The influence of the prefabricated rate on the environmental performance of buildings in a life cycle perspective

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IN

DEGREE PROJECT ENVIRONMENTAL ENGINEERING,

SECOND CYCLE, 30 CREDITS ,

STOCKHOLM SWEDEN 2020

The influence of the prefabricated

rate on the environmental

performance of buildings in a life

cycle perspective

SHAOZHE WANG

KTH ROYAL INSTITUTE OF TECHNOLOGY

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The influence of the prefabricated rate on the environmental performance of buildings in a life cycle perspective

Author: Shaozhe Wang Supervisor: Rajib Sinha Examiner: Monika Olsson

Degree project course: AL227X Industrial Ecology, 30 credits

Department of Sustainable Development, Environmental Science and Engineering School of Architecture and Built Environment

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Abstract

As the construction sector has caused significant environmental impacts, Sweden has made efforts to develop prefabricated buildings by increasing their prefabricated rates. In a life cycle perspective, this thesis examines how the overall environmental impacts during the construction processes are influenced when the prefabricated rate increases. By modelling in SimaPro version 9, this thesis conducts a life cycle assessment (LCA) of a concrete framed reference building with a prefabricated rate of 26% located in Stockholm Royal Seaport. Nine scenarios with increasing prefabricated rates varying from 6% to 96% are compared in the thesis. The results show that when the prefabricated rate increases, the total water consumption of the building is optimized, while the overall energy consumption and GHG emissions have increased. For other environmental impacts, the thesis finds the terrestrial ecotoxicity is the most affected impact category when the prefabricated rate rises. The thesis indicates that water consumption of the building is mainly influenced by material extraction and processing stage, and the transport is the dominating factor to energy consumption, GHG emissions and terrestrial ecotoxicity as the prefabricated rate rises. The sensitivity analysis in the thesis also reveals that the energy consumption and GHG emissions of the prefabricated building are sensitive to transport scheme, such as transport distance and vehicle types.

Keywords

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Sammanfattning

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Acknowledgements

I would like to thank my supervisor Rajib Sinha for his patient guidance and timely feedback during my master thesis. I would also like to thank my examiner Monika Olsson for her advice and support for me to finish the thesis. I appreciate the technical help from my LCA course teacher Anna Björklund and the student group for LCA thesis.

During the special outbreak of covid-19, I am grateful to my loving parents and grandparents for their support from China. I am also grateful to my friends and neighbours who gave me courage to complete my master thesis.

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IV

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4.2.3 Uncertainties ... 33

5 Results and analysis ... 35

5.1 Life cycle assessment of the reference building ... 35

5.1.1 Environmental impacts of different construction work types ... 35

5.1.2 Environmental impacts of different building materials ... 36

5.1.3 Environmental impacts during different lifecycle stages ... 36

5.2 Life cycle assessment of the scenarios ... 38

5.2.1 Water consumption changes in the scenarios ... 39

5.2.2 Energy consumption changes in the scenarios ... 40

5.2.3 GHG emission changes in the scenarios ... 42

5.2.4 Other environmental impact changes in the scenarios ... 43

6 Discussion ... 47

6.1 Interpretation and analysis of the results ... 47

6.2 Sensitivity analysis ... 48

6.2.1 The sensitivity analysis of vehicle type ... 48

6.2.2 The sensitivity analysis of geographical transport distance ... 50

6.3 Uncertainty analysis ... 52

6.3.1 Qualitative uncertainty analysis ... 52

6.3.2 Quantitative uncertainty analysis ... 53

6.4 Applicability of the results ... 56

6.5 Limitation of the study and possible future work ... 57

7 Conclusion ... 59

References ... 61

Appendix A Data collection ... 71

Appendix B Results for LCA of the reference building ... 77

Appendix C Data for LCA results of the reference building ... 89

Appendix D Data for sensitivity analysis ... 95

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VI

Abbreviations

LCA – Life Cycle Assessment

UNEP – United Nations Environmental Programme GHG – Greenhouse gas

CO2 – Carbon Dioxide NOx – Nitrogen Oxides EU – European Union

BIM – Building Information Modelling

HVAC – Heating, cooling, lighting and ventilation RCA – Recycled concrete aggregates

CED – Cumulative energy demand GWh – Gigawatt hours

CO2 eq – Carbon Dioxide equivalent PCM – Phase change material GJ – Gigajoule

ISO – International Standard Organization LCI – Life cycle inventory

Glulam – Glued laminated timber IGU – Insulated glass units

MPA – Mineral Products Association kWh – Kilowatt hours

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VII

List of Figures

Fig. 1 The schematic diagram of the method applied in this thesis ... 14

Fig. 2 Simplified diagram of the life cycle of the reference building in this “cradle to gate” LCA ... 17

Fig. 3 The waste disposal processes for precast concrete and conventional concrete in Sweden ... 19

Fig. 4 The illustration of the “cut-off” principle related to recycling in this LCA. ... 20

Fig. 5 The process flowchart for precast concrete work of the reference building ... 24

Fig. 6 The process flowchart for conventional concrete work of the reference building ... 25

Fig. 7 The process flowchart for other building material work of the reference building ... 27

Fig. 8 The transport during the construction processes for the reference building ... 31

Fig. 9 Contributions of different work types to environmental impacts of the reference building ... 35

Fig. 10 The contributions of different materials to the environmental impacts ... 36

Fig. 11 The contributions of different lifecycle stages to the environmental impacts ... 37

Fig. 12 Changes in environmental impacts of different scenarios compared with the reference building ... 38

Fig. 13 Water consumption caused by each lifecycle stage of different lifecycle scenarios ... 39

Fig. 14 Energy consumption caused by each lifecycle stage of different lifecycle scenarios ... 41

Fig. 15 GHG emissions caused by each lifecycle stage of different lifecycle scenarios ... 42

Fig. 16 Changes of 18 impact categories in Method ReCiPe 2016 Midpoint (H) in the scenarios ... 44

Fig. 17 Terrestrial ecotoxicity caused by each lifecycle stage of different lifecycle scenarios ... 44

Fig. 18 Changing rates of environmental impacts in scenarios when vehicle type is replaced ... 50

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List of Tables

Table 1: The concrete information of the reference building ... 15

Table 2: Reasons for excluding the use phase of the reference building in this LCA ... 17

Table 3: The list of the cut-off criteria for the system model of the reference building ... 21

Table 4: The list of general assumptions for data collection ... 21

Table 5: Other building materials used in the reference building ... 26

Table 6: Theoretical material consumption for concrete work of the reference building ... 28

Table 7: Raw material consumption considering loss rates for the reference building ... 29

Table 8: Qualitative uncertainties during each lifecycle stage in the data collection process ... 34

Table 9: The total environmental impacts per m2_{floor area in Scenario 1 to Scenario 9 ... 38}

Table 10: Process contribution data for water consumption during material production (m3_/m2_{) ... 39}

Table 11: Process contribution data for water consumption during on-site work stage (m3_/m2_{) ... 40}

Table 12: Process contribution data for energy consumption during in-plant processing (MJ/m2_{) ... 41}

Table 13: Environmental impacts of “FMX MethaneDiesel (LBG)” compared with “FMX (B0)” ... 48

Table 14: Lifecycle environmental impacts of “FMX MethaneDiesel (LBG)” compared with “FMX (B0)” ... 49

Table 15: Lifecycle environmental impacts when precast concrete suppliers are changed ... 51

Table 16: Uncertainty analysis results of energy consumption ... 54

Table 17: Uncertainty analysis results of GHG emissions ... 54

Table 18: Uncertainty analysis results of water consumption ... 55

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1 1 Introduction

Under the trend of urbanisation and the increasing demand for construction activities, the construction sector has become one of the industries contributing to significant environmental impacts. Statistics show that buildings around the world have caused about 40% of primary energy consumption, 25% to 40% of GHG emissions and about 17% of freshwater use (Schiavoni et al., 2016; Nuaimi et al., 2019). For Sweden, the total climate impact caused by the construction sector is about 10 million tons of CO2 eq each year, which approximately equals to the total emissions caused by all the vehicles in the country (Royal Swedish Academy of Engineering Sciences, 2016). To cope with environmental problems caused by construction processes, the Swedish parliament has decided to decrease construction-related energy consumption by 20% before 2030. In order to reach the goal, the prefabricated building is mentioned as one way to improve energy efficiency and the overall environmental performance of the construction sector (Janson, 2008).

Prefabricated building refers to the building whose construction components and structures are manufactured in prefabrication factory completely or partly completely, then transported to the construction site, and finally assembled to finish the building (Tam et al., 2007). Depending on the extent of how much prefabrication is applied to the building, prefabricated buildings can be categorised into buildings with prefabricated components (such as beams, columns and slabs), buildings with panelised pre-assembly (such as wall panels), buildings with volumetric pre-assembly (such as kitchens and toilets) and modular buildings (Kamali & Hewage, 2016; Winch, 2013). Among these categories, the most common category in practical construction activities is the building with prefabricated components, although it has the lowest level of prefabrication. In comparison, other categories still have a low application rate, or even remain in the research stage or conceptual exhibition stage (Arlandastad Holding, 2019; Nord, 2008).

In practical construction designs, the term “prefabricated rate” is put forward to further describe how much prefabrication is used in the buildings with prefabricated components. Prefabricated rate describes the ratio between the weight of prefabricated structural components and the total weight of all the material used for the building structure (Hong et al., 2015). More specifically, for a concrete-framed building, the prefabricated rate of this building refers to the ratio between the concrete weight in prefabricated components and the total concrete weight consumed by the entire building (Liu & Chen, 2019). In order to optimize the environmental performance of the construction sector, Sweden promotes the development of prefabricated buildings. More specifically, Sweden fosters the increasing of prefabricated rate throughout the construction activities (European Commission, 2018; Roland Berger, 2018).

Life cycle assessment (LCA) is a quantitative tool which provides a comprehensive environmental impact assessment of a service or a product throughout its life cycle (United Nations Environmental Programme (UNEP), 2004). The entire life cycle of a system starts from raw material extraction, via manufacture and transport, and then to use and waste management (UNEP, 2004). By analysing environmental impacts, resource consumption and human health, etc., LCA identifies impact hotspots and possible improvements for the production system. Most importantly, the life cycle perspective tries to avoid shifting the environmental burdens from one stage to another or from one region to another, which benefits the decision making for enhancing the production system’s environmental behaviour (Finnveden et al., 2009).

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prefabricated buildings (Jaillon & Poon, 2008). Given this fact, the effectiveness of improving the environmental impacts of buildings by increasing prefabricated rates requires further study.

There is a research gap between the existing literature and the need to examine whether the increase of prefabricated rates can improve the environmental performance of the buildings, especially during construction processes. First of all, lots of researches about the environmental impacts of buildings focus more on the use and operation stage of the buildings without a life cycle perspective (Thewes et

al., 2014; Ruuska & Häkkinen, 2015; Hong et al., 2015; dos Santos et al., 2015). Besides, many LCAs

about environmental problems of buildings talk about buildings in a general way, instead of distinguishing conventional buildings and prefabricated buildings (Santos et al., 2019; Dylewski & Adamczyk, 2014; Hollberg & Ruth, 2016). In this case, the differences between conventional and prefabricated buildings cannot be concluded. For the literature comparing these two kinds of buildings, many of them choose to compare conventional buildings and modular buildings, which are still theoretical concepts (Kim, 2008; Kamali & Hewage, 2016). Since modular buildings have not been put into practical construction yet, the results can offer little guidance to real construction practice. Although some literature do compare conventional buildings and the buildings with prefabricated components, most of them choose buildings with fixed prefabricated rates as the case (Wen et al., 2015; Cao et al., 2015; Pons & Wadel, 2011; Omar et al., 2014). These results are not comprehensive enough to show whether a higher prefabrication rate can result in a better environmental performance of buildings. Only a small part of literature have discussed the variation of prefabricated rate, but most of them are addressing cost and social issues (Liu & Chen, 2019; Shen et al., 2019; Dong & Ng, 2015).

1.1 Aim and objectives

The trend of raising the prefabricated rate of the prefabricated building and the research gap identified above indicate that there is a need to conduct an LCA which includes various prefabricated rate scenarios, to analyse the environmental impact changes. This thesis aims to compare the environmental performance of the prefabricated building with different prefabricated rates by applying LCA and examine whether the increasing prefabricated rate has a positive effect on making the construction processes of the buildings more environmental friendly. Focusing on the situation in Sweden, two specific objectives are put forward to achieve this aim:

(1) To conduct an LCA of one reference building with a certain prefabricated rate, as a baseline of the environmental impact analysis.

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3 2 Background

2.1 Environmental problems related to the construction sector

Compared with other industries such as manufacturing and agriculture, some unique characteristics of the construction sector have added to its environmental impacts. Firstly, high-level complexity and project specificity of building designs make it difficult to achieve efficient mass production in the construction sector. The repeated design processes for each construction project requires lots of resources (Khasreen et al., 2009). Secondly, construction projects usually have a much longer lifespan than other products or services, which can be more than 50 years (Scheuer et al., 2003). Throughout the lifespan of a building, different kinds of changes related to design, structure, function, construction methods etc. may happen to the building. In this case, it is difficult to manage all steps and control the environmental problems throughout the entire lifespan of a building. Thirdly, construction projects are influenced by various stakeholders. Some stakeholders only take part in one single stage of the construction processes of the projects, for example, architects and structural engineers focus on design schemes and building material selection, and clients and facility managers focus on the use and maintenance of the buildings. These stakeholders do not take part in the practical construction of the buildings (Olander & Landin, 2005). This fact also adds to the complexity of the construction sector and requires much more efforts to cope with the environmental impacts of the buildings.

Globally, the construction sector is one of the significant factors causing environmental problems. As the construction of residential buildings, schools, commercial and business buildings, hospitals and infrastructure etc. is critical to the development of countries, the global construction sector is rapidly growing. It is estimated that the total output of all kinds of construction worldwide will increase by 85% until 2030 (Afable, 2019). This growth may improve social welfare and global economy, but also lead to the environmental impacts of the construction sector. For example, the growing demand for building materials requires a larger scale of mining projects, material processing and transport. It is estimated that the greenhouse gases (GHG) such as CO2, N2O and methane emitted by construction projects make up 25% to 40% of global GHG emissions, and the construction sector contributes to about 40% of the worldwide energy consumption (Nuaimi et al., 2019). Also, construction activities are related to particle pollution, waste, water consumption and pollution (Wu et al., 2016).

Chen et al. (2010) emphasize that the construction sector is one of the primary natural resource consumers and is responsible for significant GHG emissions in America. It is estimated that construction activities make up 38.9% of primary energy consumption, 38% of CO2 emissions and 30% of waste generation in America (Jackson et al., 2013). In China, the urbanisation rate across the country jumps from 17.9% to 47.6% from 2006 to 2011 after the implementation of the 11th_{Five-Year Plan (Yuan} & Wang, 2011). Due to the quick development of construction activities, the construction sector in China accounts for 35% of national energy consumption, 41.5% of GHG emissions and 27% of dust pollution (Wang, 2014). Also, the recovery rate of construction waste in China is less than 5%, and most of the construction waste is landfilled (Shi et al., 2013). It is also pointed out that the construction sector takes responsibility for approximately 40% of the total environmental burdens in the European Union (EU) (Khasreen et al., 2009). According to the report from the Federation for the European Concrete Industry (2019a), buildings make up the largest share of total energy consumption and CO2 emissions in EU during 2016, with a ratio of 40% and 36% respectively. EU (2019) also reports that the construction sector is responsible for around 50% of raw material extraction and about one-third of total water consumption.

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Swedish National Report shows that residence and construction sector consumes around 40% of the total national energy consumption in 2016 (Moisio et al., 2016). It is also emphasised that the emissions of volatile organic compounds and semi-volatile organic compounds from construction materials can cause risks to human health (The OECD Environmental Programme, 2001). While conducting a framework to map the hazardous substances during all construction processes in Sweden, a total of 46 hazardous substances are identified (Swedish Chemicals Agency, 2016).

2.2 Prefabricated buildings

Different from the conventional buildings whose structures are mainly manufactured on the construction site, prefabricated buildings are first produced in prefabrication factories in forms of building components, separate units or modular etc. Then parts of the buildings are transported and assembled on the construction sites (Mapston & Westbrook, 2010).

2.2.1 Different prefabrication levels and the concept of prefabricated rate

Prefabricated buildings are further categorised into different levels of prefabrication. The first category is the building with prefabricated components such as beams, columns and slabs. The prefabricated components are first manufactured in prefabrication factories as individual units and then transported to the construction site where all the components are completely assembled. This is the basic level of prefabrication and is also the most widely used (Winch, 2013). The second category is the building with panelised pre-assembly, such as panels and walls. After being manufactured in the factories, some components are partly pre-assembled inside of the factories into panels and walls with 2D panelised structures, and then transported and assembled on the construction site (Bertram et al., 2019). However, the diverse combinations of different prefabricated components increase the difficulties for the operation, management as well as internal transport in the prefabrication factories. As a result, this category is less popular than the building with prefabricated components despite its slightly higher prefabrication level (Said et al., 2017). The third category is the building with volumetric pre-assembly, which means that an entire enclosed unit is manufactured inside of the prefabrication factory, such as kitchens and bathrooms. Then all enclosed units are transported to the construction site and are installed there (Winch, 2003). The last category with the highest level of prefabrication is the modular building, which means the entire building is assembled volumetrically in the prefabrication factories. After being transported to the construction site, the modular building is fixed onto the foundation on the construction site (Kamali & Hewage, 2016). Prefabricated buildings with volumetric pre-assembly and modular buildings are still in researching or conceptual stage due to less flexibility, high requirements for building technology and difficulties for logistics (Bertram et al., 2019). In real practical construction projects around the world, the building with prefabricated components is still the most common category of prefabricated buildings.

To further describe the prefabrication level of buildings with prefabricated components in a more quantitative way, the concept of “prefabricated rate” is put forward, which describes the ratio of prefabrication weight in the prefabricated components to the total weight of the component material used in the entire building (Hong et al., 2015).

2.2.2 The history and development of prefabricated buildings

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spread to other parts of Europe, Northern America and Asia (Yuan et al., 2018). The report shows that the average ratio of prefabricated buildings to conventional buildings in the EU was about 18% in 1996. The highest prefabrication level was in Denmark (43%), following by Netherlands (40%), Germany (31%) and Sweden (30%) (Chen et al., 2010). Until 2011, the production of precast concrete components in EU had reached 24 billion Euro and more than 5500 prefabrication companies had developed in EU (Federation for the European Concrete Industry, 2019b). Having been developed for more than 130 years, the ratio of prefabricated buildings to conventional buildings has reached 20% to 40% in Europe, 35% in America and more than 50% in Japan (Lindblad, 2019).

In Sweden, increased labour costs since 1950 triggered the rapid development of machinery production to save construction time and heavy manual operation (Wästlund, 1965). This marked the beginning stage of prefabricated buildings in Sweden. Until 1965, the annual production of precast concrete was around 125 million Swedish krona, with an annual increase rate of 5% (Wästlund, 1965). In the first decade after the Second World War, Sweden witnessed fast urbanisation, growing prosperity and increasing demand for housing. Due to the housing shortage, the Swedish parliament started the “Million Homes Programme”, which aimed to construct one million dwellings from 1965 to 1974 (Hall & Viden, 2005). After the implementation of this construction programme, about 920000 dwellings within 40000 apartment blocks were constructed around the country. The large demand for new buildings within one decade had encouraged the development of prefabrication in Sweden (Hall & Viden, 2005). Besides the growing demand for housing supply, the goal to construct buildings with better quality and more efficient energy consumption also required the promotion of prefabricated buildings in the following years. Since 2009, Swedish large construction companies such as Skanska and NCC have started to test out precast concrete sandwich elements in their apartments, to achieve energy efficiency in a cost-effective way (Sundberg, 2013). From 2010 to 2016, the production value of precast concrete components had risen from 107.2 million euro to 339.8 million euro in Sweden, which had increased by more than 200% (European Commission, 2018). From 2012 to 2015, the production of precast concrete in value in Sweden had increased from 643.5 billion to 937.8 billion Swedish krona (Federation for the European Concrete Industry, 2019a). Studies show that large concrete slabs, staircases and balconies are the most popular prefabrication parts for the design of prefabricated buildings in Sweden (Wang et al., 2017).

2.2.3 The benefits of prefabricated buildings

Given the fact that parts of prefabricated buildings are manufactured in standardised and controllable factories, and then assembled on the construction sites, there are several advantages of prefabricated buildings compared to conventional buildings.

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repetitive (Li et al., 2013). It is estimated that the on-site accidents of prefabricated building projects are about 80% less than those of conventional building projects (Lawson et al., 2012). Last but not least, prefabricated building projects have higher productivity. The standardised assembly line in the prefabricated factories enables more organised production, regular supervision, stable working environment and parallel operations, which contributes to much higher productivity than conventional building projects (Blismas et al., 2006).

The above benefits add to the potential of prefabricated buildings to improve the environmental performance of building projects. By moving more construction activities into controllable and standardised prefabrication factories, it is promising to improve the overall production efficiency and resource consumption efficiency. Less disturbance on the construction site may also be achieved, such as reduced noise, waste, dust and on-site congestion.

2.2.4 Environmental goals and government policies related to prefabricated buildings

Throughout the efforts to make a more environmentally sustainable construction sector, global organisations and national governments of countries around the world are setting goals and policies both for the country in general and for building projects. Some of the goals and policies are also directly related to the development of prefabricated buildings. This section reviews the goals and policies adopted in America and China, which can be the examples of developed countries and developing countries respectively. The goals and policies in EU and Sweden are also reviewed, since Sweden is the targeted region studied in the thesis.

Since the 1980s, America has started to encourage the development of prefabricated building for residential building projects. At the same time, the application of digitalisation is also motivated in the construction processes of prefabricated buildings to increase construction efficiency. In 2003, America started to invest 200 million dollars annually to provide down payment funding for the families who purchase prefabricated buildings, which also stimulated the prefabrication market (Wang & Grace, 2017). In 2005, the American Institute of Architects put forward the target to construct carbon-neutral buildings by 2030. Further, the Department of Energy in America also signed the goal for constructing net-zero-energy buildings and eliminating the consumption of fossil fuel by 2030 (Commission for Environmental Cooperation, 2008). A program called Building America is also launched by Department of Energy to encourage prefabricated building projects on a community scale, to achieve an average energy consumption around 30% less than conventional buildings (Office of Energy Efficiency & Renewable Energy, 2019). It is estimated that the value for prefabricated buildings in the American real estate market will reach 130 billion dollars by 2030 (Bertram et al., 2019).

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China needs to develop 50 pilot cities for prefabricated building projects and 30 prefabrication technical bases before 2022 (Xu et al., 2019).

Since 2009, the Energy Performance Directive for Buildings in European Community has urged European governments to take actions to cope with high energy consumption, fast resource depletion and other negative environmental impacts within the construction sector (Khasreen et al., 2009). EU has set the goal to reduce GHG emissions by about 80% to 90% compared to the emission level in 1990 by 2050 (Tettey et al., 2018). Many countries started to require a sustainable assessment of the newly constructed buildings based on LCA. For example, the Netherlands released the regulations in 2012 that all new buildings more than 100 m2_{must have a life cycle emission and resource consumption} analysis to get the planning permission for the project. Similarly, new construction projects must provide lifecycle carbon footprint report in Finland (EU, 2019). Furthermore, the architect council of Europe has also worked out a framework attempting to achieve sustainable buildings, which includes increasing public awareness, enabling certification, making corresponding policies and developing construction technology and digital tools (EU, 2019).

In Sweden, the 4th_{National Energy Efficiency Action Plan set the target for the country to reduce energy} intensity by 20% between 2008 to 2020. In 2017, Swedish National Board of Housing, Building and Planning (Boverket in Swedish) opened an information centre to integrate all the resources better to construct energy-efficient buildings with lifelong lower climate impacts (European Commission, 2018). Also, in 2017, Sweden released the regulations for buildings requiring all newly constructed buildings should be near-zero energy consumption since 2021 (Energy Performance of Buildings, 2016). In January 2018, Swedish Parliament also approved the goal to have zero net GHG emissions by 2045 (Paananen & Burom, 2019). At the same time, the growing housing market put many challenges to the above environmental goals. An EU report indicates that the supply of residential dwellings still cannot meet the demand and at least 60000 new houses are needed to be finished by 2025. Swedish government plans to invest an extra 622.5 billion Swedish krona for building and infrastructure from 2018 to 2029 (European Commission, 2018). Besides, according to an estimation made by the Swedish National Board of Housing, Building and Planning, approximately 75% of existing houses require major renovation or reconstruction before 2050 (Berggren & Wall, 2019). To achieve the environmental goals within the construction sector, the Swedish government encourages technical development of prefabrication conducted by major contractors in Sweden. For example, Skanska adopted a reusable technological platform in their prefabrication factories to increase quality and avoid duplication of work (Sundberg, 2013). The Swedish government also funds research related to prefabricated technologies, such as the research conducted by NCC and Lund University about the improvement of thermal comfort and energy performance of precast concrete elements (Boqvist, 2010; Flodberg, 2012). Between 2004 to 2017, Swedish National Board of Housing, Building and Planning released more than 18 quality regulations for different kinds of precast elements such as beams, columns, stairs and walls. These regulations and guidance include the assessment of factorised production control, sampling control and the performance of precast concrete products (Boverket, 2017).

2.3 Literature review on environmental impacts of conventional and prefabricated buildings

2.3.1 Literature on environmental impacts of buildings in general

Since the construction sector is one of the industries that have the greatest contribution to global environmental problems, researches point out the environmental impacts of residential as well as non-residential buildings, and study the possible optimisation measures. Among all kinds of environmental impacts studied in these literature, energy consumption, GHG emissions and water consumption are the most common topics.

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insulation in these components is applied, the building energy consumption for heating can be reduced by 21% to 42%. Seem (2007) developed a technology using intelligent data analysis to study energy consumption in the buildings and detect abnormal energy consumption during the day, which helped digitally detect faults in the buildings and conserve the operation energy. Ramesh et al. (2010) conducted a review targeting on the total energy use for 73 projects in 13 different countries, including residential projects and office projects. They found that the operation stage of buildings consumed 80% to 90% of the total energy use and the embodied energy in building materials accounted for about 10% to 20% of the total energy use. Pérez-Lombard et al. (2008) concluded that the energy consumption of the building during the use stage contributes to 20% to 40% of total energy use in developed countries. Especially, the energy use of heating, ventilation and air conditioning (HVAC) system is significant, which accounted for 50% of the total building energy consumption and about 15% of total energy consumption in the USA. To optimise the energy consumption during the use stage of the buildings, Thewes et al. (2014) made a field study on school buildings in Luxembourg. They found that better airtightness and higher wall insulation could approximately save annual heating energy by 17 GWh, which equalled to about 1% of the annual national fuel oil and gas consumption in the tertiary sector. They also pointed out that lower lighting power inside of the buildings could save primary energy by about 17% to 37%.

By conducting a literature review and studying multi-storey residential buildings with similar project characteristics in Finland, Ruuska and Häkkinen (2015) analysed the major factors for total GHG emissions of buildings. They found that the solar systems, building cooling systems, surface materials and windows are significant factors contributing to material-related GHG emissions. Hong et al. (2015) utilised on-site detailed process data related to machinery construction work and human activities to study both direct and indirect GHG emissions in the construction industry. The result showed that electricity consumption on-site and building material production were the two major factors for building GHG emissions, which approximately generated 385 tCO2 eq annually in China. Some literature also explored the possible solutions to ease GHG emissions caused by buildings. Karan et al. (2016) studied different kinds of electric vehicles to reduce transport-related GHG emissions for the construction processes of buildings. They found that the electric vehicle powered by a hybrid solar system with both solar panels and electricity storage capacity had better GHG emission reduction effect than other types of electric vehicles. Lai (2014) studied the requirement to mandatorily report GHG emissions from buildings in Hong Kong by conducting surveys among different stakeholders. The study found out that the mandatory reporting policy might have a significant effect to reduce scope 1 and scope 2 GHG emissions but less impact to scope 3 emissions unless more financial and technical supports were provided. Chen and Ng (2016) studied the current green building rating tools including LEED, BREEAM, Green Star, BEAM Plus and GBES, and concluded their limitation regarding measuring GHG emissions caused by buildings. They recommended adding “embodied GHG emissions from building materials” as the 8th_{credit in the LEED system to reduce building GHG emissions.} Dos Santos et al. (2015) documented monthly water consumption data of a construction project in Brazil to study the water consumption on the construction site. They found that about 20% of the total water consumption happened directly on the construction site, and indirect construction activities consumed more than 50%. They also indicated that structure construction and waterproofing processes consumed more water than other activities. Bardhan (2011) studied indirect water consumption during material production and building construction for six residential building projects in Iran. They concluded that the average indirect water consumption was about 20.8 m3_/m2_{, and the energy used to} provide the water causes a total of 13.7 million tons of GHG emissions. Some literature also attempted to find potential optimisation to water consumption problem in the construction sector. Esmaeilifar et

al. (2014) surveyed contractors and designers in Malaysia. They concluded that a proper site location

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to increase the water quality of the building. Some literature also focused on water consumption during the use stage of the buildings. Terés-Zubiaga et al. (2018) conducted a case study in Spain about the requirement of individual measuring and charging of domestic hot water consumption. They concluded that by monitoring individual domestic hot water consumption, the total water consumption in the buildings during the use stage could be reduced by 15% to 20%.

2.3.2 Literature on LCAs of conventional buildings

To study the environmental problems of the construction sector more comprehensively, researchers have applied the life cycle perspective regarding the environmental impact assessment of conventional buildings. Many studies conducted LCAs for building design selection and the evaluation of construction processes. Russell-Smith et al. (2015) combined LCA and target value design to quantify design targets on energy consumption, GHG emissions and ozone depletion to make a balance between design schemes and environmental performance of buildings. Werner and Richter (2017) made a comparative LCA about different kinds of wood materials for the construction of buildings based on extensive literature review in Europe, Northern America and Australia. They found out that particleboard was the principle wood material causing fossil energy consumption and impregnated wood products had a significant contribution to toxicological impacts. Dylewski and Adamczyk (2014) studied five different kinds of building thermal insulation materials such as extruded polystyrene and mineral wool based on LCA analysis from both ecological and economic perspectives. They found that the application of insulation materials could reduce energy consumption during the use stage, but had the potential to increase environmental and economic costs during the production stage. Hollberg and Ruth (2016) designed a parametric LCA model which could simplify the data input for construction projects to benefit architects without a detailed understanding of the life cycle perspective. The model could also automatically optimise the environmental impacts of building designs based on evolutionary algorithms.

Many other studies tried to compare different LCA databases for building evaluation to give a more accurate LCA of conventional buildings. Emami et al. (2019) examined the Ecoinvent database with SimaPro and Gabi software using a concrete-framed residential building and a wooden building in Finland. For climate change, the results from SimaPro and Gabi showed 15% differences, and for other impact categories, about 40% variation was estimated. Takano et al. (2014) analysed the GHG emissions of two wooden building and one concrete building. They compared the LCA results coming from five different databases, including Gabi, Ecoinvent, IBO (Austrian), Synergia (Finnish) and CFP (Japan). The results based on different databases showed the same trend and in the same order of magnitude, but the differences in value between each database were substantial. Martínez-Rocamora

et al. (2016) compared 10 different LCA databases in Europe and America regarding the scope,

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Taking projects in five different climate zones in Spain as an example, they concluded that the surrounding climate and the thermal characteristics of the PCM could influence the overall environmental behavior of the buildings.

2.3.3 LCAs of prefabricated buildings

As the prefabricated building is becoming one of the most popular building innovations to cope with environmental problems within the construction sector, researchers have conducted many studies related to prefabricated buildings, many of which have applied the life cycle perspective.

Many of the existing LCAs of prefabricated buildings are related to different design choices. Bonamente

et al. (2014) conducted a cradle to grave LCA to analyse the carbon footprint and energy footprint of

four prefabricated buildings with similar properties but various floor areas from 1000 m2_{to 20000 m}2_. The study showed that the building with a larger floor area has smaller carbon footprint and energy consumption, especially the energy efficiency during the use stage. Faludi et al. (2012) conducted an LCA of a prefabricated commercial building to study the environmental impacts of different material substitutions in building design. The study showed that if 25% of cement in concrete was replaced by fly ash , the total GHG emissions could be reduced by 11% to 14%. Hong et al. (2015) studied 8 prefabricated building projects in China, with prefabricated rates varying from 15% to 59%. They measured the average energy consumption of the precast façades, precast forms, semi-precast slabs, precast balconies, precast staircases and precast air-conditioning panels in the projects. They found that the precast staircase was the most energy-efficient prefabricated component, which has an average energy footprint of 7.33 GJ/m3_.

Among the existing literature about prefabricated buildings, another part is related to the LCAs comparing prefabricated buildings and conventional buildings. Some of the studies compared the general environmental performance of prefabricated buildings and conventional buildings. Kim (2008) conducted a cradle to grave comparative LCA between a modular house and a conventional building in Michigan over a 50 year’s life span. The study showed that the modular house reduced about 2.5 times of construction waste, 5% of total energy consumption and 4 to 5 days of production time. Cao et al. (2015) studied a prefabricated residential building with a prefabricated rate of 38% and a conventional residential building in China throughout its entire lifecycle. The results showed that prefabricated building reduced 35.82% of resource consumption, 6.61% of health damage and 3.47% of ecosystem damage. Some of the comparative LCAs of prefabricated buildings and conventional buildings further divided the prefabricated building projects into projects with different prefabricated technologies. Pons and Wadel (2011) studied 138 school buildings from 2002 and 2006 in Spain and categorised them into the precast concrete structure, precast steel structure, precast timber structure and conventional structure. By comparing three major prefabrication technologies with conventional technology, the study found out that the total waste could be reduced by 60% but still far away from the material circular economy. Omar et al. (2014) compared the construction technology of conventional concrete structure, precast concrete structural and industrialised steel structure on a 2-storey building in Malaysia by conducting a cradle to gate LCA. The result showed that the precast concrete structure could achieve total GHG emission reduction of 26.27%.

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a social life cycle assessment comparing the prefabricated buildings and conventional buildings in the public rental housing project in Hong Kong. The study showed that the overall health and safety of workers were improved in prefabricated building projects. At the same time, the fair salary and local employment were negatively influenced because the prefabricated components were imported from outside of Hong Kong.

2.3.4 The research gap

The above literature review shows that although lots of researches studied the environmental impacts of the construction sector, many of them did not have a life cycle perspective when discussing the impacts such as energy consumption, GHG emissions and water consumption of buildings (Ruuska & Häkkinen, 2015; Hong et al., 2015; Chen & Ng, 2016; dos Santos et al., 2015; Esmaeilifar et al., 2014). Many studies attached more importance to the use and operation stage of a building project and often simplified or omitted the impacts during material production or construction processes (Seem, 2007; Ramesh et al., 2010; Lombard et al., 2008; Thewes et al., 2014; Lai, 2014; Hashemi et al., 2015; Terés-Zubiaga et al., 2018). Some literature conducted LCAs and attempted to find optimization methods to improve the environmental behaviour of buildings. However, the focuses were on specific building materials, special design or construction technology instead of prefabrication (Richter, 2017; Dylewski & Adamczyk, 2014; Proietti et al., 2013; Sartori & Hestnes, 2007; Aranda et al., 2013). Some LCAs mentioned prefabrication to some extent, but these studies illustrated the environmental impacts of buildings in general or included both conventional and prefabricated buildings in their case studies without distinguishing them apart (Balaras et al., 2000; Karan et al., 2016; Bardhan, 2011; Hollberg & Ruth, 2016). In this case, it cannot be concluded in these researches whether prefabricated buildings have an advantage over conventional buildings.

For the LCAs related to prefabricated buildings, many studies applied the life cycle perspective for making design decisions, such as choosing a more environment-friendly building material, prefabricated structures or technology (Bonamente et al., 2014; Faludi et al., 2012; Hong et al., 2015). However, there is no direct link between their results and the environmental impacts caused by different prefabrication level. For the LCAs comparing prefabricated buildings and conventional buildings, many studies have compared modular buildings with the highest prefabrication level and conventional buildings without any prefabrication (Kim, 2008; Pons & Wadel, 2011; Omar et al., 2014). As the modular buildings are still conceptual theories and have not yet been applied in practical construction processes, the results cannot offer guidance to specific construction activities in practice. Some literature did compared conventional buildings and buildings with prefabricated components, however, most researches focus on the buildings with fixed prefabricated rates (Wen et al., 2015;Cao et al., 2015). In this case, these researches have only compared the environmental behaviour of buildings with or without prefabrication, and not yet sufficient to identify the environmental benefits of higher prefabricated rates. A small part of literature have considered the change of prefabricated rate, but most researches focus on cost management or environmental cost-benefit analysis (Liu & Chen, 2019; Shen

et al., 2019; Dong & Ng, 2015).

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13 3 Method

To study the environmental performance of a prefabricated building and how its environmental impacts may change if the prefabricated rate varies, this thesis conducts a life cycle assessment. Generally speaking, LCA is a tool to compile and evaluate the inputs and outputs of a production system or a service throughout its entire life cycle and to assess its potential lifecycle environmental impacts (International Standard Organization (ISO), 2009). Among its various applications, LCA is commonly used to identify opportunities to enhance the environmental performance of the production system or service and to guide decision-makers to conduct more environment-friendly planning (Curran, 2015). According to ISO 14040:2006, there are four main steps to perform an LCA: goal and scope definition, inventory analysis, impact assessment and impact interpretation (ISO, 2009). In this thesis, the LCA about the prefabricated building is also performed according to these four steps.

The identification of the goal and scope of the study is the first step of an LCA. This step defines the purpose of the LCA and the main problem to be answered, which is of vital importance and can further affect the LCA results (Selmes, 2005). Within this step, the system boundary will be identified, including which specific prefabricated building project to focus as reference building in the LCA and how many lifecycle stages of the target system will be considered. Especially, several LCA scenarios with the prefabricated rates changing between 0% to 100% will also be studied, by adding or subtracting 10% intervals to the prefabricated rate of the reference building. Besides, the audience of the study, functional unit, delimitation and assumptions will also be set for the LCA. Further, to guide the following calculations and assessments, the allocation method, impact categories and assessment methods are also selected in this step.

The second step is inventory analysis, which includes data collection and calculation processes (ISO, 2009). When studying the prefabricated building project, the detailed process flowcharts for the prefabrication work as well as non-prefabrication work of the building project are made to illustrate all inputs, outputs and procedures related to the construction activities. After that, data related to material consumption for structures and components are collected both from the specific information provided by project documents and from external sources. Data related to energy and water consumption during transport and construction processes are also collected. Further, all kinds of data will be converted into each unit process according to the chosen functional unit, and the data uncertainties are also going to be summarized.

The third step is the impact assessment to evaluate the targeted production system or service from an environmental point of view (Goedkoop et al., 2016). Using conversion indicators for each impact category, the characterization process assigns the environmental contribution caused by each kind of input and output in the life cycle inventory to the selected impact categories. Then the results are compiled and summed up for the selected impact categories (Menoufi, 2011). To study the environmental impact changes of the prefabricated building with various prefabricated rates, the impact assessment for different life cycle scenarios of the chosen prefabricated building is also performed during this step. The model for the evaluation and calculation is made in SimaPro software version 9.

The final step is the impact interpretation, during which the environmental assessment results are analysed to reach the goal identified in the first step. Based on data uncertainties and all the assumptions, this thesis also performed uncertainty analysis and sensitivity analysis to provide more convincing and consistent results.

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Fig. 1 The schematic diagram of the method applied in this thesis

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15 4 System description of LCA

4.1 Goal and scope

Since conventional construction projects have relatively lower construction efficiency, higher defective risks, more extended construction period and more environmental concerns, there is an increasing trend to promote prefabricated buildings as a sustainable solution (Chen et al., 2010). The global market for prefabricated building projects is predicted to be continuously brisk from 2018 to 2025, especially in Northern America and Western Europe (Anon, 2019). Among all building materials, concrete is the dominant material consumed in the construction sector, with 25 billion tons being processed annually (Salesa et al., 2017). Precast concrete components and structures are growingly applied in various prefabricated building projects, with an increased awareness of their environmental, economic and social benefits (Yee & Eng, 2001).

While promoting prefabricated building projects, the most common method is to raise the prefabricated rate, since this is more realistic to be implemented on a large scale compared to other categories of prefabricated buildings. By increasing the ratio of precast concrete consumption to total concrete consumption, the sustainability of the entire building projects is expected to be improved (Hwang, 2018). A variety of stakeholders are involved throughout the supply chain of prefabricated buildings, such as architects, structure designers, transporters, clients, principal contractors and sub-contractors (Luo et al., 2019). In addition, many background processes, such as the work related to raw materials and transportation, are “hidden” behind the manufacture of precast concrete components in the factories and the on-site construction of the buildings. In this case, the audiences of this LCA are building designers and project managers, who have the responsibility to understand the “hidden impacts” of the full life cycle of prefabricated buildings with different prefabricated rates before making decisions. The goal of this LCA is to figure out the environmental hotspots of the chosen reference building and to study how prefabricated rates can influence the environmental behaviour of the building.

4.1.1 The description of the reference building

The City of Stockholm held a design competition for a positive net-energy residential building. The high standards call for energy efficiency and outstanding innovation, which requires that the building can generate more energy than the energy it consumes during its occupation calculated throughout a one-year cycle (Dubbeldam Architecture and Design, 2018).

The chosen reference building is the winning design scheme in the competition, located at Stockholm Royal Seaport. The reference building, with 43-unit rooms is constructed with a mix of conventional concrete and precast concrete frame, and the prefabricated rate is about 26%. More specific information about the concrete type and concrete weight of the reference building is shown in Table 1 (Stockholmshem, 2012). Detailed information about building materials, transport and construction activities of the reference building will be further illustrated in Section 4.2.

Table 1: The concrete information of the reference building

Concrete type Structure Concrete weight

Conventional concrete Bearing walls, groundwork panel and hollow ground floor

3121000 kg

Precast concrete (with reinforcement bars)

Concrete slab floors, external gable, attic floor and balcony floor

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There are three reasons to choose this building as the reference building. Firstly, this project has a relatively high availability of data, which can contribute to the reliability of this LCA. Since this is the winning design in the competition held by the City of Stockholm, specific information such as design drawings, material consumption, suppliers, transport scheme etc. are available on the project websites and the documents provided by the energy consultancy company (Stockholmshem, 2012; Dubbeldam Architecture and Design, 2018). Secondly, the prefabricated rate of 26% in this project is representative. Studies show that more than 80% of newly constructed building projects have adopted prefabrication in Europe, and the average prefabricated rate of these projects is around 20% to 40% (Larsson et al., 2014). Thirdly, the LCA results of this building may set an example for future sustainable building development. The building itself, as well as the entire Stockholm Royal Seaport region, are aiming at creating an environment-friendly built environment with advanced urban planning, energy-efficient designs and systematic region management etc. The results related to the relationship between prefabricated rate and environmental impacts will further add to the efforts towards construction sustainability.

4.1.2 Functionalunit

According to Curran (2015), the functional unit gives a quantitative description of the product or the service provided by a production system, which helps identify the environmental impacts caused by each unit of the product or the service. While studying the environmental impacts within the production system or among different production systems, a reasonable choice of functional unit offers a common baseline for all kinds of inputs and outputs (Curran, 2015).

Same with all residential buildings, the function of the chosen reference building is to provide an area for residents to live, rest and conduct daily indoor activities. As the goal of this LCA is related to the construction activities of the building, the choice of the functional unit should not only include the residential living areas, some other aspects concerning the frame of building structure should also be considered. In this case, 1 m2 _{of floor area is chosen as the functional unit in this LCA. According to the} Swedish Standard SS 21054:2009 for area and volume of buildings, the floor area (bruttoarea in Swedish) refers to the measurable area of the floor plan within the building, limited by the outmost of exterior walls and considering the thickness of walls (Swedish Institutes for Standards, 2009). Based on the design drawing of the reference building, the total floor area of the reference building is 4500 m2_.

4.1.3 System boundaries

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Fig. 2 Simplified diagram of the life cycle of the reference building in this “cradle to gate” LCA

The first lifecycle stage is “raw material extraction and processing”, during which the raw materials such as primary sand and metal ores are directly obtained from nature. Raw materials are then processed into raw material products which can be further processed by the next lifecycle stage. Since the reference building also reuses recycled metals as building materials, the first lifecycle of these recycled materials is determined as “recycled material collection and processing” instead. The second lifecycle stage is “in-plant processing”. During this stage, ready-mixed concrete is processed for conventional concrete work, and precast concrete components with reinforcement bars are manufactured for precast concrete work. Other building materials such as glasses and laminated wood are processed into required sizes in the manufacturing factories. The third lifecycle stage is “on-site work”. Precast concrete components are directly assembled on the construction site, while the ready-mixed concrete is strengthened by reinforcement bars and is cast in place for conventional concrete components on the construction site. Other building materials are constructed and installed in required locations according to construction specifications and design drawings. Between each stage, necessary transport (Tr. In Fig. 2) is also considered in the system.

The use stage of the reference building is not included in the system of this LCA, such as building maintenance, heat and moisture controlling, electricity consumption and ventilation. There are three main reasons for excluding the use stage in the system, as is shown in Table 2.

Table 2: Reasons for excluding the use phase of the reference building in this LCA

Energy-efficiency design Project-specific characteristics Resident-related behaviours - Thermal bridge

- Solar panels

- 30-degree roof slope

- Geological location - Building orientation - Wind pattern

- Interaction with advanced building technology Precast concrete work Conventional concrete work Other material work

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- Green roof for rainwater - Rainfall pattern - Building envelope area

- Personal habits (e.g. turning off the lights, opening windows) - Characteristics (e.g. age,

gender, education level, income)

The first reason is that the design of the reference building is energy-efficient, and the building is ought to generate more energy than it consumes. According to the design scheme, special thermal bridges are created between the balconies and the interior area of the building, which can avoid unnecessary heat and cold exchange between the building and the environment. The roof and façade of the building are also equipped with solar panels to store solar energy during daytime and release the heat at night, which reduces more than 30% of the energy consumption. The roof of the building faces south with a 30-degree slope to maximize energy production from the solar panels, and green plants growing on the roof can also limit the rainwater run-off to conserve freshwater (Dubbeldam Architecture and Design, 2018). In this case, the energy consumption and other environmental impacts during the use stage are not the primary factors influencing the environmental behaviour of the reference building.

The second reason to exclude the use stage is that use stage is usually project-specific. Theories of building energy consumption point out that the energy and environmental performance of a building during use stage can be influenced by various project-specific factors, such as the geological location, building orientation, wind and rain pattern, the ratio between building envelope area and building volume (Fabbri et al., 2012). The project specification may influence the transferability and generalization of the study results.

The third reason to not consider the use stage is that the environmental behaviour of buildings during this stage is also resident-related in no small extent. Residents need to interact with advanced HVAC systems to achieve the sustainable goals of the high-tech buildings. However, it is usually difficult for residents to learn and control everything correctly in the apartments (Liedtke et al., 2012). The environmental behaviour of the building can be influenced by many habits of the residents, such as turning off the light, controlling space heating temperature, opening windows and using domestic appliances (Fabi et al., 2012). Studies even show that characteristics of the residents, including the age, gender, culture, education level and income etc., can complexify the environmental impacts during the use stage of the building. (Wei et al., 2014).

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Fig. 3 The waste disposal processes for precast concrete and conventional concrete in Sweden

(modified from Sadagopan et al., n.d.)

Shown in Fig. 3, around 40% of the waste concrete is landfilled (De Juan & Gutiérrez, 2009). The other 60% of the precast concrete waste and cast-in-place concrete waste is collected and transported to factories producing recycled concrete aggregates (RCA), where they are crushed into smaller pieces. The reinforcement steels are separated with concrete by large magnet machines. Other impurities such as shattered glass and wood are screened out before reprocessing the waste concrete into RCA. The RCA produced from waste concrete are applied to lower-grade construction applications with less-strict requirements such as the subbase of roads (Sadagopan et al., n.d.). Since the waste disposal stages of precast concrete and conventional concrete are identical, the waste disposal stage is not included in the life cycle of the target system.

The reference building locates inside of Stockholm Royal Seaport, the geographical boundary of on-site work is in 115 41, Stockholm, Sweden (Stockholmshem, 2012). Most of the raw materials are produced in different cities within Sweden and then transported by in-land trucks to Stockholm Royal Seaport. The reinforcement bars consumed by the building are produced from a factory adjacent to Riga in Latvia and then shipped to Sweden. The cement boards for gable façade are transported from the factory in Muijala, Finland to the construction site via the port of Turku (Stockholmshem, 2012). The temporal boundary for this LCA is the period since the production of raw material begins until the construction of the reference building finishes. For the data collected in this LCA, the period is limited to the last ten years to ensure data quality.

4.1.4 The allocation method

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Fig. 4 The illustration of the “cut-off” principle related to recycling in this LCA.

In SimaPro, Ecoinvent version 3 provides a corresponding system model: “allocation, cut-off by classification”. This system model handles the recycled material as burden-free since the environmental burdens of collecting and reprocessing recycled materials are much fewer than the production processes consuming virgin materials. In this case, the resource consumption and emissions of the recycling process are excluded when constructing the LCA model in SimaPro version 9 (SimaPro Help Center, 2017).

4.1.5 Scenario setting

To analyse how the different prefabricated rates of the building can cause changes in its environmental impacts, this study includes nine scenarios with the prefabricated rates changing from 0% to 100% by adding or subtracting 10% intervals to the prefabricated rate of the chosen reference building. As is introduced before, the prefabricated rate of the reference building is about 26%. In this case, the nine scenarios included in this study, have the prefabricated rates of 6%, 16%, 36%, 46%, 56%, 66%, 76%, 86% and 96% respectively, and are numbered from Scenario 1 (S1) to Scenario 9 (S9),. With the total concrete weight unchanged, the weight of the concrete for the manufacturing of precast concrete components in each scenario is calculated by multiplying the total concrete weight with the prefabricated rate of each scenario.

4.1.6 Delimitations and assumptions

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Table 3: The list of the cut-off criteria for the system model of the reference building No. The cut-off criteria

1)

The manufacture of production factories, machinery and transport vehicles is excluded because they can be used for an extended period by different building projects and their environmental impacts are not directly related to the reference building project itself.

2)

The cleaning processes during the in-plant processing stage and on-site work stage are not included, such as the cleaning of machinery and the daily cleaning of the construction site.

3)

The system model only considers the structural elements which support the frame of the reference building, since this is where the calculation of the quantities of building materials starts. In this case, the materials related to foundation works are not considered, such as drainage layers, drainage lines and moisture protective coatings.

4)

The façade glazing is excluded in the reference building’s façade structure. The roof drainage system is also excluded. For the balcony of the reference building, the non-bearing parts such as light construction made in wood are also not considered. 5) The decoration and furniture in the reference building and the sidewalks outside of

the reference building are also omitted.

6) The environmental impacts of the workers’ and project managers’ activities are not considered.

7)

Other material work remains the same for all lifecycle scenarios, and the weight of other building materials only accounts for less than 5% of the total material weight in the reference building. In this case, the environmental impacts caused by other material work are omitted when comparing different lifecycle scenarios.

When defining and modelling the target system of the reference building in SimaPro version 9, some assumptions are made to guide data collection, which can impact the study results to some extent. Some general assumptions for data collection and target system modelling are listed below in Table 4, while other detailed assumptions and specific information related to each lifecycle process are further explained in Section 4.2.2.

Table 4: The list of general assumptions for data collection

No. Assumptions

1)

It is assumed that no design change will happen during all lifecycle stages in the target system, such as the changes related to the material type and weight, transport methods and distance, construction methods or building structure etc. The construction of the reference building will be conducted according to the design scheme.

The influence of the prefabricated rate on the environmental performance of buildings in a life cycle perspective

The influence of the prefabricated

rate on the environmental

performance of buildings in a life

cycle perspective

SHAOZHE WANG

I

Abstract

Keywords

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Sammanfattning

III

Acknowledgements

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Table of Contents

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Abbreviations

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List of Figures

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List of Tables

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1 Introduction

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2 Background

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3 Method

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4 System description of LCA

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