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Temperature reduction factor for compressive strength parallel to the grain
Fire Safety Journal 83 (2016) 99–104
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Temperature reduction factor for compressive strength parallel to the grain Manuel Jesús Manríquez Figueroa a,n, Poliana Dias de Moraes b,n a
Department of Building Engineering, University of Magallanes- Avenida Bulnes, 01855, Cx Postal 113-D, Punta Arenas, Chile Department of Civil Engineering, Federal University of Santa Catarina - Rua João Pio Duarte Silva, s/n, Cx. Postal 476, Córrego Grande, Florianópolis 88040-970, SC, Brazil
b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2015 Received in revised form 25 May 2016 Accepted 29 May 2016 Available online 7 June 2016
The aim of this work is to evaluate the effect of the temperature on the compressive strength parallel to grain of Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda from cultivated forests. The Schizolobium amazonicum sample was formed by 105 clear specimens assembled in 15 groups, the Eucalyptus saligna and Pinus taeda were formed by 150 clear specimens assembled in 15 groups each. Each group of samples was formed by 7, 10 and 10 specimens, respectively. The specimens were tested in a temperature range from 20 to 230 °C. The Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda samples present a decrease in the compressive strength, reaching 70%, 79% and 62% of the compressive strength at room temperature, respectively. The compressive strength decrease of the samples can be associated to the thermal degradation of wood polymers and the moisture content. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Timber Temperature Reduction factor Compressive strength Planted forests
1. Introduction Many studies have shown the influence of temperature on the mechanical properties of wood [1–26]. However, some results, so far, do not completely agree in some respects, especially with regard to the benefits or losses caused by temperature. This difference may be related to various test methods used in the development of the studies. The uniformization of test conditions to determine wood strength under thermal action and actual moisture content is required in order to avoid discrepancies in test results, since these methods differ as regards to the size, moisture content and heating of the specimens. Gerhards [8] summarizes the significant researches reported in the literature on the effects of moisture content and temperature on mechanical properties of wood, in which the cross section of specimens varies in the range from 4.8 mm 4.8 mm to 50.0 mm 50.0 mm. The specimen size interferes directly with the process of heating and drying of the specimens, affecting the time required to obtain a homogeneous temperature and humidity in its inner part. Schaffer [4] and Knudson and Schniewind [5] used samples of Pseudotsuga menziesii (Douglas fir). The former used dried samples, with specimens of 25.4 mm 3.2 mm 95.2 mm while the latter used 12% moisture content samples, with specimens with dimensions n
Corresponding author. E-mail addresses: [email protected] (M.J. Manríquez Figueroa), [email protected] (P. Dias de Moraes). http://dx.doi.org/10.1016/j.firesaf.2016.05.005 0379-7112/& 2016 Elsevier Ltd. All rights reserved.
of 4.8 mm 4.8 mm 19 mm, heated by hot plates placed on the side surfaces of the specimens during 5–20 min. Gerhards [8] presented results obtained from dried samples, with 5 mm 5 mm of cross section, while Young and Clancy [14] used dried and 12% moisture content specimens of 90 mm 90 mm 300 mm of Pinus radiata and different heating periods. Manríquez et al. [26] used air dried and saturated samples with specimens of 5 cm 5 cm 15 cm, heated in the electric oven for 180 min. The study of the influence of temperature on mechanical properties of wood with normal moisture content is a difficult task, since the specimen heating tends to dry it. The variation of moisture content can be avoided by performing the tests in saturated or completely anhydrous conditions. The tests with saturated specimens provide a lower limit resistance while dried specimens provide an upper limit. However, in the latter condition they are more exposed to thermal degradation of wood polymers during the heating process. The exposure of wood to high temperatures causes transient and permanent effects. The timber heating for short periods of time causes a temporary reduction in mechanical strength which is restored when the temperature returns to normal. If high temperature exposure is maintained for an extended period of time, the effects on the timber are permanent [27–30]. Salamon [3] noted that, between 25 °C and 100 °C, the effects of the temperature on mechanical properties of wood vary according to the species and the applied load. Some species show no loss of strength, while others have between 7% and 20% of reduction compared with values obtained at room temperature. Chafe [7]
The specimens, with 12% equilibrium moisture content at ambient temperature, were heated at a constant temperature during 180 min in an electric drying oven with automatic temperature
434 421 428 427 431 422 424 433 423 432 424 430 426 430 427
Mass Density (kg/m3) Mass (kg/m ) Mass density (kg/m )
25 25 24 23 22 25 24 25 24 22 22 24 25 25 23 776 788 772 771 775 776 774 770 774 769 778 772 777 773 771 38 37 37 36 35 37 35 35 34 36 38 38 38 41 40 331 334 337 336 338 347 348 349 351 355 356 343 360 367 369
Schizolobium amazonicum [24]
3
20 40 50 60 70 80 90 100 110 130 150 170 190 210 230
2.2. Samples heating
Table 1 Average mass density of specimen groups at 20 °C and 12% of moisture content.
Three timber samples were used in this study, all from planted forests. The sample of the Schizolobium amazonicum was formed by 105 specimens [24], while the samples of the Eucalyptus saligna and Pinus taeda were formed by 150 specimens each, tested at different levels of temperatures. The physical properties and characteristics of the samples and the temperature range are shown in Table 1. The specimens were prepared with timber classified visually according to the specifications of the Brazilian standard [31]. They were clear, defect-free, straight-grained material cut from regions distant at least 30 cm from the tip. The specimen dimensions, 50 mm 50 mm 150 mm, are those defined in the Brazilian standard [31]. Cross-section dimensions and length were measured with accuracy of 0.01 mm. Special care was taken in preparing the specimen to ensure that the end surfaces are parallel and that both are aligned perpendicular to the longitudinal axis. The distribution of specimens in sets was done in order to constitute groups with densities statistically homogeneous. The homogeneity was verified by variance analysis, using statistically significant differences. The method used to compare the means was Fisher least significant difference (LSD) procedure, with a confidence level of 95% [32].
3
2.1. Samples and specimens
Standard deviation (kg/m )
2. Materials and methods
3
Eucalyptus saligna
3
Standard deviation (kg/m )
Pinus taeda
Standard deviation (kg/m3)
reports that the wood strength reduction is related to the thermal degradation of its polymers. According to Gerhards [8], Kollmann and Côté [2] and Green et al. [12], the timber strength is connected in reverse order at room temperature. However, little influence was found in static bending [9]. Milota [13] heated samples of Western hemlock (Tsuga heterophylla) and Spruce Fir (Abies spp.) at 80 °C, 116 °C and 132 °C and observed a reduction on the strength between 40% and 50%. Frühwald [21] conducted tests with samples of Norway spruce and Larch, with 6% of moisture content. The specimens were heated at 80 °C, 120 °C and 170 °C. Samples of the same species were heated at 190 °C and 210 °C. The results indicated that the temperature affects the compressive and tensile strength parallel to the grain, but the results were different for the various species. Manríquez [23] studied the paricá (Schizolobium amazonicum) from planted forests in the Amazon region. In this study, compression, tensile, shear and embedment tests were performed in the range from 20 °C to 230 °C, leading to the conclusion that the temperature directly influences the reduction of the mechanical strength of the timber. Further studies, using specimens with dimensions specified by international standards, may be required in order to evaluate the strength reduction factor and to find a behavior pattern. The aim of this work is to evaluate the effect of the temperature on wood of Schizolobium amazonicum (paricá), Eucalyptus saligna (Sydney bluegum) and Pinus taeda (loblolly pine) from cultivated forests and to determine the temperature reduction factor for the compressive strength parallel to grain of wood in a temperature range from 20 to 230 °C. This paper is organized as follows: a section on materials and methods used to carry out the experiments, a results and discussion section and a section presenting the conclusions of this research.
46 42 45 45 45 42 44 45 44 45 44 45 45 45 45
M.J. Manríquez Figueroa, P. Dias de Moraes / Fire Safety Journal 83 (2016) 99–104
Temperature (°C)
100
M.J. Manríquez Figueroa, P. Dias de Moraes / Fire Safety Journal 83 (2016) 99–104
Fig. 3. Force displacement curve.
Fig. 1. Specimens used to determine the heating period.
control and internal dimension of 90 cm 100 cm 66 cm, in order to ensure homogeneous temperature inside the specimens. The heating time of the specimens was determined from previous study, in which the temperature of the specimen center was monitored until it reached the furnace temperature. The temperature level and stabilization were checked by means of a type K thermocouple, inserted into a hole of 2.0 mm diameter, whose end was located at the geometric center of the specimen (Fig. 1).
101
the ultimate compression force as illustrated in Fig. 3.
fc 0, θ =
Fc 0, max , A
(1)
where Fc0,max is the ultimate force, in N, applied to the specimens at temperature level θ and A is the cross section of the specimen after the heating, in mm2. 2.5. Determination of the moisture content
2.3. Mechanical tests The heated specimens were placed in an environmental chamber with internal dimension of 37 cm 50 cm 52 cm (Fig. 2), both at the same constant temperature, as specified in Table 1. The tests were performed with a load cell of 200 kN 72 kN and the load was applied continuously increasing at a constant rate of displacement of 2 mm min 1. The tests ended with the specimen rupture, which took 5–10 min. 2.4. Determination of compressive strength parallel to grain Compressive strength values (fc0,θ) for different levels of temperature were calculated according to Eq. (1) and determined from
The moisture content of the specimens before the preheating was 12%. The determination of the specimen moisture content after the mechanical test was carried out using the same specimen. After the rupture, the specimen was immediately weighted for the determination of the final mass (mf), and then submitted to drying at a maximum temperature of 103 72 °C, as required by the Brazilian Standard [30], until the difference between two consecutive measures was less or equal to 0.5% of the last measured mass. In this condition, the measured mass is considered to be the dry mass (md). The moisture content at the end of the tests was determined by Eq. (2)
mc =
mf − md ⋅100. md
(2)
2.6. Determination of characteristic values of the compressive strength The determination of characteristic values of strength, at different temperature levels, was performed according to the statistical methodology of Student t distribution, with ν ¼ n 1 degrees of freedom [32], using Eq. (3). The characteristic value is defined as the smallest value that has only 5% probability of being exceeded in a given lot.
fkθ = f¯θ − t⋅Sn,
(3)
where f¯θ is the mean value of the strength at temperature θ, in MPa; t¼1.833 for 9 degrees of freedom and t¼1.943, for 6 degrees of freedom; Sn is the sample standard deviation at temperature θ; n is the number of specimens of the sample at temperature θ. 2.7. Determination of the reduction factor Fig. 2. Specimen in the environmental chamber attached to the testing machine after a test at 190 °C.
The reduction factors as functions of the temperature of the
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compressive strength parallel to the grain of threes wood species were determined by Eq. (4)
kθ =
fkθ , fk,20 oC
(4)
where fkθ is the characteristic strength property at temperature level θ, in MPa; fk,,20 °C is the characteristic strength property at room temperature, in MPa.
3. Results and discussion In this section, the results on the types of failure and the compressive strength parallel to grain of the Schizolobium amazonicum [24], Eucalyptus saligna and Pinus taeda are presented. All mechanical data concerning the compressive strength were statistically treated. The analysis of variance was carried out in order to verify statistically significant differences, with 95% reliability, between the average mechanical values at a given temperature and those at room temperature [32]. 3.1. Moisture content In Fig. 4, the moisture content of the specimens of the 3 samples is shown. It decreases linearly with the temperature increase, presenting 0% moisture content at 150 °C for both samples. At temperatures higher than 150 °C, the specimens were completely dry. According to Schaffer [4], at 140 °C, the water contained in the components of wood begins to be released and the polymer begins to degrade. As the temperature increases, so does the energy level of the molecules and constituents. This energy is enough to modify the wood internal structure, decreasing the fiber moisture content. 3.2. Specimen failure During the compression parallel to grain tests, failures occurred at different temperature levels. From Fig. 5a, in which the results for the compression parallel to grain tests of Schizolobium amazonicum is presented [24], it can be concluded that the form of rupture varies with the test temperature. According to Bodig and Jayne [10], the rupture of the specimens under compression parallel to grain is caused by the maximum shear forces at an angle with the loaded surface and associated with a weaker plan, as presented by the theory of Solid Mechanics. The types of failures of the specimens of Eucalyptus saligna are presented in Fig. 5b. It was observed that during the tests, there were two types of failures. The first occurred between 20 °C and
130 °C, located at the center of the specimen. The second occurred near the ends of the specimens in the temperature range from 150 °C to 230 °C. In this temperature range, deep cracks occur on the faces of the specimens due to severe heating. These cracks cause planes of weakness in the specimens, which lead to its rupture. Longitudinal cracks were also observed on the faces of the specimens of Eucalyptus saligna (Fig. 5b) for the same temperature range. These may be due to the sudden heating and, especially, the high values of the specimens density. All specimens with density above 700 kg/m3 presented these types of failure and fracture. The types of failures in the specimens of Pinus taeda are presented in Fig. 5c. It was observed that this kind of failure is affected by the test temperature. At room temperature, the failure line is nearly horizontal, while, at 230 °C, the failure line is slightly above 45°. The results obtained by Young and Clancy [14] for the Pinus radiata, in the range of 20 °C and 250 °C, in compression parallel to grain tests, are similar to those obtained in this study with Pinus taeda. 3.3. Compressive strength parallel to the grain In Fig. 6, the compressive strength results for the 3 samples are presented. At room temperature, the Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda samples present an average strength and standard-deviation of 32 MPa74 MPa [24]; 51 MPa7 4 MPa; 24 MPa72 MPa, respectively. The results show that a continuous strength decrease exists with the increase of temperature up to 80 °C; afterwards, an increase exists up to 150 °C, when the moisture content of the specimens is 0% (Fig. 4). For the Schizolobium amazonicum sample, the analysis of variance indicates that there is a statistically significant difference, with 95% confidence, between the compressive strength parallel to grain at 20 °C and at other temperature levels, except at 150 °C [24]. For the Pinus taeda and Eucalyptus saligna samples, a difference is found between the data obtained at room temperature and for those heated specimens. 3.4. Deformation at maximum compression force The deformation results, at maximum compression force, for the three samples are presented in Fig. 7, including the deformation by crushing in the specimens ends. They vary with the increase of the temperature. The highest deformation was presented by the Pinus taeda, indicating that it is the softest sample since it has the lowest strength (Fig. 6) while the lowest total deformation was presented by the Schizolobium amazonicum sample indicating that it is the stiffest sample. The deformation remains constant between 40 and 60 °C and between 130 and 170 °C, as shown by the plateaux in Fig. 7. These plateaux may be related to the lignin softening as remarked by Schaffer [4]. From 170 °C onwards, the deformation decreases again. It is important to remark that specimens had 0% of moisture content from 150 °C onwards (Fig. 4), when the total deformation of the three samples decreases, indicating the influence of moisture content in the behavior of wood. 3.5. Temperature reduction factor for compressive strength parallel to grain
Fig. 4. Moisture content.
The temperature reduction factors for compressive strength parallel to grain of three species studied in this research and those found in the literature and proposed by NDS [18], NCh 1198 [22] and EN 1995 1-2 [19] are presented in Fig. 8. The results of the Pinus taeda samples are similar to those obtained by Knudson and Schniewind [5] for Pseudotsuga menziesii (Douglas fir). Both tests used samples of conifer species with 12% initial moisture content. After 150 °C, the results agree well with the values
M.J. Manríquez Figueroa, P. Dias de Moraes / Fire Safety Journal 83 (2016) 99–104
20 0 ºC 230 ºC (aa) S. amazonicuum [24].
20-130 ºC
C 150-230 ºC (b) E. salignna
150-230 ºC
103
20 ºC 230 0 ºC (c) P. taeda
Fig. 5. Patterns of rupture.
Fig. 6. Compressive strength parallel to the grain.
Fig. 7. Deformation at maximum compression force.
presented by Schaffer [4] using dry samples of Pseudotsuga menziesii, with specimens dimension of 25.4 mm 3.2 mm 95.2 mm and with the results of Young and Clancy [14], using oven dry samples of Pinus radiata with specimens dimension of 90 mm 90 mm 300 mm. At 100 °C, the results of this study are similar to those found by Young and Clancy [14], using 12% moisture content samples of Pinus radiata, by Manríquez et al. [26] using water saturated samples of Schizolobium amazonicum. Above 150 °C, the reduction factor values of the Schizolobium amazonicum and Pinus taeda samples, both low mass density woods (Table 1), decrease with the temperature increase and tend
Fig. 8. Reduction factor of compressive strength parallel to the grain.
to zero near 250 °C, while the EN 1995 1-2 standard [19] considers zero strength at 300 °C. At 230 °C, the maximum reduction for the Schizolobium amazonicum is 70%, for the Eucalyptus saligna, is 79% and for the Pinus taeda is 62%. Among the three species studied, Eucalyptus saligna was the species most sensitive to the effect of temperature. The values found by Young and Clancy [14], using dry oven samples of Pinus radiata, are the upper limit of the reduction factors. The values presented by the NDS and EN 1995 1-2 standards have a similar behavior between 20 and 70 °C [18,19]. However, they are lower than the values recommended by the NCh 1198 standard [22]. For the same temperature range, the temperature reduction factors obtained in this study for the hardwood and softwood species have a similar behavior as predicted by those standards although they are higher than those presented by the European standard [19]. In the range from 70 to 230 °C, the reduction factors obtained are higher than those presented by EN 1995 1-2 [19] and lower than the values found by Young and Clancy [14], using dry oven samples. For heavy sections, EN 1995 1-2 [19] consider that the gradient temperature in a timber section is very steep and only a small part of the section under the charred layer is heated and has its strength altered, which has a small effect on the resistance of the whole residual section. However, as demonstrated by Young and Clancy [14], for light-framed timber structures, the temperature affected zone may represent a significant percentage of the cross section of the structural element. Further studies may be needed for other hardwood and softwood species in order to find a behavior pattern.
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4. Conclusions This study evaluated the effect of the temperature on the compressive strength parallel to the grain of Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda. For the three samples used, the results have shown that: – the rupture patterns of the specimens are associated with the temperature levels of the tests. – at ambient temperature, the Eucalyptus saligna has a greater compressive strength parallel to the grain than the other studied species, and also the greatest reduction of mechanical strength; – Pinus taeda sample is the softest while the Schizolobium amazonicum is the stiffest of the studied species. – in the temperature range from 20 to 70 °C, the reduction factors have a similar behavior as given by the standards but, between 70 and 230 °C, the reduction factors are higher than those presented by EN 1995 1-2 [19], though extensive studies are needed to allow the generalization of this behavior for hardwood and softwood species. – although for heavy sections only a small part of the section under the charred layer has its strength altered, for light-framed timber structures, the temperature affected zone may represent a significant percentage of the cross section of the structural element.
Acknowledgments The authors thank Dr. Rodrigo Figueiredo Terezo, the timber industries Caswood and Klabin for providing the wood used in the tests and the Brazilian Research Council (CNPq) for sponsoring this research (Grant no. 303223/2013-8).
References [1] E. Sano, Effects of temperature on the mechanical properties of wood. I. Compression parallel to grain. II. Tension parallel to grain. III. Torsion test, J. Jpn. Wood Res. Soc. 7 (4) (1961) 147–150. [2] F.F.P. Kollmann, W.A. Côté Jr., Principles of Wood Science and Technology, in: Solid wood, Springer, Berlin Heidelberg New York, 1968, p. 592. [3] M. Salamon, High temperature and its effect on wood properties, For. Prod. J. 19 (3) (1969) 27–34. [4] E.L. Schaffer, Effect of pyrolytic temperatures on the longitudinal strength of dry Douglas fir, J. Test. Eval. 1 (4) (1973) 319–329. [5] R.M. Knudson, A.P. Schniewind, Performance of structural wood members exposed to fire, For. Prod. J. 25 (2) (1975) 23–32. [6] C.C. Gerhards, Effect of high temperature drying on tensile strength of Douglas fir 2 4’, For. Prod. J. 29 (3) (1979) 39–46. [7] S.C. Chafe, Growth stresses in trees, Aust. For. Res. 9 (1979) 203–223. [8] C.C. Gerhards, Effect of the moisture content and temperature on the mechanical properties of wood an analysis of immediate effects, Wood Fiber 14
(1) (1982) 4–36. [9] C.C. Gerhards, Effects of high temperature drying on bending strength of yellow poplar 2 by 4's, For. Prod. J. 33 (29) (1983) 61–67. [10] J. Bodig, B.A. Jayne, Mechanics of Wood and Wood Composites, Nostrand Reinhold, New York, 1993. [11] A.M. Olsson, L. Salmén, Humidity and temperature affecting hemicellulose softening in wood. in: International Conference of COAST Action E8, Denmark, 1997, pp. 269–279. [12] D.W. Green, J.E. Winandy, D.E. Kretschmann, Mechanical properties of wood, in: Wood handbook wood as an engineering material. Gen Tech Rep FPL-GTR113, US Department of Agriculture Madison, 1999, pp. 1–45. [13] M.R. Milota, Warp and shrinkage of hem-fir stud lumber dried at conventional and high temperatures, For. Prod. J. 50 (11) (2000) 79–84. [14] S.A. Young, P. Clancy, Compression mechanical properties of wood at temperatures simulating fire conditions, Fire Mater. 25 (2001) 83–93, http://dx. doi.org/10.1002/fam.759. [15] P. Bekhta, P. Niemz, Effect of temperature on color and strength of spruce wood, Holzforschung 57 (5) (2003) 539–546. [16] P.D. Moraes, et al., Influence of temperature on the Modulus of Elasticity (MOE) of Pinus sylvestris L, Holzforschung 58 (2004) 143–147. [17] P.D. Moraes, Y. Rogaume, P. Triboulot, Influence of temperature on the embedding strength, Holz als Roh-und Werkstoff 63 (2005) 297–302, http://dx. doi.org/10.1007/s00107-004-0568-9. [18] National Design Specification (NDS) For Wood Construction, American Forest e Paper Association, 2005, p. 174. [19] European Standard Eurocode Eurocode 5, Design of timber structures, Part 1-2: General rules-Structural fire design, European Committee for standardization, Brussels, 2005. [20] S. Poncsák, et al., Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera), Wood Sci. Technol. 40 (2006) 647–663, http://dx.doi.org/10.1007/s00226-006-0082-9. [21] E. Frühwald, Effect of high-temperature drying on properties of Norway spruce and Larch, Holz als Roh-und Werkstoff 65 (2007) 411–418, http://dx. doi.org/10.1007/s00107-007-0174-8. [22] Instituto Nacional de Normalizacion NCh 1198, Diseño e cálculo de estructuras en maderas, Santiago, Chile, 2006. [23] M.J. Manriquez, Influência da temperatura sobre as propriedades mecânicas do paricá, Dissertação de mestrado. Programa de Pós-Graduação em Engenharia Civil. Universidade Federal de Santa Catarina, Florianópolis, Brasil 2008, p. 111. [24] M.J. Manriquez, P.D. Moraes, Influence of the temperature on the compression strength parallel to grain of paricá, Constr. Build. Mater. 24 (2010) 99–104, http://dx.doi.org/10.1016/j.conbuildmat.2009.08.003. [25] M.J. Manriquez, P.D. Moraes, Embedding strength parallel and perpendicular to the grain under thermal action. In: IRF2013 Recent Advances in Integrity, Reliability, Failure, Funchal, Portugal, June 23–27, 2013, pp. 645–646. [26] M.J. Manríquez, P.D. Moraes, F.A. Maestri, Temperature and moisture content effects on compressive strength parallel to the grain of Paricá Porto Alegre, Ambiente Construído 15 (1) (2015) 43–53, http://dx.doi.org/10.1590/ S1678-86212015000100003. [27] A. Atreya, Pyrolysis, ignition, and flame spread on horizontal surfaces of wood (Ph.D. thesis), Harvard University, Cambridge, MA 1983, p. 112. [28] F. Shafizadeh, Pyrolytic reactions and products of biomass, In: Overend PP, Milne TA, 1985. [29] R.S. Miller, J.A. Bellan, Generalized Biomass Pyrolysis Model Based on Superimposed Cellulose, Hemicellulose and Lingnin Kinetics, Westerm States Section of the Combustion Institute, 1996, Paper 96F-057. [30] K.M. Bryden, Computational Modeling of Wood Combustion (Ph.D. thesis), University of Wisconsin-Madison, 1998. [31] Associação Brasileira de Normas Técnicas, NBR 7190, Projeto de estruturas de madeira, Anexo G: Classificação visual de madeira serrada de coníferas, Projeto CE-02:126.10. 997 Rio de Janeiro. [32] D.C. Montgomery, G.C. Runger, Applied Statistics and Probability for Engineers, 3rd ed., Wiley, New York, 2003.
Contents lists available at ScienceDirect
Fire Safety Journal journal homepage: www.elsevier.com/locate/firesaf
Temperature reduction factor for compressive strength parallel to the grain Manuel Jesús Manríquez Figueroa a,n, Poliana Dias de Moraes b,n a
Department of Building Engineering, University of Magallanes- Avenida Bulnes, 01855, Cx Postal 113-D, Punta Arenas, Chile Department of Civil Engineering, Federal University of Santa Catarina - Rua João Pio Duarte Silva, s/n, Cx. Postal 476, Córrego Grande, Florianópolis 88040-970, SC, Brazil
b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 December 2015 Received in revised form 25 May 2016 Accepted 29 May 2016 Available online 7 June 2016
The aim of this work is to evaluate the effect of the temperature on the compressive strength parallel to grain of Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda from cultivated forests. The Schizolobium amazonicum sample was formed by 105 clear specimens assembled in 15 groups, the Eucalyptus saligna and Pinus taeda were formed by 150 clear specimens assembled in 15 groups each. Each group of samples was formed by 7, 10 and 10 specimens, respectively. The specimens were tested in a temperature range from 20 to 230 °C. The Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda samples present a decrease in the compressive strength, reaching 70%, 79% and 62% of the compressive strength at room temperature, respectively. The compressive strength decrease of the samples can be associated to the thermal degradation of wood polymers and the moisture content. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Timber Temperature Reduction factor Compressive strength Planted forests
1. Introduction Many studies have shown the influence of temperature on the mechanical properties of wood [1–26]. However, some results, so far, do not completely agree in some respects, especially with regard to the benefits or losses caused by temperature. This difference may be related to various test methods used in the development of the studies. The uniformization of test conditions to determine wood strength under thermal action and actual moisture content is required in order to avoid discrepancies in test results, since these methods differ as regards to the size, moisture content and heating of the specimens. Gerhards [8] summarizes the significant researches reported in the literature on the effects of moisture content and temperature on mechanical properties of wood, in which the cross section of specimens varies in the range from 4.8 mm 4.8 mm to 50.0 mm 50.0 mm. The specimen size interferes directly with the process of heating and drying of the specimens, affecting the time required to obtain a homogeneous temperature and humidity in its inner part. Schaffer [4] and Knudson and Schniewind [5] used samples of Pseudotsuga menziesii (Douglas fir). The former used dried samples, with specimens of 25.4 mm 3.2 mm 95.2 mm while the latter used 12% moisture content samples, with specimens with dimensions n
Corresponding author. E-mail addresses: [email protected] (M.J. Manríquez Figueroa), [email protected] (P. Dias de Moraes). http://dx.doi.org/10.1016/j.firesaf.2016.05.005 0379-7112/& 2016 Elsevier Ltd. All rights reserved.
of 4.8 mm 4.8 mm 19 mm, heated by hot plates placed on the side surfaces of the specimens during 5–20 min. Gerhards [8] presented results obtained from dried samples, with 5 mm 5 mm of cross section, while Young and Clancy [14] used dried and 12% moisture content specimens of 90 mm 90 mm 300 mm of Pinus radiata and different heating periods. Manríquez et al. [26] used air dried and saturated samples with specimens of 5 cm 5 cm 15 cm, heated in the electric oven for 180 min. The study of the influence of temperature on mechanical properties of wood with normal moisture content is a difficult task, since the specimen heating tends to dry it. The variation of moisture content can be avoided by performing the tests in saturated or completely anhydrous conditions. The tests with saturated specimens provide a lower limit resistance while dried specimens provide an upper limit. However, in the latter condition they are more exposed to thermal degradation of wood polymers during the heating process. The exposure of wood to high temperatures causes transient and permanent effects. The timber heating for short periods of time causes a temporary reduction in mechanical strength which is restored when the temperature returns to normal. If high temperature exposure is maintained for an extended period of time, the effects on the timber are permanent [27–30]. Salamon [3] noted that, between 25 °C and 100 °C, the effects of the temperature on mechanical properties of wood vary according to the species and the applied load. Some species show no loss of strength, while others have between 7% and 20% of reduction compared with values obtained at room temperature. Chafe [7]
The specimens, with 12% equilibrium moisture content at ambient temperature, were heated at a constant temperature during 180 min in an electric drying oven with automatic temperature
434 421 428 427 431 422 424 433 423 432 424 430 426 430 427
Mass Density (kg/m3) Mass (kg/m ) Mass density (kg/m )
25 25 24 23 22 25 24 25 24 22 22 24 25 25 23 776 788 772 771 775 776 774 770 774 769 778 772 777 773 771 38 37 37 36 35 37 35 35 34 36 38 38 38 41 40 331 334 337 336 338 347 348 349 351 355 356 343 360 367 369
Schizolobium amazonicum [24]
3
20 40 50 60 70 80 90 100 110 130 150 170 190 210 230
2.2. Samples heating
Table 1 Average mass density of specimen groups at 20 °C and 12% of moisture content.
Three timber samples were used in this study, all from planted forests. The sample of the Schizolobium amazonicum was formed by 105 specimens [24], while the samples of the Eucalyptus saligna and Pinus taeda were formed by 150 specimens each, tested at different levels of temperatures. The physical properties and characteristics of the samples and the temperature range are shown in Table 1. The specimens were prepared with timber classified visually according to the specifications of the Brazilian standard [31]. They were clear, defect-free, straight-grained material cut from regions distant at least 30 cm from the tip. The specimen dimensions, 50 mm 50 mm 150 mm, are those defined in the Brazilian standard [31]. Cross-section dimensions and length were measured with accuracy of 0.01 mm. Special care was taken in preparing the specimen to ensure that the end surfaces are parallel and that both are aligned perpendicular to the longitudinal axis. The distribution of specimens in sets was done in order to constitute groups with densities statistically homogeneous. The homogeneity was verified by variance analysis, using statistically significant differences. The method used to compare the means was Fisher least significant difference (LSD) procedure, with a confidence level of 95% [32].
3
2.1. Samples and specimens
Standard deviation (kg/m )
2. Materials and methods
3
Eucalyptus saligna
3
Standard deviation (kg/m )
Pinus taeda
Standard deviation (kg/m3)
reports that the wood strength reduction is related to the thermal degradation of its polymers. According to Gerhards [8], Kollmann and Côté [2] and Green et al. [12], the timber strength is connected in reverse order at room temperature. However, little influence was found in static bending [9]. Milota [13] heated samples of Western hemlock (Tsuga heterophylla) and Spruce Fir (Abies spp.) at 80 °C, 116 °C and 132 °C and observed a reduction on the strength between 40% and 50%. Frühwald [21] conducted tests with samples of Norway spruce and Larch, with 6% of moisture content. The specimens were heated at 80 °C, 120 °C and 170 °C. Samples of the same species were heated at 190 °C and 210 °C. The results indicated that the temperature affects the compressive and tensile strength parallel to the grain, but the results were different for the various species. Manríquez [23] studied the paricá (Schizolobium amazonicum) from planted forests in the Amazon region. In this study, compression, tensile, shear and embedment tests were performed in the range from 20 °C to 230 °C, leading to the conclusion that the temperature directly influences the reduction of the mechanical strength of the timber. Further studies, using specimens with dimensions specified by international standards, may be required in order to evaluate the strength reduction factor and to find a behavior pattern. The aim of this work is to evaluate the effect of the temperature on wood of Schizolobium amazonicum (paricá), Eucalyptus saligna (Sydney bluegum) and Pinus taeda (loblolly pine) from cultivated forests and to determine the temperature reduction factor for the compressive strength parallel to grain of wood in a temperature range from 20 to 230 °C. This paper is organized as follows: a section on materials and methods used to carry out the experiments, a results and discussion section and a section presenting the conclusions of this research.
46 42 45 45 45 42 44 45 44 45 44 45 45 45 45
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Temperature (°C)
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Fig. 3. Force displacement curve.
Fig. 1. Specimens used to determine the heating period.
control and internal dimension of 90 cm 100 cm 66 cm, in order to ensure homogeneous temperature inside the specimens. The heating time of the specimens was determined from previous study, in which the temperature of the specimen center was monitored until it reached the furnace temperature. The temperature level and stabilization were checked by means of a type K thermocouple, inserted into a hole of 2.0 mm diameter, whose end was located at the geometric center of the specimen (Fig. 1).
101
the ultimate compression force as illustrated in Fig. 3.
fc 0, θ =
Fc 0, max , A
(1)
where Fc0,max is the ultimate force, in N, applied to the specimens at temperature level θ and A is the cross section of the specimen after the heating, in mm2. 2.5. Determination of the moisture content
2.3. Mechanical tests The heated specimens were placed in an environmental chamber with internal dimension of 37 cm 50 cm 52 cm (Fig. 2), both at the same constant temperature, as specified in Table 1. The tests were performed with a load cell of 200 kN 72 kN and the load was applied continuously increasing at a constant rate of displacement of 2 mm min 1. The tests ended with the specimen rupture, which took 5–10 min. 2.4. Determination of compressive strength parallel to grain Compressive strength values (fc0,θ) for different levels of temperature were calculated according to Eq. (1) and determined from
The moisture content of the specimens before the preheating was 12%. The determination of the specimen moisture content after the mechanical test was carried out using the same specimen. After the rupture, the specimen was immediately weighted for the determination of the final mass (mf), and then submitted to drying at a maximum temperature of 103 72 °C, as required by the Brazilian Standard [30], until the difference between two consecutive measures was less or equal to 0.5% of the last measured mass. In this condition, the measured mass is considered to be the dry mass (md). The moisture content at the end of the tests was determined by Eq. (2)
mc =
mf − md ⋅100. md
(2)
2.6. Determination of characteristic values of the compressive strength The determination of characteristic values of strength, at different temperature levels, was performed according to the statistical methodology of Student t distribution, with ν ¼ n 1 degrees of freedom [32], using Eq. (3). The characteristic value is defined as the smallest value that has only 5% probability of being exceeded in a given lot.
fkθ = f¯θ − t⋅Sn,
(3)
where f¯θ is the mean value of the strength at temperature θ, in MPa; t¼1.833 for 9 degrees of freedom and t¼1.943, for 6 degrees of freedom; Sn is the sample standard deviation at temperature θ; n is the number of specimens of the sample at temperature θ. 2.7. Determination of the reduction factor Fig. 2. Specimen in the environmental chamber attached to the testing machine after a test at 190 °C.
The reduction factors as functions of the temperature of the
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compressive strength parallel to the grain of threes wood species were determined by Eq. (4)
kθ =
fkθ , fk,20 oC
(4)
where fkθ is the characteristic strength property at temperature level θ, in MPa; fk,,20 °C is the characteristic strength property at room temperature, in MPa.
3. Results and discussion In this section, the results on the types of failure and the compressive strength parallel to grain of the Schizolobium amazonicum [24], Eucalyptus saligna and Pinus taeda are presented. All mechanical data concerning the compressive strength were statistically treated. The analysis of variance was carried out in order to verify statistically significant differences, with 95% reliability, between the average mechanical values at a given temperature and those at room temperature [32]. 3.1. Moisture content In Fig. 4, the moisture content of the specimens of the 3 samples is shown. It decreases linearly with the temperature increase, presenting 0% moisture content at 150 °C for both samples. At temperatures higher than 150 °C, the specimens were completely dry. According to Schaffer [4], at 140 °C, the water contained in the components of wood begins to be released and the polymer begins to degrade. As the temperature increases, so does the energy level of the molecules and constituents. This energy is enough to modify the wood internal structure, decreasing the fiber moisture content. 3.2. Specimen failure During the compression parallel to grain tests, failures occurred at different temperature levels. From Fig. 5a, in which the results for the compression parallel to grain tests of Schizolobium amazonicum is presented [24], it can be concluded that the form of rupture varies with the test temperature. According to Bodig and Jayne [10], the rupture of the specimens under compression parallel to grain is caused by the maximum shear forces at an angle with the loaded surface and associated with a weaker plan, as presented by the theory of Solid Mechanics. The types of failures of the specimens of Eucalyptus saligna are presented in Fig. 5b. It was observed that during the tests, there were two types of failures. The first occurred between 20 °C and
130 °C, located at the center of the specimen. The second occurred near the ends of the specimens in the temperature range from 150 °C to 230 °C. In this temperature range, deep cracks occur on the faces of the specimens due to severe heating. These cracks cause planes of weakness in the specimens, which lead to its rupture. Longitudinal cracks were also observed on the faces of the specimens of Eucalyptus saligna (Fig. 5b) for the same temperature range. These may be due to the sudden heating and, especially, the high values of the specimens density. All specimens with density above 700 kg/m3 presented these types of failure and fracture. The types of failures in the specimens of Pinus taeda are presented in Fig. 5c. It was observed that this kind of failure is affected by the test temperature. At room temperature, the failure line is nearly horizontal, while, at 230 °C, the failure line is slightly above 45°. The results obtained by Young and Clancy [14] for the Pinus radiata, in the range of 20 °C and 250 °C, in compression parallel to grain tests, are similar to those obtained in this study with Pinus taeda. 3.3. Compressive strength parallel to the grain In Fig. 6, the compressive strength results for the 3 samples are presented. At room temperature, the Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda samples present an average strength and standard-deviation of 32 MPa74 MPa [24]; 51 MPa7 4 MPa; 24 MPa72 MPa, respectively. The results show that a continuous strength decrease exists with the increase of temperature up to 80 °C; afterwards, an increase exists up to 150 °C, when the moisture content of the specimens is 0% (Fig. 4). For the Schizolobium amazonicum sample, the analysis of variance indicates that there is a statistically significant difference, with 95% confidence, between the compressive strength parallel to grain at 20 °C and at other temperature levels, except at 150 °C [24]. For the Pinus taeda and Eucalyptus saligna samples, a difference is found between the data obtained at room temperature and for those heated specimens. 3.4. Deformation at maximum compression force The deformation results, at maximum compression force, for the three samples are presented in Fig. 7, including the deformation by crushing in the specimens ends. They vary with the increase of the temperature. The highest deformation was presented by the Pinus taeda, indicating that it is the softest sample since it has the lowest strength (Fig. 6) while the lowest total deformation was presented by the Schizolobium amazonicum sample indicating that it is the stiffest sample. The deformation remains constant between 40 and 60 °C and between 130 and 170 °C, as shown by the plateaux in Fig. 7. These plateaux may be related to the lignin softening as remarked by Schaffer [4]. From 170 °C onwards, the deformation decreases again. It is important to remark that specimens had 0% of moisture content from 150 °C onwards (Fig. 4), when the total deformation of the three samples decreases, indicating the influence of moisture content in the behavior of wood. 3.5. Temperature reduction factor for compressive strength parallel to grain
Fig. 4. Moisture content.
The temperature reduction factors for compressive strength parallel to grain of three species studied in this research and those found in the literature and proposed by NDS [18], NCh 1198 [22] and EN 1995 1-2 [19] are presented in Fig. 8. The results of the Pinus taeda samples are similar to those obtained by Knudson and Schniewind [5] for Pseudotsuga menziesii (Douglas fir). Both tests used samples of conifer species with 12% initial moisture content. After 150 °C, the results agree well with the values
M.J. Manríquez Figueroa, P. Dias de Moraes / Fire Safety Journal 83 (2016) 99–104
20 0 ºC 230 ºC (aa) S. amazonicuum [24].
20-130 ºC
C 150-230 ºC (b) E. salignna
150-230 ºC
103
20 ºC 230 0 ºC (c) P. taeda
Fig. 5. Patterns of rupture.
Fig. 6. Compressive strength parallel to the grain.
Fig. 7. Deformation at maximum compression force.
presented by Schaffer [4] using dry samples of Pseudotsuga menziesii, with specimens dimension of 25.4 mm 3.2 mm 95.2 mm and with the results of Young and Clancy [14], using oven dry samples of Pinus radiata with specimens dimension of 90 mm 90 mm 300 mm. At 100 °C, the results of this study are similar to those found by Young and Clancy [14], using 12% moisture content samples of Pinus radiata, by Manríquez et al. [26] using water saturated samples of Schizolobium amazonicum. Above 150 °C, the reduction factor values of the Schizolobium amazonicum and Pinus taeda samples, both low mass density woods (Table 1), decrease with the temperature increase and tend
Fig. 8. Reduction factor of compressive strength parallel to the grain.
to zero near 250 °C, while the EN 1995 1-2 standard [19] considers zero strength at 300 °C. At 230 °C, the maximum reduction for the Schizolobium amazonicum is 70%, for the Eucalyptus saligna, is 79% and for the Pinus taeda is 62%. Among the three species studied, Eucalyptus saligna was the species most sensitive to the effect of temperature. The values found by Young and Clancy [14], using dry oven samples of Pinus radiata, are the upper limit of the reduction factors. The values presented by the NDS and EN 1995 1-2 standards have a similar behavior between 20 and 70 °C [18,19]. However, they are lower than the values recommended by the NCh 1198 standard [22]. For the same temperature range, the temperature reduction factors obtained in this study for the hardwood and softwood species have a similar behavior as predicted by those standards although they are higher than those presented by the European standard [19]. In the range from 70 to 230 °C, the reduction factors obtained are higher than those presented by EN 1995 1-2 [19] and lower than the values found by Young and Clancy [14], using dry oven samples. For heavy sections, EN 1995 1-2 [19] consider that the gradient temperature in a timber section is very steep and only a small part of the section under the charred layer is heated and has its strength altered, which has a small effect on the resistance of the whole residual section. However, as demonstrated by Young and Clancy [14], for light-framed timber structures, the temperature affected zone may represent a significant percentage of the cross section of the structural element. Further studies may be needed for other hardwood and softwood species in order to find a behavior pattern.
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4. Conclusions This study evaluated the effect of the temperature on the compressive strength parallel to the grain of Schizolobium amazonicum, Eucalyptus saligna and Pinus taeda. For the three samples used, the results have shown that: – the rupture patterns of the specimens are associated with the temperature levels of the tests. – at ambient temperature, the Eucalyptus saligna has a greater compressive strength parallel to the grain than the other studied species, and also the greatest reduction of mechanical strength; – Pinus taeda sample is the softest while the Schizolobium amazonicum is the stiffest of the studied species. – in the temperature range from 20 to 70 °C, the reduction factors have a similar behavior as given by the standards but, between 70 and 230 °C, the reduction factors are higher than those presented by EN 1995 1-2 [19], though extensive studies are needed to allow the generalization of this behavior for hardwood and softwood species. – although for heavy sections only a small part of the section under the charred layer has its strength altered, for light-framed timber structures, the temperature affected zone may represent a significant percentage of the cross section of the structural element.
Acknowledgments The authors thank Dr. Rodrigo Figueiredo Terezo, the timber industries Caswood and Klabin for providing the wood used in the tests and the Brazilian Research Council (CNPq) for sponsoring this research (Grant no. 303223/2013-8).
References [1] E. Sano, Effects of temperature on the mechanical properties of wood. I. Compression parallel to grain. II. Tension parallel to grain. III. Torsion test, J. Jpn. Wood Res. Soc. 7 (4) (1961) 147–150. [2] F.F.P. Kollmann, W.A. Côté Jr., Principles of Wood Science and Technology, in: Solid wood, Springer, Berlin Heidelberg New York, 1968, p. 592. [3] M. Salamon, High temperature and its effect on wood properties, For. Prod. J. 19 (3) (1969) 27–34. [4] E.L. Schaffer, Effect of pyrolytic temperatures on the longitudinal strength of dry Douglas fir, J. Test. Eval. 1 (4) (1973) 319–329. [5] R.M. Knudson, A.P. Schniewind, Performance of structural wood members exposed to fire, For. Prod. J. 25 (2) (1975) 23–32. [6] C.C. Gerhards, Effect of high temperature drying on tensile strength of Douglas fir 2 4’, For. Prod. J. 29 (3) (1979) 39–46. [7] S.C. Chafe, Growth stresses in trees, Aust. For. Res. 9 (1979) 203–223. [8] C.C. Gerhards, Effect of the moisture content and temperature on the mechanical properties of wood an analysis of immediate effects, Wood Fiber 14
(1) (1982) 4–36. [9] C.C. Gerhards, Effects of high temperature drying on bending strength of yellow poplar 2 by 4's, For. Prod. J. 33 (29) (1983) 61–67. [10] J. Bodig, B.A. Jayne, Mechanics of Wood and Wood Composites, Nostrand Reinhold, New York, 1993. [11] A.M. Olsson, L. Salmén, Humidity and temperature affecting hemicellulose softening in wood. in: International Conference of COAST Action E8, Denmark, 1997, pp. 269–279. [12] D.W. Green, J.E. Winandy, D.E. Kretschmann, Mechanical properties of wood, in: Wood handbook wood as an engineering material. Gen Tech Rep FPL-GTR113, US Department of Agriculture Madison, 1999, pp. 1–45. [13] M.R. Milota, Warp and shrinkage of hem-fir stud lumber dried at conventional and high temperatures, For. Prod. J. 50 (11) (2000) 79–84. [14] S.A. Young, P. Clancy, Compression mechanical properties of wood at temperatures simulating fire conditions, Fire Mater. 25 (2001) 83–93, http://dx. doi.org/10.1002/fam.759. [15] P. Bekhta, P. Niemz, Effect of temperature on color and strength of spruce wood, Holzforschung 57 (5) (2003) 539–546. [16] P.D. Moraes, et al., Influence of temperature on the Modulus of Elasticity (MOE) of Pinus sylvestris L, Holzforschung 58 (2004) 143–147. [17] P.D. Moraes, Y. Rogaume, P. Triboulot, Influence of temperature on the embedding strength, Holz als Roh-und Werkstoff 63 (2005) 297–302, http://dx. doi.org/10.1007/s00107-004-0568-9. [18] National Design Specification (NDS) For Wood Construction, American Forest e Paper Association, 2005, p. 174. [19] European Standard Eurocode Eurocode 5, Design of timber structures, Part 1-2: General rules-Structural fire design, European Committee for standardization, Brussels, 2005. [20] S. Poncsák, et al., Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera), Wood Sci. Technol. 40 (2006) 647–663, http://dx.doi.org/10.1007/s00226-006-0082-9. [21] E. Frühwald, Effect of high-temperature drying on properties of Norway spruce and Larch, Holz als Roh-und Werkstoff 65 (2007) 411–418, http://dx. doi.org/10.1007/s00107-007-0174-8. [22] Instituto Nacional de Normalizacion NCh 1198, Diseño e cálculo de estructuras en maderas, Santiago, Chile, 2006. [23] M.J. Manriquez, Influência da temperatura sobre as propriedades mecânicas do paricá, Dissertação de mestrado. Programa de Pós-Graduação em Engenharia Civil. Universidade Federal de Santa Catarina, Florianópolis, Brasil 2008, p. 111. [24] M.J. Manriquez, P.D. Moraes, Influence of the temperature on the compression strength parallel to grain of paricá, Constr. Build. Mater. 24 (2010) 99–104, http://dx.doi.org/10.1016/j.conbuildmat.2009.08.003. [25] M.J. Manriquez, P.D. Moraes, Embedding strength parallel and perpendicular to the grain under thermal action. In: IRF2013 Recent Advances in Integrity, Reliability, Failure, Funchal, Portugal, June 23–27, 2013, pp. 645–646. [26] M.J. Manríquez, P.D. Moraes, F.A. Maestri, Temperature and moisture content effects on compressive strength parallel to the grain of Paricá Porto Alegre, Ambiente Construído 15 (1) (2015) 43–53, http://dx.doi.org/10.1590/ S1678-86212015000100003. [27] A. Atreya, Pyrolysis, ignition, and flame spread on horizontal surfaces of wood (Ph.D. thesis), Harvard University, Cambridge, MA 1983, p. 112. [28] F. Shafizadeh, Pyrolytic reactions and products of biomass, In: Overend PP, Milne TA, 1985. [29] R.S. Miller, J.A. Bellan, Generalized Biomass Pyrolysis Model Based on Superimposed Cellulose, Hemicellulose and Lingnin Kinetics, Westerm States Section of the Combustion Institute, 1996, Paper 96F-057. [30] K.M. Bryden, Computational Modeling of Wood Combustion (Ph.D. thesis), University of Wisconsin-Madison, 1998. [31] Associação Brasileira de Normas Técnicas, NBR 7190, Projeto de estruturas de madeira, Anexo G: Classificação visual de madeira serrada de coníferas, Projeto CE-02:126.10. 997 Rio de Janeiro. [32] D.C. Montgomery, G.C. Runger, Applied Statistics and Probability for Engineers, 3rd ed., Wiley, New York, 2003.