- Cross-laminated timber (CLT) is a massive engineered wood product made of orthogonally bonded layers of solid-sawn lumber, and is intended for roof, floor, or wall applications. Although it was developed in Europe in the early 90s, CLT is relatively new to North America.
CLT products must be certified for structural use. First North American product standard stipulating test methods and qualification criteria for benchmark structural properties and adhesive bond integrity in structural CLT is ANSI/APA PRG320-2012. These methods and criteria have been adapted from existing laminated timber products (glulam), sometimes disregarding substantial differences between parallel laminates and CLT, in which layers are perpendicular to each other.
From the point of view of long term sustainability of the CLT industry in North America, the critical questions are:
1. Is it possible to use low-grade timber harvested in the Pacific Northwest region in CLT products without compromising critical engineering parameters? Utilization of low-
grade lumber, which is typically under-valued, in value-added engineered products should reduce the pressure on the high end structural lumber supply and may also provide a substantial outlet for lower-grade lumber timber species, including beetle-killed pine (BKP) harvested in the affected areas.
2. Can alternative adhesive systems, currently used in related engineered wood products and manufactured by domestic industry, be successfully used in CLT production? This is an important question, and is related to the fact that polyurethane (PUR) is the primary adhesive currently used by CLT manufacturing industry, and is supplied worldwide by a single Europe-based company. This adhesive is optimized for the species commonly used in CLT products to-date. ANSI/APA PRG320-2012 standard allows alternative adhesive types (PRF and EPI are specifically named), but to-date, only one alternative (MUF) has been used in commercial products.
The objective of this project is to determine effective adhesive systems and bonding pressures for the hybrid cross-laminated timber (CLT) combinations. A secondary objective is to evaluate the testing methods prescribed in PRG 320-2012 for cross-laminated bond integrity.
Integrity of hybrid CLT layups was evaluated on small specimens derived from CLT billets fabricated in-house using test procedures and qualification criteria specified in ANSI/APA PRG 320-2012 section 8.2.3. Test results were compared to prescribed qualification criteria.
The Hybrid CLT combinations for this study include both structural grade lumber and low-grade lumber. For a reference species, lodgepole pine was selected, since it is a member of the US-SPF group closely related to the European species commonly used for CLT construction. The structural-grade, local species will be represented by Douglas-fir, while the low-grade species will be represented by low-grade lodgepole Pine, Douglas-fir, and Western Hemlock. The two adhesive systems investigated were 1) polyurethane-based PUR (currently the most common adhesive used by the CLT industry), which will serve as a reference system, and 2) phenol-resorcinol formaldehyde (PRF), which will represent a potential domestic alternative. PRF was chosen because it is a cold setting adhesive commonly used by the engineered wood products industry in North America; however, no CLT manufacturers utilize this adhesive system.
The variables included species combinations (6), adhesive types (2), and clamping pressures (3), with repetition of 9 specimens per combination coming from at least three different CLT billets. The specimen’s bond integrity was assessed by the qualification panel requirements in PRG 320-2012 section 8.2. The qualification tests are block shear and cyclic delamination. A combination must pass both of the test requirements to qualify.
The results of the study show that, of the 36 combinations, six failed the block shear test requirements and twenty-five failed the delamination test requirements. The 10 variable combinations that passed both requirements were DDL10F, DDL40F, DPL40F, PPH10F, PPH69F, PPH10U, PPH40U, PPL10U, PPL69U, and PHL69U.
Initial inspection of test results show that no single variable that seems to make a significant impact on the bond integrity. It did reveal that no combinations with the use of Douglas-fir as a face material and PUR as an adhesive met the requirement, and only one combination with western hemlock as a core material met the requirements. It is evident that the delamination test was the major restriction on whether or not a combination passes the bond qualification. We believe that the adaption of a delamination test standard designed for layers with parallel grains makes the passing requirement too strict for an orthogonally bonded product.
In conclusion, there were 10 combinations that passed both bond integrity test requirements. It was unclear whether the species and/or grade combination, adhesive system, or clamping pressure made the biggest impact on the bond integrity. Relative to the reference adhesive (PUR), and species combination (lodgepole pine), the hybrid panels performed similarly and showed that certain species and/or grade combinations could pass the qualification requirements for specific requirements.
The knowledge gained by this screening study will allow further qualification testing of the passing combinations per PRG320-2012. This also has the potential to supply the CLT manufacturing community with greater flexibility of manufacturing techniques and materials, as well as offer value to underutilized lumber.
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