Developing structurally reliable cross-laminated timber panel made from fibre-managed plantation Eucalyptus nitens

With the increasing availability of fibre-managed plantation hardwood resources in Australia, the timber product manufacturing sector has become increasingly interested in determining its potential as a feedstock for a range of mass timber products. The predominant species in Australia’s hardwood plantations are Eucalyptus globulus (E. globulus) and Eucalyptus nitens (E.nitens), and these are mainly managed in unthinned and unpruned stands to produce a high volume fibre resource for pulp and woodchips. Developing value-added engineered timber products from this resource could provide a sustainable alternative solution for the timber production industry and the building design and construction sectors. Cross-laminated timber (CLT) production has gained interest within the Australian timber industry and plantation hardwoods can provide a potential resource and sustainable supply to make CLT for commercial buildings. Till now, CLT panels are generally manufactured from softwood species, and current guidelines, standards, and test methods support softwood timber only. However, over the last three years, Cusp Building Solutions (Cusp), a Tasmanian base company, has developed and manufactured engineered timber products from plantation E.nitens. The research presented in this thesis, started at about the same time and occurred in parallel with Cusp's development of its CLT product, but took the opportunity to further explore how the potential performance could be enhanced through a greater optimisation process. The opportunities presented included heterogeneous panel configurations and assemble the timber in the panel layout using most of the boards recovered from the resource. Hence, the research in this doctoral thesis aimed to develop and optimise reliable structural CLT, specifically for use in floor panel from fast-growing, fibre-managed plantation eucalypt timber. Developing and optimising a structurally reliable CLT panel from fibre-managed plantation hardwood boards requires: confirming the feedstocks’ physical and mechanical properties, determining the structural performance of CLT panels assembled from graded batches of the feedstock, and investigating the rolling shear properties of the CLT panels. To determine whether CLT panel made from this material can satisfy the performance requirement for structural applications in buildings, an experimental study was performed in order to generate the necessary research data. The experiments in this thesis were performed in three phases:
• Characterisation of feedstock material and evaluate the assessment methods available to grade the feedstock into structural groups relevant for CLT production.
• Developing, manufacturing, and evaluating heterogeneous CLT panels assembled from the graded feedstock.
• Evaluation of the rolling shear properties of the CLT panels
The first assessment phase aimed to evaluate the methods suitable for the structural grading of the material. It aimed to determine the modulus of elasticity (MoE) of plantation eucalypt sawn timber, as this can assist in sorting the material into relevant structural groups for developing CLT panel laminations.
A range of destructive and non-destructive techniques (NDT) were performed on sawn E. nitens timber boards in order to understand and characterise the material’s physical and mechanical properties relevant to the performance of CLT panels. These included the NDT, machine stress grading (MSG), and mechanical four-point bending test. Assessment of the results showed that the dynamic MOE (MoE_dyn) obtained from NDT mechanical test and MSG were in agreement with the results of the mechanical four-point bending test. This supported the idea that these techniques can be used to effectively predict the mechanical properties of E. nitens timber and sort it into batches useful for CLT laminations. The results from all methods were in agreement with those from the static four-point bending test. A profile of boards' MoE population was determined to establish nominal boundaries for categorising the boards to be used in CLT panel lamination.
The second phase had two aims. The first aim was to determine the bending properties and structural capacity of heterogenous CLT panels laminated and manufactured from the graded boards. Unlike the generally homogenous CLT produced internationally, this research used three different board grades obtained from Australian-grown hardwood species E. nitens to make heterogeneous CLT configurations within various combinations of board stiffness (MoE; from 7 GPa to 21 GPa) in the inner and outer layers. This was to make full use of the range of timber grades produced from the material, in addition to maximising material utilisation, improving efficiency from timber processing and optimising cost- to performance ratio. The panels were then subjected to and tested under the four?point bending test. In this phase, the bending properties of all panel configurations were evaluated and compared.
The phase’s second aim was to assess the reliability of analytical methods such as gamma and shear analogy methods to estimate the bending properties of fibre-managed E. nitens CLT panels. In addition to the static modulus of elasticity (MoE_s) values used in those analytical methods, the MoE_dyn of the boards obtained from the NDT method through AWV were applied. Applying the MoEdyn in the theoretical methods could be an appropriate method in terms of less time and energy consumption of testing for estimating the mechanical performance of CLT elements without mechanical test. The results demonstrated that CLT made from fibre-managed plantation E. nitens has satisfactory bending and structural performance to meet serviceability requirements for reliable and structural CLT panels.
In addition, analytical methods using both MoE_s and MoE_dyn were in good agreement with the results of the mechanical test and confirmed the appropriate method for assessing the E. nitens CLT panels. The results prove that the design of the panels is driven by stiffness and that these CLT elements, as a structural component of the floor system showed satisfactory results for both domestic and commercial applications.
The aim of the third phase was to evaluate the rolling shear properties of the E. nitens CLT. Shear of the transverse layers, known as rolling shear, is one of the governing factors in serviceability and limits state design for CLT under out-of-plane loading. The experimental investigation of the rolling shear properties of the CLT panels was performed through two methods: short-span three-point bending and planar shear test. This phase of the study provides an overview of the rolling shear properties of hardwood species and contributes to a better understanding of the mechanical behaviour of CLT subjected to out-of-plane loading. The results demonstrated that CLT made from fibre-managed plantation E. nitens has satisfactory shear performance to meet serviceability requirements for reliable and structural CLT panels. The results were also comparable to those recommended in European standards for softwood CLT, demonstrating the potential use for eucalypt timber boards in CLT. The results were also in agreement with those under short-span bending tests in the literature for other species. The results of this phase provide an important insight into using Australian-made E. nitens CLT panels, demonstrating that they have a great potential for use in a wide range of construction applications.
Overall, the results of this research found that CLT panels manufactured from a fibre-managed plantation E. nitens feedstock could satisfy the performance requirements for structural applications in building and that heterogeneous CLT panels could efficiently use the recovered resources while providing robust solutions for specific building applications. The results demonstrate that this material could replace imports and create an opportunity locally grown and processed plantation hardwood timber in the Australian building sectors.