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Grain angle and temperature effect on embedding strength
Construction and Building Materials 150 (2017) 442–449
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Grain angle and temperature effect on embedding strength Eduardo Schneid a,⇑, Poliana Dias de Moraes b a b
Federal University of Santa Catarina – UFSC, 205 João Pio Duarte da Silva St., Florianópolis, SC 88037-000, Brazil Federal University of Santa Catarina – UFSC, Department of Civil Engineering, 205 João Pio Duarte da Silva St., Florianópolis, SC 88037-000, Brazil
h i g h l i g h t s A continuous function of the reduction factor of the embedding strength is proposed. The influence of the temperature on the embedding strength for different grain angles. The relation between the glass transition temperature and the decrease of the embedding strength.
a r t i c l e
i n f o
Article history: Received 13 October 2016 Received in revised form 18 May 2017 Accepted 2 June 2017
Keywords: Embedding tests High temperature Bolted timber joints
a b s t r a c t The objective of this research is to evaluate the effect of temperature and grain angles on the wood embedding strength in the range from 20 to 140 °C. To this end, a reduction factor of the wood embedding strength is proposed as a continuous function of the grain angle and the temperature, for use in modeling of timber bolted connections. Embedding tests were carried out with 252 specimens of Eucalyptus grandis, according to ASTM D 5764-97a [35] standard. The embedding strength, reduction factor, glass transition temperature and moisture content were determined. The minimum values of the characteristic embedding strengths, parallel (fh,0,k) and perpendicular (fh,90,k) to the grain, were observed at 60 and 80 °C, respectively. For all grain angles, except 15°, the embedding strength presented the first relative minimum at 60 °C. Based on the experimental results, a continuous function of the reduction factor of the embedding strength was then determined. To predict the load-bearing capacity of bolted joints through numerical models, the reduction factor of the embedding strength from the present paper can be used in future research. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sample and specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Determination of the heating time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Heating of the specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Embedding test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Determination of embedding strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Embedding strength adjustment for 12% moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Mean embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10. Characteristic reduction factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11. Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12. Statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail addresses: [email protected] (E. Schneid), [email protected] (P.D. de Moraes). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.015 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
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4.
3.2. Effect of the heating on the specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Mean embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Reduction factor of the characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Usually, the load-bearing capacity of the wood bolted connection is estimated by the mathematical model of a beam under a deformable foundation, formulated by Johansen [27] or by the finite elements method [28–31]. In the Johansen model [27], the limit analysis theory defines the load-bearing capacity of a single connector, in which the ideal plasticity of both steel and timber is assumed and, besides that, the dowel is assumed rigid-plastic. In this theory, the stress level of the wood plastic failure is represented by the embedding strength, while, in the many models based on finite elements, the wood compressive strength is used as failure parameter. From Hong and Barret [32], the finite element models are based on compressive strength. However, the wood behavior in compression differs from the one in embedding situation, due to the stresses at the contact zone between the wood and the bolt. In bolted joints under bending moment, the direction of the efforts on the bolts varies in relation to the grain. In order to estimate the joint load-bearing capacity, the embedding strength should be considered as a function of the loading direction according to the grain [33]. Therefore, the objective of this research is to evaluate the effect of the temperature and the grain angles on the wood embedding strength in the range from 20 to 140 °C. To that end, a reduction factor of the wood embedding strength is proposed as a continuous function of the grain angle and the temperature, which can be used in modeling of timber bolted connections.
1. Introduction The wood embedding strength is the main parameter in order to assess the load-bearing capacity for the design of bolted connections. In normal reference conditions, the wood embedding strength, parallel and perpendicular to the grain, is determined at 20 °C and 12% moisture content. However, the physical, chemical and mechanical wood properties are modified under thermal action [1–7]. Some studies have shown that the embedding strength, parallel and perpendicular to the grain, and the loadbearing capacity of timber connections decrease when subjected to high temperatures [8–14]. Manriquez [7] evaluated embedding strengths parallel and perpendicular to the grain of the Eucalyptus saligna in the range from 20 to 230 °C. The author observed decreases of embedding strength by 33% at 70 °C in the parallel direction and by 43% at 100 °C in the perpendicular direction. As a similar study, Moraes et al. [8] noticed that the embedding strength parallel to the grain of the Pinus sylvestris drops by 30% at 80 °C. The reduction of the mechanical properties as function of the temperature is not taken into consideration in the EN 1995-1-1: 2004 standard [15]. However, it is contemplated for structural fire design in the EN 1995-1-2 standard [16], in which the reduction factor for strength and modulus of elasticity parallel to the grain of softwood are presented as function of the temperature. The reduction of the mechanical properties with increase of the temperature may be related to the glass transition temperature of wood. Conform with the literature [17–19], the lignin glass transition temperature is between 60 and 90 °C. Within this temperature range, the lignin softening may arise, causing then the reduction of the wood strength. Standards only describe experimental procedures to determine the wood embedding strengths parallel (0°) and perpendicular (90°) to the grain of the wood. For other grain angles, the Hankinson equation is used, requiring the wood strengths perpendicular and parallel to the grain [15,20–26]. As this equation was initially developed for room temperature (25 °C), embedding strength behavior in other temperatures and grain angle conditions are unknown.
2. Material and methods 2.1. Sample and specimens In this research, 252 clear specimens of Eucalyptus grandis, with density of 718 kg/m3 and mean moisture content of 15%, were used. The specimens were assembled in order to present sets with statistically homogeneous densities (Table 1). The homogeneity was verified by variance analysis (ANOVA) with 95.0% confidence level [34]. The experimental program considers 6 temperature levels and 7 grain angles. For each combination of one temperature level and one grain angle, 6 specimens were used. The specimens are featured by rectangular parallelepiped elements with half a dowel hole across one face, by modifying the grain angles a with respect to the vertical loading applied on the fastener as shown in Fig. 1. Conform with ASTM D 5764-
Table 1 Mean densities of the samples. Grain angle (°)
Density (kg/m3) Temperature (°C) 20
0 15 30 45 60 75 90
740 730 720 716 724 731 733
60 (78) (85) (80) (84) (75) (84) (72)
Values between parentheses are the standard deviations.
719 699 735 726 734 715 713
80 (75) (84) (93) (92) (92) (92) (88)
739 714 749 735 745 723 726
100 (75) (71) (84) (90) (91) (87) (86)
706 709 719 721 736 710 706
120 (120) (96) (114) (112) (119) (120) (119)
706 697 712 703 705 693 713
140 (90) (79) (101) (100) (100) (106) (92)
713 702 712 714 724 705 712
(85) (69) (77) (84) (96) (85) (87)
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Fig. 1. Embedding test specimens front view: (a) 0°, (b) 15°, (c) 30°, (d) 45°, (e) 60°, (f) 75°, (g) 90° and (h) side view. Dimensions in mm.
Table 2 Heating time determination. Temperature level (°C)
Heating time (min)
60 80 100 120 140
90 120 150 150 120
Fig. 3. Embedding test setup. (a) support; (b) specimen; (c) bolt; (d) loading plate. Fig. 2. Heating study. (a) specimen; (b) thermocouple. 97a [35], the dimensions of samples were checked properly. The tests were carried out by using partially threaded bolts characterized by: 8.8 and 10 mm diameter, yield strength of 640 MPa and tensile strength of 800 MPa [36]. Note that the smooth shank of the bolt must carefully fit into the full slot length for each sample. Therefore, the threaded part does not come into contact with the wood specimens.
The maximum heating time of 150 min was obtained at the temperature levels of 100 and 120 °C (Table 2) and it was adopted for all samples. A shorter heating time at 140 °C may be attributed to a quicker increase of the wood surface temperature, leading to a faster drying and higher rate of the internal temperature rise. Conform with the literature [37–40], the drying rate actually increases with the temperature.
2.2. Determination of the heating time 2.3. Heating of the specimens The heating time was determined by a preliminary study, in which specimens were heated at five temperature levels in an oven at a constant temperature (Table 2), until the specimens internal temperature reached the oven temperature. The temperature was measured by a K thermocouple (Fig. 2), placed inside an orifice with 3 mm-diameter in the specimen center and connected to a data acquisition system.
The specimens were previously heated in an electric oven at the temperature test during 150 min, in order to homogenize their inner temperature. After the specimen reached the specified temperature (Table 2), it was removed from the oven and then performed under embedding test in a universal testing machine provided with a heating chamber at the same oven temperature.
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2.8. Mean embedding strength The mean embedding strength (f h;ah;m ) was calculated by Eq. (4) as the arithmetic average of the sample strength values obtained for each combination of temperature and grain angle.
f h;ah;m ¼
X f h;ah n
;
ð4Þ
in which fh,ah,m is the mean embedding strength at a grain angle a and a temperature h, in MPa and n is the number of specimens. 2.9. Characteristic embedding strength The characteristic embedding strength is defined as the lower 5th percentile. This means that at least, 95% of the embedding strength values should be higher than the characteristic value [16]. The embedding strength values are normally distributed and the characteristic values at the grain angle a and the temperature h were determined using the Student’s t distribution [34], with n-1 degrees of freedom [41], according to Eq. (5).
Fig. 4. Force-displacement curve.
The electric oven, with internal dimensions of 99 cm 100 cm 66 cm, had, besides natural ventilation, an automatic temperature control with accuracy of ±1 °C.
2.4. Embedding test Embedding tests were carried out in a universal testing machine with a heating chamber (Fig. 3), according to the ASTM D 5764-97a standard [35]. A monotonic loading in the compression direction was applied at a rate of 1.9 mm/min. The load-rate was determined from preliminary tests, so that each test takes between 1 and 10 min [35]. The force was measured by a load cell with capacity of 200 kN and the displacement was measured by a system of the testing machine.
s2 f h;ah;k ¼ f h;ah;m t: pffiffiffi ; n
in which fh,ah,k is the characteristic embedding strength at the grain angle a and the temperature h, in MPa; fh,ah,m is the mean embedding strength at the grain angle a and the temperature h, in MPa; t is parameter of Student’s t distribution (t = 2.0150 for n1 = 5); s2 is the standard deviation and n is the number of specimens. 2.10. Characteristic reduction factor The characteristic reduction factor (kah;k ) is the ratio between the characteristic embedding strength at the grain angle a and the temperature h and the characteristic embedding strength parallel to the grain at 20 °C. It was determined by Eq. (6).
kah;k ¼ 2.5. Determination of embedding strength
F ah ; d:t
2.11. Differential Scanning Calorimetry (DSC)
2.6. Moisture content After the experimental assessment of the embedding strength, the moisture content of the tested specimens was determined by Eq. (2). The procedure is an adaptation of the oven-drying method proposed by the NBR 7190 standard [20]. The specimens were weighed and dried in an electric oven at a temperature of 103 ± 2 °C. Assuming that the endpoint has been reached when the mass loss is equal or less than 0.5%, the last measurement is considered an oven-dry mass.
mwet mdry ð100Þ; mdry
ð2Þ
in which mwet is the wet mass, in g, and mdry is the oven-dry mass, in g.
Conform with the NBR 7190 standard [20], the embedding strength adjustment for 12% moisture content was calculated by Eq. (3), only for the wood specimens performed under room temperature tests.
f 12
in which f% is the embedding strength, in MPa, at moisture content of U%.
The Differential Scanning Calorimetry test (DSC) was performed to identify the glass transition temperature of the wood and the possible influence of this parameter on the embedding strength. A 10 mg sample of ground wood with 15% moisture content and a Setaram SetSys equipment were used for the test. The analysis was performed at a temperature range from 25 to 150 °C, with heating and cooling steps at a rate of 15 °C/min under an argon atmosphere (flow rate of 50.0 mL/min). 2.12. Statistical analysis Statistical analysis was applied to assess the influence of temperature on the embedding strength for each grain angle. In this analysis, the dependent variable was the embedding strength at a grain angle while the independent variable was the temperature. Parametric or nonparametric analyses were applied, depending on the assumption of independence, normality and homogeneity of variance of data [34]. The parametric test for the analysis of variance (ANOVA) was used based on the assumption of data independence, normality and homogeneity of variance, valid for the angles 0°, 60° and 90°. The F test, at 5% probability of error, was used to compare the means. When the null hypothesis of F test was rejected, the Tukey’s test, at 5% probability of error, was applied to verify if the means are significantly different. The nonparametric Kruskal-Wallis test was applied when the assumption of normality and homogeneity of variance was violated, which occurred for angles of 15°, 30°, 45° and 75°. In case of rejection of the null hypothesis of the nonparametric test, multiple comparisons tests were applied [34].
3. Results and discussion
2.7. Embedding strength adjustment for 12% moisture content
3ðU 12Þ ¼ f% 1 þ 100
ð6Þ
ð1Þ
in which fh,ah is the embedding strength at a grain angle a and a temperature h, in MPa; Fah is the embedment force at a grain angle a and a temperature h, in N; d is the bolt diameter, in mm; and t is the specimen width, in mm. The force value used for determining the embedding strength of the specimen was defined by the intersection of a line parallel to the linear segment of the force-displacement curve and offset of 5% of the diameter of the bolt with the same curve (Fig. 4), according to the ASTM D 5764-97a standard [35]. In those cases where the offset line does not intersect the force-displacement curve, the maximum load is used.
MCð%Þ ¼
f h;ah;k ; f h;020;k
in which f h;ah;k is the characteristic embedding strength at the grain angle a and the temperature h and f h;020;k is the characteristic embedding strength parallel to the grain at 20 °C.
The embedding strength of each specimen was determined by Eq. (1).
f h;ah ¼
ð5Þ
ð3Þ
3.1. Moisture content The mean values of the moisture content for each grain angle and temperature are presented in Fig. 5. The moisture content decreases when temperature rises, as expected. Due to the heating time of 150 min, at 140 °C, the specimens were basically anhydrous. Between 20 and 100 °C, the specimens can contain bound
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Fig. 5. Mean moisture content of the specimens.
ferences of moisture between the surface and the center of the specimens [44,45]. These differences lead to the emergence of tensile stress perpendicular to the grain which cause cracks in the structure of the wood if they exceed the resistance of the ligneous tissue [44,45]. Thereby, the small dimensions of wood specimens from the present research may reduce the difference of moisture content between the surface and the center, by preventing then withdrawal cracks. 3.3. Differential Scanning Calorimetry (DSC)
Fig. 6. DSC curve: (A) First endothermic peak and (B) second endothermic peak.
water and water of constitution while the latter only remains inside the wood samples for temperatures above 100 °C. According to Moreschi [42], bound water occurs in moisture content ranging from 6% to 28%, while water of constitution occurs between 0% and 6%. From the author [42], loss of bound water modifies the weight of wood and draws closer micelle, microfibril and macrofibril, by raising then the wood rigidity. Therefore, the wood strength can vary due to loss of bound water. Bodig and Jayne [43] concluded that loss of water of constitution can cause the reduction of the wood mechanical properties. 3.2. Effect of the heating on the specimens When heating the wood specimens in the oven, no cracking or warping was noticed. However, Manriquez [7] observed the appearance of small cracking at the edge of wood specimens heated at a temperature higher than 40 °C. This can be explained by the dimensions of the specimens used by Manriquez [7], 25 mm 60 mm 140 mm, while in this research they were 20 mm 50 mm 50 mm. Cracks appear due to the difference of retraction in the wood radial and tangential directions and the dif-
The result of DSC tests of ground Eucalyptus grandis wood is presented in Fig. 6. Two endothermic peaks, indicated by letters A and B, are observed. The first one (letter A) occurs at 57 °C and is attributed to the glass transition temperature (Tg) of Eucalyptus grandis wood. This result was similar to those found by Irvine [17] for Eucalyptus regnans, whose glass transition temperature of ground wood with 15% moisture content was about 60 °C. According to Kelley et al. [46], the first endothermic peak is due to the relaxation enthalpy and is related to the a thermal transition, corresponding to the Tg of the wood. Lisperguer et al. [47] and Roig et al. [48] also observed this phenomenon. The second endothermic peak (B) occurs at 135 °C and is associated to the activation energy necessary for wood drying and for breaking the chemical bonds of wood components [49,50]. In dry conditions, the wood polymers presented different glass transition temperatures. For the cellulose, it is between 200 and 250 °C; for hemicellulose, between 150 and 220 °C and, for lignin in situ, above 205 °C [19]. However, the glass transition temperature is affected by the wood moisture content and decreases as the humidity increases [51]. At fiber saturation point (FSP), the glass transition temperature of lignin is between 60 and 90 °C; while for hemicellulose, the temperature is near 25 °C [51,17]. 3.4. Mean embedding strength The results of embedding strength are presented in Table 3. For grain angles of 0, 60 and 90, the mean embedding strengths at 20 °C are significantly different from those for other temperature levels, as shown by the different lowercase letters following the mean values. For all grain angles, except 45°, the mean embedding strength at 100 °C is lower than at 20 °C. Moraes et al. [8], Man-
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E. Schneid, P.D. de Moraes / Construction and Building Materials 150 (2017) 442–449 Table 3 Experimental results of the embedment test for different angle to the grain. Grain angle (°)
fh,ah,m (MPa) Temperature (°C)
0 15 30 45 60 75 90
20
60
80
100
120
140
44.0 a (4.4) 41.7 a (7.6) 29.8 a (7.6) 27.7 a (5.7) 27.6 a (3.6) 26.5 a (6.2) 26.7 a (4.1)
31.0 b (7.6) 26.6 bc (2.7) 22.6 ab (2.8) 19.8 a (3.1) 19.8 b (4.4) 19.2 ab (4.0) 19.4 b (3.8)
31.6 b (4.4) 26.9 bc (2.1) 22.2 ab (1.6) 20.9 a (1.8) 20.4 b (3.6) 18.9 ab (1.8) 17.1 b (2.9)
30.2 b (4.8) 27.0 bc (2.4) 18.8 b (3.3) 18.7 a (1.7) 19.0 b (3.0) 16.9 b (1.9) 17.3 b (2.4)
33.4 b (5.0) 26.9 bc (4.1) 23.5 ab (3.7) 21.8 a (7.8) 19.6 b (4.5) 18.1 ab (1.6) 19.9 b (4.0)
33.6 b (5.6) 30.3 ac (1.6) 22.7 ab (3.0) 20.6 a (3.6) 19.9 b (2.7) 19.0 ab (2.1) 19.5 b (3.9)
fh,ah,m is the mean value of the embedding strength at the grain angle a and at the temperature h; values between parenthesis are the standard deviation. If the letter combinations corresponding to two mean values, in the same line, have a common letter, they have no statistic difference at a 5% probability of error by the Tukey Test.
Fig. 7. Characteristic embedding strength.
riquez and Moraes [9] and Manriquez [7] observed the same behavior for the embedding strengths parallel (fh,0,m) and perpendicular (fh,90,m) to the grain. The analysis of each angle a time showed that the mean embedding strengths at 60, 80, 100, 120 and 140 °C did not present significant differences between the means. 3.5. Characteristic embedding strength The results of the characteristic embedding strength for each grain angle at different temperatures are presented in Fig. 7. The minimum value of the characteristic embedding strength, parallel (fh,0,k) and perpendicular (fh,90,k) to the grain, were respectively observed at 60 and 80 °C. From the experimental results, two typical evolution curves of embedding strength according to the temperature can be pointed out (Fig. 7). The former is featured by two minimal peaks of embedding strength, for all the grain angles apart from 15° and 90°. The first peak arises for wood specimens at a temperature of 60 °C while the second one occurs between 100
and 120 °C. Note that the first minimal peak of embedding strength is related to the decrease of moisture content by 3.0% with respect to wood samples tested at room temperature (Fig. 5). The latter typical curve is characterized by a single minimal peak of embedding strength for 15° and 90° grain angles, at temperatures of 120 and 80 °C respectively. By comparing the results with respect to each grain angle (Fig. 7), it appears that the impact of the temperature on the embedding strength is higher for lower grain angles of wood specimens. At room temperature (20 °C), the reduction of the moisture content would cause an increase of the wood strength [43,42]. However, the results on specimens at temperature below 100 °C show that the wood strength reduction due to the heating prevails over its gain due to moisture loss. This behavior may be associated with the glass transition temperature of the Eucalyptus grandis, which occurs at 57 °C (Fig. 6A). The glass transition temperature is related to the softening of the polymers present in the wood, leading to a plastic behavior [51]. Further studies should be conducted to investigate the influence of the temperature on the embedding strength at 15° and 90° grain angles.
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Fig. 8. Surface of the reduction factor of the characteristic embedding strength.
3.6. Reduction factor of the characteristic embedding strength As shown in Fig. 8, the reduction factor of the characteristic embedding strength calculated by Eq. (6) from experimental results varies as a function of grain angles and temperature levels of wood specimens. Being between 45 and 65%, the largest embedding strength reductions can be observed for wood specimens featured by grain angles higher than 30° at all temperature levels. As a reminder, the reference embedding strength (f h;020;k ) for which the reduction factor is equivalent to the unity, comes from wood specimens tested at room temperature (h = 20 °C) under loading parallel to the grain (a = 0°). The adjusted surface of the reduction factor is described by Eq. (7) with a coefficient of determination (R2) of 0.87 (Fig. 6). This equation can be used in numerical models to predict the bolted joint load capacity, by a continuous function of the property reduction regarding the embedding strength value at 20 °C and 0° grain angle (f h;020;k ).
ature and grain angle on the wood embedding strength. From these results, the embedding strength could be expressed as a continuous function of the grain angle and temperature. The results lead to the following conclusions: & &
&
&
&
kah;k ¼ 1:00749 0:00655:h 0:00986:a þ 3:49583 105 :h2 þ 6:82688 105 :a2
ð7Þ
&
the moisture content decreases when the temperature rises, as expected [1,7,8,]; the glass transition of Eucalyptus grandis wood occurs at 57 °C. For glass transition temperature, the results in this paper are similar to those found in the literature [17]; the reduction of the embedding strength at 60 °C can be attributed to the glass transition temperature (Tg) of the wood, since it causes its softening; the mean and characteristic embedding strength values decrease with the increase of the temperature at all wood grain angles. However, this reduction is not monotonic; the reduction factor equation can be used in numerical models to predict the load capacity of timber connections. However, this equation is valid only for Eucalyptus grandis and temperatures at the range from 20 to 140 °C; other species and temperatures should be tested in order to corroborate the results obtained in this research.
in which kah,k is the reduction factor, b is the temperature in °C, and
a is the grain angle in degrees.
At 60 °C, the reduction factor of embedding strength parallel to the grain was 0.58. As this temperature is easily reached in tropical regions, particularly in structural elements of roofs [52,53], the influence of temperature on the embedding strength might be a significant parameter to be considered. The proposed formula should be used with caution for design, since a larger number of specimens should be tested in order to corroborate the results obtained in this research. Additional tests with other species and at higher temperature levels are also required. 4. Conclusions The results of the embedding strength tests with specimens of Eucalyptus grandis, in the range from 20 to 140 °C and grain angles from 0° to 90°, allowed the evaluation of the influence of temper-
Acknowledgements The authors thank the Brazilian Research Council (CNPq) for sponsoring this research through grants number 140116/2015-0 and 303223/2013-8. References [1] C.C. Gerhards, Effect of moisture content and temperature on the mechanical properties of wood: an analysis of immediate effects, Wood Fiber. 14 (1) (1982) 4–36. [2] B.A. Ostman, Wood tensile strength at temperature and moisture contents simulating fire conditions, Wood Sci. Technol. 19 (1985) 103–116. [3] M. Thiam, M.R. Milota, R.J. Leichti, Effect of high-temperature drying on bending and shear strengths of Western Hemlock lumber, Forest Prod. J. 52 (4) (2002) 64–68. [4] P. Bekhta, P. Niemz, Effect of high temperature on the change in color dimensional stability and mechanical properties of spruce, Wood Holzforschung 57 (2003) 539–546, http://dx.doi.org/10.1515/HF.2003.080.
E. Schneid, P.D. de Moraes / Construction and Building Materials 150 (2017) 442–449 [5] 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. [6] Forest Products Laboratory, Wood Handbook-Wood as an engineering material. Madison, WI: U. S. Department of Agriculture, Forest Service, Forest Products Laboratory, 1999. (Gen. Tech. Rep. FPL-GTR-113), 463p. [7] M.J. Manriquez, Coeficiente de modificação das propriedades mecânicas da madeira devidos à temperatura Tese de Doutorado, Programa de PósGraduação em Engenharia Civil 2012 Universidade Federal de Santa Catarina, Florianópolis, Brasil, 2012. [8] 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. [9] M.J. Manriquez, P.D. Moraes, Influência da temperatura sobre a resistência ao embutimento do paricá, in: XI Encontro Brasileiro em Madeira e Estruturas de Madeira, 2008, Londrina, Julho, 2008. [10] M. Audebert et al., Experimental and numerical analysis of timber connections in tension perpendicular to grain in fire, Fire Saf. J. 63 (2014) 125–137, http:// dx.doi.org/10.1016/j.firesaf.2013.11.011. [11] N.F. Correia, Comportamento de ligações em estruturas de madeira sob influência de temperaturas elevadas, in: Dissertação de Mestrado Universidade de Coimbra, Coimbra, Portugal, 2009, p. 2009. [12] R.G.M. Santos, Comportamento de ligações parafusadas simples em estruturas de madeira sob ação da temperatura. 2012, Trabalho de conclusão de curso – Universidade Federal de Santa Catarina, Florianópolis, Brasil, 2012. [13] P.D. Moraes, N.D.F. Correia, J.P.C. Rodrigues, Behavior of bolted timber joints subjected to high temperatures, Eur. J. Wood Prod. 70 (2012) 225–232, http:// dx.doi.org/10.1007/s00107-011-0539-x. [14] E. Schneid, Análise de ligações parafusadas em estruturas de madeira sob ação de temperaturas elevadas, in: XIV Encontro Brasileiro em Madeiras e em Estruturas de Madeira 28-30/Abril, 2014, Natal, RN, Brasil. [15] European Standard Eurocode, in EN 1995-1-1 Eurocode 5: Design of timber structures – Part 1–1: General – Common rules and rules for buildings, European Committee for standardization, Brussels, 2004, p. 123p. [16] European Standard Eurocode, EN 1995-1-2 Eurocode 5: Structural fire designPart 1–2: General – Common rules and rules for buildings, European Committee for standardization, Brussels, 2004, p. 123p. [17] G. Irvine, The glass transitions of lignin and hemicellulose and their measurement by differential thermal analysis, Tappi J. 67 (5) (1984) 116–121. [18] 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. [19] E.L. Back, L. Salmén, Glass transition of wood components hold implications for molding and pulping processes, Tappi J. 65 (7) (1982) 107–110. [20] Associação Brasileira de Normas Técnicas, Norma de projeto de estruturas de madeira, in NBR 7190, Rio de Janeiro, 1997. [21] National Design Specification For Wood Construction, American Wood Council, Leeslburg, VA (2015) 202p. [22] J. Ehlbeck, H. Werner, Softwood and hardwood embedding strength for doweltype fasteners, in: Proceedings of the cib-w18 meetings, 1992. [23] L.R.J. Whale, I. Smith, H.J. Larsen, Design of nailed and bolted joints proposal for the revision of existing formulae in draft Eurocode 5 and the CIB code, in: Proceedings of the cibw18 meetings, 1987. [24] L. Bleron, G. Duchanois, Angle to the grain embedding strength concerning dowel type fasteners, Forest Prod. J. 3 (2006) 44–50. [25] U. Hübner et al., Embedding Strength of European hardwoods. In: International council for research and innovation in building and construction, CIB-W18, Paper 41-7-5, St. Andrews, Canada, 2008. [26] S. Franke, P. Quenneville, Embedding behaviour of lvl and radiata pine lumber, in.: Wood conference on timber engineering, 2010. Riva del Gard, Itália, 2010. [27] K.W. Johansen, Theory of timber connections, Int Assoc Bridge Struct Eng. 9 (1949) 249–262. [28] M. Patton-mallory, The three-dimensional mechanics and failure of single bolt wood connections Ph.D dissertation, Civil Engineering Department, Colorado State University, Fort Collins, CO, USA, 1996. [29] M.D. Moses, Constitutive and analytical models for structural composite lumber with applications to bolted connections Ph.D dissertation, Department
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
Grain angle and temperature effect on embedding strength Eduardo Schneid a,⇑, Poliana Dias de Moraes b a b
Federal University of Santa Catarina – UFSC, 205 João Pio Duarte da Silva St., Florianópolis, SC 88037-000, Brazil Federal University of Santa Catarina – UFSC, Department of Civil Engineering, 205 João Pio Duarte da Silva St., Florianópolis, SC 88037-000, Brazil
h i g h l i g h t s A continuous function of the reduction factor of the embedding strength is proposed. The influence of the temperature on the embedding strength for different grain angles. The relation between the glass transition temperature and the decrease of the embedding strength.
a r t i c l e
i n f o
Article history: Received 13 October 2016 Received in revised form 18 May 2017 Accepted 2 June 2017
Keywords: Embedding tests High temperature Bolted timber joints
a b s t r a c t The objective of this research is to evaluate the effect of temperature and grain angles on the wood embedding strength in the range from 20 to 140 °C. To this end, a reduction factor of the wood embedding strength is proposed as a continuous function of the grain angle and the temperature, for use in modeling of timber bolted connections. Embedding tests were carried out with 252 specimens of Eucalyptus grandis, according to ASTM D 5764-97a [35] standard. The embedding strength, reduction factor, glass transition temperature and moisture content were determined. The minimum values of the characteristic embedding strengths, parallel (fh,0,k) and perpendicular (fh,90,k) to the grain, were observed at 60 and 80 °C, respectively. For all grain angles, except 15°, the embedding strength presented the first relative minimum at 60 °C. Based on the experimental results, a continuous function of the reduction factor of the embedding strength was then determined. To predict the load-bearing capacity of bolted joints through numerical models, the reduction factor of the embedding strength from the present paper can be used in future research. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Material and methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Sample and specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Determination of the heating time. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Heating of the specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Embedding test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Determination of embedding strength. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Embedding strength adjustment for 12% moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Mean embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10. Characteristic reduction factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11. Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12. Statistical analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail addresses: [email protected] (E. Schneid), [email protected] (P.D. de Moraes). http://dx.doi.org/10.1016/j.conbuildmat.2017.06.015 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
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4.
3.2. Effect of the heating on the specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Differential Scanning Calorimetry (DSC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Mean embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Reduction factor of the characteristic embedding strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Usually, the load-bearing capacity of the wood bolted connection is estimated by the mathematical model of a beam under a deformable foundation, formulated by Johansen [27] or by the finite elements method [28–31]. In the Johansen model [27], the limit analysis theory defines the load-bearing capacity of a single connector, in which the ideal plasticity of both steel and timber is assumed and, besides that, the dowel is assumed rigid-plastic. In this theory, the stress level of the wood plastic failure is represented by the embedding strength, while, in the many models based on finite elements, the wood compressive strength is used as failure parameter. From Hong and Barret [32], the finite element models are based on compressive strength. However, the wood behavior in compression differs from the one in embedding situation, due to the stresses at the contact zone between the wood and the bolt. In bolted joints under bending moment, the direction of the efforts on the bolts varies in relation to the grain. In order to estimate the joint load-bearing capacity, the embedding strength should be considered as a function of the loading direction according to the grain [33]. Therefore, the objective of this research is to evaluate the effect of the temperature and the grain angles on the wood embedding strength in the range from 20 to 140 °C. To that end, a reduction factor of the wood embedding strength is proposed as a continuous function of the grain angle and the temperature, which can be used in modeling of timber bolted connections.
1. Introduction The wood embedding strength is the main parameter in order to assess the load-bearing capacity for the design of bolted connections. In normal reference conditions, the wood embedding strength, parallel and perpendicular to the grain, is determined at 20 °C and 12% moisture content. However, the physical, chemical and mechanical wood properties are modified under thermal action [1–7]. Some studies have shown that the embedding strength, parallel and perpendicular to the grain, and the loadbearing capacity of timber connections decrease when subjected to high temperatures [8–14]. Manriquez [7] evaluated embedding strengths parallel and perpendicular to the grain of the Eucalyptus saligna in the range from 20 to 230 °C. The author observed decreases of embedding strength by 33% at 70 °C in the parallel direction and by 43% at 100 °C in the perpendicular direction. As a similar study, Moraes et al. [8] noticed that the embedding strength parallel to the grain of the Pinus sylvestris drops by 30% at 80 °C. The reduction of the mechanical properties as function of the temperature is not taken into consideration in the EN 1995-1-1: 2004 standard [15]. However, it is contemplated for structural fire design in the EN 1995-1-2 standard [16], in which the reduction factor for strength and modulus of elasticity parallel to the grain of softwood are presented as function of the temperature. The reduction of the mechanical properties with increase of the temperature may be related to the glass transition temperature of wood. Conform with the literature [17–19], the lignin glass transition temperature is between 60 and 90 °C. Within this temperature range, the lignin softening may arise, causing then the reduction of the wood strength. Standards only describe experimental procedures to determine the wood embedding strengths parallel (0°) and perpendicular (90°) to the grain of the wood. For other grain angles, the Hankinson equation is used, requiring the wood strengths perpendicular and parallel to the grain [15,20–26]. As this equation was initially developed for room temperature (25 °C), embedding strength behavior in other temperatures and grain angle conditions are unknown.
2. Material and methods 2.1. Sample and specimens In this research, 252 clear specimens of Eucalyptus grandis, with density of 718 kg/m3 and mean moisture content of 15%, were used. The specimens were assembled in order to present sets with statistically homogeneous densities (Table 1). The homogeneity was verified by variance analysis (ANOVA) with 95.0% confidence level [34]. The experimental program considers 6 temperature levels and 7 grain angles. For each combination of one temperature level and one grain angle, 6 specimens were used. The specimens are featured by rectangular parallelepiped elements with half a dowel hole across one face, by modifying the grain angles a with respect to the vertical loading applied on the fastener as shown in Fig. 1. Conform with ASTM D 5764-
Table 1 Mean densities of the samples. Grain angle (°)
Density (kg/m3) Temperature (°C) 20
0 15 30 45 60 75 90
740 730 720 716 724 731 733
60 (78) (85) (80) (84) (75) (84) (72)
Values between parentheses are the standard deviations.
719 699 735 726 734 715 713
80 (75) (84) (93) (92) (92) (92) (88)
739 714 749 735 745 723 726
100 (75) (71) (84) (90) (91) (87) (86)
706 709 719 721 736 710 706
120 (120) (96) (114) (112) (119) (120) (119)
706 697 712 703 705 693 713
140 (90) (79) (101) (100) (100) (106) (92)
713 702 712 714 724 705 712
(85) (69) (77) (84) (96) (85) (87)
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Fig. 1. Embedding test specimens front view: (a) 0°, (b) 15°, (c) 30°, (d) 45°, (e) 60°, (f) 75°, (g) 90° and (h) side view. Dimensions in mm.
Table 2 Heating time determination. Temperature level (°C)
Heating time (min)
60 80 100 120 140
90 120 150 150 120
Fig. 3. Embedding test setup. (a) support; (b) specimen; (c) bolt; (d) loading plate. Fig. 2. Heating study. (a) specimen; (b) thermocouple. 97a [35], the dimensions of samples were checked properly. The tests were carried out by using partially threaded bolts characterized by: 8.8 and 10 mm diameter, yield strength of 640 MPa and tensile strength of 800 MPa [36]. Note that the smooth shank of the bolt must carefully fit into the full slot length for each sample. Therefore, the threaded part does not come into contact with the wood specimens.
The maximum heating time of 150 min was obtained at the temperature levels of 100 and 120 °C (Table 2) and it was adopted for all samples. A shorter heating time at 140 °C may be attributed to a quicker increase of the wood surface temperature, leading to a faster drying and higher rate of the internal temperature rise. Conform with the literature [37–40], the drying rate actually increases with the temperature.
2.2. Determination of the heating time 2.3. Heating of the specimens The heating time was determined by a preliminary study, in which specimens were heated at five temperature levels in an oven at a constant temperature (Table 2), until the specimens internal temperature reached the oven temperature. The temperature was measured by a K thermocouple (Fig. 2), placed inside an orifice with 3 mm-diameter in the specimen center and connected to a data acquisition system.
The specimens were previously heated in an electric oven at the temperature test during 150 min, in order to homogenize their inner temperature. After the specimen reached the specified temperature (Table 2), it was removed from the oven and then performed under embedding test in a universal testing machine provided with a heating chamber at the same oven temperature.
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2.8. Mean embedding strength The mean embedding strength (f h;ah;m ) was calculated by Eq. (4) as the arithmetic average of the sample strength values obtained for each combination of temperature and grain angle.
f h;ah;m ¼
X f h;ah n
;
ð4Þ
in which fh,ah,m is the mean embedding strength at a grain angle a and a temperature h, in MPa and n is the number of specimens. 2.9. Characteristic embedding strength The characteristic embedding strength is defined as the lower 5th percentile. This means that at least, 95% of the embedding strength values should be higher than the characteristic value [16]. The embedding strength values are normally distributed and the characteristic values at the grain angle a and the temperature h were determined using the Student’s t distribution [34], with n-1 degrees of freedom [41], according to Eq. (5).
Fig. 4. Force-displacement curve.
The electric oven, with internal dimensions of 99 cm 100 cm 66 cm, had, besides natural ventilation, an automatic temperature control with accuracy of ±1 °C.
2.4. Embedding test Embedding tests were carried out in a universal testing machine with a heating chamber (Fig. 3), according to the ASTM D 5764-97a standard [35]. A monotonic loading in the compression direction was applied at a rate of 1.9 mm/min. The load-rate was determined from preliminary tests, so that each test takes between 1 and 10 min [35]. The force was measured by a load cell with capacity of 200 kN and the displacement was measured by a system of the testing machine.
s2 f h;ah;k ¼ f h;ah;m t: pffiffiffi ; n
in which fh,ah,k is the characteristic embedding strength at the grain angle a and the temperature h, in MPa; fh,ah,m is the mean embedding strength at the grain angle a and the temperature h, in MPa; t is parameter of Student’s t distribution (t = 2.0150 for n1 = 5); s2 is the standard deviation and n is the number of specimens. 2.10. Characteristic reduction factor The characteristic reduction factor (kah;k ) is the ratio between the characteristic embedding strength at the grain angle a and the temperature h and the characteristic embedding strength parallel to the grain at 20 °C. It was determined by Eq. (6).
kah;k ¼ 2.5. Determination of embedding strength
F ah ; d:t
2.11. Differential Scanning Calorimetry (DSC)
2.6. Moisture content After the experimental assessment of the embedding strength, the moisture content of the tested specimens was determined by Eq. (2). The procedure is an adaptation of the oven-drying method proposed by the NBR 7190 standard [20]. The specimens were weighed and dried in an electric oven at a temperature of 103 ± 2 °C. Assuming that the endpoint has been reached when the mass loss is equal or less than 0.5%, the last measurement is considered an oven-dry mass.
mwet mdry ð100Þ; mdry
ð2Þ
in which mwet is the wet mass, in g, and mdry is the oven-dry mass, in g.
Conform with the NBR 7190 standard [20], the embedding strength adjustment for 12% moisture content was calculated by Eq. (3), only for the wood specimens performed under room temperature tests.
f 12
in which f% is the embedding strength, in MPa, at moisture content of U%.
The Differential Scanning Calorimetry test (DSC) was performed to identify the glass transition temperature of the wood and the possible influence of this parameter on the embedding strength. A 10 mg sample of ground wood with 15% moisture content and a Setaram SetSys equipment were used for the test. The analysis was performed at a temperature range from 25 to 150 °C, with heating and cooling steps at a rate of 15 °C/min under an argon atmosphere (flow rate of 50.0 mL/min). 2.12. Statistical analysis Statistical analysis was applied to assess the influence of temperature on the embedding strength for each grain angle. In this analysis, the dependent variable was the embedding strength at a grain angle while the independent variable was the temperature. Parametric or nonparametric analyses were applied, depending on the assumption of independence, normality and homogeneity of variance of data [34]. The parametric test for the analysis of variance (ANOVA) was used based on the assumption of data independence, normality and homogeneity of variance, valid for the angles 0°, 60° and 90°. The F test, at 5% probability of error, was used to compare the means. When the null hypothesis of F test was rejected, the Tukey’s test, at 5% probability of error, was applied to verify if the means are significantly different. The nonparametric Kruskal-Wallis test was applied when the assumption of normality and homogeneity of variance was violated, which occurred for angles of 15°, 30°, 45° and 75°. In case of rejection of the null hypothesis of the nonparametric test, multiple comparisons tests were applied [34].
3. Results and discussion
2.7. Embedding strength adjustment for 12% moisture content
3ðU 12Þ ¼ f% 1 þ 100
ð6Þ
ð1Þ
in which fh,ah is the embedding strength at a grain angle a and a temperature h, in MPa; Fah is the embedment force at a grain angle a and a temperature h, in N; d is the bolt diameter, in mm; and t is the specimen width, in mm. The force value used for determining the embedding strength of the specimen was defined by the intersection of a line parallel to the linear segment of the force-displacement curve and offset of 5% of the diameter of the bolt with the same curve (Fig. 4), according to the ASTM D 5764-97a standard [35]. In those cases where the offset line does not intersect the force-displacement curve, the maximum load is used.
MCð%Þ ¼
f h;ah;k ; f h;020;k
in which f h;ah;k is the characteristic embedding strength at the grain angle a and the temperature h and f h;020;k is the characteristic embedding strength parallel to the grain at 20 °C.
The embedding strength of each specimen was determined by Eq. (1).
f h;ah ¼
ð5Þ
ð3Þ
3.1. Moisture content The mean values of the moisture content for each grain angle and temperature are presented in Fig. 5. The moisture content decreases when temperature rises, as expected. Due to the heating time of 150 min, at 140 °C, the specimens were basically anhydrous. Between 20 and 100 °C, the specimens can contain bound
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Fig. 5. Mean moisture content of the specimens.
ferences of moisture between the surface and the center of the specimens [44,45]. These differences lead to the emergence of tensile stress perpendicular to the grain which cause cracks in the structure of the wood if they exceed the resistance of the ligneous tissue [44,45]. Thereby, the small dimensions of wood specimens from the present research may reduce the difference of moisture content between the surface and the center, by preventing then withdrawal cracks. 3.3. Differential Scanning Calorimetry (DSC)
Fig. 6. DSC curve: (A) First endothermic peak and (B) second endothermic peak.
water and water of constitution while the latter only remains inside the wood samples for temperatures above 100 °C. According to Moreschi [42], bound water occurs in moisture content ranging from 6% to 28%, while water of constitution occurs between 0% and 6%. From the author [42], loss of bound water modifies the weight of wood and draws closer micelle, microfibril and macrofibril, by raising then the wood rigidity. Therefore, the wood strength can vary due to loss of bound water. Bodig and Jayne [43] concluded that loss of water of constitution can cause the reduction of the wood mechanical properties. 3.2. Effect of the heating on the specimens When heating the wood specimens in the oven, no cracking or warping was noticed. However, Manriquez [7] observed the appearance of small cracking at the edge of wood specimens heated at a temperature higher than 40 °C. This can be explained by the dimensions of the specimens used by Manriquez [7], 25 mm 60 mm 140 mm, while in this research they were 20 mm 50 mm 50 mm. Cracks appear due to the difference of retraction in the wood radial and tangential directions and the dif-
The result of DSC tests of ground Eucalyptus grandis wood is presented in Fig. 6. Two endothermic peaks, indicated by letters A and B, are observed. The first one (letter A) occurs at 57 °C and is attributed to the glass transition temperature (Tg) of Eucalyptus grandis wood. This result was similar to those found by Irvine [17] for Eucalyptus regnans, whose glass transition temperature of ground wood with 15% moisture content was about 60 °C. According to Kelley et al. [46], the first endothermic peak is due to the relaxation enthalpy and is related to the a thermal transition, corresponding to the Tg of the wood. Lisperguer et al. [47] and Roig et al. [48] also observed this phenomenon. The second endothermic peak (B) occurs at 135 °C and is associated to the activation energy necessary for wood drying and for breaking the chemical bonds of wood components [49,50]. In dry conditions, the wood polymers presented different glass transition temperatures. For the cellulose, it is between 200 and 250 °C; for hemicellulose, between 150 and 220 °C and, for lignin in situ, above 205 °C [19]. However, the glass transition temperature is affected by the wood moisture content and decreases as the humidity increases [51]. At fiber saturation point (FSP), the glass transition temperature of lignin is between 60 and 90 °C; while for hemicellulose, the temperature is near 25 °C [51,17]. 3.4. Mean embedding strength The results of embedding strength are presented in Table 3. For grain angles of 0, 60 and 90, the mean embedding strengths at 20 °C are significantly different from those for other temperature levels, as shown by the different lowercase letters following the mean values. For all grain angles, except 45°, the mean embedding strength at 100 °C is lower than at 20 °C. Moraes et al. [8], Man-
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E. Schneid, P.D. de Moraes / Construction and Building Materials 150 (2017) 442–449 Table 3 Experimental results of the embedment test for different angle to the grain. Grain angle (°)
fh,ah,m (MPa) Temperature (°C)
0 15 30 45 60 75 90
20
60
80
100
120
140
44.0 a (4.4) 41.7 a (7.6) 29.8 a (7.6) 27.7 a (5.7) 27.6 a (3.6) 26.5 a (6.2) 26.7 a (4.1)
31.0 b (7.6) 26.6 bc (2.7) 22.6 ab (2.8) 19.8 a (3.1) 19.8 b (4.4) 19.2 ab (4.0) 19.4 b (3.8)
31.6 b (4.4) 26.9 bc (2.1) 22.2 ab (1.6) 20.9 a (1.8) 20.4 b (3.6) 18.9 ab (1.8) 17.1 b (2.9)
30.2 b (4.8) 27.0 bc (2.4) 18.8 b (3.3) 18.7 a (1.7) 19.0 b (3.0) 16.9 b (1.9) 17.3 b (2.4)
33.4 b (5.0) 26.9 bc (4.1) 23.5 ab (3.7) 21.8 a (7.8) 19.6 b (4.5) 18.1 ab (1.6) 19.9 b (4.0)
33.6 b (5.6) 30.3 ac (1.6) 22.7 ab (3.0) 20.6 a (3.6) 19.9 b (2.7) 19.0 ab (2.1) 19.5 b (3.9)
fh,ah,m is the mean value of the embedding strength at the grain angle a and at the temperature h; values between parenthesis are the standard deviation. If the letter combinations corresponding to two mean values, in the same line, have a common letter, they have no statistic difference at a 5% probability of error by the Tukey Test.
Fig. 7. Characteristic embedding strength.
riquez and Moraes [9] and Manriquez [7] observed the same behavior for the embedding strengths parallel (fh,0,m) and perpendicular (fh,90,m) to the grain. The analysis of each angle a time showed that the mean embedding strengths at 60, 80, 100, 120 and 140 °C did not present significant differences between the means. 3.5. Characteristic embedding strength The results of the characteristic embedding strength for each grain angle at different temperatures are presented in Fig. 7. The minimum value of the characteristic embedding strength, parallel (fh,0,k) and perpendicular (fh,90,k) to the grain, were respectively observed at 60 and 80 °C. From the experimental results, two typical evolution curves of embedding strength according to the temperature can be pointed out (Fig. 7). The former is featured by two minimal peaks of embedding strength, for all the grain angles apart from 15° and 90°. The first peak arises for wood specimens at a temperature of 60 °C while the second one occurs between 100
and 120 °C. Note that the first minimal peak of embedding strength is related to the decrease of moisture content by 3.0% with respect to wood samples tested at room temperature (Fig. 5). The latter typical curve is characterized by a single minimal peak of embedding strength for 15° and 90° grain angles, at temperatures of 120 and 80 °C respectively. By comparing the results with respect to each grain angle (Fig. 7), it appears that the impact of the temperature on the embedding strength is higher for lower grain angles of wood specimens. At room temperature (20 °C), the reduction of the moisture content would cause an increase of the wood strength [43,42]. However, the results on specimens at temperature below 100 °C show that the wood strength reduction due to the heating prevails over its gain due to moisture loss. This behavior may be associated with the glass transition temperature of the Eucalyptus grandis, which occurs at 57 °C (Fig. 6A). The glass transition temperature is related to the softening of the polymers present in the wood, leading to a plastic behavior [51]. Further studies should be conducted to investigate the influence of the temperature on the embedding strength at 15° and 90° grain angles.
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Fig. 8. Surface of the reduction factor of the characteristic embedding strength.
3.6. Reduction factor of the characteristic embedding strength As shown in Fig. 8, the reduction factor of the characteristic embedding strength calculated by Eq. (6) from experimental results varies as a function of grain angles and temperature levels of wood specimens. Being between 45 and 65%, the largest embedding strength reductions can be observed for wood specimens featured by grain angles higher than 30° at all temperature levels. As a reminder, the reference embedding strength (f h;020;k ) for which the reduction factor is equivalent to the unity, comes from wood specimens tested at room temperature (h = 20 °C) under loading parallel to the grain (a = 0°). The adjusted surface of the reduction factor is described by Eq. (7) with a coefficient of determination (R2) of 0.87 (Fig. 6). This equation can be used in numerical models to predict the bolted joint load capacity, by a continuous function of the property reduction regarding the embedding strength value at 20 °C and 0° grain angle (f h;020;k ).
ature and grain angle on the wood embedding strength. From these results, the embedding strength could be expressed as a continuous function of the grain angle and temperature. The results lead to the following conclusions: & &
&
&
&
kah;k ¼ 1:00749 0:00655:h 0:00986:a þ 3:49583 105 :h2 þ 6:82688 105 :a2
ð7Þ
&
the moisture content decreases when the temperature rises, as expected [1,7,8,]; the glass transition of Eucalyptus grandis wood occurs at 57 °C. For glass transition temperature, the results in this paper are similar to those found in the literature [17]; the reduction of the embedding strength at 60 °C can be attributed to the glass transition temperature (Tg) of the wood, since it causes its softening; the mean and characteristic embedding strength values decrease with the increase of the temperature at all wood grain angles. However, this reduction is not monotonic; the reduction factor equation can be used in numerical models to predict the load capacity of timber connections. However, this equation is valid only for Eucalyptus grandis and temperatures at the range from 20 to 140 °C; other species and temperatures should be tested in order to corroborate the results obtained in this research.
in which kah,k is the reduction factor, b is the temperature in °C, and
a is the grain angle in degrees.
At 60 °C, the reduction factor of embedding strength parallel to the grain was 0.58. As this temperature is easily reached in tropical regions, particularly in structural elements of roofs [52,53], the influence of temperature on the embedding strength might be a significant parameter to be considered. The proposed formula should be used with caution for design, since a larger number of specimens should be tested in order to corroborate the results obtained in this research. Additional tests with other species and at higher temperature levels are also required. 4. Conclusions The results of the embedding strength tests with specimens of Eucalyptus grandis, in the range from 20 to 140 °C and grain angles from 0° to 90°, allowed the evaluation of the influence of temper-
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