Physical and mechanical properties of Eucalyptus saligna wood for timber structures

ue 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 fc0 and fc0,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

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, 2016GUO, , 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 ( All the results from these sixteen physicalmechanical 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 <fn) for the calculation of the characteristic strength of the wood (fk), 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).

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.
Basic density decreased from 30 to 12% in about 1.69%.Through the t-test, the ρb 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 (ρap = 0.61 g/cm³).However, the basic density of E. benthamii (ρb = 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 (ρap = 0.57 g/cm³), and E. dunnii (ρap = 0.77 g/cm³), E. urophylla (ρap = 0.55 g/cm³) and E. tereticornis (BHAT et al., 1987;EVANGELISTA et al., 2010).
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 ft0, ft90 and fc0 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 fc90, 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 EM.The analysis of t-test identified the Ec0 and Et0 had significant influence in their properties with moisture content decrease (P-value ≤ 0.05), whereas Ec90 and EM did not have any significant differences with regard to the decrease of moisture content (Table 4).
The batch of Eucalyptus saligna wood presented a compression strength parallel to grain characteristic value (fc0,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).

Conclusion
The present study concluded that: (a) 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; (b) Eucalyptus saligna showed fc0 equal to 46.80 MPa and apparent density 0.58 g/cm³ at 12% m.c.; and (c) at this reduced moisture content, 11 from 14 evaluated mechanical properties presented significant differences.Four strength properties showed an improvement in their mean values: fc0, ft0, ft90 and fM.Ec0 and Et0 showed an increasing trend in their average values with increasing m.c.

Table 6 -Average (x) and characteristic strength (fk) values of Eucalyptus saligna
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 fc0 mean value higher than the batch of Eucalyptus saligna.