Carbon Emission Reduction Evaluation of End-of-Life Buildings Based on Multiple Recycling Strategies

Abstract

With the promotion of sustainability in the buildings and construction sector, the carbon saving strategies for the end-of-life (EoL) phase have been receiving increasing attention. In this research, life cycle assessment (LCA) theory was employed to study and compare the carbon savings benefits of three different management strategies (i.e., recycling, remanufacturing, and reuse) on the EoL phase of various buildings (including residential, office, commercial, and school buildings). Moreover, the carbon savings potential (CSP) was calculated and analyzed, which is defined as the percentage of the actual carbon savings to the sum of the total embodied carbon of the building. Results show that compared with traditional demolition and landfill treatment, the implementation of integrated management strategies for residential, office, commercial, and school buildings can reduce carbon emissions by 193.5–526.4 kgCO₂-e/m². Among the building materials, steel bar, structural steel, and concrete account for the major proportion of the total carbon savings of buildings (81.5–93.2%). The sequence of the CSPs for the four types of buildings, in descending order, is school, residential, commercial, and office buildings. A building with a life span of 50 years has the greatest CSP. The results of the study can be used to reduce environmental impacts, and have broad positive implications in terms of sustainable construction.

1. Introduction

The buildings and construction sector is responsible for 37% of energy and process-related emissions [1]. In addition, a large amount of construction waste is generated in this sector [2,3], which is approximately one-third of the waste stream [4], and these construction solid wastes are primarily derived from demolition during the end-of-life (EoL) phase of buildings and construction [5]. Many studies on the environmental impacts of the life cycle of buildings have been conducted [6,7,8]. The life cycle of a building can be divided into five stages: the product stage, construction process stage, use stage, EoL stage and the beyond the life cycle stage [9]. Considering that energy consumption in the use stage accounts for 52% to 82% of the total life cycle energy consumption during a 40- to 50-year life span [10], energy consumption in this stage has always been the focus of research. As a result, there are relatively few studies on the end of the life cycle of buildings. With the continuous development of passive and active energy-saving methods and renewable energy technologies, energy consumption in the use stage is becoming lower [11,12,13]. The proportion of environmental impacts of the EoL phase of buildings is increasing, and more attention must be paid to energy and carbon saving strategies at this stage.

Over the past decade, the principle of sustainability has become a fundamental requirement for modern building projects [14]. Through field sampling and research on C stocks and driving factors of large-scale ecosystems in the Loess Plateau, Yang et al. [15] proposed the C stocks estimation method, in order to achieve carbon saving targets in the Loess Plateau. Shang and Luo [16] used cloud computing to calculate a city’s carbon footprint and characterize its carbon sequestration capacity, providing a compelling perspective on carbon reduction strategies in complex urban environments. Based on a life cycle assessment, Han et al. [17] proposed a calculation method for carbon emissions of road infrastructure in demonstration zones to predict emission trends under different development scenarios. Guo et al. [18] improved the carbon trading market and implemented differentiated carbon emission reduction strategies, which have certain reference value for promoting the development of a low-carbon economy. The conversion of waste into resources to reduce carbon emissions is one of the elements within the framework of a circular economy system. Thus, special attention must be paid to innovation in building recycling, reuse of materials, and waste management.

Conventional buildings in the EoL phase are dealt with by direct demolition; that is, bulldozers, wrecking balls, explosives, and other extrusion forces are used to knock down and demolish buildings. In studies of the building demolition stage, most of the demolished building materials were directly buried as waste. For example, Junnila et al. [19] considered direct landfill as the material disposal method after demolition when assessing the life cycle impact of buildings in Europe and the United States. Although direct demolition and landfill is a simple way to dispose of building wastes, its adverse environmental and economic impacts are enormous. Construction waste accounts for approximately 10–30% of the waste disposed in landfills around the world [4]. In the UK, more than half of the construction and demolition wastes (C&DW) are transported directly to landfills [20]. Building deconstruction, which is a sustainable EoL disposal method used to replace building demolition, is the one-by-one decomposition of buildings to reuse materials to their maximum extent [5]. Building deconstruction emphasizes the deconstruction and decomposition of buildings, as well as the recovery and reuse of old materials [21]. The basic deconstruction requirement is to separate different kinds of components or materials to maintain the integrity of components for easy reuse [22]. The concept of deconstruction is developed to reduce the adverse environmental impact on the EoL phase by closing the material loop. Coelho analyzed the economic benefits of building deconstruction, and showed that some deconstruction schemes do present economic advantages [23]. Building deconstruction can save energy and reduce the burden of landfills, carbon emissions, and pollution [24,25,26]. Kibert and Languell [27] emphasized that approximately 80% of the total materials recovered from deconstructed buildings can be reused or recycled. Changing the trend from demolition to deconstruction is imperative in the EoL phase of buildings.

The energy-saving and carbon reduction benefits brought about by the implementation of reasonable EoL management strategies play a key role in reducing the adverse environmental impacts of the construction industry. The concepts of recycling, remanufacturing, and reuse are explained in the current research for the recovery of various buildings, building components, and materials. Recycling refers to the breaking and decomposition of building components into basic materials after deconstruction to replace raw materials for the production of building materials. For example, removed concrete can be broken and processed into aggregate, which is used to partially replace natural aggregates to produce recycled concrete [28], as shown in Figure 1a. Remanufacturing refers to the decomposition of building components into basic components after deconstruction, and the usable parts are reused in new buildings after simple processing [29]. For example, part of walls is broken down into blocks and used in walls of new buildings (Figure 1b). Reuse refers to the reuse of building components or materials in accordance with their original purposes after deconstruction [30]. For example, some steel structural components are directly reused as components of new buildings (Figure 1c). Scheuer et al. [31] analyzed the life cycle impact of buildings, assuming that some of the demolished building materials were recyclable. However, the specific recycling scheme and recycling ratio of each material were not elaborated further. Gao et al. [32] studied the recycling potential of building materials at the end life of buildings, and concluded that the recycling of steel structural building materials could save 13% of energy consumption, whereas the recycling and reuse of materials could save 25% of energy consumption. Gian [33] established a life cycle assessment (LCA) model for the EoL stage of building by analyzing the actual recycling potential of building materials from economic and environmental perspectives. Brown and Buranakarn [34] evaluated the energy consumption of the recovery of construction waste and determined that recycling aluminum and steel had the greatest advantage. The reuse of cork frames and hardwood flooring can save 6467 MJ/m³ and 7763 MJ/m³ of raw material production energy, respectively [35]. Queheille et al. [36] analyzed 40 types of waste and 7 waste treatments, namely reuse, recycling, backfilling, composting (biological waste), sorting, incineration, and landfill, in order to comprehensively assess the environmental impacts of the EoL stage of buildings. Recycling and reusing construction wastes can reduce the impacts of energy consumption and carbon emissions in the EoL stage of building, and can reduce the greenhouse gas emissions generated by material production and transportation by approximately 28% [37]. The reuse and recycling of construction wastes can produce good environmental benefits because it will enter the life cycle of the new subsequent buildings, thereby avoiding the consumption of natural resources [31]. The concept of remanufacturing, which is between reuse and recycling, has been proposed in the existing literature. However, its environmental effect evaluation has not been reported.

Currently, studies have been comparing the energy saved by implementing different recovery strategies at the end of life of different building structures, but the carbon saving effects of different building types have rarely been studied. Aye et al. [38] studied the energy consumption of an eight-story concrete structure with prefabricated steel and timber systems, and found that 32.3%, 69.1%, and 81.3% of the total embodied energy could be saved if each of these building materials was reused, respectively. Thormark [30] established two recycling modes for the EoL of low-energy buildings, material recycling and incineration with heat recovery and material reuse, and concluded that reuse treatment could save 39% of the embodied energy, whereas material recycling and incineration with heat recovery could save 35% of the embodied energy. Dewulf et al. [39] analyzed the recovery potential of some materials in the demolition stage of large townhouses, and proposed treatment methods such as reuse, recycling, and incineration with heat recovery. The embodied energy can be reduced by 15% by combining various treatment methods.

In general, current studies on management strategies at the EoL stage of a building mainly focus on energy savings. Systematic studies on deconstruction management strategies during the EoL phase of different types of buildings, including remanufacturing and the analysis of carbon saving benefits, have not been reported in the literature. Therefore, this research intends to study the management strategies (e.g., recycling, remanufacturing, and reuse) and carbon saving benefits of four types of buildings at their end-of-life stage (e.g., residential, office, commercial, and school buildings), obtain the carbon saving potential (CSP) of different types of buildings and building materials, and then determine the building types and materials with the greatest CSP at the end of life of a building. The influence of building life span on the CSP is also analyzed.

2. Methodology

In this study, the LCA is used to assess the environmental impact of the deconstruction of different buildings. LCA, as a comprehensive environmental management tool, can be used to evaluate the overall environmental impact of products related to input and output. LCA has been widely used to assess the environmental impacts of products and processes in the construction industry [40,41,42]. The LCA framework is divided into four steps: goal and definition, inventory analysis, impact assessment, and interpretation [43]. In this study, the application of LCA is limited to the EoL stage of a building.

2.1. Goal and Definition

According to the Inventory of Carbon and Energy (ICE) database [44], the embodied carbon of building materials refers to the carbon emissions generated from the “cradle to gate” process of building materials, including the extraction of raw materials, transportation to the manufacturer, and production of building materials. The purpose of this study is to evaluate the carbon saving benefits of different EoL management strategies for four types of buildings, including residential, office, commercial, and school buildings. The boundary includes the building deconstruction, the transportation from the deconstruction site to the disposal site, the processing of building materials and the landfill. According to the system boundary defined in this study, the C&DW obtained from the EoL stage is deconstructed, transported, and processed to form usable building materials. This is consistent with the embodied carbon boundary of natural building materials, in order to evaluate the carbon saving benefits of recycling C&DW compared with the embodied carbon of natural building materials. Figure 2 defines the system boundaries of this study. The material consumption of different types of buildings is different, which produces different quantities of C&DW materials such as concrete, reinforcing bar, structural steel, and so on. The specific parameters corresponding to different types of C&DW are different in the boundaries, including carbon emissions in the recycling process, carbon emissions in the remanufacturing process (e.g., recycling, remanufacturing, reuse and landfill), embodied carbon of building materials, the life span of building materials, and the proportional distribution of different management strategies. The respective environmental impacts of different types of C&DW materials will be discussed in the following chapter. Greenhouse gases emitted in the production and transportation stage of building materials are diverse. To measure the overall greenhouse effect uniformly, a measurement unit that is capable of comparing emissions of different greenhouse gases is needed. Considering that CO₂ contributes the most to global warming, carbon dioxide equivalent is defined as the basic unit for measuring the greenhouse effect [45]. Furthermore, the carbon dioxide equivalent is generated per unit building area (kgCO₂-e/m²) as the functional unit. Based on the uniform standard for civil building design (GB50352—2019) [46], the designed service life of ordinary buildings is usually specified at 50 years. Therefore, the life span of the four various types of buildings is assumed to be unified to 50 years, which may facilitate the assessment of the carbon saving benefits of different types of buildings.

2.2. Inventory Analysis

In view of the large amount of data required for LCA, data uncertainty is a critical problem that must be addressed [47]. To ensure the accuracy and reliability of the research, this study extracted the information of material consumption per unit building area of the four types of buildings from the large database of the Shenzhen Sware Budget Platform [48]. The data are representative and practical, as they are taken from the material consumption of a large number of different buildings. Therefore, the building type involved in this study is not a single building structure; it includes a variety of building structure types, such as frame structure, frame structure, light steel structure, and so on. The details are presented in

Table 1. Material consumption per floor area of different types of buildings (kg/m²).

Name of the Materials	Residential	Office	Commercial	School
Concrete	1204.19	1371.83	1142.80	1473.36
Reinforcing bar	52.00	69.00	58.00	69.00
Structural steel	26.00	11.00	10.00	103.00
Cement	12.00	33.00	13.00	38.00
Mortar	183.00	158.00	88.00	209.00
Sand	22.40	107.80	40.60	74.20
Insulation material	16.94	13.94	10.30	14.36
Blocks	28.80	51.60	91.80	48.60
Bricks	117.00	135.00	88.20	131.40
Aluminum windows	2.24	1.64	2.13	2.65
Plywood doors	0.08	0.04	0.08	0.03
Ceramics	14.59	16.37	6.10	19.10
Lead	0.05	0.17	0.05	0.08
PVC pipes	2.00	1.50	1.60	1.20

. For example, the amount of materials used in each unit of floor area of residential buildings is the average amount of materials used in each unit of floor area of residential buildings (including different structures) in the statistical data. The statistics in Table 1 show that residential, office, and commercial buildings are mainly concrete structures, and school buildings are usually light steel structures.

2.3. Calculation Method

Some building materials, such as doors, windows, and lead, must be replaced in the life span of the building. To obtain the carbon savings of material i with the number of times of replacement needed for material i, the number of times of replacement of material i ($n_i$) is defined as follows [49]:

$$n_i=\frac{x}{{\mu }_i}\left\{\begin{array}{c}{n}_i\le 1, n_i=0\\ n_i>1, n_i=\left[n_i+0.5\right]\end{array}\right.$$

(1)

where $n_i$ is the number of times of replacement needed for material I throughout the building life span, x is the building life span, and ${\mu }_i$ is the life expectancy of material i. When $n_i$ is less than or equal to 1, the building life span is less than or equal to the life expectancy of material i, and the material i does not need to be replaced, $n_i$ = 0. When $n_i$ is greater than 1, the building life span is longer than the life expectancy of material i, and material i needs to be replaced (if it is a decimal, then round to the nearest integer).

2.3.1. Carbon Savings Due to Recycling

The carbon emissions of material i from the recycling strategy are estimated as follows:

$$C_{recycle,proci}=C{D}_i+C{T}_{recycle,i}+C_{recycle,i}$$

(2)

where $C_{recycle,proci}$ is the carbon emissions of material i involved in the recycling strategy, $C{D}_i$ is the carbon emissions of material i generated during the deconstruction of construction, $C{T}_{recycle,i}$ is the transportation carbon emissions of material i from the deconstruction site to the recycling processing plant, and $C_{recycle,i}$ is the carbon emissions generated during the recycling processing of material i.

The carbon savings of material i, by adopting a recycling strategy, are estimated as follows:

$$C{S}_{recycle,i}=R_{recycle,i}(E{C}_{0,i}-C_{recycle,proci})(n_{\mathrm{i}}+1)$$

(3)

where $C{S}_{recycle,i}$ is the carbon savings of material i by adopting a recycling strategy, $R_{recycle,i}$ is the recycling fraction of material i (the ratio of the amount of material i recycled to its total amount), and $\mathrm{E}{\mathrm{C}}_{0,\mathrm{i}}$ is the embodied carbon of material i.

2.3.2. Carbon Savings Due to Remanufacturing

The carbon emissions of material i from the remanufacturing strategy are estimated as follows:

$$C_{remanufacture,proci}=C{D}_i+C{T}_{remanufacture,i}+C_{recmanufacture,i}$$

(4)

where $C_{remanufacture,proci}$ is the carbon emissions involved in the remanufacturing strategy of materials i, $C{T}_{remanufacture,i}$ is the transportation carbon emissions of material i from the deconstruction site to the remanufacturing processing plant, and $C_{remanufacture,i}$ is the carbon emissions generated during the remanufacturing processing of material i.

The carbon savings of material i, by adopting a remanufacturing strategy, are estimated as follows:

$$C{S}_{remanufacture,i}=R_{remanufacture,i}(E{C}_{0,i}-C_{remanufacture,proci})(n_i+1)$$

(5)

where $C{S}_{remanufacture,i}$ is the carbon savings of material i by adopting the remanufacturing strategy, and $R_{remanufacture,i}$ is the remanufacturing fraction of material i (the ratio of the amount of material i remanufactured to its total amount).

2.3.3. Carbon Savings Due to Reuse

The carbon emissions of material i from a reuse strategy are estimated as follows:

$$C_{reuse,proci}=C{D}_i+C{T}_{reuse,i}$$

(6)

where $C_{reuse,proci}$ is the carbon emissions involved in a reuse strategy of material i, and $C{T}_{reuse,i}$ is the transportation carbon emissions of material i from the deconstruction site to the storage sites.

The carbon savings of material i, by adopting a reuse strategy, are estimated as follows:

$$C{S}_{reuse,i}=R_{reuse,i}(E{C}_{0,i}-C_{reuse,proci})(n_i+1)$$

(7)

where $C{S}_{reuse,i}$ is the carbon savings of material i by adopting the reuse strategy, and $R_{reuse,i}$ is the reuse fraction of material i (the ratio of the amount of material i reused to its total amount).

2.3.4. Carbon Emissions in Landfill

After the building is demolished, the C&DW will be transported to a landfill, which leads to energy waste and carbon emissions. The carbon emissions of material i generated by landfill are estimated as follows:

$$C_{landfill,i}=C_{Demolition,i}+C{T}_{landfill,i}$$

(8)

where $C_{landfill,i}$ is the carbon emissions generated by the landfill of material i, $C_{Demolition,i}$ is the carbon emissions generated during the demolition of material i, and $C{T}_{landfill,i}$ is the transportation carbon emissions of material i from the deconstruction site to the landfill.

2.3.5. Parameter Determination

To evaluate the carbon saving benefits of different EoL management strategies for building materials effectively, the use of materials is assumed to remain unchanged before and after recovery. Construction waste includes inert and non-inert components. Inert components include concrete, cement, mortar, blocks, bricks, and other materials. Non-inert components include metals, wood, plastics, and other materials. Compared with non-inert components, the recovery of inert materials in construction waste requires more energy consumption and produces more carbon dioxide. Due to the lack of carbon emission factors for building materials recycling in China, this research chose the ICE database [44]. According to the ICE database, the recycling carbon emissions of non-inert materials such as aluminum, brass, lead, steel, and zinc account for 17.85% of their natural embodied carbon value EC_0,i on average. Therefore, the carbon emissions of non-inert material i for recycling processing $C_{recycle,i}$ without carbon emission data information are 17.85% EC_0,i. According to study [48], the carbon emissions of concrete recycling accounted for 93.73% of the natural embodied carbon value EC_0,i. Therefore, the carbon emissions of inert material i for recycling processing $C_{recycle,i}$ without carbon emission data information are 93.73% EC_0,i. By definition, recycling emphasizes crushing and consumes more energy. Remanufacturing emphasizes decomposition and microprocessing. Compared with the energy consumption generated by recycling, remanufacturing consumes less energy, and reuse does not generate energy consumption. Due to the lack of engineering data for the remanufacturing process, the carbon emissions of material i for the remanufacturing process are assumed to be half of that of the recycling process, according to the aforementioned definition of material remanufacturing and recycling. The carbon emissions of non-inert material i for the remanufacturing process $C_{remanufacture,i}$ are 8.93%EC_0,i, and those of inert material i for the remanufacturing process $C_{remanufacture,i}$ are 46.83%EC_0,i. The embodied carbon, carbon emissions of recycling processing, carbon emissions of remanufacturing processing, and life expectancy of building materials are summarized in

Table 2. Carbon equivalents per unit mass and life expectancies (kgCO₂-e/kg) of building materials (year) [44,49,50].

Name of the Materials	Embodied Carbon	Carbon Emissions of Recycling Processing	Carbon Emissions of Remanufacturing Processing	Life Expectancy
Concrete	0.107	0.1	0.050	75
Reinforcing bar	2.09	0.373	0.187	71
Structural steel	2.89	0.47	0.235	71
Cement	0.74	0.694	0.347	50
Mortar	0.221	0.207	0.104	50
Sand	0.0051	0.0047	0.0025	50
Insulation material	1.86	0.332	0.166	50
Blocks	0.084	0.079	0.039	100
Bricks	0.240	0.225	0.113	100
Aluminum windows	1.160	0.616	0.308	27
Plywood doors	1.838	0.626	0.313	31
Ceramics	0.7	0.656	0.328	75
Lead	3.37	0.58	0.290	20
PVC pipes	3.32	0.593	0.296	30

.

In addition to the carbon emissions data of material processing with different EoL management strategies, this study included the carbon emission data related to material deconstruction, demolition, and transportation. According to study [36], the combined demolition method of manual and mechanical was used to dismantle and deconstruct a building with a total construction area of 15,473 m². The specific carbon emissions generated by manual and mechanical processes are shown in

Table 3. The carbon emissions of building demolition and deconstruction.

	Employee × Day	Artificial Carbon Emission Factor (kgCO2-e/Employee × Day) [44]	Artificial Carbon Emissions (kgCO2-e)	Mechanical Energy Consumption (Diesel, L)	Diesel Carbon Emission Factor (kgCO2-e/L) [45]	Mechanical Carbon Emissions (kgCO2-e)	Total Carbon Emissions (kgCO2-e)	Carbon Emissions per Building Area (kgCO2-e/m2)
Demolition	419	4.16	1743.04	18,675	3.178	70,591.5	72,334.54	4.67
Deconstruction	588	4.16	2446.08	17,066	3.178	54,235.748	56,681.82	3.66

. The carbon emissions generated by building demolition and deconstruction are 4.67 kg/m² and 3.66 kg/m², respectively. For a particular city, processing plants and landfills should be located close to the deconstruction site to reduce the carbon emissions generated during transportation. The average transportation distance associated with different EoL management strategies is shown in

Table 4. Transportation distances of different EoL management strategies.

Process	Transport Distance
Storage sites for reuse materials	30
Recycling/remanufacturing plants (e.g., cement, concrete, sand)	30
Recycling/remanufacturing (e.g., other materials)	50
Landfill sites	30

[30]. A medium diesel truck (with a load of 8 t) is assumed to be used in the transportation process, and its unit carbon emissions are 0.179 kgCO₂/(t • km) [51].

3. Results and Analysis

3.1. Single Management Strategy

In LCA, a scenario is defined as “a description of possible future situations related to a particular LCA application based on specific assumptions about the future” [52]. The scenario based on a single management strategy corresponds to building deconstruction in which the building materials are taken for 100% recycling, 100% remanufacturing, or 100% reuse. A total of 100% recycling refers to transporting all materials to a recycling plant for crushing processing to obtain basic materials after the building deconstruction. A total of 100% remanufacturing refers to the decomposition of all materials into basic components after the building deconstruction for reuse in a new building. A total of 100% reuse refers to the reuse of all materials after the building deconstruction. Figure 3 shows the carbon savings of three management strategies for different buildings in the scenario based on a single management strategy. Compared with 100% waste landfill, the three management strategies bring substantial carbon savings benefits. Among the three management strategies, reuse is the best, which is followed by remanufacturing and recycling. Furthermore, the building type with the largest carbon savings per unit building area is school buildings. This is reasonable that the amount of structural steel used in school buildings is 5–10 times that of other buildings when the consumption of other materials is similar and the embodied carbon of structural steel is larger.

Figure 4 shows the carbon savings of four types of building materials under different management strategies and landfill disposal. As shown in Figure 4, under the 100% recycling management strategy, for all four types of buildings, the materials with significant carbon saving benefits from the largest to the smallest are steel bar, structural steel, and insulation materials. Under the 100% remanufacturing management strategy, for all four types of buildings, the materials with significant carbon saving benefits are steel bar, structural steel, and concrete. Under the 100% reuse management strategy, the materials with significant carbon saving benefits are concrete, steel bar, and structural steel for residential, commercial, and school buildings; for office buildings under the 100% reuse management strategy, the materials with significant carbon saving benefits are concrete, steel bar, and mortar. The reason for the difference in the materials with significant carbon saving benefits between office buildings and other buildings is that the amount of mortar used in office buildings is approximately 14 times that of structural steel, and the embodied carbon of structural steel is approximately 13 times that of mortar. Hence, the carbon saving benefits of mortar are greater than that of structural steel for office buildings.

3.2. Integrated Management Strategy

The actual treatment of materials after building deconstruction is unlikely to adopt a single management strategy but a combination of various management strategies (recycling, remanufacturing, reuse, and landfill disposal). To this end, this study combined these three management strategies and landfill disposal methods in different proportions to study, namely the following:

$$R_{landfill,i}+R_{recycle,i}+R_{reuse,i}+R_{remanufacture,i}=1$$

(9)

where $R_{landfill,i}$ is the proportion of landfill disposal of the material i (that is, the percentage of the landfill disposal of material i in the total amount).

According to study [53], the proportions of treatment methods of various building materials under the integrated management strategy are summarized in

Table 5. Proportion distribution of integrated management strategy for building materials.

Name of the Materials	Rrecycle,i	Rremanufacture,i	Rreuse,i	Rlandfill,i
Concrete	51.99%	21.62%	11.31%	15.08%
Steel bar	67.07%	21.62%	11.31%	—
Structural steel	33.33%	—	65.71%	0.96%
Cement	—	—	—	100.00%
Mortar	80.00%	—	—	20.00%
Sand	—	—	—	100.00%
Insulation material	51.99%	21.62%	11.31%	15.08%
Blocks	80.00%	—	—	20.00%
Bricks	80.00%	—	—	20.00%
Aluminum windows	100.00%	—	—	—
Plywood doors	100.00%	—	—	—
Ceramics	80%	—	—	20%
Lead	51.99%	21.62%	11.31%	15.08%
PVC pipes	51.99%	21.62%	11.31%	15.08%

.

Under the integrated management strategy, when three management strategies are adopted in a certain proportion, the carbon savings of material i are as follows:

$$C{S}_{recycle,i}+C{S}_{remanufacture,i}+C{S}_{reuse,i}$$

(10)

Accordingly, when three management strategies are adopted for material i, the carbon savings of the remaining part by landfill disposal are presented as follows:

$$-R_{landfill,i}{C}_{landfill,i}(n_i+1)$$

(11)

At the same time, due to the three management strategies adopted by material i, it can also save the carbon emissions generated by landfill disposal after traditional demolition:

$$(R_{recycle,i}+R_{remanufacture,i}+R_{\mathrm{reuse},i})C_{landfill,i}(n_i+1)=(1-R_{landfill,i})C_{landfill,i}(n_i+1)$$

(12)

The actual carbon savings of material i can be obtained by adding Equations (10)–(12):

$$C{S}_{real,i}=C{S}_{recycle,i}+C{S}_{remanufacture,i}+C{S}_{reuse,i}+(1-2R_{landfill,i})C_{landfill,i}(n_i+1)$$

(13)

where $C{S}_{real,i}$ is the actual carbon savings of material i.

Figure 5 shows the carbon savings of the main materials of different types of buildings under the integrated management strategy. For residential buildings, the materials with significant carbon saving benefits from large to small are steel bar, structural steel, and concrete. For office and commercial buildings, the materials with significant carbon saving benefits from large to small are steel bar, concrete, and structural steel. For school buildings, the materials with significant carbon saving benefits from large to small are structural steel, steel bar, and concrete. The materials of the four types of buildings with significant carbon saving benefits are steel bar, concrete, and structural steel. Figure 6 shows the carbon savings of different types of buildings under the integrated management strategy. In general, the carbon savings of implementing integrated management strategy are 16.8–34.9 times that of the carbon emissions generated by landfill. Among the four types of buildings, school buildings have the greatest carbon saving benefits, whereas commercial buildings have the least carbon saving benefits. As a result of the consumption of structural steel in school buildings being about 5–10 times that of other types of buildings, and the reuse rate of structural steel is high under the integrated management strategy, school buildings have the greatest carbon savings benefits, whereas the consumption of concrete and structural steel in commercial buildings is lower than those of the other types of buildings. Hence, commercial buildings have the least carbon savings benefits.

3.3. Carbon Saving Potential

Existing studies have proposed the concepts of material recovery and energy savings potential when evaluating the impact of recovery and disposal of construction waste on the environment [28,44]. To facilitate the quantification and comparison of carbon savings of different materials, the concept of carbon savings potential of material i (CSP_i), which is the ratio of actual carbon reduction in material i to the sum of embodied carbon of all building materials, is presented in this research as follows:

$$CS{P}_i=\frac{C{S}_{real,i}}{{\displaystyle \sum E{C}_{0,i}(n_i+1)}}$$

(14)

Figure 7 shows the CSPs of different major building materials under the integrated management strategy. The CSPs of materials in office and commercial buildings have the same rule. In descending order, their CSPs are steel bar, concrete, structural steel, insulation materials, and PVC pipes. Although the amount of concrete is about 20 times that of steel bar, the embodied carbon of a steel bar is nearly 20 times that of concrete, and the recycling rate of steel bar is greater than that of concrete under the integrated management strategy. Thus, the CSP of steel bar is greater than that of concrete. In descending order, the CSPs of residential building materials are steel bar, structural steel, concrete, insulation materials, and PVC pipes. The reason for the regular difference between residential buildings and office and commercial buildings is that the consumption of structural steel in residential buildings is around twice as much as those in office and commercial buildings, and structural steel has more embodied carbon and a higher reuse rate. Hence, the CSP of structural steel in residential buildings is greater than those in office and commercial buildings. In school buildings, the CSP of structural steel is the largest, accounting for 62.2% of the total CSP of school buildings, followed by steel bar, concrete, insulation materials, and PVC pipes.

3.4. Impact of Building Life Span

The carbon saving potential (CSP) of a building is influenced not only by management strategies and building types, but also by the life span of the building, which affects the number of times building materials are replaced. Considering that the building is composed of various building materials, and different building materials have dissimilar service lives, the influence of different building life spans on the CSP of buildings can be studied by collecting and analyzing the replacement times of building materials within the building life span. The average life span of buildings is expected to be 25–30 years in China [54,55], and the minimum life span is assumed to be 20 years. Figure 8 shows the influences of building life span changes from 20 to 70 years on the CSPs of four types of buildings under the integrated management strategy. As the life span of a building increases from 20 years to 50 years, the CSP of the building rises due to the increased replacement of doors, windows, lead, and PVC pipes, as well as the high recycling rate and low abandonment rate of these materials. With the life span of a building increasing from 50 years to 70 years, the CSP of the building decreases because of the increased replacement of cement, mortar, and sand with a low recycling rate and high abandonment rate. Buildings have the greatest CSP when the life span is 50 years. Among the four types of buildings, school buildings have the highest CSP (57.18–64.01%), followed by residential buildings (48.74–51.03%), commercial buildings (48.66–51.14%), and office buildings (43.70–47.41%).

4. Discussion

It was found that 32.3%, 69.1%, and 81.3% of the total embodied energy consumption could be saved if all building materials were reused after deconstructing the concrete structure, prefabricated steel structure, and wood structure at the end of a building’s life [38,56]. The material in Table 1 shows that compared with other buildings, school buildings have the largest content of structural steel. The carbon savings potential of school buildings under the integrated management strategy is 66%, which is consistent with the literature [38]. The amount of building materials used in different structures, the embodied carbon of materials, and the processing carbon emissions of materials adopted by management strategies lead to differences in the carbon savings benefits of materials among various buildings and different management strategies. In this research, three different management strategies are adopted to improve the recovery rates of construction and demolition wastes, avoid the accumulation of construction and demolition wastes, and replace natural resources to avoid the environmental impact brought by the use of natural resources. The recycling, remanufacturing, and recycling of construction and demolition wastes will reduce the impact on the natural environment to the lowest possible degree, in order to obtain the best technical and environmental benefits, as well as social sustainability.

5. Conclusions

In this study, the EoL management strategies (e.g., recycling, remanufacturing, and reuse) and carbon saving benefits of four types of buildings of residential, office, commercial, and school buildings were studied. The main conclusions are presented as follows:

1.

Compared with traditional demolition and landfill disposal, the implementation of three management strategies (e.g., recycling, remanufacturing, and reuse) for deconstructed building materials at the end of life of residential, office, commercial, and school buildings can produce significant carbon saving benefits. The carbon savings of three management strategies are 15.9–33.1 times that of the carbon emissions generated by landfill.

2.

In comparing the CSPs of different types of buildings, the buildings with the highest CSP are school buildings. Among the building materials, the CSPs of steel bar, structural steel, and concrete accounted for the largest proportion in the total CSP of buildings, with CSPs of 81.5–93.2%. Accordingly, more attention should be paid to those three materials in the actual construction deconstruction to improve the recovery rate of those materials.

3.

When the building life span increases from 20 years to 70 years, the CSP of the buildings gradually increases first, and then decreases gradually. When the building life span is 50 years, the CSP of the building reaches its maximum value. The optimal building life span during the building deconstruction is 50 years. Therefore, from the perspective of reducing carbon emissions, it is better to deconstruct the building when its service life reaches 50 years, which is conducive to improving the CSP of the building.