1. Introduction

Wood is used as raw material in several sectors of economy, such as cellulose and paper production [1] and oriented strand board panels (OSB) [2]. For each purpose, their properties should be estimated, due to the variability found among different species [3] and, within the species itself, due to anatomical factors [4], edaphoclimatic [5] and moisture content [6].

For wood use as structural element in construction of bridges [7], trusses [8] among others [9], physical and mechanical properties are the most important [10-12].

In Brazil, the code ABNT NBR 7190:1997 [13] governs design of timber structural elements considering physical and mechanical properties determined by the procedures in its annex B.

Among species that can be used in civil construction, the most important are those belonging to genus Eucalyptus and Corymbia [14, 15]. Because of their rapid growth, eucalypt species have emerged among the major alternatives for lumber production in Brazil for the coming years [16].

Eucalyptus umbra is a species from Australia, with little diametric, Specific gravity, chemical constituents and anatomical characteristics variabilities [17]. In addition, Eucalyptus umbra is resistant to attack by bio-deterioration agents [18].

This study aims to investigate the possibility of Eucalyptus umbra wood use in timber structures, through its physical and mechanical properties, estimated in two different moisture contents (12% and 30%) according to Brazilian code [13].

2. Materials and Methods

Eucalyptus umbra wood samples had origin in five logs collected in a planted forest at Rio Claro, Sao Paulo state, Brazil. These trees presented diameters from 0.17 to 0.26 m and 28 years old.

According to experimental procedures recommended by Brazilian code [13], the following physical and mechanical properties were estimated for Eucalyptus umbra wood:

- Specific gravity (ρ_b);

- Density (ρ_ap);

- Compression parallel to grain strength (f_c0);

- Compression perpendicular to grain strength (f_c90);

- Tension parallel to grain strength (f_t0);

- Tension perpendicular to grain strength (f_t90);

- Shear parallel to grain strength (f_v0);

- Cleavage parallel to grain strength (f_s0);

- Modulus of rupture in static bending (f_M);

- Modulus of elasticity in compression parallel to grain (E_c0);

- Modulus of elasticity in compression perpendicular to grain (E_c90);

- Modulus of elasticity in tension parallel to grain (E_c0);

- Modulus of elasticity in static bending (E_M);

- Hardness parallel to grain (f_H0);

- Hardness perpendicular to grain (f_H90);

- Toughness (W).

Two moisture content conditions were verified for wood samples: above saturation fiber point (30% moisture content), and standardized point for structures [13] (12% moisture content). For each property and density 6 specimens were made.

Considering that results for two moisture contents are independent and normal distributed, t-test was performed to verify differences among groups averages. Statistical hypotheses of t-test are established by two conditions, where averages do not differ (H₀: µ₁ = µ₂) and the opposite (H₁: µ₁ ≠ µ₂). In this study, data variances were considered different and unknown. Decision was focused on P-value and t-test significance level (5%). H₀ hypothesis was rejected when P-value was lower than 5%. Statistical analyzes were carried out using R version 3.4.2 software.

3. Results and Discussions

Table 1 shows, for all physical and mechanical properties of Eucalyptus umbra wood, and both moisture content (Mc), the average values (x_m), standard deviations (Sd) and t-test performed P-values.

Decreasing moisture content of Eucalyptus umbra wood, from 30% to 12%, ρ_ap decreased 0.13 g/cm³, or 13% of its initial value. T-test analysis for ρ_ap was rejected the null hypothesis of averages equivalence, i.e., density presented significant difference when moisture content decreased from 30% to 12% (P-value < 0.05).

Along moisture content decrease condition, f_c0 presented decrease of 6% (2.7 MPa) and f_c90 40% (2.1 MPa), as well as f_t90 presented increase of 60% (1.8 MPa) and f_M increase of 7% (5.7 MPa). f_t0 did not present any variation. Through t-test, only f_t90 presented significant P-value, i.e., Tension parallel to grain strength presented significant difference when moisture content changed (P-value < 0,05).

For decreasing condition for moisture content, E_c0 increased 6% (853.1 MPa) and E_M increased 3% (394.3 MPa). E_c90 and E_t0 decreased around 41% (211.1 MPa) and 7% (1277.3 MPa), respectively. T-test applied for modulus of elasticity indicated that all properties did not present significant P-value, i.e., modulus of elasticity did not present significant difference when moisture content changed (P-value > 0,05).

f_s0 decreased 20% (0.17 MPa). f_v0, f_H90, f_H0 and W increased 16% (2.5 MPa), 64% (1343 N), 28% (324 N) and 16.89% (2.5 Nm), respectively. T-test for f_s0, f_v0, f_H90, f_H0 and W resulted significant difference with moisture content decrease (P-value < 0,05).

Gonçalves et al. (2009) [19] studied mechanical properties of a hybrid Eucalyptus urophylla x Eucalyptus grandis species, finding average values of f_M, E_M, f_c0 and ρ_ap (12% moisture content) equal to 103.2 MPa, 12.474 MPa, 67.38 MPa and 0.56 g/cm³, respectively. Eucalyptus umbra wood presented higher density and lower f_c0 and f_M values compared to hybrid Eucalyptus urophylla x Eucalyptus grandis.

Table 1. Physical and mechanical properties of Eucalyptus umbra wood at 12% and 30% moisture contents

Lima et al. (2011) [17] evaluated for 25 years-old Eucalyptus umbra trees from a forest area at Piracicaba city, in Sao Paulo State, ρ_ap 0.621 g/cm³ (12% moisture content). Comparing literature values to the obtained in this study, their values were about 10% lower than the obtained here (0.69 g/cm³).

Torres et al. (2016) [20] determined ρ_b, ρ_ap, f_c0 and E_c0 for Eucalyptus camaldulensis wood, resulting 0.47 g/cm³, 0.581 g/cm³, 25.31 MPa and 12,379 MPa, respectively. These results are lower than the ones determined for Eucalyptus umbra wood.

4. Conclusions

Through decrease of moisture content and emphasized condition of this study for Eucalyptus umbra wood, was verified the increasing of eight mechanical property values. In other hand five mechanical properties of this eucalypt species decreased their values. Furthermore, tension parallel to grain strength remained constant.

Statistically, among 14 mechanical properties of Eucalyptus umbra wood, only three properties (f_t90, f_H0 and W) increased values significantly with moisture content change.

According to presented results for Eucalyptus umbra wood, it is possible to conclude the feasibility of its use as raw material for design of timber structures.

ACKNOWLEDGMENTS

Authors thank to Wood and Timber Structures Laboratory (LaMEM), Structural Engineering Department (SET), Sao Carlos Engineering School (EESC), Sao Paulo University, by the materials and resources used in this study.