Time of exposure at 60 °C service temperature: Influence on strength and modulus of elasticity in compression parallel to the grain of hardwood species

de Almeida, T. H., de Almeida, D. H., Arroyo, F. N., De Araújo, V. A., Chahud, E., Branco, L. A. M. N., Vasconcelos Pinheiro, R., Christoforo, A. L., and Rocco Lahr, F. A. (2019). "Time of exposure at 60 °C service temperature: Influence on strength and modulus of elasticity in compression parallel to the grain of hardwood species," BioRes. 14(1), 207-219.

Abstract

The aim of this research was to determine the influence of exposure time at 60 °C in the compression strength parallel to the grain (f_c0) and modulus of elasticity in compression parallel to the grain (E_c0) for cupiúba (Goupia glabra), eucalypt (Eucalyptus saligna), garapeira (Apuleia leiocarpa) and jatobá tamarindo (Hymeneae sp.) wood species. The factor investigated in this research was the residence time in kiln (exposure at controlled temperature 60 °C) of specimens: 0 (wood tested at room temperature and moisture content around to 12%), 168, 456, 720, and 2160 hours, simulating the reaction of the material in confined roof structures. Compression parallel to the grain tests were conducted according to the methods of the Brazilian Standard Code ABNT NBR 7190 (1997). ANOVA results indicated that the temperature (60 °C) had a significant influence in the five exposure times. All studied wood species showed an increase in the f_c0 value with increasing time of exposure at 60 °C. For E_c0, this phenomenon occurred only for the jatobá tamarindo and cupiúba wood species.

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Time of Exposure at 60 °C Service Temperature: Influence on Strength and Modulus of Elasticity in Compression Parallel to the Grain of Hardwood Species

Tiago H. Almeida,^aDiego H. Almeida,^b Felipe N. Arroyo,^c Victor A. De Araujo,^dEduardo Chahud,^e Luiz A. M. N. Branco,^fRoberto V. Pinheiro,^gAndré L. Christoforo,^h,* and Francisco A. R. Lahr ⁱ

Keywords: Amazon forest; Mechanical properties; Planted forest; Tropical wood; Wood

Contact information: a: Department of Materials Engineering, Engineering School of São Carlos, University of São Paulo (EESC/USP), São Carlos, Brazil; b: Department of Civil Engineering, Federal University of Rondônia (UNIR), Porto Velho, Brazil; c: Department of Civil Engineering, Integrated Colleges of Cacoal(UNESC), Cacoal, Brazil; d: State University of São Paulo, Research Group on Development of Lignocellulosic Products (LIGNO), Itapeva, Brazil, e: Department of Civil Engineering, Federal University of Minas Gerais (UFMG), Belo Horizonte, Brazil; f: Department of Civil Engineering, FUMEC University, Belo Horizonte, Brazil; g: Department of Civil Engineering, Mato Grosso State University (UNEMAT), Sinop, Brazil; h: Department of Civil Engineering, Federal University of São Carlos (UFSCar), São Carlos, Brazil; i: Department of Structures Engineering, Engineering School of São Carlos, University of São Paulo (EESC/USP), São Carlos, Brazil;

* Corresponding author: christoforoal@yahoo.com.br

INTRODUCTION

Wood is one of the main raw materials used in the world, from small artifacts with little technology, for example, a teaching kit for children apprentices (Vieira et al. 2010), to research for their use in aggregates for higher value purposes (Song et al. 2018). Wood used in industry may come from native forests (Ter Steege et al. 2016; Barbosa et al. 2017) or planted forests (Bäcklund et al. 2017; De Araújo et al. 2017).

Brazil has within its territory different biomes with numerous species of native trees that can supply quality raw material for several industrial sectors. One such biome is the Amazon Forest (Almeida et al. 2017).

According to Ter Steege et al. (2016) and Cardoso et al. (2017), it is estimated that only in the Brazilian territory, the Amazon Forest presents around 12,000 tree species that have yet to be catalogued. Therefore, the study of the properties of the wood is important for its rational use (Top et al. 2018) and forest sustainability (Ilha et al. 2018; Metcalfe et al. 2018).

As pointed out by De Araújo et al. (2016) and Sotsek and Santos (2018), wood is used as a structural member in bridges (Cheung et al. 2017), footbridges (Robbers et al. 2018), and other civil construction projects (Chen and Guo 2016).

For the use of wood as a structural member, its physical and mechanical properties should be determined according to standardized laboratory procedures (Kloiber et al. 2015; Mahmud et al. 2017).

Strength in compression parallel to the grain (f_c0) and the modulus of elasticity in compression parallel to the grain (E_c0) are the main mechanical properties to classify some wood species for the construction of structures according to the Brazilian Standard Code ABNT NBR 7190 (1997) “Design of Wooden Structures” (Almeida et al. 2013; Christoforo et al. 2017; Lahr et al. 2017).

Wood varies in its properties according to the place of origin (Csordós et al. 2014; Silva et al. 2018), tree species (Matos and Molina 2016), moisture content (Stolf et al. 2014), age (Lourençon et al. 2014), and other edaphoclimatic factors (Vásquez-Cuecuecha et al. 2015). However, the use of wood in structures is influenced by weather conditions (Guo et al. 2018; Herrera et al. 2018; Reinprecht et al. 2018), even when previously chemically treated against the attack of decay organisms (Pigozzo et al. 2017).

Temperature is another factor that can influence the wood properties. Some thermal treatments change the surface wood coloration to increase its value added to the market (Lopes et al.2014; Santos et al. 2014; Zanuncio et al. 2015; Yang et al. 2015). The mechanical properties of wood can be improved with heat-thermal treatments (Cademartori et al. 2015; Anjos and Sousa 2015; Zhang et al. 2015; Holocek et al. 2017). Timber members exposed to fire conditions are also research topics that will increase the knowledge of the behavior of the wood in this situation (Moreno Junior and Molina 2012; Moreno Junior et al. 2013).

According to Coelho et al. (2017), asbestos-free fiber cement roof tiles with a high rate of solar radiation can reach 60 ºC. In service, the wood used in the roof structures is exposed to this temperature of service.

The aim of this research was to determine the influence of time of exposure to the service temperature on the compression parallel to the grain strength and modulus of elasticity in compression parallel to the grain for cupiúba (Goupia glabra), eucalypt (Eucalyptus saligna), garapeira (Apuleia leiocarpa), and jatobá tamarindo (Hymeneae sp.) wood species.

EXPERIMENTAL

Three homogeneous batches of cupiúba (Goupia glabra), garapeira (Apuleia leiocarpa), and jatobá tamarindo (Hymeneae sp.) from different extraction sites in the Amazon Forest (certified areas on North of Brazil) and one homogeneous batch of eucalypt (Eucalyptus saligna) from a planted forest area in the state of São Paulo, Brazil, were used. In this research, hardwood types were studied because they are more often used in roof structures in Brazil. All wood batches were dried at room temperature until they reached a moisture content of 12%, as designated for manufacturing specimens in the ABNT NBR 7190 (1997) standard. All specimens had a length in the grain direction of 15 cm and a square cross section with an edge of 5 cm.

Compression parallel to the grain tests were carried out in an AMSLER universal testing machine (Schaffhausen, Switzerland) at 250 kN capacity. f_c0 and E_c0 values were obtained according to ABNT NBR 7190 (1997).

The factor investigated in this research was the time inside the kiln (Marconi, Piracicaba, Brazil), with an exposure at controlled temperature 60 ºC of specimens: 0 (wood tested at room temperature and moisture content close to 12%), 168 (7 days), 456 (19 days), 720 (30 days), and 2160 h (90 days), simulating the reaction of the material in confined roof structures (according to Coelho et al. 2017).

For mechanical properties of wood species, 24 values were determined for specimens at room temperature and 12% moisture content and six values determined for the four other exposure times evaluated (168, 456, 720, and 2160 h), resulting in a total of 384 values. It is noteworthy that specimens for each experimental condition were made in pairs, implying that possible differences in results are explained mainly by wood exposure time, at 60 °C, decreasing intrinsic variability of wood.

To investigate the effect of exposure time (0, 168, 456, 720, and 2106 h) of specimens in the kiln in the f_c0 and E_c0 values, the analysis of variance (ANOVA) was applied, using the software Minitab®14 (Minitab, State College, PA, USA), considering a 5% significance level (α). The null-hypothesis consisted of means equivalence (H₀) and their non-equivalence (at least two) as an alternative hypothesis (H₁). By hypotheses formulation, a P-value greater than the significance level implies accepting H₀ (means of the investigated treatments are equivalent), and lower values (P-value < 0.05) refute H₀ (at least one the means differs significantly).

For ANOVA’s validation, normal distributions of f_c0 and E_c0 values and variance homogeneity of treatment using Anderson-Darling (AD) and Bartlett (Bt) tests, respectively, were investigated, both at 5% significance. To test formulation, a P-value exceeding 5% means that answers that present a normal distribution and variance of treatments are equivalent, thus validating ANOVA model. If considered significant, the influence of exposure time at 60 ºC, by ANOVA, graphs of main effects (visual inspection), and, subsequently, the multiple comparison Tukey test (contrast test) to group the levels of factor investigated were applied. In the Tukey test, means in descending order are labeled A and B, with A being the highest average. It is noteworthy that factor levels investigated with the same letters show statistically similar means.

RESULTS AND DISCUSSION

Table 1 presents the f_c0 and E_c0 values and coefficient of variation (CV) for four wood species in the five conditions of exposure at a controlled temperature of 60 ºC. Silva et al. (2018) found that for cupiúba wood from different provenances and ages in Brazil, average f_c0 values were equal to 62.07 (Caracaraí, state of Roraima), 47.73 (Bonfim do Sul, state of Roraima), and 57.42 MPa (Claúdia, state of Mato Grosso). All these values were lower in comparison with the present research. The E_c0 value of cupiúba from Caracaraí was equal to 15,976 MPa, higher that the E_c0 determined in this research in the same condition, without time exposure at 60 ºC.

Table 1. The f_c0 and E_c0 Values for Hardwoods at Different Exposures to 60 ºC

Moreira et al. (2017) determined f_c0 and E_c0 values equal to 57 and 12,970 MPa for cupiúba, both values being lower than this research. Almeida and Dias (2016) found cupiúba wood f_c0and E_c0 values equal to 44.50 and 11,114 MPa, for wood at 12% of moisture content. The E_c0 value determined by Almeida and Dias (2016) was close to the determined value in this research, but the f_c0 value was much lower. Jesus et al. (2015) determined the value of the modulus of elasticity in compression parallel to the grain (E_c0) equal to 13,900 MPa. Ferro et al.(2015) found E_c0 values of 14,600 and 17,800 MPa for cupiúba wood. Cupiúba wood studied in this research at first condition of exposure presents a lower E_c0 value compared to values determined by Ferro et al. (2015). Dias and Lahr (2004) classified Cupíuba wood in C30 strength class for Brazilian native hardwood according ABNT NBR 7190 (1997), with f_c0 and E_c0values equal to 54 and 14,125 MPa.

ABNT NBR 7190 (1997) showed for garapeira (Apuleia leiocarpa) f_c0 and E_c0values equal to 78.40 and 18,359 MPa. The values determined in this research were lower. Dias and Lahr (2004) classified garapa wood (Apuleia leiocarpa) in the C60 strength class for Brazilian native hardwood according to ABNT NBR 7190 (1997), with f_c0 and E_c0 average values equal to 73 and 17,718 MPa.

Lahr et al. (2016) investigated jatobá wood from different provenances in Brazil, finding average f_c0 values equal to 93.91 (Caracaraí, state of Roraima), 93.42 (Bonfim do Sul, state of Roraima), and 94.38 (Alta Floresta, state of Mato Grosso), all values being lower than in this research. For the E_c0, Lahr et al. (2016) found the following values for jatobá wood: 22,482 (Caracaraí, state of Roraima), 21,403 (Bonfim do Sul, state of Roraima), and 21,759 MPa (Alta Floresta, state of Mato Grosso), and all values were higher than in this research.

Dias and Lahr (2004) classified jatobá wood in the C60 strength class for Brazilian native hardwood according to ABNT NBR 7190 (1997), with f_c0 and E_c0 values equal to 91 and 22,967 MPa. The f_c0 and E_c0 values determined that the Brazilian native hardwood species studied in this research show the variability in both mechanical properties. There are few other reports on the properties of cupiúba, garapeira, and jatobá tamarindo wood, reinforcing the need for more research on the physical and mechanical properties of wood from Amazonian forest trees (Ter Steege et al. 2016; Cardoso et al. 2017).

Serpa et al. (2003) found the Eucalyptus saligna wood value for f_c0 equal to 60 MPa, which is above that determined in this reseach; however, the value determined in this research was higher than the f_c0 determined for Eucalyptus grandis, also by Serpa et al. (2003). ABNT NBR 7190 (1997) shows f_c0 and E_c0 values to Eucalyptus saligna equal to 46.80 and 14,933 MPa, respectively, which is close to the results determined in this research.

Nogueira et al. (2018a,b) studied the mechanical properties of Eucalyptus umbra and Eucalyptus camaldulensis, respectively. Eucalyptus umbra presents f_c0 and E_c0 values equal to 42.70 and 14,579 MPa; for Eucalyptus camaldulensis, the f_c0 and E_c0 average values were equal to 48 and 13,286 Mpa, respectively. Lahr et al. (2017) determined f_c0 and E_c0 for Eucalyptus urophylla equal to 46 and 13,391 MPa, respectively, and both values are lower compared to values determined in this research for Eucalyptus saligna. The f_c0 and E_c0 values of Eucalyptus saligna wood reported here were lower than that of Eucalyptus benthamii wood as determined by Müller et al. (2014). Lima et al. (2014) reported the f_c0 value of Eucalyptus resiniferawood equal to 55.87 MPa, which was higher than that determined for Eucalyptus saligna wood in this research. Almeida and Dias (2016) determined the f_c0 and E_c0 for Lyptus wood (Eucalyptus hybrid) equal to 53.6 and 22,559 MPa, respectively. Lyptus wood presented a higher f_c0 value than the batch of Eucalyptus saligna.

Table 2 presents ANOVA and test validations (AD and Bt) for the evaluated mechanical properties. The distributions by property and for all wood species were normal and the variances between the treatments were homogeneous, validating the ANOVA model (P-value > 0.05). The P-values of ANOVA for both properties and wood species were lower than the significance level considered (0.05), implying the influence of the exposure time at 60 ºC.

Table 2. ANOVA and Validation Tests Results

Note: FD, freedom degrees; AD, Anderson-Darling test; Bt, Bartlett test

Table 3 presents the results of Tukey’s multiple comparison test, and Fig. 1 shows the results of the main effects graphs. For all studied wood species, there was an increase in the value of f_c0 with the increase of the time of exposure at 60 ºC. For E_c0 values, this phenomenon occurred only for the species jatobá tamarindo and cupiúba.

Almeida et al. (2018) studied the influence of exposure time (0, 168, 456, and 720 h) at 60 ºC in shear parallel to the grain strength (f_v0) of softwoods Pinus taeda and Pinus elliottii from planted forests. Similar to hardwoods studied in this research, times of exposure at temperature service had an influence in the f_v0 softwoods.

According to Figueroa and Moraes (2009), in heat-treatment between 0 and 200 ºC, wood presents the following phenomena: the process called slow pyrolysis; water vapor and gas release; wood is not ignited; existence of some exothermic oxidative reactions; and color change. From 55 ºC, lignin and hemicellulose is altered, which explains the change in the values of f_c0 obtained in the present research, but, just after 100 ºC, the wood chemical compounds (lignin, cellulose, and hemicellulose) are negatively affected.

Fig. 1. Graphs of main effects of exposure time factor on mechanical properties for the four evaluated wood species: (a) cupiúba; (b) eucalypt; (c) garapeira; (d) jatobá tamarindo

Table 3. Tukey Test Results

Jatobá wood (Hymenaea courbaril) has a lignin content equal to 21% (Almeida et al. 2015); Eucalyptus saligna: 21.6% (Foelkel et al. 1975); and cupiúba (Goupia glabra): 35% (Duarte 2017). The exposure time of wood to 60 ºC might influence the material chemical structure, affecting its mechanical properties.

First, tests were carried out with specimens that exhibited a 12% moisture content and were not treated in the kiln. After a long time in the 60 ºC kiln (in this work, 2160 h), moisture content of specimens decreased, as water vapor was released (Figueroa and Moraes 2009; Phonetip et al. 2018), allowing an increase in the f_c0 and E_c0 values (Silva et al. 2012; Branco et al. 2014; Nogueira et al. 2018a, b).

CONCLUSIONS

Jatobá wood presented the highest f_c0 and E_c0 values of the species studied.
The f_c0 and E_c0 values of all hardwood species (cupíuba, eucalypt, garapeira, and jatobá tamarindo) were influenced by exposure time at 60 ºC.
For all wood species, there was an increase in the f_c0 value with the increase of the time of exposure at 60 ºC. For E_c0, this phenomenon occurred only for jatobá tamarindo and Cupiúba.

ACKNOWLEDGMENTS

The authors are grateful for LaMEM (Wood and Timber Structures Laboratory), the Department of Structural Engineering (SET), São Carlos Engineering School (EESC), and University of São Paulo (USP), Laboratory of Federal University of São Carlos (UFSCar) and Federal University of Minas Gerais (UFMG), for providing facilities and inputs required for this study.

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Article submitted: July 21, 2018; Peer review completed: October 20, 2018; Revised version received: November 8, 2018; Accepted: November 11, 2018; Published: November 14, 2018.

DOI: 10.15376/biores.14.1.207-219