Physical and mechanical properties of Eucalyptus saligna wood for timber structures

Propriedades físicas e mecânicas da madeira de Eucalyptus saligna para estruturas de madeira

Abstract

Due to its great availability in planted forests in Brazil, Eucalyptus saligna appears as a good species to be exploited, in order to assist in the consumption of wood for construction purposes. The aim of this research was to determine the physical and mechanical properties of Eucalyptus saligna wood species for its use in civil construction. The evaluation was based on 16 physical and mechanical properties obtained according to NBR 7190 (1997) standard. Two values of moisture content were considered: 30% (above the fiber saturation point) and 12% (equilibrium moisture content) according to NBR 7190 (1997). All obtained results obtained were statistically analyzed according to the t-test at the 5% level of significance. In addition, the characteristic strength properties were also determined, for batch classification in the strength classes recommended by the Brazilian standard. Eucalyptus saligna had an apparent density of 0.58 g/cm³ and a basic density of 0.73 g/cm³. The mechanical properties, presented f_c0 and f_c0,k equal to 46.80 and 32 MPa, respectively. The results indicated that Eucalyptus saligna wood can be used in the construction of timber structures as structural member

Keywords: Eucalypt; Strenght; Stiffness; Timber structures

Resumo

A fim de auxiliar no consumo de madeira pela construção civil, o Eucalyptus saligna surge como uma boa oportunidade a ser explorada, devido à sua grande disponibilidade em florestas plantadas no Brasil. O objetivo dessa pesquisa foi determinar as principais propriedades físicas e mecânicas da madeira de Eucalyptus saligna para sua utilização na construção civil. Esta avaliação baseou-se em 16 propriedades físicas e mecânicas obtidas de acordo com a norma NBR 7190 (1997). Dois valores de teor de umidade foram considerados: 30% (acima do ponto de saturação das fibras) e 12% (teor de umidade de equilíbrio) de acordo com a NBR 7190 (1997). Todos os resultados obtidos foram analisados estatisticamente de acordo com o teste t ao nível de 5% de significância. Além disso, também foram determinadas as resistências características para classificação do lote nas classes de resistência recomendadas pela norma. O Eucalyptus saligna apresentou densidade aparente igual a 0,58 g/cm³ e densidade básica igual a 0,73 g/cm³. Em relação às propriedades mecânicas, apresentou f_c0 e f_c0,k iguais a 46,80 e 32 MPa, respectivamente. De acordo com os resultados, foi possível concluir que o Eucalyptus saligna pode ser utilizado na construção de estruturas de madeira como elemento estrutural.

Palavras-chave: Eucalipto; Resistência; Rigidez; Estruturas de madeira

Introduction

Wood is a natural raw material obtained from natural or planted forests (MAZURANA et al., 2011; MATOS et al., 2012; TER STEEGE et al., 2016; TUISIMA-CORAL et al., 2017). In Brazil, Eucalyptus tree genus are utilized for a variety of applications, among them: pulp and paper, charcoal, essential oils, boards, firewood, construction, and furniture (GUIMARÃES JUNIOR et al., 2009, 2011; ALMEIDA; BRITO; PERRÉ, 2010; IWAKIRI et al., 2013; SOARES et al., 2015; MATTOS et al., 2014; NEIVA et al., 2015; BALLESTEROS et al., 2015).

For timber building structures, the knowledge of physical and mechanical properties of wood are very important, for its rational use (CALIL JUNIOR; LAHR; DIAS, 2003; CALIL JUNIOR; MOLINA, 2010; ANDRADE JUNIOR et al., 2014; LAHR et al., 2017a). The very purpose of wood characterization is to evaluate its possible use as a structural member (VIVIAN et al., 2010; CHEN; GUO, 2016, 2017; LAHR et al., 2017b). For this purpose, Brazilian Standard Code NBR 7190 (Associação Brasileira de Normas Técnicas (ABNT, 1997) presents all experimental procedures for the conduction of the testing of the specimens.

Another important characteristic of wood for timber structures is its moisture content (KOLMANN; CÔTÈ, 1968; HERZOG et al., 2000). In general, wood with higher moisture content presents lower mechanical properties (MATOS; MOLINA, 2016; ALMEIDA et al., 2016) and dimensional stability that is also influenced by moisture content (ALMEIDA et al., 2017).

Eucalyptus genus presents a fast growth that causes internal tension (growth stress) (BELTRAME et al., 2015), and besides this, the dry process, must be carefully performed to minimize the occurrence of drying defects, for examples, bows, crooks, top cracks, cups and twists (ELEOTÉRIO et al., 2014; REZENDE et al., 2015). These defect types influence the physical and mechanical properties of wood (LIMA et al., 2004).

Due to the scarce amount of reports available for this species, this research was aimed at determining the physical and mechanical properties of Eucalyptus saligna wood from different planted forests at two moistures content regimes (12 and 30%) for its use as structural timber.

Materials and methods

Eucalyptus saligna wood from a planted forest area in the State of São Paulo, Brazil was used for this research. 20-year old Eucalyptus saligna logs with an average 35 cm diameter were selected for the study. After the primary processing of the logs, the sawn timber boards were submitted to natural drying of the moisture contents adopted for this research: first above the fiber saturation point (30%) and at 12% moisture content as prescribed in NBR 7190 (1997).

Test specimens were prepared for physical and mechanical properties (Table 1) according to the Brazilian Standard, NBR 7190 (1997). For each property studied, moisture content was determined for 12 specimens according to NBR 7190 (1997) (except for the apparent density (ρ_ap) which is always determined at 12% m.c). For this purpose, 372 specimens were prepared. Toughness tests were carried out in Charpy machine according to D143 (AMERICAN…, 1999) and, another test specimen was carried out in AMSLER with load capacity equal to 25,000 kgf.

Table 1
Physical and mechanical properties of Eucalyptus saligna

Properties	Symbol
Apparent density	ρap
Basic density	ρp
Compression parallel to the grain	fc0
Compression perpendicular to the grain	fc90
Tension parallel to the grain	ft0
Tension perpendicular to the grain	ft90
Modulus of rupture in static bending	fM
Modulus of elasticity in compression parallel to the grain	Ec0
Modulus of elasticity in compression perpendicular to the grain	Ec90
Modulus of elasticity in tension parallel to the grain	Et0
Modulus of elasticity in static bending	EM
Shear parallel to the grain	fv0
Cleavage parallel to the grain	fs0
Parallel hardness	fH0
Perpendicular hardness	fH90
Toughness	W

All the results from these sixteen physical-mechanical properties tests, with the exception of the apparent density (considered at 12% moisture content), were analyzed through t-test at a significance level of 5% (or P-value ≤ 0.05), investigating the moisture content influence.

The characteristic strengths were determined using Equation 1, according to NBR 7190 (1997). After determining the strength properties of 12 specimens, the results were placed in ascending order (f₁ <f₂ <f₃ <f_n) for the calculation of the characteristic strength of the wood (f_k), the value of the characteristic strength could not be less than f₁ and 70% of the average value of strength. Characteristic strength values were determined only to Eucalyptus saligna wood at 12% moisture content at 12%, according to NBR 7190 (1997).

Eq. 1 $f_k=\left(2.\frac{f_1+f_2+\dots .+f_{\left(\frac{n}{2}-1\right)}}{\left(\frac{n}{2}-1\right)}-f_{\frac{\mathit{n}}{2}}\right).1,1$

Results and discussions

Tables 2, 3, 4 and 5 present mean values (x), standard deviation (Sd), number of specimens (n) and P-values to physical and mechanical properties for each moisture content (MC), 12 and 30%, for Eucalyptus saligna.

Table 2
Densities of Eucalyptus saligna

Properties	MC (%)	n	x	Sd	P-value
ρap (g/cm3)	12	77	0.73	0.16	---
ρb (g/cm3)	30	73	0.59	0.16	0.6670
	12	77	0.58	0.12

Table 3
Strength properties of Eucalyptus saligna

Properties	MC (%)	n	x	Sd	P-value
fc0 (MPa)	30	75	37.70	7.90	0.0000
	12	71	46.80	14.30
fc90 (MPa)	30	74	4.60	2.10	0.5711
	12	75	4.80	2.20
ft0 (MPa)	30	74	74.50	33.40	0.0014
	12	74	95.50	44.40
ft90 (MPa)	30	77	3.10	1.20	0.0000
	12	77	4.10	1.70
fM (MPa)	30	73	74.10	14.10	0.0004
	12	74	91.60	38.20

Table 4
Stiffness properties of Eucalyptus saligna

Properties	MC (%)	n	x	Sd	P-value
Ec0 (MPa)	30	75	13,076	4,021	0.0023
	12	71	15,261	4,442
Ec90 (MPa)	30	74	458	212	0.5739
	12	75	478	217
Et0 (MPa)	30	74	13,682	3,765	0.0018
	12	74	15,981	4,939
EM (MPa)	30	73	12,306	2,640	0.0666
	12	74	13,313	3,852

Table 5
Mechanical properties of Eucalyptus saligna

Properties	MC (%)	n	x	Sd	P-value
fv0 (MPa)	30	73	11.00	2.10	0.0000
	12	73	14.00	3.90
fs0 (MPa)	30	75	0.65	0.18	0.0000
	12	76	0.88	0.27
fH90 (kN)	30	73	5,160	1,330	0.0047
	12	73	6,330	3,150
fH0 (kN)	30	76	5,580	1,830	0.0095
	12	73	6,770	3,020
W (N∙m)	30	76	10.8	4.20	0.0018
	12	76	13.3	5.40

Basic density decreased from 30 to 12% in about 1.69%. Through the t-test, the ρ_b not suffered the influence by the variation in moisture content (Table 2). Eucalyptus saligna wood studied in this research had an apparent density at 12% moisture content lower than the one determined by Müller (2014) for Eucalyptus benthamii wood (ρ_ap = 0.61 g/cm³). However, the basic density of E. benthamii (ρ_b = 0.52 g/cm³) was lower than the one in this study. The density values obtained in this study were close to those of other Eucalyptus wood species, such as: Eucalyptus grandis (ρ_ap = 0.57 g/cm³), and E. dunnii (ρ_ap = 0.77 g/cm³), E. urophylla (ρ_ap = 0.55 g/cm³) and E. tereticornis (BHAT et al., 1987; EVANGELISTA et al., 2010).

The average density values obtained for Eucalyptus saligna wood species compare well with Brazilian native species, such as Cedro-amargo (Cedrela odorata), Cedro-doce (Cedrella sp.) and Cedrorana (Cedrelinga catenaeformis) (DIAS; LAHR, 2004).

The strength properties of Eucalyptus saligna indicated increases with the moisture content decrease (30 to 12%) such as 19.44% (9.1 MPa) compression parallel to grain, 0.2% (4.17 MPa) compression perpendicular to grain, 21.99% (21 MPa) tension parallel to grain, 24.39% (1 MPa) tension perpendicular to grain and 19.10% (17.5 MPa) in modulus of rupture in static bending. Thus, the t-test analysis showed f_t0, f_t90 and f_c0 suffered influence in their mean values when these properties were submitted to the moisture decrease (P-value ≤ 0.05), similar to the results obtained by Almeida et al. (2016). This situation was not similar in f_c90, because it did not present significant difference in their mean values with the decrease of moisture content (Table 3).

The modulus of elasticity of Eucalyptus saligna indicated increases with the moisture content decrease (30 to 12%) such as 14.31% (2,185 MPa) in compression parallel to grain, 4.15% (20 MPa) in compression perpendicular to grain, 14.39% (2,299 MPa) in tension parallel to grain and 7.56% (1,007 MPa) in E_M. The analysis of t-test identified the E_c0 and E_t0 had significant influence in their properties with moisture content decrease (P-value ≤ 0.05), whereas E_c90 and E_M did not have any significant differences with regard to the decrease of moisture content (Table 4).

Finally, Table 4 revealed increases in the properties of shear parallel to the grain (20.74% or 3 MPa), cleavage parallel to the grain (26.14% or 0.23 MPa), perpendicular hardness (17.58% or 1.19 kN), parallel hardness (18.48% or 1.17 kN) and toughness (18.80% or 2.5 N∙m) with reduced moisture content from 30% to 12% (KOLMANN; CÔTÈ, 1968). The t-test, indicated that all these five properties had significant effect with the reduced moisture content (P-value ≤ 0.05).

The batch of Eucalyptus saligna wood presented a compression strength parallel to grain characteristic value (f_c0,k) equal to 32 MPa (Table 6). Therefore, it was possible to classify this batch into class C30 of the dicotyledons according to the Brazilian Standard Code NBR 7190 (1997). The strength values of Eucalyptus saligna wood reported here was higher than that of Eucalyptus benthamii wood as determined by Müller et al. (2014).

Table 6
Average (x) and characteristic strength (f_k) values of Eucalyptus saligna

Properties	x	fk
fc0 (MPa)	46.80	32.00
fc90 (MPa)	4.80	3.40
ft0 (MPa)	95.50	66.00
ft90 (MPa)	4.10	3.00
fM (MPa)	91.60	64.00
fs0 (MPa)	0.88	0.60
fv0 (MPa)	14.00	12.00

The batch of Eucalyptus saligna wood presented a compression strength parallel to grain characteristic value (f_c0,k) equal to 32 MPa (Table 6). Therefore, it was possible to classify this batch into class C30 of the dicotyledons according to the Brazilian Standard Code NBR 7190 (1997). The strength values of Eucalyptus saligna wood reported here was higher than that of Eucalyptus benthamii wood as determined by Müller et al. (2014).

Lima et al. (2014) reported basic density, compression parallel to the grain and shear parallel to the grain of Eucalyptus resinifera wood, equal to 0.74 g/cm³, 55.87 and 16.16 MPa, respectively, all of which are higher than those determined for Eucalyptus saligna wood.

Almeida and Dias (2016) determined the apparent density and the compression parallel to grain of Lyptus ^® wood species (Eucalyptus hybrid) equal to 0.55 g/cm³ and 53.6 MPa, respectively. Even though the density was low when compared to Eucalyptus saligna, Lyptus presented a f_c0 mean value higher than the batch of Eucalyptus saligna.

Conclusion

The present study concluded that:

1. the batch of Eucalyptus saligna was classified under strength class C30 of the dicotyledons, after a large test campaign, and thus, it can be used safely as a structural member on timber structures;

2. Eucalyptus saligna showed f_c0 equal to 46.80 MPa and apparent density 0.58 g/cm³ at 12% m.c.; and

3. at this reduced moisture content, 11 from 14 evaluated mechanical properties presented significant differences. Four strength properties showed an improvement in their mean values: f_c0, f_t0, f_t90 and f_M. E_c0 and E_t0 showed an increasing trend in their average values with increasing m.c.

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Publication Dates

• Publication in this collection
  Apr-Jun 2019

History

• Received
  17 Dec 2017
• Accepted
  14 July 2018