LCA MODELLING FOR BUILDING CONSTRUCTION PROCESSES

Transcription

1 LCA MODELLING FOR BUILDING CONSTRUCTION PROCESSES Ya Hong Dong and S. Thomas Ng 1 Department of Civil Engineering, The University of Hong Kong, HKSAR 1 Corresponding Author tstng@hku.hk, Tel: (852) , Fax: (852)

2 LCA MODELLING FOR BUILDING CONSTRUCTION PROCESSES ABSTRACT On-site construction processes are complicated as they include the delivery, assembling and installation of components. Meanwhile, energy is consumed and pollutants are emitted. Although the environmental impact of construction is not as much as that of the operation and maintenance stages, it has been agreed that the emissions during the construction phase is very intensive since a large amount of work is complemented within a limited time frame. Life cycle assessment (LCA) is an effective tool that evaluates the entire life cycle of a product. Most studies attempted to investigate the upstream (i.e. the manufacturing of materials) and downstream (i.e. the operation) processes within a building s life cycle, while few models are capable of calculating the environmental impact of construction. This problem can also be found in local academy, as previous work mainly focused on building materials rather than the construction phase. Construction activities are intensively carried out in Hong Kong and China to comply with big demands for accommodation and the development of the local society. A LCA model is, therefore, desired for assessing the pollutant emissions in construction. This study develops a cradle-to-site LCA model, namely Environmental Modelling of Construction (EMoC), which accounts the environmental impact before or during construction. Localized data of concrete products, transportation, electricity, and emission factors are adopted in addition to the overseas database. A case study on a residential building project is further provided to test the model performance. While EMoC is a holistic and up-to-date model, it is likely that it can assist construction industry with decision making in the design stage. Keywords: building; construction management; environment; LCA. 1. INTRODUCTION Sustainable development was proposed by the World Commission on Environment and Development (currently Brundtland Commission) in 1987 as the development that meets the need of present without compromising the ability of future generations to meet their own needs (Brundtland, 1987). Three baseline areas are usually considered in sustainable development, i.e. environment, society and economy. Environmental problems such as climate change, ozone depletion, acidification, eutrophication, resource depletion, etc. have tremendous influence on the social and economic development. Globally, the carbon dioxide concentration exhibited an increase from the pre-industrial level of 280 ppm to 379 ppm in 2005, mainly attributed to the combustion of fossil fuel (IPCC, 2007). The rise in carbon dioxide concentration is corresponding to temperature increase, sea level rise, and snow cover decrease (IPCC, 2007). Ozone depletion caused by abundant use of halocarbon in refrigerants leads to high frequency of skin cancer apart from damaging the ecosystems. Regionally, algal blooms occur frequently in coastal waters of China, Gulf of Mexico, and fresh water systems of lakes and reservoirs. The accumulation of algae in aquatic system may destroy the ecosystem and kill the fishes and other ocean lives. Hong Kong being a coastal city was suffering from an algal bloom in 1998 leading to economic loss of HK$312 million (Dong, 2011). Other environmental issues such as a lack of landfill space and a decrease of agriculture farm are confronted by Hong Kong as well. To protect the environment, 190 countries agreed to reduce carbon emission under the Kyoto 2

3 Protocol. Local environmental protection acts also take place. For example, the China s 12 th Five Year Plan aims at a percent reduction in carbon intensity from 2005 to Hong Kong should, therefore, take opportunity to control carbon emissions, waste disposal, water quality, etc. Building and construction industry consumes a large amount of materials in the construction stage. The manufacturing of construction materials contributes significantly to environmental pollution. For instance, cement industry accounts for 5% of greenhouse gas emissions (Huntzinger and Eatmon, 2009). Other environmental impacts during construction include greenhouse gas emissions due to fuel combustion, water consumption, dust emission, resource depletion, solid waste, etc. Previous studies reveal that the environmental impact during construction is not as significant as in the operation and maintenance stage (Bilec, 2007), while the intensity of emissions due to construction is quite large since considerable work should be completed within the limited time. Rating systems such as the Leadership in Energy and Environment Design (LEED) by the U.S. Green Building Council (USGBC) and the Building Environment Assessment Method (BEAM) by Hong Kong Green Building Council (HKGBC) play a critical role in controlling the on-site environmental performance. Life cycle assessment (LCA) software programs, e.g. Athena and Eco-Quantum involve on-site processes in the assessment. Specific LCA models are developed in U.S. for on-site construction (Bilec, 2007, Guggemos, 2003). In Hong Kong, a LCA model to assess the energy consumption of commercial buildings is established (EMSD, 2006). Another study for Hong Kong context is on the LCA and life cycle cost (LCC) of building materials (HKHA, 2005). Recent construction projects have adopted precast elements more frequently. In the public rental housing projects, precast concrete accounts for 17% of the total concrete volume on average, while the adoption of precast concrete can be extended to 65% (Jaillon and Poon, 2009). Previous studies, however, have excluded the environmental performance of precast concrete elements in their LCA models. The ignorance of the use of precast concrete components may lead to inaccurate results. Furthermore, the LCA studies in Hong Kong are mostly focusing on the stages of material and operation, while the construction processes are simply included with few details or breakdowns. A holistic and up-to-date LCA model which can help evaluate environmental performance of construction is hence in lack. To bridge the knowledge gap and assist the industry to better interpret their construction projects performance, a cradle-to-site LCA model, namely Environmental Modelling of Construction (EMoC) has been developed. EMoC covers the processes occurred before or during construction from raw material extraction, through material manufacturing to on-site construction. Life cycle impact assessment (LCIA) is conducted in EMoC and a method which as received an increasing attention called ReCiPe is selected to conduct the LCIA calculation. The model provides analysis at both the midpoint and endpoint levels under the 18 impact categories. The rest of the article is structured as follows. The basic concept and recent development of LCA are introduced in the next section. Then, the model structure of EMoC is described, which is followed by a case study of a public housing project in Hong Kong. 2. LIFE CYCLE ASSESSMENT Life cycle analysis or life cycle assessment (LCA) was first implemented by Coca-Cola in 1960s to seek for alternative containers besides glass bottles. LCA differentiates 3

4 from other methods in its ability to cover a product s partial or entire life cycle. The outcomes of a LCA study may be distinct from the results of traditional analysis which focuses on certain stages, as LCA can provide a more comprehensive overview of the whole system. A LCA study is composed of four stages: (i) goal and scope definition, (ii) life cycle inventory, (iii) life cycle impact assessment, and (iv) interpretation (ISO, 2006). The first phase defines essential information for a LCA study, such as objectives, audience, study system, etc. In the second phase of life cycle inventory (LCI), model structure is established and a functional unit is defined. In terms of LCI, there are two types of LCA models: the process LCA and input-output LCA. The process LCA tracks emissions of processes within the studied system and collect data of each process. On the other hand, the input-output LCA relies on the economic input-output table in which the environmental impact is given in a sector-to-sector basis. A hybrid LCA is evolved from the process and input-output LCA as it integrates the characteristics of the two types (Bilec, 2007). The LCI result is a list of substance emissions and fuel consumption that can be used in the next phase, i.e. the life cycle impact assessment (LCIA). LCA studies can terminates at LCI without further analysis on LCIA (e.g. Guggemos, 2003). LCIA converts LCI results to various levels of results through certain calculation methods. Several LCIA methods are available, including the midpoint methods (e.g. CML, TRACI, EPD, etc), endpoint methods (e.g. Eco-indicator, EDIP, etc) and combined methods that can provide analysis in both the midpoint and endpoint levels (e.g. ReCiPe ). The midpoint methods result in values of indicators of impact categories. The endpoint methods further evaluate the damage impact caused by emissions. The calculation in LCIA is complicated with several steps to select the impact categories, as well as to derive results of characterization, normlization, and weighting. The details of the LCIA procedures are given in ISO 14040s. ReCiPe is adopted following the conclusion from a previous study (Dong et al., 2013a). ReCiPe is able to calculate results in both the midpoint and endpoint levels. Eighteen impact categories are provided in the midpoint version of ReCiPe. Characterization and normalization are available in the midpoint version. In the endpoint version, damage assessment is given at characterization, normalization and weighting. In addition, a single score is calculated to represent the total environmental performance. 3. MODEL DEVELOPMENT Environmental modelling of construction (EMoC) is designed to cover the cradle-tosite processes related to construction. The operation, maintenance and demolition stages are excluded. The model studies four groups of on-site activities, including ground work, concrete work, masonry work, and other work. The following items related to the construction activities are estimated in EMoC: material, transportation, energy, labor, equipment, and waste. Precast and cast-in-situ concrete methods are separately evaluated so that users can understand which concrete construction method is more environmental friendly. The activities and elements within EMoC are illustrated in Figure 1. The model established in Microsoft Excel composes of eleven worksheets as given in Figure 2. Users can enter data in the Input worksheet and obtain the results in the Result worksheet. Calculation is conducted in two worksheets: Concrete and Other Work. The construction activities of ground work, masonry work and other work are incorporated in the worksheet of Other Work. Background data is obtained from published research, LCI database, government report, manufacturer website, as well as field surveys. 4

5 Figure 1: Schematic illustration of the processes and elements considered in EMoC model Figure 2: Model structure of EMoC. Arrows represent data flow The LCIA results of individual products are generated from Ecoinvent database in SimaPro (a LCA software program) and utilized as background data. Materials evaluated in EMoC account for over 80% of the materials used in a general building project according to EMSD (2006). The LCI of precast concrete elements in the Hong Kong context is obtained from a previous research (Dong et al., 2013b). The emission standard can be chosen for trucks with three tiers of emissions, Euro III, Euro IV and 5

6 Euro V for various types of trucks as well as passenger cars being included in the model. Three energy types are evaluated in EMoC: electricity, gasoline and diesel. The fuel mix of electricity generation in Hong Kong is also provided in the model. For gasoline and diesel, the environmental impacts due to manufacture and combustion of fuels are separately calculated so that users can estimate the on-site emissions due to fuel combustion. The energy consumed by equipments is calculated by referring to a list of plants obtained from website of manufacturers. Three waste treatment methods of construction waste: recycle / reuse, landfill for noninert waste and public fill for inert waste have been considered in the model. Users can select if a material is recycled or not in the Input worksheet. The recycled or reused wastes are not responsible for any environmental impact as it is assumed that the impact is allocated to the next usage stage outside the system boundary. On-site dust emission is further studied in addition to the particulate emission as calculated in LCIA results in the Background Data worksheets. Users can choose from the drop-down list among the options of No control, Partially controlled and Highly controlled. The corresponding total suspended solid (TSP) value will then be provided in the Result worksheet and users can understand the dust control level of the studied project based on the results. 4. CASE STUDY The studied construction project is a public rental housing (PRH) project developed by the Hong Kong Housing Authority (HKHA). The project is to provide about 13,300 flats for 34,000 residents. The PRH project implements several green techniques, such as the reuse of marine mud, light emitting diode (LED), rainwater recycling system, precast components, etc. Precast components in this project accounts for about 35% of the concrete volume. The precast components include façade, bathroom, refuse chute, slab, etc. The environmental impact of the PRH project is evaluated in EMoC. The input data is collected through a questionnaire survey and entered into EMoC by the authors. The results are then generated in the Result worksheet of EMoC. The midpoint results are summarized in Table 1. Ng and Kwok (2013) studied the carbon emission at Kai Tak Site 1A and the value is 544 kg CO 2 eq per GFA (m 2 ). The larger value of 631 kg CO 2 eq per GFA (m 2 ) in Table 1 is due to the extra processes and materials being considered in EMoC. As the characterization value is hard to interpret, the normalization results are further provided. The normalization results are calculated as a ratio of the characterization result to intervention caused by one person in reference year. For example, a person generates about 6,891 kg CO 2 eq in 2000, thus the carbon emission of 1 m 2 GFA of the studied PRH project is equivalent to about one month emissions due to one person 1. By using the endpoint method, the single score of the project is 74 per GFA (m 2 ), mainly contributed from damage to human health (46) and resource depletion (24). 1 The value of 6,891 kg CO 2 eq per capita includes emissions due to manufacturing, construction, and other human activities that can generate greenhouse gases. Physiologically, a person generates about 1 kg CO 2 per day. 6

7 Table 1: Midpoint results of the PRH project Impact category Unit per GFA (m 2 ) Characterization Normalization Climate change kg CO 2 eq Ozone depletion kg CFC-11 eq 4.15E Human toxicity kg 1,4-DB eq Photochemical oxidant formation kg NMVOC Particulate matter formation kg PM10 eq Ionizing radiation kg U235 eq Terrestrial acidification kg SO 2 eq Freshwater eutrophication kg P eq Marine eutrophication kg N eq Terrestrial ecotoxicity kg 1,4-DB eq Freshwater ecotoxicity kg 1,4-DB eq Marine ecotoxicity kg 1,4-DB eq Agricultural land occupation m 2 a Urban land occupation m 2 a Natural land transformation m Water depletion m Metal depletion kg Fe eq Fossil depletion kg oil eq CONCLUDING REMARK A novel assessment tool namely the Environmental Modelling of Construction (EMoC) is developed to cover the cradle-to-site processes and estimates material consumption, energy usage, transportation, and waste treatment. The model is mainly based on the context of Hong Kong and China, while the coverage of regions can be potentially expanded by considering the fuel mix of electricity generation in other areas. A test case is provided in this study and the results are consistent with previous research. The advantages of EMoC include: its ability to estimate the environmental impact of the precast and cast-in-situ concrete methods; the possibility to consider several waste treatment approaches; a separate estimation on manufacturing and combustion of fuels; an utilization of local concrete inventory; a comprehensive coverage on construction materials; an analysis on eighteen impact categories; the implementation of both midpoint and endpoint methods; the implementation of newly developed LCIA method ReCiPe ; and a detailed breakdown of results. 7

8 While EMoC is an up-to-date model that provides holistic evaluation on the environmental performance of construction projects, it can be implemented in the early stage of a construction project for selecting a more environmental friendly design option. ACKNOWLEDGEMENT The authors would like to thank the Hong Kong Housing Authority for their support on this research. The financial support of The University of Hong Kong through the CRCG Seed Funding for Basic Research (Grant Nos.: and ) should be gratefully acknowledged. REFERENCES Bilec, M. M., A Hybrid Life Cycle Assessment Model for Construction Processes. Thesis (PhD), University of Pittsburgh. Brundtland, G. H., World commission on environment and development. Our common future. Oxford University Press, Oxford. Dong, Y., Analysis of stratification and algal bloom risk in Mirs Bay. Thesis (M.Phil), The University of Hong Kong. Dong, Y. H., Ng, S. T. and Kumaraswamy, M. M., 2013a. Critical analysis of the life cycle impact assessment methods. Environmental Engineering and Management Journal. (In press) Dong, Y. H., Wong, C. T. C., Ng, S. T. and Wong, J. M. W., 2013b. Life Cycle Assessment of precast concrete Units. International Conference on Civil, Environmental and Architectural Engineering, Madrid, Spain, March. EMSD, Consultancy Study on Life Cycle Energy Analysis of Building Construction. Guggemos, A. A., Environmental Impacts of On-site Construction Processes: Focus on Structural Frames. Thesis (PhD), University of California, Berkeley. HKHA, Life Cycle Assessment (LCA) and Life Cycle Costing (LCC) Study of Building Materials and Components. Huntzinger, D. N. and Eatmon, T. D., A life-cycle assessment of Portland cement manufacturing: comparing the traditional process with alternative technologies. Journal of Cleaner Production, 17, IPCC, The Physical Science Basis. Cambridge University Press, Cambridge. ISO, International Standard. In: Environmental Management - Life Cycle Assessment - Requirements and Guidelines. Geneva, Switzerland: International Organisation for Standardisation. Jaillon, L. and Poon, C. S., The evolution of prefabricated residential building systems in Hong Kong: A review of the public and the private sector. Automation in Construction, 18, Ng, T. K. and Kwok, S. M., Carbon emission estimation - a design verification tool for new public housing developments in Hong Kong. HKU-HKHA International Conference 2013, Hong Kong, 2-3 May. 8