Bonding Characteristics of CLT Made from Silver Birch (Betula pendula Roth.), European Aspen (Populus tremula L.) and Norway Spruce (Picea abies (L.) H. Karst.) Wood

Abstract

This paper deals with the bonding characteristics of cross-laminated timber (CLT) panels made of Silver birch (Betula pendula Roth.), European aspen (Populus tremula L.), and Norway spruce (Picea abies (L.) H. Karst.) wood. Three-layered single-species CLT panels were manufactured using birch, aspen, and spruce lamellae bonded with a one-component polyurethane (PUR) adhesive. Spruce CLT panels were used as reference. The bonding characteristics of CLT were assessed based on bond shear strength, total and maximum delamination, and wood failure percentage. The reference spruce CLT met both criteria (Delam_tot ≤ 10%, Delam_max ≤ 40%) for passing the delamination test, where up to 80% of the test samples passed. The aspen and birch CLTs met the criterion for maximum delamination (26.5% and 33.2%, respectively), but exceeded the maximum allowed value for total delamination (12.7% and 13.2%, respectively). However, the minimum requirement of 70% wood failure percentage (WFP) was met for all CLT types, with aspen CLTs achieving 83.7% and birch CLTs 76.9%. The spruce CLTs achieved an average bond shear strength of 1.9 N/mm², while both hardwood CLTs had significantly higher values, with the aspen CLT at 3.3 N/mm² and the birch CLT at up to 3.9 N/mm². Based on the results obtained, it can be concluded that cross-laminated timber (CLT) made from hardwoods like aspen and birch is suitable for environments with low humidity fluctuations.

1. Introduction

Cross-laminated timber (CLT) is a prefabricated structural building material composed of odd layers of sawn timber that are face-bonded together. These layers are arranged perpendicular to each other. The unique orientation of the layers ensures uniform mechanical properties in the panel’s plane, including strength and rigidity, making CLT a viable alternative to other building materials such as steel, concrete, or masonry [1,2,3]. Cross-laminated timber (CLT) can be utilized for entire buildings or for specific elements that constitute load-bearing walls, internal partitions, floors, and roofs. Its versatility and structural properties make it a compelling choice in modern construction [2,4].

Softwoods, such as spruce, fir, pine, and larch, are most commonly used for CLT production in Europe and North America. However, there are requirements for the use of hardwoods in CLT production [2,4,5,6]. In Europe, for example, these requirements are driven by the limited utilization of hardwoods in industry and a change in forestry policy that has led to an increase in the planting of hardwoods rather than softwoods [6]. These requirements primarily target hardwoods with appropriate strength properties and satisfactory bonding characteristics [7,8]. Another reason for the increased use of hardwoods is the desire to utilize underutilized or low-value hardwoods for structural purposes, such as for glulam and CLT [9].

In recent years, researchers have extensively investigated the bonding properties of hardwood cross-laminated timber (CLT). Several studies suggest that low- or medium-density hardwoods, such as poplar, aspen, or birch, may be suitable for CLT production due to their improved adhesion properties, resulting in reduced delamination in the glue line [2,10]. The bonding ability of CLT depends primarily on the wood species, its properties, and the type of adhesive used. This principle holds particularly true for hardwoods, which exhibit greater variability in anatomical structure and physical-mechanical properties [5,6]. According to the standard EN 16351 [11], bonding properties are assessed based on bond shear strength, maximum and total delamination, and wood failure percentage. Another property, describing the strength characteristics of CLT elements from a different perspective, can be dowel-bearing strength. Dowel-bearing strength describes the strength of the connection between individual CLT elements, which are joined together using fasteners (bolts, dowels, screws, etc.), considering the influence of the material properties (e.g., fastener diameter, moisture content, density, and grain direction) [12,13].

Brunetti et al. tested the bonding parameters of beech and mixed beech–spruce CLT glued with polyurethane (PUR) and melamine–urea–formaldehyde (MUF) adhesives [5]. They found that none of the adhesives met the minimum delamination criteria required by the standard for coniferous CLT. Specifically, the one-component PUR adhesive exhibited poorer performance in the production of beech CLTs due to very high delamination and low shear strength [5]. Das et al. investigated the bonding properties of single-species and mixed CLT made from poplar (Populus nigra L.) and maple (Acer platanoides L.) wood bonded with melamine and PUR adhesives [4]. Their results indicate that all single species poplar CLTs met the minimum requirements for delamination. However, only 50% of the mixed maple–poplar CLTs bonded with melamine adhesive met the minimum requirements for total delamination. Additionally, both single poplar and mixed maple–poplar CLTs met the minimum requirements for shear strength of the glue line, but CLTs glued with melamine adhesive achieved higher values [4]. Musah et al. investigated the quality of adhesive bonds in cross-laminated timber (CLT) made from both single species and mixed hardwoods [14]. They used melamine and phenol-resorcinol adhesives to bond several hardwood species, including sugar maple (Acer saccharum Marshall), red maple (Acer rubrum L.), northern red oak (Quercus rubra L.), white ash (Fraxinus americana L.), yellow birch (Betula alleghaniensis Britton), American basswood (Tilia americana L.), and aspen (Populus tremuloides Michx.). Their research revealed that single-species CLT panels made from yellow birch, aspen, and white ash exhibited a high failure delamination rate (≥50%). In contrast, mixed-species CLT panels performed better in the delamination test compared to single-species CLT [14]. Purba et al. assessed the bonding properties of cross-laminated timber (CLT) made from single oak and mixed poplar–oak wood [15]. These CLT panels were glued using polyurethane (PUR) and melamine–urea–formaldehyde (MUF) adhesives and pressed with varying pressures. They found that only 27% of the CLT combinations met the minimum requirements for both total and maximum delamination. In contrast, mixed-species CLT panels performed better in this regard. Single oak CLTs bonded with MUF glue achieved higher shear strength values; however, they did not meet the minimum requirements for delamination, unlike the mixed poplar–oak CLT [15].

In this study, European birch (Betula pendula Roth.), European aspen (Populus tremula L.), and Norway spruce (Picea abies (L.) H. Karst.) wood were bonded using polyurethane adhesive (PUR) to produce cross-laminated timber (CLT). The primary objective was to evaluate the suitability of hardwoods for CLT and compare them with the commonly used spruce CLT. The bonding characteristics were assessed by determining maximum and total delamination, wood failure, and bond shear strength for CLTs made from birch, aspen, and spruce wood.

2. Materials and Methods

2.1. CLT Preparation

The European birch (Betula pendula Roth.), European aspen (Populus tremula L.), and Norway spruce (Picea abies (L.) H. Karst.) lumbers were purchased from a local supplier in the Czech Republic. All lumber were processed into lamellae with final dimensions of 20 mm × 75 mm × 2200 mm. Subsequently, all the lamellae were visually graded according to EN 14081+A1 [16]. Only defect-free lamellae (without knots, small holes, wane, etc.) were used for the outer and inner layers (lamellae) of the CLT.

For this research, single-species three-layered CLT panels were prepared by joining three layers of lamellae, which were glued together side by side as well as face to face, with grain directions perpendicular to each other. The nominal dimensions of these panels were 60 mm × 300 mm × 2000 mm (t × w × l). The prepared lamellae from all wood species were glued using a one-component polyurethane (PUR) adhesive Kestopur 1010 (Kiilto Oy, Lempäälä, Finland). A glue spread of 200 g/m² was applied to the surface of the lamellae using a roller-coaster. Then, the CLT panels were pressed for 1 h using a hydraulic pressure machine SCM GS 6/90 (SCM Group, Emilia, Italy) with a pressure of 1 MPa.

2.2. Methods

2.2.1. Delamination

The delamination samples, which were prepared from cross-laminated timber (CLT), had dimensions of 60 mm × 100 mm × 100 mm (t × w × l). Twenty samples were assessed for each wood species out of a total of 60 samples following the guidelines specified in EN 16351 [11]. The samples were weighed, placed in a pressure vessel, and soaked in water at a temperature of 10–20 °C until they were completely submerged. A vacuum was drawn to 85 kPa and held for 30 min. Subsequently, the vacuum was released, and a pressure of 680 kPa was applied for the next 2 h. After pressure release, the samples were dried for 15 h in a drying chamber at a temperature of 70 °C. Delamination was determined when the mass of the sample returned to 100%–110% of the original mass.

The total and maximum delamination were computed in compliance with EN 16351 [11] using Equations (1) and (2):

$${Delam}_{tot}={\displaystyle \frac{Ltot.delam}{Ltot.glueline}}\times 100$$

(1)

$${Delam}_{max}={\displaystyle \frac{Lmax.delam}{Lglueline}}\times 100$$

(2)

where Delam_tot is the total delamination (%), Delam_max is the maximum delamination (%), L_tot.delam is the total delamination length (mm), L_tot.glueline is the sum of the perimeter of all glue lines in samples (mm), L_max.delam is the maximum delamination length (mm), and L_glueline is the perimeter of one glue line in a delamination sample (mm).

The results of delamination were assessed based on the criteria outlined in Annex C of EN 16351 [11]. Bonding strength of CLT is proven to be sufficient if it achieves the following:

• The total delamination (Delam_tot) length does not surpass 10% of the sum of both glue lines;
• The maximum delamination (Delam_max) length of each sample does not exceed 40% of the overall length of the single glue line.

If the maximum delamination length or the total delamination exceeds the limits (Delam_tot ≤ 10%, Delam_max ≤ 40%) or if the delamination cannot be determined due to improper surface quality of the end grain surface, then wood failure percentage should be calculated. Bonding strength of CLT is proven to be sufficient if it achieves the following:

• The minimum wood failure (WF_min) percentage of each split glued area must be at least 50%;
• The minimum wood failure (WF_tot) percentage of the sum of all split glued areas must be at least 70%.

2.2.2. Bond Shear Strength

The bond shear test was conducted following the draft standard FprEN 14732 [17], utilizing compression loading. The bond shear samples (Figure 1), measuring 40 mm × 60 mm × 80 mm (w × t × l), featured a double shear area of 40 mm × 40 mm. For each type of wood species and adhesive, a total of 20 samples were used. All samples underwent air-conditioning under specific conditions (20 °C temperature and 65% relative air humidity) until they achieved an equilibrium moisture content of 12%.

The compression loading (Figure 2) was carried out using the universal testing machine UTS 50 (TIRA, Schalkau, Germany). The load application rate was adjusted to induce failure within 30 to 90 s. After failure occurred, the compression (shear) force was recorded.

The bond shear strength was calculated in line with Annex A of the draft standard FprEN 14732 [17], as per Equation (3):

$$f_v={\displaystyle \frac{Fu}{2\times A}}$$

(3)

where f_v is the bond shear strength (N/mm²), F_u is the ultimate load (N), and A is the total shear area (40 × 40 mm) (mm²).

The experimental values were analyzed using multifactorial analysis (ANOVA) with Statistica 13 software (TIBCO Software Inc., Palo Alto, CA, USA). Statistical analysis relied on 95% confidence intervals of the means using Fisher’s F-test.

3. Results and Discussion

3.1. Delamination and Wood Failure

The statistical analysis of CLT delamination was performed using the ANOVA method, with results presented in

Table 1. Evaluation of total delamination in CLT using analysis of variance (ANOVA).

Effect	Sum of Squares	Degree of Freedom	Variance	Fischer’ F-Test	p-Value
Intercept	7487.57	1	7487.57	102.9893	0.000000 **
Wood species	382.8500	2	191.4250	2.6330	0.080590 NS
Error	4144.0370	57	72.7020

NS: Not statistically significant at p > 0.5, ** Statistically significant at p ≤ 0.05.

and

Table 2. Evaluation of maximum delamination in CLT using analysis of variance (ANOVA).

Effect	Sum of Squares	Degree of Freedom	Variance	Fischer’ F-Test	p-Value
Intercept	48,474.13	1	48,474.13	278.2345	0.000000 **
Wood species	681.6400	2	340.8200	1.9563	0.150763 NS
Error	9930.5600	57	174.2200

NS: Not statistically significant at p  >  0.5, ** Statistically significant at p  ≤  0.05.

, as well as Figure 3 and Figure 4. These results, based on a 95% confidence interval and p-value, indicate that the type of wood species used in CLT does not have a statistically significant impact (p-value > 0.05). Both the total and maximum delamination of CLT were found to be statistically insignificant.

The EN 16351 stipulates that the total delamination (Delam_tot) should be ≤10% for cross-laminated timber (CLT) to be deemed suitable in terms of bonding quality [11]. Based on this criterion, only the spruce CLT met the minimum requirements, as it achieved a total delamination value of 7.61%. On the other hand, the hardwood CLTs exceeded the total delamination values, with aspen CLT and birch CLT registering 12.69% and 13.21%, respectively (Figure 3). This corresponds to an increase of 66.8% and 73.6%, respectively.

The EN 16351 establishes a limit value for maximum delamination (Delam_max) of ≤40% and the previously mentioned required value of total delamination (Delam_tot) ≤ 10%, both of which must be adhered to simultaneously [11]. All types of CLT satisfied this minimum requirement for maximum delamination (Figure 4). The lowest average value of maximum delamination, 25.63%, was observed for spruce CLT. Aspen CLT had a marginally higher average value of 26.48%, and birch CLT had a value up to 33.17%, corresponding to increases of 3.3% and 29.4% respectively.

The methodology of delamination testing, which involves a cycle of vacuum, pressure, and drying, and its subsequent results are intrinsically linked to the density of the wood utilized in CLT. The process of wood swelling and drying, which induces stresses in the CLT guidelines, is directly proportional to the wood’s density [18].

Consequently, wood with a higher density undergoes more pronounced dimensional changes and experiences greater stresses in the glue lines during alternating cycles of swelling and drying [2]. However, the mean value of total or maximum delamination does not provide a clear picture of the percentage of CLT samples in each set that meets the minimum requirements. Therefore, these data are depicted in Figure 5.

The delamination values required by the EN 16351 [11] standard (Delam_tot ≤ 10%, Delam_max ≤ 40%) were met to varying degrees by different types of CLT (Figure 5). For spruce CLTs, 70% of the samples passed the delamination test (80% meeting the Delam_tot requirements and 95% meeting the Delam_max requirements). In the case of aspen CLTs, 60% of the samples passed the delamination test (60% meeting both the Delam_tot and Delam_max requirements). For birch CLTs, 45% of the samples passed the delamination test (65% meeting the Delam_tot requirements and 80% meeting the for Delam_max requirements).

Although CLT generally satisfies the minimum required values for maximum delamination as per the EN 16351 [11] standard, aspen and birch CLTs have been observed to exceed the highest average allowable values for total delamination. In such instances, it becomes essential to evaluate the wood failure percentage (WFP). The statistical analysis of the impact of wood species on the WFP of CLTs is presented in

Table 3. Evaluation of wood failure in CLT using analysis of variance (ANOVA).

Effect	Sum of Squares	Degree of Freedom	Variance	Fischer’ F-Test	p-Value
Intercept	420,023.40	1	420,023.40	4727.8224	0.000000 **
Wood species	428.7648	2	214.3824	2.4131	0.098631 NS
Error	5063.9240	57	88.8407

NS: Not statistically significant at p  >  0.5, ** Statistically significant at p  ≤  0.05.

and Figure 6. This analysis found that the influence of wood species on the WFP was not statistically significant.

The highest average wood failure percentage (WFP) of 86.9% was observed for spruce CLT, while birch CLT achieved the lowest WFP values of 76.9%. Aspen CLTs had a mean WFP value of 83.7%. These results affirm the suitability of all three wood species for CLT production. Although aspen and birch CLTs did not meet the minimum required values for delamination (Delam_tot ≤ 10%, Delam_max ≤ 40%), they achieved mean WFP values higher than 70%, which is considered satisfactory.

Other studies have reported different results for the delamination and wood failure in CLT made from spruce, aspen, and birch wood. For instance, a study conducted by Betti et al. examined the bonding characteristics of 5-layer spruce CLT bonded with PUR adhesive [19]. Their findings revealed that the average total delamination (Delam_tot) was 3.5%, the maximum delamination (Delam_max) was 6.2%, and the wood failure percentage (WFP) was 85%. Furthermore, 88% of all the test samples successfully passed the delamination test [19]. Knorz et al. examined the delamination properties of 3-layer spruce CLT bonded with PUR adhesive [20]. They found that the average total delamination (Delam_tot) was 5%, and the maximum delamination (Delam_max) was 8%. Additionally, 70% of the samples passed the delamination test. Interestingly, they also discovered that the wood failure percentage (WFP) values of spruce CLTs were, on average, significantly higher than the requirements set by the EN 16351 [11] standard [20]. On the other hand, Sikora et al., who studied the total and maximum delamination of CLT made from Sitka spruce and bonded with PUR adhesive, report that no CLT met both conditions set by the EN 16351 [11] standard, i.e., Delam_tot ≤ 10% and Delam_max ≤ 40% [21]. However, the second condition (WFP of each split glued area must be ≥50% and for the sum of all split glued areas must be ≥70%), set by the EN 16351 [12] standard, was met [21]. Morin-Bernard et al., in their study on glued-laminated timber (glulam) made from yellow birch wood bonded with two-component polyurethane (PUR) adhesive, reported an average wood failure percentage (WFP) of 76% [18]. Meanwhile, Weidman, who investigated CLT properties using hybrid poplar wood and PUR adhesive, found an average WFP of 64.6% [22].

The delamination test poses a significantly greater challenge for CLT compared to dry-state bond shear testing. While most hardwood CLTs meet the bond shear strength requirements of the European standard EN 16351 [11], they often fail the delamination test [12]. In our study, the delamination failure modes were not particularly evident (Figure 7). However, it is common to observe that when EPI adhesives or medium-to-high density hardwoods (such as birch, beech, or oak) are used in CLTs, the individual layers of the CLT samples can become entirely separated from each other across the full extent of the glued surface following the delamination test. Betti et al. argue that the delamination test, as per EN 16351 [11], poses a significant challenge even for coniferous CLTs [19]. They suggest that it would be more practical to substitute this test with a shear test, which is a simpler and more reliable method [19]. Furthermore, Betti et al. propose that in certain instances, the bond shear test should be conducted on samples post their exposure to the delamination test (wetting and drying process) [19]. This approach, they believe, provides a more accurate and objective assessment of the durability and adhesive strength.

In general, 1C PUR adhesives designed for load-bearing glued wooden elements are intended for bonding softwoods [23] because softwoods have a more homogeneous structure and grow faster than hardwoods [24]. The structure of softwood, due to tracheids, which are considered more homogeneous cells than those in hardwoods, is more suitable for uniform adhesive penetration [25]. For this reason, 1C PUR adhesives are used for bonding softwoods without special property adjustments (viscosity, additives), and there is no need to modify the wood surface. On the other hand, hardwoods generally have higher density and low adhesion penetration, leading to lower bond shear strength and especially low resistance to delamination [26]. Additionally, the presence of sapwood and extractives in hardwoods causes problems during bonding [27]. Therefore, surface pretreatment and the application of a primer are recommended for hardwoods to improve the strength of the bonded joint [24].

While hardwoods enhance the mechanical properties of CLT, the quality of bonding is frequently a matter of concern. The production of CLTs from higher density hardwoods (e.g., beech or oak) necessitates the application of higher pressures due to the lower porosity and thicker cell walls of the wood, resulting in less effective adhesive penetration into the wood structure [14,18]. The principle of delamination (vacuum-pressure-drying) causes significant swelling of the wood, which is more pronounced in the tangential and radial directions than in the longitudinal direction. This leads to substantial internal shear stresses in the bonded surfaces. Therefore, hardwoods exhibit a higher rate of swelling and drying, which induces greater stresses in the glued joint during the delamination test, leading to an undesirably high level of delamination [15]. Additionally, if the individual lamellae in CLT are not edge-bonded, small gaps can be created between adjacent lamellae in each layer [21], which could compromise the integrity of the CLT. However, when using hardwoods with a higher density for the production of CLT, their total costs of production and weight increase, which needs to be taken into account during the handling and transport of panels, as well as in the design of the load-bearing structure of a building.

3.2. Bond Shear Strength

The statistical assessment of bond shear strength is depicted in

Table 4. Evaluation of bond shear strength in CLT using analysis of variance (ANOVA).

Effect	Sum of Squares	Degree of Freedom	Variance	Fischer’ F-Test	p-Value
Intercept	557.2443	1	557.2443	5362.9983	0.00000 **
Wood species	37.6666	2	18.8333	181.2543	0.00000 **
Error	5.9226	57	0.1039

** Statistically significant at p  ≤  0.05.

and Figure 8. The findings indicate that the influence of individual wood species used in CLT production is statistically significant (with a p-value < 0.05).

The bond shear strength of spruce CLT was the lowest, registering at 1.9 N/mm². In contrast, the higher density of hardwood CLTs had a beneficial impact. Specifically, aspen CLT demonstrated a bond shear strength that was 73.7% greater than that of the reference spruce CLT. The highest average bond shear strength was observed in birch CLTs, which was 105.3% higher than that of spruce CLTs. Despite the notable differences in mean values, all CLTs satisfied the minimum required value of 1 N/mm² as stipulated in Annex D of EN 16351 [11], which can be confirmed by the average values presented in

Table 5. Average values of the bonding characteristics of the CLT.

Wood Species of CLT	Bonding Characteristics
	Maximum Delamination Delammax (%)	Total Delamination Delamtot (%)	Wood Failure Percentage WFP (%)	Bond Shear Strength fv (N/mm2)
Spruce	25.63 (10.25)	7.61 (3.73)	86.9 (7.45)	1.9 (13.31)
Aspen	26.48 (12.06)	12.69 (3.31)	83.7 (13.99)	3.3 (8.97)
Birch	33.17 (6.24)	13.21 (8.10)	76.9 (15.66)	3.9 (17.49)

The values in parentheses are the coefficients of variation (COV) in %.

.

Li et al., who tested the bond performance of CLT made from Japanese larch (Larix kaempferi (Lamb.) Carrière) wood bonded with PUR adhesive, found similar bond shear strength values (1.98 N/mm²) as in our research for spruce CLT [3]. Jeitler and Augustin reported an average bond shear strength of 4.5 N/mm² for birch CLT, a value that closely aligns with our findings [28]. Morin-Bernard et al. discovered that glulam made from yellow birch wood and bonded with 2C PUR adhesive achieved an average bond shear strength of 17.4 N/mm² [18]. Engelhardt et al., who examined the bonding capability of silver birch wood using PUR adhesive, recorded an average tensile shear strength of 11.3 N/mm² [29]. Weidman reported an average bond shear strength of 3.7 N/mm² for CLT constructed from hybrid poplar wood and glued with PUR adhesive [22]. Kramer et al. conducted tests on CLT made from hybrid poplar wood and discovered that the average bond shear strength was 3.1 N/mm² [30]. Numerous studies have confirmed the suitability of hardwoods as the central layer in CLT construction, primarily due to their positive impact on bond shear strength. For instance, Li and Ren explored various compositions of 3-layer CLT (with the middle layer composed of OSB, larch, and poplar) and found that the bond shear strength values were highest when poplar wood was used for the middle layer [31].

In our study, the bond shear failure modes were very similar for birch and aspen CLT (Figure 9). The interlayer shear strength and failure mode of CLT vary depending on the cross-layers used. The interlayer shear strength in CLT is mainly influenced by the bending strength of the cross-laminated lamellae, and the failure mode is the same as that of the lamellae.

Hardwoods are suitable for these cross-layers of CLT, which is confirmed by several studies examining the use of birch, beech, or poplar wood for the inner layers of mixed CLT. For example, Gong et al. found that aspen and birch wood are suitable as cross-layers in CLT because they enhance its planar shear performance [32]. Additionally, aspen, being a lower-density timber, exhibits a smaller contact angle and superior wettability. In general, these properties accelerate the absorption of the adhesive, which in most cases helps to increase the bond shear strength [33].

Bond shear strength in hardwoods is not as significantly affected as resistance to delamination and often meets the minimum strength requirements of the bonded joint. Konnerth et al. examined the bond quality of various hardwoods bonded with 1C PUR adhesive, finding satisfactory tensile–shear strength values for ash, beech, birch, hornbeam, and black locust in dry conditions, but encountered problems in wet conditions, with only oak, beech, and ash meeting the requirements [34]. Additionally, the use of low-grade or low-density hardwoods results in lower bond shear strength values, which can be improved. Modifying the properties of 1C PUR adhesives enhances not only resistance to delamination and wood failure percentage but also bond shear strength. For example, Lima García et al. reported that adding a certain amount of lignin to 1C PUR adhesive improves all these necessary properties when bonding beech wood [35]. Kläusler et al., who tested the effect of N,N-dimethylformamide (DMF) primer on the tensile shear strength of beech wood, found an improvement in the wettability of the wood surface and overall bonding quality [36]. Lu et al. tested the use of N,N-dimethylformamide (DMF) and hydroxymethylated resorcinol (HMR) primers on eucalyptus lamellas (Eucalyptus urophylla × E. grandis) bonded with 1C PUR adhesive to improve the bond performance of CLT [37]. They found that DMF significantly increased bond shear strength in dry conditions, while HMR had an excellent effect on bond shear strength in wet conditions [37].

4. Conclusions

The study assesses the suitability of European aspen (Populus tremula L.) and Silver birch (Betula pendula Roth.) wood for CLT bonded with 1C-PUR adhesive as a possible replacement or alternative to the commonly used CLT made from Norway spruce (Picea abies (L.) H. Karst.) wood. The key findings of this study are outlined below:

• The minimum criteria for maximum and total delamination (Delam_tot ≤ 10%, Delam_max ≤ 40%), as defined by EN 16351 [11], were variably met. 70% of spruce CLTs passed the delamination test (80% meeting the Delam_tot requirements and 95% meeting the Delam_max requirements). However, aspen CLTs only met the minimum delamination criteria in 60% of cases (60% meeting both the Delam_tot and Delam_max requirements), while only 45% of birch CLTs passed the delamination test (65% meeting the Delam_tot requirements and 80% meeting the Delam_max requirements).
• Although aspen and birch CLTs did not meet the minimum delamination requirements, they met the minimum requirement of 70% wood failure percentage (WFP). Spruce CLT reached the highest average WFP.
• The aspen and birch CLTs met the minimum criterion for bond shear strength of 1 N/mm² set by EN 16351 [11]. The bond shear strength values were 73.7% (aspen) and 105.3% (birch) higher than for the spruce CLT.
• In general, it can be stated that hardwoods such as aspen and birch can be used for CLT. However, it is recommended for less demanding conditions of use in terms of humidity fluctuations, or it is necessary to use a modified adhesive for hardwood that ensures higher resistance to delamination.