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Article

The Effect of Different Densification Levels on the Mechanical Properties of Southern Yellow Pine

by
Suman Pradhan
,
Aadarsha Lamichhane
,
Dalila Belaidi
and
Mostafa Mohammadabadi
*
Department of Sustainable Bioproducts, Forest and Wildlife Research Center (FWRC), College of Forest Resources (CFR), Mississippi State University, Starkville, MS 39762, USA
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(15), 6662; https://doi.org/10.3390/su16156662
Submission received: 30 May 2024 / Revised: 25 June 2024 / Accepted: 1 August 2024 / Published: 4 August 2024
(This article belongs to the Section Sustainable Engineering and Science)
Browse Figures
Versions Notes

Abstract

:
Plantations, typically involving the cultivation of fast-growing trees like southern yellow pine, offer avenues to enhance sustainability and manage limited resources more effectively. However, fast-growing trees suffer from low mechanical properties due to less dense wood. Densification and the development of engineered wood products represent approaches to developing high-performance products from fast-growing tree species. In this study, the correlation between the densification levels and mechanical properties of a fast-growing species, loblolly pine (Pinus taeda L.), was established to improve resource utilization. Wood specimens were densified at three compression ratios: 16.67%, 33.33%, and 50.00%. The impact of densification levels on bending strength, bending stiffness, shear strength, and hardness was studied. The findings highlighted the positive impact of densification on structural integrity, as bending stiffness consistently improved, eventually reaching a 42% enhancement at a compression ratio of 50.00%. However, bending strength showed an initial increasing trend but reached a plateau at higher densification levels. Densification levels showed minimal changes in shear strength parallel to the grain. Notably, densification significantly enhanced hardness properties, particularly on the tangential surface, where a fourfold increase was observed at a 50% compression ratio. Overall, these findings reveal the relation between the compression ratio and the mechanical properties of lumber and are beneficial for utilizing lower-quality wood species in construction and engineering applications.

1. Introduction

The current rapid growth rate of the world’s population, which is projected to exceed 11 billion by 2100, has raised global concern [1]. The population increases the demand for construction activities and amplifies environmental challenges stemming from the extraction and utilization of construction materials. According to the United Nations Environment Programme (UNEP) [2], the buildings sector—which encompasses energy used for construction, heating, cooling, lighting, and operating appliances and equipment—accounts for approximately 39% of energy-related CO2 emissions and over 36% of global energy demand. Over the next 40 years, there will be a huge surge in construction worldwide, adding an astonishing 2.6 trillion square feet of new space, which equates to constructing the whole of New York City every month for four decades [3]. With the advancement of construction technology, concrete and steel have emerged as dominant construction materials worldwide, as evidenced by prevailing architectural trends [4,5]. However, both cement and steel industries play a significant role in contributing to global carbon emissions, with cement being responsible for 5–8% and steel contributing 7–9% of greenhouse gas emissions, underscoring notable environmental concerns [6,7]. In contrast, wood and its derivatives are recognized as carbon-neutral or carbon-negative materials owing to their ability to sequester carbon during growth; stored carbon makes up about 50% of the dry weight of wood [8]. Approximately one cubic meter of wood can sequester around one metric ton of carbon dioxide through photosynthesis [9]. In addition, wood possesses numerous benefits as being a natural and renewable construction material, paving the way for sustainable construction solutions to meet the needs of a growing population in the future.
In the current scenario, the development of engineered wood products (EWPs) like Glulam and CLT (Cross-Laminated Timber) has enabled the wood industry to compete with steel and concrete in medium-to-high-rise construction projects [10]. Currently, Ascent, a 25-story timber–concrete hybrid building in Milwaukee, WI, is the world’s tallest mass timber building, which was possible due to the development of high-performance engineered wood products [11]. However, the number of large-diameter trees with higher yields is being rapidly depleted across different regions worldwide, which poses a significant threat to ecosystem stability, biodiversity, and sustainability [12]. Due to the limited availability of high-quality solid wood resources worldwide, there has been a push to create valuable products using rapidly growing wood instead [13]. Bendtsen and Senft observed that despite the accelerated growth in plantations with short rotations, the subsequent young trees frequently possess a considerable proportion of juvenile wood, leading to adverse effects on end-product performance [14]. Several ongoing studies aim to improve the mechanical properties of wood products through the reinforcement of engineered wood using materials such as chemically synthesized fibers (including glass, carbon, aramid, and basalt) [15], natural fiber-reinforced polymer composites (NFRP), or hybrid fibers, which combine synthetic and natural fibers [16,17,18]. However, the prevalent use of these reinforcing materials, which emit carbon and are environmentally unfriendly, presents a significant obstacle to sustainability initiatives and carbon emission reduction. Densified timber offers a more sustainable alternative by improving mechanical properties without additional chemicals and reducing environmental and health impacts [19].
Densification is a process aimed at enhancing timber density, which has a positive effect on the mechanical properties of timber, such as its modulus of rupture (MOR), modulus of elasticity (MOE), stiffness, and hardness [19,20,21]. Wood can be densified through mechanical methods like thermo-hydro-mechanical densification, viscoelastic thermal compression, and surface densification, as well as chemical processes such as acetic anhydride, phenol formaldehyde, and melamine formaldehyde [19,22,23,24]. Huang et al. improved the tensile strength of basswood through a process involving a three-step method, which includes partially removing hemicellulose and lignin using a NaOH and Na2SO3 solution, followed by a high-pressure steam treatment and then hot-pressing for densification resulting in tensile strength of approximately 420 MPa, significantly higher than natural basswood [25]. In a study by Pertuzzatti et al., compressing pine (Pinus elliottii) by 40% significantly enhanced its mechanical properties, with the modulus of elasticity increasing by 85% and the modulus of rupture by 294% [26]. Pradhan et al. [27] experimented to optimize lumber densification to mitigate rolling shear failure in CLT, demonstrating a 48% increase in rolling shear strength. Several studies have also been carried out to investigate the effect of densification on termites, fungi, decay, and the fire resistance of densified wood. Prolonged heat treatment at higher temperatures, integral to the densification process, increases the resistance of sugi sapwood against white rot [28]. In several wood species, heat treatment significantly improved resistance to all types of fungi, especially enhancing resistance to brown rot fungi, and increased resistance to white rot and soft rot fungi [29]. Thermo-mechanically densified wood pretreated with fire retardant resulted in enhanced ignition resistance and hardness, while maintaining minimal effects on moisture stability [30]. A key environmental concept for achieving a more sustainable future is the circular economy, which aims to maximize resource use, minimize waste and pollution, and improve material recovery [19]. Due to its manufacturing process, densified timber is highly compatible with this model. Its capacity to be reused and repurposed supports a cradle-to-cradle approach to sustainability, in contrast to the linear or cradle-to-grave models.
Utilizing low-grade wood is crucial for sustainability, reducing the reliance on high-grade timber and large-diameter trees. Densification has shown promising results in improving lumber’s mechanical properties and durability. However, densification processes involve multiple steps—from softening to hot-pressing—leading to significant energy consumption and financial costs. Understanding the optimal densification level for enhancing lumber stiffness and strength can provide valuable insights into energy efficiency and sustainability. This study aims to address this by examining bending, shear, and hardness properties across different densification levels.

2. Materials and Methods

2.1. Materials

Flat-sawn dimensional lumber of the loblolly pine species (Pinus taeda L.) with average dimensions of 38.1 mm in thickness, 139.7 mm in width, and 3.65 m in length, commonly referred to as ‘2 by 6-dimensional lumber’, was utilized for the study. To have a better understanding and reduce variability, each dimensional lumber was processed to obtain specimens for every densification level and each test. Six pieces of dimensional lumber were used, and from each piece, samples were prepared for three different compression ratios and one control. This ensured consistency across groups, as each group, taking into account the compression ratio and the experiment, possessed a specimen cut from each lumber with a distinct grain pattern and initial density.

2.2. Densification Process

In the study, the densification process was guided by employing a specified compression ratio, calculated as shown in Equation (1).
C o m p r e s s i o n   R a t i o = t i n i t i a l − t f i n a l t i n i t i a l
where, t i n i t i a l = the initial thickness of non-densified lumber, and t f i n a l = the final thickness of lumber after densification.
Densification was performed on the tangential face of the lumber to achieve higher compression levels without causing any damage using the thermo-hydro-mechanical densification technique. Three distinct compression ratios were used for analysis: 16.67%, 33.33%, and 50% referred to as D1, D2, and D3, respectively, in this study. To prevent the possibility of structural damage during compression, the lumber underwent a softening procedure before thermal densification. This was achieved by immersing the specimens in boiling water for a duration of 10 min. The subsequent densification step involved hot pressing the lumber using a Clifton hydraulic press (Clifton, NJ, USA) to a target thickness corresponding to the compression ratio within a 5 min period at a temperature of 140 °C. Then, the heat source was turned off, and the hot press was allowed to cool down until it reached a temperature of below 70 °C, at which point the densified specimen was removed to avoid springing back [31]. After densification, all the specimens were kept in ambient conditions for two weeks and were tested afterwards.
Taking into account the experimental testing, different initial and final thicknesses were aimed for during the densification process. For the shear test and hardness test, densification was performed on specimens with the same initial thickness of dimensional lumber, resulting in different final thicknesses depending on compression ratios. In contrast, for the bending test, densification was carried out on specimens with different initial thicknesses depending on compression ratios, resulting in densified specimens with the same final thickness. It enables the preparation of bending specimens with the same span length/depth ratio which has a significant effect on bending behavior. The average density, as well as initial and final thicknesses, of all non-densified (control) and densified groups are given in Table 1.

2.3. Experimental Methods

2.3.1. Bending Test

A flatwise static bending test was conducted to evaluate the effect of compression ratios on bending behavior following code ASTM D198 [32]. A four-point bending test (third point) was conducted with the load applied at two points positioned equidistant, one-third of the span (L/3), from the support reactions. The bending specimens, across all groups, had a width (b) of 66 mm, a length of 558 mm, and a thickness (t) close to 12.7 mm.
Drawing conclusions from the bending results of densified specimens with different thicknesses may not offer meaningful insights due to dimensional variations that could influence overall bending behavior. Consequently, a consistent thickness and span length/depth ratio were employed for bending specimens across all groups. In order to achieve a consistent thickness of 12.70 mm for all densified bending specimens, as shown in Figure 1, the initial thicknesses of 15.24 mm, 19.05 mm, and 25.4 mm corresponding to compression ratios of 16.67%, 33.33%, and 50%, respectively, were computed using Equation (1). Likewise, in the case of the non-densified control samples, they were planned to have an equal thickness (t) of 12.70 mm. A consistent span length (L) of 457.2 mm was maintained across all specimens, resulting in a span-to-depth ratio of 36. For each group (i.e., D1, D2, D3, and control), 12 specimens were subjected to a bending test, making a total of 48 bending specimens.
The bending test utilized displacement-controlled loading, applying a consistent rate to reach the maximum loading time of around 5 min. The bending test was performed using the Tinius Olsen Satec Universal Mechanical Testing Machine (Tinius Olsen Testing Machine Company Inc., Horsham, PA, USA). The bending deflection (Δ) was determined using an LVDT placed at the bottom center of the sample. Geometric parameters and recorded maximum load (Pmax) at failure were used to compute the mechanical properties of each specimen, namely the modulus of rupture (MOR) and the modulus of elasticity (MOE) from Equations (2) and (3), respectively.
M O R = P m a x × L b × t 2
M O E = 23 × P × L 3 108 × b × t 3 × Δ
Note that Pmax represents the maximum bending load (in Newtons) at failure, L denotes the span length (in millimeters), b indicates the width of the panel (in millimeters), t stands for the thickness of the sample (in millimeters), and Δ signifies the mid-span deflection at the moment of bending load P. P/Δ is the slope of the load–defection curve in the linear region.

2.3.2. Shear Block Test

A shear test parallel to the grain was conducted to evaluate the effect of different densification levels on the shear behavior. For this test, the initial thickness (t1) of all densified and non-densified (control) specimens was equal to 38.1 mm. However, they had different final thicknesses (t2): 31.74 mm, 25.40 mm, and 19.05 mm for specimens compressed at 16.67%, 33.33%, and 50% ratios, respectively. Following ASTM D143 [33], the shear test specimens were prepared by laminating each specimen between two outer layers with different thicknesses to reach the final dimensions of 50.8 mm × 50.8 mm × 63.5 mm (2 in × 2 in × 2½ in), as shown in Figure 2a. The two outer layers were bonded using a one-component polyurethane adhesive. A primer solution consisting of 5% Loctite PR 3105 PURBOND primer (Henkel) diluted in water was applied at a rate of 20 g per square meter. Subsequently, the adhesive, Loctite HB X202 PURBOND (Henkel), was applied at 180 g per square meter. A notch of 19.05 mm was made on the final sample so that the plane of shear was at the mid-layer of the densified specimens. This ensured that the specimens were thick enough and the plane of shear fell within the specimen itself. The shear plane of each specimen was precisely aligned to accurately test the shear strength, as illustrated in the accompanying Figure 2. For each group of densified and control specimens, 12 specimens were tested, making a total of 48 samples.

2.3.3. Hardness Test

The hardness test was conducted on specimens measuring 50.8 mm (2 inches) in width and 152.4 mm (6 inches) in length. Similar to the shear block test, densification was carried out on lumber with an initial thickness of 38.1 mm. Depending on the compression ratio, the post-densification thickness varied, resulting in final thicknesses of 31.74 mm, 25.40 mm, and 19.05 mm for compression ratios of 16.67%, 33.33%, and 50%, respectively. Each group, densified and controlled, consisted of 6 specimens, resulting in a total of 24 specimens as shown in Figure 3. The test procedure complied with ASTM D143, utilizing an 11.3 mm (0.444 inches) diameter ball, which resulted in a total projected area of 1 cm2. The load was applied at a rate of 6 mm/min, and the load was recorded until the ball penetrated halfway into the specimen, corresponding to half of its diameter. For each specimen, two penetrations were made on the tangential surface, with two on the radial surface, and one on each end.

3. Results and Discussion

Table 2 summarizes the results from all three test methods, i.e., bending, shear block, and hardness, for all groups. Detailed analyses are provided in the corresponding subsections, supported by graphical representations.

3.1. Bending Properties

The failure mode for all the groups was observed to be simple tension, with an initial small crack developing at the bottom mid-layer of the specimen, which propagated to cause an ultimate failure, as shown in Figure 4.
The bending properties improved as the compression ratio increased as shown in Figure 5. Group D1, which was densified at a 16.67% compression ratio, exhibited an 11% increase in the MOE compared to that of the control group. This increase was even more pronounced in group D2, with a 33.33% compression ratio, showing a 29% higher MOE than the control. Group D3, with the highest compression ratio of 50%, demonstrated the greatest improvement in bending properties, particularly in the MOE, which was 42% higher than the control, 28% higher than D1, and 10% higher than D2. These results indicate a positive correlation between the compression ratio and the MOE.
The MOR showed a different trend. Group D1, with 16.67% densification, showed a 30% increase in the MOR compared to that of the control group, which represents a more substantial increase than that observed for the MOE. At a 33.33% compression ratio, the MOR increased by 46% compared to the control. Specimens densified at a 50% compression ratio showed a slight decline of 2% in the MOR compared to specimens densified at a 33.33% compression ratio. The slight decrease at 50% densification can be related to inherent variations in the wood as a natural product or damage caused in the wood cell wall at such a high compression ratio. Densifying specimens at a compression ratio of more than 50%, followed by SEM analysis, is required to verify this.
As wood is densified to a 50% compression ratio, the wood fibers become more compacted, resulting in increased stiffness. However, the MOR reached a plateau at a compression ratio of around 33%, and further densification did not result in observable increases. These findings enable us to efficiently modify the densification process for specific improvements in bending properties, whether it is the MOR or the MOE.

3.2. Shear Strength Parallel to the Grain

In the case of the shear block test, the failure planes for all samples were within the layer itself, as illustrated in Figure 6. In the control group and group D1, the shear crack was small and did not propagate through the plane. However, for groups D2 and D3, a more defined crack was observed, which propagated through the failure plane. Such failure shows brittle behavior for densified wood at high compression ratios of 33.33% and 50%.
The shear strength parallel to the grain increased with densification which can be observed in Table 2, but the increase was relatively modest. Sample group D2 showed a 12% increase compared to the control, while D3 exhibited an 18% increase. For group D1, the shear strength remained almost unchanged. This limited increase is attributed to the high stress concentration at the growth ring boundaries [34], which is the dominant factor in shear failure. This suggests that densification has minimal effect on altering the behavior of this boundary layer.

3.3. Hardness Property

In this study, densification was performed on the tangential surface. The hardness of the tangential surface showed a significantly higher increase compared to the radial and longitudinal (edge) surfaces, graphically represented in Figure 7. The hardness of the tangential surface increased by 54%, 103%, and 223% for samples D1, D2, and D3, respectively, compared to the hardness of the non-densified control specimens. Similarly, the radial surface also experienced an increase in hardness, but to a lesser extent, with increases of 24%, 80%, and 108% for samples D1, D2, and D3, respectively, compared to that of the control specimens. The longitudinal (edge) surface showed the smallest increase in hardness, with values of 9%, 44%, and 68% for samples D1, D2, and D3, respectively, compared to the non-densified control specimens. The 9% increase, though modest, underscores the tangible impact of the densification process, highlighting measurable hardness improvements even at lower levels of densification.
The increase in hardness within each surface can be readily justified by linking it to the enhancement in density, as hardness has been correlated with density [35]. However, the question arises regarding variation in the degree of increase in hardness across different surfaces. The findings of the vertical density profile have shown that surfaces attached to the hot platens during surface densification exhibit higher density [36]. In this study, those surfaces are referred to as tangential surfaces, which had a higher hardness compared to other surfaces. Because both the radial and edge surfaces share the same mid-plane where the hardness test was conducted, their hardness values are very close, owing to their similar density.

4. Conclusions

Densification has demonstrated potential for enhancing the mechanical properties of lumber. This study explored the relationship between material properties and densification levels using three different test methods: bending, shear block, and hardness tests. The research focused on loblolly pine, one of the species collectively referred to as southern yellow pine (SYP). Through these tests, this study aimed to provide a comprehensive understanding of how different densification levels impact the performance of lumber, particularly in terms of strength, stiffness, and hardness.
The results from all tests indicated that densification positively impacts the strength, stiffness, and hardness properties of lumber. The MOE consistently increased with all levels of densification, reaching a maximum of 42% higher at a 50% compression ratio. In contrast, the MOR showed a 46% increase at a 33.33% compression ratio but plateaued at higher densification levels. In contrast, shear strength parallel to the grain showed only minimal increases with densification due to stress concentration at the growth ring boundaries. Densification had a more pronounced effect on hardness properties, particularly on the tangential surface, where hardness increased by four times with a 50% compression ratio. The radial surface and edge also experienced increases in hardness due to densification but to a lesser extent compared to the tangential surface.

Author Contributions

Conceptualization, S.P. and M.M.; methodology, S.P. and M.M.; validation, S.P. and M.M.; formal analysis, S.P., M.M., A.L. and D.B.; investigation, S.P., M.M., A.L., and D.B.; resources, M.M.; data curation, S.P. and M.M.; writing—original draft preparation, S.P. and A.L.; writing—review and editing, M.M.; visualization, S.P. and M.M.; supervision, M.M., project administration, M.M.; funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Acknowledgments

This material is based upon work supported by McIntire-Stennis, accession number 7003668, and is a contribution of the Forest and Wildlife Research Center, Mississippi State University.

Conflicts of Interest

The authors declare no conflicts of interest.

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Sustainability 16 06662 g001
Figure 1. Bending specimens with constant thickness (starting from left, compression ratios are 50%, 33.33%, 16.67%, and control).
Figure 1. Bending specimens with constant thickness (starting from left, compression ratios are 50%, 33.33%, 16.67%, and control).
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Figure 2. (a) A schematic view of a laminated specimen depicted in ASM D143; (b) shear specimens having the shear plane at the mid-layer of densified and non-densified specimens (from the left, the compression ratios are 50%, 33.33%, 16.67%, and control).
Figure 2. (a) A schematic view of a laminated specimen depicted in ASM D143; (b) shear specimens having the shear plane at the mid-layer of densified and non-densified specimens (from the left, the compression ratios are 50%, 33.33%, 16.67%, and control).
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Figure 3. Hardness test samples (from left, compression ratios are 50%, 33.33%, 16.67%, and control).
Figure 3. Hardness test samples (from left, compression ratios are 50%, 33.33%, 16.67%, and control).
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Figure 4. Failure mode in bending test.
Figure 4. Failure mode in bending test.
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Figure 5. A graphical representation of the percentage of increment in the MOR and the MOE compared to the control (non-densified) group.
Figure 5. A graphical representation of the percentage of increment in the MOR and the MOE compared to the control (non-densified) group.
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Figure 6. Failure in shear block test of D2 specimen.
Figure 6. Failure in shear block test of D2 specimen.
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Figure 7. Bar graph showing the hardness at different surfaces.
Figure 7. Bar graph showing the hardness at different surfaces.
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Table 1. Density and thickness of samples for bending, shear, and hardness test.
Table 1. Density and thickness of samples for bending, shear, and hardness test.
Group IDBending TestBlock Shear/Hardness
Initial Density
(Kg/m3)		Final Density
(Kg/m3)		Initial Thickness
(mm)		Final Thickness
(mm)		Initial Density
(Kg/m3)		Final Density
(Kg/m3)		Initial Thickness
(mm)		Final Thickness
(mm)
Control663.95 (7%)-12.7012.70647.63 (6%)-38.10-
D1755.85 (6%)15.24746.68 (6%)31.74
D2958.07 (6%)19.05923.80 (6%)25.40
D31280.55 (3%)25.401199.92 (6%)19.05
Note: the percentage in parentheses represents the coefficient of variation (COV).
Table 2. Summary of test results.
Table 2. Summary of test results.
Group IDModulus of Rupture Modulus of Elasticity Hardness (N)Block Shear
(MPa)(MPa)Tangential SurfaceRadial SurfaceEdge End (MPa)
Control103.77 (11.53%)18,990.93 (10.48%)4802.27 (25.66%)4107.39 (13.62%)4813.10 (14.35%)11.65 (8.51%)
D1134.98 (14.8%)21,076.95 (19.22%)7392.97 (17.15%)5103.69 (14.87%)5266.89 (16.79%)11.19 (10.21%)
D2151.37 (12.65%)24,592.10 (10.25%)9739.62 (10.97%)7387.50 (17.32%)6916.52 (15.16%)13.00 (13.39%)
D3148.14 (7.19%)27,060.61 (7.73%)15,491.79 (7.94%)8543.17 (18.82%)8087.30 (8.79%)13.69 (10.82%)
Note: the percentage in parentheses represents the coefficient of variation (COV).
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Pradhan, S.; Lamichhane, A.; Belaidi, D.; Mohammadabadi, M. The Effect of Different Densification Levels on the Mechanical Properties of Southern Yellow Pine. Sustainability 2024, 16, 6662. https://doi.org/10.3390/su16156662

AMA Style

Pradhan S, Lamichhane A, Belaidi D, Mohammadabadi M. The Effect of Different Densification Levels on the Mechanical Properties of Southern Yellow Pine. Sustainability. 2024; 16(15):6662. https://doi.org/10.3390/su16156662

Chicago/Turabian Style

Pradhan, Suman, Aadarsha Lamichhane, Dalila Belaidi, and Mostafa Mohammadabadi. 2024. "The Effect of Different Densification Levels on the Mechanical Properties of Southern Yellow Pine" Sustainability 16, no. 15: 6662. https://doi.org/10.3390/su16156662

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