Cross-laminated timber (CLT) is becoming increasingly adopted into North American construction, yet little is known about the impacts of environmental exposure (e.g., to rain during construction) on its long-term performance. The lack of protocols for on-site moisture protection in North America makes it a pressing matter to determine general moisture responses of this material in order to establish a behavioral baseline for practitioners and future researchers.
A CLT floor panel sample was exposed to cycles of wetting and drying in an environmental chamber. During these cycles, physical and geometrical properties of the panel were monitored. Testing results indicate that discontinuities in the layup CLT affects the hygroscopic behavior of the product. While the panel showed high dimensional stability, it also exhibited checking, cupping, and interfacial shearing after cycling. Bending test results before and after cycling indicated a reduction of the structural capacity due to the weathering.
Currently, design of tall wood buildings is generally accomplished in the USA through the so-called alternate means process, with requires extensive testing, engineering analysis, and a stringent peer review process. As it pertains to cross-laminated timber (CLT), it is critical to develop effective performance prediction models, through laboratory testing elaborating on material behaviors (e.g. hygrothermal, vibrational, etc.) as well as monitoring data on the mid- to long-term performance of timber structures in situ. This paper presents the scope and preliminary outcomes of a project aiming to cross reference laboratory research and in-situ monitoring to establish a holistic performance-monitoring protocol for mass timber buildings; this protocol can later serve to define standards for mid- to long-term monitoring as well as to develop guidelines for the design of mass timber structures.
There are currently no standards regulating water management for mass timber elements during construction, little knowledge of impacts of moisture exposure (wetting and drying performance, dimensional stability, checking), and few precedents serving as guidelines for monitoring moisture response of mass timber. To address these gaps, a hygrothermal monitoring study was devised to track moisture performance of U.S. made cross laminated timber (CLT) and glulam at a three-story mass timber building. This paper discusses moisture measurements that were collected during the first six months of construction at a CLT rocking shear wall and a timber floor connection. Despite the limited number of structural systems monitored during construction, the distribution and number of sensors in these elements allow to draw some important conclusions. The data confirmed that moisture distribution and wetting/drying rates varied based on local conditions and details (aspect, coatings, connections, etc.), with measurements at an uncoated, north-facing area showing the highest moisture levels (reaching fiber saturation at multiple ply depths and locations). Most locations rarely exceeded 16% moisture content for more than a few months. Certain moisture-trapping details consistently showed higher moisture levels (i.e., above 16%) and poorer drying. Some interior plies continued to show slow increases in MC even after months of drying conditions. These observations suggest preventative approaches implementable in the design (e.g., avoiding moisture trapping details), during fabrication (e.g., localized coating), and construction (e.g., sequencing installation to minimize exposure and allow drying).
Value and use of the data are listed below:
1. Facilitate research advancement in the assessment of hygrothermal performance of mass timber products and the development of models (i.e., hygrothermal models, service life prediction models, etc.) that account for different factors (at the scale of the building, structural system and materials) affecting variability of moisture data.
2. Educate mass timber industry and the AEC (architecture, engineering and construction) industry on the moisture behavior of CLT structural elements exposed to precipitations for a prolonged time during construction.
3. Support development of refined moisture management practices of mass timber buildings.
4. Support development of future hygrothermal monitoring projects of mass timber buildings.
5. Provide a basis for development of models to reference sensor reliability.