Physical and Mechanical Properties Performance between Untreated and Treated with CCA Treatment at Different Age Groups of Fast-Growing Acacia Hybrid of Sarawak

Abstract

An effort was carried out to fully utilise fast-growing Acacia hybrid usage in the timber engineering field; however, the research data are still lacking. This paper aims to evaluate the physical and mechanical properties performance between untreated (control) and treated with 10% copper chrome arsenic of Acacia hybrid collected from Daikin Plantation Sdn. Bhd. Bintulu, Sarawak at air-dry condition at different age groups using the small clear method. Mechanical properties test refers to shear parallel to grain (tangential and radial directions), cleavage (tangential and radial directions), and tension parallel to grain test. Meanwhile, the physical properties test refers to moisture content (MC) and density test. After treatment, mechanical properties increase with an average of 13.67%; meanwhile, moisture content decreased with an average of 0.58% or 0.09% MC, and density slightly increased with an average of 0.44% or 0.002 g/cm³. Results indicate that 10-year-old Acacia hybrid exhibits the highest strength values in shear parallel to the grain, tension parallel to the grain, and cleavage, followed by 13-year-old and 7-year-old. Treated samples in the tangential direction performed better with consistent mean results than that of the untreated samples, while radial direction gave a high average strength increment when treated.

1. Introduction

Timber was used throughout the history of mankind and provided humans with a broad range of building products and construction materials [1]. It is the most sustainable construction material, as it is renewable and absorbs carbon dioxide as it grows [1]. Malaysia is one of the leading producers of the world’s good quality timbers, which are very highly demanded all over the globe [2]. However, over the year, Malaysia was unable to accommodate the huge demand for timber, especially primary hardwood timber, due to a shortage of timber resources. In conjunction with that, Malaysia introduced Acacia mangium for forest plantation species due to its fast-growing rate. Sarawak’s effort, by planting the fast-growing species tree, began in the 1980s, and includes plantations of Acacia mangium with the largest forest plantation area in the country. However, the effort plantations grew and turned out to be prone to several diseases [3].

In a serious effort to meet the current and future raw material demand, the Acacia hybrid was introduced in Sarawak. Acacia hybrid is a cross-breed of Acacia mangium × Acacia auriculiformis and tends to possess many desirable features, such as less tapering, straight bole, fast growth, and heart rot resistance [4]. It was first reported in Sabah in the late 1970s. The wood properties of the Acacia hybrid are similar to those of A. mangium [5]. Its morphological traits, such as flower colour, pod aspect, leaf shape and size, bark aspect, and wood density, are generally an even mixture between its pure parent species [6]. However, it also differs from its parents in several ways. The tree has many small and light branches that can be easily pruned, and the main stem is not as straight as that of A. mangium, but it is much straighter than the main stem of A. auriculiformis [5]. It is reported that the Acacia hybrid shows more excellent resistance to diseases, a higher growth rate and better adaptation to different soil types than the parental species [7]. In Malaysia, heart rot disease was frequently observed in A. mangium, but it was never reported in Acacia hybrid [8]. Acacia hybrid also showed the ability to improve soil properties both physically and chemically [9]. The research on improved species with fast-growing time, durable and high engineering properties, viz., mechanical and physical properties, were carried out to perceive the suitability and potential of the species. Research conducted by Choong et al. [10] found that the improvement of Acacia mangium species to the Acacia hybrid improved the wood quality. The Acacia hybrid is more durable and less susceptible to heart rot disease [11]. The density of the Acacia hybrid was reported as slightly higher; the shape of the log is almost completely round and not susceptible to termite attack [4]. Study in determining the engineering properties of Sarawak species, especially fast-growing timber, to determine its strength grouping still at the beginning stage. There are more species to be explored and timber engineering research approaches to be done before the timber engineering properties of the species can be introduced in engineering design or other related fields for its various utilisation and application. Thus this has led to lacking knowledge in timber properties and design among architects and engineers [2]. Little is known about the mechanical and physical properties of the Acacia hybrid, especially in the timber engineering field, and the utilisation of this species is only limited to furniture, pulp, and paper industries. It is an alternative to evaluate the mechanical properties of Acacia hybrid and introduce this species as a building material in the timber engineering field. As mentioned by Treza et al. [12], fast-growing wood is an alternative solution for replacing the function of broadleaf plants as a material for floor, furniture, interior elements, and as structural components. Additionally, this research information will be used to utilise timber species more efficiently and effectively. Wood’s strength properties are classified according to its moisture content, density, durability, and grain direction [13]. Timber’s strength properties are essential to determining timber applications in real life, whether suitable for furniture, outdoor, or structural. The stronger the strength of the timber, the heavier the loading it will be able to withstand.

This study was conducted to evaluate and compare the behaviours of Acacia hybrid’s engineering properties between untreated and treated samples under 10% of the copper chrome arsenic (CCA) treatment at different age groups using a small clear method. Copper chrome arsenic (CCA) treatment was used in this study since this treatment is widely used in the timber industry in the country [14]. Due to the environmental impact of timber engineering applications, understanding the treated timber for durability is very important. To achieve the aim of this study, several objectives should be considered. Firstly, evaluate the behaviours of physical and mechanical strength performance between untreated and treated Acacia hybrid in different age groups at air-dry conditions. Secondly, compare the mechanical properties of Acacia hybrid in different age groups and grain directions. Third, identify the age group with the best performance in mechanical and physical properties treated or untreated.

2. Materials and Methods

The sample for the experiment used was Acacia hybrid species, and it was taken from Daikin Plantation Sdn. Bhd. located at Bintulu, Sarawak (Figure 1a). There are three age groups of Acacia hybrid collected, which are the age groups of 7-year-old, 10-year-old, and 13-year-old. The selected trees must be free from any decay, termites, and defects. However, there is an exception for the knots on the trees. The trees were cut approximately 30 centimetres above the ground, with the diameter at breast height over the bark of trees approximately 20 cm at the height of 1.3 m from the root collar.

All the collected logs were transferred to Samling Plywood Bintulu Sdn. Bhd. for the sawing process. The processed samples were in plank size and were transferred to Sarawak Forestry Corporation, Kuching (Figure 1b). All of the processed green condition samples were appropriately stacked and allowed for the natural air-drying process placed in a shed air-drying room at AFSID Sarawak Forestry Corporation. The samples were stacked according to their age group. The natural air-drying process was chosen for this study to minimise the strength of the wood failures, such as various splits and cracks [15], as shown in Figure 1c. This air-drying process for this species took about one year to reach an approximate moisture content of 19%. The air-drying process depends on the type of species. Ismaili [16] conducted a study on Acacia mangium where the air-drying process took more than nine months to reach 19% moisture content. The plank’s moisture content (MC) under natural air-drying needs to be checked using electric moisture metres before cutting it into test samples. The moisture content (MC) must be less than 19% for the test at air-dry condition.

Two types of samples were prepared for each age group shear parallel to grain test, tension parallel to grain test, and cleavage test. These samples were prepared in small clear sizes under untreated and treated samples at air-dry condition. It was understood that timber is a heterogeneous or nonhomogeneous material; thus, the most suitable sample to be tested was suggested using a small clear sample, which is defect-free [17,18,19,20]. The samples are prepared in accordance with Tan et al. [21], which is adopted from BS 373:1957 [22]. The samples for shear (Figure 2a), cleavage (Figure 3a), and tensile (Figure 4a) are cut from a stick with a cross-sectional dimension of 30 mm × 30 mm. The sticks are yielded from the flitches by reaping the 2.5 m of log sample using a band saw. Due to its hygroscopic behaviour [18,23,24], to prevent the dry condition of timber, the flitches are left in the air-conditioned room and air-dried to ensure its moisture content is less than 19%. Samples will be prepared in solid untreated and solid treated, with different age groups (Figure 5). A total of 3600 samples for untreated and treated samples were prepared for these three tests. The mechanical properties test was designated with 1200 samples, with 40 samples for each test and 2400 samples for the physical properties test, with 200 samples for each test.

2.1. Moisture Content Determination

The moisture content is according to the BS 373: 1957 [22] and Tan et al. [21]. Samples were dried in an oven with a temperature of 103 ± 3 °C for 24 h, or until a constant weight was obtained. In order to achieve accurate results, the sample should be kept in a desiccator before reweighing. The samples were weighed again in order to obtain the oven-dry weight with MC ≤ 19% by using an electronic balance to obtain the initial weight of Acacia hybrid samples. Samples of 1200 test pieces of size 20 × 20 mm were used. The moisture content was calculated using the formula given:

MC = (Initial weight − Oven-dried weight (g))/(Oven-dried weight (g)) × 100%

(1)

2.2. Density Determination

According to BS 373: 1957 [22], a density determination test was conducted using the moisture content determination test samples. The prepared samples’ dimensions are measured using vernier calipers to obtain the green volume of the Acacia hybrid. A total of 1200 samples were dried in an oven with a temperature of 103 ± 3 °C for 24 h or until a constant weight was obtained. The samples were weighed to obtain oven-dry weight with MC ≤ 19%. The basic density was calculated using the formula given:

Density = (Oven-dried weight (g))/(Green volume of sample (cm³))

(2)

2.3. Treatment Process

The treatment process of the sample using copper chrome arsenic (CCA) is according to Tan et al. [21]. A total of 1800 samples of Acacia hybrid at three different age groups were treated using copper chrome arsenic (CCA) treatment, as shown in Figure 5a,b. Test samples underwent a full-cell process using 10% of the copper chrome arsenic (CCA) solution to achieve maximum absorption into the wood. The process was started by performing the initial vacuum, where −85 kPa was subjected to the samples for 1 h. The preservative pumped into the treatment cylinder with the vacuum was maintained at −85 kPa. Then, 200 psi (14 bars) of hydraulic pressure was subjected to samples for 2 h. The copper chrome arsenic (CCA) solution was drained out from the treatment cylinder after the pressure period was completed. The final vacuum was subjected to samples at −85 kPa for 30 min to remove excess copper chrome arsenic (CCA) solution in the samples.

2.4. Mechanical Testing

All testing was conducted based on BS 373: 1957 [22] and Tan et al. [21]. This study has three types of mechanical strength tests: shear parallel to grain, tension parallel to grain, and cleavage test. The tests were performed using the Instron 5569 universal testing machine to determine the samples’ strength value. The shear parallel to grain test was carried out by a constant loading speed at 0.6 mm per minute on the samples. This test aims to measure the maximum dividing load by the cross-section area when the load is applied parallel to grain direction of timber. The samples are prepared with dimensions 20 mm × 20 mm × 20 mm. The shear failures are shown along a plane parallel to the tangential and radial directions. Shear stress at maximum load in megapascal (MPa) is calculated by using the formula as follows:

Shear stress at maximum load = (Force (N))/(Area (mm²)).

(3)

The cleavage test aims to measure the resistance of timber to splitting based on the failure caused by the maximum load on the test samples. The samples are prepared with a dimension of 20 mm × 20 mm × 45 mm. The load is applied to the cleavage sample at a constant crosshead speed of 2.5 mm per minute to give a failure along the tangential and radial surface. The test layout is shown in Figure 3a,b. The maximum splitting load in N/mm is calculated by using the formula given:

Load per mm of width to resist splitting = (Max. load (N))/(breath (mm)).

(4)

The tension parallel to grain test is conducted to determine the maximum loadable to cater by sample before tensile failure occurs. This test will give the maximum load and maximum tensile stress exerted by the samples. In correlation with that, the samples are subjected to load at constant head speed until the samples break. Both ends of the test samples are held by toothed and self-aligning grips, as shown in Figure 4a,b. The samples are a break in 1.5 to 2 min from the start of loading. The failure of tension parallel to grain happened at the minimum cross-section of the test sample. Tensile stress at maximum load in MPa is calculated by using the formula given:

Tensile stress at maximum load = ((Force (N))/((Area (mm²)).

(5)

3. Results

From this study,

Table 1. Summary of the mean mechanical properties of Acacia hybrid.

Condition		Unit	Controlled			Treated
Age Groups			7	10	13	7	10	13
Mechanical properties	Shear parallel to grain (tangential)	Nos	40	40	40	40	40	40
		MPa	17.06	18.18	17.61	17.52	18.59	17.67
		SD	1.55	1.85	2.62	1.85	1.86	2.17
		CV %	9.1	10.18	14.91	10.55	10	12.3
		SE	0.25	0.29	0.42	0.29	0.29	0.34
		95% CI	0.50	0.59	0.84	0.59	0.59	0.70
	Shear parallel to grain (radial)	Nos	40	40	40	40	40	40
		MPa	16	17.46	16.62	16.78	17.92	16.88
		SD	2.06	2.09	2.64	1.74	1.92	2.2
		CV %	12.86	11.95	15.88	10.38	10.71	13.06
		SE	0.33	0.33	0.42	0.28	0.30	0.35
		95% CI	0.66	0.67	0.84	0.56	0.61	0.71
	Cleavage (tangential)	Nos	40	40	40	40	40	40
		N/mm	15.82	17.66	16.19	16.69	18.12	16.86
		SD	2.68	2.52	1.98	2.27	3.16	2.1
		CV %	16.94	14.29	12.21	13.62	17.43	12.43
		SE	0.42	0.40	0.31	0.36	0.50	0.33
		95% CI	0.85	0.80	0.63	0.72	1.00	0.66
	Cleavage (Radial)	Nos	40	40	40	40	40	40
		N/mm	13.67	15	14.19	14.61	16.41	15.41
		SD	2.57	2.09	2.16	2.75	2.75	1.68
		CV %	18.78	13.9	15.18	18.79	16.77	10.91
		SE	0.41	0.33	0.34	0.43	0.44	0.27
		95% CI	0.81	0.66	0.68	0.87	0.87	0.53
	Tension parallel to grain	Nos	40	40	40	40	40	40
		MPa	141.8	148.99	144.19	158.4	176.44	160.26
		SD	24.26	22.33	17.49	17.56	27.08	11.94
		CV %	17.11	14.99	12.13	11.08	15.35	7.45
		SE	3.84	3.53	2.77	2.78	4.28	1.89
		95% CI	7.67	7.06	5.53	5.55	8.56	3.78

and

Table 2. Summary of the mean moisture content and density of Acacia hybrid.

Condition		Unit	Controlled			Treated
Age Groups			7	10	13	7	10	13
Physical properties	Moisture content	Nos	200	200	200	200	200	200
		%	15.56	15.44	15.52	15.48	15.32	15.45
		SD	0.31	0.27	0.28	0.31	0.29	0.20
		CV %	2.01	1.74	1.80	1.99	1.87	1.32
		SE	0.02	0.02	0.02	0.02	0.02	0.01
		95% CI	0.04	0.04	0.04	0.04	0.04	0.03
	Density	Nos	200	200	200	200	200	200
		g/cm3	0.503	0.479	0.507	0.503	0.482	0.511
		SD	0.09	0.09	0.08	0.08	0.08	0.08
		CV %	17.03	18.83	16.67	15.91	17.37	16.42
		SE	0.01	0.01	0.01	0.01	0.01	0.01
		95% CI	0.01	0.01	0.01	0.01	0.01	0.01

show the mean value with a 95% coefficient interval of physical and mechanical properties taken during the experiment accordingly to the condition of the samples that are untreated and treated. The results of the Acacia hybrid at an untreated condition show that the 7-year-old age group sample recorded a higher mean value of 15.56% MC with 95% coefficient interval value of 0.04, followed by the 13-year-old sample that recorded 15.52% MC with 95% coefficient interval value of 0.04, and the 10-year-old sample that recorded 15.44% MC with 95% coefficient interval value of 0.04. The results of the Acacia hybrid under the treated condition show a similar pattern, where the 7-year-old age group sample recorded a higher mean value of 15.48% MC with 95% coefficient interval value of 0.04, followed by the 13-year-old sample that recorded 15.45% MC with a 95% coefficient interval value of 0.03, and the 10-year-old sample that recorded 15.32% MC with 95% coefficient interval value of 0.04. After the samples were treated with 10% copper chrome arsenic, the moisture content value for the Acacia hybrid at air-dry condition from each testing sample at different age groups showed a decrement with an average of 0.58% or 0.09% MC. It was recorded that moisture content in the age group 10-year-old sample decreased more moisture content with 0.76% or 0.12% MC, followed by the 7-year-old sample with 0.52% or 0.08% MC, and the 13-year-old sample with 0.45% or 0.07% MC, which can be clearly observed in Figure 6. The decreasing moisture content from untreated to treated samples was observed in this study. The study by Ferreira et al. [25] shows that CCA treatment increased the moisture content from an untreated to treated sample of plywood panel from Pinus taeda wood. However, previous work by Epmeier et al. [26] and Bruno et al. [27] has similar results to this study, where pine species were treated by furfurylation treatment, showing that the moisture content decreased when treated.

For density, untreated Acacia hybrid shows that the 13-year-old sample recorded higher mean value 0.507 g/cm² with a 95% coefficient interval value of 0.01, followed by the 7-year-old sample, which recorded 0.503 g/cm² with 95% coefficient interval value of 0.01, and the 10-year-old sample, which recorded 0.479 g/cm² with 95% coefficient interval value of 0.01. The mean results of the Acacia hybrid under treated condition show a similar pattern as of untreated Acacia hybrid, with the 13-year-old age group sample recording a higher mean value of 0.511 g/cm² with a 95% coefficient interval value of 0.01, followed by the 7-year-old age group sample, which recorded 0.503 g/cm² with a 95% coefficient interval value of 0.01, and the 10-year-old age group sample recorded a lower mean value of 0.482 g/cm² with s 95% coefficient interval value of 0.01. As a result of the decrement in moisture content value, the density of samples increased after the sample was treated. The density result observed for each testing sample in different age groups is in the range of 0.479 g/cm³ to 0.507 g/cm³ for untreated and 0.482 g/cm³ to 0.511 g/cm³ for untreated samples. It was recorded that 13-year-old density increased more by 0.78% or 0.004 g/cm^3, followed by the 10-year-old increase by 0.62% or 0.003 g/cm³, and the 7-year-old age group sample remained unchanged, which can be clearly observed in Figure 7. From these results, we can observe that the reduction percentage in treated samples’ moisture content led to an increase in density. According to Ismaili [16], the moisture content is reported to greatly influence the mechanical strength of the timber. The more the timber dried, the greater the timber’s strength. This shows that changing moisture content directly influences the basic density, and this is supported by the study conducted by Alik and Naohiro [28]. The increment of strength is due to the reduction in moisture content in timber because of the shortening and, consequently, strengthening of hydrogen bonds linking together the microfibrils in the timber [17,29]. Therefore, high basic density has low moisture content and vice versa. Although the timber density is relatively reflected in the strength of the timber, it should not be the definitive measurement of its strength [19].

3.1. Shear Parallel to Grain

As shown in Table 1, the mean value was observed with a 95% coefficient interval. For a shear parallel to grain tangential direction, the 10-year-old group exhibited the highest shear strength for both untreated and treated samples. For the untreated 10-year-old sample, the mean value was recorded as 18.18 MPa with a 95% coefficient interval value of 0.5 and the shear strength increased by 2.26% or 0.41 MPa to 18.59 MPa with a 95% coefficient interval value of 0.59. This was followed by the untreated 13-year-old and 7-year-old samples. The untreated 13-year-old sample was recorded with a mean value of 17.61 MPa with a 95% coefficient interval value of 0.84 and slightly increased by 0.34% or 0.06 MPa to achieve 17.67 MPa with a 95% coefficient interval value of 0.7 when treated. The 7-year-old untreated sample recorded a mean value of 17.06 MPa with a 95% coefficient interval value of 0.5 and increased by 2.7% or 0.46 MPa higher than the 10-year-old to achieve 17.52 MPa with a 95% coefficient interval value of 0.59 when treated.

The results in shear parallel to grain radial direction show a similar pattern, where the untreated 10-year-old sample recorded a higher mean value of 17.46 MPa with a 95% coefficient interval value of 0.67. The shear strength increased by 2.63% or 0.46 MPa to achieve 17.92 MPa with a 95% coefficient interval value of 0.61 when being treated. This was followed by an untreated 13-year-old sample with a mean value of 16.62 MPa with a 95% coefficient interval value of 0.84, and shear strength increased by 1.56% or 0.26MPa to achieve 16.88 MPa when treated. Meanwhile, the untreated 7-year-old recorded 16 MPa with a 95% coefficient interval value of 0.66, and a huge increment was spotted with 4.88% or 0.78 MPa to achieve 16.78 MPa with a 95% coefficient interval value of 0.56 when the sample was treated. From this result, it can be concluded that the CCA treatment improves the shear strength parallel to grain for both grain directions when samples were treated for all age groups, which can be observed clearly in Figure 8. The previous study by Andy [30] also shows a similar trend, where southern pine shear strength increased when treated with CCA. The outcome from this study also shows a similar strength pattern for each age group to the study conducted by Alik et al. [31], where the highest shear strength value is the 10-year-old group, followed by the 13-year-old and 7-year-old group.

From observation, the mean values obtained from this study show that shear parallel to grain tangential direction gives higher shear strength than the radial direction, and this finding was supported by Ismaili et al. [3] and Gaddafi [18] in their research. Compared to the percentage difference of strength increment, shear parallel to grain at tangential direction increased by 1.76% or 0.31 MPa after treatment, which is slightly higher than at radial direction with an increment of 1.43% or 0.19 MPa. The result also shows an improvement in shear strength in both directions, especially in the radial direction. This means the treatment process improved the shear parallel to the grain of the Acacia hybrid species, especially in the radial direction. As Ismaili et al. [3] mentioned in their research, this could be due to differences in the proportion of major wood constituents, such as cellulose, hemicelluloses, and lignin present in the woods or differences in extractive contents. Riyanto and Gupta [32] also explained that, although ring orientation was strong, inconsistent factors affected shear strength parallel to the grain. Moreover, other factors affect shear strength, such as moisture content. According to Table 2, the 10-year-old Acacia hybrid sample exhibited the lowest moisture content, where moisture content is reported to significantly influence timber’s mechanical strength [28,31,33,34]. Madsen [35], in his study, found that moisture content significantly affects shear strength. According to Jamil [36], density is greatly influenced by the amount of moisture content in the timber. The presence of moisture in the wood not only increases the mass, but also the volume of timber. From this study, the 7- and 13-year-old age group sample has higher density compared to the 10-year-old age group sample. Similarly, the percentage of moisture content for both age groups was also higher compared to 10 years old. However, compared to mechanical or strength properties, the 10-year-old Acacia hybrid sample recorded higher values for all tests. Thus, it is agreed that the density of wood should not be the definite measurement of its strength [37]. McKenzie [33] reported that heartwood tissue’s presence provides mechanical rigidity and strength to timber. From this study, the age group of 10 years old possessed the highest strength properties value as compared to other age groups. Therefore, from this study, 10-year-old timber has more heartwood tissue presence than 13-year-old and 7-year-old timber. From this study, the highest mean value of shear parallel to grain is from the 10-year-old age group. Further analysis was conducted using statistically significant difference (p < 0.5) to observe the error bars between the 10-year-old age group and other age groups, grain directions, and treatment conditions of Acacia hybrid samples, as shown in Figure 8. The observation will later be computed with the number of frequencies in percentage having the condition statistically significant difference (p < 0.5), and the results will be tabulated in

Table 3. ANOVA on the effect of age groups, treatment conditions, and grain directions with 10-year-old untreated and treated for shear parallel to grain of Acacia hybrid samples at air-dry condition.

Age Group	Treatment Condition	Shear Parallel to Grain Direction	Age Groups	Treatment Condition (UT, TD)	Significant Difference p-Values
10	UT	T	7	UT	0.0044 *
			10	TD	0.3157 ns
			13	UT	0.2687 ns
		R	7	UT	0.0023 *
			10	TD	0.3040 ns
			13	UT	0.1191 ns
	TD	T	7	TD	0.0112 *
			13	TD	0.0432 *
		R	7	TD	0.0067 *
			13	TD	0.0275 *
Number of frequencies having condition statistically significant difference (p < 0.05), %					60.0

T—Tangential direction; R—Radial direction; UT—Untreated (Control); TD—Treated; ns—not significant; * Statistically significant difference at 95% (p < 0.05).

. For a shear parallel to grain at tangential direction, the untreated 10-year-old sample has a statistically significant difference (p < 0.5) with the 7-year-old (p = 0.0044). A similar pattern was observed for the 10-year-old Acacia hybrid untreated sample at a radial direction has a statistically significant difference (p < 0.5) with the 7-year-old (p = 0.0023) sample. The treated 10-year-old Acacia hybrid sample in the tangential direction also has a statistically significant difference (p < 0.5) when compared with the 7-year-old (p = 0.0112) and 13-year-old (p = 0.0432) treated samples at the tangential direction. Meanwhile, shear parallel to grain in the radial direction, indicated that the 10-year-old Acacia hybrid treated sample has a significant difference with the 7-year-old (p = 0.0067) and 13-year-old (p = 0.0275) samples. Statistically significant difference analysis observed no significant difference (p = 0.3157) between the treated 10-year-old sample compared with the untreated 10-year-old Acacia hybrid of shear parallel to grain in the tangential direction sample. Similarly, the treated 10-year-old sample compared with the untreated 10-year-old sample has no significant difference for shear parallel to grain in radial direction. The analysis shows that a 10-year-old Acacia hybrid sample at shear parallel to grain yield a 60% statistically significant difference (p < 0.05) when compared with other age groups, grain directions, and treatment conditions, which can be clearly observed in Table 3. Therefore, concrete evidence from this analysis shows that the 10-year-old Acacia hybrid of shear parallel to grain sample has a statistically significant difference (p < 0.05), especially in the tangential direction at the treated condition.

3.2. Cleavage

Figure 9 was observed with a 95% coefficient interval for cleavage tangential direction. It was reported that the untreated 10-year-old sample exhibited the highest cleavage strength of 17.66 N/mm, with a 95% coefficient interval value of 7.06, followed by the 13-year-old and 7-year-old samples, with a mean value of 16.19 N/mm with a 95% coefficient interval value of 0.563 and 15.82 N/mm, with a 95% coefficient interval value of 0.85, respectively. The cleavage strength increased when samples were treated. This can be observed in the 10-year-old age group sample, which achieved a higher mean value of 18.12 N/mm with a 95% coefficient interval value of 1 with 2.60% or 0.46 N/mm increment after the samples were treated. A similar pattern was observed in the treated 13-year-old sample with an increment of 4.14% or 0.67 N/mm to achieve 16.86 N/mm with a 95% coefficient interval value of 0.66. This is followed by a treated 7-year-old sample with an increment of 5.50% or 0.87 N/mm to achieve 16.69 N/mm with a 95% coefficient interval value of 0.72. These results are supported by the findings from Alik et al. [31], which also stated that the 10-year-old age group sample exhibited the highest cleavage strength at air-dry condition, followed by the 13-year-old and 7-year-old age group samples. This gave an average increment after the sample was treated with 4.08% or 0.66 N/mm.

For cleavage in the radial direction, the same trend was observed correspondingly with the results in the tangential direction, where the untreated 10-year-old sample recorded a higher mean value of 15 N/mm with a 95% coefficient interval value of 0.66 followed by the 13-year-old and 7-year-old sample, each recorded with 14.19 N/mm with a 95% coefficient interval value of 0.68 and 13.67 N/mm with a 95% coefficient interval value of 0.81, respectively. For the treated sample, the 10-year-old was also recorded with a higher mean value with an increment of 9.40% or 1.41 N/mm to achieve 16.41 N/mm with a 95% coefficient interval value of 0.87, followed by the 13-year-old and 7-year-old age groups, each recorded with an increment of 8.6% or 1.22 N/mm to achieve 15.41 N/mm with a 95% coefficient interval value of 0.53 and an increment of 6.88% or 0.94 N/mm to achieve 14.61 N/mm with a 95% coefficient interval value of 0.87, respectively. It can be observed that the cleavage results show a high percentage of increment in radial direction after the sample was treated with 8.33% or 1.19 N/mm compared with the tangential direction, which is 4.08% or 0.67 N/mm. However, comparing the results obtained between the mean value of cleavage in both radial and tangential for untreated and treated samples, the mean value obtained in the tangential direction for all age groups was higher than in the radial direction. From the results, it can be concluded that the 10-year-old age group sample had the highest mean value for both untreated and treated at cleavage in the tangential direction. This was confirmed by Moya and Muñoz [38] in their research, where cleavage in the tangential direction gives a higher strength mean value compared with the radial direction. According to Wallis [34], the lowest value cleavage in the radial direction compared with the tangential direction was due to the relationship between air-dry density and the cleavage strength of timber along the fibres.

The study indicated that the highest mean value of cleavage is from the 10-year-old age group. When the results were analysed using statistically significant difference (p < 0.05), a similar pattern as in the shear parallel to grain results was observed in

Table 4. ANOVA on the effect of age groups, treatment conditions, and grain directions with 10-year-old untreated and treated cleavage of Acacia hybrid samples at air-dry condition.

Age Group	Treatment Condition	Cleavage to Grain Direction	Age Groups	Treatment Condition (UT, TD)	Significant Difference p-Values
10	UT	T	7	UT	0.0023 *
			10	TD	0.4788 ns
			13	UT	0.0046 *
		R	7	UT	0.0128 *
			10	TD	0.0117 *
			13	UT	0.0921 ns
	TD	T	7	TD	0.0227 *
			13	TD	0.0383 *
		R	7	TD	0.0045 *
			13	TD	0.0527 ns
Number of frequencies having condition statistically significant difference (p < 0.05), %					70.0

T—Tangential direction; R—Radial direction; UT—Untreated (Control); TD—Treated; ns—not significant; * Statistically significant difference at 95% (p < 0.05).

. The statistically significant difference (p < 0.5) analysis will be carried out with the highest mean value obtained by the 10-year-old sample compared with other age groups for both grain directions, untreated, and treated samples. For the untreated Acacia hybrid of cleavage in the tangential direction sample, the 10-year-old sample has a significant difference (p < 0.5) when compared with the 7-year-old (p = 0.0023) and 13-year-old (p = 0.0046). Meanwhile, for the untreated Acacia hybrid of cleavage in the radial direction sample, the 10-year-old sample has a significant difference (p < 0.5) when compared with untreated 7-year-old (p = 0.0128) and treated 10-year-old (p = 0.0117) Acacia hybrid sample. Furthermore, for the treated Acacia hybrid of cleavage in the tangential direction sample, the 10-year-old sample has a statistically significant difference (p < 0.5) when compared with the 7-year-old (p = 0.0227) and 13-year-old (p = 0.0383). Meanwhile, for the treated Acacia hybrid of cleavage in the radial direction sample, the 10-year-old sample significantly differs only from the 7-year-old (p = 0.0045) sample. Statistically significant difference analysis observed no significant difference (p = 0.4788) between the treated 10-year-old sample compared with the untreated 10-year-old Acacia hybrid of cleavage in the tangential direction sample. However, there is a significant difference (p = 0.0117) between treated 10-year-old samples compared with the untreated 10-year-old Acacia hybrid of cleavage in the radial direction sample. The analysis shows that the 10-year-old Acacia hybrid for the cleavage sample yields a 70% statistically significant difference (p < 0.05) when compared with other age groups, grain directions, and treatment conditions, which can be clearly seen in Table 4. Therefore, it can be concluded that the 10-year-old Acacia hybrid of the cleavage sample has a statistically significant difference (p < 0.05), especially in the tangential direction at the treated condition.

3.3. Tension Parallel to Grain

As shown in Figure 10, the mean results observed with a 95% coefficient interval reported that the 10-year-old has the highest strength mean value for the untreated sample with 148.99 MPa with a 95% coefficient interval value of 0.8, followed by the 13-year-old and 7-year-old samples with 144.19 MPa with a 95% coefficient interval value of 5.53 and 141.80 MPa with a 95% coefficient interval value of 7.67, respectively. When the sample was treated, the treated 10-year-old sample also recorded with the highest mean value increased by 18.42% or 27.45 MPa to achieve 176.44 MPa with a 95% coefficient interval value of 8.56, followed by 13-year-old and 7-year-old age group samples, which each recorded increments of 11.15% or 16.07 MPa to achieve 160.26 MPa with a 95% coefficient interval value of 3.78 and increment of 11.71% or 16.6 MPa to achieve 158.40 MPa with a 95% coefficient interval value of 5.55, respectively. The treated 10-year-old sample shows excellent increment value with an average mean difference of 10.7% or 17.11 MPa more compared to both the 13-year-old and 7-year-old samples. As expected, untreated samples in all age groups increased their strength value when treated. It can be observed that the average increment in strength value from the untreated sample to the treated sample for all age groups increased with the average increment in strength value of 13.8% or 20.04 MPa, whereby the 10-year-old age group sample is more prominent when being treated. These results, supported by findings from Alik et al. [31], stated that the 10-year-old age group sample exhibited the highest strength at the air-dry condition, followed by the 13-year-old and 7-year-old age group.

Similarly, this study’s 10-year-old age group sample also recorded the highest tensile strength value. Thus, the statistically significant difference (p < 0.5) analysis will be carried out with the highest mean value obtained by the 10-year-old age group sample compared with other age groups, untreated, and treated samples. For the untreated Acacia hybrid sample, the 10-year-old age group sample has no statistically significant difference (p < 0.5) when compared with 7-year-old and 13-year-old age group samples. However, for untreated 10-year-old Acacia hybrid tensile sample has a statistically significant difference (p < 0.5) when compared with the treated 10-year-old (p = 0.0000) sample. Meanwhile, for the treated Acacia hybrid sample, the 10-year-old age group sample has a statistically significant difference (p < 0.5) when compared with 7-year-old (p = 0.0007) and 13-year-old (p = 0.0009) samples. Therefore, from this analysis, the 10-year-old age group Acacia hybrid of the tensile sample yields a 60 % statistically significant difference (p < 0.05) when compared with other age groups and treatment conditions, which can be observed in

Table 5. ANOVA on the effect of age groups and treatment conditions with 10-year-old Acacia hybrid untreated and treated for tensile samples at air-dry condition.

Age Group	Treatment Condition	Age Groups	Treatment Condition (UT, TD)	Significant Difference p-Values
10	UT	7	UT	0.1719 ns
		10	TD	0.0000 *
		13	UT	0.2879 ns
	TD	7	TD	0.0007 *
		13	TD	0.0009 *
Number of frequencies having condition statistically significant difference (p < 0.05), %				60.0

T—Tangential direction; R—Radial direction; UT—Untreated (Control); TD—Treated; ns—not significant; * Statistically significant difference at 95% (p < 0.05).

. From the study conducted, we can observe that the 10-year-old age group sample constantly recorded with higher strength value as compared with the 7-year-old and 13-year-old age group samples, although the density of the 10-year-old age group sample was slightly lower with an average of 5.03%. However, the result contradicts the study conducted by Alik and Kuroda [28], where basic density is a strong indicator correlated to mechanical strength properties. Therefore, this revealed that timber density should not be the definitive measurement of its strength [19].

4. Conclusions

Based on the mean results obtained in this study for the moisture content, density, shear parallel to grain, cleavage, and tensile properties of Acacia hybrid at different age groups under 10% of the copper chrome arsenic (CCA) treatment at air-dry condition, the following conclusions are made.

• The moisture content for the Acacia hybrid decreases when the sample is treated with an average 0.58% or 0.09% MC, whereas the treated 10-year-old sample recorded a higher mean value due to a decrement of 0.76% or 0.118% MC. The density of Acacia hybrid shows that, as a result of the sample being treated, the mean density increased with an average of 0.44% or 0.002 g/cm³, whereas the treated 13-year-old sample recorded a higher increment value of 0.74% or 0.004 g/cm³. Similarly, mechanical properties also increased with an average 6.18% increment of strength value when treated. Tension value recorded a huge average increment of 13.67% when treated.
• The tangential direction gives a high mean value in both untreated and treated samples for shear parallel to grain and cleavage. However, in terms of the percentage difference of strength increment from untreated to treated samples, radial direction samples show significant improvement where it recorded a higher percentage increment with an average of 3.02% or 0.5 MPa and 8.29% or 1.19 MPa for shear parallel to grain and cleavage, respectively. The 10-year-old age group sample showed a significant increment of strength in both the radial and tangential direction when treated, followed by 13-year-old and 7-year-old age group.
• Findings revealed that the mechanical and physical properties of Acacia hybrid obviously perform better in the 10-year-old age group, followed by the 13-year-old and 7-year-old age groups. It is demonstrated that the mechanical properties of Acacia hybrid serve better after being treated using CCA treatment.
• It is recommended that the air-dried 10-year-old Acacia hybrid is the most suitable with consistent physical property values and mechanical strength values in the shear parallel to grain at the tangential direction, cleavage in the tangential direction, and tension when treated with copper chrome arsenic (CCA) treatment. However, the radial direction selection can also be considered due to its greater percentage difference in strength increment when being treated.

To predict the treated sample’s physical and mechanical properties under 10% chrome arsenic (CCA) treatment at air-dry condition, it can be calculated using the mean value of the untreated sample using the equation in

Table 6. Calculation of mean value of engineering properties of treated Acacia hybrid sample under 10% chrome arsenic (CCA) treatment at air-dry condition using the mean value of the untreated sample.

Engineering Properties	Mean Value for Untreated Sample		Unit	Age of Sample Factor, Yi			Mean Value for Treated Sample with 10% CCA
				Y7	Y10	Y13
Physical properties (Air-dry)	MCC	Moisture content	%	0.995	0.992	0.996	MCC × Yi
	DC	Density	g/cm3	0.999	1.006	1.007	DC × Yi
Strength Properties (Air-dry)	S//TC	Shear Parallel to Grain (Tangential)	MPa	1.027	1.023	1.003	S//TC × Yi
	S//RC	Shear Parallel to Grain (Radial)	MPa	1.049	1.026	1.016	S//RC × Yi
	TC	Tension	MPa	1.117	1.184	1.111	TC × Yi
	CLTC	Cleavage (Tangential)	N/mm	1.055	1.026	1.041	CLTC × Yi
	CLRC	Cleavage (Radial)	N/mm	1.069	1.094	1.086	CLRC × Yi

below.