Durability of Laminated Veneer Lumber Made from Blackbutt (Eucalyptus Pilularis)

10DBMC International Conference On Durability of Building Materials and Components LYON [France] 17-20 April 2005

Durability of Laminated Veneer Lumber made from Blackbutt (Eucalyptus Pilularis)

J. Carrick, K. Mathieu University of New South Wales UNSW, Sydney NSW 2052 Australia [email protected]

TT2-55

ABSTRACT

Blackbutt (Eucalyptus Pilularis) is a common plantation hardwood in New South Wales, Australia, highly regarded for its strength and durability but considered difficult to laminate for "engineered timber" because of chemicals present in the timber (extractives). While previous work by State Forests New South Wales pointed to a viable lamination method using phenolic tannin glues, plywood made from Blackbutt has shown glue failure in exterior applications. At the University of New South Wales (UNSW), a lamination technique was investigated for Blackbutt Laminated Veneer Lumber (LVL) and a cleavage fracture toughness method was adapted to quantify the toughness of its glue-lines.

The durability of Blackbutt LVL with differing extractive content was explored by assessing fracture toughness of glue-lines after exposure to one of three artificial weathering environments and a marine inter-tidal zone. Specimens subjected to the most aggressive laboratory environment and those immersed in the inter-tidal zone showed some loss of fracture toughness with increasing exposure, however Blackbutt LVL was shown to be much more durable than Pine LVL. Exposure to the adverse environments had not compromised the nature of the glue-lines and the mean toughness values remained relatively high at approximately 400 J/m2. The results suggest that Blackbutt veneer is capable of being glued for application as a durable structural LVL.

KEYWORDS

Blackbutt, Cleavage Fracture, LVL .

10DBMC International Conference On Durability of Building Materials and Components LYON [France] 17-20 April 2005

2 INTRODUCTION

There are many reasons for the increasing popularity of engineered timber in structural applications: it is sourced from a renewable resource which absorbs carbon, its material properties are less variable than those of sawn timber and its timber elements can be obtained from younger plantation trees. Engineered timbers however, are generally made up by the gluing together of components and glue-line failures have contributed to durability problems.

This project examined “Laminated Veneer Lumber” (LVL) made from the Australian hardwood, Blackbutt. Unlike plywood, the grains of adjacent veneers are parallel. The randomisation of defects leads to better and more uniform mechanical properties than those of the parent timber. In Australia, although pine LVL is now commonly used for structural elements in protected environments, it is not sufficiently durable to be used outside. Blackbutt LVL offers the promise of strength and durability.

Blackbutt is a common plantation species in New South Wales - it produces a strong and durable timber. Although Blackbutt is difficult to glue because of natural chemicals (extractives), previous work at State Forests NSW (SFNSW) has shown that with tight process control, poly-phenolic tannin glues cured with para-formaldehyde can adhere to Blackbutt veneers. Were Blackbutt suitable for durable structural laminates, value would be added to the forest resource. "High" and "Low" extractive veneers were identified using a scanning technique for colour variation and became a variable in the specimen sets.

After examining work at the US department of Agriculture [Scott et al., 1992]; [River and Okkonen, 1993] and at Monash University [Milner, 1996], the project adapted a cleavage fracture toughness method to evaluate the glue-lines in LVL (the current Australian Standard method of evaluating LVL glue-lines does not test fracture toughness of correctly orientated veneers). Those researchers used fracture mechanics to assess joints in the larger laminate structure of “GlueLam” timber beams - some modification of the technique was required to test 3 mm veneers in hardwood LVL. Cleavage was thought to best represent the stresses induced by dimensional change in the timber. A scanning process was evolved to quantify the proportion of wood failure on the separated glue-line faces.

Blackbutt LVL specimens were made up at UNSW on two scales: beam-sized elements (12 ply, 36 x 120mm x 3.6m) for testing mechanical properties and two-ply strips, 6 x 20 x 300mm for fracture testing of glue-lines. The strip specimens were tested either soon after manufacture or after varying periods of exposure to one of three accelerated-aging environments. After testing to determine the considerable mechanical properties of Blackbutt LVL, undamaged parts of the larger specimens were placed in an inter-tidal zone in Sydney Harbour. Some specimens were removed after fourteen months of this cyclic salt-water immersion for glue-bond evaluation using the fracture toughness method.

The results suggest that the most severe of the accelerated aging environments and the intertidal marine exposure have produced a discernable weakening of the LVL glue-lines however because the mean fracture toughness values remain relatively high (approximately 400 J/m2) and the wood-fibre percentages are similar to those of unexposed specimens, the glue-lines can be said to have maintained their integrity.

3. METHOD

3.1. Extractive assessment:

Yazaki [1994] found a correlation between colour redness and the quantity of hydrolisable tannins known as “extractives” on the veneer surface. Both 250mm square surfaces of 148 veneer sheets were analysed.

TT2-55, Durability of laminated Veneer lumber made from black butt (Eucalyptus Pilularis) J. Carrick, K. Mathieu 10DBMC International Conference On Durability of Building Materials and Components LYON [France] 17-20 April 2005

A digital image of each surface was analysed using “Adobe Photo Shop”. A mean redness number was obtained (approximately 135 for a low (L) extractive veneer , 140 for high (H) extractive).

3.2 Strip specimen manufacture for glue-line analysis:

Pairs of 250 mm square x 3.2 mm thick veneers were glued and pressed (tight side to loose side) using the Poly-Phenol Tannin (PFT) adhesive, "Bondtite 245" to form two-ply LVL. Adhesive was spread on both faces of adjacent veneers using a rubber roller spreader. The following gluing parameters were used:

Veneer moisture content : 10% Glue mix (Bontite 245) : 100 parts resin, 36 parts hardener, 77.3 parts water,16 parts cure retardant. Spread rate : 300 g/m2 (per glue line) Glue-line moisture : 22% Open assembly time : 25 minutes Closed assembly time: 10 minutes Pre-press settings : 35 minutes @ 1200 kPa Hot press settings : 35 minutes @ 1350 kPa and 138°C

High extractive veneers were glued only to high extractive veneers (H) - low extractive veneers (L) were similarly paired. Parallel-grain strip specimens, 20mm wide were cut from each sheet to allow for at least five replicate glue-line tests under each condition.

Veneers used to make the larger 12-ply structural specimens were assessed visually and laid up alternatively with high and low extractive. After exposure to the inter tidal zone, a band saw was used to cut two-ply strip specimens from the larger samples.

3.3 Fracture toughness test procedure

To assess the Mode-1 (cleavage) fracture toughness of its glue-line, each strip specimen was progressively pulled apart in a contoured double cantilever beam apparatus (CDCB) after ASTM D3433-93 (Fig.1). Before testing, the strip specimens were conditioned for two days to P 25°C and 60% relative humidity. The top and bottom surfaces of a veneer-pair strip specimen were each glued to an aluminium cantilever whose dimensions were contrived to maintain a constant separating

P effort in the fracturing glue-line as the cantilever pair was forced open. A saw-cut stiffness calibration after the procedure of [River and Figure 1. Contoured Double Okkonen, 1993] of the aluminium cantilever arms showed the Cantilever Beam Test correspondence relation, dC/da to be 0.00001916 (N-1).

Because the relative strength of the Blackbutt LVL glue-line caused problems in attaining adhesion between LVL specimen and aluminium beam, the receiving edges of the aluminium CDCB were recess-milled to a depth of 2 mm (Fig.2) thus providing a lip for increased surface area and a shear component for the aluminium-wood bond - failure was directed to the timber glue-line. Strip specimens were bonded to the aluminium using either a strong epoxy resin or a ductile cyano-acrylate Figure 2. Recess-milled Edge adhesive For lab-made strip specimens, failure within the glue-line was initiated by the inclusion of a tab of Teflon tape at the start of the glue- line. For strips obtained from larger specimens, a hacksaw cut initiated failure in the glue-line.

The ends of the cantilever were opened at a rate of 0.5 mm/minute (Photo 1) causing the glue-line failure to progress smoothly along the specimen - propagation and arrest points were not evident. The force required to open the cantilever pair was continuously sampled at a rate of 11hz. allowing a load-deflection curve to be plotted. The relationship between fracture toughness of the glue-line and load on the separating double cantilever is :

GI = P2/2t * (dC/da) where : GI = Mode I fracture toughness (J/m2) P = Fracture load (N) t = Width of strip specimen (m) dC/da = 0.00001916 (N-1)

After testing, separated faces of the glue-line were scanned and assessed using "Adobe Photo Shop" to quantify the proportion of wood-fibre failure. Fibres could be discerned on the faces of dry specimens but discolouration of some weathered specimens caused problems.

3.4 Fracture test data handling As the cantilever pairs were separated, load increased to a peak value at initial fracture and dropped to a relatively constant value as the crack propagated along the glue-line (Fig 3). Towards the end of the test, a hinging action affects the load readings [Mostovoy et al 1971]. In this work, the area of interest is the stable crack propagation region of the load/deflection curve, after the initial peak and before the subsequent drop off. The staccato, propagation and arrest crack pattern seen in other Mode-1 fracture records was not observed in the Blackbutt LVL tests.

Figure 3. Crack tip and load deflection curve To determine toughness from fracture load data, all load readings in a selected range of the anomaly-

Load vs Delta (S-H-E3-P5-1) free zone were used (Fig.3). To enable statistical comparison, the selected range contained the same 2000 number of points for all tests examined in the series 1600 - it was selected to include a balance of higher and 1200 lower values reflecting the statistical qualities of the 800

Load (N) whole anomaly-free zone. Its size (793 points) was 400 determined by the full length of the anomaly-free 0 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 zone seen in the shortest test to be compared Delta (mm) (Fig.4). Using the expression of 3.3, fracture Figure 4. Shortest range load-deflection curve toughness, GI was calculated for force readings over this range and presented as a box plot. The box plot sets out the median, spread and quartiles of the fracture toughness values in the selected range. Outliers are represented as small circles above or below the spread lines (eg. Fig.5).

3.5 Weathering environments

In addition to being tested at ambient conditions (dry, D), Blackbutt LVL strips were held in one of three environments [Milner et al, 1996] for one of four periods: 5, 10, 20, or 40 days before reconditioning for three days at 25°C and 60%RH, and testing. The environments were:

• Environment 1. (E1) Constant low temperature and high humidity (30°C, 90% RH) • Environment 2. (E2) Constant high temperature and high humidity (100°C, 100% RH) • Environment 3. (E3) High temperature constant, varying humidity (100°C, 20 - 100% RH) The exposure conditions were achieved using controlled temperature and humidity chambers. For Environment 2, the specimens were left above boiling water in an autoclave. With increasing exposure to moist environments, the two-ply Blackbutt strips tended to change colour and warp out of plane however they were straightened during gluing to the aluminium cantilever arms.

Generally, five replicate strips of each variable were tested. The variables investigated in the Blackbutt LVL fracture toughness tests were : • Extractive level as determined by redness scanning "High" or "Low" (H or L) • Type of environment endured, "Dry" (D) or one of three environments (E1, E2 or E3) • Period of exposure, (P1 – P4): 5 days, 10 days, 20 days and 40 days.

4. RESULTS AND OBSERVATIONS

Fracture Toughness of High & Low Extractive Veneer 1500 4.1. Unweathered (Dry) Specimens

The central tendency and spread of fracture toughness of 1200 each replicate of dry strip specimens are shown in the box plots of Figure 5. High and low extractive specimens have 900 been grouped. Trends evident from the dry plots are: • Fracture toughness values between 700 and 1500 j/m2 are 2 600 high for glue-lines in timber (cf. USA results of 80j/m obtained for Urea Formaldehyde on Yellow Birch [Scott 300 et al, 1992]) . Fracture Toughness (J/sqm) Toughness Fracture • Low extractive veneers showed tougher glue-line with 0 slightly less variability. (Although variation within the High Extractive Low Extractive high and low categories make these assessments problematic, the trend is expected and very clear for dry Figure 5. Box plots of fracture toughness for specimens) individual dry strip glue-lines

Fracture Toughness of Every Individual Test 4.2 Weathering Environment 1:

of High Extractive Veneer in Environment 1 1800 The first of the durability environments (constant 30°C and 95%RH) was relatively benign. Box plots 1500 for individual toughness tests of high extractive specimens after each exposure period are shown in 1200 Fig. 6. Fig. 7 shows the same results where the

900 results for all five high extractive strip replicates have been combined for each time of exposure to E1 - Fig 600 8. shows similar record for low extractives. While

Fracture Toughness (J/sqm) low extractive veneer bonds show less variability, the 300 plots do not indicate a clear trend towards decreased

0 bond fracture toughness with increasing exposure to Day 0 Day 5 Day 10 Day 20 Day 40 Environment 1.

Figure 6. Box plots of fracture toughness for high extractive individual strip glue-lines exposed to Environment 1

Combined Fracture Toughness of Combined Fracture Toughness of

High Extractive Veneer in Environment 1 Low Extractive Veneer in Environment 1 1800 1800

1500 1500

1200 1200

900 900

600 600 Fracture Toughness (J/sqm) Fracture Toughness (J/sqm) Toughness Fracture 300 300

0 0 Day 0 Day 5 Day 10 Day 20 Day 40 Day 0 Day 5 Day 10 Day 20 Day 40

Figure 7. Replicate-combined box plots of fracture toughness Figure 8. Replicate-combined box plots of fracture for High extractive strips exposed to E1 toughness for Low extractive strips exposed to E1

4.3 Weathering Environment 2

Environment, E2 (constant 100°C and 100% RH in an autoclave) represented very hostile conditions for glue-lines in timber. All specimens were distorted and discoloured on removal. Figures 9 and 10 show plots for replicate-combined fracture toughness tests of high and low extractive strips respectively. The toughness of glue lines in both high and low extractive specimens trended down with increased exposure to this environment. After exposure to E2, a greater variability can be discerned in the plots of fourth- spreads for high-extractive specimens.

Combined Fracture Toughness of High Extractive Veneer Combined Fracture Toughness of Low Extractive Veneer

in Environment 2, with 14 Month Marine Exposure in Environment 2, with 14 Month Marine Exposure 1800 1800

1500 1500

1200 1200

900 900

600 600 Fracture Toughness (J/sqm) Fracture Frcature Toughness (J/sqm) Frcature 300 300

0 0 Da Da D Da Da Tidal Day 0 Day 5 D Day 20 Day 40 T ay a i y y y y y da 0 5 1 2 40 1 l 0 0 0

Figure 9. Replicate-combined box plots of toughness for Figure 10. Replicate-combined box plots of toughness for High-extractive strips exposed to E2 or tidal sea-water Low-extractive strips exposed to E2 or tidal sea-water

4.4 Weathering Environment 3

Environment 3 (temperature constant, 100°C and humidity cycled between 30% & 90% RH every five days) was explored to investigate the effect of shrink/swell dimension changes. Figures 11 and 12 show plots for individual and replicate combined tests separated for high and low extractive. A slight trend down in the toughness of glue-lines can be discerned in most specimens with increasing exposure to this environment. No clear trend to greater variability is shown between high and low extractive specimens - both are more variable than dry (Day 0) or Environment 1.

Combined Fracture Toughness of High Extractive Combined Fracture Toughness of Low Extractive

in Environment 3, with 14 Month Marine Exposure in Environment 3, with 14 Month Marine Exposure 1500 1500

1200 1200

900 900

600 600

Fracture Toughness (J/sqm) Toughness Fracture 300 (J/sqm) Toughness Fracture 300

0 0 Da Da Day 10 Da Da Tidal D Da Da Da D T a a id y y y y y 0 y y 10 y y 40 a 0 5 2 4 5 20 l 0 0 Figure 11. Replicate-combined box plots of toughness for Figure 12. Replicate-combined box plots of toughness for High-extractive strips exposed to E3 or tidal sea-water Low-extractive strips exposed to E3 or tidal sea-water

4.5 Cyclic tidal immersion

The box plots of glue-line fracture toughness of two-ply strips taken from immersed Blackbutt LVL specimens are appended to the plots of Environments 2 and 3 shown in Figures 9 to 12 above. Because extractive levels were not identified in the veneers used to make the immersed specimens, the results are appended to both replicate combined sets. While the plots suggest that compared to unweathered specimens, immersion has diminished glue-line toughness, the value of mean toughness of approximately 750 J/m2 and the high level of wood fibre failure observed indicate a strong glue bond has been maintained after 14 months in the adverse climate of Sydney Harbour's inter-tidal zone.

Photo 1 Strip test Photo 2 Blackbutt LVL after 14 months of Photo 3 Radiata Pine LVL after 14 months of apparatus and tested glue- inter- tidal immersion inter- tidal immersion line faces

5. DISCUSSION AND CONCLUSIONS

The fracture toughness results did not show a dramatic breakdown of any of the LVL glue-lines tested. Although after considerable exposure to very hostile environments, some downward trends were evident in glue-line toughness, those values remained relatively high and unlike pine, the natural durability of the timber prevented its breakdown (Photos 2 and 3). The lowest fracture toughness observed on the most degraded specimen was approximately 300 J/m2, greater than the unweathered fracture toughness in yellow birch laminates reported by US researchers - these results suggests that a durable structural LVL can be made from Blackbutt.

Milner [1996] proposed a cyclic environment such as E3 to investigate the effect of shrink/swell dimension changes. The slenderness of 6mm thick two-ply strip specimens did not restrain one side of the glue-line sufficiently to build up shear and opening stresses to the extent that a trend could be established. The intertidal zone specimens, at 36mm thick provide more restraint and could have achieved more shrink/swell glue-line stresses with varying moisture content but the degree of drying under the wharf may not have been great over the low tide. Subsequent work needs to log moisture content each side of the relatively impermeable barrier set up by the glue line.

The relatively good durability shown by the UNSW Blackbutt LVL compared to plywood which we understand delaminated in service could be attributed to two factors: • Parallel-grain LVL veneers may settle together more intimately under pressure than do the orthogonal grains of plywood. • While the gluing parameters were similar, the lab specimens spread the PFT adhesive on both sides of the line and controlled the process very closely whereas at the plywood mill, one side of the line only was thickly coated. The glue was intended to be transferred to the other side in the stack.

ACKNOWLEDGEMENTS

The Blackbutt LVL project was carried out as a joint venture between the Building Research Centre (now the Australian Centre for Construction Innovation ACCI in the school of Civil and Environmental Engineering) at the University of New South Wales and State Forests New South Wales. Project funds were supplied by the Forest and Wood Products Research and Development Corporation. Materials were supplied by Big River Timbers Pty Ltd and Bondtite Pty Limited.

REFERENCES

Milner, H R and Ward, C: The Evaluation of Structural Adhesives, 26th Biennial Forest Products Conference, Clayton, 2/3 (1996) Mostovoy, S, Ripling, E J and Corten, H T: Fracture Mechanics: A Tool for Evaluating Structural Adhesives, Journal of Adhesion, 3, 107 (1971) River, B H and Okkonen, E A: Contoured Wood Double Cantilever Beam Specimen for Adhesive Joint Fracture Tests, ASTM Journal of Testing and Evaluation, 21, 21 (1993) Scott, C T, River, B H and Koutsky, J A: Fracture Testing Wood Adhesives with Composite Cantilever Beams, ASTM Journal of Testing and Evaluation, 20, 259 (1992) Yazaki, Y, Collins, P J, and McCombe, B “Variations in Hot Water Extractive Content and Density of Commercial Wood Veneers from Blackbutt” Holzforsprung V.48 1994 W de Gruyter, Berlin, 1994 ASTM D3433-93 "Standard Test Method for Fracture Strength in Cleavage of Adhesives in Bonded Metal Joints", American Society of Testing Materials

TT2-55, Durability of laminated Veneer lumber made from black butt (Eucalyptus Pilularis) J. Carrick, K. Mathieu