Mass timber structures have the potential to change wooden construction on a global scale. Numerous mass timber high-rise buildings are in planning, under development or already built and their performance will alter how architects and engineers view wood as a material. To date, the discussion of material durability and biodegradation in these structures has been limited. While all materials can be degraded by wetting, the potential for biodegradation of wood in a mass timber building requires special consideration. Identifying and eliminating the conditions that might lead to this degradation will be critical for ensuring proper performance of wood in these structures. This article reviews and contrasts potential sources of biodegradation that exist for traditional wood construction with those in mass timber construction and identifies methods for limiting the degradation risk. Finally, future research needs are outlined.
Current design standards incorporate the use of preservative treated and naturally durable wood where conditions are suitable for deterioration, but treatment options for mass timber products, particularly, cross laminated timber (CLT) remain to be addressed. Termiticide treatment is a necessity for CLT structures, especially in southeastern climates. Wood species currently used to fabricate CLT are non-durable, and current design standards do not properly address incorporation of preservative treatments into these systems. In this study, 12” x 14” x 4” Douglas-fir CLT pieces were installed in a ground proximity protected test at the Harrison Experimental Forest (HEF), (Saucier, MS). Test samples were placed in sets of two in 30” x 30” subplots on bricks approximately 3-4” above soil and covered with ventilated waterproof covers. A total of 20 test pairs (40 total samples) with four different treatments were installed. The treatments consisted of a soil termiticide treatment, a preventive borate spray treatment at initiation, a remedial treatment with the spray-on borate one year post initiation and untreated controls. Soil below five pairs of samples was treated the with a soil termiticide (Termidor® SC). For the preventive borate at initiation treatment, one sample per pair of five other sets was treated with a spray-on borate preventative treatment (Bora-Care®). The remaining ten pairs were left as untreated controls. Temperature and humidity inside some of the covered units is being monitored throughout the test. The ten control sets will be examined for termite attack 12 months after installation. Five of the attacked sets will be treated with spray-on borate as a remedial treatment for active termite attack.
Cross-laminated timber (CLT) market demand is on the rise in the United States. Adequate protective measures have not been extensively studied. The objective of this study was to investigate the weathering performance of exterior wood coatings. We evaluated coated CLT sample surfaces based on visual appearance, color change (CIE*L*a*b), gloss changes, and water intrusion. From the five exterior wood coatings evaluated, only two showed adequate performance after twelve months field exposure. Based on visual ratings following the ASTM procedures, coating failure occurs more quickly in Mississippi than in Wisconsin, due to its greater decay zone. Both location and coating type impacted the aging of the samples. Artificial weathering results were consistent with natural weathering indicating the two adequate coatings were the most resistant to failure, color, and gloss change. For future studies, new coatings designed for the protection of end-grain in CLT panels should be a target of research and development.
