BONDING HARDWOOD LUMBER FOR CROSS LAMINATED TIMBER: PROPERTIES AND ENVIRONMENTAL IMPACTS

Cross-laminated timber (CLT) is a prefabricated engineered wood product that are made of at least three orthogonal layers of graded sawn lumber or structural composite lumber (SCL) that are laminated by gluing with structural adhesives. The concept of CLT has been developed into a global adaptable building material. The advancement rates of CLT as mass building material in production volume and distribution has unwrapped up new opportunities for hardwood species, which traditionally have not been extensively used for structural engineering applications. The objectives of this research are to provide key information on engineering performance, as well as baseline data on the adhesion properties of bonding northern hardwood species and their environmental impacts. The methodology used in this study followed specific standard guidelines including; the Standard for performance-rated CLT panels (ANSI/APA PRG-320-2012; 2016), the Standard Test Method for Strength Properties of Adhesive Bonds in Shear by Compression Loading (ASTM D905), Standard Practice for Estimating the Percentage of Wood Failure in Adhesive Bonded Joints (ASTM D5266), the Standard Specification for Adhesives for Bonded Structural Wood Products for Use Under Exterior Exposure Conditions (ASTM D2559), Test Methods for Structural Glued Laminated Timber (AITC Test 2007) and ISO 14044 on the environmental management- life cycle assessment requirements and guidelines. The results showed that bonding strength is positively related to wood density; high density diffuse-porous species (such as hard maple and yellow birch) have low percentage of wood failure when used in single species or mixed with each other for CLT production. The average bonding shear strength of cross-laminated mixed hardwoods was 5% (P85%) from the small block test was very high. Although, there was record of individual hard maple sample blocks recording very low percentage wood failure (\u3e25%). The results from the bending tests are promising since the experimental results show that both hard maple CLT and hard maple-spruce hybrid CLT meets and exceed the shear and bending strength requirements in ANSI/APA PRG-320-2012 CLT Grade E3. The major failure modes found were shear failure, bottom splint and delamination. Finally, the cradle-gate life cycle analysis of hardwood and softwood CLT indicated that resorcinol (PRF) and hardwood combinations have the highest environmental impact (10300 and 60900 Kg CO2 eq; 1890000 MJ and 1140000 MJ of total energy consumed respectively). Comparably, softwood has less environmental impact compared to hardwood in CLT manufacturing and melamine base adhesive has a reduction of 34% of energy demand and 11% in GWP when bonded with softwood compared to a combination with hardwoods bonded with resorcinol. Over 70% of the energy in CLT was from non-renewable sources

Integrated experimental and numerical study on flexural properties of cross laminated timber made of low-value sugar maple lumber

The objective of this study is to examine the mechanical performance of cross laminated timber (CLT) panels made of low-value sugar maple under out of plane loads through mechanical tests and numerical simulation. The laminations were sorted into High and Low classes based on the measured modulus of elasticity (MOE). Two 3-layer sugar maple CLT layups as High-Low-High and Low-High-Low glued with resorcinol-based adhesive and one CLT layup of High-Low-High glued with melamine-based adhesive were prepared. Block shear, long-span bending (span-to-depth ratio of 33:1) and short-span bending (5.5:1) tests were conducted to evaluate the bonding, flexural and shear behavior of these low-value sugar maple CLTs. With a limited sample size, the lab-manufactured low-value sugar maple CLT provided a 50% to 80% higher MOE and at least two times higher MOR than CLT type E1 from APA/PRG 320. Similar MOE and MOR improvements were found by comparing CLT made with other species from literatures. The finite element simulation of bending tests was conducted with the orthogonal constitutive law and the progressive damage model based on the calibrated material properties parameters from lumber rating and references. The simulation results on each CLT panel type have a reasonable comparison with experimental test data. Therefore, these integrated experiment and simulation methods can provide detailed mechanical behaviors of the low-value sugar maple CLT, which can also be applied to other CLT species and layup

Stomatal density in Quercus petraea and Q. robur natural populations in Northern Turkey

This study examined variation of stomatal density in two populations of Quercus petraea (Matt.) Liebl. and two populations of Q. robur L. from northwestern Turkey. Stomatal density was determined in fully expanded and dried leaf samples that were collected from trees under natural conditions. Stomatal densities of Q. petraea and Q. robur varied from 186 to 459 per mm2 (mean value: 333 stomata per mm2) and from 397 to 826 per mm2 (mean value: 517 stomata per mm2), respectively. Significant differences in stomatal density were found between these two oak species in Turkey as well as between populations within species. Strong and significant negative correlations were observed between stomatal density and leaf length within each species and across the species. While in Central Europe Q. petraea occurs in drier environments than Q. robur, in the present study Q. robur populations grow in more arid environments and have smaller leaves and a higher stomatal density than Q. petraea. Stomatal density had negative correla¬tions with each of the other leaf characters apart from sinus width. In addition, the interspecific PST value (0.48) for stomatal density was relatively high compared to the mean genetic differentiation calculated at eight microsatellite loci (FST = 0.030), suggesting different local adaptations of populations. Further studies that include additional populations will be necessary to associate genetic variation at candidate genes with phenotypic and environmental variation

Face bonding strength of cross laminated northern hardwoods and softwoods lumber

Producing cross-laminated timber (CLT) has opened a new market for the lumber industry in North America, while few hardwood species have been studied in the U.S. for CLT production. Combining hardwood species in mixed hardwood CLT or in hybrid CLT can be a solution to boost the market of the undervalued hardwoods. However, the knowledge gap on bonding hardwoods needs to be filled to provide evidence of feasibility. This study focused on the face bonding properties of the cross laminations made of seven hardwood species and two softwood species from the Great Lakes region using two commercial structural adhesives, the phenol resorcinol adhesive (the Resorcinol) and the melamine urea formaldehyde adhesive (the Melamine). A total of 45 combinations of the selected species were studied for the bonding strength (shear under compression) and the percentage of wood failure. For single species samples, the bond strength was positively related to the specific gravity of wood and the bond strength of hardwoods was 33% and 82% stronger than that of softwoods bonded with the Resorcinol and the Melamine respectively. The mixed hardwoods showed higher bonding strength (the Melamine: 5.45 MPa, the Resorcinol: 5.28 MPa) than mixed softwoods (the Melamine: 2.8 MPa and the Resorcinol: 2.7 MPa). Among the Resorcinol bonded mixed combinations, 54% had a percentage of wood failure less than 80%, while the Melamine bonded had only 4 out of 22 which had a percentage of wood failure less than 80%. The overall bonding strength of the hybrid combinations was 5% weaker than that of the mixed hardwoods. All hybrid combinations bonded with the Melamine met the 80% wood failure criterium, but some bonded with the Resorcinol had a percentage of wood failure less than 80%. Anatomical features, especially pore distribution, played a key role in the bonding performance. However, the Melamine adhesive consistently achieved over 80% wood failure across all pore distribution types

Mechanical Property Evaluation of Hybrid Mixed-Species CLT Panels with Sugar Maple and White Spruce

This study evaluated the mechanical properties of hybrid (mixed-species) cross-laminated timber (CLT) panels made of low-value sugar maple (Acer saccharum) and white spruce (Picea glauca). The modulus of elasticity (MOE) of the laminations was measured with a nondestructive method. Three-layer hybrid CLT panels with layup combinations of sugar maple-white spruce-sugar maple and white spruce-sugar maple-white spruce were prepared to evaluate the effects of layups on the performance of the CLT samples. The mechanical properties of hybrid CLT panels were evaluated with different layups and both melamine- and resorcinol-based adhesives. Both long-span and short-span third-point bending tests were conducted to study the flexural and shear behavior of each CLT panel type. It was found that the influence of adhesive types was not significant. The mechanical properties of the hybrid CLT panels with sugar maple surface layers were improved significantly compared with those of the current standard layups. Both bending tests were simulated with finite-element analysis based on measured and reference material properties. The simulated results of each case were in good agreement with the test results

Durability of the adhesive bond in cross-laminated northern hardwoods and softwoods

In this study, we investigated the durability of adhesive bonds in the cross-laminated lumber of seven hardwood and two softwood species from the Great Lakes region. The 2-layered cross-laminations were glued using phenol resorcinol- and melamine-based structural adhesives. A total of 720 cross-laminated wood blocks were tested for delamination by exposing the samples to cyclic (wet-dry) conditions. Distribution of the adhesive on the bondlines was also studied to understand the effect of adhesive penetration on bond durability. The results indicated that mixed hardwood cross-laminations generally produced better bonds than single hardwood species cross-laminations. Hardwood and softwood hybrid cross-laminations were found to have better bond durability in dry-wet cycles. A high failure rate (≥ 50%) was found in the following single species cross-laminations: aspen, white ash, white pine, and yellow birch. Similarly, several mixed species cross-laminations resulted in a delamination rate of 50% and higher, which raises caution in their use in CLT manufacturing. In addition, the viscosity of the adhesive influences the maximum depth of penetration, which tends to affect the durability of adhesive bonds

Field Assessment of Downed Timber Strength Deterioration Rate and Wood Quality Using Acoustic Technologies

Hurricane and tornado events cause significant damage to high-value timber in the United States each year. Forest managers and landowners are keenly interested in finding solutions to salvage and repurpose these downed timbers before they cause pest infestations and fire outbreaks, completely losing their value or increasing processing costs. To better understand the wood quality of the downed timber, we used acoustic waves techniques as a nondestructive testing approach to assess the wood degradation rate of downed trees and determine the extent of fracture and voids in the damaged regions. We periodically monitored the acoustic velocity of the downed trees for 12 consecutive months using a time of flight (TOF) acoustic method. Acoustic measurements were conducted using three different techniques—longitudinal, transverse, and off-set methods. Wood density, age, and the diameter at breast height (dbh) class measurement for southern timber (chip-n-saw for dbh 8″–11″ and sawtimber with dbh 12″ and up) were used as the predictive parameters of the downed trees. The results indicated positive relationships between dbh class, stand age, and acoustic velocity measurement (R2 > 65%). The TOF acoustic velocity was indicated to potentially separate higher-stiffness timber from lower-stiffness timber in a hurricane event for structural or non-structural applications. The regression coefficient from the repeated measurements indicated that both age and diameter class strongly impacted the acoustic properties of the downed trees (p-value ≤ 0.001). The sawtimber dbh class recorded a higher acoustic velocity compared to the chip-n-saw type. Fracture, voids, and massive decay in downed trees were detected beyond the visible inspection, features that often are identified by loggers in lower quality wood; however, TOF showed a weak response in picking up incremental deterioration due to changes in specific environmental factors that affected acoustic readings. This study showed that acoustic wave methods could potentially be used as a field evaluation tool for assessing the quality of downed trees