- Email: [email protected]
Effects of logs steaming, veneer drying and aging on the mechanical properties of laminated veneer lumber (LVL)
ARTICLE IN PRESS
Building and Environment 42 (2007) 93–98 www.elsevier.com/locate/buildenv
Effects of logs steaming, veneer drying and aging on the mechanical properties of laminated veneer lumber (LVL) Semra C - olak, Gu¨rsel C - olakog˘lu, Ismail Aydin Forest Industry Engineering Department, Faculty of Forestry, Karadeniz Technical University 61080 Trabzon, Turkey Received 13 December 2004; received in revised form 21 July 2005; accepted 2 August 2005
Abstract In this study the effects of steaming and drying condition on the mechanical properties and durability of laminated veneer lumber (LVL) and solid sawn lumber were investigated in a comparative way. Steamed beech and steamed and non-steamed spruce logs were used and two different veneer drying temperatures (20 and 110 1C) were selected for this aim. Aging test was applied according to EN 321 to determine the durability of LVL and solid wood samples. Steaming decreased considerably all investigated strength properties of LVL panels and the least affected was the compression strength. The compression strength and the static bending strength values of both beech and spruce LVL panels were higher than those of the solid wood groups obtained from the same logs. The impact strength values of LVL panels, unlike the static bending strength and the compression strength, were lower than those of the solid samples, which were not steamed and aged. r 2005 Elsevier Ltd. All rights reserved. Keywords: Logs steaming; Veneer drying; Aging of spruce/beach; LVL; Mechanical properties
1. Introduction Laminated veneer lumber (LVL) is an engineered wood product manufactured from veneers that are rotary peeled, dried and laminated together with parallelly oriented grains under heat and pressure with a waterproof adhesive. A higher strength product can be produced by this method from low-grade logs, due to the dispersion of defects from veneer to veneer, than could be realized by sawing the same low-grade log [1]. One of the most significant technical advantages of LVL is that specific performance characteristics can be incorporated into its design. By the nature of their manufacturing process, large defects such as knots and other strength-reducing characteristics are either eliminated or dispersed throughout the cross-section to Corresponding authors. Tel.: +90 462 377 3244, +90 462 377 3258; fax: +90 462 3257499. E-mail addresses: [email protected] (G. C - olakog˘lu), [email protected] (I. Aydin).
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.08.008
produce a more homogenous product [2,3]. Due to the uniformity in properties, high strength and availability in virtually unlimited length and size, LVL is used in a variety of products, such as commodity structural components, floor beams, garage door, window and door headers, valley rafters, scaffold planking, and the flange material for prefabricated wood I-joists and ridge and hip beams. Before the development of engineered wood products, builders were limited to large, solid sawn timber which often do not have the dimensional stability and uniformity required. The LVL manufacturing process creates a strong and stable product that can reliably support large areas. A second important benefit of LVL is that the veneering and gluing process enables large beams to be made from relatively small diameter logs of many species, thereby providing for efficient use of forest resources. Due to their advantages mentioned above, LVL has been suggested as a good alternative for structural purposes. However, the results of researches about
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
94
exposure durability of this material are contradictory to each other [4,5]. Green et al. [4] investigated the flexural properties of structural lumber products after long-term exposure to 65 1C and 75% relative humidity. The results of their study showed that for 2–3 years of continuous exposure to heated air at 65 1C and 75% relative humidity in a conditioning room, the reduction in bending strength of LVL is similar to that of solidsawn spruce, pine and fir lumber, but after 3 years, the bending strength of LVL decreased faster than that of solid sawn lumber. Different physical and mechanical properties of LVL manufactured from rubber wood were compared with teak by Kamala et al. They stated that LVL of rubber wood can be compared with teak in many properties and has got strength equivalent to that of gamari, poon, lakooch and benteak which are used as door and window frames, flooring and transport vehicles [5]. Investigations showed that durability, mechanical properties and engineering performance of LVL are affected by many factors such as wood species, veneer thickness and quality, processing variables, member size, in service environment and loading types [3]. For instance, steaming is one of the processing variables that most of wood species need to be steamed for peeling and it has negative effect on the mechanical properties of wood. Lumbers of some species are also used as steamed such as beech or sometimes as air-dried. Efficient usage of LVL in the construction industry requires an understanding of structural behavior of numerous species and knowledge about to the effects of log pre-treatment process on the mechanical properties and durability of LVL. Therefore, the aims of this study were put forward to the effects of steaming and drying condition on the mechanical properties and durability of LVL and solid-sawn lumber by comparing these two materials.
2. Material and method Steamed beech (Fagus orientalis Lipsky) log with 45 cm diameter and 260 cm length obtained from the region near the border of Turkey–Georgia and steamed and non-steamed spruce logs with 40 cm diameter and 260 cm length obtained from Trabzon region located on the
north cost of Turkey were used in this study. Both places where the logs were taken are located in the warm climate zone. The beech and one of the spruce logs having about 70% initial moisture content were steamed at 60 1C temperature for 20 and 12 h, respectively. The average core temperatures of beech and spruce logs were 50 and 32 1C, respectively (during rotary peeling). Each log was divided into two equal parts to get six logs with 65 cm length. One each part of these logs (three of them) was used for veneer manufacturing while the others were used for preparing solid wood test samples. Only the sapwood portions of the logs were taken for the tests. The horizontal opening (distance from the leading edge of pressure bar to a plane extended from the ground surface of the knife) was 85% of the veneer thickness and the vertical opening was 0.5 mm in the peeling process and 2mm-thick rotary cut veneers with dimensions of 55 55 cm were obtained. After rotary peeling, beech veneer sheets were dried for 5 min from 40–50% to 5–7% final moisture content at 110 1C in a veneer dryer. Veneers obtained from non-steamed logs having 50–60% moisture content were dried at 20 1C while the veneers obtained from steamed logs were dried at 20 1C (for 2 weeks) and 110 1C temperature (for 5 min). Nine-layer LVL panels with 16 mm thickness were manufactured from the veneer sheets. Three replicate panels were produced. Approximately 180 g/m2 adhesive mixture of phenol formaldehyde (PF) having 47% solid content was applied on single bonding surfaces of veneers. Hot press temperature, pressure and duration were 140 1C, 1.4 N/mm2 for beech and 0.8 N/mm2 for spruce and 10 min, respectively. Test groups were formed as shown in Table 1. All test specimens prepared for static bending strength, compression strength parallel to the grain direction and impact strength were classified into two groups after the tests groups were formed. The samples in the first group were tested after conditioning at 20 1C air temperature and 65% relative humidity. The test specimens in the other group were subjected to the aging process with three stage cycles described in EN 321 standard to determine the durability of these materials. The cycle phases were as follows: 1. waiting in water having 2072 1C for 7271 h, 2. freezing at 15 1C for 24 h, 3. drying at 7071 1C temperature for 7271 h.
Table 1 Test groups
Drying temperature Solid wood groups LVL panel groups
Beech
Spruce
Steamed
Non-steamed
Steamed
20 1C SW1 LVL1
20 1C SW2 LVL2
20 1C BW —
110 1C — B-LVL
110 1C — LVL3
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
574
604
645
Test
460
469
471
500
593
Control
490
600
578
Density (kg/m3)
700
587
800
681
723
Air-dry density values of solid wood and LVL test specimens are given in Fig. 1. According to the test results, the density of solid beech specimens decreased as only 1.5% after the aging exposure. The density of nonsteamed solid spruce (490 kg/m3) was higher by 4.5% than that of steamed spruce (469 kg/m3). It is known that log steaming process causes weight loss, dependent on the steaming period and temperature [6]. Although the logs were steamed for only 12 h, the decrease in density value of spruce was measurable. After the aging process, the density losses of non-steamed and steamed spruce solid specimens were 4% and 2%, respectively. Both beech and spruce LVL groups had higher density values and also density losses than those of solid wood from which LVLs were produced. In LVL production, veneers are laminated together under pressure. Thickness of the panel decreases with the pressure applied depending on wood species. The reason for higher density of LVL is related to the thickness loss of the panel due to the compression rate during hot pressing
582
3. Results and discussion
and the higher density of adhesive being used in panel production. In addition, the density values of both solid and LVL groups decreased after the accelerated aging process. The reason of the density losses was the weight losses of specimens occurring in each step of aging treatment. Because the weight losses occurred at the end of the steaming process, no considerable change was obsvered in the density of beech solid-sawn lumber. The decreasing effect of hydrothermal processes on the weight losses were reported in many studies [6–8]. Higher density losses of both non-steamed solid spruce wood and LVL1 sample determined after the aging process confirm this explanation. The mechanical properties of beech solid wood and LVL groups determined before and after aging exposure were given in the Table 2. Static bending and compression strength values parallel to grain of solid wood groups were found to be lower than those of LVLs produced from the same log. On the contrary, impact strength of beech solid wood was significantly higher than that of LVL. No significant change was found in static bending strength and compression strength parallel to grain while a 33% loss was determined in the impact strength of solid wood groups after the aging treatment. The aging treatment had no considerable effect on the compression strength parallel to the grain. The decrease in the bending strength was quite less. The most considerable decrease (25%) caused by aging treatment was obtained for the impact strength. When steamed and non-steamed spruce solid wood samples were compared, it was concluded that steaming decreased considerably all strength properties investigated and the least affected was the compression strength. It was stated in the literature that the mostaffected strength properties from thermal processes were bending strength and impact strength [9,10]. Similar results were found in our previous study for the test samples with 20 20 mm cross-section dimensions [11]. The compression strength mean value of the steamed solid wood samples was lower than that of non-steamed test specimens as much as nearly, 5% (given in Fig. 2).
572
After this cycle was applied for three periods (21 days), aged samples were kept in an autoclave for 2 h under 120 1C temperature and 1.2 atm pressure. Test samples were conditioned to achieve equilibrium moisture content at 20 1C temperature and 65% relative humidity prior to testing. All tests were achieved according to the DIN standards described for solid wood (DIN 52182 for density, DIN 52185 for compression strength, DIN 52186 for static bending strength, DIN 52189-1 for impact bending strength). However, the thickness of solid wood samples was taken as 16 mm (in radial direction) to compare with the LVL panels. Bending strength test was carried out with 10 samples having the dimensions of 300 20 16 mm cut from each manufactured panel. For compression strength parallel to grain, test specimens with dimension of 20 16 30 mm were used.
95
400 300 200 100 0 BW
B-LVL
SW-1
SW-2 Test Groups
LVL-1
LVL-2
Fig. 1. Density mean values of test samples (n ¼ 30).
LVL-3
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
96
Table 2 Mechanical properties of beech solid and LVL test groups before and after aging (n ¼ 30)
Beech–solid wood Beech–LVL a
Compression strength (N/mm2)
Bending strength (N/mm2)
Impact strength (kg m/cm2)
Control
Test
Control
Test
Control
Test
47.4 (1.5)a 52.4 (1.6)
47.1 (2.3) 51.8 (1.8)
98.2 (5.8) 107.8 (6.1)
97.8 (5.1) 104.4 (4.8)
0.91 (0.15) 0.69 (0.03)
0.61 (0.09) 0.52 (0.07)
Values in parenthesis are standard deviations.
Compression Strength (N/mm2)
70 60 50
Control
Test
43.2 39.8 (3.4) (4.0)
41 40 (2.6) (2.7)
SW-1
SW-2
55.1 (5.6) 49.7 (3.1)
49.5 (2.9) 44.4 (2.8)
49.7 47.3 (2.6) (2.3)
LVL-2
LVL-3
40 30 20 10 0 LVL-1 Test Groups
Bending Strength (N/mm2)
Fig. 2. Compression strength in longitudinal direction of spruce wood and LVL groups (n ¼ 30).
120 100 80
Control Test 88.0 (8.4) 80.0 (5.0) 69.3 71.0 (7.4) (7.7)
98.4 99.6 (6.0) (8.1)
92.8 93.7 (4.3) (10.7)
91.4 94.0 (4.8) (7.0)
LVL-2
LVL-3
60 40 20 0 SW-1
SW-2
LVL-1 Test Groups
Fig. 3. Bending strength mean values of spruce solid wood and LVL groups (n ¼ 30).
After the accelerated aging test, the compression strength in longitudinal direction of non-steamed solid test specimens decreased as much as 8%, whereas that of the steamed group did not change significantly. LVL panels produced from the same log had considerably higher compression strength than those of the solid wood groups as shown in Fig. 2. The steaming process causeda similar effect on the compression strength of LVL panels. The highest compression strength was found for the spruce LVL panels manufactured from non-steamed and air-dried (at 20 1C) veneers. The drying temperatures selected in this study
had no clear effect on the compression strength of the spruce LVL panels, which were not subjected to aging process. The decreases in the compression strength values of all LVL groups due to the aging process were higher than those of solid wood samples. Among the spruce LVL groups, the lowest decrease in compression strength was found for the panels manufactured from the veneers dried at 110 1C, which is a mild drying schedule from the industrial point of view. The static bending strength of steamed spruce solid wood samples was 21% lower than that of non-steamed samples (Fig. 3). A decrease of 9% in static bending
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
Impact Strength (Kg.m/cm2)
0.6 0.5 0.4
0.48 (0.06) 0.38 (0.09)
Control 0.38 0.39 (0.05)(0.09)
0.38 0.38 (0.07)(0.08)
0.39 0.39 (0.04) (0.03)
SW-2
LVL-1 Test Groups
LVL-2
97
Test 0.34 0.32 (0.06) (0.04)
0.3 0.2 0.1 0 SW-1
LVL-3
Fig. 4. Impact strength mean values of spruce solid wood and LVL panels.
strength of the non-steamed solid spruce samples was determined after the aging process, whereas no clear difference was obtained for the static bending strength of the steamed samples. The bending strength values of the LVL groups were clearly higher than those of the solid wood groups just like the compression strength. Among the spruce LVL groups, the highest bending strength value was obtained for the panels produced from non-steamed and air-dried veneers (control 98.4 N/ mm2, test 99.6 N/mm2). When samples LVL-2 and LVL3 were compared, it can be stated that veneer-drying temperatures selected in this study had no significant effect on the static bending strength of the panels. Aging process also did not affect the bending strength of the LVLs. Compression strength and the static bending strength values of both beech and spruce LVL panels were higher when compared to the solid wood groups obtained from the same logs and this can be related to the production conditions of the LVL panels. The density of the panel increases with the pressure during hot pressing in panel production and it is known that increasing the density improves the mechanical properties (bending strength) of the wood materials [12,13]. Because woodbased panels are bonded materials; glue-type and glue line thickness also affect positively their mechanical properties. The decrease in impact strength of spruce solid samples after steaming was about 21% (Fig. 4). The impact strength of non-steamed solid wood samples decreased as much as 21%, after the aging process. The impact strength values of LVL panels, unlike the bending strength and the compression strength, were lower than those of the solid samples, which were not steamed and aged. The lowest impact strength was obtained for LVL-3 group. Lower impact strength of LVL panels may also be related to the elastic or brittle nature of glue line after the curing reaction. Brittle glue line might cause a decrease in impact strength of the LVL panels.
The substance’s losses and changes in chemical structure caused by hydrothermal processes may be the reasons for the decrease in mechanical properties. It is known that steaming has a positive effect on the water absorption and swelling properties, whereas a negative effect on the strength properties of wood. These effects are attributed to hydrolysis and extraction of hemicellulose caused by steaming [14]. Because the hemicelluloses are composed of a shorter chain of molecules and have a more branched structure, they are generally easier to hydrolyze than other wood components. Strength losses in thermal degradation were attributed to acetic acid derivated from acetyl groups in the chemical structure and acts as a catalyst to speed the rate of degradation further. Hardwoods have much more acetyl groups than softwoods. Resinous acids generated during thermal processes in some softwood were also stated to be the source of greater thermal sensitivity [4]. The reason why the aging process did not have a significant effect on the investigated mechanical properties of steamed specimens may be the removal of resinous acids during the steaming. Steaming can cause a loosening in wood structure and thus allow freer movement of the acids. To support this opinion, the pH values of the steamed and non-steamed spruce solid samples were measured, they were found to be 4.79 and 5.27, respectively. The higher pH values of steamed test specimens confirm this idea.
4. Conclusion The following points were concluded from this study: 1. Both beech and spruce LVL samples had higher density values than those of solid wood from which LVLs were produced. The increased ratios in average density values were determined as 23% for beech LVL and 31% and 28% for non-steamed and steamed spruce LVLs, respectively. After the aging process, similar results were found.
ARTICLE IN PRESS 98
S. C - olak et al. / Building and Environment 42 (2007) 93–98
2. Static bending strength values parallel to grain of solid wood samples were found to be lower by 10.6% for non-steamed spruce and 25.3% for steamed spruce than those of LVLs produced from the same log. Similarly, compression strength values of solid wood samples were found to be lower as mujch as 21.6% and 17.2% for non-steamed and steamed spruce, respectively. On the contrary, impact strength of beech solid wood was 24% higher than that of LVL. 3. The most considerable decrease caused by aging treatment was obtained for the impact strength of 25%. 4. Steaming process of logs for veneering considerably decreased all strength properties investigated and the least affected was the compression strength. 5. LVL panels produced from the same log had considerably higher compression strength than beams made of the solid wood samples. The drying temperatures selected in this study had no clear effect on the compression strength of the spruce LVL panels, which were not subjected to aging process. 6. The bending strength values of the LVL samples were clearly higher than those of the solid wood samples. Among the spruce LVL sample, the highest bending strength value was obtained for the panels produced from non-steamed and air-dried veneers. 7. The impact strength values of LVL panels, unlike the bending and the compression strength, were lower than those of the solid samples, which were not steamed and aged.
Acknowledgment S. C - olak thanks The Scientific and Technical Research Council of Turkey—Directorate of Human Resources Development for the scholarship during her post doctoral study.
References [1] Llaufenberg T. Exposure effects upon performance of laminated veneer lumber and glulam materials. Forest Products Journal 1982;32(5):42–8. [2] Wang X, Ross RJ, Brashaw BK, Verhey SA, Formsan JW, Ericson JR. Flexural properties of laminated veneer lumber manufactured from ultrasonically rated red maple veneer. Forest Product Laboratory, FPL-RN-0288, 2003. [3] Hing PS, Paridah MT, Zakiah A. Edgewise bending properties of LVL: effects of veneer thickness and species. In: The proceedings of USM-JIRCAS joint international symposium, lignocellulosematerial of the millennium: technology and application, 20–22 March 2001. [4] Green DW, Evans JW. Flexural properties of structural lumber products after long-term exposure to 1501F and 75% relative humidity. In: The proceedings of 35th international particleboard/ composites materials symposium. Washington State University. Washington, USA, 3–5 April 2005, p.3–14 [5] Kamala BS, Kumar P, Rao RV, Sharma SN. Performance test of laminated veneer lumber (LVL) from rubber wood for different physical and mechanical properties. Holz als Roh- und Werkstoff 1999;57(2):114–6. [6] Kollmann FFP, Cote WA. Principles of wood science and technology—I: Solid wood, 1968. [7] Santos JA. Mechanical behaviour of Eucalyptus wood modified by heat. Wood Science and Technology 2000;34:39–43. [8] Yılgo¨r N, U¨nsal O, Kartal SN. Physical, mechanical, and chemical properties of beech wood. Forest Product Journal 2001;51(11–12):89–93. [9] MacLean JD. Effect of steaming on the strength of wood. A. Wood Preservers’ Association Proceedings 1953;49:88–112. [10] Aydın I. Effects of some manufacturing conditions on wettability and bonding of veneers obtained from various wood species. PhD, Thesis, Graduate School of Natural and Applied Sciences, Karadeniz Technical University, Trabzon, 2004 [in Turkish]. [11] Aydin I, C - olak S. The changes in some physical and mechanical properties of steamed spruce (Picea orientalis L) wood. Journal of Artvin Forestry Faculty of Kafkas University 2004 [Turkish]. [12] Liu JY, Mcnatt JD. Thickness swelling and density variation in aspen flakeboards. Wood Science and Technology 1991;82:25–33. [13] Nemli G, Kırcı H, Temiz A. Influence of impregnating wood particles with mimosa bark extract on some properties of particleboard. Industrial Crops and Products 2004;20:339–44. [14] Carll CG. Review of thickness swell in hardboard siding. General technical report FPL-GTR-96, Department of Agriculture, Forest Service, Forest Product Laboratory, 1996.
Building and Environment 42 (2007) 93–98 www.elsevier.com/locate/buildenv
Effects of logs steaming, veneer drying and aging on the mechanical properties of laminated veneer lumber (LVL) Semra C - olak, Gu¨rsel C - olakog˘lu, Ismail Aydin Forest Industry Engineering Department, Faculty of Forestry, Karadeniz Technical University 61080 Trabzon, Turkey Received 13 December 2004; received in revised form 21 July 2005; accepted 2 August 2005
Abstract In this study the effects of steaming and drying condition on the mechanical properties and durability of laminated veneer lumber (LVL) and solid sawn lumber were investigated in a comparative way. Steamed beech and steamed and non-steamed spruce logs were used and two different veneer drying temperatures (20 and 110 1C) were selected for this aim. Aging test was applied according to EN 321 to determine the durability of LVL and solid wood samples. Steaming decreased considerably all investigated strength properties of LVL panels and the least affected was the compression strength. The compression strength and the static bending strength values of both beech and spruce LVL panels were higher than those of the solid wood groups obtained from the same logs. The impact strength values of LVL panels, unlike the static bending strength and the compression strength, were lower than those of the solid samples, which were not steamed and aged. r 2005 Elsevier Ltd. All rights reserved. Keywords: Logs steaming; Veneer drying; Aging of spruce/beach; LVL; Mechanical properties
1. Introduction Laminated veneer lumber (LVL) is an engineered wood product manufactured from veneers that are rotary peeled, dried and laminated together with parallelly oriented grains under heat and pressure with a waterproof adhesive. A higher strength product can be produced by this method from low-grade logs, due to the dispersion of defects from veneer to veneer, than could be realized by sawing the same low-grade log [1]. One of the most significant technical advantages of LVL is that specific performance characteristics can be incorporated into its design. By the nature of their manufacturing process, large defects such as knots and other strength-reducing characteristics are either eliminated or dispersed throughout the cross-section to Corresponding authors. Tel.: +90 462 377 3244, +90 462 377 3258; fax: +90 462 3257499. E-mail addresses: [email protected] (G. C - olakog˘lu), [email protected] (I. Aydin).
0360-1323/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2005.08.008
produce a more homogenous product [2,3]. Due to the uniformity in properties, high strength and availability in virtually unlimited length and size, LVL is used in a variety of products, such as commodity structural components, floor beams, garage door, window and door headers, valley rafters, scaffold planking, and the flange material for prefabricated wood I-joists and ridge and hip beams. Before the development of engineered wood products, builders were limited to large, solid sawn timber which often do not have the dimensional stability and uniformity required. The LVL manufacturing process creates a strong and stable product that can reliably support large areas. A second important benefit of LVL is that the veneering and gluing process enables large beams to be made from relatively small diameter logs of many species, thereby providing for efficient use of forest resources. Due to their advantages mentioned above, LVL has been suggested as a good alternative for structural purposes. However, the results of researches about
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
94
exposure durability of this material are contradictory to each other [4,5]. Green et al. [4] investigated the flexural properties of structural lumber products after long-term exposure to 65 1C and 75% relative humidity. The results of their study showed that for 2–3 years of continuous exposure to heated air at 65 1C and 75% relative humidity in a conditioning room, the reduction in bending strength of LVL is similar to that of solidsawn spruce, pine and fir lumber, but after 3 years, the bending strength of LVL decreased faster than that of solid sawn lumber. Different physical and mechanical properties of LVL manufactured from rubber wood were compared with teak by Kamala et al. They stated that LVL of rubber wood can be compared with teak in many properties and has got strength equivalent to that of gamari, poon, lakooch and benteak which are used as door and window frames, flooring and transport vehicles [5]. Investigations showed that durability, mechanical properties and engineering performance of LVL are affected by many factors such as wood species, veneer thickness and quality, processing variables, member size, in service environment and loading types [3]. For instance, steaming is one of the processing variables that most of wood species need to be steamed for peeling and it has negative effect on the mechanical properties of wood. Lumbers of some species are also used as steamed such as beech or sometimes as air-dried. Efficient usage of LVL in the construction industry requires an understanding of structural behavior of numerous species and knowledge about to the effects of log pre-treatment process on the mechanical properties and durability of LVL. Therefore, the aims of this study were put forward to the effects of steaming and drying condition on the mechanical properties and durability of LVL and solid-sawn lumber by comparing these two materials.
2. Material and method Steamed beech (Fagus orientalis Lipsky) log with 45 cm diameter and 260 cm length obtained from the region near the border of Turkey–Georgia and steamed and non-steamed spruce logs with 40 cm diameter and 260 cm length obtained from Trabzon region located on the
north cost of Turkey were used in this study. Both places where the logs were taken are located in the warm climate zone. The beech and one of the spruce logs having about 70% initial moisture content were steamed at 60 1C temperature for 20 and 12 h, respectively. The average core temperatures of beech and spruce logs were 50 and 32 1C, respectively (during rotary peeling). Each log was divided into two equal parts to get six logs with 65 cm length. One each part of these logs (three of them) was used for veneer manufacturing while the others were used for preparing solid wood test samples. Only the sapwood portions of the logs were taken for the tests. The horizontal opening (distance from the leading edge of pressure bar to a plane extended from the ground surface of the knife) was 85% of the veneer thickness and the vertical opening was 0.5 mm in the peeling process and 2mm-thick rotary cut veneers with dimensions of 55 55 cm were obtained. After rotary peeling, beech veneer sheets were dried for 5 min from 40–50% to 5–7% final moisture content at 110 1C in a veneer dryer. Veneers obtained from non-steamed logs having 50–60% moisture content were dried at 20 1C while the veneers obtained from steamed logs were dried at 20 1C (for 2 weeks) and 110 1C temperature (for 5 min). Nine-layer LVL panels with 16 mm thickness were manufactured from the veneer sheets. Three replicate panels were produced. Approximately 180 g/m2 adhesive mixture of phenol formaldehyde (PF) having 47% solid content was applied on single bonding surfaces of veneers. Hot press temperature, pressure and duration were 140 1C, 1.4 N/mm2 for beech and 0.8 N/mm2 for spruce and 10 min, respectively. Test groups were formed as shown in Table 1. All test specimens prepared for static bending strength, compression strength parallel to the grain direction and impact strength were classified into two groups after the tests groups were formed. The samples in the first group were tested after conditioning at 20 1C air temperature and 65% relative humidity. The test specimens in the other group were subjected to the aging process with three stage cycles described in EN 321 standard to determine the durability of these materials. The cycle phases were as follows: 1. waiting in water having 2072 1C for 7271 h, 2. freezing at 15 1C for 24 h, 3. drying at 7071 1C temperature for 7271 h.
Table 1 Test groups
Drying temperature Solid wood groups LVL panel groups
Beech
Spruce
Steamed
Non-steamed
Steamed
20 1C SW1 LVL1
20 1C SW2 LVL2
20 1C BW —
110 1C — B-LVL
110 1C — LVL3
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
574
604
645
Test
460
469
471
500
593
Control
490
600
578
Density (kg/m3)
700
587
800
681
723
Air-dry density values of solid wood and LVL test specimens are given in Fig. 1. According to the test results, the density of solid beech specimens decreased as only 1.5% after the aging exposure. The density of nonsteamed solid spruce (490 kg/m3) was higher by 4.5% than that of steamed spruce (469 kg/m3). It is known that log steaming process causes weight loss, dependent on the steaming period and temperature [6]. Although the logs were steamed for only 12 h, the decrease in density value of spruce was measurable. After the aging process, the density losses of non-steamed and steamed spruce solid specimens were 4% and 2%, respectively. Both beech and spruce LVL groups had higher density values and also density losses than those of solid wood from which LVLs were produced. In LVL production, veneers are laminated together under pressure. Thickness of the panel decreases with the pressure applied depending on wood species. The reason for higher density of LVL is related to the thickness loss of the panel due to the compression rate during hot pressing
582
3. Results and discussion
and the higher density of adhesive being used in panel production. In addition, the density values of both solid and LVL groups decreased after the accelerated aging process. The reason of the density losses was the weight losses of specimens occurring in each step of aging treatment. Because the weight losses occurred at the end of the steaming process, no considerable change was obsvered in the density of beech solid-sawn lumber. The decreasing effect of hydrothermal processes on the weight losses were reported in many studies [6–8]. Higher density losses of both non-steamed solid spruce wood and LVL1 sample determined after the aging process confirm this explanation. The mechanical properties of beech solid wood and LVL groups determined before and after aging exposure were given in the Table 2. Static bending and compression strength values parallel to grain of solid wood groups were found to be lower than those of LVLs produced from the same log. On the contrary, impact strength of beech solid wood was significantly higher than that of LVL. No significant change was found in static bending strength and compression strength parallel to grain while a 33% loss was determined in the impact strength of solid wood groups after the aging treatment. The aging treatment had no considerable effect on the compression strength parallel to the grain. The decrease in the bending strength was quite less. The most considerable decrease (25%) caused by aging treatment was obtained for the impact strength. When steamed and non-steamed spruce solid wood samples were compared, it was concluded that steaming decreased considerably all strength properties investigated and the least affected was the compression strength. It was stated in the literature that the mostaffected strength properties from thermal processes were bending strength and impact strength [9,10]. Similar results were found in our previous study for the test samples with 20 20 mm cross-section dimensions [11]. The compression strength mean value of the steamed solid wood samples was lower than that of non-steamed test specimens as much as nearly, 5% (given in Fig. 2).
572
After this cycle was applied for three periods (21 days), aged samples were kept in an autoclave for 2 h under 120 1C temperature and 1.2 atm pressure. Test samples were conditioned to achieve equilibrium moisture content at 20 1C temperature and 65% relative humidity prior to testing. All tests were achieved according to the DIN standards described for solid wood (DIN 52182 for density, DIN 52185 for compression strength, DIN 52186 for static bending strength, DIN 52189-1 for impact bending strength). However, the thickness of solid wood samples was taken as 16 mm (in radial direction) to compare with the LVL panels. Bending strength test was carried out with 10 samples having the dimensions of 300 20 16 mm cut from each manufactured panel. For compression strength parallel to grain, test specimens with dimension of 20 16 30 mm were used.
95
400 300 200 100 0 BW
B-LVL
SW-1
SW-2 Test Groups
LVL-1
LVL-2
Fig. 1. Density mean values of test samples (n ¼ 30).
LVL-3
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
96
Table 2 Mechanical properties of beech solid and LVL test groups before and after aging (n ¼ 30)
Beech–solid wood Beech–LVL a
Compression strength (N/mm2)
Bending strength (N/mm2)
Impact strength (kg m/cm2)
Control
Test
Control
Test
Control
Test
47.4 (1.5)a 52.4 (1.6)
47.1 (2.3) 51.8 (1.8)
98.2 (5.8) 107.8 (6.1)
97.8 (5.1) 104.4 (4.8)
0.91 (0.15) 0.69 (0.03)
0.61 (0.09) 0.52 (0.07)
Values in parenthesis are standard deviations.
Compression Strength (N/mm2)
70 60 50
Control
Test
43.2 39.8 (3.4) (4.0)
41 40 (2.6) (2.7)
SW-1
SW-2
55.1 (5.6) 49.7 (3.1)
49.5 (2.9) 44.4 (2.8)
49.7 47.3 (2.6) (2.3)
LVL-2
LVL-3
40 30 20 10 0 LVL-1 Test Groups
Bending Strength (N/mm2)
Fig. 2. Compression strength in longitudinal direction of spruce wood and LVL groups (n ¼ 30).
120 100 80
Control Test 88.0 (8.4) 80.0 (5.0) 69.3 71.0 (7.4) (7.7)
98.4 99.6 (6.0) (8.1)
92.8 93.7 (4.3) (10.7)
91.4 94.0 (4.8) (7.0)
LVL-2
LVL-3
60 40 20 0 SW-1
SW-2
LVL-1 Test Groups
Fig. 3. Bending strength mean values of spruce solid wood and LVL groups (n ¼ 30).
After the accelerated aging test, the compression strength in longitudinal direction of non-steamed solid test specimens decreased as much as 8%, whereas that of the steamed group did not change significantly. LVL panels produced from the same log had considerably higher compression strength than those of the solid wood groups as shown in Fig. 2. The steaming process causeda similar effect on the compression strength of LVL panels. The highest compression strength was found for the spruce LVL panels manufactured from non-steamed and air-dried (at 20 1C) veneers. The drying temperatures selected in this study
had no clear effect on the compression strength of the spruce LVL panels, which were not subjected to aging process. The decreases in the compression strength values of all LVL groups due to the aging process were higher than those of solid wood samples. Among the spruce LVL groups, the lowest decrease in compression strength was found for the panels manufactured from the veneers dried at 110 1C, which is a mild drying schedule from the industrial point of view. The static bending strength of steamed spruce solid wood samples was 21% lower than that of non-steamed samples (Fig. 3). A decrease of 9% in static bending
ARTICLE IN PRESS S. C - olak et al. / Building and Environment 42 (2007) 93–98
Impact Strength (Kg.m/cm2)
0.6 0.5 0.4
0.48 (0.06) 0.38 (0.09)
Control 0.38 0.39 (0.05)(0.09)
0.38 0.38 (0.07)(0.08)
0.39 0.39 (0.04) (0.03)
SW-2
LVL-1 Test Groups
LVL-2
97
Test 0.34 0.32 (0.06) (0.04)
0.3 0.2 0.1 0 SW-1
LVL-3
Fig. 4. Impact strength mean values of spruce solid wood and LVL panels.
strength of the non-steamed solid spruce samples was determined after the aging process, whereas no clear difference was obtained for the static bending strength of the steamed samples. The bending strength values of the LVL groups were clearly higher than those of the solid wood groups just like the compression strength. Among the spruce LVL groups, the highest bending strength value was obtained for the panels produced from non-steamed and air-dried veneers (control 98.4 N/ mm2, test 99.6 N/mm2). When samples LVL-2 and LVL3 were compared, it can be stated that veneer-drying temperatures selected in this study had no significant effect on the static bending strength of the panels. Aging process also did not affect the bending strength of the LVLs. Compression strength and the static bending strength values of both beech and spruce LVL panels were higher when compared to the solid wood groups obtained from the same logs and this can be related to the production conditions of the LVL panels. The density of the panel increases with the pressure during hot pressing in panel production and it is known that increasing the density improves the mechanical properties (bending strength) of the wood materials [12,13]. Because woodbased panels are bonded materials; glue-type and glue line thickness also affect positively their mechanical properties. The decrease in impact strength of spruce solid samples after steaming was about 21% (Fig. 4). The impact strength of non-steamed solid wood samples decreased as much as 21%, after the aging process. The impact strength values of LVL panels, unlike the bending strength and the compression strength, were lower than those of the solid samples, which were not steamed and aged. The lowest impact strength was obtained for LVL-3 group. Lower impact strength of LVL panels may also be related to the elastic or brittle nature of glue line after the curing reaction. Brittle glue line might cause a decrease in impact strength of the LVL panels.
The substance’s losses and changes in chemical structure caused by hydrothermal processes may be the reasons for the decrease in mechanical properties. It is known that steaming has a positive effect on the water absorption and swelling properties, whereas a negative effect on the strength properties of wood. These effects are attributed to hydrolysis and extraction of hemicellulose caused by steaming [14]. Because the hemicelluloses are composed of a shorter chain of molecules and have a more branched structure, they are generally easier to hydrolyze than other wood components. Strength losses in thermal degradation were attributed to acetic acid derivated from acetyl groups in the chemical structure and acts as a catalyst to speed the rate of degradation further. Hardwoods have much more acetyl groups than softwoods. Resinous acids generated during thermal processes in some softwood were also stated to be the source of greater thermal sensitivity [4]. The reason why the aging process did not have a significant effect on the investigated mechanical properties of steamed specimens may be the removal of resinous acids during the steaming. Steaming can cause a loosening in wood structure and thus allow freer movement of the acids. To support this opinion, the pH values of the steamed and non-steamed spruce solid samples were measured, they were found to be 4.79 and 5.27, respectively. The higher pH values of steamed test specimens confirm this idea.
4. Conclusion The following points were concluded from this study: 1. Both beech and spruce LVL samples had higher density values than those of solid wood from which LVLs were produced. The increased ratios in average density values were determined as 23% for beech LVL and 31% and 28% for non-steamed and steamed spruce LVLs, respectively. After the aging process, similar results were found.
ARTICLE IN PRESS 98
S. C - olak et al. / Building and Environment 42 (2007) 93–98
2. Static bending strength values parallel to grain of solid wood samples were found to be lower by 10.6% for non-steamed spruce and 25.3% for steamed spruce than those of LVLs produced from the same log. Similarly, compression strength values of solid wood samples were found to be lower as mujch as 21.6% and 17.2% for non-steamed and steamed spruce, respectively. On the contrary, impact strength of beech solid wood was 24% higher than that of LVL. 3. The most considerable decrease caused by aging treatment was obtained for the impact strength of 25%. 4. Steaming process of logs for veneering considerably decreased all strength properties investigated and the least affected was the compression strength. 5. LVL panels produced from the same log had considerably higher compression strength than beams made of the solid wood samples. The drying temperatures selected in this study had no clear effect on the compression strength of the spruce LVL panels, which were not subjected to aging process. 6. The bending strength values of the LVL samples were clearly higher than those of the solid wood samples. Among the spruce LVL sample, the highest bending strength value was obtained for the panels produced from non-steamed and air-dried veneers. 7. The impact strength values of LVL panels, unlike the bending and the compression strength, were lower than those of the solid samples, which were not steamed and aged.
Acknowledgment S. C - olak thanks The Scientific and Technical Research Council of Turkey—Directorate of Human Resources Development for the scholarship during her post doctoral study.
References [1] Llaufenberg T. Exposure effects upon performance of laminated veneer lumber and glulam materials. Forest Products Journal 1982;32(5):42–8. [2] Wang X, Ross RJ, Brashaw BK, Verhey SA, Formsan JW, Ericson JR. Flexural properties of laminated veneer lumber manufactured from ultrasonically rated red maple veneer. Forest Product Laboratory, FPL-RN-0288, 2003. [3] Hing PS, Paridah MT, Zakiah A. Edgewise bending properties of LVL: effects of veneer thickness and species. In: The proceedings of USM-JIRCAS joint international symposium, lignocellulosematerial of the millennium: technology and application, 20–22 March 2001. [4] Green DW, Evans JW. Flexural properties of structural lumber products after long-term exposure to 1501F and 75% relative humidity. In: The proceedings of 35th international particleboard/ composites materials symposium. Washington State University. Washington, USA, 3–5 April 2005, p.3–14 [5] Kamala BS, Kumar P, Rao RV, Sharma SN. Performance test of laminated veneer lumber (LVL) from rubber wood for different physical and mechanical properties. Holz als Roh- und Werkstoff 1999;57(2):114–6. [6] Kollmann FFP, Cote WA. Principles of wood science and technology—I: Solid wood, 1968. [7] Santos JA. Mechanical behaviour of Eucalyptus wood modified by heat. Wood Science and Technology 2000;34:39–43. [8] Yılgo¨r N, U¨nsal O, Kartal SN. Physical, mechanical, and chemical properties of beech wood. Forest Product Journal 2001;51(11–12):89–93. [9] MacLean JD. Effect of steaming on the strength of wood. A. Wood Preservers’ Association Proceedings 1953;49:88–112. [10] Aydın I. Effects of some manufacturing conditions on wettability and bonding of veneers obtained from various wood species. PhD, Thesis, Graduate School of Natural and Applied Sciences, Karadeniz Technical University, Trabzon, 2004 [in Turkish]. [11] Aydin I, C - olak S. The changes in some physical and mechanical properties of steamed spruce (Picea orientalis L) wood. Journal of Artvin Forestry Faculty of Kafkas University 2004 [Turkish]. [12] Liu JY, Mcnatt JD. Thickness swelling and density variation in aspen flakeboards. Wood Science and Technology 1991;82:25–33. [13] Nemli G, Kırcı H, Temiz A. Influence of impregnating wood particles with mimosa bark extract on some properties of particleboard. Industrial Crops and Products 2004;20:339–44. [14] Carll CG. Review of thickness swell in hardboard siding. General technical report FPL-GTR-96, Department of Agriculture, Forest Service, Forest Product Laboratory, 1996.