- Each year the United States: Forest Service (USFS) spends billions of dollars fighting forest fires. One strategy used by the USFS to prevent catastrophic fires is through forest restoration programs, in which potential forest fuel is removed through mechanized thinning. This program is expensive and generates high volumes of small-diameter logs (less than 6” at the small end). This material is often converted to low-value wood chips, pulp, or biomass. Some of these small-diameter logs can be processed in local specialized sawmills capable of processing it into lumber. What is not utilized locally from the restoration programs, is transported at a cost to the USFS to the nearest facilities that can use or process the produced products from the thinning. Creating a value-added product using low-grade lumber produced from small-diameter timber would improve the economic balance for forest restoration operation.
The general aim of this research was to increase or stimulate markets for wood products utilizing low-value small-diameter material generated in National Forest System restoration programs. Our hypothesis is that low-value lumber cut from small-diameter logs (4”-6” at the small end) could be successfully utilized in core layers of structural cross laminated timber (CLT) panels. Cross -laminated timber is an engineered wood panel composed of solid-sawn lumber, such as 2x6 or 2x8, laid up in perpendicular directions and used as prefabricated walls and floors.
However, to be qualified for structural uses, CLT must meet standard minimum bond integrity criteria specified by the North American product standard (ANSI/APA PRG 320-2012), determined through laboratory testing for delamination (≤5%) and shear resistance (≥80% wood failure).
The objective of this project was to determine the feasibility of small-diameter logs harvested from National Forest System restoration programs in 3- and 5ply CLT panels. Adding value to low-value timber harvested from USFS lands by using it within CLT applications is expected to increase profitability of the harvested timber, offsetting costs for the restoration programs.
The specific objectives were to: (1) build and test CLT panels utilizing lumber from forest restoration operations in core layers of panels against the certification criteria per PRG 320-2012 to allow low-grade lumber in cores of structural CLT; (2) based on findings, propose respective changes to the current North American standard PRG 320-2012; and (3) investigate the efficiency of the primary processing of small-logs from the thinnings and lamination options with lumber produced from these small logs.
The approach was to incorporate the forest restoration material harvested in the larger Pacific Northwest region into the cores of 3- and 5-layer hybrid CLT panels and assess the technical viability of these panels by testing layup samples against the standard adhesive bond integrity criteria, and by comparing the characteristic engineering properties of the material (E, MOR, and rolling shear) with the standard CLT grade benchmarks. All tests were performed following the standard test protocols of the ANSI/APA PRG 320-2012.
Blue Mountain Region of Eastern Oregon, and the Fremont-Winema forest in Southern Oregon were selected as representative forest restoration sites. The species harvested were Ponderosa pine, White fir, and Douglas-fir. The small logs processed at Idaho Forest Group (IFG) Lewiston, ID facility mainly produced a nominal 2x4, mostly No.2 & BTR visual grade lumber; Collins Co. donated Utility grade 2x4s. The 3- and 5-layer hybrid test panels were manufactured at D.R. Johnson in Riddle, OR consisting of No. 2 visual grade Douglas-fir as the panel’s faces and mixed species from the restoration thinnings in the core layers. The hybrid panels were used to determine E and MOR () using third-point bending, rolling shear ( through center-point bending, as well as adhesive bond integrity via block shear and cyclic delamination tests.
In addition, 3- and 5-layer homogeneous panels consisting of Ponderosa pine, White fir and Douglas-fir were produced to detect and separate the potential effects of individual species on the adhesive bond integrity in the layups with mixed species in the core. These homogeneous panels were only used to determine the rolling shear strength in the core layers and to evaluate the integrity of the adhesive bonds. Homogeneous Douglas fir laminations constructed with standard lamstock used in commercial CLT production at DR Johnson were used as control material.
The efficiency of primary and secondary manufacturing processes was also evaluated by company interviews and on-site visits.
The control sample group, the homogeneous 3- and 5--ply No.2 visual grade Douglas-fir, met the minimum qualifications for the PRG 320-2012. The CLT test panels incorporating mixed species material from forest restoration programs in the core layers have shown good strength and elastic properties (compared to the standard E3 pre-defined CLT grade). However, in contrast to the reference commercial all Douglas-fir panels, none of the CLT panels with mixed species material from restoration programs passed the delamination test for bond integrity. Of the additional homogeneous layups, only 3-ply White fir combination passed the delamination test.
Potential causes of failure might have been related to processing issues: 1) inconsistent thickness tolerances of laminations and 2) incompatibility of species-specific adhesive system with the species mix used in the tests.
In the light of the current findings, none of the sample groups with material from forest restoration programs qualified for structural CLT per PRG 320-2012 standard criteria.
Further investigation is needed to identify factors affecting the delamination failures, which both appear to be related to the manufacturing process and, thus, possible to mitigate.
Regarding the efficiency of production of lumber and CLT panels from small logs, additional presorting during harvesting and mill processing steps may help increase process efficiencies during breakdown manufacturing steps. The efficiency of the IFG primary saw line was substantially lower when processing logs of diameters below 6 inches at the small end than normal production; however, with increased familiarity of the project’s thinned material, production efficiency should increase through additional pre-sorting and machine system settings.
The economic feasibility side of using the harvested material conducted by Lawrence (2017), who found the material to not have significant to persuade CLT manufactures in the use of the material (Lawrence 2017).