Abstract

Cross-laminated timber (CLT)—PRG 320 certified

CLT panels are prefabricated large-scale solid engineered wood panels used for long spans in walls, floors, and roofs. These panels can range in size, with widths between 2 and 10 feet (0.61 m and 3.05 m), lengths of up to 60 feet (18.29 m), and thickness of up to 20 inches (50.8 cm; The Engineered Wood Association 2022). CLT panels are created using laminations of dimension lumber or structural composite lumber (SCL), such as laminated veneer lumber, oriented strand lumber, or laminated strand lumber, that is orthogonally bonded with a structural adhesive (Yeh et al. 2013).

According to the current standard for CLT, ANSI/APA PRG 320-2019, any lumber that is a softwood species with a minimum published specific gravity of 0.35 is permitted for use (ANSI/APA 2020). It is recommended that the lumber maintain a moisture content of 12 percent ± 3 percent to reduce the development of internal stresses due to shrinkage, and to ensure proper bond quality of the product. E-Class lumber, lumber that is graded based on the modulus of elasticity, or V-class lumber, which is visually graded, can be used in the construction of CLT panels (Yeh et al. 2013). The minimum grade of lumber in the longitudinal layers of CLT must have a visual grade of No. 2, or machine stress rating (MSR) of 1.2E (1.2 × 10⁶ psi). The transverse layers do not require an MSR rating, and only require a visual grade of No. 3 (ANSI/APA 2020).

Once the lumber has been graded and dried to the desired moisture content, machining and adhesive application must occur. The lumber must be planed or surfaced on all four sides to improve the effectivity of bonding and ensure dimensional uniformity (Yeh et al. 2013). After surfacing, the longitudinal and transverse pieces must be cut to fit the dimensions of the press. Adhesive application should occur shortly after planing (i.e., surfaced 4 sides) to reduce problems associated with oxidation, aging, and dimensional instability. The bonding surface should be free of any substances such as oils, greases, or release agents, which would have negative effect on bond quality. The adhesive spread rate should be applied in accordance with adhesive manufacturer’s recommendations. Generally, the correct spread rate can be seen when a slight and even squeeze-out is present along the entire bond line, as illustrated in Figure 1 (Yeh et al. 2013).

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After the adhesive has been spread on each board, panel layup and assembly pressing occur. Lumber defects should be carefully examined while producing the layup because the panel must have a minimum effective bonding area of 80 percent (ANSI/APA 2020). A hydraulic press is used to generate the vertical clamping force necessary to bond each layer. The amount of vertical pressure applied is dependent on the adhesive used, and the adhesive manufacturer should be consulted to determine the optimum level.

Side clamping, with a pressure between 40 and 80 psi (276 and 550 kPa), is also recommended (Yeh et al. 2013), because it ensures that edge joint gaps between laminations do not exceed 1/4 inch (6.4 mm) for face layers, or 3/8 inch (9.5 mm) for all other layers (ANSI/APA 2020). Pressing times vary by the adhesive used. In general, phenol–resorcinol formaldehyde (PRF) systems have the longest pressing times, followed by polyurethane (PUR), and emulsion polymer isocyanate (EPI; Yeh et al. 2013).

There are additional steps that must be taken in order to create a structural panel according to PRG 320. Based on the scope of this research, it is irrelevant to discuss topics such as quality control, machining, grade stamps, and packaging, for nonstructural products such as timber mats.

Timber mats—non-PRG 320 certified product

Timber mats, also known as access, crane, dragline, construction, or environmental mats, are often used in heavy industrial applications to reduce ground pressure and decreases rutting, erosion, and other soil disturbances associated with heavy equipment use. General uses for mats include temporary roadways, platforms, or bridges, and they are deployed by the mining, power transmission, general construction, and logging industries (Shmulsky et al. 2021).

Traditional timber mats are constructed of solid wood timbers that have a thickness between 8 inches (20.3 cm) and 12 inches (30.5 cm; Spartan Mat 2020b), and are held together with steel bolts or rods (Herberg 2018). Dimensions for mats vary by manufacturer, but the most common sizes include widths of 4 feet (1.2 m) or 8 feet (2.4 m), lengths between 14 feet (4.3 m) and 20 feet (6.1 m), in 2-foot (0.61 m) increments, and a thickness of 3, 5, 7, or 9 plies. The mat market has expanded to include mats of other constructions including composite, laminated, and CLT (Spartan Mat 2020b).

Other types of mats have been researched and developed, such as the bolt-laminated hardwood mats evaluated by Owens et al. (2020); however, they are beyond the scope of this study, so this literature review will compare the features of traditional bolted mats and CLT mats. Traditional timber mats use large members of green solid mixed hardwoods, mixed oak species, or Douglas fir (Pseudotsuga menziesii) that are between 8 inches (20.3 cm) and 12 inches (30.5 cm) thick (Shmulsky et al. 2021). CLT timber mats, such as those manufactured by Spartan Mat and Sterling Solutions, are generally constructed with 2-inch by 8-inch (5.1 cm by 20.3 cm) southern yellow pine lumber and bonded with a structural adhesive (Mahamid et al. 2017).

Spartan Mat currently produces CLT mats from southern yellow pine, using grades of Southern Pine Inspection Bureau (SPIB) #2 and better for the parallel layers, and SPIB #3 and better for the perpendicular layers. The adhesive used to bond the panels is Loctite UR 5151, a polyurethane manufactured by Henkel AG & Co. KGaA (Düsseldorf, Germany). The mats are produced in various dimensions with lengths ranging from 14 feet (4.3 m) to 28 feet (8.5 m), widths of 4 feet (1.2 m) or 8 feet (2.4 m), and thickness of 3, 5, or 7 plies (Spartan Mat 2020a).

CLT mats may have additional upfront manufacturing costs when compared with traditional mats because of the need for adhesives, kiln drying, and specialized pressing and processing equipment, whereas traditional mats require significant amounts of manual labor combined with common hand tools. Although a higher initial investment is required to manufacture CLT mats, these mats can be made faster, with significantly reduced manual labor. These benefits allow manufacturers to reduce costs when compared with traditional mats.

Red oak

Within the matting industry, mixed oak species are one of the most commonly used species groupings (Shmulsky et al. 2021). According to the 2021 edition of Standard Grading Rules for Northeastern Lumber (NELMA 2021a), northern red oak is included as a mixed oak species. Red oak, specifically northern red oak, serves as the primary wood species for this study because of its popularity as a matting material, availability throughout most of Appalachia, and high strength characteristics.

Northern red oak is a ring porous hardwood species that exhibits nearly white sapwood, and heartwood that is brown with a red tinge. The wood is heavy and exhibits a high volumetric shrinkage value of 13.7 percent (Glass and Zelinka, 2010). This species is commonly found in the eastern United States and Canada and serves as an important commercial species, often used to make furniture, cabinets, flooring, interior trim, and general joinery (Sander 1990).

Red oak has been used for a variety of purposes for its appearance, but the mechanical and bonding properties of this particular species are what make it versatile. Red oak is a strong species, with a modulus of rupture (MOR) of 57,000 kPa (8,300 psi) and modulus of elasticity (MOE) of 9,300 MPa (1.35 × 10⁶ psi) when green (Senalik and Farber 2021). When dried to a moisture content of 12 percent the MOR increases to 99,000 kPa (14,300 psi), while the MOE increases to 12,500 MPa (1.82 × 10⁶ psi; Senalik and Farber 2021).

Red oak is defined as bonding satisfactorily, which indicates it will bond well with high-quality adhesives when bonding conditions are well controlled. High density and strength of oak allows higher loads to be placed on the bondline, resulting in higher strength values for the glued product. Although the higher density of oak can increase the strength of the final glued product, it can also cause bonding difficulties because high-density woods have thicker cell walls and smaller diameter lumens, potentially resulting in poor adhesive penetration. However, this can be overcome through the use of higher pressures to bring contact between the wood surfaces and adhesive (Frihart and Hunt 2010). Care must be taken when bonding this species because overpenetration of adhesives can occur as a result of the large number of radially oriented rays as well as easy flow within the vessels (Frihart and Hunt 2021). Additionally, red oak can be difficult to bond with some phenol–formaldehyde adhesives (Frihart and Hunt 2010).

Overall, red oak has high strength characteristics, high density, an ability to bond well, high levels of durability, and a history of being used in mats, making it an ideal species for use in CLT mats if the bonding issues can be resolved.

Adhesives

Adhesives used for commercial CLT panels that meet the criteria set by ANSI APA/PRG 320 include polyurethanes, emulsion polymer isocyanates, melamine, and phenolic-based adhesives (Yeh et al. 2013). Structural adhesives that are capable of withstanding long-term cycles of soaking and drying include phenol–formaldehyde, resorcinol–formaldehyde, phenol–resorcinol–formaldehyde, emulsion polymer isocyanate, melamine formaldehyde, and isocyanate type adhesives. Additional adhesives that are capable of withstanding short periods of soaking water include melamine–urea formaldehyde, epoxy, and polyurethane (Frihart and Hunt 2010).

Based on their structural use, ability to withstand long periods of soaking and drying, and availability within the United States, three adhesive options—emulsion polymer isocyanate (EPI), phenol–resorcinol–formaldehyde (PRF), and polyurethane (PUR)—were the focus of this study.

EPI adhesives are generally used for laminated beams, lamination of plywood to steel, metals, and plastics, as well as doors and other architectural materials (Frihart and Hunt 2010). EPI adhesives are two-part systems that contain a liquid emulsion and separate isocyanate hardener. The hardener reacts with emulsion and is capable of curing at room or elevated temperatures (Sellers et al. 1988). High pressure is required when using EPI adhesives. This adhesive type displays high wet and dry strength while also being very resistant to water and damp atmospheric conditions. Additionally, EPI adhesives show high resistance to prolonged and repeated wetting and drying cycles (Frihart and Hunt 2010).

PRF adhesives are primarily used for laminated timbers and assembly joints that must endure severe service conditions (Sellers et al. 1988). PRF systems are two-part systems that contain a liquid resin and powder hardener. The hardener reacts with the liquid resin and cures between 70° and 150°F (21°C and 66°C; Frihart and Hunt 2010). This system is characterized as having high wet and dry strength, while also being unaffected by water, molds, grease, oil, and most solvents (Ebnesajjad 2011).

Polyurethane adhesive is typically used for general home and shop purposes, as well as panelized walls and floors, specialty laminates, and the installation of gypsum board. This adhesive is supplied as a one- or two-component system. The adhesive is directly applied to one surface that has been treated with water or a primer. The contact with water activates the adhesive and allows it to cure at room temperature (Ebnesajjad 2011). PUR type adhesives have high wet and dry strength and are resistant to water and damp atmospheres, but have limited resistance to prolonged and repeated wetting and drying cycles (Frihart and Hunt 2010).

Bonding studies

Few studies have evaluated the bond performance of hardwoods in CLT applications, however, Michigan Technological University in collaboration with the USDA Forest Service, and the New Zealand Forest Research Institute examined the durability of adhesive bonds in cross-laminated northern hardwoods and softwoods (Musah et al. 2021). The study involved creating single-species cross-laminations, hardwood cross-laminations, softwood cross-laminations, and hybrid cross-laminations that combined hardwood and softwood species. Two adhesives were used to bond each setup, a melamine formaldehyde and a phenol–resorcinol formaldehyde. Both adhesives were applied using each manufacturer’s parameters, respectively.

All panels were subjected to cyclic delamination testing according to AITC Test T110-2007 (American Institute of Timber Construction 2007), and the results of using both adhesives were compared. The results for single-species red oak laminations showed that no delamination was found when using the melamine adhesive system, but the delamination failure rate was 37.5 percent when using the phenol–resorcinol formaldehyde (Musah et al. 2021). Although this study provides results for numerous CLT setups, it provides limited results because it evaluates the two adhesives, but no other application variables. Evaluating additional variables such as moisture content, spread rate, pressure, press time, as well as additional adhesives would aid in providing CLT manufacturers crucial information when attempting to determine optimal setups. Adjusting any of these variables has the potential to change performance with any adhesive.

Lumber

In order to evaluate the bonding performance of HCLT constructed with northern red oak (Quercus rubra), lumber was obtained from a large sawmill in northern West Virginia. The acquired lumber was initially unsurfaced (rough), kiln-dried, and consisted of a mixture of flooring and pallet grade material; and all boards had dimensions of 4/4 (1 in.; 2.5 cm) thick, 10 feet in length (3.1 m), and width of approximately 7.25 inches (18.42 cm). These board dimensions are a result of the mill’s standard procedure for sawing a combination of grade lumber and pallet boards; process logs to a 7.25-inch flitch and then process the flitch through a gangsaw. The boards were effectively mill-run, with no visual override applied for structural grading purposes.

The boards were first surfaced on the two wide faces by the cooperating mill and shipped to the West Virginia University Appalachian Hardwood Center (AHC) labs where the lumber was ripped to reduce the width of each board to 6.25 inches (15.9 cm) using a straight line rip saw. After ripping, the lumber was graded in accordance with the National Hardwood Lumber Association (NHLA 2019) and Northeastern Lumber Manufacturers Association (NELMA 2021b) standards to determine the appearance and structural grades of each board, respectively.

Prior to manufacturing the 1-foot by 1-foot sample panels the boards were conditioned to a moisture content of 8, 12, or 16 percent depending on the requirements of the setup. Directly before panel manufacturing started, each board was planed to a width of 6 inches (15.2 cm) and thickness of 0.75 inch (19.1 mm). After planing, the boards were cut into 1-foot (30.5 cm) lengths using a miter saw. If sanding was required for the setup based on the experimental design, a belt sander was used directly after cutting to length.

Adhesives

Three adhesive systems were studied to determine how effectively they bond red oak CLT. Each adhesive system represents an EPI, PRF, or PUR type adhesive. The EPI used for this study is a two-component system (Anonymous 2023). An additional two-component system serves as the PRF (Anonymous 2004). The PUR selected for this study is a one-component PUR (Anonymous 2015). For specifics regarding each adhesive, refer to Table 1. All adhesives were applied and pressed under ambient temperature conditions.

Table 1. Specifications for the emulsion polymer isocyanate (EPI), phenol–resorcinol formaldehyde (PRF), and polyurethane (PUR) adhesives following the recommendations of each manufacturer.

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Experimental design

An L9 Taguchi design was implemented to determine the factors of each adhesive type that influence the results of cyclic delamination and shear testing. This method, developed by Genichi Taguchi, is used to design experiments to produce high-quality products at a low cost for the manufacturer by evaluating how different parameters affect the mean and variance of process performance characteristics. The goal of this experimental design is to build quality into a product by minimizing the deviation from a target and making it immune to uncontrollable environmental factors (Fraley et al. 2023). The L9 Taguchi Method was selected for use in this study to reduce the time and large number of combinations that result from a full factorial model. The Taguchi method optimizes the performance characteristics of a process by identifying the signal to noise (S/N) ratio of a factor at a certain level. The S/N ratio is a ratio of the determinate, or fixed contribution (signal) to an indeterminate, or random contribution (noise) of the analyzed substance (Harvey 2023). Within this study the S/N ratio for each analysis is maximized, but the equation for the S/N ratio changes based on the desired response. When the goal is to minimize the response (Eq. 1), the S/N ratio is calculated as

$\frac{S}{N}r\mathit{atio}=-10\mathit{lo}{\mathit{g}}_{10}\frac{1}{n}{{\displaystyle \sum }}_{i=1}^n{y}_i{}^2$ (1)

Equation 1: The equation to calculate S/N ratio when the goal is to minimize the response (Sabarish et al. 2019).

When the goal is to maximize the response y_i² in Equation 1, it is replaced with ,where,

S = Signal

N = Noise

n = Number of repetitions for the experimental combinations

y = Measured value of a characteristic

i = Trial number

The objective of the Taguchi analysis is to determine the optimal setup to minimize the response of delamination in cyclic delamination testing, or to maximize the response of wood failure in shear block testing. The equations will ultimately determine the optimal settings for resin spread, moisture content, pressure, and surface roughness to achieve lower levels of delamination or higher levels of wood failure based on the results obtained from the setups of the L9 Taguchi design.

The resulting S/N ratios and Desirability value provide the necessary evaluation of each adhesive. An S/N ratio of zero is desirable when the goal is to minimize the response because any other result would be negative and indicate levels of variability within the results. When maximizing the response, the highest positive S/N ratio is desirable because it also indicates low levels of variability. Desirability values are shown on the chart for each factor and indicate the individual desirability of that particular setting. The numerical value listed for desirability is the composite desirability, which indicates how favorable the results are for all responses as a whole. The highest achievable value for desirability is one.

For each tested resin, four factors were considered; resin spread, moisture content, pressure, and surface roughness. Each factor was divided into three levels, allowing a low, medium, and high level for evaluation. An example of these values using moisture content would be 8, 12, and 16 percent for the low, medium, and high values, respectively, for each factor. The levels for resin spread and pressure were determined based on the parameters recommended by the adhesive manufacturer (Table 1). All factors, except pressure, remained the same for each adhesive and its respective setup because each individual adhesive has different pressing requirements. In summary, nine setups were created for each adhesive (Table 2) to determine which factors influence the bonding quality for HCLT mats made with red oak lumber. The setups can be illustrated using PUR setup 1, where A1 is 150 g/m², B1 is 8 percent moisture content (MC), C1 is 70 psi, and D1 is a planed surface (Table 2).

Table 2. Summary of delamination testing by adhesive type and Taguchi setup.

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Manufacturing and testing sample panels

Small three-layer sample panels were manufactured based on the setups generated through the Taguchi design. The panels had a length and width of 1 foot (30.5 cm) and overall thickness of approximately 2.25 inches (5.72 cm). The steps involved in constructing each sample panel included conditioning for MC, planing, cutting, gluing, layup and pressing.

Adhesive was applied according to the individual manufacturer’s recommendations. With the adhesive applied, the panel was inserted into a Carver Bench Top Auto Press and programmed to the force required by the experimental design and the press time recommended by the adhesive manufacturer.

All samples in this study were subjected to the same testing procedures. Each sample was evaluated using cyclic delamination and shear block (see Fig. 2) tests according to the methods described in sections 8.2.6 and 8.2.4 of PRG 320 (ANSI/APA 2020), respectively. Upon completion of the cyclic delamination process, the percentage of delamination was visually estimated on each bond line using a 0.003-inch (0.076-mm) feeler gauge to identify areas of delamination, and a digital caliper to measure the length of delamination. Once the shear block tests were complete, the wood failure percentage was visually estimated.

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It is necessary to note that the sample size for both the shear and delamination specimens is unequal. In setups that exhibited poor initial results for delamination, only one panel was used to obtain the results, while setups that performed well used three panels.

Cyclic delamination

Cyclic delamination testing in accordance with PRG 320 standards yielded various results across each adhesive and associated setup. Based on PRG 320 criteria, the average delamination of all bond lines within a specimen cannot exceed 5 percent. This value was used as a guide to evaluate the potential of a tested adhesive in bonding red oak (ANSI/APA 2020).

Best delamination setups for each adhesive

The results revealed that none of the setups utilizing the EPI and PUR adhesives met APA standards. It is important to note that the standards set by PRG 320 do not have to be met because of this product is an uncertified product. However, the references to PRG 320 serve to guide our understanding of how a CLT mat might perform in the field.

The PRF adhesive exhibited the lowest average delamination percentages and met the requirements of the standard in seven out of the nine setups. PRF adhesive setup #9 displayed an average delamination of 1.26 percent with a standard deviation of 3.20 percent and proved to exhibit the least amount of delamination and variability. PRF setup #8 performed in a similar manner with an average delamination of 1.76 percent and standard deviation of 3.39 percent. A summary of the delamination results for each adhesive and their associated setups is provided in Table 2.

Optimized settings to reduce delamination

Analysis of the PRF test results (see Fig. 3) suggest that optimal settings include a resin spread rate of 250 g/m², 12 percent lumber MC, pressure of 150 psi (1.03 Mpa), and a pre-layup surface roughness of 150 grit. These settings resulted in a predicted mean value of −8.00 percent delamination and an S/N ratio of −0.69

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Optimal settings for the PUR setups include a resin spread rate of 200 g/m², MC of 8 percent, pressure of 70 psi (0.48 Mpa), and surface roughness of 60 grit. These settings resulted in a predicted mean value of 5.93 percent delamination and an S/N ratio −23.69.

The optimal settings of the EPI adhesive, include 200 g/m² for resin spread rate, 16 percent MC, pressure of 150 psi (1.03 Mpa), and surface roughness of 60 grit. These settings resulted in a predicted mean value of 16.28 percent delamination and an S/N ratio of −24.42.

A summary of the optimal settings to reduce delamination for each adhesive can be found in Table 3.

Table 3. Summary of the optimal settings to improve delamination performance.

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Shear block tests

The shear block testing followed procedures outlined in PRG 320 and produced diverse results among each adhesive and associated setup. Section 6.3.3 of PRG 320 requires the average wood failure of tested specimens to be greater than or equal to 80 percent (ANSI/APA 2020). A summary of the results detailing the shear stress and percentage of wood failure is available in Table 4.

Table 4. Summary of shear testing by adhesive type and Taguchi setup.

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Best performing shear block setups for each adhesive

The results of shear block testing produced a broad range of results. None of the setups were able to meet the 80 percent threshold for wood failure. The best performing adhesive was the PRF using setup #7, which produced an average of 76.35 percent wood failure with a standard deviation of 18.81 percent. PUR setup #7 also performed well, with an average of 75.42 percent and standard deviation of 23.69 percent.

Optimized settings to increase wood failure

Optimal settings for the EPI setup were defined by the Taguchi analysis to be 200 g/m² for resin spread rate, 8 percent MC, pressure of 150 psi (1.03 Mpa), and surface roughness set at ‘planed.’ These settings resulted in a predicted mean value of 70.67 percent wood failure and an S/N ratio of 46.41.

For PUR, the optimal settings predicted by the Taguchi analysis include a resin spread rate of 200 g/m², MC of 8 percent, pressure of 70 psi (0.48 Mpa), and surface roughness of ’60 grit.’ These settings resulted in a predicted mean value of 84.39 percent wood failure and an S/N ratio 43.44 (Fig. 4).

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For PRF, a resin spread rate of 250 g/m², 8 percent MC, pressure of 150 psi (1.03 Mpa), and surface roughness of ’60 grit’ were considered optimal through Taguchi analysis. These settings resulted in a predicted mean value of 76.96 percent wood failure and an S/N ratio of 37.69.

A summary of the optimal settings to improve shear performance for each adhesive can be found in Table 5.

Table 5. Summary of optimal settings to improve shear performance.

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Delamination

Of the 27 total setups tested, 7 setups using the PRF adhesive met the delamination requirements of PRG 320. None of the setups using the two other adhesive types were able to reach the requirements of the standard. Fortunately, the Taguchi analysis shows that significant reductions in delamination can be achieved using the predicted optimal settings, especially for the PUR.

The delamination results when using the EPI adhesive with setup #6 matches the optimized results of the Taguchi analysis. This indicates that 16.28 percent approximates the lowest amount of delamination that can be achieved with this adhesive, and that, for the variables being considered, it cannot be further optimized. Currently, there are no studies that evaluate the use of EPI to bond red oak, and the reason for its performance cannot be explained. As a result of the results obtained using this adhesive, it cannot be recommended to bond red oak lumber to manufacture HCLT mats.

The predicted optimal settings for the PUR found that 5.93 percent delamination could be achieved, indicating a significant reduction in delamination when using this adhesive. Although the PUR did not achieve the 5 percent delamination threshold, its use in HCLT mats constructed with red oak should be investigated further. Even though this does not satisfy PRG 320, the amount of delamination is low, and the standard does not have to be fulfilled because of the nonstructural nature of this product. Additionally, using the PUR allows for a 2-hour press time, which is half the time required for the PRF. The reduced press time would allow manufacturers to create more mats per day, without a significant compromise in quality when compared with using the PRF. There may also be some environmental benefits of using PUR instead of PRF, although these issues were not addressed by our study.

The lowest average delamination of the PRF setups was 1.26 percent. The Taguchi analysis predicted the optimal settings for the PRF to produce −8.00 percent delamination, indicating that no delamination should occur with the optimized settings.

The study conducted by Musah et al. (2021) utilized Cascophen G-1131A, a phenol–resorcinol-based adhesive, achieving a delamination rate of 37.5 percent (Musah et al. 2021). The difference in the results obtained from this study, compared with theirs, could be associated with the different formulas used to manufacture the respective adhesives. Additionally, Musah et al. did not evaluate the performance of each factor at different levels, only using a MC of 12 percent, planed surface, pressure of 125 psi, and spread rate of 50 pounds per thousand square feet (Musah et al. 2021).

Shear

None of the adhesives or test setups were able to reach the 80 percent threshold for wood failure while subjected to shear block testing. However, the Taguchi analysis was capable of finding setups for each adhesive that would significantly improve the predicted amount of wood failure.

The EPI setup with the highest average wood failure produced 52.50 percent. The analysis produced an optimized setting that predicts a mean wood failure of 70.67 percent, an increase of 18.17 percent over the initial setup with the best performance.

The PUR with the largest percentage of wood failure yielded an average of 75.42 percent. The optimal settings suggest that a wood failure of 84.39 percent can be reached, making it the only setup with the potential to exceed the requirements of PRG 320. The optimal settings with the PUR insinuate that it can compete with the PRF adhesive, as seen previously in the results for delamination testing.

The Taguchi analysis was able to slightly optimize the settings for the PRF, going from a peak average of 76.35 percent to a predicted average of 76.96 percent. This indicates that the best performing setup was close to its maximum potential and there was little room for optimization.

Future research

Many opportunities exist to research the use of hardwoods in mass timber applications because hardwood utilization in mass timber products, particularly using red oak, is virtually nonexistent. Many of the adhesives developed for CLT products were specifically made for species such as spruce (Picea spp.), pine (Pinus spp.), and fir (Abies spp.). They may work for these species, but additional research must include the development of adhesives specific to hardwoods. One such method for red oak could include increasing the molecular weight of the resin to reduce the absorption of resin due to the large vessels. This would greatly increase the bonding properties of hardwoods and could potentially create new markets for HCLT products. Additional adhesive types that could be tested for exterior structural applications would include melamine–formaldehyde and isocyanate adhesives because they are rated for these uses (Frihart and Hunt 2010).

Although the PRF produced the best results, the PUR should still be considered for use once the optimized results have been verified. The optimized results suggest that an average delamination of below 6 percent can be achieved, indicating that it could be used in place of the PRF. Additionally, using the PUR would allow manufacturers to produce more mats per day as a result of the lower cure time of this adhesive.

At this time, the optimal settings generated through the Taguchi analysis have not been specifically verified. Future research requires using the optimized settings to create test specimens that can verify the predicted results, especially for the PRF and PUR adhesives.

Additionally, the red oak HCLT mats must undergo field testing to evaluate the performance of the mats to understand how they perform in real-world applications. At this time, red oak HCLT mats with a length of 9 feet (2.74 m) and width of 3 feet (0.91 m) have been produced using the optimized settings for the PRF and await field testing through the AHC.

Outside of noncertified products, red oak shows potential as a raw material for CLT in structural applications where certification is required. The predicted delamination results from the L9 Taguchi analysis fulfill the 5 percent requirement set by PRG 320 when using the PRF and are within 1 percent of satisfying the requirements when using the PUR. The shear testing results are also favorable for certification because the predicted wood failure using the PUR exceeds the 80 percent threshold set by PRG 320, while the PRF comes within 3 percent of meeting that requirement. Both delamination and shear properties have proven that red oak serves as a viable species for structural CLT applications but would require additional research into properties such as bending, thermodynamics, fire resistance, and earthquake resistance before red oak could enter the structural market.

Finally, red oak could serve to increase the aesthetic value of CLT panels. This could be achieved through three different approaches. One approach would be to use red oak as the sole species for the panel. Another would be to create a mixed-species panel that uses traditional spruce, pine, and fir lumber for the interior layers, with higher grade red oak used for the face layers. The problem with a single-species red oak panel would be the weight, because red oak is significantly denser than softwood species. The mixed-species panels, using red oak lumber or veneer, would greatly reduce the weight when compared with single-species panels, but would also require additional research to determine how to bond the different species together. Mixed-species CLT, specifically combinations of red pine (Pinus resinosa) and red oak, as well as eastern white pine (P. strobus) and red oak have been studied by Musah et al. (2021), but the study only evaluated two adhesives, and did not use methods conducive to commercial manufacturing.