Abstract
Cross-laminated timber (CLT) is increasingly used in building construction worldwide. Durability of CLT against fungal attack has yet to be fully explored. Water intrusion in mass timber can yield dimensional changes and microbial growth. This study evaluated the performance of CLT coated with various water- and solvent-based stains commercially available in the United States. Twelve coatings were tested for moisture excluding effectiveness, water repellency effectiveness, volumetric swelling, and anti-swelling efficiency. Only five coatings repelled water, limiting dimensional changes. A modified version of AWPA E10-16 (2016) was performed to evaluate decay of the coated CLT samples. Weight losses were recorded after 18 weeks’ exposure to the brown-rot decay fungus Gloeophyllum trabeum. In accelerated mold testing, coated CLT samples were grown in chambers containing spores of Aspergillus sp., Rhizopus sp., and Penicillium sp. for 29 d and assessed visually for mold growth. In both tests, coating C (transparent, water-based, alkyd/acrylic resin) performed the best among the tested coatings. Mold growth was completely prevented, and weight loss caused by G. trabeum was approximately 1.33%. Although coating C prevented decay for 18 weeks, coatings are not intended to protect against decay fungi. However, they may offer short-term protection during transport, storage, and construction.
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Effect of Exterior Wood Coatings on the Durability of Cross-laminated Timber against Mold and Decay Fungi
Gabrielly S. Bobadilha,a,* C. Elizabeth Stokes,a Grant Kirker,b Sheikh Ali Ahmed,c Katie M. Ohno,b and Dercilio Junior Verly Lopes a
Cross-laminated timber (CLT) is increasingly used in building construction worldwide. Durability of CLT against fungal attack has yet to be fully explored. Water intrusion in mass timber can yield dimensional changes and microbial growth. This study evaluated the performance of CLT coated with various water- and solvent-based stains commercially available in the United States. Twelve coatings were tested for moisture excluding effectiveness, water repellency effectiveness, volumetric swelling, and anti-swelling efficiency. Only five coatings repelled water, limiting dimensional changes. A modified version of AWPA E10-16 (2016) was performed to evaluate decay of the coated CLT samples. Weight losses were recorded after 18 weeks’ exposure to the brown-rot decay fungus Gloeophyllum trabeum. In accelerated mold testing, coated CLT samples were grown in chambers containing spores of Aspergillus sp., Rhizopus sp., and Penicillium sp. for 29 d and assessed visually for mold growth. In both tests, coating C (transparent, water-based, alkyd/acrylic resin) performed the best among the tested coatings. Mold growth was completely prevented, and weight loss caused by G. trabeum was approximately 1.33%. Although coating C prevented decay for 18 weeks, coatings are not intended to protect against decay fungi. However, they may offer short-term protection during transport, storage, and construction.
Keywords: Surface treatment; Coatings; Mass-timber; Cross laminated timber deterioration; Mold
Contact information: a: Department of Sustainable Bioproducts, Forest and Wildlife Research Center (FWRC), Mississippi State University, Mississippi State, MS, USA; b: U.S. Forest Products Laboratory, Madison, WI, USA; c: Department of Forestry and Wood Technology, Linnaeus University, Växjö, Sweden;
* Corresponding author: gd450@msstate.edu
INTRODUCTION
The use of mass timber in building construction has increased dramatically over the last decade (Harte 2017). Cross-laminated timber (CLT) panels present numerous advantages compared to construction materials traditionally used for mid- and high-rise structures, such as masonry, concrete, and steel (Smith et al. 2018). Safe and reliable progress of these products necessitates development and adoption of techniques to extend their durability. Cross-laminated timber is usually made of softwood lumber that is considered non-durable (Clausen 2010). As the use of mass timber increases throughout North America, protective methods are critical to extending their service life, especially in regions with accentuated hazards, such as termites and fungal degradation. Although they are not designed to be used in ground contact, careful precautions are needed to minimize the risk of decay and other economic losses.
Among many factors that may contribute to deterioration of building materials, water is one of the most detrimental in mass timber products. Moisture control is essential to the proper functioning of any building (Trechsel 2002). Wang et al. (2018) noted that any material can experience some type of moisture issue, which might be caused by vapor condensation, roof leaks, failures at building envelope protection, or wicking from wet foundation. Moisture exposure can occur due to numerous reasons, such as excessive wetting during or after construction (Bora et al. 2019). The material configuration inherent in CLT design contributes to water absorption throughout the panel. The high absorption capability of wood grain may result in warping of the CLT laminas, caused by moisture differences in the layers. Shrinkage and swelling are likely to cause separation from the adhesive layer of the CLT. According to Carll and Wiedenhoeft (2009), the integrity and strength of bonded wood and progressive deflection of wood composites can be impaired by swelling-induced stresses caused by moisture and by repeated cycles of drying and wetting. Even mechanical connections may be compromised by moisture exposure.
Dimensional changes, moisture damage, and microbial growth can eventually occur with short-term wetting or high relative humidity (RH) (80% to 95%) (Schmidt and Riggio 2019). Cappelazzi et al. (2020) noted that in North America, moisture control during construction is minimal, independently of the material used. Water intrusion in CLT is generally proportional to the volume of wood present. Thus, panel thickness influences the rate of moisture uptake. Cross-laminated timber flooring exposed to rainfall in Oregon, USA, absorbed from 12% to 27%, close to a point where fungal growth begins (Morrell et al. 2018).
Moistened materials are more likely to experience fungal growth. Mold is a persistent issue due its possible occurrence at any manufacturing stage of wood products or when the product is in use if it is wet enough (Clausen 2010). Molds are likely to occur on coated wood surfaces in either indoor or outdoor conditions. Although mold fungi do not affect the strength of wood materials, they are considered a major maintenance concern and are usually associated with respiratory issues in building occupants (Bornehag et al. 2001; Purokivi et al. 2001). Conversely, decay fungi (except for dry rot fungi) are more likely to attack wood materials when free water is available, which is normally at approximately 30% moisture content (MC). When the MC increases to 60% to 80%, the decay rate increases (Stienen et al. 2014; Brischke et al. 2017,). As decay progresses, significant deterioration of the wood is seen to a point where the mechanical and physical properties are completely compromised.
There are several protection methods designed to protect timber products against microbiological deterioration; the most common is the use of pressure-treated wood. However, because mass timber dimensions are incompatible with currently available pressure-treatment cylinders, pressure treatment of the finished product is not suitable (Cappelazzi et al. 2020). Consequently, surface coatings have become popular as a potential solution for extending mass timber service life because of both the water repellent and anti-fungal chemicals found within some coatings (Rosu et al. 2018, 2020).
Currently, there is no available study on the performance of exterior wood coatings on mass timber exposed to water, mold, and decay fungi. To address this issue, coated and uncoated CLT samples were tested in this work based on their water properties and their ability to control mold growth. Based on the progression of decay on control samples, a modified soil block test was developed to investigate the ability of coatings to prevent fungal degradation.
EXPERIMENTAL
Materials and Methods
Moisture properties
To determine water repellency effectiveness (WRE) and anti-swelling efficiency (ASE), two CLT panels were manufactured using a Dieffenbacher laboratory hydraulic press. Six No. #2 2 in × 4 in Southern yellow pine (Pinus spp.) lumber pieces were planed (within 12 h), trimmed, and cut at two different lengths: 762 mm (outer layer) and 305 mm (core layer). The layers were glued together (glue-spread rate of 147 g/m2) with polyurethane resin (PUR) and cold pressed (23 °C) for 3 h at 738 kPa.
The criteria for using Southern yellow pine CLT for this experiment was based on two factors. First, to perform a reliable and precise moisture properties evaluation is critical to have uniform and free of defects material. Second, in the local market, Southern yellow pine boards are easily found. Furthermore, the exterior wood coatings could be applied to manufactured panels and compared with less variation due to substrate.
Seventy-eight samples (free of knots, resin pockets, cracks, and end joints) measuring 110 mm × 50 mm × 25 mm (length × width × height) were selected for testing based on absence of defects, similarity in size, direction of growth rings, and wood density. The samples were randomized and distributed to each treatment. The treatments comprised 12 US commercially available water- and solvent-based coatings/stains: transparent, semitransparent, and white paint. The specimens were coated according to manufacturer instructions, and a set of samples was left uncoated (Table 1).
Table 1. Description of Selected Coatings System
W1 – water based; S2 – solvent based; Alk – alkyd; Acr – acrylic
After being treated, the samples were air dried for 3 d at room temperature, weighed, and conditioned in an environmental chamber at 66% RH and 24 °C (12% equilibrium moisture content) until the samples reached a consistent weight. Then, the moisture excluding effectiveness (MEE) was calculated as follows (Eq. 1), based on Feist et al. (1985),
(1)
where MU is the equilibrium moisture content of the untreated samples, and MT is the equilibrium moisture content of the treated samples.
To determine the water uptake capacity, after being conditioned at 66% RH and 24 °C and weighed, the samples were submerged into a water bath and weighed at the following intervals: 30 min, 1 h, 2 h, 24 h, 48 h, and 72 h. The WRE was determined using Eq. 2,
(2)
where WU is the water uptake of the untreated samples, and WT is the water uptake of the treated samples.
Dimensional changes due to moisture uptake were determined by measuring the volume after periods of 24 h, 48 h, and 72 h. The volumes of the CLT pieces were obtained by the caliper method (measurement at the same spots for height, width, and thickness for error reduction), and the volumetric swelling coefficient (S) was calculated from Eq. 3,
(3)
where V2 is the wood volume after humidity conditioning or wetting with water, and V1 is the wood volume of the air-dried sample before conditioning or wetting.
Anti-swelling efficiency was calculated for each time period (24 h, 48 h, and 72 h) based on the volumetric swelling (Eq. 4),
(4)
where S2 is the treated volumetric swelling coefficient, and S1 is the untreated volumetric swelling coefficient.
Accelerated mold growth
A mold growth test was performed on samples generated from the three-ply southern yellow pine panel described earlier to determine the ability of the coatings to inhibit mold growth.
Table 2. Specifications of Tested Coating Systems
Number of coats applied was determined by manufacturer recommendations. W1 – water based; S2 – solvent based; Alk – alkyd; Acr – acrylic
Based on the MEE, WRE, and ASE, 30 samples: five per coating treatment were selected to be tested against mold growth. The samples were randomized and distributed to each treatment according to dry weight to minimize sources of variation. The treatments consisted of five US commercially available water- and solvent-based coatings/stains. The coating types were transparent and semitransparent (Table 2). The specimens were coated according to manufacturer instructions, with some control samples left uncoated.
The accelerated mold growth test was performed at the Department of Forestry and Wood Technology, Linnaeus University (Växjö, Sweden). The test was conducted in a climate chamber (Memmert HCP 246, Memmert GmbH, Schwabach, Germany) under non-sterile conditions. Temperature and RH in the chamber were monitored throughout the experimental period. Samples of pine sapwood naturally infected by Aspergillus sp., Rhizopus sp., and Penicillium sp. were used as inocula sources. Over a period of 14 d, the chamber was kept at temperature less than 27 °C and 95% RH to be infested with spores. Afterwards, the test samples were hung edgewise from the top through aluminum bars spaced to allow a minimum 10-mm gap between two samples (Fig. 1).