Calculating and Analyzing Carbon Emission Factors of Prefabricated Components

Abstract

The construction industry’s carbon emissions have a considerable impact on the environment, and all countries have pledged to reduce them to achieve low-carbon transformation. The use of prefabricated components is widely recognized as a crucial measure for mitigating carbon emissions. However, there is a scarcity of existing data on the carbon emission factors of prefabricated components (CEFoPC), and few studies have focused on calculating and reducing their carbon emissions. This study presents a novel approach to calculating CEFoPC, which involves analyzing the production process, defining the carbon emission range, identifying the sources of carbon emissions, and establishing measurement equations for the carbon emissions of prefabricated components. The proposed approach is demonstrated using six typical prefabricated components in Nanjing, China, namely, prefabricated exterior and interior wall panels, stairs, laminated panels, balconies, and air conditioning panels. After decomposing the carbon emission factors and exploring carbon emission reduction strategies, the findings suggest that the production and transportation of raw materials are the primary contributors to carbon emissions associated with prefabricated components. Additionally, the most effective carbon emission reduction strategy involves the use of green and recycled raw materials. Furthermore, the framework for calculating CEFoPC proposed in this study is considered a significant contribution, as it can facilitate future research and the enrichment of carbon emission factor databases.

1. Introduction

Countries across the globe are endeavoring to tackle the daunting challenge of climate change by curbing carbon emissions, with the aim of mitigating the environmental impact of national development. Reducing carbon emissions also has implications for the sustainability and resilience of cities [1,2]. China, as the foremost producer and consumer of construction materials worldwide, has committed to reaching a “carbon peak” by 2030 and attaining “carbon neutrality” by 2060 [3,4]. The building materials sector stands out as one of the largest contributors to carbon emissions [5]. To this end, the “China Building Materials Industry Carbon Emissions Report (2020)” published by the China Building Materials Federation highlights that the carbon emissions from China’s building materials industry in 2020 amounted to 1.48 billion tons [6]. Consequently, analyzing and computing the carbon emissions of building materials constitutes crucial groundwork in the quest to lower carbon emissions.

Prefabricated buildings can be modularized to save resources and shorten construction time [7]. Prefabricated components are materials used in the construction of prefabricated buildings. Prefabricated component carbon emissions during the production stage account for a significant portion of the carbon emissions of prefabricated buildings [8]. Therefore, the precise calculation of carbon emissions from prefabricated components and the exploration of carbon emission reduction strategies are crucial for mitigating carbon emissions from prefabricated buildings at the source, thereby facilitating the low-carbon transformation of the construction industry.

The carbon emission factor method is widely utilized for computing the carbon emissions of buildings [9]; however, there is a dearth of information on the carbon emission factors of prefabricated components (CEFoPC). Some international organizations and regions have already established carbon emission factor databases and are continually improving them [10]. The prevailing carbon emission factors are derived from various sources, including, but not limited to, the Intergovernmental Panel on Climate Change (IPCC), the European Monitoring and Evaluation Programme (EMEP), the United States Environmental Protection Agency (US EPA), and the Department for Environment, Food and Rural Affairs (DEFRA). Most of these databases primarily consider the carbon emission factors of energy sources (e.g., anthracite, coking coal, lignite, coke, crude oil, kerosene, diesel, and natural gas), while few include CEFoPC [10].

In response, this study will devise a calculation approach to CEFoPC and reveal the composition of CEFoPC. Six typical prefabricated components, with the largest usage in prefabricated buildings, have been selected to demonstrate the proposed CEFoPC calculation approach. This work shall enrich the carbon emission factor database of prefabricated components and explore the carbon reduction potential of prefabricated components by analyzing the carbon emission factors of typical prefabricated components.

2. Literature Review

2.1. Existing Research on Carbon Emission Calculation and Analysis

In terms of calculation boundaries, most scholars focus on the carbon emissions of building materials throughout their life cycle. Robati Mehdi et al. used uncertainty analysis to study the lifetime implied carbon emissions of building materials for net-zero-energy buildings [11]. Ben-Alon Lola et al. conducted a comparative study of the environmental impact of natural building materials and traditional building materials from the perspective of the whole life cycle [12]. Meanwhile, few studies focus on the production phase of building materials. Geng Yuanbo et al. calculated carbon emission factors at the cement production stage [13]. Orsini Federico and others have creatively proposed low-carbon production methods for building materials to reduce their environmental impact at the production stage [14]. Previous studies have not specifically addressed carbon emissions associated with prefabricated components, and the computational boundary has not been fully applied. Therefore, taking into account the production process of prefabricated components, this study provides a detailed summary of the carbon emission boundary.

In terms of calculation methods, the most widely used method is the inventory statistics method, which can be calculated from the micro-building level and macro-city level; it is easy and fast to calculate, but entails high data requirements. Luo Zhixing et al. studied the carbon emissions of different types of building material based on inventory calculation [9]. Liu Ke et al. used building material consumption at the design stage to calculate implied carbon emissions, highlighting the carbon reduction potential of locally sourced raw materials and using recycled materials [15]. In addition, the actual measurement method has become the focus of some scholars’ research because of its high calculation accuracy, but it has drawbacks, such as high labor and material consumption. Seo Min-Seop et al. conducted on-site measurements of carbon emission data for the production of building materials in their study of carbon emission in the construction phase of buildings [16]. Liu Guiwen et al. proposed real-time carbon emission monitoring for the whole industrial chain of prefabricated buildings, which included real-time data collection and calculation analysis of the carbon emissions of building materials [17]. While numerous calculation methods for carbon emissions have been proposed in previous studies, they are not entirely applicable to the unique characteristics of prefabricated components. This study improves the calculation method for carbon emission factors and provides equations for calculating the carbon emissions associated with prefabricated components.

2.2. Existing Research on Carbon Emission Factor Database

The carbon emission factor is an important factor affecting low-carbon transition, and has been reflected in existing studies [18]. A carbon emission database is a collection of carbon emission coefficients. Most of the existing carbon emission factor databases have been established and updated by researchers from international organizations or regions. The IPCC aims to assess information on climate change science, impacts, adaptation measures, and mitigation strategies. the IPCC’s Carbon Emission Factor Database is the most widely used database in the world, providing countries with carbon emission factors for energy, industry, transportation, agriculture, and other sectors [19]. The EMEP provides a database of carbon emission factors and mainly serves European countries, covering energy, transportation, industry, agriculture, and other sectors, and providing European policymakers with the most up-to-date and reliable environmental information to support sustainable development and environmental protection [20]. The US EPA provides a database of carbon emission factors and serves the U.S., covering energy, transportation, industry, agriculture, and other sectors, and is designed to help governments, businesses, and the public understand and reduce greenhouse gas emissions [21]. DEFRA has released a database of carbon emission factors for the UK, covering energy, transport, industry, and agriculture, which can be used for carbon management, policy development, and regulation by the UK government and businesses [22]. The current major databases for carbon emission factors lack information specifically related to prefabricated components. Therefore, this paper proposes a method for calculating carbon emission factors associated with prefabricated components, with the aim of contributing to future research and enriching the carbon emission factor database.

2.3. Existing Research on Carbon Emissions of Prefabricated Components

As for prefabricated components, some research is based on life cycle theory an dstudies the carbon emissions of prefabricated components at different stages of prefabricated buildings. Li Xiaojuan et al. calculated and analyzed the carbon emissions of precast concrete stair components throughout their whole life cycle [23]. Xue Li et al. studied the carbon emissions of precast concrete columns during the construction stage [24]. Some scholars focus on the carbon emissions of prefabricated components in the production stage. Liu Guiwen et al. proposed a carbon emission calculation model for a precast component production line and conducted a case study with precast concrete wall panels [25]. It can be seen that carbon emissions of prefabricated components have already received research attention and accumulated a certain research foundation. Despite previous research on the carbon emissions associated with prefabricated components, the current literature still lacks depth. A standardized approach to calculating carbon emission factors has not been established, and there is a dearth of discussion on effective strategies for reducing carbon emissions.

2.4. Summary of Existing Research

According to our review of existing studies, it can be found that research on carbon emission calculation has advanced, and research on carbon emission factors is constantly carried out to enrich the carbon emission factor databases. Of course, some pioneering research has involved certain explorations into CEFoPC, but currently, CEFoPC data remain quite scarce in carbon emission factor databases; thus, it is important to propose a CEFoPC calculation approach that can enrich the carbon emission factor databases. Furthermore, existing studies mostly focus on whether or not to use prefabricated components as a means of reducing buildings’ carbon emissions, but this study focuses on reducing the carbon emissions of prefabricated components themselves, which can fundamentally reduce carbon emissions.

3. Methodology

This paper focuses on the carbon emission factor measurement and analysis of the prefabricated components of prefabricated buildings. The prefabricated components are made a prefabricated component factory according to the design specifications of the prefabricated buildings. The carbon emissions of prefabricated components studied in this paper refer to their carbon emissions before they are put to use, meaning the carbon emissions at the production stage. The establishment of a carbon emission factor calculation framework for prefabricated components is anticipated to contribute towards the expansion of carbon emission factor databases in the future.

3.1. Range of Prefabricated Components at the Production Stage and Sources of Carbon Emissions

This paper focuses on the production stage of prefabricated components for the assembly of buildings. The process flow of prefabricated component production includes formwork processing, steel processing, concrete processing, and prefabricated component processing. The process flow indicates that prefabricated component production requires specific raw materials, which need to be produced where the raw materials are produced and transported to the prefabricated component factory to be useful. The process flow indicates that the raw materials have to be processed to make prefabricated components, and this process needs to be carried out in a prefabricated component factory. Therefore, the range of the prefabricated component production stage considered in this paper includes three regions: the raw material production location, the transport process, and the prefabricated component factory.

The production elements in these three regions generate carbon emissions through material consumption, energy consumption, etc.: (i) raw material carbon emissions $C_{sc1}$, which is generated through the consumption of raw materials such as cement, sand, stone, additives, and steel reinforcement during the production of prefabricated components [26]; (ii) raw material transportation carbon emissions $C_{sc2}$, which is generated through the energy consumption caused by the transportation of raw materials from the extraction site to the prefabricated component factory using transportation equipment [11]; (iii) carbon emissions from machinery and equipment $C_{sc3}$, generated through the energy consumption of mechanical equipment such as steel cutting machines, steel bending machines, concrete mixers, concrete placing machines, etc., in the prefabricated component factory [27]; (iv) carbon emissions from labor $C_{sc4}$, generated through the consumption of various resources generated by the production workers in their offices and who live within range of the prefabricated component factory. This aspect of carbon emissions is not addressed in most studies, but this study considers human-induced carbon emissions to ensure the integrity of carbon emission sources. The range and carbon emission sources of prefabricated components at the production stage are shown in Figure 1.

3.2. Process for Calculating Carbon Emissions at the Production Stage of Prefabricated Components

Considering the range of the carbon emissions of prefabricated components delineated in Figure 1, the total carbon emissions of prefabricated components at the production stage are calculated according to Equation (1).

$$C_{sc}=C_{sc1}+C_{sc2}+C_{sc3}+C_{sc4}$$

(1)

where: $C_{sc}$—carbon emissions at the production stage of one unit of prefabricated components (kgCO₂); $C_{sc1}$—carbon emissions caused by raw material consumption at the production stage of one unit of prefabricated components (kgCO₂); $C_{sc2}$—carbon emissions caused by raw material transportation at the production stage of one unit of prefabricated components (kgCO₂). $C_{sc3}$—carbon emissions caused by energy consumption by machinery and equipment at the production stage of one unit of prefabricated components (kgCO₂); $C_{sc4}$—carbon emissions caused by labor consumption at the production stage of one unit of prefabricated components.

3.2.1. Carbon Emissions Caused by Raw Material Consumption

The production of prefabricated components involves the extraction and production of raw materials. This aspect of carbon emissions is calculated according to Equation (2).

$$C_{sc1}={{\displaystyle \sum }}_{i=1}^n{M}_i{F}_i$$

(2)

where: $M_i$—the consumption of the $i\mathrm{th}$ raw material at the production stage of one unit of prefabricated components; $F_i$—the carbon emission factor of the $i\mathrm{th}$ raw material (kgCO₂/one unit of raw material quantity); $i$—the $i\mathrm{th}$ raw material; n—there are $i$ raw materials in total.

The consumption of various types of raw material in the prefabricated component factory is calculated in accordance with the production volume of various types of prefabricated component to be apportioned, and the consumption of raw materials of one unit of prefabricated components is calculated according to Equation (3).

$$M_i=\frac{M_{r,i}}{{{\displaystyle \sum }}_{j=1}^n{Q}_{r,j}}$$

(3)

where: $M_{r,i}$—the monthly consumption of the $i\mathrm{th}$ raw material in the prefabricated component factory; $Q_{r,j}$—the monthly production of the $j\mathrm{th}$ prefabricated component in the prefabricated component factory.

3.2.2. Carbon Emissions Caused by Raw Material Transportation

The carbon emissions of prefabricated component production involve the transportation of raw materials. This aspect of carbon emissions is calculated according to Equation (4).

$$C_{sc2}={{\displaystyle \sum }}_{i=1}^n\left(M_i\times D_i\times T_i\right)$$

(4)

where: $D_i$—the average transportation distance of the $i\mathrm{th}$ raw material (km); $T_i$—the carbon emission factor under the $i\mathrm{th}$ raw material transportation mode (kgCO₂/(t·km)).

3.2.3. Carbon Emissions Caused by Energy Consumption by Machinery and Equipment

The machinery and equipment used to process raw materials in the prefabricated component factory involve energy consumption, which mainly comprises fuel, electricity, heat, and water. This aspect of carbon emissions is calculated according to Equation (5).

$$C_{sc3}=E_f+E_e+E_h+E_w$$

(5)

where: $E_f$—carbon emissions caused by the combustion of fossil fuels at the production stage of one unit of prefabricated components (kgCO₂); $E_e$—carbon emissions caused by the consumption of electricity at the production stage of one unit of prefabricated components (kgCO₂); $E_h$—carbon emissions caused by the consumption of heat at the production stage of one unit of prefabricated components (kgCO₂); $E_w$—carbon emissions caused by the consumption of water at the production stage of one unit of prefabricated components (kgCO₂).

$E_f$ is calculated according to Equation (6).

$$E_f=A{D}_f\times E{F}_f$$

(6)

where: $A{D}_f$—the amount of fossil fuel combusted at the production stage of one unit of prefabricated components (kg or m³); $E{F}_f$—the carbon emission factor of the $i\mathrm{th}$ fossil fuel (kgCO₂/kg or kgCO₂/m³).

$E_e$ is calculated according to Equation (7).

$$E_e=A{D}_e\times E{F}_e$$

(7)

where: $A{D}_e$—the amount of electricity consumed at the production stage of one unit of prefabricated components (kWh); $E{F}_e$—the carbon emission factor of electricity (kgCO₂/kWh).

$E_h$ is calculated according to Equation (8).

$$E_h=A{D}_h\times E{F}_h$$

(8)

where: $A{D}_h$—the amount of heat consumed at the production stage of one unit of prefabricated components (GJ); $E{F}_h$—the carbon emission factor of heat (kgCO₂/GJ).

$E_w$ is calculated according to Equation (9).

$$E_w=A{D}_w\times E{F}_w$$

(9)

where: $A{D}_w$—the amount of water consumed at the production stage of one unit of prefabricated components (t); $E{F}_w$—the carbon emission factor of water (kgCO₂/t).

The energy resource consumption of a prefabricated component factory is calculated in accordance with the production volume of various types of prefabricated component to be apportioned, and the consumption of energy resource of one unit of prefabricated components is calculated according to Equation (10).

$$A{D}_i=\frac{A{D}_{r,i}}{{{\displaystyle \sum }}_{j=1}^n{Q}_{r,j}}$$

(10)

where: $A{D}_i$—the amount of the $i\mathrm{th}$ energy resource (fossil fuel, electricity, heat, or water) consumed at the production stage of one unit of prefabricated components; $A{D}_{r,i}$—the monthly consumption of the $i\mathrm{th}$ energy resource of the prefabricated component factory; $Q_{r,j}$—the monthly production of the $j\mathrm{th}$ type of prefabricated component in the prefabricated component factory.

3.2.4. Carbon Emissions Caused by Labor Consumption

The carbon emissions caused by labor consumption at the production stage of one unit of prefabricated components are calculated according to Equation (11).

$$C_{sc4}=N\times F_r$$

(11)

where: $N$—the amount of labor consumption at the production stage of one unit of prefabricated components (working day); $F_r$—the carbon emission factor of labor (kgCO₂/working day).

3.3. Selection of Low-Grade Carbon Emission Factors

The CEFoPC in this study is calculated using the carbon emission factor method, which means that the CEFoPC is calculated using raw material carbon emission factors, such as energy carbon emission factors, etc. The carbon emission factors were obtained according to the priority levels in

Table 1. Carbon emission factor acquisition methods and priorities.

Data Type	Interpretation
Measured/mass-energy-balanced emission factors	Emission factors obtained via direct measurement or usinf mass-energy balancing methods
Equipment empirical emission factors	Equipment-specific emission factors, but not directly measured
Emission factors provided by the manufacturer	Emission factors based on those obtained at the manufacturer’s level
Regional emission factors	Emission factors based on regional characteristics
National emission factors	Emission factors based on national characteristics
International emission factors	Internationally common emission factors

Note: The priority of selecting carbon emission factors in the table decreases from top to bottom.

. Data on raw material production, data on raw material transportation, data on the energy consumption of machinery and equipment, and data on labor consumption were obtained through field research.

4. Demonstrative Cases

4.1. Basic Information on the Case

Prefabricated components can be divided into vertical components and horizontal components. Vertical components include prefabricated exterior wall panels (PEWPs) [28], prefabricated interior wall panels (PIWPs) [29], etc. Horizontal components include prefabricated stairs (PS) [23], prefabricated laminated panels (PLP) [30], prefabricated balconies (PB) [31], prefabricated air conditioning panels (PACP) [32], etc. These six types of prefabricated component are used in large quantities and contribute to large amounts of carbon emissions in prefabricated buildings. In this paper, carbon emission factors are measured and analyzed for these six common types of prefabricated component.

4.2. Measurement of CEF of Typical Prefabricated Components

(1)

Carbon emissions caused by raw material consumption

The carbon emission factors of the five raw materials are shown in

Table 2. Carbon emission factors of raw material consumption.

Raw Material Type	Carbon Emission Factor	Factor Unit	Data Source
Cement	0.735000	kgCO2/kg	a
Sand	0.002510	kgCO2/kg	a
Gravel	0.002180	kgCO2/kg	a
Tap water	0.000168	kgCO2/kg	a
Reinforcement	3.030000	kgCO2/kg	a

Note: ‘a’ refers to GBT 51366-2019 Carbon Emission Calculation Standard for Buildings [33].

. The consumption of different raw materials in the six prefabricated components is shown in

Table 3. Calculation results of raw material consumption and carbon emission.

Type of Prefabricated Component	Raw Material Consumption (kg)					Carbon Emission (kgCO2)
	Cement	Sand	Gravel	Tap Water	Reinforcement
PEWP	360.00	804.00	1066.00	165.00	80.00	511.37
PIWP					80.00	511.37
PS					120.00	632.57
PLP					160.00	753.77
PB					54.00	432.59
PACP					148.00	717.94

Note: Raw material consumption data were provided by the prefabricated component factory.

. Carbon emission calculation is carried out according to Equation (2), and the results are shown in Table 3.

(2)

Carbon emissions caused by raw material transportation

The prefabricated component factory used in this study uniformly uses 30-ton diesel vehicles to transport raw materials, and its carbon emission factor is given as 0.078 kgCO₂/(t·km) in GBT 51366-2019. In this paper, the consumption of five kinds of raw material is set equal to their transportation quantity, and the distance from the production site to the prefabricated component factory varies for different raw materials. The transportation distances of five kinds of raw material are shown in

Table 4. Calculation results of transportation distance of raw materials and carbon emissions.

Type of Prefabricated Component	Raw Material Transportation Volume (km)					Carbon Emission (kgCO2)
	Cement	Sand	Gravel	Tap Water	Reinforcement
PEWP	35.00	500.00	150.00	0	40.00	45.06
PIWP						45.06
PS						45.19
PLP						45.31
PB						44.98
PACP						45.27

Note: The transportation distances of raw materials were provided by the prefabricated component factory. Among them, the purchasing place of sand is often changed, so its transportation distance takes a default value of 500 km. tap water is transported via pipeline, which is not related to diesel vehicles, so it takes a value of 0 km.

. The carbon emission calculation is carried out according to Equation (4), and the results are shown in Table 4.

(3)

Carbon Emissions caused by energy consumption by machinery and equipment

The machinery and equipment that operate in this prefabricated component factory mainly consume tap water, electricity, and fuel oil, and their carbon emission factors are shown in

Table 5. Carbon emission factors of energy consumption.

Energy Type	Carbon Emission Factor	Factor Unit	Data Source
Tap water	0.000168	kgCO2/kg	a
Electricity	0.570300	kgCO2/kwh	b
Fuel oil	0.848009	kgCO2/kg	c

Note: ‘a’ refers to GBT 51366-2019 Carbon Emission Calculation Standard for Buildings, ‘b’ refers to the “Notice on the Management of Greenhouse Gas Emissions Reporting by Enterprises in the Power Generation Industry for 2023–2025” issued by the General Office of the Ministry of Ecology and Environment, and ‘c’ refers to the “Greenhouse Gas Emissions Accounting Methodology and Reporting Guide for Chemical Production Enterprises in China”.

. In this study, energy consumption apportionment is used to calculate the energy required to produce a unit of prefabricated components, and the energy consumption of a unit of prefabricated components is calculated according to Equation (10), as shown in

Table 6. Calculation results of energy consumption and carbon emissions of machinery and equipment operation.

Type of Prefabricated Component	Energy Consumption Type			Carbon Emission (kgCO2)
	Water	Electricity	Fuel Oil
PEWP	1620 kg	30.1 kwh	0.006375 kg	18.03
PIWP				18.03
PS				18.03
PLP				18.03
PB				18.03
PACP				18.03

Note: The energy consumption data were provided by the prefabricated component factory.

. The carbon emission calculation was carried out according to Equations (6), (7) and (9), and the calculation results are shown in Table 6.

(4)

Carbon emissions caused by labor consumption

This study uses labor consumption apportionment to calculate the workdays required to produce a unit of prefabricated components, and its carbon emission factor is 3.77 kgCO₂/workday according to a field measurement. The workdays required to produce a unit of prefabricated components are shown in

Table 7. Calculation results of labor consumption and carbon emissions of labor.

Type of Prefabricated Component	Labor Consumption (Workday)	Carbon Emission (kgCO2)
PEWP	0.55	0.61
PIWP		0.61
PS		0.61
PLP		0.61
PB		0.61
PACP		0.61

Note: The labor consumption data were provided by the prefabricated component factory.

. The carbon emission calculation was calculated according to Equation (11), and the calculation results are shown in Table 7.

4.3. Analysis of Carbon Emission

According to the process of calculating the carbon emissions of prefabricated components in the previous section, the carbon emissions are summarized and calculated, and the summary results are shown in Figure 2.

By calculating and summarizing the carbon emission factors of six types of prefabricated component, it can be seen that the carbon emission factors of two vertical prefabricated components (prefabricated exterior wall panels and prefabricated interior wall panels) are both 575.07 kgCO₂/m³, and the carbon emission factors of four horizontal prefabricated components (prefabricated stairs, prefabricated laminated panels, prefabricated balconies, and prefabricated air conditioning panels) are 696.4 kgCO₂/m³, 817.72 kgCO₂/m³, 496.21 kgCO₂/m³, and 781.85 kgCO₂/m³. The carbon emission factors of vertical components are smaller than those of horizontal ones, except for prefabricated balconies. In a previous study, the carbon emission value from interior prefabricated wall panels with volumes of 0.609 m³ was 427 kg [25], which is similar to the calculation results in this study.

Based on the aggregated calculation results, the range of carbon emission contribution of the four stages (raw material consumption, raw material transportation, machinery and equipment, and labor) in prefabricated components can be derived, as shown in Figure 3.

Of the four stages of the prefabricated component production process, the largest proportion of carbon emission contribution is attributed to raw material consumption, accounting for 87.18% to 92.18%, so to promote the low carbonization of prefabricated components at the production stage, we should focus on the low carbonization of raw material consumption. The carbon emission contribution of the transportation of raw materials accounts for 5.54% to 9.06%, indicating that the low carbonization of transportation will also have an important impact on the carbonization of prefabricated components at the production stage. In addition, the contribution of the energy consumption of in-factory machinery and equipment to carbon emissions accounts for 2.20% to 3.63%, with certain carbon emission reduction potential, and the contribution of labor consumption to carbon emissions only accounts for 0.07% to 0.12%, with small carbon emission potential.

5. Discussion

According to the calculation results of this case study, it is found that carbon emissions at the production stage of prefabricated components are mainly composed of two aspects: raw material consumption and raw material transportation; these contribute more than 90% of carbon emissions. An existing study found that materials contribute 96.2% of carbon emissions, which is consistent with this study [25]. It is valuable to conduct carbon emission reduction strategy research and effect evaluation for these two aspects of carbon emissions.

5.1. Carbon Emission Reduction Strategies for Raw Material Consumption

According to the existing research, the strategy for the carbon reduction of raw material consumption mainly includes: the use of green raw materials, high-performance raw materials, and recycled raw materials for prefabricated component processing and production.

Green raw materials are those with low carbon emissions and minimal environmental impact. Our workshop discussed low-carbon cement as a typical example. It is widely used in new buildings in China, such as the exhibition hall of the Shanghai World Expo. Compared with conventional cement, low-carbon cement reduces carbon emissions by 25% in its own production process [34]. It can lead to an effective reduction in carbon emissions from the raw material consumption of prefabricated components. Figure 4 shows the carbon emissions of low-carbon cement used as a raw material to produce six types of prefabricated component.

High-performance raw materials are more functional and efficient raw materials that can achieve the same performance goals with less consumption. Our workshop discussed high-strength reinforcement as a typical example. It is widely used in various large public buildings, such as the Shanghai World Financial Center. The use of high-strength reinforcement can save at least 10% of reinforcement consumption [35]. It can effectively reduce carbon emissions from the raw material consumption of prefabricated components. Figure 5 shows the carbon emissions of producing six types of prefabricated component with and without the use of high-strength reinforcement as a raw material.

Recycled raw materials are raw materials formed by recycling and processing construction and demolition materials, etc., and can make full use of existing resources to achieve the goal of environmental protection. Our workshop discussed the recycling of steel as a typical example, the use of which is encouraged in construction in China. The recycling rate of steel is as high as 95%, and the carbon emission factor of producing recycled steel is 0.393 kgCO₂/kg, which is much lower than the 3.03 kgCO₂/kg used in the factory studied in this paper. It can effectively reduce the carbon emissions of the raw material consumption of prefabricated components. Figure 6 shows the carbon emissions of the six types of prefabricated component produced with and without the use of recycled reinforcement as a raw material.

5.2. Carbon Emission Reduction Strategies for Raw Material Transportation

According to existing studies, logistics-derived carbon emissions from prefabricated components are also of concern [36]. The main carbon reduction strategies for raw material transportation include the use of new transportation tools and the optimization of raw material transportation routes.

New transport tools are those that optimize their energy consumption structure to achieve environmental goals. Our workshop discussed electric loaders as a typical example. These have been widely used in cities such as Nanjing, where the construction industry is more developed. Electric loaders can effectively reduce carbon emissions during the raw material transportation phase by consuming electricity instead of fossil fuels when transporting the same raw materials. Their carbon emission factor is given as 0.01 kgCO₂/(t·km) in GBT 51366-2019. Figure 7 shows the carbon emissions of the six types of prefabricated component with and without the use of electric loaders as transporters.

Raw material transportation route optimization aims to reduce the distance of raw material transportation in order to reduce the mileage of vehicles, and thus, achieve environmental protection goals. Our workshop ascertained that a typical example is the local procurement of raw materials. The local procurement of raw materials in Nanjing can shorten the distance traveled by means of transportation, assuming that the transportation routes of cement and steel reinforcement are shortened by 10 km each, and the transportation routes of sand and gravel are both shortened to 150 km. This can effectively reduce carbon emissions at the transportation stage of raw materials. Figure 8 shows the carbon emissions of the six types of prefabricated component with and without the procurement of raw materials nearby.

5.3. Sensitivity Analysis of Carbon Reduction Strategies

Based on five carbon emission reduction strategies, the carbon emission reduction sensitivity of the six prefabricated components was calculated. The sensitivity was calculated according to Equation (12), and the calculated results are shown in

Table 8. Sensitivity of six prefabricated components to five carbon emission reduction strategies.

Carbon Reduction Strategy	PEWP	PIWP	PS	PLP	PB	PACP
Adopt green raw materials	13.00%	13.00%	10.50%	8.80%	15.38%	9.24%
Adopt high-performance raw materials	4.40%	4.40%	5.51%	6.30%	3.41%	6.08%
Adopt recycled raw materials	9.22%	9.22%	11.68%	13.48%	7.07%	12.98%
Adopt new transportation tools	7.33%	7.33%	6.00%	5.08%	8.58%	5.31%
Optimize raw material transportation routes	2.49%	2.49%	2.05%	1.74%	2.89%	1.82%

.

$$S_{ij}=\left(E_{1ij}-E_{2ij}\right)/E_{2ij}$$

(12)

where: $S_{ij}$—the sensitivity of the $i\mathrm{th}$ prefabricated component to the $j\mathrm{th}$ carbon emission reduction strategy; $E_{1ij}$—carbon emission of the $i\mathrm{th}$ prefabricated component before adopting the $j\mathrm{th}$ carbon emission reduction strategy; $E_{2ij}$—carbon emission of the $i\mathrm{th}$ prefabricated component after adopting the $j\mathrm{th}$ carbon emission reduction strategy.

Regarding existing studies on carbon emission reduction, in terms of raw material consumption, some studies have focused on how to develop raw materials for low-carbon production, and some studies have focused on how to optimize the process to reduce raw material consumption, so as to reduce carbon emissions. In terms of the transportation of raw materials, some studies have focused on how to develop new tools for the transportation of raw materials, and some studies have focused on how to plan routes to minimize carbon emissions. This paper considers green raw materials, high-strength raw materials, and recyclable raw materials, as well as new transportation vehicles and transportation routes; then, it calculates and analyzes their sensitivity to the carbon emission reduction of prefabricated components.

We have identified several potential sources of endogeneity that may exist in our study, which we will explain below. Firstly, missing variables could be a concern. To address this, we conducted extensive field research to collect data on carbon emission activities, thereby minimizing the risk of data omission. Secondly, two-way causality is not a factor in our study, as it is focused on one-way causality—that is, the production activities of prefabricated components result in carbon emissions. Finally, measurement errors in data collection are possible, but we have taken rigorous measures to ensure that the collected data accurately reflect the production activities of prefabricated components over a specific time period, reducing the likelihood of measurement errors on a large scale.

6. Conclusions

This study proposes a CEFoPC calculation approach and discloses the constitutions of CEFoPC. This approach is demonstrated using the CEFs of six types of prefabricated component, and the carbon emission source of each CEFoPC is calculated, analyzed, and compared. The carbon emission reduction strategies are discussed for raw material consumption and transportation, which are responsible for the majority of carbon emissions, and sensitivity analysis is conducted to evaluate the potential of each strategy. The main findings of this study are as follows.

(1)

The carbon emissions of prefabricated components mainly include carbon emission from the consumption of raw materials, carbon emission from the transportation of raw materials, carbon emission from the operation of machinery and equipment, and carbon emission from labor; therefore, calculation of the carbon emission factor of prefabricated components should be carried out based on these four aspects, and the statistical analyses should be carried out according to the regional characteristics.

(2)

The carbon emission decomposition of prefabricated components shows a larger contribution of carbon emission from raw material consumption and transportation and a smaller contribution from others. The contribution percentages of the carbon emission sources of prefabricated components studied in this project are as follows: raw material consumption (87.18% to 92.18%), raw material transportation (5.54% to 9.06%), energy consumption caused by the use of machinery and equipment (2.20% to 3.63%), and labor consumption (0.07% to 0.12%).

(3)

Carbon reduction strategies for prefabricated components should focus on the decarbonization of raw material consumption and raw material transportation. The ranking of the sensitivity of the carbon emission factors of precast components to specific strategies, from largest to smallest, is as follows: using green raw materials (8.80–15.38%), using recycled raw materials (7.07–13.48%), using new transportation means (5.08–8.58%), using high-performance raw materials (3.41–6.30%), and raw material transportation path optimization (1.74–2.89%).

This study delves into the carbon emission factors present during the production of prefabricated components utilized in building assembly. Specifically, it selects six types of typical prefabricated component from a factory located in Nanjing to conduct experimental calculations. This expands the database of local carbon emission factors, providing a solid foundation and theoretical references for subsequent research in this field. Furthermore, drawing on the findings of previous research and the outcomes of our workshop, this study proposes a more effective carbon reduction strategy; specifically, it proposes the utilization of environmentally friendly and recycled raw materials.

However, there are certain limitations to this study. For instance, the original data sources used to measure the carbon emission factors are not extensive enough. This study employed a workshop to facilitate the discussion of carbon emission reduction strategies. The workshop participants were primarily comprised of professionals from academia and the construction industry, including several university professors and doctoral researchers, as well as industry experts. Future research could consider comprehensively incorporating carbon emission data from the production stages of prefabricated components in multiple regions to address these limitations and improve the universality of the research conclusions. Moreover, this study’s length limits its discussion on carbon emission reduction strategies. Future studies could explore the impacts of various strategies on decarbonized prefabricated component production to enhance current research in this field.