The interest in softwood-based cross-laminated timber (CLT) production has in turn generated a great deal of interest in producing CLT from hardwood species. This prospect of a new market for hardwood lumber is a significant driver behind efforts to gain certification of hardwoods within American Panel Association (APA) PRG 320, Standard for Performance-Rated Cross-Laminated Timber. However, a number of challenges present significant hurdles for gaining acceptance of hardwoods in CLT manufacturing. These include differences in how softwoods and hardwoods are produced and marketed (e.g., structural lumber markets [softwoods] vs. appearance-grade markets [hardwoods]), lumber grading differences, available sizes of hardwood lumber, drying differences between hardwoods and softwoods, and gluing particularities with hardwoods. This paper identifies the various issues involved with introducing hardwoods into a softwood-dominated market and what it will take to be competitive within the overall CLT market.

The use of cross-laminated timber (CLT) has significantly grown in North America, but hardwood species have not yet been deemed a viable raw material for manufacturing CLT panels. Therefore, softwood species continue to serve as the only approved material for CLT in structural applications according to ANSI/APA PRG-320. Nonstructural CLT products that utilize low-grade lumber from hardwood species, are a good option for introducing hardwoods into the CLT market. Of the hardwood species located within Appalachia, northern red oak (Quercus rubra) is readily available. The purpose of this research was to develop hardwood cross-laminated timber mats utilizing low-grade red oak lumber. In order to manufacture red oak CLT mats, the best adhesive and bonding parameters had to be identified. Overall, sample CLT panels were made using three adhesives with nine different setups for each adhesive. The sample panels were processed into smaller blocks and separated for cyclic delamination and shear-block tests following the ANSI/APA PRG-320 standards. This research determined that a phenol–resorcinol formaldehyde (PRF) adhesive produced the lowest percentage of delamination, satisfying the delamination requirements. The PRF adhesive also produced the largest percentage of wood failure in shear-block testing, however, the results fell short of meeting the requirements. A Taguchi statistical analysis was used to predict the optimal bonding parameters for each adhesive. The optimized bonding parameters for the polyurethane (PUR) adhesive produced favorable results, indicating the delamination results have the potential to nearly meet the standard requirements, while the predicted shear results would exceed the requirements.

The densification of yellow poplar (Liriodendron tulipifera) has emerged as a critical area of research, driven by its desirable properties and broad potential applications. This study investigated the effects of four densification parameters using a 2⁴ full factorial design to evaluate their impact on physical and mechanical properties. Analysis of variance (ANOVA) and Pareto analyses identified the compression ratio as the most influential factor, significantly affecting bending strength, compression strength, hardness, and spring-back behavior. Pressing temperature was the second most significant factor, with higher levels positively impacting mechanical properties. However, increasing the pre-steaming treatment duration from 30 to 60 min at 130 °C had a detrimental effect on strength and spring-back performance, particularly at a 50% compression ratio. Pressing time showed no significant effect on strength properties but contributed positively to hardness and spring-back behavior at higher levels. Several significant factor interactions were observed, further influencing the outcomes. Differences in density profiles were notable across compression ratios, with higher ratios producing more uniform distributions. Under optimal parameters, densification increased compression strength by 117%, bending strength by 60%, and hardness by 154% compared to undensified control samples, demonstrating the potential of densified yellow poplar (Liriodendron tulipifera) for high-performance applications.

The modulus of elasticity (MOE) of structurally graded one-inch-thick red oak (Quercus rubra) and red maple (Acer rubrum) lumber was measured in this work. The center-point, third-point static loading tests, and the stress wave timer methods were used. The objective was to determine if there are statistical differences between three structural lumber grades based on their MOE values. The study considered both the within separated grades and the across combined grades. For red oak and red maple, significant differences in MOE values from center-point static loading tests were observed solely between Select Structural and Below-grade lumber. With the dynamic method, no significant differences were found between any visual grades, including Below-grade lumber. Regardless of the MOE determination method used, the MOE value was not useful for distinguishing the structural, No. 2, and No. 3 visual grades. The strongest correlation existed between the global MOE and the dynamic MOE, which was even higher when the analyses were conducted on separated visual grades. In the case of red maple, stronger correlations between the dynamic MOE, local MOE, and global MOE were observed when separated by visual classes, compared to the analysis conducted on the combined grades. The global MOE was found to be a better predictor of the local MOE than the dynamic MOE.