Does the Modular Construction Project Outperform the Traditional One? A Comparative Life Cycle Analysis Study in Hong Kong

Abstract

Hong Kong faces critical construction challenges, including workforce aging, land shortages, and near-capacity waste disposal. Modular Integrated Construction (MiC) offers a promising solution. As Hong Kong has just recently adopted the MiC, quantitative studies that explore the actual performance differences between MiC projects and conventional on-site construction projects in Hong Kong are lacking. To fill this knowledge gap, this study utilizes an extended life cycle assessment–Life Cycle Performance Assessment to conduct on-site investigations and case studies on a MiC pilot residential project and a conventional on-site construction residential project in Hong Kong from multiple dimensions: cost, time, safety, and environment. The assessment indicators include five types of greenhouse gas emissions, cost performance, schedule performance, and safety-level index. This study found that the greenhouse gas emissions of the MiC project during the entire construction period were reduced by approximately 21.60% compared to traditional on-site construction projects. The most significant part of the greenhouse gas emissions of the two methods was the embodied emissions of construction materials, accounting for 83.11% and 87.17%. Compared with the conventional construction project, the factors that actively promote the reduction of greenhouse gas emissions in the MiC project are the embodied greenhouse gas emissions of building materials, the transportation of construction waste, and the resource consumption of equipment. In addition, there is no significant difference in the safety performance index of the two construction methods, but MiC projects have more efficient schedule performance management. Surprisingly, the cost control of MiC projects is not as good as that of conventional construction projects, which differs from existing research results in other regions.

1. Introduction

The construction industry is an important cornerstone of Hong Kong’s economy. According to the Hong Kong Construction Industry Council, the sector contributed to 4.45% of the Gross Domestic Product in 2023 [1]. The production performance of the construction industry is critically influencing Hong Kong’s economic development and people’s livelihood. Nevertheless, there have been some obstacles in recent years, such as a lack of skilled labor [2], the aging problem of labor [3], higher construction costs [4], and so on. The construction industry often faces delays caused by numerous factors, such as a shortage of workers or disruptions caused by extreme weather conditions and other unexpected events, like COVID-19 [5]. As the demand for construction services continues to grow, the situation will get worse in the coming years.

To address these issues, many countries began to adopt and promote prefabricated buildings, enabling most of the components of buildings to be completed in off-site factories [6]. For instance, the Building and Construction Authority of Singapore states that prefabricated structural technology is a core strategy to improve construction productivity [7]. The Australian government regards prefabricated technology as a key breakthrough direction for the construction industry [8]. Government support and customer satisfaction have made Sweden a global leader in modular construction, accounting for 80% of the residential market [9]. A study in Italy also confirmed that the prefabricated structure reinforcement solution based on dissipation has better environmental performance than the traditional concrete mezzanine [10]. However, the development of modular buildings in Hong Kong is facing significant challenges, such as transportation restrictions and costs, negative perceptions in the market, and a lack of knowledge and experience [2,11,12].

In order to drive economic growth and improve financial competitiveness, the HKSAR Government and Hong Kong Construction Industry Council strongly support the promotion and development of innovation and advanced technologies. The Development Bureau of HKSAR established the “Construction Industry Innovation and Technology Fund” in October 2018 [13], allocating HKD billion to enhance the capacities of construction companies and practitioners and encourage the industry in using innovations and advanced technologies. In this regard, Modular Integrated Construction (MiC) is one of the main technologies supported and promoted in this plan.

As a cutting-edge and innovative building technique, MiC has been demonstrated by global practices to successfully mitigate the challenges encountered by the construction sector and has become a key policy support project. The principle of MiC is to manufacture independent volume modules (such as finishes, fixtures, accessories, and so on) made off-site and transfer the modules to the construction site for assembly afterward [14,15]. The main components of MiC include steel-frame modules, reinforced-concrete modules, and hybrid modules [6]. The researchers analyzed the advantages and current challenges of MiC [16]. MiC can enhance the safety of a site, enable more productive quality control, shorten the period of construction time, and reduce the construction refuse [17,18,19]. By shifting the procedure of on-site construction to a governed manufacturing plant environment, it can minimize the unfavorable factors of the on-site construction, such as the effect of detrimental weather, insufficient worker resources, site hindrances, and so on [20,21,22]. Based on the experience of international practices, MiC has significant advantages in regard to many aspects when compared with conventional construction methods, helping to alleviate Hong Kong’s construction challenges [23]. In Hong Kong, The MiC process typically includes seeking statutory approvals, project design and approval, module production, module transportation, and on-site installation [24,25,26]. However, the injection of incentive policies has not effectively stimulated the modular building market, and various stakeholders cannot comprehensively analyze and compare the social and economic benefits brought by MiC [27].

Considering the actual problems and needs of the construction industry in Hong Kong, the localization of MiC in Hong Kong has also gained the attention of researchers. Although existing research has identified key success factors [2] and policy-driver factors [5] of MiC projects in Hong Kong, there is a lack of case studies and quantitative analysis of actual MiC projects. In the research on strategy formulation, Choi, et al. [28] discussed the challenges and opportunities of implementing modular architecture in dense urban environments. Further, effective MiC supply chain management [6] and an IoT BIM platform [3] were also proposed. When considering formulating a groundbreaking strategy or policy, performance analysis is always an element that cannot be ignored. However, since MiC projects have just been implemented and landed in Hong Kong for a short period, quantitative studies that explore the actual performance gap between MiC projects and conventional on-site construction projects in Hong Kong are lacking.

The life cycle assessment (LCA) is an analytical tool [29] used to evaluate the environmental dimensions of a project in the construction sector during all phases of a given period [30]. The results of the LCA show the overall situation of the assessment project and are helpful for decision-making and improvement [31,32,33]. During the construction process, GHG emissions are commonly made to affect the environment in different stages [34,35]. The accuracy of LCA is very high, and it can provide decision-makers who intend to improve environmental performance and understand the potential of construction GHG emission reductions to respond to actions and provide important information. In addition, as a new technology in Hong Kong, MiC lacks actual data analysis of the project. Therefore, it is valuable to adopt the LCA approach to comprehensively evaluate and analyze MiC projects in Hong Kong.

Many studies exist on the LCA of residential buildings in the existing research. However, only a few studies have compared different types of buildings [36]. Quale, et al. [37] compared the environmental impact of wood-frame houses built by modular and traditional construction methods in the United States. Paya-Marin, et al. [38] performed an LCA analysis of two modular schools to determine energy environmental and impact consumption. Kamali, et al. [39] analyzed the differences in environmental impact between modular buildings and traditional buildings using LCA. However, researchers have not yet quantified the environmental LCA performance of MiC residential projects in Hong Kong. Furthermore, LCA is limited to the environmental dimension and needs support from other dimensions, such as finance [40].

To address the identified knowledge gap, this research aims to answer the following research inquiries:

• What are the quantitative differences in GHG emissions, cost, time efficiency, and safety performance between MiC and traditional construction methods in Hong Kong’s residential projects?
• Which specific life cycle stages contribute most significantly to the environmental and economic disparities between the two methods?
• How do Hong Kong’s high-density urban constraints influence MiC performance outcomes compared to other findings?

Therefore, this study utilizes an extended life cycle assessment–Life Cycle Performance Assessment (LCPA) to examine and compare a MiC pilot residential project and a conventional on-site construction residential project in Hong Kong from multiple dimensions: cost, time, safety, and environment. This paper proceeds as follows. Section 2 details the methodology used in this paper and elaborates on the calculation method and boundary of LCPA. Section 3 is a case study of the target projects. Section 4 analyzes the results and provides a discussion of the case analysis. Concluding remarks comprise the final section.

2. Methodology

This study adopted mixed methods, integrating qualitative and quantitative approaches. Firstly, the development of MiC in Hong Kong was analyzed and reviewed through desktop research, including a literature and policy review and a collection and review of government information and data reports. Secondly, the on-site investigation and case study of the MiC project and the conventional construction project in Hong Kong were conducted. The data of the project comparison group in the construction stage were collected through a questionnaire survey. Finally, the collected data were compared and analyzed using the LCPA method. The comparison of these two different construction methods can reflect the adaptability of MiC to residential projects. Figure 1 illustrates the research framework.

2.1. Life Cycle Performance Assessment (LCPA)

This research broadened LCA to LCPA and conducts a detailed analysis of the performance of the MiC project in Hong Kong. This study is based on the recognized construction project key performance indicators proposed by Radujkovic, et al. [41], the project performance indicators summarized by Kamali and Hewage [36], and the measurement forms of construction projects introduced by Al-Jibouri and Mawdesley [42]. Based on the original LCA method, the calculable cost, time, and safety dimensions of the field project were added to measure the project’s LCPA. The time dimension, as an important component of construction project management, is measured by the schedule performance index. This is because almost all articles discussing the advantages of MiC point out that modular construction will shorten the time cost. In addition, the typhoon season in Hong Kong, from May to September each year, affects outdoor construction activities. MiC’s factory-based workflow minimizes weather delays and enhances flexibility. The safety dimension is a top priority for project management. There are three limitations of data accessibility and contextual realities: (1) there are no accidental deaths in either of the two items, while the general safety accident records at specific locations cannot be disclosed publicly; (2) the two projects share the same safety certifications and agreements enforced by the contractor; and (3) as MiC’s security database in Hong Kong is just getting started, we have prioritized the quantifiable and auditable metrics recorded in the project logs. Therefore, the two indicators “exposure to bad weather” and “quantity of dangerous activities” were selected as safety indicators for evaluation. These indicators were measured using the following approaches:

• Cost: Measured by cost performance index (CPI). The cost performance increases with a decreasing CPI value.
• Time: Measured by the schedule performance index (SPI). Better time performance is indicated by a higher SPI value.
• Safety: The duration of exposure to bad weather and the quantity of dangerous activities (including elevated work) are the indicators used to determine the degree of safety.
• Environment: Quantitative analysis of environmental sustainability is achieved by measuring GHG emissions.

The purpose of this study is to evaluate and analyze the comprehensive performance of the MiC project during the construction phase, including the transportation and consumption related to the production of raw construction materials, the transportation and energy consumption of construction materials, and the construction equipment and workers in the project construction process, and excluding the materials and energy related to the operation and maintenance stage and demolition stage. Therefore, the system boundary was established prior to the usage phase, as shown in Figure 2.

2.2. Calculation Boundaries

This research mainly analyzed four varieties: cost of construction, schedule of construction, safety level, and environmental sustainability.

2.2.1. Cost of Construction

Firstly, the cost of construction is one of the three primary constraints in a project, along with time and quality. As a result, we use the cost performance index (CPI) to evaluate the efficiency of project expenditure. The CPI explains the relationship between planned and actual costs at the project or task level, which can be calculated by Formula (1). A value greater than 1 indicates a good condition, while values less than 1 will be considered unfavorable.

$$\mathrm{CPI}={\displaystyle \frac{\mathrm{Budget} \mathrm{cost} \mathrm{of} \mathrm{activity}}{\mathrm{Actual} \mathrm{cost} \mathrm{of} \mathrm{activity}}},$$

(1)

2.2.2. Schedule of Construction

Secondly, in construction management, schedule adherence ranks among the most critical aspects of project execution. A meticulously developed schedule minimizes operational downtime while also ensuring that all project components are delivered within both schedule and budget constraints. Correspondingly, the SPI shows the degree of actual work completed compared to the schedule. Thus, SPI herein was calculated by Formula (2). A value higher than 1 indicates the project is progressing well, a value equal to 1 indicates the project proceeds as planned, and a value lower than 1 indicates that the project is running as planned.

$$\mathrm{SPI}={\displaystyle \frac{\mathrm{Budget} \mathrm{cost} \mathrm{of} \mathrm{work} \mathrm{performed}}{\mathrm{Budget} \mathrm{cost} \mathrm{of} \mathrm{work} \mathrm{scheduled}}},$$

(2)

2.2.3. Level of Safety

Thirdly, the safety level is also a factor of concern in the construction process. It can be divided into two indicators for measuring the level of safety, time of exposure to bad weather and the quantity of dangerous activities. These two safety issues mainly occurred during the construction project. As a result, safety is one of the important criteria for the feasibility of MiC for the residential building.

2.2.4. Sustainability of Environmental

Lastly, for the sustainability of the environment, LCPA has mainly discussed the environmental burdens related to material manufacturing and the construction stage. Residential construction GHG emissions primarily comprise CO₂, CH₄, and N₂O [43]. The Global Warming Potential (GWP) metric enables comparative assessment of climatic impacts across gases [44], quantifying relative radiative forcing per ton of emissions against CO₂ over specified time horizons [44]. The value of GWP is usually calculated over a period of 100 years. CO₂ has a GWP value of 1 by definition. CH₄ exhibits 28–36 GWP, while N₂O reaches 265–298 times CO₂-equivalence [45]. Therefore, according to the formula of Mao, Shen, Shen and Tang [43], GHG emission factors derive from Formula (3),

$$f_{ k}^{ t}={\sum }_{\mathrm{x}=1}^{\mathrm{x}}{\mathrm{F}}_{\mathrm{x}} \times {\mathrm{GWPV}}_{\mathrm{x}},$$

(3)

where F_x represents a given emission factor per unit of gas, x; and GWPV_x represents the GWP of gas, x.

According to studies by Mao, Shen, Shen and Tang [43] and Yan, et al. [46], the six emission sources determined during the construction stage were construction materials production and transportation, construction machinery transportation and energy expenditure, labor transportation, and removal of construction waste.

The GHG emissions of the MiC project can be divided into two categories based on the system boundary shown in Figure 1: off-site prefabricated modular components and on-site casting and assembly. For conventional construction methods, the GHG emissions only come from the on-site casting process.

According to Mao, Shen, Shen and Tang [43], this study counts the following five emission sources:

• E₁: Embodied carbon in main permanent construction materials;
• E₂: Transport emissions from construction materials logistics;
• E₃: Waste/soil transport fuel combustion;
• E₄: Modular components delivery emissions;
• E₅: Operational energy consumption (equipment/technologies), covering water, electricity, diesel, and oil.

Therefore, Formulas (4)–(8) compose the quantitative model of GHG emissions E₁–E₅ throughout the construction phase of the MiC project.

$${\mathrm{E}}_{1 }={\sum }_{\mathrm{a}=1}^{\mathrm{a}}{\mathrm{M}}_{\mathrm{a}} \times {\mathrm{f} }_{\mathrm{a}}^{\mathrm{b}} \times \left(1+{\mathrm{\epsilon }}_{\mathrm{a}}\right),$$

(4)

where $\mathrm{M}\mathrm{a}$ represents the quantity of construction material a (tons), ${\mathrm{f} }_{\mathrm{a}}^{\mathrm{b}}$ represents the GHG emission factor of a (kg CO_2-e/kg), and ${\mathrm{\epsilon }}_{\mathrm{a}}$ represents the factor that causes the waste of materials a during construction or transportation. According to Mao, Shen, Shen and Tang [43], Alcorn [47], and Chen, et al. [48], and using

Table 1. The GHG emission factors of the core construction materials.

Construction Materials	${\mathit{\epsilon }}_{\mathit{a}}$	CO2 Emission Factor (kg CO2-e/kg)	${\mathit{f} }_{\mathit{a}}^{\mathit{b}}$ (kg CO2-e/kg)
Cement	2.5%	0.653	0.698
Brick	2.5%	0.230	0.246
Steel	5.0%	0.352	0.367
Glass	0.5%	1.735	1.854
Ready-mixed concrete	2.5%	0.113	0.120
Insulating materials	5.0%	0.140	0.145

, we calculated the related ${\mathrm{f} }_{\mathrm{a}}^{\mathrm{b}}$ for the six main construction materials.

$${\mathrm{E}}_2={\sum }_{\mathrm{s}=1}^{\mathrm{k}}{\sum }_{\mathrm{a}=1}^{\mathrm{a}}{\mathrm{M}}_{\mathrm{a}} \times {\mathrm{L}}_{\mathrm{a}}^{\mathrm{m}} \times {\mathrm{f} }_{\mathrm{s}}^{\mathrm{t}}/1000,$$

(5)

where ${\mathrm{L}}_{\mathrm{a}}^{\mathrm{m}}$ represents the transportation distance (km); and ${\mathrm{f} }_{\mathrm{s}}^{\mathrm{t}}$ represents the GHG emission factor from fuel combustion generated by transport modes (kg CO_2-e/ton km), such as trucks, trains, or ships. According to Zhu and Chen [49], the ${\mathrm{f} }_{\mathrm{s}}^{\mathrm{t}}$ of different transportation methods are shown in

Table 2. The GHG emission factors of the different modes of transportation.

Modes of Transportation	Type of Energy	Energy Consumption (MJ/ton km)	Emission Factors (g/MJ)			${\mathit{f} }_{\mathit{s}}^{\mathit{t}}$ (kg CO2-e/ton km)
			CO2	CH4	N2O
Truck	Gas	3.663	74.800	0.010	0.012	0.288
Truck (>16t)	Diesel	2.423	74.800	0.070	0.030	0.207
Train	Diesel	0.362	74.800	0.010	0.086	0.036
Ship	Diesel	0.468	74.800	0.007	0.002	0.035

.

$${\mathrm{E}}_3={\sum }_{\mathrm{b}=1}^{\mathrm{b}}{\sum }_{\mathrm{c}=1}^{\mathrm{c}}{\sum }_{\mathrm{k}=1}^{\mathrm{k}}{\mathrm{W}}_{\mathrm{b}} \times {\mathrm{L}}_{\mathrm{c}}^{\mathrm{w}} \times {\mathrm{f} }_{\mathrm{s}}^{\mathrm{t}}/1000,$$

(6)

where W_b represents the quantity of construction waste and soil (tons), and ${\mathrm{L}}_{\mathrm{c}}^{\mathrm{w}}$ represents the transportation distance (km).

$${\mathrm{E}}_{4 }=\mathrm{P} \times {\mathrm{L}}^{\mathrm{p}} \times { \mathrm{f} }_{\mathrm{s}}^{\mathrm{t}}/1000,$$

(7)

where P represents the total amount of prefabricated modular components (tons), and $L^p$ represents the transportation distance (km).

$${\mathrm{E}}_5={\sum }_{\mathrm{r}=1}^{\mathrm{r}}{\sum }_{\mathrm{v}=1}^{\mathrm{v}}{\mathrm{R}}_{\mathrm{r}} \times {\mathrm{f} }_{\mathrm{n}}^{\mathrm{v}},$$

(8)

where R_r represents the resource consumption or energy usage (r) during the construction stage. Generally, construction equipment comprises concrete mixers, pumps, cranes, forklifts, welding machines, and construction elevators. Primary resource consumption spans three categories: diesel, electricity, and water.

According to EMSD [50], the ${\mathrm{f} }_{\mathrm{n}}^{\mathrm{v}}$ of resource consumption for construction equipment is shown in

Table 3. The GHG emission factors of the resource consumption by construction equipment.

Energy Types	Emission Factors			${\mathit{f} }_{\mathit{s}}^{\mathit{t}}$
	CO2	CH4	N2O
Diesel/oil	2.614 (kg/L)	0.024 (g/L)	0.007 (g/L)	${\mathit{f} }_1^{\mathit{v}}$ = 2.617 (kg CO2-e/L)
Electricity	0.500 (kg/kWh)	-	-	${\mathit{f} }_2^{\mathit{v}}$ = 0.500 (kg CO2-e/kWh)
Water	-	-	-	${\mathit{f} }_3^{\mathit{v}}$ = 0.414 (kg CO2-e/m3)

.

3. Case Study

3.1. Description of the Targeted Projects

This section compares these two varieties of housing projects with the requirements of process-based cost, time, safety, and environment, thereby conveying comprehensive calculations and preliminary research. Two varieties of projects are Project A, which adopted the method of MiC, and Project B, which adopted the conventional method.

Project A is located in the Sha Tin District of Hong Kong. It is a high-rise permanent MiC residential project. The structural system of this multi-story building uses a frame shear wall structure with MiC modules. The total construction area of Project A is approximately 15,300 m², with a building height of 59.4 m. The construction period was from August 2019 to August 2020. The entire project is overseen by a project manager and surveyors who provide reports through site surveys and regular inspections. The project, which won the 2019 Green Building Award, has a prefabricated level of 70% by volume, of which 13.2% is concrete volume. In addition to concrete, there are also varieties of off-site precast components, such as reinforced concrete, structural steel, and glass. The remaining parts are cast in place. In addition, since there are not many cases and data on adopting MiC methods in Hong Kong, data monitoring and collection were carried out through a combination of field surveys, archival analysis of project documentation, and semi-structured interviews with senior project managers. The layout of the MiC modules of Project A is shown in Figure 3. The size of each module is 7350 mm on the long side and 3100 mm on the wide side.

Project B represents a characteristic public rental housing project located on Chung Nga Road East in Hong Kong’s Tai Po District. According to the Housing Department, the permanent public housing is to hold 45% of the population by type of housing. It is powerful evidence that identifies the importance of public housing in Hong Kong. As a result, using the public housing to compare with Project A was more persuasive. Project A and Project B have some similar characteristics in terms of building type, structural frame, unit building area, and number of basements. However, Project B is not only one domestic building with 30 floors but also has a one-floor carpark and one six-story social welfare block. This study only examined the residential part of this project, using the data provided by the construction project manager.

The motivation for choosing these two projects for comparative analysis are as follows: (1) both projects were executed by the same contractor, Hip Hing Construction Company Limited, which controls a key variable and ensures that differences in management expertise, safety protocols, and operational efficiency do not affect the comparison between MiC and conventional methods; (2) both projects are public residential buildings in the unique context of Hong Kong (high-rise buildings, land scarcity, and logistics constraints), with similar geographical characteristics and social uses; (3) the structural attributes and sizes of the two projects are very similar, and this alignment minimizes functional and structural biases; and (4) the two projects are consistent and fair in terms of data source and collection, with data from the same construction company and original data recorded on site, as well as consistent LCPA boundaries and unified emission factors and calculation formulas.

The summary of the construction panel information for Project A and Project B is shown in

Table 4. A summary of the information between Project A and Project B.

Items	Project A: MiC	Project B: Conventional
Building types	Residential	Residential
Location	Hong Kong	Hong Kong
Total floor area	15,300.00 m2	44,640.00 m2
Floor area of each unit	17.00–46.17 m2	9.40–23.60 m2
Floor to floor height	3.05 m	2.75 m
Number of basements	1 Floors	1 Floor
Foundations	Socket-H piles	Bored pile foundation
Structure system	Frame–shear wall structure with MiC modules	Frame–shear wall structure
Height	59.40 m	104.00 m
Precast level	70.00%	16.31%
Structure frame	Beams, columns, slabs, and structure wall (cast-in-place reinforced concrete); MiC modules (off-site precast reinforced concrete and structural steel)	Cast-in-place reinforced concrete
Structure frame (for staircase and corridor slabs)	Cast-in-place reinforced concrete	Off-site reinforced concrete
External works (for facades)	Off-site prefabricated aluminum and glass	Off-site reinforced concrete
External works (for roof)	Cast-in-place reinforced concrete	Cast-in-place reinforced concrete
Internal works (for partition wall)	Partition walls laid on site (dry wall and brick)	Off-site reinforced lightweight wall panels (dry wall)

.

3.2. Data Acquisition on Environmental Sustainability

This study collected three types of data on environmental sustainability during the construction phase of Project A and Project B: (1) transportation and consumption of construction materials; (2) resource utilization of construction equipment; and (3) transportation of modular components and construction waste.

In this research, we mainly aimed to conduct an LCPA to determine the differences between these two projects. All formulas are used mainly to calculate the GHG emissions between the traditional method and the MiC method in the whole process of construction. Although these two projects are not the same as each other and assume some varied data, such as the total floor area, the distance of transportation, and the consumption of construction materials, the data are part of the research on the characteristics between these two methods, allowing people to understand the comprehensive of MiC method and the adaptability of MiC applied to residential housing in Hong Kong.

3.2.1. Transportation and Consumption of Construction Materials

This research examined six materials, namely ready-mixed concrete, cement, steel, glass, brick, and insulating materials. In addition, to ensure cross-project comparability, Project B’s total floor area was standardized to match Project A, with this adjusted scenario designated as Project B₀. The data acquisition will be illustrated in detail in the subsequent sections of this article.

Table 5. The consumption and transportation distance of construction materials.

Materials	Consumption (tons)						Transportation Distance (km)
	Project A			Project B0			Project A		Project B0
	On-Site	Off-Site	Total	On-Site	Off-Site	Total	${\mathit{L}}_1^{\mathit{m}}$ *	${\mathit{L}}_2^{\mathit{m}}$ **	${\mathit{L}}_1^{\mathit{m}}$	${\mathit{L}}_2^{\mathit{m}}$
Ready-mixed concrete	9877	6971	16,848	21,795	2543	24,338	4	27	4	27
Cement	400	0	400	527	0	527	-	20	-	20
Steel	1015	1466	2481	2446	109	2555	1600	6330	1600	6330
Glass	26	61	87	218	0	218	160	100	-	100
Brick	134	38	172	544	0	544	315	20	-	20
Insulating materials	143	484	627	54	0	54	130	9	-	9

* The transportation distance between the prefabrication components factory and the distribution center. ** The transportation distance between the construction site and the distribution center.

demonstrates the consumption and transportation distance of six construction materials during the construction period of Project A and Project B₀ (unit: 15,300 m²). Except for steel transported by ship between the distribution center and the construction site, all construction materials were transported by truck on the road.

3.2.2. Resource Utilization for Construction Equipment

At these two construction sites, the equipment that consumes diesel mainly includes excavators, bulldozers, cranes, and concrete pumps. The diesel consumption of the concrete pump transporting 1 m³ concrete on the construction site is about 0.80~1.20 L, and the standard value is 1 L/m³. The fuel consumption rate of excavators, bulldozers, and cranes is about 20 L/h.

The data in

Table 6. Resource utilization of equipment in the construction process.

Resource	Project A		Project B0
	Off-Site	On-Site	Off-Site	On-Site
Diesel/oil (L)	84.00	115,000.00	0.00	157,661.00
Electricity (kWh)	22.00	185,431.00	14,567.00	69,218.00
Tap water (m3)	3642.00	6315.00	685.00	9940.00

are provided by the project managers of these two projects, showing the total amount of resource usage, such as diesel, electricity, and water.

3.2.3. Transportation of Construction Waste and Prefabricated Components

Construction waste was transported using heavy trucks. Prefabricated components are mainly transported via shipping. The transportation volume and distance of construction waste and prefabricated components are shown in

Table 7. Transportation volume and distance of construction waste and prefabricated components.

Project	Construction Waste		Prefabricated Components (tons)
	Volume (tons)	Distance (km)	Volume (tons)	Distance (km)
Project A	852.00	27.00	6971.00	180.00
Project B0	16,508.00	27.00	2543.00	180.00

.

4. Result and Discussion

4.1. CPI, SPI, and Safety Level

According to the interview with the project manager of Project A, the total budgeted cost of Project A is on par with the actual total cost, which is HKD 700 million, and the CPI is approximately equal to 1, which means that the project is executed according to the budget. The total budgeted cost of Project B was HKD 768 million, while the actual total cost of Project B was HKD 720 million. CPI is approximately equal to 1.067. The CPI for Project B reflects that the project has performed very well within the budget. This study found that conventional construction performed better than MiC in terms of cost management. Since MiC is still in the pilot stage in Hong Kong, there may not be much cost data for reference, resulting in less precise cost management control. In contrast, Project B exemplifies Hong Kong’s standardized public housing program, comprising approximately 800,000 with consistent design parameters. Such construction uniformity facilitates precise budgetary control throughout project implementation.

During the design stage, according to the prediction of the contractor of Project A, if the traditional concrete structure construction is used, the construction period will be about 550 days, but the use of a modular building system can shorten the construction period to 397 days, saving about 30% of the construction period. The project has been successfully completed. According to official data, compared with traditional construction methods, MiC technology has shortened the construction period by 40%. The estimated construction period of Project A is 395 days, and the actual construction period is 330 days. Therefore, according to Formula (2), the SPI of Project A is 1.197. The estimated construction period of Project B is 1140 days, and the project was completed 30 days ahead of schedule. Therefore, the SPI of Project B is 1.027. Compared with conventional construction, MiC has better and more efficient schedule performance management.

Consider that the contractors for both projects are the same, with consistent safety procedures and training, as well as construction compliance and safety certifications. Therefore, the safety level of the construction phase in this study is mainly measured based on the number of dangerous activities of the project and the time exposed to severe weather. According to the data of Project A and Project B, there are no records of hazardous activities and accidents, such as high-altitude scaffolding work, unsafe conditions, etc. This shows that the safety equipment of Projects A and B is fully prepared, workers comply with work procedures, and the safety level is high. Regarding the exposure time to severe weather, Project A is 24 days, accounting for 7.27% of the total actual construction period, and Project B is 86 days, accounting for 7.74% of the total actual construction period. It can be concluded that the safety levels of these two projects are almost the same.

4.2. Environmental Sustainability

4.2.1. Total GHG Emissions

The total GHG emissions of Project A and Project B are calculated according to Formulas (3)–(8), as shown in

Table 8. The GHG emissions between Project A and Project B (unit: tonCO_2-e).

	Project A			Project B			Reduction of GHG Emissions
	Off-Site	On-Site	Total (x)	Off-Site	On-Site	Total (y)	y-x
E1	1619.29	1710.01	3329.29	354.79	4194.25	4549.04	1219.75
E2	508.83	281.44	790.27	38.32	670.59	708.91	−81.36
E3	0.03	4.58	4.62	0.00	92.26	92.26	87.65
E4	43.92	0.00	43.92	16.02	0.00	16.02	−27.90
E5	1.74	396.28	398.02	7.57	451.32	458.89	60.87
Sum			4566.12			5825.13	1259.01

. The total GHG emissions of Project A and Project B during the entire construction period are 4566.12 tonsCO_2-e and 5825.13 tonsCO_2-e, respectively (total floor area: 15,300 m²), equivalent to 298.44 kg/m² and 380.73 kg/m². Therefore, Project A reduces 8.23 tonsCO_2-e GHG emissions per 100 m² more than Project B, a reduction of approximately 21.6%.

The study by Zhang, Lee, Jaillon and Poon [4] mentioned that higher prefabrication levels can bring greater benefits to the environment, and their study showed that a 10.5% prefabrication level reduced GHG emissions by 3.2%. In this study, the prefabrication level of Project A was 54% higher than that of Project B, and GHG emissions were reduced by 21.6%. These data demonstrate a positive correlation between prefabrication levels and GHG emissions.

4.2.2. Embodied GHG Emissions of Construction Materials

Material embodied GHG emissions (E₁) constitute the dominant GHG source for both the MiC project and the conventional construction project, accounting for over 70% of total construction-phase carbon footprint. The E₁ of the MiC project is 217.60 kg/m², which is 26.8% lower than the conventional construction project. Ready-mix concrete is an important component in E₁, with both exceeding 60%, followed by steel, glass, and bricks.

On-site construction of conventional construction projects requires a large amount of concrete because the main structure is built of steel and concrete, such as facade components, floor slabs, and stairs. On-site construction and transportation processes often cause concrete waste, so the GHG emissions of ready-mixed concrete in conventional construction projects will be higher than in MiC projects.

The modular structure of the MiC project is a steel frame rather than reinforced concrete. Many projects use steel to make each module easy to transport. However, the MiC project also uses a lot of concrete on the modular interior and steel frames instead of concrete on the modular exterior walls, and this is why the steel GHG emissions between Project A and Project B are similar.

In addition, since the exterior wall uses a steel frame instead of concrete, the thickness of the steel-frame exterior wall is thinner than that of the concrete exterior wall, resulting in a decrease in thermal insulation performance. Therefore, the module needs to be installed with internal wall insulation to enhance its insulation. Therefore, the insulation material usage and corresponding GHG emissions of Project A exceed those of Project B.

Project A’s managerial report indicates that the MiC approach enhances materials’ efficiency through standardized designs protocols, precise dimensions, and systematic production control, effectively minimizing construction material waste generation via lean manufacturing principles. Therefore, the MiC approach is effective in reducing embodied greenhouse gas emissions from building materials.

4.2.3. GHG Emissions of Transportation

Three GHG emission factors related to transport are construction materials (E₂), construction waste (E₃), and modular components (E₄). The total GHG emissions from transportation are 838.81 tonsCO_2-e for the MiC project and 817.19 tonsCO_2-e for the conventional construction project, as shown in Figure 4.

Firstly, MiC projects have higher GHG emissions from the transportation of construction materials. The reason is that MiC projects have a lot of off-site processes, and many building materials and precast components go through distribution centers to precast component factories. Project A’s management team reports that the MiC implementation incorporates diverse prefabricated components and steel in off-site manufacturing, driven by evolving specifications and standardized requirements.

Secondly, regarding the GHG emissions generated by the transportation of construction waste, considering that construction soil has a high recycling rate of more than 90% in Hong Kong in the past decade, it can provide good carbon offsets, so it is not included in this calculation. Project B’s construction waste includes inserted construction waste and non-inserted construction waste. The types of construction waste inserted include concrete, rock, sand, and discarded bricks. Non-inserted construction waste includes wood, organic materials, vegetation, and other waste. The project manager of Project A only provided the amount of construction waste in the precast plant and the amount of construction waste on-site but did not involve the proportion of inserted and non-inserted construction waste. Since E₃ contributes only a small amount to the total GHG emissions during the construction phase, these data are only used as a reference value. In addition, according to contractor payment data, MiC technology can reduce construction waste by more than 50%, so the MiC project has a good carbon reduction effect on construction waste transportation.

Thirdly, regarding the GHG emissions generated by the transportation of prefabricated components, the number of prefabricated components in the MiC project is 6971 tons, while the number of prefabricated components in Project B is only 2543 tons. Therefore, the greenhouse gas emissions of the MiC project will be higher than those of the conventional construction project. Most of Hong Kong’s prefabricated-component factories are located in the coastal area of Guangdong Province, China. Therefore, large-scale prefabricated components and modular units can be completed via ship transportation, which greatly reduces transportation costs and GHG emissions generated by transportation.

4.2.4. Operational Carbon and Energy Demand of Construction Equipment (E₅)

The operational carbon footprint of construction equipment (diesel, electricity, and water) totaled 398.02 and 458.89 tons of CO_2-e for Projects A and B, respectively, reflecting a 13.3% reduction. Project A’s management reported that modular fabrication necessitates intensive electrical infrastructure—including welding systems, power tools, distribution panels, and cabling—to facilitate off-site equipment integration. Each installed component undergoes mandatory electrical safety testing. In addition, the components of the MiC methods are manufactured by factories. Many tasks can be simplified, such as integrating construction links and interfaces to reduce unnecessary construction waste from each link and interface.

Key LCPA indicators for both projects are comparatively presented in

Table 9. The key LCPA indicators of Project A and Project B.

Dimensions	Indicators	Project A	Project B
Cost	Total budget cost	HKD 700 million	HKD 768 million
	Actual total cost	HKD 700 million	HKD 720 million
	CPI	1.000	1.067
Time	Estimated construction period	395 days	1140 days
	Actual construction period	330 days	1110 days
	SPI	1.197	1.027
Safety	The number of dangerous activities	0	0
	The time of exposure to bad weather (as a proportion of the actual construction period)	24 days (7.27%)	86 days (7.74%)
Environment	E1	3329.29 tonsCO2-e	4549.04 tonsCO2-e
	E2	790.27 tonsCO2-e	708.91 tonsCO2-e
	E3	4.62 tonsCO2-e	92.26 tonsCO2-e
	E4	43.92 tonsCO2-e	16.02 tonsCO2-e
	E5	398.02 tonsCO2-e	458.89 tonsCO2-e

.

It is worth noting that MiC has not led to a reduction in costs in Hong Kong. On the contrary, the cost-control ability of traditional construction methods is better. This results from Hong Kong’s elevated logistics expenses and underdeveloped supply chain infrastructure. Therefore, Zhang, Pan, Pan and Wu [12] suggest conducting regular evaluations to monitor, analyze, and improve the performance of the supply chain. From the data of the time dimension in Table 9, it can be calculated that, compared with the conventional construction method, MiC can shorten the construction time by 14.20%, which is quite different from the 30% estimated value of the contractor. According to the interview with the project manager of Project A, as MiC is just getting started in Hong Kong, the supporting knowledge training and on-site management are facing challenges, and the production, transportation, and installation of irregular-shaped components are also the sinking points of time. In the subsequent MiC project, the construction period was advanced by more than 30%, making the time investment in the early stage worthwhile.

Therefore, during the design stage, projects that can be repeatedly laid out are conducive to standardization and economies of scale, thereby significantly saving costs. The number of newly planned public housing estates in Hong Kong is extremely large, and the number of floors is relatively high, which has a high degree of compatibility with the MiC method. The appropriate project must provide sufficient lead time for the customized MiC process [51].

In addition, a simple and flat terrain site is conducive to the efficient transportation and movement of the modules. The transportation of MiC components poses a challenge to a high-density and road-restricted city like Hong Kong. The urban roads in Hong Kong are relatively narrow and have certain height differences, which will generate additional carbon emissions during transportation. Consequently, geographical constraints and site-specific conditions influence MiC-implementation efficacy [52].

5. Conclusions

This study utilizes an extended life cycle assessment–LCPA to examine and compare a MiC pilot residential project and a conventional residential project in Hong Kong from multiples dimensions: cost, time, safety, and environment. Specifically, this study determined the computational boundaries of MiC based on textual analysis, including the cost performance index, schedule performance index, level of safety, embodied GHG emissions from construction materials, transportation of construction materials, transportation of construction waste, transportation of modular components, and equipment operation and construction technologies. I have revised it.

This study found that the GHG emissions of the MiC project and the traditional on-site construction project during the entire construction period were 298.44 kg/m² and 380.73 kg/m², respectively, witnessing a reduction of approximately 21.6%. Compared with the traditional construction project, the factors that actively promote the reduction of GHG emissions in the MiC project are the embodied GHG emissions of building materials, the transportation of construction waste, and the resource consumption of equipment, which reduced emissions by 1219.75 tonsCO_2-e (26.8%), 87.65 tonsCO_2-e (95.0%), and 60.87 tonsCO_2-e (13.3%), respectively. In addition, there was no significant difference in the safety performance of the two construction methods, but MiC projects have more efficient schedule performance management. Surprisingly, the cost control of MiC projects is not as good as that of conventional construction projects, which differs from some existing research results in other regions. Although the progress performance of MiC projects has increased by 14.20%, it has not yet reached the international leading level of 30%. Therefore, Hong Kong can choose a repeatable project design to improve standardization and save time and cost. In addition, the transportation of MiC components in Hong Kong has higher GHG emissions due to the narrow and large transition differences of Hong Kong’s urban road system.

This study has some limitations, and future research can be further improved and expanded in regard to several aspects. First, as the MiC project in Hong Kong is still in the pilot stage, this study only quantitatively analyzed the LCPA indicators in a limited number of case studies, and the conclusions focused on relative performance rather than absolute generalizations across the industry, thus limiting the generalizability of the findings. Although the differences between the two comparable projects were reduced by normalization for scale differences, this needs to be validated in more projects in the future. We agree that as the Hong Kong MiC market matures, case studies need to be expanded and guide future work in this direction. In addition, due to the limitations of data accessibility and contextual realities, the selection and evaluation of indicators for the safety dimension in LCPA are limited, and more abundant indicators (e.g., lost time, injury frequency, safety training time, etc.) need to be proposed in future studies to achieve repeatable research.