A Comparative study of housing life-cycle carbon emissions for the characteristics of structural materials

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1 A Comparative study of housing life-cycle carbon emissions for the characteristics of structural Ji-yeon Park 1, Chang-u Chae 2, Sung-hee Kim 3 1 Korea Institute of Civil Engineering and Building Technology, Republic of Korea 2 Korea Institute of Civil Engineering and Building Technology, Republic of Korea 3 Hanyang University, Republic of Korea Abstract: Buildings composed of various construction and components manufactured through various processes, induce enormous environmental pollution, such as global warming throughout the entire life cycle from production to disposal stages. In particular, cement, concrete and steel which are major construction constituting a building, are regarded as typical CO₂ generating and it is necessary to develop an alternative to those to reduce greenhouse gas. In the assessment, the LCCO₂ technique as an evaluation tool for life-cycle CO₂ emissions is adopted for the comparative analysis of the environmental impacts. Through the comparative analysis of the houses based on the evaluation results, this study attempts to identify potential carbon emission reductions from the reuse and reduction of housing and propose ecofriendly development potentials in the housing market. Low Cabon Technology, Life Cycle Assessment, Green Building, Building 1. Introduction 1.1. Background of the study While multi-faceted efforts for greenhouse gas reduction are being carried out worldwide, environmental measures have been advanced from various sectors in Korea to reduce greenhouse gas emissions.the greenhouse gas emissions in domestic construction sector accounts for about 40% of total emissions for material production and building management and various efforts for developing technologies are also being accelerated to reduce the emissions. In particular for building, concrete, cement, and steel are regarded as typical CO₂ generating and it is necessary to develop low-carbon or an alternative to those to reduce greenhouse gas. Also, for the sustainable and eco-friendly development of the construction sector, efforts should be made for eco-friendly material and technology development to reduce its environmental impacts Necessity and purpose of the study This study intends to analyze modular houses that tend to increase in a compact housing market and conventional concrete houses from the perspective of LCA, and compare their environmental impacts generated throughout whole life cycle. A modular house is an industrialized house that can be mass-produced in the form of a unit in a factory and 1

2 prefabricated on site for being produced in large quantities for a short period. The modular house is designed with a prefabricated structure, allowing recycling and reuse of building through the assembly and dismantling of its structural members to reduce waste material. Besides, it has a number of advantages over conventional construction types including cost savings and site personnel reduction and a wide variety of configuration and styles in the building layout as a result of the lightweight construction and large supply of modular houses. Also, the modular house is expected to be effective in reducing and energy consumption compared to conventional construction types such as reinforced concrete structures, eventually reducing greenhouse gas emissions and the environmental impact. Through the comparative analysis of the houses based on the evaluation results, this study aims to identify potential carbon emission reductions from the reuse and reduction of housing and to propose eco-friendly development potentials in the housing market. 2. Life cycle assessment (LCA) methodology for building 2.1. Overview of LCA methodology for buildings In the assessment, environmental impacts are evaluated by comparing reinforced concrete structure houses to modular houses from the perspective of LCA. As an evaluation tool for environmental impacts of environmental burden that occur in the life cycle of products and systems, LCA is intended to evaluate quantitatively potential impacts associated with inputs and outputs generated from the life cycle of a product on the environment in the production, construction, use and disposal stages.among the tools available to evaluate environmental performance, LCA provides a basic structureand principle to evaluate environmental performance and impacts on sustainable development by considering the potential environmental impacts from all stages of manufacture, product use and end-of-life stages, and as a result, to establish criteria for evaluating the environmental impacts in the building life cycle LCA methodology A building LCA includes setting a functional unit for a product system, providing a reference flow to which the inputs and outputs can be related, and system boundaries to define the scope of data collection for the product. An LCA starts with an explicit statement of the goal and scope of the study, and then sets the functional unit and reference flow. The functional unit quantifies the function and scope delivered by the product to be assessed, setting the amount of product required for conducting the assessment. Further, the functional unitis set in consideration of the life time and performance of a target and the end user's perspective. Reference flow is also taken into account to set the functional unit for the comparative evaluation. The reference flow is determined by the final outcome necessary for the functional unit by identifying the process flow of a building. To develop a LCA, system boundaries for the life cycle of a product is to be defined. A clear definition is required for the system boundaries to be assessed and a clear explanation is required for the portion not to be assessed. The system boundary for the LCA of a building comprises all unit processes from 2

3 the entire life cycle to be evaluated, including raw material extraction, transportation and product manufacturing stages. The unit process included in a system boundary is referenced based on the inputs and outputs in a system boundary. Because raw, products and energy that are inputted vary with the unit process of the life cycle, data categories are chosen to collect input and output data. If recycled reusable are to be reintroduced to the process within a system boundary, reuse recycling processes are included in the system boundary for evaluation. Using the corresponding LCI DB module on the basis of the calculation formula of the data, collected data are multiplied by a greenhouse gas emission factor to calculate the greenhouse gas emissions for each data set. Carbon dioxide emissions per data to be calculated include the amount of carbon dioxide emissions in each stage of the life cycle and the one emitted during the life cycle of the building. 3. Overview of assessment targets and assessment methods 3.1. Overview of assessment targets The buildings to be compared include a reinforced concrete apartment house and a modular house. The reinforced concrete apartment house uses a electric boiler heating system in 98 square meters with a 4-person household. The modular house with for a 1-person household uses an electric boiler heating system in a unit type prefabricated structure. Type R.C. Apartment Housing Modular Housing Building type Apartment Building Modular Unit Area 98m2 22m2 Number of occupants 4 persons 1 person Heating system electric boiler electric boiler Heating energy usage 54 kwh/m2 year 49kwh/m2 year Table 1 Overview of target buildings 3.2. Assessment methods For the houses to be compared that differ in housing area and number of occupants, the basis of comparison for analysis results is required. For the LCA analysis in this study, greenhouse gas emissions during the life cycle are analyzed from one household house. In the case of a modular house, because the life-span of a house varies with the used, it is difficult to define the life cycle of the house. Thus, in consideration of the service life of a concrete building, the life cycle was set to 30 years. The reference flow was set for the analysis on the amount of energy and that are inputted in the production, construction, use, and disposal stages of a one household residential building to be used for 30 years. The system boundaries for the reinforced concrete house and modular house are set differently in the production and construction stages of LCA, For the reinforced concrete house, the production stage includes material production and all the processes before are delivered to the construction site after processed, and the construction stage 3

4 comprises the processes in which are transported to the construction site and a house is built. The processes of house repair maintenance and energy usage by residents are included in the use stage. The disposal stage is associated with the amount of energy of equipments caused by house dismantling and waste material transportation, and all the processes for the reuse of waste, recycling, incineration and landfill. To conduct a LCA, the data of the raw and transportation of the life cycle is calculated by setting a scenario. Data is calculated within the range that does not affect the evaluation results. The scope of data collection is typically determined by 95% or 99% of the cumulative mass contribution. For this assessment, the corresponding to 99% of the cumulative mass contribution are used for data calculation. For this assessment, the top-down approach is chosen to determine the data values by reducing the unit of comparison targets. For the criteria that can compare by quantifying targets, data is compared by setting a comparison unit for area and number of occupants, eventually, for emissions per household, unit area, and person. 4. Analysis of life cycle carbon emissions 4.1. Life cycle CO emissions The analysis results of the life cycle total emissions showed the modular house generated emissions of 39,375kgCOeq when using general and 37,976kgCOeq when using reusable recycled. For the reinforced concrete house, the emissions were 113,321kgCOeq from general and 90,798kgCOeq from reusable recycled. Modular house using general Modular house using reusable recycled Reinforced concrete house using general Reinforced concrete house using recycled Type kgcoeq/household Emissions kgcoeq/m2 kgcoeq/person Production stage 6, , Construction stage Use stage 32,820 1,492 32, Disposal stage Production stage 5, , Construction stage Use stage 32,820 1,492 32, Disposal stage Production stage 233,475 2,382 58, Construction stage 6, , Use stage 197,505 2,015 49, Disposal stage 16, , Production stage 143,383 1,463 35, Construction stage 6, , Use stage 197,505 2,015 49, Disposal stage 16, , Table 2 Total carbon emissions by stage during life cycle 4.2 Comparative Analysis % Total emissions kgcoeq/person 39,375 37, ,321 90,798 4

5 From the perspective of LCA, the carbon emissions of a reinforced concrete house and a modular house were compared and analyzed for each stage, based on the life cycle of 30 years and one household. Life cycle greenhouse gas emissions were analyzed and eventually a comparative analysis was applied by dividing into emissions per unit area and per person. Type Modular house using general Modular house using reusable recycled Reinforced concrete house using general Reinforced concrete house using recycled kgcoeq/ household Reductions against RC house using general kgcoeq / m2 Total emissions Reductions against RC house using general kgcoeq/ person Reductions against RC house using general 39, ,908 1,790 2,835 39,375 73,945 37, ,307 1,726 2,899 37,976 75, , , , ,191 90,092 3, ,798 22,522 Table 3 Comparison of carbon emissions during life cycle (1) Comparative analysis of emissions per household The comparative analysis of life cycle carbon emissions showed that the total carbon emissions of the modular house was 39,375kgCOeq per household when using general and 37,976kgCOeq when using reusable recycled. The reinforced concrete house using general showed the total emissions of 453,283kgCOeq, indicating the modular house was more effective by more than 80% in reducing the emissions than the reinforced concrete house. Further, the emissions from the reinforced concrete using reusable recycled were reduced by more than 20%, compared to the reinforced concrete house using general. (2) Comparative analysis of emissions per unit area The analysis results of the life cycle total emissions per unit area for 30 years showed the reinforced concrete house using general generated emissions of 4,625kgCOeq. Also, the emissions from the modular house were 1,790kgCOeq when using general and 1,726kgCOeq when using reusable recycled. It is also shown that the total emissions of the modular house were reduced by approximately 60% compared to the reinforced concrete house. (3) Comparative analysis of emissions per person The comparative analysis of life cycle carbon emissions per person showed that the total carbon emissions of the reinforced concrete house was 113,321kgCOeq per person when using general and 90,798kgCOeq per person when using reusable recycled. The modular house showed the total emissions of 39,375kgCOeq per person for using general, and 37,976kgCOeq for using reusable recycled. This indicates the modular house with reusable recycled was more effective by approximately 66% in reducing the emissions than the reinforced concrete house. Further, the emissions from the reinforced concrete with reusable recycled were reduced by approximately 20%, compared to the reinforced concrete house with general. 5. Analysis of assessment results 5

6 From the perspective of LCA, the carbon emissions of a reinforced concrete house and a modular house were compared and analyzed in each stage. Depending on the assessment method, the life cycles of 30 years for two houses to be compared were divided into production, construction, use and disposal stages to collect data, and the emissions were calculated by using an emission factor. From the results regarding major environmental impact factors in the life cycle of each evaluated house, it was analyzed that the outputs of concrete in the production and construction stages of the reinforced concrete house, gas usage by heating in the use stage and the amount of recycled waste concrete in the disposal stage were major environmental impact factors. For the modular house, the amount of steel frame and plasterboard was regarded as a major environmental impact factor in the production stage, but the emissions were significantly reduced when reusable and recycled steel frames were used. In the use stage, electricity usage by using an electric furnace, and the carbon emissions due to the landfill of in the disposal stage were regarded as major environmental impact factors. However, it was analyzed the reduction of outputs due to the reuse of in the disposal stage exerted the greatest effect on the reduction of emissions. It is shown that the modular house using general can reduce the emissions by 413 tons over 30 years, compared to the reinforced concrete house with general, and the modular house with reusable recycled can reduce the emissions by 415 tons. The modular house with reusable recycled is shown to be more effective in reducing 415 tons of greenhouse gas emissions over 30 years than the reinforced concrete house with general, which is equivalent to the amount of carbon dioxide that 28, years old pine trees absorb for one year. When using these modular houses for 90 years, it is expected that 1,200 tons of greenhouse gas emissions can be reduced, as compared to reinforced concrete houses with general. Therefore, by the improvement of the performance of modular houses resulting from future performance development of modular houses and the introduction of energysaving technologies, potential reduction is expected to be larger. 6. Conclusions In this study, to reduce greenhouse gas in the life cycle of a building, we analyzed the environmental performance due to building by comparing and assessing the life cycle of a reinforced concrete house and a modular house to provide the direction of ecofriendly development of building. A LCA was applied to the reinforced concrete house using general concrete and a recycled material of slag concrete, and the modular house using general and reusable recycled, to compare their environmental impacts arising from the use of reusable recycled. The analysis results of life cycle carbon emissions per person over 30 years showed that the total carbon emissions of the reinforced concrete house was 113,321kgCO₂eq when using general and 90,798kgCO₂eq per person when using 6

7 reusable recycled. Further, the emissions from the use of a recycled material of slag concrete were shown to decrease by approximately 20%, compared to the use of general. The modular house showed the total emissions of 39,375kgCO₂eq per person over 30 years for using general, and 37,976kgCO₂eq for using reusable recycled, resulting in a reduction of carbon emissions by 1.4 tons. This indicates the modular house with general was more effective by approximately 65% in reducing the emissions than the reinforced concrete house. Factors affecting the emissions of greenhouse gases appeared slightly different in the two types of house. The use of concrete and rebar in the production of the reinforced concrete house and gas usage by heating in the use stage were shown to be major environmental impact factors. For the modular house, steel frame and plasterboard in the production stage and electricity usage in the use stage were regarded as major environmental impact factors. Difference in the heating system in the use stage was also found to affect the difference in greenhouse gas emissions. It is also found that the reuse or recycling of building concrete and steel is capable of reducing greenhouse gas emissions in the production stage. Further, modular houses are expected to contribute more actively to environmental protection and resource saving compared to existing reinforced concrete houses, by reducing the emissions and relatively reducing energy consumption in the production stage from the environmental aspect related to the response to climate change. However, there is a weakness of modular house, such as fire safety performance and durability as compared to conventional reinforced concrete buildings, and it is necessary to solve the technical challenges including noise and indoor environment to address the need for technology development associated with performance improvement. As a result, it is needed to consider the development of a building that can reduce the environmental impact by applying the modular housing method to slag concrete and complementing its performance and safety. In future, ongoing technology development and research of building complemented with environmental advantages are essential to cope with climate change and reduce an environmental impact. References Seol Dongpil, (2012), A study on the efficiency of housing supply policy with increasing of 1~2 person household, Seoul Kim Gyuntae et al, (2011), Scenario development of Korean modular house construction, Architectural Institute of Korea Heo SungHoon et al, (2011), A study on the small sized housing supply according to the changes in family composition, Architectural Institute of Korea Heo Tak et al, (1995), Principle of life cycle assesment, Korea employer s federation 7