Project contacts are Grant Kirker (Forest Products Laboratory), Katie Ohno (Forest Products Laboratory) and C. Elizabeth Stokes (Mississippi State University)
Mass timber, as a renewable prefabricated structural panel material, is seen as highly desirable in the “green” building movement and has excellent thermal insulation, sound insulation, and fire restriction qualities. CLT is one of the more recent additions to the mass timber market worldwide, and although the product has undergone structural property testing in several laboratories, degradation testing of this non-preservative-treated product has only recently been initiated (Singh and Page 2016). Preliminary testing with exposure to Oligoporus placenta and Antrodia xantha indicated that untreated CLT is susceptible to the spread of mold and decay fungi, while treatment with boron somewhat reduced the extent of the decay fungus spread (Singh and Page 2016). These panels are easily handled on-site and have a much higher strength-to-weight ratio than their precast concrete competitors, which make them ideal for rapid construction of modular buildings, including apartment/condominium structures (Van de Kuilen et al. 2011). However, installations using CLT as a primary structural component in humid/damp climates, such as the southeastern United States, may be heavily affected by molds and decay fungi, and effects on CLT strength should be determined prior to widespread use of the product in these areas.
The aim of this study was to assess the durability of commercially available coatings on cross- laminated timber (CLT) during natural and artificial weathering and against wood decay fungus. The CLT samples coated with twelve coatings were tested based on their moisture exclusion, water repellency, volumetric swelling and anti-swelling efficiency. Among all the tested coatings, only five (A, C, F, I and J) were able to promote water repellency and limiting dimensional changes. The top five coatings were then tested on CLT blocks exposed to natural (Starkville-MS and Madison-WI) and artificial weathering conditions and brown-rot fungi (G. trabeum). Variables such as visual ratings, water uptake, color and gloss change were determined during both weathering procedures. Damage caused by Gloeophyllum trabeum on uncoated and coated CLT was analyzed based on visual appearance and weight loss. For the coatings C and F, the visual rakings and color change results indicated high consistency during outdoor exposure. The artificial weathering showed that coating C and F were the most resistant to chalking, lightness, color and gloss change. In the soil block test, coating C obtained satisfactory performance against G. trabeum with weight loss of 1.33%. Coatings F and J did not offer any protection to water penetration, which eventually contributed to fungal development. For future, new coatings specifically designed for the protection of high percentages of end-grain in CLT panels should be a target of research and development.
Canadian Conference on Building Science and Technology
Cross laminated timber (CLT) and mass timber construction is a promising structural technology that harnesses the advantageous structural properties of wood combined with renewability and carbon sequestering capacities not readily found in other major structural materials. However, as an organic material, mass timber is susceptible to biodeterioration, and when considered in conjunction with increased use of engineered wood materials, particularly in more extreme environments and exposures, it requires careful assessments to ensure long-term performance.
A promising approach towards reducing construction moisture in CLT and other mass timber assemblies is to protect the surfaces with a water-resistant coating. To assess this approach, a calibrated hygrothermal model was developed with small and large scale CLT samples, instrumented with moisture content sensors at different depths, and treated with different types of water resistant coatings exposed to the Vancouver climate. The models were further validated with additional moisture content sensors installed in a mock-up floor structure of an actual CLT building under construction. Biodeterioration studies assessing fungal colonization were undertaken using the modified VTT growth method and a Dose-Response model for decay potential.
The research indicates that CLT and mass timber is susceptible to dangerously high moisture contents, particularly when exposed to liquid water in horizontal applications. However, a non-porous, vapour impermeable coating, when applied on dry CLT, appears to significantly reduce the moisture load and effectively eliminate the risk of biodeterioration. This work strongly suggests that future use of CLT consider applications of a protective water-resistant coating at the manufacturing plant to resist construction moisture. The fungal study also highlights the need for a limit state design for biodeterioration to countenance variance between predicted and observed conditions.
Project contact is Arijit Sinha at Oregon State University
This project will undertake a comprehensive analysis of the effects of water exposure, in various forms, on mass timber building elements. Water intrusion is mostly commonly seen during construction, but can also occur during failure of roofs or external facades or as a result of internal plumbing failures. The research team will employ CAT-scan imaging, vibrational testing, non-destructive and small-scale physical tests to assess the effects of moisture intrusion and any subsequent biodegradation on the structural performance and aesthetic characteristics of the building elements and connections. This analysis will include investigating the effects of cracking and delamination that may occur as a result of wetting and drying. The project will facilitate development of guidelines on moisture control during construction, help identify suitable methods for protecting mass timber products where required and highlight design features that can be used to mitigate the risk of fungal and insect attack.