Determination of Strength Improvements in the Acacia Hybrid Through the Combination of Age Groups at the Air-Dry Conditioning Stage

Abstract

Acacia hybrid is an important plantation species in Malaysia, but its use in structural applications is still limited due to the lack of comprehensive data on its engineering properties. This study evaluated the physical and mechanical properties of laminated or glulam Acacia hybrid timber in an air-dried condition for three age group combinations (7//10, 10//13, and 7//13 years old) to determine the optimal combination for structural applications. The results showed that the 10//13-year-old combination had the best mechanical performance, along with the highest basis density (0.7099 g/cm³), highest modulus of elasticity (MOE) (16,335.6 N/mm²), and highest parallel compressive strength (56.998 N/mm²), while the 7//10-year-old combination showed the highest moisture content (14.94%) and highest perpendicular compressive strength (8.9256 N/mm²). This study demonstrated that the combination of juvenile wood (7 years old) with mature wood (10 or 13 years old) increased strength by up to 43.06%, thus optimising the potential of Acacia hybrid in the construction industry. All combinations meet SG5 standards, with the 10//13-year-old combination recommended as the best choice for high-performance applications of glulam products.

1. Introduction

Timber has become a vital construction material in Sarawak due to its abundant rainforest-based resources. According to Mohd Yusof et al. [1], Acacia mangium is considered one of the major wood species in Malaysian forest plantations. The Acacia hybrid, a cross between Acacia mangium and Acacia auriculiformis, is a fast-growing species widely planted in Malaysia. However, its utilisation is largely confined to pulp and paper, with limited applications in structural components. In the timber industry throughout Malaysia, it is asserted that the development industry is not sufficiently adept at utilising timber and timber items as structural materials. According to Duju [2], metals and concrete have been favoured over wood for structural components because of the homogeneity of metals and concrete with respect to their properties. To make judicious use of the abundant resources provided, some fundamental standards should be determined; for example, an assortment of decisions might be made as to proper use, strength, stability, durability, cost, readiness to provide support, and amenability to the specialised abilities required for applications. According to Milner [3], engineered wood products (EWPs) contain interconnected wood-based components that are utilised as auxiliary structure components. Ramage et al. [4] added that the benefits of using engineered wood manufactured from laminated timbers, adhesives, and other materials include increased dimensional stability, mechanical properties that are more homogeneous, and greater durability. As described in a relevant engineering bulletin [5], the glulam product is applicable in the context of large structural elements, for instance, beams, columns, trusses, bridges, portal frames, posts, and beam structures. Engineered wood products (EWPs) should be harnessed as a structural material due to the advantages they provide. For instance, their use can reduce timber waste. Timber is reusable and can be processed into strands and fibres, which are then reassembled to create engineered wood materials. Moreover, EWPs are able to enhance the strength and stiffness of a given building’s structures. By combining multiple smaller components to form a structural section, the natural variability of timber can be minimised, resulting in enhanced strength and stiffness in the composite material. Some examples of EWP are glued laminated timber (glulam), laminated veneer lumber (LVL), laminated strand lumber (LSL), parallel strand lumber (PSL), and OSB (oriented strand board). In the context of the evolution of engineered wood-based building materials, glulam is an engineered wood product produced by arranging small layers of wood in parallel and bonding them together using high-performance adhesives under controlled pressure. This process produces large-sized structural components that have high strength and good dimensional stability and are suitable for construction applications requiring high load-bearing capacity and flexible geometries, including curved structures [6]. A research initiative by Yang et al. [7] reported that the compression strength of GLT (62.7 N/mm²) was higher, when compared with lumber (61.5 N/mm²). Meanwhile, cross-laminated timber (CLT) has dominated as the primary structural solution, with a configuration of 3–7 alternating layers [1], reaching monumental dimensions (500 mm thick, 20 m long) that rival conventional materials [8]. The development of laminated veneer lumber (LVL), through the strategic arrangement of parallel veneers [9], has produced materials with strength-to-weight ratios that surpass steel, particularly for long-span applications. The PSL paradigm, with its long strand geometry (300:1 ratio), offers a unique solution to the deformation problem [10], while OSB has emerged as the industry’s ’dark horse’ through the transformation of wood waste into cost-effective structural panels [11], addressing the global plywood supply crisis.

In Malaysia, the strength classification of tropical hardwood species is based on the Malaysian Standard Code of Practice for Structural Use of Timber [12]. The ultimate strength values obtained from tests, namely, the modulus of rupture, modulus of elasticity, compression, shear, and tension, can be compared with the tabulated values in the Malaysian Standard Code of Practice for Structural Use of Timber [12], which derived from ultimate strength values obtained in air-dry specimen tests [13,14]. The dry ultimate stress values for the species representing each strength group, namely, SG1 to SG7 in MS544:2017 [12], are obtained from Timber Trade Leaflet No. 34, “The Strength Properties of Some Malaysian Timber” [15], the Handbook of Some Sarawak Timber [16], and the TRTTC Technical Report TR/1 “Strength Properties of Dipterocarp Timbers of Sarawak” [17]. According to the MTIB [18], the strength grouping can be classified based on the ultimate compression parallel to grain in a dry condition, as tabulated in

Table 1. Strength groups of timber in dry conditions based on ultimate compression parallel to grain [18].

Strength Group	Dry Ultimate Compression Parallel to Grain (N/mm2)
A	>55.2; extremely strong
B	41.2–55.2; very strong
C	27.6–41.4; strong
D	<27.6; weak

. Meanwhile, each strength group is designated according to its application, as suggested in Table 2. Currently, there is no classification of strength groups for the Acacia hybrid.

The selection of age groups in this study represents different stages of wood maturity. The combination of the age groups 7//10 years old and 7//13 years old represents juvenile wood combined with transition wood and juvenile wood combined with mature wood, respectively. Meanwhile, the combination of the age groups 10//13 years old represents transition wood combined with mature wood. A study conducted by Gaddafi et al. [19] on the age comparison of solid wood at 7, 10, and 13 years found that the 10-year-old samples exhibited superior mechanical and physical properties, suggesting that this age represents a transitional phase towards maturity. This aligns with findings from Dang et al. [20], who reported that the density of the Acacia hybrid aged 9 to 12 years old ranged from 0.58 to 0.69 g/cm³, consistent with the characteristics of mature wood. The presence of juvenile wood in younger trees is known to influence mechanical properties, making it essential to assess its impact on laminated timber performance [21]. A study by Dinh Kha et al. [22] reported a survival rate of Acacia hybrid trees that decreased to 68.7% at 9 years of age. This finding was corroborated by Jusoh et al. [23], who found that only 41% of Acacia hybrid trees survived at 10.3 years of age. Furthermore, Afifi et al. [24] observed an even more pronounced downward trend in the survival rate of Acacia hybrid parent species, which declined to 27.4% at 12.7 years of age in stands. Based on their analysis, this decline is closely linked to the increase in stand age, where natural factors such as nutrient competition, pathogen attack, and environmental stress contribute to the reduction in stem density and tree vitality. The primary issues identified include the absence of an official strength classification for Acacia hybrid wood in standards such as MS 544:2017 [12], alongside a lack of studies assessing the effects of combined wood age groups on engineering performance. Therefore, this study aims to evaluate and compare the physical and mechanical properties of laminated Acacia hybrid wood derived from a combination of different age groups (7, 10, and 13 years) to identify the optimal configuration in terms of structural strength. Additionally, some of the objectives of this study are to ensure the appropriate use of wood resources, reduce waste, and optimise the utilisation of wood by-products. This study not only contributes to scientific knowledge but also possesses the potential to support the development of local policies and industries towards more the sustainable and effective use of wood in modern construction. This study was conducted with the following main objectives:

• To evaluate the physical and mechanical properties of solid and laminated Acacia hybrid wood from different age group combinations;
• To identify the ideal age combination that offers the best structural performance;
• To contribute to the development of a technical database for strength classification in accordance with standards.

Table 2. Timber strength groups and their applications [25].

Application	Strength Group	Strength Group
Structural Components
Columns, beams, bearers, studs, joist, ties, and struts	A and B	SG1, SG2, SG3, and SG4
Form work	A, B, and C	SG1, SG2, SG3, SG4 and SG5
Roofing
Rafters, ties, struts, purlins, and bracing	A and B	SG1, SG2, SG3, and SG4
Battens	A, B, and C	SG1, SG2, SG3, SG4, and SG5
Staircase
Stringers, treads, trimmer beam, and handrail	A and B	SG1, SG2, SG3, and SG4
Balustrades	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Flooring
Floor boarding and parquetry	A and B	SG1, SG2, SG3, and SG4
Skirtings	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Walling
Wall, partition, framing, and external wall boarding	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Internal wall boarding and slate screens	A, B, C, and D	SG1, SG2, SG3, SG4, SG5, SG6, and SG7
Facia boards	A and B	SG1, SG2, SG3, and SG4
Ceiling frames
Battens to cover jointing of ceiling sheets	A and B	SG1, SG2, SG3, and SG4
Ceiling strips and soffit battens	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Door and window frames
Door, window, and vent frames, including their stops and grounds	A and B	SG1, SG2, SG3, and SG4
Door leaves, window, and vent sashes	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Furniture
Built-in fittings, furniture generally, and workshop furniture	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Science laboratory tops	A and B	SG1, SG2, SG3, and SG4
Beading fillets and edgings generally	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6

Table 2. Timber strength groups and their applications [25].

Application	Strength Group	Strength Group
Structural Components
Columns, beams, bearers, studs, joist, ties, and struts	A and B	SG1, SG2, SG3, and SG4
Form work	A, B, and C	SG1, SG2, SG3, SG4 and SG5
Roofing
Rafters, ties, struts, purlins, and bracing	A and B	SG1, SG2, SG3, and SG4
Battens	A, B, and C	SG1, SG2, SG3, SG4, and SG5
Staircase
Stringers, treads, trimmer beam, and handrail	A and B	SG1, SG2, SG3, and SG4
Balustrades	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Flooring
Floor boarding and parquetry	A and B	SG1, SG2, SG3, and SG4
Skirtings	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Walling
Wall, partition, framing, and external wall boarding	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Internal wall boarding and slate screens	A, B, C, and D	SG1, SG2, SG3, SG4, SG5, SG6, and SG7
Facia boards	A and B	SG1, SG2, SG3, and SG4
Ceiling frames
Battens to cover jointing of ceiling sheets	A and B	SG1, SG2, SG3, and SG4
Ceiling strips and soffit battens	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Door and window frames
Door, window, and vent frames, including their stops and grounds	A and B	SG1, SG2, SG3, and SG4
Door leaves, window, and vent sashes	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Furniture
Built-in fittings, furniture generally, and workshop furniture	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6
Science laboratory tops	A and B	SG1, SG2, SG3, and SG4
Beading fillets and edgings generally	A, B, and C	SG1, SG2, SG3, SG4, SG5, and SG6

2. Materials and Methods

2.1. Preparation of Materials

The preparation of materials followed the Testing Methods for Plantation-Grown Tropical Timbers [26]. Samples were collected from Daiken Sarawak Sdn Bhd at the Bintulu plantation site. Eight trees from each age group available at the plantation site (7, 10, and 13 years), with an average diameter at breast height (DBH) of 26–30 cm, were selected. For the planted trees, each tree was cut at approximately 0.3 m from ground level [19]. Each section was cut to a length of 1.5 m, and the circumference of the bole was approximately 0.7 m. The mean diameter at breast height of 1.3 m (DBH) of the enumerated healthy trees exceeds 22 cm. Sample trees must be selected from among those with a minimum diameter of no less than 18 cm. If the average diameter is below 22 cm, sample trees with a minimum diameter of 14 cm should be chosen. The sample must be healthy, showing no visible defects, apart from knots, and feature a clear bole of at least 6 m from the root collar. The samples were collected from various plot numbers based on the available age at the plantation site for this study. The selection of timber according to their plot number and age is as follows: Plot 1H7: 7 years old; Plot 4H10: 10 years old; and Plot 6H13: 13 years old. Trees from one age category chosen for felling on Plot 1H7, 7 years old, were located on Block No. 38, GPS location N 03° 19′02.2″ E 113° 25′52.9″, with an elevation of 57 m on a sharp incline. Plot 4H10, 10 years old, was located on Block No. 58, GPS location N 03° 18′41.0” E 113° 26′51.7”, with an elevation of 68 m on a steep slope. Furthermore, Plot 6H13, 13 years old, was situated on Block No. 7, GPS location N 03° 21′19.5” E 113° 27′33.0”, with an elevation of 25 m on flat terrain.

The selected felled trees were then sent to Samling Plywood Bintulu Sdn Bhd and placed in a sawing machine to cut the logs into 2 by 4-inch flitches before being sent to the Sarawak Forestry Corporation (SFC) laboratory in Kuching. At the initial stage, the trees were kept in green conditions or wet conditions. For the purpose of this study, the tree samples were prepared in air-drying conditions through a natural drying process or an air-drying process. This natural drying approach was chosen to reduce the likelihood of structural defects, such as fractures and separations, due to the drying process being sudden [19].

Before the natural air-drying process occurred, according to BS 373:1957 [27], the 2 by 4-inch flitches were cut into four portions of planks at a thickness of 20 mm to facilitate the air-drying process. The planks were arranged in stacks and left to air-dry in a covered yet ventilated area, gradually reaching a moisture level equal to or below 19%, which took approximately a year. Due to the hygroscopic behaviour of timber, after the natural air-drying process was completed, the wood was stored in a conditioned area until it reached a moisture content of 12% prior to the sample preparation process, ensuring that the sample was maintained in a dry state. When the moisture content was below 19 percent, the plank-sized samples were selected, and sample preparation commenced. According to BS 373:1957 [27], the required dimensions for a small clear specimen, also known as a defect-free specimen, are 20 mm by 20 mm. Consequently, the thickness of the plank-sized specimen was checked, and the surface was designed to meet the requirements. For solid specimens, the surface of the samples aged 7, 10, and 13 years old were designed to have a thickness of 20 mm. In contrast, for the laminated specimens, the surface was designed to have a thickness of 10 mm (Figure 1). This is for the glueing process later on, which combines planks with respective thicknesses of 10 mm to create a 20 mm by 20 mm plank (Figure 2).

The lamination process follows ASTM D 5751–99 [28] and MAFF [29]. In this study, the action of glue is a constant variable. The glued specimens were pressed at 1.5 MPa and 25° C for 24 hours, followed by a curing period of 7 days at room temperature [30]. The laminated materials, consisting of planks measuring 10 × 155 × 1300 mm³ (depth × width × length) per age group, were glued together using a Type C79-A press (Tahei Machinery Work LTD, Komaki, Aichi, Japan). Figure 3 illustrates the general workflow process for sample preparation. The small clear specimen method, with a cross-section of 20 × 20 mm², was employed to prepare laminated products by glueing together planks from two different age groups using phenol–resorcinol–formaldehyde with a 15% hardener, in accordance with Tan et al. [26], and this method was adopted from BS 373:1957 [27]. This study categorises the samples by the combination of age groups: 7//10 years old, 7//13 years old, and 10//13 years old. After the glueing process was completed, the planks were cut using the Altendorf-C45 machine (Altendorf GmbH & Co. KG Maschinenbau, Minden, Nordrhein-Westfalen, Germany) into sticks measuring 20 × 20 mm² and 50 × 50 mm², as required by the British Standard for test samples [27]. A sample size of 20 × 20 × 300 mm³ for static bending and 20 × 20 × 60 mm³ for compression parallel to the grain was prepared. A total of 90 laminated samples for the compression parallel to the grain test, 60 samples for the compression perpendicular to the grain test, and 180 samples for the static bending test were prepared, resulting in approximately 330 test samples for this study.

2.2. Laboratory Work

The Instron 5569 Universal Testing Machine (Instron, Norwood, MA, USA) was utilised to conduct mechanical testing, specifically static bending and compression tests, to determine the ultimate stresses of timber properties. For each test type and age category, 150 laminated small clear specimens were prepared for compression testing, while 180 laminated small clear specimens were employed for static bending. All specimens were tested under air-dry conditions with a moisture content of 19%. Only defect-free samples from a total of 330 specimens were selected.

2.3. Testing of Physical Properties

The testing of physical properties conducted for this research included moisture content and basic density in air-dry conditions according to Standards MS544:2017 [12], which were adopted from Standards BS 373:1957 [27] and Testing Methods for Plantation-Grown Tropical Timbers [26].

2.3.1. Moisture Content Test

The samples for this study were set up in air-dry conditions. Once the samples reached constant readings under air-dry conditions using a moisture meter, they were prepared for testing. This moisture content assessment was conducted after mechanical testing had been completed. Each specimen was first weighed and then placed in an oven set to 103 ± 2° C until a stable weight was achieved. The reduction in mass, expressed as a percentage relative to the final oven-dried weight, was recorded as the specimen’s moisture content. The following equation was used for computation:

$$\mathrm{MC}={\displaystyle \frac{{{\mathrm{W}}_1-\mathrm{W}}_0}{{\mathrm{W}}_0}}\times 100$$

(1)

where MC represents moisture content (%), W₀ denotes the oven-dried mass (g), and W₁ indicates the initial mass (g).

2.3.2. Basic Density Test

The basic density (BD) was determined after mechanical testing. It was calculated as the ratio of the oven-dried weight to the green volume. To obtain the green volume, the water displacement method was employed, in which each specimen was coated with paraffin wax before being submerged in water. The sample’s weight was measured using an electronic balance. The formula used for basic density was

$$\mathrm{BD}={\displaystyle \frac{{\mathrm{W}}_0}{{\mathrm{V}}_{\mathrm{g}}}}$$

(2)

where BD is the basic density (g/cm³), W₀ is the oven-dried weight (g), and V_g denotes the volume under green conditions (cm³).

2.4. Testing of Mechanical Properties

Mechanical properties were evaluated in accordance with ASTM D143-14 [31] and MS544:2017 [7], which were derived from BS 373:1957 [20] and Testing Methods for Plantation Grown Tropical Timbers [26]. The tests included static bending: the modulus of rupture (MOR) and the modulus of elasticity (MOE) were measured using a three-point bending method, along with compression parallel and perpendicular to the grain. Compressive stress was determined using an Instron 5569 Universal Testing Machine (Instron, Norwood, MA, USA).

2.4.1. Static Bending Test

The experiment utilised the three-point flexural testing technique. The dimensions of the test piece were 20 × 20 × 300 mm³. The distance between the points of support on the test piece was set at 280 mm, and the load was applied at a constant loading speed of 6.0 mm/min throughout the test. The contour of the loading head, which was in contact with the beam, had a radius of 30 mm. The deflection of the beam at mid-length was measured with reference to the outer loading points. MOR and MOE, expressed in megapascals (1 MPa = 1 N/mm²), were calculated using the following formulae, respectively:

$$\mathrm{M}\mathrm{O}\mathrm{R}={\displaystyle \frac{3}{2}}\left({\displaystyle \frac{\mathrm{FL}}{\mathrm{W}{\mathrm{T}}^2}}\right)$$

(3)

where F is the maximum load (N), L is the length of the span (mm), W indicates the width (mm), and T represent the depth (mm).

$$\mathrm{M}\mathrm{O}\mathrm{E}=\left({\displaystyle \frac{{\mathrm{L}}^3}{4{\mathrm{WT}}^2}}\right)\left({\displaystyle \frac{∆\mathrm{F}}{∆\mathrm{l}}}\right)$$

(4)

where L represents the length of the span (mm), ${\displaystyle \frac{∆\mathrm{F}}{∆\mathrm{l}}}$ denotes the slope of the graph (N/mm), W indicates the width (mm), and T represent the depth (mm).

2.4.2. Compression Parallel to Grain Test

In timber, compressive strength is typically assessed along the grain direction, meaning that the applied force aligns with the natural grain orientation when subjected to compression loading. The equation employed for this evaluation is as follows:

$$\mathrm{C}\mathrm{o}\mathrm{m}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s}\mathrm{i}\mathrm{v}\mathrm{e} \mathrm{s}\mathrm{t}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{s} \mathrm{a}\mathrm{t} \mathrm{m}\mathrm{a}\mathrm{x}\mathrm{i}\mathrm{m}\mathrm{u}\mathrm{m} \mathrm{l}\mathrm{o}\mathrm{a}\mathrm{d}={\displaystyle \frac{\mathrm{F}}{\mathrm{A}}}$$

(5)

where F represents maximum load (N) and A indicates the cross-sectional area (mm²).

Test specimens for compression along the grain measured 20 mm × 20 mm × 60 mm, with force applied in alignment with the grain at a consistent loading rate of 0.6 mm/min.

2.4.3. Compression Perpendicular to Grain Test

The test involved loading the specimen between parallel compression plates in the tangential directions, applying a constant speed of 0.6 mm/min throughout the test. The load compression curve was plotted until the compression of the test piece reached 2.54 mm. The maximum load and its associated strain were also recorded. The dimensions of the prepared specimen were 50 mm × 50 mm × 50 mm.

2.5. Statistical Analysis

Statistical analysis was conducted to obtain the mean value of physical and mechanical properties with a 95% confidence interval across combined age groups. Further analysis was undertaken using a statistically significant difference (p < 0.05) to observe the error bars between the combined age groups, and regression analysis was also employed to examine the data for potential correlations. The Garrett Ranking analysis method was implemented to further support the data analysis and determine the ranking, as guided by Dhanavan [32]. This process involves assigning ranks to various factors, calculating percent positions, and then converting these positions into scores using Garrett’s table. The factors are ranked based on their mean scores to identify the most significant issues. Thus, the analysis assists in choosing the best combination of age groups that possesses the greatest strength among the combined age groups in terms of their mechanical and physical properties for engineered timber products for the Acacia hybrid.

3. Results and Discussion

3.1. Moisture Content Results

The mean moisture content results were observed with 95% confidence intervals calculated based on the measured oven-dried weight, as listed in

Table 3. Moisture content of solid and laminated/glulam samples for the respective age groups.

Moisture Content (%)
Age groups	Solid			Laminated (Glulam)
	7	10	13	7//10	10//13	7//13
Sample size	30	30	30	30	30	30
Mean (%)	15.32	15.95	15.06	14.94	14.27	13.75
Std Dev	0.53	0.53	0.76	2.12	0.93	0.71
CV (%)	3.48	3.45	5.02	14.19	6.49	5.17
Standard error, SE	0.10	0.10	0.14	0.39	0.17	0.13
95% confidence interval, CI	0.19	0.19	0.28	0.77	0.34	0.26

. Generally, the moisture contents among species vary significantly. The ability of species to retain certain percentages of moisture content directly depends on their wood cells. Specifically, this research recorded a notable moisture content in the small clear solid samples, where 13-year-old samples exhibited lower moisture content than 7- and 10-year-old samples, with recorded values of 15.06%, 15.32%, and 15.95%, respectively. Meanwhile, the laminated/glulam sample from the 7//13-year-old age combination had the lowest recorded moisture content of 13.75%, with a 95% confidence interval value of 0.26, followed by the 10//13-year-old combination, which recorded 14.27% with a 95% confidence interval value of 0.34, and the 7//10-year-old combination, which recorded 14.94% with a 95% confidence interval value of 0.77. However, all specimens from all age groups complied with the requirement for a moisture content of 19% or below, indicating dry conditions as required by the standard. Thus, from this study, the moisture content decreased by up to 10% for the laminated sample from the 7//13-year-old combination compared to the 10//13-year-old and 7//10-year-old samples, which exhibited decreases of 8% and 4.3%, respectively, when the solid samples were laminated.

A study by Rokeya et al. [33] in Bangladesh also revealed that the lowest moisture content obtained for Acacia mangium from a combination of juvenile and mature wood in age groups ranging from 9 to 12 years old, with a mean age of 10.5 years old, was between 12% and 14%. A study conducted by Zobel and Sprague [34] on Pine species found that the juvenile wood had a higher moisture content and more shrinkage than mature wood; when these types of wood are combined to form a wood product, it can lead to more stable drying and, consequently, reduced overall moisture content. Walker et al. [35], in their study, showed a similar trend in moisture content for Douglas Fir species when juvenile wood was blended with mature wood; furthermore, this combination can reduce drying defects. This is due to the balancing effect of denser mature wood, which leads to a decline in moisture content [36].

As observed in a previous study regarding the effect of moisture content on the strength of a timber species, the strength and stability of timber are significantly influenced by its moisture content [37]. As mentioned by Gerhards [38], moisture content greatly affects mechanical properties, with lower moisture content generally leading to higher strength. The decrease in moisture content, which results in wood with a higher density and better structural integrity, significantly impacts the increase in mechanical strength. Generally, timber increases in strength as it dries, although this effect does not commence until the water within the cell has evaporated. However, according to a study by Gaddafi [39], not all mechanical properties of timber improve as moisture content decreases. The moisture content recorded for the respective samples varies because it is well acknowledged that wood is a highly hygroscopic material [40]. Thus, most of its properties are considerably influenced by the moisture content. According to Panshin and Zeeuw [41], most of the strength properties of wood vary inversely with the MC of the wood below the fibre saturation point. Above the saturation point, the mechanical properties remain constant despite changes in moisture content.

3.2. Basic Density Results

The basic density mean results, observed with 95% confidence intervals, are presented in

Table 4. Basic density of solid and laminated/glulam samples for respective age groups under dry conditions.

Basic Density (g/cm3)
Age groups	Solid			Laminated (Glulam)
	7	10	13	7//10	10//13	7//13
Sample size	30	30	30	30	30	30
Mean (g/cm3)	0.693	0.694	0.678	0.69	0.710	0.708
Std Dev	0.06	0.09	0.10	0.063	0.066	0.08
CV (%)	9.30	12.98	15.28	9.11	9.25	11.11
Standard error, SE	0.01	0.02	0.02	0.0115	0.0119	0.0143
95% confidence level, CI	0.02	0.03	0.04	0.023	0.024	0.029

of this study. It can be seen that the small clear solid samples of 7-year-old specimens recorded the highest value, followed closely by those of 10- and 13-year-old specimens, with values of 0.693, 0.694, and 0.678 g/cm³, respectively. Regarding the small clear laminated glulam samples, the 10//13-year-old specimens exhibited the highest recorded basic density of 0.71 g/cm³, followed by the 7//13-year-old specimens at 0.708 g/cm³ and the 7//10-year-old specimens recorded 0.69 g/cm³. A study by Jusoh et al. [42] reported that the density of a 7-year-old solid Acacia hybrid was 0.43 g/cm³. Thus, this study shows that the basic density increased by 3.4% for the laminated 10//13-year-old sample, followed by the 7//13-year-old sample at 3.2%. Conversely, the 7//10-year-old laminated sample displayed a decrease of 0.32% when compared to the solid samples. Cown [21] highlighted that density serves as a reliable predictor of strength, with greater density correlating with enhanced mechanical performance. This property is influenced by the ratio of wood cell walls to the void spaces within and between cells. Consequently, key factors affecting density include cell dimensions, wall thickness, void distribution, and the arrangement of various cell types [43]. In a broader context, density remains a vital parameter not only for strength but also for properties such as rigidity, joint durability, hardness, workability, fire resistance, and drying behaviour [44,45]. According to Mohd Jamil [44], timber, as a natural material, demonstrates considerable variability in its mechanical properties, making strength assessment particularly challenging. Additionally, Desch and Dinwoodie [46] emphasise that timber’s strength and surface hardness are significantly influenced by its density. However, factors such as grain deviation, knots, and brittle heart can considerably weaken its overall strength. A study conducted by Jusoh et al. [42] revealed that the Acacia hybrid recorded the highest basic density, followed closely by the second-generation Acacia mangium, where their basic density measurements did not differ at the 5% level. He further stated that the results for the two species indicate that the basic density of the Acacia hybrid falls between the parental species’ (Acacia mangium and Acacia auriculiformis) basic densities, but statistically, its basic density did not differ from that of Acacia mangium.

3.3. MOR and MOE

A total of 90 laminated samples were used, of which 30 samples were tested for different age groups in the static bending test to obtain the MOR and MOE results observed with 95% confidence intervals, as shown in

Table 5. (a) MOR and (b) MOE of solid and laminated/glulam samples for respective age groups under dry conditions.

(a)
Modulus of Rupture (MOR)
	Solid			Laminated (Glulam)
Age groups	7	10	13	7//10	7//13	10//13
Sample size	30	30	30	30	30	30
Mean ultimate stress (N/mm2)	89.95	99.28	97.83	108.61	109.34	110.67
Std Dev	17.09	21.44	34.71	14.52	24.96	24.71
CV (%)	19.00	21.60	35.48	13.37	22.83	22.33
Standard error SE	3.12	3.91	6.34	2.65	4.56	4.51
95% confidence level CI	6.24	7.83	12.67	5.3	9.11	9.02
(b)
Modulus of Elasticity (MOE)
	Solid			Laminated (Glulam)
Age groups	7	10	13	7//10	7//13	10//13
Sample size	30	30	30	30	30	30
Mean ultimate stress (N/mm2)	11,697.07	12,779.77	12,765.17	16,143.30	15,847.80	16,335.60
Std Dev	1498.68	1560.43	2661.22	2186.49	2774.25	2296.79
CV (%)	12.81	12.21	20.85	13.54	17.51	14.06
Standard error SE	273.62	284.89	485.87	399.2	506.51	419.33
95% confidence level CI	547.24	569.79	971.74	798.39	1013.01	838.67

. As illustrated in Figure 4a, it is evident that for small clear solid samples, the 10-year-old sample exhibited the highest MOR recorded at 99.28 N/mm², followed by the 13-year-old sample at 97.83 N/mm², while the lowest value was obtained the 7-year-old sample at 89.95 N/mm². As can be observed from Table 5 (a), laminated/glulam samples demonstrate greater MOR strength than solid specimens. Figure 4a clearly shows that the highest MOR for laminated/glulam samples is recorded at 10//13 years old with a value of 110.67 N/mm², followed by 7//13 years old at 109.34 N/mm², and the lowest value is obtained at 7//10 years old at 108.61 N/mm². A similar trend, as shown in Figure 4b, was noted for MOE in small clear solid samples. The 10-year-old sample recorded the highest MOE, at 12,779.77 N/mm², closely followed by the 13-year-old sample at 12,765.17 N/mm², while the lowest value was obtained for the 7-year-old sample at 11,697.07 N/mm². However, for laminated/glulam samples, we observed that the 10//13-year-old samples exhibited the highest MOE value, followed by the 7//10-year-old samples, with values 16,335.6 and 16,143.30 N/mm², respectively. Meanwhile, the lowest value, obtained for the 7//13-year-old sample, was 15,847.80 N/mm².

A study conducted by Jusoh et al. [42] on a solid sample of a 7-year-old Acacia hybrid reported the MOR and MOE under air-dry conditions as 85 N/mm² and 9614 N/mm², respectively. Jusoh et al. [23] also confirmed that a 7-year-old Acacia hybrid was identified to possess low strength properties compared to mature timber. This study revealed that the 7-year-old Acacia hybrid had improved strength properties when laminated with mature wood. An increased MOR was also reported when the 7-year-old Acacia hybrid was laminated with mature wood. An increase in strength and MOR was also reported when a 7-year-old Acacia hybrid was laminated with 10-year-old or 13-year-old wood, with a 27 to 28% increase in strength. Meanwhile, for MOE, an increment of about 64 to 68% was reported. Rokeya et al. [33] noted that for a solid sample of a Acacia hybrid aged 9 to 12 years old, the MOR and MOE were 74.14 MPa and 11,473.78 MPa, respectively. Therefore, this finding indicates that the combination of 10-year-old with 13-year-old Acacia wood for laminated/glulam products revealed an increase in MOR and MOE by 49.3 percent and 42.4 percent, respectively.

Both the MOE and MOR values recorded for the 10//13-year-old age group combination present the highest value compared to the others. According to Laurila [47], higher MOR and MOE values indicate that the wood can withstand significant stress and maintain rigidity, making it a viable alternative to traditional timber species. Research conducted by Mohd Yusof et al. [1] on the mechanical and physical properties of cross-laminated timber (CLT) produced from Acacia mangium reveals that the lamination process significantly enhances the wood’s bending strength and other mechanical properties. This finding is further reinforced by Yahya et al. [48], whose research on glulam/lamination demonstrated improvements in both mechanical and physical properties as a result of the lamination process. Serrano [30] stated that the benefits of using laminated wood include enhanced strength and stiffness properties, greater flexibility in geometric forms, the ability to adjust the quality of wood laminates and finger joint connections to desired stress levels, and improved dimensional stability and shape accuracy. Furthermore, by redistributing stress and minimising the effect of flaws, the cross-lamination technique enhances the performance of laminated timber.

According to Sattar and Bhattacharjee [49], physical properties are among the factors that influence data results. Generally, denser specimens exhibit a higher MOR and MOE, while increased moisture reduces both strength and stiffness. Therefore, the density of timber should not be regarded as a definitive measure of its strength [50]. The MOR results indicate that the laminated Acacia hybrid species can withstand bending strains and stresses prior to rupture. Furthermore, the MOE results obtained in this study suggest that the stiffness is high, as it yielded the highest reading for elastic modulus. Mohd Jamil [44] found that the ultimate bending strength of full-size structural specimens tends to be lower than that of defect-free small clear samples. He noted that the small clear testing method, employing the three-point bending approach, mitigates the impact of defects such as knots and cross-grain. In contrast, structural specimens tested under the third-point loading method are more susceptible to horizontal shear effects. The strength characteristics of structural-sized specimens differ significantly from those of small clear specimens due to the inherent variability in timber. A small clear specimen refers to a defect-free sample. Additionally, variations in strength properties can arise even within the same species [51]. Ismaili et al. [52] emphasised that small clear test results should not be directly applied in structural design. Instead, they must be adjusted using appropriate modification factors, as specified in the British Standard CP 112:1967 [53] and Malaysian Standard MS544: Part 2 [12], to determine basic stress and permissible stress values, and this also applies to 5 percentile ultimate strength results of structural size.

To further evaluate the data, a one-way ANOVA test was conducted to analyse variations in the modulus of rupture (MOR) and modulus of elasticity (MOE) for a laminated Acacia hybrid across different age group combinations. This analysis was performed using a statistically significant difference (p < 0.05) to observe the error bars in Figure 4 between the 10//13-year-old age combination (which recorded a higher mean) and other age group combinations of Acacia hybrid samples, with the results tabulated in

Table 6. ANOVA comparison for MOE and MOR.

Age Groups	Significant Difference p-Value (p < 0.05)
	Age Group	MOR	MOE
10//13	7//10	0.69 **	0.73 **
	7//13	0.88 **	0.52 **
7	7//10	0.00003 *	0.00000 *
	7//13	0.00087 *	0.00000 *

MOR: modulus of rupture; MOE: modulus of elasticity; *: significant at 5% level; **: not significant at 5% level.

. The findings indicate no statistically significant difference (p > 0.05) in MOR between the 10//13-year-old group and the 7//10-year-old (p = 0.69) and 7//13-year-old (p = 0.88) groups. A similar trend was observed for MOE between the 10//13-year-old group and the 7//10-year-old (p = 0.73) and 7//13-year-old (p = 0.52) groups, where no significant differences were detected between these age groups, although slight differences exist. The results indicated that combining juvenile timber with mature timber led to similar strength behaviour to combinations of mature timber alone. The percentile similarity index of the MOR results for the 10//13-year-old group compared with the 7//13-year-old and 7//10-year-old groups is 98.8 and 99.3 percentiles, respectively. Meanwhile, the percentile similarity index of the MOE results for the 10//13-year-old group compared with the 7//10-year-old and 7//13-year-old groups is 98.8 and 97 percentiles, respectively. Solid samples of Acacia hybrids aged 7 years old have been shown to have increased MOR and MOE values when combined with 10- or 13-year-old Acacia hybrids in studies. To ensure a significant change, a statistically significant difference analysis (p < 0.05) was performed to investigate the extent of significant differences between 7-year-old solid wood and 7//10- and 7//13-year-old laminated wood. The analysis indicated that, as shown in Table 6, there is a statistically significant difference (p < 0.05) in the MOR between the solid 7-year-old and laminated/glulam 7//10-year-old (p = 0.00003) and 7//13-year-old (p = 0.00087) groups. This also confirms the regression analysis shown in Figure 5a, where the best-fit age combination for the laminated/glulam product is 7 years old with 10 years old, demonstrating the best-fit correlation of R = 0.440 and y = 1.575x + 71.96. Meanwhile, for the age combination of 7 years old with 13 years old, a slightly closer fit exists, with R = 0.426 and y = 1.7537x + 69.588. However, both correlations for the best-fit age combinations for laminated/glulam products are moderate. Similarly, for MOE, a statistically significant difference (p < 0.05) exists between the solid 7-year-old and laminated/glulam 7//13-year-old (p = 0.00000) and 7//10-year-old (p = 0.00000) groups. This was also confirmed by regression analysis, as shown in Figure 5b, where the best-fit age combination for the laminated/glulam product is 7 years old with 13 years old, with a best-fit relationship of R = 0.507 and y = 270.1x + 10,084. Meanwhile, for the age combination of 7 years old with 10 years old, a slightly closer fit exists, with R = 0.4403 and y = 209.84x + 10,840. However, both correlations for the best-fit age combination for laminated/glulam products remain moderate. From the analysis, it was confirmed that the 7-year-old solid Acacia hybrid shows significant MOR and MOE strength when laminated (glulam) with either 10-year-old or 13-year-old wood.

3.4. Compression Stress Properties

The average compressive stress results concerning the combination of age groups were observed with 95% confidence intervals, as shown in

Table 7. (a) Compression parallel to the grain and (b) compression perpendicular to the grain of solid and laminated/glulam samples for respective age groups under dry conditions.

(a)
Compression Parallel to Grain (N/mm2)
Age Groups	Solid			Laminated (Glulam)
	7	10	13	7//10	7//13	10//13
Sample size	30	30	30	30	30	30
Mean ultimate stress (N/mm2)	48.020	60.275	53.474	54.09	52.44	56.99
Std Dev	5.39	8.94	10.02	5.83	6.35	6.04
CV (%)	11.23	14.83	18.74	10.78	12.12	10.6
Standard error SE	0.98	1.63	1.83	0.39	0.42	0.4
95% confidence level CI	1.97	3.26	3.66	0.78	0.84	0.8
(b)
Compression Perpendicular to Grain (N/mm2)
Age Groups	Solid			Laminated (Glulam)
	7	10	13	7//10	7//13	10//13
Sample size	30	30	30	20	20	20
Mean ultimate stress (N/mm2)	4.017	8.395	5.743	8.92	7.4237	7.4242
Std Dev	0.29	1.18	0.91	0.91	1.33	0.74
CV (%)	7.27	14.05	15.87	10.18	17.88	9.92
Standard error SE	0.48	0.94	1.06	0.2	0.3	0.16
95% confidence level CI	0.97	1.87	2.12	0.41	0.59	0.33

. Based on the data presented in Table 7 (a) for small clear solid samples, the 10-year-old sample exhibited the highest recorded compressive stress of 60.275 N/mm², followed by the 13-year-old sample with a value of 53.474 N/mm², while the lowest value was obtained for the 7-year-old sample, recorded at 48.020 N/mm². Meanwhile, for laminated/glulam samples, the 10//13-year-old sample demonstrated the highest compression stress parallel to the grain, measuring 56.99 N/mm². This was followed by the age group combinations of 7//10 years old and 7//13 years old, which recorded compression stresses of 54.09 N/mm² and 52.44 N/mm², respectively.

It has been shown that combining juvenile and mature trees, particularly the 7-year-old solid trees in this study, improved compressive strength. The increment recorded was about 11.22 percent when a 7-year-old tree was combined with a laminated/glulam tree at 10 years old; meanwhile, there was an 8.43 percent increment when it was combined with a 13-year-old tree. Similarly, the 13-year-old solid sample increased by 6.57 percent in strength when combined with a 10-year-old sample, but decreased by 1.93 percent in compressive strength when laminated with a 7-year-old sample. A decreasing pattern was also observed for the solid 10-year-old sample, where a decrease of 10.26 percent and 5.45 percent in strength was noted when lamination/glulam was used with 7-year-old and 13-year-old wood, respectively. Studies have shown that not all age groups of solid wood will see an improvement in strength when being laminated (glulam). Additionally, the densest samples were taken from the 10-year-old group, further reinforcing the correlation between increased wood density and greater strength [52].

A statistical significance analysis (p < 0.05) was performed to assess the error bars displayed in Figure 6a, comparing the mean values across these age groups. The detailed results of this analysis are provided in

Table 8. ANOVA comparison of compression strength for different age group combinations.

Compression Parallel to Grain	Age group	10//13	Age Group	7//10	7//13
			p-value	0.063 **	0.006 *
		7	Age Group	7//10	7//13
			p-value	0.000097 *	0.005226 *
Compression Perpendicular to Grain		7//10	Age Group	10 + 13	7 + 13
			p-value	0.00 *	0.00 *
		7	Age Group	7//10	7//13
			p-value	0.00000 *	0.00000 *

*: significant at 5% level; **: not significant at 5% level.

. For compression parallel to the grain, no statistically significant difference (p > 0.05) was found between the age group combinations of 10//13-year-old and 7//10-year-old samples (p = 0.063). However, a statistically significant difference (p < 0.05) was observed when comparing the age group combinations of 10//13 year-olds with those of 7//13 year-olds (p = 0.006). This indicates that the density of these age group combinations influences the compressive strength of the samples, as the age group combination of 7//13 year-olds recorded the second highest density. Studies have shown that solid samples of Acacia hybrids aged 7 years old increase in compression parallel to grain values when combined with 10- or 13-year-old Acacia hybrids. To ensure that there is a significant change, a statistically significant difference analysis (p < 0.05) was conducted to investigate the extent to which the change was significant between the 7-year-old solid wood and the 7//10- and 7//13-year-old laminated wood. The analysis found that, as shown in Table 8, there was a statistically significant difference (p < 0.05) in compression parallel to the grain between the solid 7-year-old and laminated/glulam 7//10-year-old (p = 0.000097) and 7//13-year-old (p = 0.005226) samples. This was further confirmed by the regression analysis, as illustrated in Figure 7a, where the best-fit age combination for the laminated/glulam product was 7 years old with 10 years old, with a best-fit correlation of R = 0.471 and y = 0.7258x + 38.738. Meanwhile, for the age combination of 7 years old with 13 years old, a slightly less close fit was observed, with R = 0.417 and y = 0.6355x + 38.673. However, both correlations for the best-fit age combination for the laminated/glulam product are moderate. Therefore, this analysis confirmed that a 7-year-old solid Acacia hybrid had significantly altered compression parallel to grain strength when laminated (glulam) with a 10-year-old or 13-year-old Acacia hybrid.

Table 7 (b) presents the average compressive stress perpendicular to the grain across various age groups observed with 95% confidence intervals. Figure 6b shows that among the small clear solid samples, the 10-year-old samples exhibited the highest recorded compressive strength of 8.395 N/mm², followed by the 13-year-old samples at 5.743 N/mm², and the lowest was obtained for the 7-year-old samples, recorded at 4.017 N/mm². The combined age groups of 7//10-year-old specimens displayed the highest compressive stress, followed by the combined age groups of 10//13-year-old specimens and 7//13-year-old specimens, for which values of 8.92 N/mm², 7.4242 N/mm², and 7.4237 N/mm² were reported, respectively.

Sharma et al. [54] revealed that compression perpendicular to the grain for solid samples of an 8-year-old Acacia hybrid ranged from 5.5 to 7.2 MPa, and the results obtained in this study are also within this range. Therefore, this study demonstrated that combining the juvenile wood layer with mature wood in the laminated system for compression perpendicular to the grain results in increased compressive strength. The compressive strength increased by 122.06 percent when integrating 7-year-old juvenile wood into 10-year-old mature wood. Meanwhile, a strength increase of 84.8 percent was observed when 7-year-old wood was combined with 13-year-old wood. However, when 10-year-old solid laminated (glulam) wood is compared with 13-year-old wood under the same mature wood categorisation, the compression perpendicular to the grain decreases by 11.56 percent. The results showed that when a 10-year-old sample was combined with a 13-year-old sample to produce a laminated product, its compression perpendicular to the grain decreased compared to its solid state. As mentioned by Bodig and Jayne [55], compression strength perpendicular to the grain has a stronger correlation with the density of the wood. However, this finding differs from previous studies, which have emphasised that density has a strong correlation with compression perpendicular to the grain. These differences may signify a relationship between strength and anatomical properties. It has been reported that wood density is less sensitive to the increment in strength related to the microfibril angle of wood fibres [56,57]. This occurs because, while the microfibril angle decreases with age, density increases. Therefore, as noted by Ross [58], grain angle has been identified to affect the correlation between density and mechanical properties. It has also been reported that the survival rate of the Acacia hybrid decreases at an age of 10.3 years [23]. Zobel and Buijtenen [59] also mentioned that a discrepancy arises as a result of variations in geographic locations, age, growth rates, and genetic details. The implications of the decreased survival rate of timber will lead to a decrease in its density and mechanical properties [48]. As discussed by Cown [21], the lower mechanical strength of specimens below 10 years old can be attributed to the presence of juvenile wood, which is typically less dense and weaker than mature wood.

A statistical significance test (p < 0.05) was conducted to compare the error bar margins in Figure 6b. The highest mean value for compression stress perpendicular to the grain was obtained for the 7//10-year-old group, which was then compared with the 7//13-year-old and 10//13-year-old specimens. The significance analysis results are displayed in Table 8, showing that the 7//10-year-old group had a statistically significant difference (p < 0.05) compared to both the 10//13-year-old (p = 0.00) and 7//13-year-old groups (p = 0.00). Similarly, solid samples of Acacia hybrids aged 7 years old have demonstrated an increase in compression stress perpendicular to the grain when combined with 10- or 13-year-old samples. To ensure that there was a significant change, a statistically significant difference analysis (p < 0.05) was performed to investigate the extent to which the change was significant between 7-year-old solid wood and 7//10- and 7//13-year-old laminated wood. The analysis showed, as indicated in Table 8, that there was a statistically significant difference (p < 0.05) in compression stress perpendicular to the grain between the solid 7-year-old and laminated/glulam 7//10-year-old (p = 0.00000) and 7//13-year-old (p = 0.00000) samples. This was also confirmed by regression analysis, as illustrated in Figure 7b, where the best-fit age combination for the laminated/glulam product was 7 years old with 7//10 years old, exhibiting the best-fit correlation of R = 0.519 and y = 0.221x + 5.5726. Meanwhile, for the age combination of 7 years old with 13 years old, there was a slightly closer fit, with R = 0.467 and y = 0.1815 + 4.6615. However, both correlations for the best-fit age combinations for the laminated/glulam product were moderate. Therefore, this analysis confirmed that a 7-year-old solid Acacia hybrid had significantly altered compression stress perpendicular to the grain when laminated (glulam) with 10-year-old or 13-year-old wood.

Overall, the data suggest that the grain angle influences the relationship between density and mechanical properties [58]. The findings from this research indicate that compression parallel to the grain yielded higher results compared to compression perpendicular to the grain, a trend also reported in a study by Ismaili [60]. To determine the most suitable single-laminated age group for timber products, a Garrett Ranking analysis was employed. The ranking results, as shown in

Table 9. Garett’s ranking analysis for the respective age group combinations of Acacia hybrids.

Engineering Properties	Unit	Age Group Combination (Single-Laminated)
		7//10-Year-Old	7//13-Year-Old	10//13-Year-Old
Moisture content	Mean (%)	14.94	13.75	14.27
	Percentile Position	83.33	16.67	50.00
	Garret Score	31	69	50
Basic density	Mean (g/cm3)	0.69	0.708	0.71
	Percentile Position	83.33	16.67	50.00
	Garret Score	31	69	50
Modulus of elasticity (MOE)	Mean (N/mm2)	16143.3	15847.8	16335.6
	Percentile Position	50.00	83.33	16.67
	Garret Score	50	31	69
Modulus of rupture (MOR)	Mean (N/mm2)	108.61	109.34	110.67
	Percentile Position	83.33	50.00	16.67
	Garret Score	31	50	69
Compression parallel to grain	Mean (N/mm2)	54.09	52.44	56.99
	Percentile Position	50.00	83.33	16.67
	Garret Score	50	31	69
Compression perpendicular to grain	Mean (N/mm2)	8.92	7.4237	7.4242
	Percentile Position	16.67	83.33	50.00
	Garret Score	69	31	50
Average score		43.67	46.83	59.50
Ranking		3	2	1

, indicate that the 10//13-year-old combination achieved the highest score, followed by the 7//13-year-old group, while the 7//10-year-old group had the lowest score. The strength results for the 10//13-year-old combination exceeded those of the 7//13-year-old and 7//10-year-old groups by 21.3% and 26.6%, respectively. Consequently, the most recommended single-laminated timber combination for optimal performance is the 10//13-year-old Acacia hybrid.

4. Conclusions

This study shows that laminated Acacia hybrid wood provides a significant improvement in mechanical properties when combined with different age groups. The 10//13-year-old combination was identified as the most optimal configuration, recording the highest values for the modulus of elasticity (16,335.6 N/mm²), the modulus of rupture (110.67 N/mm²), and the parallel compressive strength (56.99 N/mm²), as well as a basic density of 0.71 g/cm³. These findings indicate the high potential of Acacia hybrid wood as a structural material, especially in engineered wood applications.

According to the results of this study, making laminated products with a combination of mature wood from 10- and 13-year-old trees does not necessarily provide additional improvements in strength; on the contrary, the strength remains unchanged. However, combining juvenile wood with mature wood contributed to an increase in strength to some extent. The use of 7-year-old juvenile wood also demonstrates significant improvement when paired with mature wood, thereby creating opportunities for the reuse of young wood that is often deemed low-value. The combination of different wood ages results in comparable or superior performance compared to single wood types in previous studies. Therefore, the results of this study support the classification of Acacia hybrid wood into strength group SG5 according to the MS544 Standard and its use in small to medium-sized structural components. Further studies are advised to assess the stability of these structural properties under long-term variations in temperature, humidity, and load. Consequently, an application for each age group combination according to its strength group from this study is suggested in

Table 10. Strength group and recommendation of application for the single age for solid and age group combinations for laminated for Acacia hybrid under dry conditions.

Age	SG	MOR (N/mm2)	Comp// (N/mm2)	Comp⊥ (N/mm2)	MOE (N/mm2)	Application
10//13	SG5	110.67 (24.71)	56.99 (6.04)	7.4242 (0.74)	16,335.6 (2296.79)	Form work, battens, balustrades, skirtings, wall, partition, framing and external wall boarding, internal wall boarding, slate screens, ceiling strips and soffit battens, door leaves, window and vent sashes, built-in fittings, general furniture and workshop furniture, beading fillets, and general edgings
7//13	SG5	109.34 (24.96)	52.44 (6.35)	7.4237 (1.33)	15,847.8 (2774.25)
7//10	SG5	108.61 (14.52)	54.09 (5.83)	8.92 (0.91)	16,143.3 (2186.49)
7	SG6	89.95 (17.09)	48.020 (5.39)	4.017 (0.29)	11,697.07 (1498.68)	Balustrades, skirtings, wall, partition, framing and external wall boarding, internal wall boarding, slate screens, ceiling strips and soffit battens, door leaves, window and vent sashes, built-in fittings, general furniture and workshop furniture, beading fillets, and general edgings
10	SG5	99.28 (21.44)	60.275 (8.94)	8.395 (1.18)	12,779.77 (1560.43)	Form work, battens, balustrades, skirtings, wall, partition, framing and external wall boarding, internal wall boarding, slate screens, ceiling strips and soffit battens, door leaves, window and vent sashes, built-in fittings, general furniture and workshop furniture, beading fillets, and general edgings
13	SG5	97.83 (34.71)	53.474 (10.02)	5.743 (0.91)	12,765.17 (2661.22)

SG: strength group; MOR: modulus of rupture; MOE: modulus of elasticity; Comp//: compression parallel to grain; Comp⊥: compression perpendicular to grain; (in parenthesis): standard deviation.

.