Life cycle performance of Cross Laminated Timber mid-rise residential buildings in Australia
Introduction
The building and construction industries consume about 40% of global energy and generate 40-50% of global greenhouse gas (GHG) emissions [1], [2], [3], [4], [5]. This highlights the negative impacts of the building industry on the environment [3], [4], [5]. Thus, there is an urgent need to expedite the development of more sustainable buildings, which necessitates the use of high-performance materials and structural systems with low carbon footprints. As a result, timber has emerged as a key material in mid-to-high rise buildings due to its lightweight, fire resilience and high stiffness-to-weight ratio, to name a few [6], [7], [8]. Consequently, many countries have changed their design guidelines and regulations for timber buildings. In Australia, the national construction code (NCC) [9] amended the maximum height restriction of timber buildings to 25 m without requiring any additional approvals. This led to an increase in mid-to-high rise mass-timber buildings in Australia. Examples include International House in Sydney, 25 King Street in Brisbane and Forte Living in Melbourne.
Timber is a more sustainable material and the use of 17% of timber in construction as an alternative to brick, aluminium, steel and concrete, can reduce GHG emissions by about 20% [10,11]. Traditional construction materials such as concrete, cement and steel account for higher embodied GHG emissions (39-44%) in Australia, whilst timber contributes only 9.3% [12, 13] of life cycle GHG emissions. Further, Ximenes and Grant [14] reported that use of timber in structural floor systems could reduce GHG emissions by 31-56% compared to that of concrete slabs. This indicates that building elements produced from timber such as Australian Radiata pine cross laminated timber (CLT) can be a more sustainable alternative construction material than traditional construction materials (i.e. concrete and steel). CLT, as an engineered timber product, has been used as a major construction material for mass-timber construction in Australia. CLT is fabricated with 3, 5 or 7 layers of timber boards (or laminates). Adjacent layers are oriented in orthogonal directions and glued to one another at high pressures [15]. CLT has significant potential to address the requirements of the construction industry due to its high in-plane and out-plane strength and stiffness, good acoustic and thermal performance, and high degree of prefabrication [16], [17], [18], [19]. Most mass-timber buildings in Australia are composed of CLT products, which are mainly imported from overseas manufacturers [15]. Recently, an Australian grown and fabricated CLT product from Radiata pine has emerged in mass-timber building construction in Australia. However, limited research is available on the structural, acoustic and thermal performance, long term viability assessed through life-cycle cost, and life cycle energy and GHG emissions of Australian Radiata pine CLT. Consequently, these factors have hindered the adoption of CLT in the construction industry [20, 21]. This highlights the importance of conducting life cycle greenhouse gas emissions (LCGHGE) and life cycle cost (LCC) assessments of CLT buildings to guide the selection of environment-friendly construction materials and optimise the construction process [22, 23].
Life-cycle energy assessment (LCEA) evaluates the embodied energy (i.e. energy content of raw materials and energy acquired in processing, manufacturing, and transportation to the factory and construction site), operational energy (i.e. the energy needed for the building during the service life) and end-of-life energy (i.e. energy for demolition and material transport to land fill or recycling plants) of a building [2, 5, [22], [23], [24], [25], [26], [27]]. The embodied and operational energy of CLT building construction is lower than that of concrete and steel [28], [29], [30]. CLT buildings can save operational energy and reduce carbon emissions by about 24.6%. However, Dong et al., [24] found that CLT buildings consume more energy in summer compared to reinforced concrete (RC) buildings. Similar findings on the lower embodied energy of CLT buildings were reported by Moncaster et al. [19]. It should be noted that accurate selections of material coefficients and system boundaries are crucial in maintaining transparency in life cycle performance analyses [31]. Life-cycle cost analysis (LCCA) is an approach to determine the total cost of a building, which accounts for the cost of materials, construction, transportation, operation, maintenance and demolition [23, 26]. The cost of Australian Radiata pine CLT product is about the same as that of European spruce pine CLT products [32]. It should be noted that the cell structure of Radiata pine allows the feedstock to be treated for durability. High initial, operational and maintenance costs of CLT has hindered its adoption in buildings [32], [33], [34]. Studies by Sterner [35], David et al., [32], Teshnizi et al., [36] and Jones et al., [33] found that total cost of both CLT and conventional buildings are identical. However, these cost estimates can overstate or understate the total cost of a building by up to 10% [32]. David et al. [32] also suggested that 3D computer modelling for cost estimation can provide accurate results. Furthermore, the construction industry is facing economic pressures due to the lack of investments in CLT building projects in Australia, as there is insufficient evidence to demonstrate the LCE and LCC benefits of CLT products.
The aim of this paper is to compare the life-cycle greenhouse gas emissions (LCGHGE) and life-cycle cost (LCC) of CLT and traditional reinforced concrete residential buildings for three cities in Australia. Life cycle analyses were conducted within the boundaries of the product and construction, operational and maintenance (O&M), and end of life (EOL) phases. Melbourne, Sydney and Brisbane were selected since they are the most populated cities.
Section snippets
Method
A life cycle assessment (LCA) evaluates the environmental impact of a building over its life time [37]. This can include a set of smaller LCA stages but can be streamlined to exclude some of these stages to reduce complexity. The analysis framework developed in this study follows the principles outlined in AS ISO 14040:2019 [38] and EN 15978 [39]. The analysis consists of three main phases, namely pre-use, use and post-use. The post-use or end of life phase can be considered as the most
Results
This section presents the embodied energy, operational energy, life cycle GHG emissions and life cycle cost of the midrise residential RC and CLT buildings. The results are compared across three main cities in Australia.
Discussion
The LCGHGE and LCC of RC and CLT buildings depend on number of factors: raw material production, service life and operations of buildings, and end of life strategies. The effects of the uncertainly of these factors have been discussed. This section presents a sensitivity analysis conducted to explore the impact of variabilities and uncertainties in the key parameters.
A building's life span can be influenced by the materials being used, maintenance and other social economic factors affecting its
Conclusions
This study quantifies the life cycle GHG emissions and life cycle cost for two type of construction (RC and CLT) for midrise residential buildings, in the three cities (Melbourne, Sydney, and Brisbane). The embodied GHG emissions of the CLT building in the product and construction phase is 50 % lower than that of RC building. This is due to significant reduction of energy intensive material usage in the CLT building. The CLT building has slightly lower operational GHG emissions in Sydney and
CRediT authorship contribution statement
Amitha Jayalath: Software, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Satheeskumar Navaratnam: Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Tuan Ngo: Conceptualization, Methodology, Resources, Writing - review & editing, Supervision, Funding acquisition. Priyan Mendis: Conceptualization, Supervision, Funding acquisition. Nick Hewson: Writing - review & editing, Resources. Lu Aye: Resources, Writing - review
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was funded by the Australian Research Council (ARC)Centre for Advanced Manufacturing of Prefabricated Housing [Grant ID: IC150100023], CRC-P with CSR, Asia Pacific Research Network for Resilient and Affordable Housing (APRAH). The authors also acknowledge Xlam Australia and Infrastructure Engineering research project students for their contribution in this study.
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