Analysis of CO2 Emission Reduction Effect of On-Site Production Precast Concrete Member according to Factory Production Environment
Precast concrete (PC) method of construction is preferred for excellence in the reduction of construction period, lightweight, and durability and for PC member to be mostly transported to a site after its production in the in-plant production because the in situ production of the PC member is negatively perceived because of the limitation of space or production process being complex and difficult. However, if the PC member is produced on site and installed, it is possible to reduce the carbon dioxide emissions that are generated during shipping and loading and unloading, which are indirectly required for in-plant production. Carbon dioxide emission reduction effect due to the difference between the in situ production and in-plant production process of the PC member was confirmed by the existing studies, but the study of the carbon dioxide reduction effect according to various production environments of the in-plant production has not been performed. Therefore, the purpose of this study is to analyze the CO2 emission reduction effect of the PC member produced in site according to the in-plant production environment. As a result, it was found that when PC members were produced on site, there was an effect of reducing CO2 emissions by an average of 25.64% compared to factory production. In future, the results of this study will be used as basic data for establishing a CO2 emission reduction plan at construction sites.
1.1. Research Background and Objective
Carbon dioxide emissions are the major cause of global warming, accounting for 80% of the world’s greenhouse gas emissions designated by the United Nations Environment Programme and World Meteorological Organization in 1985 . The Ministry of Environment’s “2019 National Greenhouse Gas Inventory Report” shows that carbon dioxide emissions in Korea rose steadily from 1990 to 2017, along with industrial development . As a result, Korea ranked 11th in carbon dioxide emissions. To preserve the environment, in 2015, Korea set a goal of 37% reduction compared to emissions prospect in the year 2030 (Business As Usual (BAU)) . Construction industry is one of the industries that have a significant impact on the environment due to the high consumption of energy and raw materials, which is the requirement of the industry .
The resources consumed by the construction industry among the entire industry fields account for 40% of energy consumption, 48% of raw material consumption, and more than 42% of carbon dioxide emissions . For the preservation of the environment and prevention of global warming, efforts are needed to reduce energy consumption, raw material consumption, and carbon dioxide emissions in the construction industry that has a negative impact on environmental destruction [6, 7].
CO2 emissions continue to increase worldwide. In particular, in the recent construction field, large-scale buildings such as logistics centers are rapidly increasing. Most of these large buildings are built using the precast concrete (PC) method. However, in general, PC methods are produced in factories and transported to the site . In order to reduce CO2 emissions generated in this process, the on-site production of PC members is essential. In other words, in terms of eco-friendliness, if the PC member is produced in the field, CO2 emission generated during the transportation process can be reduced.
The installation of precast concrete (PC) after in situ production accounts for more than 14.3% of CO2 reduction compared to in-plant production. At this time, Lim and Kim  claimed that transportation of PC members makes difference in the production of PC members because in situ production is processed in the same way as in-plant production and selected a case site to support the claim. After producing 72 PC columns in the site, the difference in CO2 emissions compared to in-plant production was calculated, which led to the confirmation that in situ production was more advantageous in terms of CO2 reduction. However, the study did not report the difference in CO2 generation according to the environment of in-plant production of PC members and manpower. Therefore, it is necessary to study the CO2 emission reduction effect analysis, including the aforementioned items.
1.2. Research Method
The purpose of this study is to prove the effect of reducing CO2 through the on-site production of PC members. For this study, the CO2 reduction effect is proved through the comparison of the production environment between the factory and the construction site. Also, the CO2 emission reduction effect was confirmed through calculating the CO2 emissions by the manpower and in-plant production environment that were not considered in the preliminary study of Lim and Kim . This study was performed as shown in Figure 1.
Initially, the CO2 unit calculation method, the examination of Life Cycle Inventory (LCI) analysis database, and the preliminary study were reviewed. Then, the CO2 emission in the case site of Lim and Kim  was analyzed, and the CO2 emission generated by the manpower was calculated. They proved the CO2 reduction effect of on-site production on actual sites. However, they only analyzed the CO2 reduction effect only for the manpower deployed to the field. Rather, it is correct to analyze the effect of CO2 reduction caused by changes in the production environment of factories and sites. Therefore, in this study, the environmental factors of in-plant production were identified and the CO2 emission was calculated. Later, the calculated CO2 emissions of in situ production and in-plant production were compared and analyzed and the CO2 emission reduction effect was confirmed. Finally, evaluation and conclusion of the CO2 emission analysis results were drawn.
2. Preliminary Study
2.1. Energy Consumption and CO2 Basic Unit Calculation Method
There are survey-based approach, input-output analysis (indirect estimator), and hybrid approach as methods for calculating energy consumption and CO2 basic unit . The method used typically is input-output analysis . The comparison of these methods is as follows.
2.1.1. Survey-Based Approach
Survey-based approach is a method of surveying carbon dioxide emissions and energy consumption generated at each stage, such as the collection, transportation, and processing of natural resources using materials and energy used in the building life cycle. In the case of the survey process, the system of the analysis target is dismantled and, then, each stage is examined and evaluated comprehensively. The scope of the survey includes raw materials, energy, and materials used for the analysis target. Additionally, it is necessary to conduct a site survey of all the processes from the construction, maintenance, and dismantling stage of the building construction process [11, 12].
2.1.2. Input-Output Analysis
Input-output analysis is an analysis using an input-output table. The input-output table shows the cross-trading between the goods and service industries in the unit of one year regarding national economy. Direct and indirect correlations between industries as a unit of goods and services can be identified in the table . In Korea, the Bank of Korea drafts an input-output table on a yearly basis. It is a method of quantitatively identifying the input and emission factors generated in the trading process, calculating the mutual goods and services between each industry in monetary units .
2.1.3. Hybrid Approach
Hybrid approach is a method of calculating a basic unit by combining the survey-based approach with input-output analysis . The survey-based approach has difficulty in examining the simultaneous input of various materials. This can be supplemented using input-output analysis. Additionally, the survey-based approach is used for building materials and material and energy consumption that have a large impact on the environment load, while analyzing the detailed items of input-output analysis. A combination of the two methods is used to analyze each stage of the industry and increase the accuracy and reliability [16, 17].
2.2. Life Cycle Inventory Database
Life Cycle Inventory Database (LCI DB) is a collective data of waste and emission generated from raw materials and energy resources needed in the production of materials. LCI DB is managed and provided worldwide, including the United States, Europe, and Australia. In Korea, the Korea Institute of Environmental Industry and Technology provides LCI DB developed by the Ministry of Environment and the Ministry of Industry and Trade. The entire production stages of the product are classified into the material and component manufacturing, processing process, transportation, and disposal, listing a total of 438 data [18–20].
2.3. Carbon Dioxide Basic Unit Calculation Structure
The CO2 basic unit construction structure by work classification is calculated with the sum of CO2 basic units in equipment, manpower, production, and transportation. The output structure is the same as Figure 2.
The CO2 basic unit for the comprehensive work consists of the sum of the material CO2 unit and the construction CO2 basic unit. The construction CO2 basic unit here is a combined value of construction CO2 basic unit and CO2 basic unit for equipment operation by manpower. Material CO2 basic unit is calculated using the sum of the CO2 basic unit generated in the process of material production and the CO2 basic unit generated during the transportation of the materials. In this study, the CO2 basic unit for equipment, manpower, production, and transportation is calculated since the working CO2 unit is calculated [21, 22].
2.4. Review on the Preliminary Study of the Existing Carbon Dioxide Emissions
Many preliminary studies have been performed to calculate CO2 emissions during the stage of construction by analyzing the CO2 emitted from the materials used in the construction by means of the survey-based approach, input and output analysis, and hybrid approach [23–26]. Recently, the study related to the CO2 emissions of the buildings was conducted using PC members. Among them, the study of Lim and Kim  is a very important study that selected a case and proved the effect of reducing CO2 due to the on-site production of PC members. However, only CO2 reduction effects according to manpower input were analyzed, and changes in the production environment were not considered. We analyze in more depth the CO2 reduction effect of changes in the production environment based on Lim and Kim’s  research. Lim and Kim  selected a case site, PC columns were produced through in situ and in-plant productions, and the CO2 emissions were comparatively analyzed. As a result of the study, 211 T-CO2 (22.6%) of CO2 emission reduction effect was calculated by 72 PC columns made through in situ production, 641 T-CO2 (22.8%) by 240 PC columns, and 2,647 T-CO2 (15.7%) by 982 PC columns. The in situ production of the PC column was confirmed to be an eco-friendly way compared to in-plant production based on the calculation. To supplement this, the study on the CO2 reduction effect includes the CO2 emissions caused by manpower and the environment of in situ production.
3. CO2 Emissions Based on Precast Concrete Member Production Method
3.1. CO2 Emissions by Production, Transportation, and Equipment
According to the study results of Lim and Kim , only the amount of transportation equipment for transporting materials and equipment used in the construction stage was used to analyze the emission rate to compare and analyze the CO2 emissions . Data for CO2 emission calculation were obtained from 28 constructions sites, and the proposed CO2 unit was used . The CO2 emissions generated from the production, transportation, and equipment phases obtained from the study are shown in Table 1. It was confirmed to have the same amount of CO2 emission in the result of calculation, production, and equipment. However, the transport phase showed different results, which is because the completed PC columns were produced immediately without the need of transportation for in situ production .
3.2. CO2 Emissions by Manpower
In the site that was subjected in the study of Lim and Kim , manpower input volume was the same because the production process was proceeded equally to compare the PC column productivity of the plant and that of the site . At this time, for the process of producing one PC column, it took 3 days (8 h of work time per day) based on the formwork assembly, reinforcement, concrete pouring, curing, and follow-up work. Additionally, the manpower involved in the production of the PC column was 30 people in total. Thus, CO2 emissions by manpower appear differently for men and women. However, the manpower of the case site of Lim and Kim  was mostly male. Thus, the study also did not take into account the CO2 emissions of women. To calculate the CO2 emissions, the CO2 emission of 549.4 mg/min per male person under 25°C was applied .
CO2 emissions calculated by the manpower are shown in Table 2, i.e., the CO2 generated by the manpower is 1.71 t-CO2 in the case of producing 72 columns, 5.70 t-CO2 in the case of producing 240 columns, and 23.31 t-CO2 in the case of producing 982 columns.
3.3. CO2 Emissions per Precast Concrete Column by Production Items
By adding CO2 emissions from manpower to the preliminary study results, the results of CO2 emissions calculated in PC column production items are the same as Table 3. The results of CO2 emissions in the number of 72 PC, 240 PC, and 982 PC columns are 759.71 t-CO2, 2,226.70 t-CO2, and 14,493.31 t-CO2, respectively, in the case of in situ production and 937.71 t-CO2, 2,818.70 t-CO2, and 16,914.31 t-CO2, respectively, in the case of in-plant production. It was confirmed that the most CO2 emissions were from the production stage using the material directly; the lowest CO2 emissions were the manpower.
3.4. In-Plant Precast Concrete Production Environment
In the case of in-plant PC column production and in situ production, the CO2 emissions were the same in the stage other than transportation . This is because when producing a PC column, the material and direct energy are equally consumed in the production stage. However, in the case of in-plant production, electricity and oil are consumed indirectly for the operation of the plant. Additionally, there are various factors necessary for production, such as plant maintenance and plant facility operation for plant operations . Thus, CO2 is generated according to the in-plant production environment.
In this study, based on the cost statement provided by the Korean “Daegu New Technology Platform,” the amount of CO2 generated by the in-plant production environment was calculated. The reason why it was used as the calculation criteria was because the same PC method was used in this study, and “precast concrete beams for underground parking using a nontension lecture line-column nonseismic junction integrated method” in the same manner of the PC member production method of the study, which makes it possible to apply the original cost statement. Additionally, it was calculated based on the percentage of the amount calculated in the “2020 Construction and Industrial Environment Facilities Corporation Costing Ratio Application Criteria” of the Korean Public Procurement Agency [34, 35]. In this study, each amount ratio was applied to the CO2 emissions suitable for the criteria, and the contents calculated were CO2 emissions, as discussed in Sections 3.5 and 3.6.
3.5. CO2 Emissions from In-Plant Production Operations
The elements required in the plant to produce a PC member are divided into three parts, namely, materials, fuel consumption, and power consumption required for production. The material elements required for production were excluded from this topic because they are the same in both in situ and in-plant productions. Fuel consumption factors include oil, natural gas, and coal, and the power consumption factor is motor, fan, dryer, and burner . Therefore, the items of plant indirect expenditure related to this were classified into five categories: electricity, water, lighting, and heating, repair, supplies, and environmental conservation . To obtain the expense of each item, a cost statement provided by the Korea Daegu New Technology Platform was applied.
The cost of electricity is the cost of operating the in-plant machinery to produce a PC member and electrical energy is required. Electricity is 6.47% of the direct material cost. The water, light, and heat costs refer to the sum of the costs of electricity, gas, and water used in the office of the plant. The water, light, and heat costs amount to 2.98% of the direct material cost. Repair costs are the cost of the internal structure of the plant and equipment movement, which varies depending on the absence of the in-plant production PC. The cost was ∼2.47% of the direct material cost. In addition to direct materials for PC member production, the cost of the material is not reused as a material used for the production of the product. The consumption cost required was 2.00% of the material cost directly. Finally, the environmental conservation cost is the cost of installing and operating the facility used to prevent environmental pollution in the plant . The environmental conservation cost was 0.06% of the sum of direct labor costs, direct material costs, and expenses.
If we calculate the itemized CO2 emissions proportional to each percentage of the amount, it is the same as outlined in Table 4. CO2 emissions according to the number of PC columns 72, 240, and 982 were found to be 83.39 t-CO2, 278.06 t-CO2, and 1,889.70 t-CO2, respectively.
3.6. CO2 Emissions of Indirect Manpower Factors for In-Plant Production
In addition to the production manpower, it is necessary to operate the plant during the production period. There are manpower and plant maintenance and office manpower for maintenance such as regular inspections and safety checks [39, 40]. This manpower requirement is an indirect manpower element for PC member in-plant production. To calculate the CO2 emissions accordingly, the indirect labor cost ratio of the Korean Public Procurement Agency was utilized. In the case of the site that was used for the study of Lim and Kim , it took ∼6 months of the construction period to produce a PC member. Thus, in the case of construction sites less than 6 months and less than 5 billion won, indirect labor costs were calculated by applying 8% of the direct labor cost. Here the CO2 caused by direct labor is a manpower CO2 for PC member production. Therefore, the emissions were calculated using the CO2 generated by the manpower. The calculated CO2 emissions are shown in Table 5. CO2 emissions according to the number of 72, 240, and 982 PC columns were found to be 0.14 t-CO2, 0.46 t-CO2, and 1.86 t-CO2, respectively.
3.7. CO2 Emissions by In-Plant Production Environmental Factors
When the PC member is made by in-plant production, the generated CO2 emissions were calculated through the indirect cost ratio. The output was calculated according to the CO2 unit calculation structure of a total of six items—electricity, water or thermal, repair, supplies, environmental conservation, and indirect labor costs—as shown in Table 6. The calculated CO2 emissions are 83.57 t-CO2, 278.56 t-CO2, and 1,891.57 t-CO2, and the item showing the highest emission ratio is electricity, depending on the number of PC columns 72, 240, and 982. Based on the number of 982 PC columns, the analysis of the total CO2 emissions by the in-plant production environment itemized ratio confirmed that the power cost has the most impact on CO2 emissions to 44.26%. Items with high CO2 emissions after the power cost were analyzed for water, light, and heat, repair, and environmental conservation costs. However, the least affected by CO2 emissions has been identified as the manpower, and the itemized ratio is 0.10%. CO2 emissions were also analyzed at the lowest value of 1.86 t-CO2.
4. CO2 Emission Reduction Effect through Precast Concrete In Situ Production
In this study, CO2 emissions of in situ production and those of in-plant production were compared to PC column production. Manpower and in-plant production environmental factors that were excluded from prestudy were included, and CO2 emissions from in situ and in-plant productions are the same as Table 7.
Considering item by item, the item that produced the largest amount of CO2 based on 982 PC manpower is the production stage, and the manpower is the item that produced the least CO2 emissions. Therefore, it is confirmed that the manpower is the item that has the least CO2 emissions, which is the same as shown in the result of in-plant production environmental factors. The reason why the in situ production of PC column shows low CO2 emissions compared to in-plant production is because there is no need of vehicle movement required for PC column transportation and there is no indirect factor that is consumed in in-plant management. Considering the total CO2 emissions of Table 7, it was calculated to be 759.71 t-CO2, 2226.70 t-CO2, and 14493.31 t-CO2 in the case of in situ production on the standard of the number of 72, 240, and 982 PC columns, respectively, and in the case of in plant production, it was calculated as 1021.34 t-CO2, 3109.05 t-CO2, and 18,805.88 t-CO2, respectively.
Through the calculated CO2 emissions, it can be seen that PC column in situ production emits less CO2 than in-plant production, and comparison of CO2 reduction in in situ production and in-plant production is shown in Figure 3. In the case of 72, 240, and 982 PC columns in number, CO2 emission reduction effects by quantity were confirmed as 261.63 t-CO2 (25.62%), 882.35 t-CO2 (28.38%), and 4312.57 t-CO2 (22.93%), respectively.
Therefore, more than 22.93% of CO2 emission reduction effect compared to in-plant production can be obtained in the case of in situ production of PC column. In this study, the calculation of the CO2 emission reduction effect through the in situ production of PC members is limited to the column. However, if applied to the in situ production of other PC members beside the column, much more CO2 reduction effects will be expected.
In this study, the carbon dioxide emission generated in the environment of the in-plant production of PC members was analyzed to compare CO2 emission of in situ production with that of in-plant production. Through the results of the analysis, the reduction effect of the CO2 emission was also analyzed according to the types of production. For the analysis on the CO2 reduction effect, the extra study targeting the site of prestudy was performed. The CO2 emission and the environmental factors of in-plant production regarding the shortage of manpower in the prestudy were analyzed, on which the calculation of the CO2 emission was based. In the case of the in situ production of PC column, it was confirmed through the calculated results that there was a reduction in CO2 emission compared to in-plant production.
First, the CO2 emissions of production, transportation, and equipment as elements that are directly required for PC column production in the construction site were examined. Then, to analyze the CO2 emissions by the manpower, the data of manpower that were put in for production and the amount of CO2 emitted per person per minute were utilized. As a result of the calculation, the emissions of 1.71 t-CO2, 5.70 t-CO2, and 23.31 t-CO2 according to columns 72, 240, and 982, respectively, were calculated. Also, regarding the environment of in-plant production, the factors that are indirectly required for PC column production were analyzed, and they were classified into items of power, fuel, and manpower consumptions, and CO2 emissions of the items were analyzed. The in-plant production environment CO2 emissions were calculated through CO2 directly emitted in the production of the PC column and the proportion of the amount required per item. As a result, the emissions of 83.52 t-CO2, 278.52 t-CO2, and 1,891.57 t-CO2 were yielded according to the number of columns 72, 240, and 982, respectively. Additionally, the highest CO2 emission ratio was the item of power cost as 44.26%, and the smallest percentage was confirmed to be the manpower as 0.10%. Finally, the total CO2 emissions of the calculated in-plant and in situ productions were analyzed. The difference of CO2 emission is 261.63 t-CO2, 882.35 t-CO2, and 4,312.57 t-CO2 based on the number of columns 72, 240, and 982, respectively, through which the CO2 emission reduction effect of 25.62%, 28.38%, and 22.93% was confirmed. The items for the production of PC column were classified into production, transportation, equipment, manpower, and in-plant production environment and, then, CO2 basic unit per item was calculated. Compared to the sum of CO2 basic unit per item estimated, the CO2 emission reduction effect of average 25.64% was confirmed compared to in-plant production in the case of in situ production. The results will contribute to the reduction of CO2 emissions through the in situ production of PC members and establish a CO2 emission reduction plan for construction sites.
Data sharing is not applicable to this article.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This research was supported by grants (NRF-2018R1C1B6004123 and NRF-2021R1C1C2091677) from the National Research Foundation of Korea by Ministry of Science, ICT and Future Planning.
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