A comparative study on the mechanical properties of laminated veneer lumber (LVL) produced from blending various wood veneers

Abstract

Rotary veneers from spotted gum (Corymbia citriodora) and white cypress pine logs (Callitris glaucophylla) recovered from the native forest in Queensland, as well as Queensland plantation hoop pine (Araucaria cunninghamii) logs were used to manufacture LVL products following six different lay-up strategies including blended species LVL. The different lay-up strategies were to determine the opportunities for improving the mechanical performance of plantation softwood LVL by including native forest veneers. The manufactured products were evaluated for their bending performance, tension, bearing strength perpendicular to the grain, and longitudinal-tangential shear strength. The all-spotted gum LVL showed superior performance in all testing compared to other construction strategies. Blending even a small amount of spotted gum veneer with plantation hoop pine veneer resulted in improved mechanical performance, especially in flatwise bending. Opportunities exist to develop more optimised construction strategies that target specific product performances while optimising the use of the variable veneer qualities generated from log processing.

Full Article

A Comparative Study on the Mechanical Properties of Laminated Veneer Lumber (LVL) Produced from Blending Various Wood Veneers

Robert L. McGavin,^a,* Hoan H. Nguyen,^b Benoit P. Gilbert,^b Tony Dakin,^a and Adam Faircloth ^a

Rotary veneers from spotted gum (Corymbia citriodora) and white cypress pine logs (Callitris glaucophylla) recovered from the native forest in Queensland, as well as Queensland plantation hoop pine (Araucaria cunninghamii) logs were used to manufacture LVL products following six different lay-up strategies including blended species LVL. The different lay-up strategies were to determine the opportunities for improving the mechanical performance of plantation softwood LVL by including native forest veneers. The manufactured products were evaluated for their bending performance, tension, bearing strength perpendicular to the grain, and longitudinal-tangential shear strength. The all-spotted gum LVL showed superior performance in all testing compared to other construction strategies. Blending even a small amount of spotted gum veneer with plantation hoop pine veneer resulted in improved mechanical performance, especially in flatwise bending. Opportunities exist to develop more optimised construction strategies that target specific product performances while optimising the use of the variable veneer qualities generated from log processing.

Keywords: Laminated veneer lumber; Hardwood; Cypress; Veneer; Rotary peeling

Contact information: a: Queensland Department of Agriculture and Fisheries, Horticulture and Forestry Science, Salisbury Research Facility, Australia; b: School of Engineering and Built Environment, Griffith University, Australia; *Corresponding author: Robbie.mcgavin@daf.qld.gov.au

INTRODUCTION

Australia’s native forest resources cover approximately 137 million hectares and constitute approximately 90% of Australian forests (ABARES 2018). For many years, mainly larger diameter logs from these forests have been transformed using traditional sawmilling technologies, into a range of end-products such as beams, bridge members, flooring, decking, and landscaping timbers. However, despite there being a significant volume of small diameter logs potentially available to the timber industry from the sustainable management of these forests, these logs yield poor recovery rates when processed by traditional sawmilling technology. This has resulted in much of this resource being under-utilised and under-valued, despite the wood properties being well-suited to a range of high-value products (McGavin and Leggate 2019). Recently, spindleless veneering technologies have been capable of efficiently processing small-diameter plantation and native forest logs and hence offering the potential of utilising these resources for veneer-based products, such as laminated veneer lumber (LVL) (McGavin 2016; McGavin and Leggate 2019).

An appropriate commercialisation pathway for rotary veneer produced from small-diameter native logs may be through blending with existing commercial plantation softwoods. Blending may support efficient use of the veneer qualities while enabling high-value and high-performance LVL to be manufactured for structural applications. Blending resources in product manufacturing has been identified in previous research as advantageous, as these products have advantages when compared to traditional sawn products, including increased product performances, efficient resource utilisation, and compatibility with modern building systems (Keskin 2004; Kilic and Celebi 2006; Burdurlu et al. 2007; Kilic et al. 2012; Xue and Hu 2012). More importantly, these products allow the increased use of lower cost, low-grade, and low-density wood veneers as core veneers in mixed-species LVL products to reduce product cost (Keskin and Musa 2005; Burdurlu et al. 2007; Xue and Hu 2012; Wang and Dai 2013) and increase the mechanical properties of predominately low-density wood LVL (Wong et al. 1996; H’ng et al. 2010; Xue and Hu 2012; Bal 2016; Ilce 2018).

According to Wong et al. (1996), it is possible to increase the use of low-grade wood veneers from fast-growing trees such as rubberwood (Hevea brasiliensis) into high-performance products by processing them into mixed-species structural LVL with higher quality mangium (Acacia mangium) veneers. The study showed that the mechanical properties of rubberwood LVL can increase up to 13% in the modulus of elasticity (MOE) and 12% in the modulus of rupture (MOR) by positioning mangium veneers in the surface or face layers. Another study on manufacturing 7-ply LVL in 8 different lay-up strategies that blended higher-density Austrian pine (Pinus nigra) veneers and lower-density Lombardy poplar (Populus nigra) veneers was conducted by Kilic et al. (2010). Results showed that as the ratio of Austrian pine veneers increase in mixed-species LVLs, the MOR and MOE increased up to 40% and 69% on average, compared to LVL manufactured only with Lombardy poplar.

Burdurlu et al. (2007) investigated the MOE and MOR of LVL manufactured from beech (Fagus orientalis L.) and Lombardy poplar (Populus nigra L.) veneers through eight different lay-up strategies and reported that increasing the proportion of high-density beech veneers leads to an increase in the MOE and MOR and that the flatwise MOE and MOR of LVL with two beech veneers on each outer layer was 49% and 27% higher on average compared to the LVL manufactured from poplar alone. The results were consistent with the study conducted by Xue and Hu (2012) which considered ten-ply LVL manufactured from poplar (Populus ussuriensis Kom.) as the core layers, and birch (Betula platyphylla Suk.) as the outer layers. The authors also reported that the bending strength of LVL with high strength birch veneers on the outer layers is much greater than LVL with low strength poplar veneers on the surface layers.

Although manufacturing LVL from blending different wood species has been advanced in other countries, the opportunities for adopting this approach in Australia are not well understood. The key objectives of this study were to examine the structural performance of LVL products manufactured from rotary veneers recovered from small-diameter selected Australian native forest species, an Australian commercial plantation grown softwood, and various blends of veneers from these species. The study aims to provide an insight into the opportunities to improve the performance of plantation pine LVL through the inclusion of native forest sourced veneers.

EXPERIMENTAL

Materials

Rotary veneers

Spotted gum (Corymbia citriodora), white cypress pine (Callitris glaucophylla), and hoop pine (Araucaria cunninghamii) were selected for this study. The spotted gum (SPG) and white cypress pine (CYP) veneers were sourced from the small diameter log processing trials previously undertaken and reported by McGavin and Leggate (2019), and represent two different resources commercially available to the timber industry from Australia’s native forests. These species represent a high-density, durable hardwood (SPG) and a mid-density, durable softwood (CYP). The processing was completed using a spindleless rotary veneer lathe that targeted a nominal dried veneer thickness of 3.0 mm. The hoop pine (HP) veneers were recovered from approximately eight logs peeled by a commercial veneer producer during standard commercial operations and also targeted a nominal dried veneer thickness of 3.0 mm.

Methods

Veneer properties

To evaluate the distribution of dynamic properties such as the elastic modulus parallel to the grain direction (E_{L_Veneer}), the acoustic properties of the SPG, CYP, and HP veneers were measured using a non-destructive grading device (Brancheriau and Baillères 2002) on sample strips (approximately 1200 mm × 200 mm) removed from a subset of recovered veneers, as reported by McGavin and Leggate (2019). Sample strips were positioned on elastic supports and a simple percussion was then induced in the direction of the grain at one end of the sample, while at the other end, a Lavalier type microphone recorded the vibrations before transmitting the signal via an anti-aliasing filter (low-pass) to an acquisition card that included an analog-to-digital converter to provide a digitized signal (Fig. 1). A fast Fourier transform algorithm processed the signal to convert the information from the time to the frequency domain. The mathematical processing of selected frequencies was undertaken using BING (beam identification using non-destructive grading) software (Version 9.7.2, Montpellier, France) in combination with the geometrical characteristics and the weight of the specimen to provide the dynamic MOE, among other specific mechanical characteristics (CIRAD 2018).

Fig. 1. Experimental setup for the acoustic properties testing

Veneer grading

The veneer quality was assessed by visual grading in accordance with AS/NZS 2269.0:2012 (2012). This standard is widely used across the Australian veneer industry and follows the same principles as other international veneer visual grading classification systems. The standard separates structural veneers into four veneer surface grades with each grade corresponding to a quality group in accordance with the standard. The grading was based on visual characteristics of the veneers such as splits, various knot types, and roughness.

Target LVL construction strategies

Six different LVL construction lay-up strategies were implemented to manufacture 12-ply LVL from the three species to demonstrate the construction strategy impact on the manufactured product mechanical properties. The strategies were comprised of three single-species reference LVLs and three blended-species LVLs (Fig. 2).

Fig. 2. LVL panel construction lay-up types

The construction strategies are outlined below, with the veneer selection process explained in the following section:

• LVL1 – 12 CYP veneers throughout the panel thickness;
• LVL2 – 12 HP veneers throughout the panel thickness;
• LVL3 – 12 SPG veneers throughout the panel thickness;
• LVL4 – SPG veneers on the outside faces and 10 HP veneers for the internal core;
• LVL5 – alternating SPG and HP veneers with SPG veneers on the outside faces; and
• LVL6 – alternating CYP and HP veneers with CYP veneers on the outside faces.

Veneer selection and allocation

The strategy to select individual veneers from the available stocks and their placement within the LVL panels had the following main objectives:

• To minimise the within-species veneer MOE variation for veneers included in the LVL panel manufacturing;
• To target “average” structural quality veneers (i.e. veneers with MOEs that were similar to the mean veneer MOE of the available stocks of each species);
• To ensure individual veneers were in the optimum position within the allocated panel to maximise the panel mechanical properties (i.e. biasing higher MOE veneers towards the outer layers of the LVL panels);
• To minimise the within-species variation between LVL panels of the same construction type.

The veneer selection and placement followed these steps:

1. From the available veneers of the three species, veneers that did not achieve a visual grade of D-grade or better were discarded. While the veneer visual grading wasn’t used as the primary selection method to influence the LVL mechanical performance, the criteria of D-grade or better was adopted to ensure a commercially relevant quality criteria, in terms of surface roughness, representation of visual defects and splitting, etc.
2. For the remaining veneer sheets, the mean dynamic MOE of the veneer population was calculated and used to guide the veneer selection. Given the study objective of assessing the mechanical performance of the manufactured LVL, the veneer MOE was used as the primary veneer selection method. This approach is common practice in commercial LVL manufacture using equipment such as Metriguard produced by the Raute Group.
3. Veneers within each population were sorted by their MOE in descending order.
4. The required subset of each species (the number of veneers required from each species to manufacture the required LVL panels including contingency veneers) were taken as a series of consecutive veneers to minimise MOE variation. Then, the mean dynamic MOE (as per Step 1) was calculated for each possible subset.
5. The veneer subset that had a mean MOE closest to the entire population MOE mean were selected for panel manufacturing.
6. The veneers from each subset were systematically distributed among the final panels of each construction type. Veneers were distributed, in order of decreasing MOE, starting with the outer layers of all panels and progressing to the core. This ensured that veneers were optimally located from a structural perspective with higher MOE veneers located towards the panel periphery, and that consistency was achieved across the panels of the same construction type. Once all the veneers were assigned, the statistics for the desired combinations of panels and positions were reviewed to ensure the objectives were achieved.

LVL panel manufacturing

A total of 18 LVL panels (approximately 1200 mm × 1200 mm × 36 mm) were manufactured with three panels for each construction type. A melamine urea formaldehyde adhesive was selected to achieve a B-bond glue line, which are the service conditions outlined in AS/NZS 2754.1 (2016).

The adhesive was applied to each face of the veneers targeting a total spread rate of 400 gsm (grams per square metre) per glue line. The assembly stage included an open assembly time of approximately 22 min (measured from adhesive application to the first veneer to when pressure was applied in the press). Pre-pressing was undertaken at 1 MPa for a duration of 8 min. Once the pre-pressing was complete, the panels were transferred to the hot press and pressed at 1.1 MPa for 26 min at 135 °C.

Test samples and mechanical properties test method

Figure 3 illustrates the LVL panel cutting pattern and test sample locations. Six samples per construction lay-up type (i.e. 2 samples per panel) were cut from each panel to experimentally evaluate their static edgewise bending MOE (E_{b_e}), static flatwise bending MOE (E_{b_f}), edgewise bending MOR (f_{b_e}), flatwise bending MOR (f_{b_f}), longitudinal-tangential shear strengths (f_s), and bearing strength perpendicular to the grain strength (f_{c_⊥}). For tension perpendicular to the grain (f_{t_⊥}), nine samples per construction lay-up type (i.e. 3 samples per panel) were tested.