Development of structural laminated veneer lumber from stress graded short-rotation hem-fir veneer

Construction and Building Materials 47 (2013) 902–909

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Development of structural laminated veneer lumber from stress graded short-rotation hem-fir veneer Brad Jianhe Wang ⇑, Chunping Dai FPInnovations – Wood Products, 2665 East Mall, Vancouver, BC V6T 1W5, Canada

h i g h l i g h t s  Feasibility of short-rotation hem-fir for structural LVL is explored.  Comparison of hem-fir visual grades and stress grades is studied.  Manufacturing parameters, lay-up and performance of hem-fir LVL are examined.  New application potentials of short-rotation hem-fir are demonstrated.

a r t i c l e

i n f o

Article history: Received 3 April 2013 Accepted 16 May 2013

Keywords: Amabilis fir Laminated veneer lumber (LVL) Performance Stress grade Veneer Visual grade Western hemlock

a b s t r a c t The key objective of this work was to investigate the feasibility of using short-rotation western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) from coastal Britich Columbia, Canada for manufacturing structural laminated veneer lumber (LVL). Fourteen hem-fir logs were sampled, bucked and conditioned. Hem-fir veneer was then peeled, clipped, dried and visually graded. Combined hem-fir veneer was further segregated into three E grades based on dynamic modulus of elasticity (MOE). LVL billets were manufactured from each E grade and from two grade mixes, and evaluated for flatwise and edgewise bending MOE and modulus of rupture (MOR), and longitudinal shear strength. The results demonstrated that hem-fir veneer visual grade yield was about 15% B with the remainder C. No correlation existed between hem-fir veneer visual grades and E grades. Hem-fir LVL made from high E1 grade, medium E2 grade and low E3 grade could meet 2.2E, 1.8E and 1.5E product market requirements, respectively. Higher E grade veneer yielded higher LVL bending MOE, but not necessarily higher LVL shear strength. A good correlation was found between the LVL bending MOE and veneer mean MOE, and between the LVL bending MOR and MOE. Thus, the bending performance of the hem-fir LVL can be predicted based on veneer E grade and product lay-up. Pressing time changed clearly with the LVL lay-up with higher E grade requiring longer pressing time. Pressing time of mixed grade LVL fell within the times required for the two single grade lay-ups. Mixed grade LVL had a greater enhancement in flatwise than in edgewise, and was therefore more suitable for fabricating flanges of Ijoists. Using a grade mix in the product lay-up allows low E grade veneer to be fully utilized for increased value recovery. The results of this study implied that without species segregation, the combined shortrotation hem-fir veneer can be successfully stress graded to manufacture structural LVL for building applications. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction At present, forest resource is changing worldwide over time from old-growth to second growth with an increasing volume of shorter rotation plantations [1–4]. To maximize the value return from the resource currently available, forest stands should be managed to produce trees with desired attributes for end products. For product development and market access, characteristics of the ⇑ Corresponding author. Tel.: +1 778 388 8707. E-mail address: [email protected] (B.J. Wang). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.05.096

second growth plantations should be competitive not only to those of old-growth, but also to those of competing species. Hem-fir is combination of western hemlock (Tsuga heterophylla (Raf.) Sarg) and amabilis fir (Abies amabilis (Dougl.) Forbes) and represents the largest component of forests in coastal forest region of British Columbia (BC), Canada. These two species are generally harvested and processed together and marketed as ‘‘hem-fir’’, which are generally used to produce both solid wood products and pulp and paper. Hem-fir solid wood is used primarily as structural lumber and plywood in home and commercial building construction. Products include framing lumber, joinery, windows,

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doors, staircases, cabinet doors, ladders, floors, roof decking, railway ties, boxes, interior woodworking and finishes, veneer/plywood, and laminating stock or glulam [5]. Despite the widespread use, opportunities exist to recover more value from the short rotation hem-fir through increased utilization in engineered wood products (EWPs) because they present opportunities for potentially transformative product developments and process improvements. The key advantage of EWPs is that their performance is not necessarily limited by fiber quality. Potential EWPs include structural composite lumbers such as laminated veneer lumber (LVL), thick light-weight strand panels, and cross laminated timber (CLT) that can replace concrete slabs in residential and non-residential constructions. LVL uses multiple parallel layers of stress graded veneers assembled with adhesives. It offers several advantages over typical dimension lumber: it is stronger, straighter, and more uniform. Due to its composite nature, it is much less likely than conventional lumber to warp, twist, bow, or shrink. Made in a factory under controlled specifications, LVL products allow users to reduce the onsite labor. They are typically used for headers and beams, flanges for prefabricated wood I-joists, rafters, columns, studs, truss chords, and rim board applications for residential floor and roof systems as well as concrete forms and scaffold planks. The industry typically markets LVL headers and beams based on bending modulus of elasticity (MOE) values along with their size and allowable bending stress. Common bending moduli are 12,000 MPa (1,800,000 psi); 13,000 MPa (1,900,000 psi); 14,000 MPa (2,000,000 psi) and 15,000 MPa (2,200,000 psi); and common allowable bending stress values are 19 MPa (2800 psi); and 21 MPa (3000 psi). In North America (NA), the production of LVL bounced back in 2010 and 2011 as housing starts increased and I-joists gained share of raised floors. It is expected to total 1.2 million m3 (about 43.4 million cubic feet) in 2012, a 4% increase from 2011. Based on the recent forecast, LVL demand and production for beams and headers and I-joist flanges could approach 2 million m3 (about 70 million cubic feet) by 2016 and 2017 in NA [6]. LVL made from older second-growth hemlock has been already considered a good option for the BC coastal industry [7,8]. Those studies showed that for this hemlock, veneer recovery is a function of log diameter, and that veneer density and stiffness are affected by growth rate [1]. Generally speaking, slow growing trees produce strong veneer suitable for producing LVL and fast growing trees produce more knotty and relatively low density veneer that is suitable for plywood production [5]. The shift to harvesting increasingly younger plantations has uncertain implications for all aspects of veneer processing, including peeling, sorting, drying, grading and pressing. The successful promotion of the short rotation hem-fir for manufacturing EWPs, particularly LVL, requires veneer characterization and product performance evaluation [8–10]. The key objectives of this work were to: (1) conduct property evaluation and visual/stress grading of the short-rotation hem-fir veneer and (2) examine performance of the resulting hem-fir LVL products. The study was part of a larger second-growth hem-fir characterization project that includes quality assessment of standing trees, log grade determination, CT imaging of 5 m logs for internal-defect sawing simulation and destructive sampling at predetermined stem positions to determine basic wood properties, mechanical properties of small clears, and pulping properties [9,10]. Sample trees were harvested and bucked into bolts for conditioning and veneer peeling. Hem-fir veneer sheets were clipped, dried and visually assessed without species segregation. They were further sorted into three E grades based on veneer dynamic modulus of elasticity (MOE). Subsequently, various 13-ply hem-fir LVL lay-ups were constructed and hot-pressed. The performance of hem-fir LVL was evaluated in terms of bending MOE, modulus of rupture (MOR), and longitudinal shear strengths in both longitudinal L–X

and L–Y configurations. Based on the visual and stress grades of hem-fir veneer and product performance, their suitability for use in the manufacture of structural LVL products can be determined. The information gathered on veneer and LVL potential is useful for product certification and market access. 2. Materials and methods Study materials came from 68-year age from a stand (1020 stems per hectare) on Vancouver Island, BC, Canada. Fourteen hem-fir logs, with different diameters at breast height (DBH), were randomly selected from a concurrent study [9]. Among the total 14 logs, six were hemlock and the remaining amabilis fir. This species ratio may not well represent the tree mix in the stand; however, at this stage, the main purpose was to determine the feasibility of processing combined hem-fir for LVL manufacturing, rather than grade outturn of each species. A nominal 1.2 m long bolt was cut from each sample log for veneer tests. As shown in Table 1, among the 14 bolts, 5, 6 and 3 bolts were from 30 cm, 40 cm and 50 cm DBH classes, respectively. 2.1. Veneer peeling and drying Those 14 bolts (Table 1) were first debarked. For each bolt, the diameters at both ends were then measured to calculate its mean value. The bolts were further conditioned in a 50 °C pond for 24 h and subsequently peeled on a 1.2 m industrial lathe. The following lathe settings were used: horizontal gap (HG) = 2.7 mm; vertical gap (VG) = 10.7 mm and pitch angle (PA) = 90°. The settings were checked at three locations on the lathe before peeling: left, center and right. After peeling and clipping, each sheet was labelled sequentially from heart to sap according to the bolt number. Clipping width was about 60 cm and the core radius of each bolt was about 6 cm. A pilot plant dryer was used to dry the green hem-fir veneer to a moisture content (MC) target of 3%. The weight of each sheet was measured before and after drying to calculate green veneer MC. 2.2. Veneer grading The total number of veneer sheets obtained from each bolt was counted after drying. Each sheet was graded visually as A, B, C Face (CF), C Inside (CI) and Reject in accordance with Canadian Softwood Plywood Standard [11] and the grade was tabulated by bolt number. The main difference between CF and CI is that a greater number of knots or knot holes are allowed on the CI sheets. After visual grading, each veneer sheet was trimmed down to a size of 86 cm  61 cm to facilitate pilot-plant veneer stress grading and LVL manufacturing. The thickness, length, width and weight of each dry sheet were measured to calculate veneer density. A modified Metriguard stress wave timer was used to measure stress wave time, equivalent to ultrasonic propagation time (UPT), of each sheet [12]. Ten readings were measured with a lateral interval of 5.1 cm. The measured span was set at 76 cm. Based on the veneer UPT and density, dynamic MOE of each veneer sheet was computed as follows: 2

MOE ¼ q=g  ðL=UPTÞ

ð1Þ

where g is the gravitational constant, q is dry density and L is the span for the UPT measurement (76 cm in this case). All measurements and computed results were tabulated for property evaluation and grading analysis. A total of 439 dry hem-fir veneer sheets (86 cm  61 cm) were tested. They were then segregated into three stress grades: E1, E2 and E3, based on

Table 1 Hem-fir bolts procured for veneering. Bolt no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Species

Amabilis fir Amabilis fir Amabilis fir Hemlock Hemlock Amabilis fir Amabilis fir Hemlock Hemlock Amabilis fir Hemlock Hemlock Amabilis fir Amabilis fir

DBH (cm)

Class

32.3 29.7 40.9 40.2 39.7 41.0 39.2 39.3 31.5 52.0 32.8 51.5 30.5 47.6

30 30 40 40 40 40 40 40 30 50 30 50 30 50

904

B.J. Wang, C. Dai / Construction and Building Materials 47 (2013) 902–909 by a 30 s decompression cycle. The number and sequence of veneer sheets in each LVL lay-up were recorded. After unloading the press, the LVL billets were stacked for 48 h for post curing. The final phase of this work was to determine the properties of the hem-fir LVL with various lay-ups. The thickness, specific gravity (SG), edgewise and flatwise bending MOE and MOR, block shear strengths in both L–X and L–Y configurations and dimensional stability from 24-h soaking of each LVL billet were measured following the cutting pattern shown in Fig. 1. From each billet, four bending specimens each in edgewise and flatwise modes, and five shear blocks each in L–X and L–Y were tested. The bending tests were performed in accordance with a JAS standard for LVL [15]. For the L–X shear test, the shear plane was parallel to the plane of the glue line, whereas for the L–Y shear test the shear plane was perpendicular to the plane of the glue line [16,17]. The thickness swell test was conducted according to ASTM D1037 [18]. However, due to the limitation of the panel size and sawing kerfs, the actual specimen size for this test was 13 cm  13 cm. All test results were analyzed to establish a relationship between the performance of LVL and veneer lay-up characteristics.

Table 2 Hem-fir LVL lay-up. LVL no.

LVL lay-up

Note

1 2 3 4 5

E1–13 E2–13 E3–13 E3–9 E3–9

Single E1 grade Single E2 grade Single E3 grade 2 plies of E1 veneer on each side 2 plies of E2 veneer on each side

E1–2 E2–2

E1–2 E2–2

3. Results and discussion 3.1. Hem-fir veneer visual grading

Note: E1 - E4 for edgewise bending (86 x 3.8 x 3.8 -cm); F1 - F4 for flatwise bending (86 x 3.8 x 3.8 -cm); WA and TS for water absorption and thickness swell (12.7 x 12.7 -cm) S1 for longitudinal shear L-X (6.4 x 5.1 x 3.8 -cm) S2 for longitudinal shear L-Y (7.6 x 5.1 x 3.8 -cm) Fig. 1. Cutting pattern of the hem-fir LVL billet for property tests.

their dynamic MOE [13] for achieving equal volume breakdown. In this study, E1, E2 and E3 were simply convenient names to differentiate the grades with variable grade boundaries. The veneer properties of each E grade were evaluated.

2.3. LVL manufacturing and testing Five typical lay-ups of 13-ply hem-fir LVL were tested in the pilot plant (Table 2). A total of 15 LVL billets were made with three replicates each. Regular phenol formaldehyde (PF) glue for plywood manufacturing was used with a glue spread level of 170 g/m2 per single glueline. A new pressing method, which integrated both pressure control and thickness control in one pressing cycle, was used with the following pressing parameters: pressing temperature, 155 °C; control thickness, 37.5 cm, to achieve the target thickness of 38 cm [14]. The pressing time was controlled until the innermost glueline temperature reached 105 °C, followed

Table 3 summarizes actual bolt diameter, veneer visual grade, veneer yield and MC for each bolt. The bolt diameter ranged from 27.9 to 46.5 cm and averaged 35.7 cm. Depending on the diameter, each bolt yielded 16 to 65 sheets with a target clipping width of 61 cm. A total of 487 sheets were generated. Green veneer MC varied from log to log. On a log basis, the mean MC was from 58% to 112%. On a sheet basis (Fig. 2), however, the green veneer MC varied from 20% to 170% with mean and standard deviation being 87.5% and 27.8%, respectively. This green veneer MC distribution represents the typical MC pattern of combined sapwood and heartwood veneer [19]. For most softwood species, sapwood generally has a higher MC than heartwood, and a dual-peak normal distribution exists with one for heartwood and the other for sapwood. Fig. 3 shows the grade outturn of hem-fir veneer. About 47%, 38% and 15% of veneer sheets were classified into C grade for surface use (CF), C grade for inner use (CI) and B grade, respectively. Both A and Reject grades were negligible. These visual quality characteristics are important for manufacturing softwood plywood products [11]. However, they only served as a reference as the focus of this study was to examine the behavior and performance of LVL products made from the combined hem-fir veneer. 3.2. Hem-fir veneer property evaluation and stress grading Figs. 4 and 5 show the cumulative distribution function (CDF) of density and dynamic MOE for the combined dry hem-fir veneer, respectively. The wide range of veneer density and dynamic MOE

Table 3 Hem-fir veneer visual grade, yield and MC. Bolt no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 Sum

Actual bolt diameter

Veneer visual grade

(cm)

B

CF

CI

Reject

Number of sheets

30.7 27.9 37.8 37.1 35.6 38.1 35.6 35.1 29.0 46.5 30.0 45.2 28.7 42.7

2 5 3 5 12 6 7 6 12 0 4 0 2 10 74

7 14 19 31 23 27 28 21 4 0 14 8 7 26 229

16 0 16 0 0 10 0 0 0 65 1 46 11 17 182

1 0 0 0 0 0 0 0 0 0 0 0 0 1 2

26 19 38 36 35 43 35 27 16 65 19 54 20 54 487

Ribbon length

Green veneer MC (%)

(cm)

Mean

Std. Dev.

1493.5 1163.3 2347.0 2214.9 2174.2 2463.8 2118.4 1661.2 911.9 3998.0 1158.2 3317.2 1244.6 3106.4 29,373

84.0 62.8 70.6 58.2 90.6 111.6 65.6 97.1 63.9 76.6 59.2 104.6 65.9 83.8

16.2 16.9 18.4 7.4 17.9 22.6 28.9 29.9 6.3 25.6 9.7 30.5 13.2 23.6

B.J. Wang, C. Dai / Construction and Building Materials 47 (2013) 902–909

905

24 22

Frequencyy (%)

20 18 16 14 12 10 8 6 4 2 0 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170

Green veneer MC (%) Fig. 2. Hem-fir green veneer MC distribution. Fig. 5. Dynamic MOE distribution of combined dry hem-fir veneer.

50 45

Frequency (%)

40 35 30 25 20 15 10 5 0 A

B

CF

CI

Reject

Veneer visual grade

used: 10,550 MPa and 13,030 MPa. The grading results are shown in Table 4. The higher the E grade, the higher the veneer density. After grading, the variation in veneer density and MOE within each E grade was significantly reduced, which is beneficial to manufacture consistent LVL products [13]. As shown in Fig. 6, for this combined hem-fir veneer, there was no correlation between visual grades (B, CF and CI) and stress grades (E1, E2 and E3) in terms of veneer density and dynamic MOE. For visual grades, it seemed that CF grade yielded the highest veneer density and dynamic MOE, followed by B and CI grades. For stress grades, there was a clear descending trend in both veneer density and dynamic MOE from E1 to E2 and E3 grades. As also indicated by the error bar (±one stand deviation), compared to visual grades, stress grades generated much smaller variation within each grade for both veneer density and dynamic MOE.

Fig. 3. Hem-fir veneer visual grade distribution.

3.3. Physical and mechanical properties of hem-fir LVL 100 90 80

CDF (%)

70 60 50 40 30 20 10 0 0.26

0.3

0.34

0.38

0.42

0.46

0.5

0.54

Density of hem-fir veneer (g/cm3) Fig. 4. Density distribution of combined dry hem-fir veneer.

makes stress grading essential to reduce performance variation of LVL products. As far as veneer stress grading is concerned, the grade boundaries are generally species and product dependent. Currently, there are no grading recipes established for veneer stress grading with defined grade boundaries. To facilitate pilot plant manufacturing of hem-fir LVL, a generic stress grading was performed to segregate the combined hem-fir veneer into three E grades with approximately equal volume breakdown. Two E grading thresholds were

Table 5 shows the physical properties of each of the 15 hem-fir LVL billets. Product thickness was under good control, yielding a compression ratio (CR) from 10% to 16%. The pressing time changed clearly with the lay-up (or veneer E grade). The higher the E grade, the heavier the lay-up, and in turn the longer the LVL pressing time. This is due to the fact that heat diffusivity, a ratio of heat conductivity over specific heat and veneer density, of high E grade was lower than that of low E grade. The pressing time of mixed grade LVL fell between those of the two single grade layups. The final MC of hem-fir LVL ranged from 5.8% to 7.2%. Based on the 24-h cold water soaking tests, the water absorption (WA) and thickness swell (TS) seemed to be higher with high grade (E1) than low grade (E3), which agrees with the result reported earlier in another separate study with lodgepole pine veneer [20]. Table 6 shows the test results of hem-fir LVL flatwise and edgewise bending MOE and MOR, and longitudinal L–X and L–Y shear strengths. The glue was fully cured, yielding a perfect wood failure (100%). The results demonstrated that hem-fir LVL made from E1 grade veneer meets 2.2E (15,000 MPa) grade LVL product requirements whereas LVL made from E2 grade veneer meets 1.8E (12,000 MPa) grade requirements. The LVL made from E3 grade veneer could meet 1.5E (10,000 MPa) grade requirements [15,21]. In total, about 66% of the hem-fir veneer from this log sample can be used to manufacture higher grade LVL, meeting 1.8E (12,000 MPa) or higher grade LVL requirements. Higher E grade veneer generally yielded a higher MOE product. However, this trend was not observed in terms of LVL longitudinal shear strength. Comparatively, medium E2 grade yielded the best shear strength in both L–X and

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B.J. Wang, C. Dai / Construction and Building Materials 47 (2013) 902–909

Table 4 Hem-fir veneer stress grading for LVL manufacturing. Stress grade

E1 (high) E2 (medium) E3 (low) Unsorted population a b

Number of sheets

150 145 144 439a

Veneer UPT (ls) b

Veneer thickness (mm)

Veneer density (g/cm3)

Veneer dynamic MOE (MPa)

Mean

Std. Dev.

Mean

Std. Dev.

Mean

Std. Dev.

Mean

Std. Dev.

127.4 136.2 146.5 136.4

5.5 6.3 12.3 10.3

3.311 3.430 3.475 3.404

0.158 0.134 0.061 0.143

0.423 0.379 0.335 0.379

0.036 0.036 0.029 0.050

15169 11859 9170 12135

1568 758 965 2689

Due to some overwet sheets and breakages, the total number of dry veneer sheets used for stress grading was reduced to 439 from the 487 used for visual grading. Between – sheet variation.

Fig. 6. Comparison of hem-fir veneer properties between visual grades and stress grades.

Table 5 Hem-fir LVL lay-up and physical properties. Test no.

LVL lay-up

Billet no.

Veneer

LVL 3

Density (g/cm )

1

E1–13

E2–13

E3–13

E1–2/E3–9/E1–2

E2–2/E3–9/E2–2

Mean a b c d e

SGb 3

MCc

CRd

24 h soake

Std. Dev.

Mean

Std. Dev.

(mm)

(mm)

(min)

(g/cm )

(%)

(%)

WA (%)

TS(%)

0.037 0.034 0.025 0.032

14617 15651 14755 15031

1538 1903 1089 1510

43.4 41.9 43.7 43.2

37.6 37.8 37.8 37.8

23.5 24.2 24.3 24.0

0.501 0.509 0.500 0.504

5.9 5.9 5.7 5.8

13.2 10.1 13.8 12.4

30.5 32.3 33.1 32.0

5.8 5.7 5.5 5.7

1 2 3

0.377 0.376 0.376 0.376

0.038 0.040 0.046 0.041

12135 11859 11859 11928

703 565 696 655

45.5 45.5 44.5 45.2

38.1 38.1 38.1 38.1

22.3 22.5 21.3 22.0

0.469 0.466 0.462 0.465

6.1 6.4 6.4 6.3

16.0 16.0 14.3 15.4

28.3 27.4 26.6 27.4

5.3 5.2 5.5 5.3

1 2 3

0.344 0.344 0.337 0.341

0.039 0.035 0.033 0.036

9446 9239 8963 9239

924 883 717 841

45.2 45.7 45.5 45.5

38.4 38.6 38.4 38.4

20.8 19.6 21.4 20.6

0.434 0.427 0.417 0.426

6.8 7.4 7.6 7.2

15.4 15.5 15.4 15.5

25.3 23.2 24.8 24.4

4.9 4.8 5.1 4.9

1 2 3

0.359 0.365 0.360 0.362

0.045 0.049 0.053 0.049

10204 11101 10756 10687

3172 2951 2813 2979

45.2 44.2 44.7 44.7

38.4 38.4 38.4 38.4

21.6 21.1 21.3 21.3

0.447 0.445 0.439 0.444

6.9 7.0 6.7 6.8

15.5 13.6 14.2 14.4

28.7 27.7 25.6 27.3

5.3 5.4 5.5 5.4

1 2 3

0.355 0.345 0.343 0.348

0.041 0.029 0.017 0.029

10204 9929 10066 10066

1675 1255 1558 1496

45.2 45.2 45.0 45.2

38.1 38.4 38.1 38.1

20.7 21.7 21.3 21.2

0.445 0.421 0.427 0.431

6.9 6.9 7.2 7.0

15.8 14.9 15.3 15.3

26.5 25.3 23.1 25.0

5.2 5.6 5.3 5.4

Mean 5

Timea

0.418 0.437 0.410 0.421

Mean 4

Thick-ness

Mean

Mean 3

Lay-up thickness

1 2 3

Mean 2

MOE (MPa)

Pressing time for each LVL billet. SG, specific gravity based on oven-dry mass. MC, moisture content (oven-dry basis). CR, compression ratio = (t1  t2)/t1, where t1 is the veneer lay-up thickness, t2 is final thickness of LVL. WA, water absorption after 24-h soak; TS, thickness swelling after 24-h soak.

L–Y configurations, followed by high E1 grade and low E3 grade. Similar results were also reported in an early study with aspen veneer [23]. Fig. 7 shows the correlation between LVL MOE and veneer mean dynamic MOE in both edgewise and flatwise bending modes. There

was high correlation between edgewise bending MOE and veneer mean MOE with an R2 of 0.97, which was consistent with that from another early study with the older second-growth hemlock veneer [7]. The correlation between flatwise bending MOE and veneer mean MOE was also good, giving an R2 of 0.83. The results

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B.J. Wang, C. Dai / Construction and Building Materials 47 (2013) 902–909 Table 6 Mechanical properties of hem-fir LVL. Test no.

LVL lay-up

1

E1–13

2

Mean E2–13

3

Mean E3–13

4

Mean E1–2/E3–9/E1–2

5

Mean E2–2/E3–9/E2–2

Billet no.

1 2 3 1 2 3 1 2 3 1 2 3 1 2 3

Mean

Shear

MOE (MPa)

MOR (MPa)

MOE (MPa)

MOR (MPa)

L–X (MPa)

WFb (%)

L–Y (MPa)

16065 15789 15720 15,858 (346)a 13238 12549 12480 12,755 (654) 10825 10135 10066 10342 (558) 13169 14272 12686 13,376 (878) 11928 10204 11583 11,239 (1012)

90.7 79.6 88.7 86.3 69.6 79.8 74.9 74.7 51.2 55.5 50.4 52.4 80.2 81.1 68.3 76.5 73.1 49.1 67.4 63.2

15031 15375 15306 15,238 (307)a 12824 12549 11928 12,434 (587) 10411 9722 9308 9814 (579) 11307 11376 11032 11,239 (397) 10825 10066 9929 10273 (465)

93.1 93.4 90.7 92.4 75.0 75.4 70.1 73.5 57.2 52.3 49.3 53.0 63.3 65.2 62.1 63.5 59.5 51.6 55.5 55.5

5.2 5.9 6.7 5.9 7.0 5.4 7.0 6.5 4.4 4.2 4.4 4.4 5.1 4.8 5.3 5.1 5.3 5.9 4.8 5.4

100 100 100 100 100 100 100 100 99 100 100 100 100 100 100 100 100 100 100 100

9.1 8.2 8.4 8.6 9.1 8.9 8.8 8.9 7.1 8.8 7.4 7.8 8.1 7.6 8.3 8.0 7.5 7.1 8.6 7.7

(8.4)a

(6.8)

(6.0)

(9.1)

(13.1)

(4.9)a

(6.5)

(4.5)

(3.6)

(5.4)

(1.1)a

(1.1)

(0.5)

(1.1)

(1.0)

(1.4)a

(0.9)

(1.5)

(0.9)

(1.2)

Data in brackets represent standard deviation. Represents wood failure.

LVL Edgewise MOE (MPa)

b

Edgewise bending

17000

17000

16000

16000

15000 14000 13000 12000 11000 10000

y = 0.95 x + 1014 R² = 0.97

LVL Flatwise MOE (MPa)

a

Flatwise bending

15000 14000 13000 12000 11000

y = 0.88x + 2702 R² = 0.83

10000

9000

9000

8000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000

8000 8000 9000 10000 11000 12000 13000 14000 15000 16000 17000

Veneer MOE (MPa)

Veneer MOE (MPa)

Fig. 7. Correlation between hem-fir LVL MOE and veneer mean MOE.

1.30 1.25 1.20

MOE ratio

demonstrated that product performance can be accurately predicted from veneer grade and product lay-up. With the product lay-ups used in this study, the flatwise bending performance was generally higher than the edgewise counterpart. As also shown in Fig. 7, both hem-fir LVL edgewise and flatwise bending MOEs were generally higher than veneer mean MOE. This was seen as the enhancement resulting from the combined effect of veneer densification, glue penetration and glueline [20,22]. Based on the slope of the trend line, it can be seen that: (1) the hem-fir LVL billets with lower mean MOE of veneer assemblies yielded a greater product MOE enhancement in both edgewise and flatwise bending modes and (2) this enhancement was more significant with the flatwise mode than the edgewise mode. The results echoed those reported in the earlier studies with other two species, namely, aspen and lodgepole pine [22,23]. As veneer stress grade is generally sorted by veneer dynamic MOE, it is therefore concluded that the lower grade veneer can be enhanced more than the higher grade veneer. As employed in an earlier study [22], the MOE enhancement (or MOE ratio) can be defined as the LVL MOE over the mean dynamic

LVL Edgewise MOE / Veneer MOE LVL Flatwise MOE / Veneer MOE

1.15 1.10 1.05 1.00 E1-13

E2-13

E3-13

E1-2/E3-9/E1-2 E2-2/E3-9/E2-2

Hem-fir LVL lay-up Fig. 8. Hem-fir LVL MOE enhancement ratio in relation to veneer grades and billet lay-ups.

B.J. Wang, C. Dai / Construction and Building Materials 47 (2013) 902–909

100

100

90

90

80

70

60

y = 0.0074x -19.32 R² = 0.99

50

40 8000

Flatwise bending MOR (MPa)

Edgewise bending MOR (MPa)

908

80

70

60

y = 0.0061x -6.58 R² = 0.82

50

40 9000 10000 11000 12000 13000 14000 15000 16000 17000

Edgewise bending MOE (MPa)

8000

9000 10000 11000 12000 13000 14000 15000 16000 17000

Flatwise bending MOE (MPa)

Fig. 9. Correlation between hem-fir LVL bending MOR and MOE.

MOE of all the veneers in the billet. Shown in Fig. 8 is the MOE enhancement in relation to veneer grades and billet lay-ups. It was evident that the low grade E3 veneer can be strengthened the most, followed by the medium grade E2 and high grade E1 veneer. From the perspective of material cost, the high grade E1 veneer generally entails the highest price, followed by the medium grade E2 and low grade E3 veneer. For edgewise bending, the enhancement ratio was about 1.02, 1.04 and 1.06 for the high (E1), medium (E2) and low (E3) grades, respectively. By comparison, for flatwise bending, the enhancement ratio was increased to approximately 1.06, 1.07 and 1.12 for the same grade designations. In addition, in the edgewise bending mode, the E1–2/E3–9/ E1–2 lay-up yielded the 2nd best enhancement, slightly shy compared to the single E3 lay-up. By contrast, in the flatwise bending mode, the two mixed grade constructions yielded higher enhancement than single grade constructions. Further, among the five billet lay-ups, the mixed grade construction E2–2/E3–9/E2–2 resulted in a similar enhancement as the single E3 lay-up in the flatwise mode but relatively lower enhancement in the edgewise mode. The E1–2/E3–9/E1–2 lay-up yielded greater enhancement than the E2–2/E3–9/E2–2 lay-up. Those results indicated that from the viewpoint of MOE enhancement, the mixed grade construction is better suited for such flatwise applications as I-joist flange than edgewise applications such as headers and beams. The mixed grade construction offers a more cost-effective way for LVL manufacturing since the low grade veneer can be fully utilized for cost reduction. Among the two mixed grade constructions, the E1–2/ E3–9/E1–2 lay-up was relatively better than the E2–2/E3–9/E2–2 lay-up for both edgewise and flatwise applications. Fig. 9 depicts the correlation between LVL MOR and MOE in both edgewise and flatwise bending modes. There was very high correlation between edgewise bending MOR and edgewise bending MOE with an R2 of 0.99, which indicated that the LVL edgewise bending MOR can be accurately predicted from the edgewise bending MOE, and in turn the veneer mean dynamic MOE. Note that the correlation between LVL flatwise bending MOR and MOE was also good giving an R2 of 0.82. By comparison, the correlation with the edgewise bending mode was higher than the flatwise bending mode. Those results demonstrated that both hem-fir LVL bending MOE and MOR can be predicted from veneer grade and product lay-up. 4. Conclusions The pilot plant tests of the combined hem-fir veneer obtained from the sample of fourteen second-growth plantation logs, and

the manufacturing and testing of hem-fir LVL billets, support the following conclusions: There was no correlation between hem-fir veneer visual grades and stress grades. With the equal volume breakdown for the three hem-fir E grades (high E1, medium E2 and low E3), the LVL made from E1, E2 and E3 grade veneer could meet 2.2E, 1.8E and 1.5E grade product market requirements, respectively. The pressing time changed clearly with the LVL lay-up (or veneer E grade). The LVL made from higher E grade required longer pressing time. A good correlation was found between hem-fir LVL bending MOE and veneer mean dynamic MOE, and between hem-fir LVL bending MOR and bending MOE. However, the correlation was found to be higher with the edgewise bending mode than the flatwise bending mode. Higher hem-fir E grade veneer had higher density and yielded higher LVL bending MOE. This indicated that the bending performance of LVL products can be well predicted in terms of veneer E grade and product lay-up. However, higher E grade veneer would not necessarily yield higher LVL shear strength. The flatwise bending performance of mixed grade hem-fir LVL was enhanced more significantly than the edgewise bending performance, which demonstrated that the mixed grade LVL is more suitable for fabricating planks of I-joists. With a grade mix in the product lay-up, the low grade hem-fir veneer (such as E3 in this case) can be fully utilized to reduce material cost for increased value recovery. The results of this study implied that without species segregation, the combined hem-fir veneer can be successfully processed and graded for manufacturing structural LVL for building applications. Further studies are needed to investigate (1) the grade outturn of the typical combined hem-fir stand for LVL products and (2) the effect of separating the hem-fir veneer by species on manufacturing and performance of LVL products and determine if any potential benefits exist for this practice. Acknowledgements FPInnovations would like to thank its industry members, Natural Resources Canada (Canadian Forest Service) and the Province of British Columbia, for their guidance and financial support for this part of research. The authors thank Gerry Middleton and Dave Munro for their help throughout this work. Thanks to Ed Proteau, Peter Ens, John Hoffmann, Gordon Chow and Heng Xu for their help in arranging the logging and trucking, veneer processing and LVL manufacturing and testing.

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