of a gutter. Adapted from Jones and Brischke (2017).

Fungicides

If a wood species is not durable, durability can be enhanced by applying fungicides.

Originally, the most-used wood preservatives were Chromated Copper Arsenate (CCA), pentachlorophenol (PCP) and creosote (Coggins, 2008; Jones and Brischke, 2017).

These are very effective but also toxic for the environment (Townsend et al., 2006). In

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1998, the Biocidal Products Directive (Directive 98/8/EC, 1998) was implemented to prevent application of environmentally harmful products and only 39 out of 81 previously included active ingredients remained on the European market of wood preservation (Jones and Brischke, 2017). Creosote is still used for wood protection in the EU, but is expected to be banned once valid alternatives are in place. Wood preservatives for the Do-It-Yourself (DIY) sector contain mainly organic active ingredients like Iodopropynyl Butyl Carbamate (IPBC), tebuconazole, propiconazole or pyrethroids. In heavy-duty applications such as highway fences and utility poles (UC 3.2 and UC4, Table 1-2), mainly copper-based preservatives are used (Jones and Brischke, 2017). Wood preservatives are often applied by brushing, spraying or short-term dipping. However, the main industrial treatments require an increased penetration depth and use vacuum pressure treatment.

Hydrophobic agents and coatings

Coatings can be applied to preserve wood, to improve its aesthetic appearance or both. When a coating has a low permeability, the moisture levels in the wood are low, positively impacting its service life (Miller, 2005). Typical coatings are pigmented paints and varnishes. Oil, waxes and paraffin are hydrophobic agents. When applied to wood or bio-based building materials, the overall material becomes more water repellent.

This reduces the amount of capillary water uptake, reducing the risk of fungal decay.

Oil treatment generally does not offer long lasting protection in UC3.2 and UC4 (Table 1-2). Treating solid wood with waxes or paraffin is technically challenging, because of their high melting points, but commercial products exist in Germany and Austria.

Besides waxes and oil, other hydrophobic agents, such as silicone compounds and silane-based sol-gel coatings, can be applied for the same purpose as well (Sèbe and Brook, 2001; De Vetter et al., 2009; Wang et al., 2011). There are also several wood-fibre insulation boards on the market containing hydrophobic agents, such as paraffin and bitumen.

Wood modification

Wood modification is a complementary technology to the conventional wood preservation techniques, where the wood constituents of less-durable wood species are thermally and/or chemically altered to improve the durability (Jones et al., 2018).

The modified wood is non-toxic and should therefore not release any toxic substance during service into the environment (Hill, 2007). During thermal modification, the wood cell wall components are changed at high temperatures (above 150 °C) and under low oxygen conditions. Various chemical reactions take place, which can improve dimensional stability: removal of OH-groups, increased crosslinking within the cell wall polymers, changes in the mobility of the polymer network and bulking of thermally degraded components that have remained in the cell wall (Hill et al., 2021).

As thermal modification negatively influences strength properties, thermally modified wood is not used for structural purposes but mainly applied as cladding and decking.

During chemical modification, the number of water sorption sites is reduced by letting

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the OH-groups in the wood polymers (i.e. cellulose, hemicellulose and lignin) react with a stable, covalently bounded, less hydrophilic group (Rowell, 2012). Acetylation is a form of chemical modification, in which acetic anhydride is added to esterify OH-groups on the wood cell wall components, with acetic acid as a by-product. Acetylated wood absorbs less water than its non-modified equivalent due to this decrease in OH-group availability, not only due to the OH-substitution itself, but also due to the corresponding spatial confinement (Thybring et al., 2020). Similar as in thermally modified wood, this improves wood properties like dimensional stability and durability.

Another form of chemical modification is based on furfuryl alcohol (furfurylation).

1.5.4 Determining durability against Badiomycota by laboratory testing