Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Davis College of Agriculture, Natural Resources and Design

Department

Wood Science and Technology

Committee Chair

David DeVallance

Committee Co-Chair

Joseph McNeel

Committee Member

Greg Dahle

Committee Member

Jorge Matos

Committee Member

Gloria Oporto

Committee Member

P. V. Vijay

Abstract

Cross-Laminated Timber (CLT) panels are engineered wood products breaking into the building sector. Currently, the range of raw materials used in CLT is still limited to softwoods, while some hardwood species such as yellow-poplar could potentially be used to produce this wood product. Therefore, the main goal of this research was to assess the feasibility of using Appalachian Hardwoods in the production of Cross-Laminated Timber panels. Specifically, the objectives of the research were to 1) Determine the amount of yellow-poplar structural lumber that can be obtained from a representative population of low-grade, yellow-poplar lumber graded for appearance; 2) Evaluate whether CLT panels produced using No. 2 and No. 3 structural grade yellow-poplar in parallel and perpendicular orientations, respectively, meet the bending and bond line requirements prescribed in ANSI/APA PRG 320-2019 (2020); 3) Determine if improvements in CLT panel properties can be achieved by laying up panels based on non-destructively assessed static bending modulus of elasticity (MOEs) instead of visual structural grades; and 4) Evaluate whether or not placing lumber with high bending MOEs and visual structural grades in outside layers improves CLT properties.

The material selected for this research was low-grade yellow-poplar (Liriodendron tulipifera) that was selected for its availability. First, an initial assessment of yellow-poplar mechanical properties was required to determine feasibility. Therefore, 8,000 board feet (18.9 m3) of yellow-poplar lumber graded as No. 2 Common and No. 3 Common, according to National Hardwood Association of Lumber (NHLA) grading rules, were evaluated. The yellow-poplar lumber was regraded according to a structural visual grade outlined by the Northeastern Lumber Manufacturers Association (NELMA) rules and by non-destructive test to obtain the board’s static modulus of elasticity (MOEs) to determine their potential to meet grade requirements for CLT panels. The percentage of boards that achieved a minimum structural visual grade (No. 3 or better) was 54.6%, and 96.6% of the boards showed MOEs above the minimum 1.2 x106 psi (8,274 MPa) as specified in ANSI/APA PRG 320-2019 (2020).

Once the structural grades of the yellow-poplar boards were assigned, the material was then used to produce different sets of CLT panels. In each case, 5-layer CLT panels with dimensions of 3.7 inches thick x 18 inches wide x 120 inches long (95 mm x 457 mm x 3,040 mm) were produced using an Emulsion Polymer Isocyanates (EPI) adhesive. Three different panel configurations were produced to assess the feasibility of different layups patterns of low-grade yellow-poplar. The first layup (YP1) was based on NELMA visual structural grade selection where No. 2 lumber was used in parallel layers, while No. 3 lumber was used in perpendicular layers. The second layup (YP2) utilized boards that did not achieve a minimum visual grade according to NELMA rules (i.e., graded as Below Grade). In this section, boards were instead sorted by their MOEs,and the lumber falling within the top 40% of MOEs values was used in the panels’ two outer layers. The last layup (YP3) consisted of boards that represented the top 10% of the MOEs population that were placed in the two outer layers. In addition to this configuration, the panels were separated into two groups of five specimens that had different visual grades in the outer layers. Specifically, one group had outer layers with the two highest NELMA grades (Select Structural and No. 1), while the other group had outer layers with the three lowest NELMA grades (No. 2, No. 3, and Below Grade). All 30 CLT panels produced were evaluated in third-point flatwise bending and bond quality according to ANSI/APA PRG 320-2019 (2020).

The panel’s shear block testing resulted in values above standard requirements. However, the cyclic delamination results showed an inconsistency within the panel production, as delamination above 5% was found in the outer areas of the panel. The average bending strength of the CLT panels was found to be 5,687 psi (39.2 MPa), 6,682 psi (46.1 MPa), and 7,266 psi (50.1 MPa) for YP1, YP2, and YP3, respectively. The average MOE of the CLT panels were: 1.39 x106 psi (9584 MPa), 1.56 x106 psi (10,756 MPa), 1.68 x106 psi (11,583 MPa) for YP1, YP2, and YP3, respectively. These results indicated that a significant amount (about 96% of the NDE tested population) of low-grade yellow-poplar lumber could still be used in producing CLT panels if they met a higher level of MOEs as determined by non-destructive evaluation (NDE) proof loading. CLT panels with higher and lower NELMA grades did not present statistical differences when sorted by MOEs, indicating that the NDE is a more effective way to evaluate the material. The placement of higher MOEs boards in the two outer layers improved the panel’s bending properties. These results and the comparison to ANSI/APA PRG 320-2019 (2020) layups strongly indicate the potential of yellow-poplar to be used as raw material in the production of CLT panels.

Finally, this research results showed the potential of low-grade yellow-poplar to be used in the production of CLT panels. The prospect of using a production surplus in a promising engineered wood product should provide financial incentives for the industry to seek the development of these novelty panels.

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