Determination of the Goal and Scope

This article determines that the objective of LCA is to calculate the carbon emissions throughout the entire process of PCS, from the processing and production of prefabricated components to on-site construction and assembly. It follows the regulations of ISO 14040/14044 and selects the carbon dioxide emission equivalent as the environmental impact assessment indicator. The full life cycle of PCS is divided into six stages, namely the planning and design stage, prefabricated component production and processing stage, prefabricated component and building material transportation stage, on-site construction and assembly stage, operation and maintenance stage, and demolition and recycling stage. Since China’s PCS industry is still in its initial development stage, the PCS built earlier are just entering the operation and maintenance stage. Therefore, this paper focuses on exploring the carbon emission intensity and level of the PCS construction stage and has not yet covered the planning and design, operation and maintenance, and demolition stages. The system boundary of the PCS construction phase includes three subphases: (i) The processing and production stage of prefabricated components, including the embodied carbon emissions of prefabricated components such as columns, beams, slabs, and stairs, that is, the direct or indirect carbon emissions generated due to the consumption of resources and energy throughout the entire process from the mining, transportation, and processing of raw materials to the final production of prefabricated components such as columns, beams, slabs, and stairs. (ii) The transportation stage of prefabricated components and building materials, including the direct or indirect carbon emissions generated by the transportation of prefabricated components such as columns, beams, slabs, and stairs from the prefabrication plant or processing plant to the construction site and the fuel burning of transportation tools such as automobiles. (iii) The on-site construction and assembly stage, including the direct or indirect carbon emissions generated by the energy consumption during the on-site transportation of construction machinery, construction assembly of prefabricated components, and the use of machinery such as hoisting machinery and ac welder, as shown in Fig. 1. PCS has a wide variety of component types and different sizes. To eliminate the influences of unfavorable factors and facilitate further exploration of the emission reduction potential, this article selects the unit area (calculated as 100 m²) as the functional unit.

Carbon Emission Accounting Model

Prefabricated Components Processing Production Stage

The carbon emissions during the prefabricated component processing and production stage consist of two parts. One is the carbon emissions generated from the extraction, transportation, and processing of raw materials for component production, which is calculated by multiplying the usage or consumption amount of various building materials by the corresponding carbon emission factor and cumulative summing up. The calculation formula is as shown in (1). The other is the carbon emissions generated by machine tools and processing equipment during the process of processing and producing prefabricated components such as columns, beams, slabs, and stairs into prefabricated products, calculated through formula (2). The total carbon emissions of the prefabricated component processing and production stage, CE_P is the sum of CE_P1 and CE_P2. The carbon emissions generated from the transportation of raw materials for prefabricated component processing and production are included in the transportation stage of prefabricated components and building materials.

(1)

CEP1=∑inCi×(1+αi)×Ei

Fig. 1.

System boundary of PCS construction stage.

Where, CE_P1 is the carbon emission of prefabricated building materials, the unit is kgCO₂e. C_i is the usage amount or consumption of the i kind of building materials, the unit is m³, kg, m, m². α_i is the loss rate of the i type of building materials. E_i is the carbon emission factor of the i type of building materials, the unit is kgCO₂e/C_i. n is the quantity of building materials.

(2)

CEP2=∑j=1,k=1m,a(Cj,k×Ek+Fj×Ej)+Rj×Et

Where CE_P2 is the carbon emission in the production process of the precast component and the unit is kgCO₂e. C_j,k is the usage amount or consumption of the k kind of energy in the production of the j kind of precast component, the unit is kg, kW·h, L, m³. E_k is the carbon emission factor of the k kind of energy, the unit is kgCO₂e/C_j,k. m is the category of prefabricated components, including balconies, laminated beams, laminated panels, shear walls, stairs, etc. a is the category of energy, including diesel, gasoline, electricity, and water. F_j is the consumption of admixtures and admixtures required for the production of the j-type prefabricated components, the unit is t. E_j is the production carbon emission factor of the additive and admixture required for the i prefabricated components, the unit is kgCO₂e/t. R_j refers to the labor days required for the production of the j-type prefabricated components, the unit is d. E_t is the artificial average carbon emission factor in component production stage, the unit is kgCO₂e/d.

Life Cycle Data Inventory Analysis

This paper selects a representative prefabricated building project in Shenzhen as the research case. The above-ground building area of this project is 50,000 m², and the prefabricated components covered include types such as laminated beams, laminated slabs, stairs, shear walls, and balconies. The prefabrication rate is 50%, which is much higher than the prefabrication rate requirement in Shenzhen. Based on the bill of quantities for the project settlement, the consumption of various building materials for the production of prefabricated components is summarized from the bill of quantities for sub-projects and sub-items, as shown in Table 1. It can be seen from the consumption that steel bars, C30 concrete, steel plates, electricity, etc., are the resources or energies with large consumption in the production process of prefabricated components. Relying on the types of building materials in the bill of quantities, the carbon emission factors of building materials such as steel bars and C30 concrete are sorted out from lifecycle databases, Calculation Standard of Carbon Emission in Buildings (GB/T 51366-2019), publicly published journal papers and other materials, as shown in Table 1. The transportation tools and transportation distances are obtained through project research. The transportation vehicle for raw materials is a medium-sized diesel truck (with a load capacity of 8t and a carbon emission factor of 0.179 kgCO₂e/(t·km)), The transportation vehicle for prefabricated components is a heavy-duty diesel truck (with a load capacity of 30t and fuel consumption of 45L/100km). The distance between the construction site and the prefabrication factory is 100km. The carbon emission factors of prefabricated component transportation and transportation methods are shown in Table 2. The consumption of mechanical shifts in the on-site construction and assembly stage refers to the Shenzhen Prefabricated Building Consumption Quota (2016) and Shenzhen Construction Project Construction Machinery Shift Quota (2014), as shown in Table 3. It should be noted that since the data collection phase of the project did not consider the consumption of labor for prefabricated component production, transportation, and on-site construction personnel, the carbon emission intensity calculated in this paper did not take into account the environmental impact of labor.

During the on-site construction and assembly stage, the carbon emission factor for energy production and consumption is comprehensively considered, the data of which are derived from the journal paper published by Su Yuehua et al. in May 2022 in the Journal of Environmental Engineering [16], as shown in Table 4. Diesel generates carbon emissions both in the production and consumption stages, and the carbon emission factor data for the production stage are derived from the life cycle database. Electricity generates no carbon emissions in the consumption stage, only carbon emissions in the production process, using the benchmark emission factor of the Southern Power Grid of China.

The carbon emission factor data for the prefabricated components production stage, transportation stage, and on-site construction and assembly stage in this paper mainly come from the Calculation Standard for Building Carbon Emissions (GB/T 51366-2019), which is a national standard issued by the Ministry of Housing and Urban-Rural Development. It has authority and authenticity and can ensure the accuracy and reliability of the carbon emission factor. If there is no data in the national standard, it comes from authoritative academic journals and industry reports. If the research results are published in authoritative journals in related fields, the authors are usually experts and scholars in the field, and the data are all professional analysis and calculation results. Therefore, the data has a high degree of credibility.

Prefabricated Components Processing Production Stage

Using formulas (1), (2), and the material list and carbon emission factor in Table 1, the carbon emissions for each type of building material in the prefabricated component processing and production stage, calculated per unit area (assuming 100 m²), are shown in Fig. 2, while the carbon emissions for each type of prefabricated component are shown in Fig. 3. From Fig. 2 and 3, it can be seen that the carbon emission intensity per unit area of the prefabricated component processing and production stage is 5,215.82 kgCO₂e/100 m². According to the types of building materials, hot-rolled rebar has the highest carbon emission, reaching 2,607.07 kgCO₂e/100 m², accounting for 49.98%. Followed by C30 concrete, with a carbon emission of 1,775.78 kgCO₂e/100 m², accounting for 34.05%. Based on the high carbon emission characteristics of hot-rolled rebar and concrete, attention should be focused on the raw material processing processes of materials such as hot-rolled rebar and concrete and the production processes of prefabricated components. Especially for concrete, its constituent materials, such as cement, sand, stone, water, admixtures, and mineral admixtures, all have implicit carbon emissions to varying degrees. Timely attention should be paid to the carbon emissions in the mining and processing links of raw materials, and energy-saving and carbon reduction measures should be taken to solve the carbon emission problems brought about by the processing and production of downstream prefabricated components from the upstream industry.

From the perspective of component types, the carbon emissions of prefabricated shear walls are the highest, reaching 3,102.61 kgCO₂e/100 m², accounting for 59.48%. Followed by laminated slabs, with carbon emissions of 620.12 kgCO₂e/100 m², accounting for 11.89%. Based on the high carbon emission characteristics of prefabricated shear walls, through on-site investigations and comparisons of building materials, it is concluded that the processing process of prefabricated shear walls is complex, and the proportion of hot-rolled steel bars and C30 concrete per unit area is much higher than that of other building materials. Therefore, the production process of prefabricated shear walls should be considered for optimization, and the usage of hot-rolled steel bars and concrete per unit area should be reduced while meeting the strength requirements, thereby reducing the environmental impact it brings.

Fig. 2.

Carbon emissions in the production stage of prefabricated component processing (by type of building materials). a) Bar chart, b) Pie chart.

Fig. 3.

Carbon emissions in the production stage of precast component processing (by component type). a) Bar chart, b) Pie chart.

Prefabricated Components and Building Materials Transportation Stage

By applying Formula (3) and the basic data in Table 1-2, the carbon emission results per unit area (calculated at 100 m²) of the transportation stage of prefabricated components and various building materials are calculated as shown in Fig. 4. It can be seen from Fig. 4 that the carbon emission intensity of the transportation stage of prefabricated components is 137.985 kgCO₂e/100 m². Among them, the carbon emissions of prefabricated components such as prefabricated shear walls and laminated slabs are the highest, reaching 110.448 kgCO₂e/100 m², accounting for 80.04%. Followed by C30 concrete, with a carbon emission of 24.703 kgCO₂e/100 m², accounting for 17.90%. From the perspective of component types in Fig. 5, prefabricated shear walls have the highest carbon emissions, reaching 65.052 kgCO₂e/100 m², accounting for 58.90%. The second is the composite board, with a carbon emission of 16.770 kgCO₂e/100 m², accounting for 15.18%. Comparing the calculation results of various prefabricated components with building materials, the carbon emissions of prefabricated shear walls and C30 concrete still rank first and second in the transportation stage, and the carbon emission results in the transportation stage are closely related to transportation methods, transportation distance, etc. Based on the carbon emission levels of prefabricated shear walls and C30 concrete, the environmental impact of prefabricated shear wall components and concrete building materials during transportation should be a key concern.

Fig. 4.

Carbon emissions of prefabricated components and various building materials during transportation.

Fig. 5.

Carbon emissions of various prefabricated components during transportation.

Site Construction and Assembly Stage

Using the formula (4), the list of mechanical shifts and energy carbon emission factors in Tables 3-4 and the carbon emission results per unit area (calculated at 100 m²) during the on-site construction and assembly stages are calculated and shown in Fig. 6 and Fig. 7. As shown in Fig. 6, the carbon emission intensity during the on-site construction and assembly phase is 44.50 kgCO₂e/100 m². Among them, the carbon emissions of lifting machinery are the highest, reaching 44.12 kgCO₂e/100 m². We should focus on the environmental impact of lifting machinery during on-site construction and assembly and further explore equipment that can replace lifting machinery. The carbon emission levels of various types of prefabricated component lifting machinery indicate that the prefabricated shear wall has the highest carbon emission, reaching 11.23 kgCO₂e/100 m², accounting for 25.45%. The second is prefabricated infilled walls (exterior walls), with a carbon emission of 5.44 kgCO₂e/100 m², accounting for 12.34%.

Fig. 6.

Carbon emissions during site construction and assembly.

Fig. 7.

Carbon emissions of hoisting machinery during the assembly stage of various prefabricated components.

Overall Analysis of the Construction Stage

The construction process of PCS includes three stages, namely the prefabricated component production and processing stage, the transportation stage of prefabricated components and building materials, and the on-site construction and assembly stage. According to the above calculation results, the unit area carbon emission intensity of the construction process is 5,398.31 kgCO₂e/100 m². Among them, the highest carbon emission intensity is the prefabricated component production and processing stage, reaching 5,215.82 kgCO₂e/100 m², accounting for 96.62%. This is followed by the transportation stage of prefabricated components and building materials, with a carbon emission intensity of 137.99 kgCO₂e/100 m², accounting for 2.56%. The contribution ratio of carbon emission per unit area to site construction and assembly stage is small, about 0.82%. As shown in Fig. 8, the prefabricated component production and processing stage is the main source of carbon emissions, with great potential for emission reduction and space, but the environmental impact of the transportation stage of prefabricated components and building materials cannot be ignored. From the construction stage of PCS, the carbon emissions contribution from the prefabricated component processing and production stage is the largest, but the environmental impact of the prefabricated components and building materials transportation stage cannot be ignored. With the progress of technology and the development of the industry, the carbon emissions of PCS will gradually decrease in the long run, especially the research and application of new building materials will help reduce the carbon emissions in the material processing stage, and the widespread use of new energy vehicles and trucks can reduce the carbon emissions in the transportation stage of prefabricated components and building materials.

Fig. 8.

Carbon emission intensity per unit area during PCS construction.

Fig. 9.

Carbon emission intensity per unit area of various prefabricated components during the construction stage.

From the perspective of component types, the unit area carbon emission intensity of various prefabricated components during the construction stage is shown in Fig. 9, assuming that the environmental impact of material transportation and alternating current welding is not taken into account. Through the above calculation results, the highest is prefabricated shear wall, with a carbon emission of 3,178.88 kgCO₂e/100 m². The second is composite plate, with a carbon emission of 642.32 kgCO₂e/100 m². The third is precast infill wall (outside), with a carbon emission of 587.05 kgCO₂e/100 m². Prefabricated shear wall is the main source of carbon emissions, and its environmental impact should be closely monitored. The carbon emissions generated by composite plates and precast infill walls (outside) should not be ignored either. There are differences in carbon emissions of PCS in different regions. In economically developed areas, especially Shenzhen, where there is a large market demand for PCS, advanced technology, and complete industrial support, PCS components can be introduced with more advanced technology and equipment during production and on-site installation stages to reduce carbon emissions. Meanwhile, there are also differences in carbon emissions between different-sized construction projects. Large PCS projects usually have economies of scale, and prefabricated shear walls, stacked slabs, and balconies are already equipped with standardized molds. They can reduce carbon emissions levels through scale, standardization, and mass production.

From the perspective of building materials, since the prefabricated components during the on-site installation stage are raised to the designated position using crane equipment, no consideration has been given to the crane equipment from the perspective of building materials. As shown in Fig. 10, without considering the impact of lifting prefabricated components during the on-site installation stage, the unit area carbon emissions contribution of hot-rolled steel bars is the largest, reaching 2,609.33 kgCO₂e/100 m². The unit area carbon emissions of C30 concrete are the second largest, reaching 1,800.48 kgCO₂e/100 m², of which cement contributes the most to the unit area carbon emissions, which is consistent with the conclusions of some experts and scholars. From the research results, it can be seen that hot-rolled steel bars and concrete contribute the most to carbon emissions, accounting for 85.48%, because the production of steel bars and concrete contains carbon-intensive cement, with high energy and resource consumption.

Fig. 10.

Carbon emission intensity per unit area of various building materials during the construction stage.

Fig. 11.

Sensitivity analysis of material consumption.

Carbon Emission Sensitivity Analysis of Building Materials

The results of the research in the previous section show that the carbon emission intensity of PCS is the largest during the prefabricated component processing and production stage. Therefore, it is necessary to analyze the carbon emission sensitivity of component production to better evaluate the stability and reliability of the model. Of course, if the sensitivity analysis of prefabricated components and building materials transportation stage and the on-site construction and assembly stage are also carried out according to this idea, it would be better. According to the Calculation Standard of Carbon Emission in Buildings (GB/T 51366- 2019), the main factor affecting the carbon emissions of materials is their consumption and carbon emission factor, so this paper will conduct sensitivity analysis from the perspectives of consumption and carbon emission factor.

(1) Sensitivity analysis of consumption. Assuming the carbon emission factor is a constant value, the impact of material consumption on the PCS carbon emissions per unit area was analyzed, as shown in Fig. 11 and Table 5. From the research results, it can be seen that material consumption is positively correlated with carbon emissions per unit area, with the ranking being hot-rolled steel bars > concrete.

(2) Sensitivity analysis of carbon emission factor. Assuming a constant material consumption, the study analyzed the impact of the carbon emission factor of materials on the carbon emissions per unit area of PCS, as shown in Fig. 12 and Table 6. From the research results, it can be seen that the carbon emission factor of materials is positively correlated with the carbon emissions per unit area, with the ranking being hot-rolled steel bars > concrete.

According to the sensitivity analysis, the hot-rolled steel bars and concrete have a significant impact on the total carbon emissions, which is consistent with the conclusion of Li et al. [9], indicating that the calculated model can ensure the reliability of PCS carbon emission intensity. In addition, attention should be paid to the actual consumption of hot-rolled steel bars and cement during the construction of PCS.

Study on the Emission Reduction Potential of PCS

PCS Emission Reduction Scenario Hypothesis

The Guiding Opinions of the General Office of the State Council on Vigorously Developing Prefabricated Buildings issued at the national level proposed that the area of prefabricated buildings accounts for 30% of the new building area. The Implementation Opinions of the General Office of the People’s Government of Guangdong Province on Vigorously Developing Prefabricated Buildings issued at the provincial level proposed that by the end of 2025, the proportion of prefabricated buildings in the new building area should reach more than 35%. The 14th Five-Year Plan for the High-Quality Development of Shenzhen’s Modern Construction Industry issued at the municipal level proposed that the newly added prefabricated building area should be no less than 50 million square meters, and the prefabricated building area should account for 60% of the new building area. According to the municipal-level plan, by the end of 2025, the prefabricated building area of the whole city will increase by at least 50 million square meters, and by the end of 2035, the prefabricated building area of the whole city will account for more than 70% of the new building area. The overall construction scale of prefabricated buildings in the future will face new “carbon reduction” challenges. From the market size of the PCS industry, its market is showing a growth trend. With the development of urbanization, it is expected that the PCS market will maintain stable growth in the coming years. According to data from the National Bureau of Statistics, the market size of prefabricated buildings in China is expected to reach 650 billion yuan in 2024, and the proportion of PCS in the new construction area of prefabricated buildings is 67.68%. Regarding industry trends, intelligent, digital, green, and low-carbon have become important directions of transformation in the construction industry. PCS has measures such as standardized design of prefabricated components, efficient production technology, lean construction, and intelligent construction to reduce emissions. From the perspective of efficient production technology, high-performance concrete and recycled materials can be used in the production of prefabricated components. High-performance concrete has higher strength and durability, which can reduce the amount of material used in components to reduce material waste and carbon emissions. Recycled materials, on the other hand, use waste materials processed after being discarded as raw materials for component production, thus reducing the extraction of natural raw materials to lower energy consumption and carbon emissions.

Table 5.

Sensitivity analysis of material consumption.

Material	Change of material consumption (%)	Change of carbon emissions per unit area (%)
Hot-rolled steel bars	5	2.42
	10	4.83
	15	7.25
	20	9.67
C30 concrete	5	1.67
	10	3.34
	15	5.00
	20	6.67

Fig. 12.

Sensitivity analysis of material carbon emission factor.

According to the foregoing results, the prefabricated component processing and production stage has the largest carbon emissions during the construction process of PCS, with significant potential and space for carbon reduction. Moreover, hot-rolled rebar and concrete are the main sources of carbon emissions. The recycling of steel and concrete is regarded as one of the important measures for energy conservation and carbon reduction, which can reduce the environmental impact caused by the mining, transportation, and processing of raw materials. Therefore, this paper combines the scenario analysis method to predict the carbon reduction potential of recycled steel and recycled concrete of PCS in Shenzhen in the future and sets three scenarios, namely the baseline type, the intermediate type, and the optimistic type, as shown in Table 7. The scenario settings refer to planning documents and schemes such as the 14^th Five-Year Plan for Building Energy Conservation and Green Building Development in Guangdong Province, the 14^th Five-Year Energy Conservation and Emission Reduction Implementation Plan of Guangdong Province, and the 14^th Five-Year Plan for High-Quality Development of Modern Construction Industry in Shenzhen and so on. The research on recycled steel and recycled concrete is quite mature. The carbon emission factors of recycled building materials in this paper adopt the research results of Huang [20], Xiao [21], etc., as shown in Table 8.

Table 6.

Sensitivity analysis of material carbon emission factor.

Material	Change of material carbon emission factor (%)	Change of carbon emissions per unit area (%)
Hot-rolled steel bars	5	2.41
	10	4.83
	15	7.24
	20	9.66
C30 concrete	5	1.64
	10	3.29
	15	4.93
	20	6.58

Table 7.

Scenario assumptions for scenario analysis during the construction stage.

Scenario name and indicator	Scenario 1 (Base type)		Scenario 2 (Middle)		Scenario 3 (Optimistic)
	2025	2035	2025	2035	2025	2035
Recycled steel ratio (%)	0	0	10	20	20	30
Recycled concrete ratio (%)	0	0	15	25	30	50

Table 8.

Carbon emission factors of recycled building materials.

Name of building materials	Carbon emission factor
Hot-rolled steel bars[20]	1.25 kgCO2e/kg
Steel plates[20]	1.19 kgCO2e/kg
C30 concrete[21]	254.26 kgCO2e/m3

Carbon Reduction Potential Analysis

Based on the above scenario assumption, the adoption of recycled steel and recycled concrete can reduce the carbon emission intensity of PCS, thereby achieving the “carbon reduction” goal and green development of PCS. As shown in Fig. 13, in Scenario 1, without the use of any recycled steel and recycled concrete, by the end of 2023, the total carbon emissions of fabricated buildings already built in Shenzhen have reached 4.3652 million tCO₂e. The carbon emissions of PCS increased from 996,000 tCO₂e generated in 2023 to 2.6992 million tCO₂e in 2025 and then to 6.7263 million tCO₂e in 2035, with a growth multiple of 5.8 times that of 2023. Scenario 2 and Scenario 3 show the carbon emissions related to using recycled steel and concrete, respectively. In Scenario 2, the PCS carbon emissions increase to 2.60 million tCO₂e in 2025 and 6.25 million tCO₂e in 2035, with carbon reduction rates of approximately 3.62% and 6.99% compared to the baseline model. In Scenario 3, the PCS carbon emissions increase to 2.50 million tCO₂e in 2025 and 5.97 million tCO₂e in 2035, with carbon reduction rates of approximately 7.24% and 11.12%, as shown in Fig. 14.

Fig. 13.

Carbon emission and carbon reduction potential of PCS.

Fig. 14.

Carbon reduction rates of PCS in 2025 and 2035.