1. Introduction
The majority of glue-laminated timber (glulam, GLT) in Europe is produced from softwood, mainly Norway spruce (
Picea abies), and the existing standards, gluing systems, and sawmill equipment are set for the manufacturing of such wood types [
1,
2]. The gradual decrease of spruce covered areas in the forests of Europe and the increase in areas covered in hardwood, especially beech (Fagus sylvatica), indicates that the latter will become the local sustainable source of hardwood in Central Europe. Therefore, the properties of hardwood elements need to be sufficiently tested and the test results should contribute to updating the current standards (building code), and therefore, it will be possible to make use of the full potential of hardwood and develop quality hardwood products.
Compared to softwood, hardwood has a higher load-bearing capacity, which can be a significant factor when producing highly stressed timber structures especially for large spans. The excellent mechanical properties of hardwood are also reflected in the inclusion of hardwood in the high-strength classes. On the other hand, hardwood has a relatively high density compared to softwood. Another limitation of hardwood, especially beech wood, is its relatively high shrinkage/swelling coefficient, which may increase the risk of delamination of the glued joints. For that reason, hardwood bonding is no trivial task. Hardwood often shows different behavior when glued [
3,
4,
5] because of its different structure, wood chemistry [
6], and extractive content [
7] compared to softwood. Hardwood also has a higher susceptibility to swelling and shrinkage (especially beech wood) [
1],and a higher bearing capacity than softwood, and therefore, the loads the glue needs to transfer are very high [
2,
8]. Due to its low natural durability, the use of beech glulam is restricted to service class 1 [
9] as defined in EN 1995-1-1 [
10].
There is no building standard specified for hardwood glue-laminated timber in Europe and certainly not for GLT comprised of timber obtained from different wood species. It is possible to partially follow the standard regulations for softwood [
11,
12], although some production requirements may not be suitable for hardwood applications [
13]. It is also possible to follow some national regulations (particularly [
14]). It is possible to classify hardwood based on the standards EN 14081-1 [
15] and EN 1912 [
16] or DIN 4074-5 [
17]. The production of hardwood glue-laminated elements is possible, but in order to do so, it is necessary to have the approval of a certificate issuing institution. In order to obtain approval, it is possible to classify GLT into strength classes according to EN 338 [
18] using the characteristic values determined according to EN 384 [
19].
The aim of this research was to show the potential of timber obtained from wood species that are locally available. Thanks to extensive research, timber from these wood species could become a new source of material for the production of GLT and other related products [
1]. Beech wood appears to be the most suitable among the local wood species for the use in load-bearing elements because of the expected increase in beech-covered areas, its straight grain with a low occurrence of knots, and its comparable mechanical properties to a more expensive oak [
20]. The disadvantages of beech wood e.g. its relatively high shrinkage/swelling can be reduced by using it in glue laminated and cross-laminated elements that are used in environments with low humidity fluctuations. The great potential of its use for structural purposes is described in the literature [
21] which deals with combined beech glulam beams, where the cross-section consists of beech wood of different strength classes.
Due to the relatively high density of beech wood, the use of homogeneous beech elements brings certain drawbacks, e.g., worse handling in construction sites, more expensive transport, etc. In order to achieve a reduction in weight, another type of wood was used in order to reduce the weight of the element. Reducing the weight of hardwood elements is achieved by replacing some of its lamellae with softwood lamellae. The above-mentioned method is described in the literature, e.g., in [
8,
22,
23].
In [
22] Blaß and Frese examined the behavior of 10-layer GLT members with a cross-section depth of 300 mm made from a combination of hardwood and softwood. When beech timber was used in the top and bottom zone (i.e., the outer layers), it was found that the combined elements only showed a slightly lower bending strength than beams made entirely of beech. The 10-layer GLT beams could be classified into classes GL28 up to GL48 according to EN 14080 [
11].
Muraleedharan and Reiterer [
8] examined the bending strength and modulus of elasticity of five-lamellae glulam beams made of oak and spruce. Different lamellae layups in the cross-section were examined. The cross-section of the tested beams was 120 × 135~200 mm. The experiments were complemented with FEM analysis. The results showed that the performance of glue-laminated timber can be increased by a combination of hardwood (oak) and softwood (spruce) lamellae compared to homogeneous softwood.
Sciomenta et al. [
23] performed an experimental investigation on eight-layer homogeneous (beech only) and hybrid (beech–Corsican pine) glulam beams with a cross-section depth of 144 mm. The used lamellae had no finger joints and were 18 mm-thick. The experiments were complemented with numerical simulations and the results showed the high mechanical performance of the examined beams, both the homogeneous beams and the hybrid beams. The homogeneous beams reached a 7% higher maximum force than the hybrid ones and a bending stress 8.3% higher than the hybrid ones.
The price of spruce wood has been increasing, mainly due to the bark beetle calamity. Therefore, there is an ongoing effort to decrease the weight of the elements using other species than spruce wood and to use wood that is not sought after and whose price does not increase. A hybrid beam was designed using soft, low-density poplar timber in the inner zone of the beam. Poplar wood has similar properties to spruce; it has low strength parameters. The good availability of poplar wood in Central Europe (including its fast growth), low density, easy workability, and easy gluing [
24] were key factors for the choice. Its shrinkage/swelling coefficient is lower than beech wood, but higher than softwood [
6,
25]. It is assumed that the effect of a different shrinkage/swelling coefficient for beech wood compared to poplar wood can be partially eliminated using hybrid beams in service class 1. The effect of humidity on the behavior of hybrid beams will be analyzed in further research.
The use of poplar wood in load-bearing elements is mentioned in the literature, especially in the field of research on cross-laminated timber (CLT), where poplar wood is either used separately [
26,
27,
28] or combined with pine, fir [
29], beech [
30]. Apart from the above-mentioned studies, there are several others, esp. Timbolmas et al. [
31] in which hybrid pine–poplar glulam beams were subjected to bending. Pine was used for the outer layers was used poplar for the inner layers of the six-lamellae beams. It was discovered that the composite layups made of pine and poplar showed high stiffness, closer to that of the beams made from pine lamellae and much higher stiffness than the poplar glulam beams. In addition, the density of the pine–poplar specimens underwent a 17% increase with respect to the poplar specimens. In another study [
32], poplar was combined with eucalyptus in seven-layer glue-laminated beams, and they demonstrated good performance and structural efficiency.
Based on our literature research, it is assumed that:
A glued element from a combination of several wood species can be functional;
It is possible to combine beech and poplar wood in one element;
Decreasing the weight of an element in the inner zone should not have a major effect on its bearing capacity.
In addition to verifying these assumptions, the aim of this paper is to verify whether the gluing of beech and hybrid elements using PUR adhesive is suitable since the researchers used different adhesives in most of the above-described studies (e.g., [
2,
3,
13,
22,
23,
32]). In study [
7], PUR adhesive was used for gluing hardwood glulam members that were thermally loaded and that then underwent bending tests. In addition, gluing beech and poplar wood with PUR adhesive has not yet been described in the literature.
4. Discussion
The results of the tests of glue-laminated beams show that it is possible to successfully combine different types of hardwood in a single GLT element. The homogeneous and hybrid beams showed high-strength characteristics. The beech lamellae had an 80% higher bending strength than the poplar lamellae. However, the use of poplar lamellae in the inner zone of the beam has apparently no effect on the bending strength and modulus of elasticity of the GLT beam. Both monitored values were comparable for both types of GLT beams (homogeneous and hybrid). Based on the experimental results, the density of hybrid beech–poplar beams in comparison to homogeneous beech GLT beams was approx. one-fifth smaller.
As shown in
Figure 7, the course of the
F–
w relationship for homogeneous and hybrid elements was very similar up to the value of half of
Fmax. The difference occurred after further loading, beyond the value of half
Fmax when the hybrid GLT beams showed higher deformation than the homogeneous GLT beams.
According to the experimental test results (
Table 1) and the statistical analysis (
Table 5), it can be stated that there is no decrease in the bending strength of the hybrid beech-poplar beams compared to the homogeneous beech beams. The failure of the homogeneous GLT beech beams was (after initial pressure from the supports in compression perpendicular to the grain, which is common for bending tests) observed especially in the tensile zone, while the failure of the hybrid GLT beech-poplar samples was more often observed in the shear, as shown in
Figure 9. From the point of view of the bending strength of the GLT samples, the behavior of most hybrid beams was affected by the shear failure of the poplar zone, while the behavior of homogeneous GLT beech beams was in all cases affected by the tensile failure of the tensile beech zones. In addition, the variability in the bending strength of the lamellae was lower for poplar than for beech, while the variability in the modulus of elasticity as comparable for both the beech and poplar lamellae. The lower variability in the results for the poplar lamellae is reflected in the variability of the results of the GLT beams.
It can be assumed that the lower variability in the bending strength of the hybrid beams is due to the influence of the lower variability in the poplar shear strength. A relatively small number of samples affected the variability of the results of the GLT beams to a certain extent. However, the obtained values from the tested samples can be, in general terms, considered low values of variability. From the point of view of the results, low variability in the hybrid beams is very beneficial.
The beech homogeneous GLT beams had a 22% lower bending strength in comparison to the tested beech lamellae (
Table 2). This difference may be caused by the fact that the lamellae tests were performed with samples without knots and other imperfections (i.e., with ideal bodies), the compactness of the glued joints, or differences in the testing bodies’ dimensions (the GLT cross-section is six times higher than the cross-sections of the lamellae). According to [
42,
43], the strength of the elements decreases with the growing dimensions (the so-called size effect).
From the point of view of the modulus of elasticity, the results of the experimental tests (
Table 3) and the statistical analysis (
Table 6) show that there was no decrease in the modulus of elasticity of the hybrid beech–poplar beams compared to the homogeneous beech GLT beams. It can therefore be concluded that the low modulus of elasticity of poplar is barely reflected in the modulus of elasticity of the entire hybrid GLT beam. Most of the load is carried by the outer beech lamellae (the strongest member).
The beams showed high values of strength characteristics, but due to the small number of samples, they could not be classified into strength classes with certainty. The strength classes and appropriate characteristic properties for the glue-laminated timber are defined in EN 14080 [
11], which lists the values for homogeneous softwood. The values for hybrid glulam beams are listed in a regulation [
14]. In addition, the classification according to the characteristic values found could not be completed accurately because the standards [
11,
14] state the maximum class GL32h and GL48hyb, respectively. However, the determined values of the tested glue-laminated beams were significantly higher than the specifications for these classes in the standards [
11,
14].
The works of Blaß and Frese [
22], Muraleedharan and Reiterer [
8], and Sciomenta et al. [
23] used different types of wood. Primarily, the aim was to analyze the behavior of hybrid beams combining wood with a lower bending strength and wood with a higher bending strength. In all cases, the hybrid beams were compared to homogeneous beams comprised a lamellae wood type with higher bending strength. These results showed a lower bending strength for the hybrid elements in comparison to the homogeneous elements. In some cases, a comparison of the behavior of the hybrid beams to the behavior of the homogenous beams comprised the lamellae wood type with a lower bending strength was carried out. The results of the previously mentioned works showed that a decrease in the load-bearing capacity was present, independent of the type of wood and the cross-section dimensions used.
The reduction in the strength depends on the percentage of weight reduction and the type of wood used. The best weight decrease to strength reduction ratio was recorded by Blaß and Frese [
22], where a 60% replacement of beech wood by spruce wood (from the cross-section area) meant just a 3% reduction of its bearing capacity, and a 67% replacement meant a 5% reduction of the beam bearing capacity. Regarding the replacement of a pine element with 67% poplar wood, Timbolmas [
31] found a 14% reduction of the bearing capacity of the GLT element. Sciomenta et al. [
23] reduced the weight of a beech element by replacing 50% of the lamellae with pine lamellae and recorded a 7% lower bending strength.
In contrast to the above-mentioned studies, in our research, only 33% of the beech lamellae were replaced and a 16% weight reduction was achieved. It was observed that the achieved weight reduction had no statistically significant effect on the bending strength and the modulus of elasticity of the GLT elements. It can be stated that the percentage of lamellae replacement was relatively low compared to the lamellae replacement of the GLT elements in the above-mentioned studies. It is clear that the lower the percentage of lamellae replacement, the lower its effect on the mechanical properties of the GLT element.
It is not possible to perform a comparison with the findings of study [
8] since the mentioned study compares oak–spruce hybrid GLT beams with homogeneous spruce beams. In our research, a comparison of hybrid beech–poplar beams with homogeneous poplar beams was not carried out.
A glued element from a combination of several wood species can be functional. As stated above, it is possible to combine beech and poplar wood in one element; PUR adhesive can be used for gluing homogenous beech and hybrid beech–poplar elements. When using poplar wood, which has a comparable bending strength and a lower modulus of elasticity than spruce, 33% lamellae replacement in the inner zone had no effect on the bearing capacity of the member. This proves that the initial assumptions were correct.
5. Conclusions
This paper describes the results based on experimental tests performed on homogenous beech and hybrid beech–poplar glue-laminated beams made from local wood and glued with PUR adhesive.
Based on the results of this work, it is possible to conclude that:
- (a)
Both of the tested homogenous beech (BE (h)) and hybrid beech–poplar (BE-PO (hyb)) beams showed high values of strength characteristics. The characteristic values were determined according to the standard [
19] although the number of tested elements was lower than the standard requires. The determined characteristic values for the hybrid GLT beams were even higher than those specified by standard [
14] for class GL48hyb;
- (b)
The tested hybrid beech–poplar beams (BE-PO (hyb)) were approx. 16% lighter than the homogeneous beech beams (BE (h));
- (c)
The bending strength of the tested hybrid beech–poplar beams (BE-PO (hyb)) was comparable to the bending strength of the homogeneous beech beams (BE (h));
- (d)
The modulus of elasticity of the tested hybrid beech–poplar beams (BE-PO (hyb)) was comparable to the modulus of elasticity of the homogeneous beech beams (BE (h));
- (e)
Upon reaching approx. half of the maximum load, the homogeneous (BE (h)) and the hybrid (BE-PO (hyb)) beams behaved very similarly.
In the future, this research is to be extended to other aspects; particularly, it is necessary to examine:
The possibility of increasing the percentage of beam weight reduction;
The effect of lamellae layup on the bending strength of GLT beams;
The cohesion of glued joints, particularly the beech–poplar joint;
The effect of moisture fluctuation on the bond line.
Based on the obtained data, the current timber elements can be very successfully replaced with hardwood glulam elements. The production of glued beams from wood species other than the hitherto used wood species offers new possibilities for their use. Based on the higher load-bearing capacity of hardwood glulam elements, the cross-section depth can be reduced compared to spruce GLT beams. The reduction in the cross-section depth could be suitable for building renovations. In addition, in architectural and structural applications, beams with greater span to cross-section depth ratios are preferred by designers since they provide more open space with fewer obstacles.