Graduate Thesis Or Dissertation

Exploratory Study of Salvaged Lumber as Feedstock for Cross-Laminated Timber (CLT)

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  • The United States generates over 70 million tonnes of wood waste annually, with over 27 million tonnes still available for recovery. Current recycling rates for wood waste are in the range of 10% to 15%, which is much lower than other common building materials and indicative of an underutilized resource. Available markets for wood waste depend on material quality, quantity, and accessibility and are primarily for lower value uses. The greatest barrier identified to better wood recovery and reuse is a lack of end markets and market development. Creating a value-added product, in this thesis cross-laminated timber (CLT), using salvaged lumber could expand market opportunities for wood waste. CLT is an engineered wood panel made using layers of lumber glued in perpendicular directions and used as prefabricated walls and floors. For salvaged lumber to be qualified for use in structurally rated CLT panels, it must be graded and selected based on Standards for Performance-Rated Cross-Laminated Timber ANSI/APA PRG 320-2018 lamination requirements. Likewise, American National Standard ANSI/APA PRG 320-2018 specifies the required qualification criteria and benchmarks that panels must meet to be approved as a structurally rated CLT panel. The specific objectives of this thesis were to: (1) determine the residual mechanical properties of salvaged lumber from Portland residential building deconstruction; (2) manufacture different experimental CLT panels using salvaged lumber, virgin lumber, and medium density fiberboard (MDF) at small-scale; and (3) examine and compare experimental CLT panels with ANSI/APA PRG 320-2018 Standards for Performance-Rated Cross-Laminated Timber. These objectives were chosen to answer our hypothesis that salvaged lumber can be used as feedstock for structural cross-laminated timber (CLT). Salvaged lumber provided by Portland deconstruction contractors was evaluated using a Metriguard system for determining stiffness. Grading was successful on 265 boards and 96% met the minimum stiffness requirements for E3 CLT grade laminations in the major direction according to ANSI/APA PRG 320-2018. Three different exploratory 3-ply CLT panels (all salvaged lumber, salvaged lumber outer plys with MDF core, and virgin lumber outer plys with salvaged core) were investigated. All panels were manufactured and tested in accordance with ANSI/APA PRG 320-2018. Testing results and calculations were compared to ANSI/APA PRG 320-2018 qualification criteria and benchmarks for 3-ply E3 grade CLT. All panels met reference values for E3 grade 3-ply CLT in effective flatwise bending moment resistance ((FbS)eff), effective flatwise bending stiffness ((EI)eff), and percent wood failure (WF%). The overall mechanical performance of manufactured panels was good, with an average (FbS)eff of 60.6 10⁶ N-mm/m of width, (EI)eff of 212 10¹⁰ N-mm²/m of width, Vs of 51.1 kN/m of width, and WF% above 99%, but delamination was a problem. Percent delamination calculated from cyclic delamination test measures manufacturing process and quality, and it is possible to achieve better results if panels were made in a professional CLT manufacturing facility. Hence, there is potential that CLT panels made with salvaged lumber are capable of meeting ANSI/APA PRG 320-2018 reference design values for 3-ply E3 grade structural CLT panels, but better manufacturing practices need to be implemented for improved delamination results and more samples need to be tested for stronger statistics.
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