Carbon Emission Analysis of RC Core Wall-Steel Frame Structures

Abstract

The development of super high-rise building projects has become crucial for mitigating land shortages in rapidly growing urban areas. Super high-rise steel structures, particularly RC core wall-steel frame systems, have become the preferred choice due to their superior performance, high prefabrication level, and construction efficiency. Despite their benefits, super high-rise buildings face challenges related to higher energy consumption and carbon emissions. Consequently, it is important to analyze the carbon emissions of these buildings throughout their lifecycle and propose low-carbon construction strategies. A carbon emission analysis focused on super high-rise buildings with RC core wall-steel frame structures is conducted in this study. A carbon emission analysis model is constructed based on BIM-enabled LCA through a real-world case study. The emission factor method is combined with the BIM model to calculate carbon emission. Furthermore, carbon emissions across various construction strategies are compared, with a particular focus on the manufacturing processes of the main materials. The results indicate that incorporating admixtures in concrete, along with adopting the electric arc furnace (EAF) method and utilizing recycled scrap steel in steel manufacturing, significantly reduces the carbon emissions of the buildings. Lastly, effective low-carbon approaches for these buildings are proposed.

1. Introduction

With the decrease in natural resources and the global variation in climate, the construction industry, as a significant factor impacting the global environment, faces many challenges. In response, the engineering field has developed numerous new technologies [1]. Advancements, including high-performance novel structures [2,3] and digital design methods [4,5], are being extensively researched to achieve the goal of sustainable development. Furthermore, the construction industry is responsible for about 30–40% of total carbon emissions across all industries [6]. Therefore, the calculation and analysis of carbon emissions of buildings is crucial for achieving energy conservation, emission reduction, and sustainability.

Currently, the development and promotion of super high-rise building projects have become key measures to alleviate the land shortage problem caused by the rapid population growth in urban communities [7]. These projects are effective in optimizing architectural functions and increasing property value [8]. The super high-rise steel structure system has gradually become the preferred structural option, attributed to its superior structural performance, high degree of prefabrication, and high construction efficiency. Khan et al. [9] reviewed the development and potential of super high-rise prefabricated steel structures, while Gong et al. [8] elaborated on the development process of engineering technology for super high-rise buildings in China. In particular, RC core wall-steel frame structures have emerged as the mainstream structural form due to their structural stability and scalability. Many studies have been conducted focusing on these structures from aspects including health monitoring [10], structural components [11], and mechanical response [12]. Additionally, the parallel construction mode of the steel frame and RC core tube significantly accelerates the construction progress and effectively shortens the construction period.

However, due to their huge material usage and complex energy requirements, super high-rise buildings often entail higher energy consumption and carbon emissions [13]. Therefore, from the perspective of long-term sustainable development, reducing the energy consumption and greenhouse gas emissions of super high-rise buildings is crucial. Measures related to different elements throughout the life cycle of the building need to be utilized to achieve energy conservation and emission reduction. Significant progress has been made in the research on green design for high-rise and super high-rise buildings. Some advancements have improved the energy efficiency [14,15] of super high-rise buildings through energy-saving designs [16], including utilizing adaptive building skins [17,18,19]. Tabadkani et al. [16] developed and optimized an origami-based adaptive solar façade system utilizing parametric design tools aiming at enhancing comfort and sustainability. Progress has also been made by achieving sustainability goals through the optimization of structural designs [20,21]. For instance, Choi et al. [22] proposed a multi-objective green design model aimed at minimizing construction costs and reducing carbon emissions. Gan et al. [23] conducted comparative analyses of carbon emissions in high-rise buildings under different design parameters, including building materials, recycled components, structural forms, and building height. Choi et al. [24] proposed an optimal design method with a genetic algorithm to reduce cost and CO₂ emissions from structural materials in high-rise buildings. The method was successfully applied to a 35-story building.

In addition to the overall control during the building design phase, some works have explored the potential to reduce carbon emission of building materials [25,26] and components [27,28]. For instance, Tae et al. [29] evaluated the environmental performance of high-strength concrete as an environmental load-reducing material in super high-rise buildings. By converting the compressive strength of concrete in existing buildings to 40 MPa high-strength concrete, they have achieved a reduction in the quantity of concrete and steel, simultaneously enhancing the lifespan of the building. Oh et al. [30] proposed an optimal design model for the low-carbon construction of reinforced concrete two-way slabs. Compared to traditional design methods, this optimal sustainable design approach considers various design scenarios, resulting in a reduction in carbon emissions for office and commercial buildings.

Building information modeling (BIM) is a digital modeling technique that centers on the fundamental elements of building components, integrating material visual characteristics, physical properties, and geometric data to achieve a detailed representation of data at various levels [31]. Several studies have been conducted for further enhancement of the utilization of BIM in practical engineering to achieve more efficient smart construction settlements [32]. For instance, Hong et al. [33] analyze the key factors influencing the acceptance of mobile BIM technology among construction practitioners and propose a model for promoting mobile BIM usage based on a survey and structural equation modeling. In addition, BIM has been proven to be efficient in sustainability analyses and management [34]. For instance, Sun et al. [35] conducted an assessment of CO₂ emissions during the tunnel construction process based on BIM. Yevu et al. [36] conducted a systematic review on the integration of BIM and prefabrication to advance low-carbon efforts in building delivery. Many scholars have integrated BIM technology to achieve life cycle assessment (LCA) of carbon emissions [37,38]. Utilizing BIM models as data carriers significantly enhances the reuse rate of building information and achieves the assessment of environmental impacts throughout the lifecycle of buildings [39,40]. For instance, Cheng et al. [41] combined the theory of LCA with BIM to propose a green building assessment method, leveraging the rich engineering information provided by BIM. Ding et al. [42] proposed a carbon emission measurement system based on BIM focusing on prefabricated residential buildings. Cang et al. [43] proposed a calculation method based on BIM utilizing “building element” as the basic unit suitable to provide design feedback during the schematic design stage. These BIM-enabled LCA carbon emission analyses encompass various stages of building development and different building types. Additionally, some studies have integrated BIM with GIS [44,45] and machine learning [46,47] technologies to enhance the performance of the analysis framework. In particular, Liu et al. [48] integrated BIM with various intelligent algorithms to optimize design parameters with a focus on sustainability.

However, previous studies have primarily concentrated on the environmental impacts of smaller-scale or conventional high-rise structures, lacking the specificity and detail needed for super high-rise buildings with unique structural systems. The related analysis and evaluation systems focusing on super high-rise buildings with RC core wall-steel frame structures are still underdeveloped and are mostly limited to qualitative analysis. Additionally, the emission reduction strategies for super high-rise buildings during the materialization stage are currently not well developed. Our aim is to offer a comprehensive quantitative model that specifically addresses these gaps. Consequently, this work specifically focuses on the carbon emission analysis of super high-rise buildings with RC core wall-steel frame structures. By examining the energy consumption and carbon emission characteristics of super high-rise buildings, key influencing factors are identified. Combining these insights with real-world case studies, a carbon emission analysis model based on BIM-enabled LCA focusing on the materialization stage and the waste recycling stage is constructed. Additionally, a comparative analysis of carbon emissions under different construction strategies is conducted, summarizing and outlining low-carbon strategies for these buildings.

2. Materials and Methods

In this study, the primary focus is on analyzing carbon emissions in super high-rise buildings with RC core wall-steel frame structures. Our aim is to develop a carbon emissions analysis framework based on BIM-enabled LCA focusing on the materialization stage and the waste recycling stage. Key influencing factors can be identified by examining the energy consumption and carbon emission characteristics specific to these buildings. The comprehensive framework for carbon emission calculation presented in this paper is depicted in Figure 1.

2.1. Inventory Analysis

In the general standard titled “Standard for building carbon emission calculation” implemented in China (GB/T51366-2019) [49], the emission factor method is employed to measure carbon emissions. The carbon emission factors (CEFs), which are also called the carbon emission coefficient (CEC), represent the amount of carbon emissions generated per unit of consumption. Research on CEFs of buildings has made significant progress, aiming to simplify the application of the emission factor method and enhance the accuracy and efficiency of carbon emission calculations [50,51].

Here, two critical stages that significantly affect life-cycle carbon emissions are mainly considered: the materialization stage and the waste recycling stage. The materialization stage is the primary source of carbon emissions in the building life cycle, including building material production, transportation, and construction stages. Low-carbon construction and design approaches can directly influence material costs, thereby significantly impacting carbon emissions during the materialization stage. The CEF method is utilized in this work for quantitative analysis of carbon emissions through BIM models. The CEF database from GB/T51366-2019 [49] is mainly utilized.

Additionally, for inventory analysis, Autodesk Revit 2018 software is employed to create the BIM model, which includes graphic information, materials data, and building components. The BIM model is then imported into Chenxi Quantity Calculation 2021 software [52] to obtain the material list and quantities for the building. By combining this with the CEF database, the carbon emissions of the building life cycle can be calculated. The analysis method will be detailed in the next section.

2.2. Carbon Emission Analysis

The total carbon emission of the life cycle C_tot in this work is calculated by subtracting the reduction in carbon emissions in the waste recycling stage from the carbon emissions generated during the materialization stage, including building material production, transportation, and construction, as follows:

$$C_{\mathrm{tot}}=C_{\mathrm{prod}}+C_{\mathrm{trans}}+C_{\mathrm{cons}}-C_{\mathrm{rec}}$$

(1)

where C_prod, C_trans, C_cons, and C_rec denote the carbon emissions corresponding to the carbon emissions of the building material production stage, the transportation stage, the construction stage, and the waste recycling stage, respectively.

Inventory analysis is primarily utilized to calculate carbon emissions during the building material production stage. Consequently, we have:

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

(2)

where M_i is the quality of the material i (t), F_i is the CEF for the material i (kgCO₂e/t), and n is the number of types of materials.

In the construction of super high-rise buildings, various building materials are utilized. The carbon emissions for the main building materials are calculated in this work. CEFs for the involved building materials are shown in

Table A1. Carbon emission factors of main materials at production stage.

Material Category	Material Name	Carbon Emission Factor	Unit
Steel	Large-scale steel	2380.00	kgCO2e/t
	Medium- and small-scale steel	2350.00	kgCO2e/t
	Steel bar	2340.00	kgCO2e/t
Concrete	C25 concrete	285.00	kgCO2e/m3
	C30 concrete	295.00	kgCO2e/m3
	C40 concrete	321.40	kgCO2e/m3
	C50 concrete	385.00	kgCO2e/m3
	C60 concrete	416.76	kgCO2e/m3
Gypsum	Waterproof putty for plastering	458.00	kgCO2e/m2
Wood	Wood	178.00	kgCO2e/m3
Glass, doors, windows, etc.	Flat glass	1130.00	kgCO2e/t
	Aluminum alloy window	254.00	kgCO2e/m2
	Light steel joists	149.20	kgCO2e/t
	Fire doors	125.00	kgCO2e/m2
Coatings and membranes	Modified asphalt waterproofing membranes	0.320	kgCO2e/m2
	Epoxy resin paint for staircase treads	0.680	kgCO2e/m2

in Appendix A.

For the transportation stage, the primary consideration is the carbon emissions generated during the transportation of building materials from the production plant to the construction site. This includes both the direct carbon emissions from the transportation process and the carbon emissions from the production of the consumed energy. The carbon emissions are mainly positively correlated with the distance between the production site and the construction site, the weight of the transported building materials, and the type of transportation used. Therefore, the carbon emissions during the building material transportation stage C_trans can be derived as:

$$C_{\mathrm{trans}}={\displaystyle \sum _{i=1}^n{M}_i{D}_i{T}_i}$$

(3)

where D_i is the average transportation distance for the material i (km), and T_i is the CEF per unit weight per unit distance for the transportation method of the material i (kgCO₂e/(t·km)). The CEFs listed in [49] are utilized for the calculation of carbon emissions during the transportation stage, as shown in

Table A2. Carbon emission factors of main materials in transportation stage.

Transportation Method	Carbon Emission Factor (kgCO2e/(t·km))
Road transportation (gasoline)	0.104
Road transportation (diesel)	0.162
Rail transportation	0.010
Diesel locomotive transportation	0.010
Waterway transportation	0.019

in Appendix A.

The carbon emissions during the construction stage consist of emissions from the energy consumption in construction process C_{cons, e} and emissions from formwork C_{cons, f}, as follows:

$$C_{\mathrm{cons}}=C_{\mathrm{cons}, \mathrm{e}}+C_{\mathrm{cons}, \mathrm{f}}$$

(4)

The estimation method proposed in [53] is utilized to calculate the carbon emissions for the energy consumption during the construction process C_{cons, e}, as follows:

$$C_{\mathrm{cons}, \mathrm{e}}=(x+1.99)·S$$

(5)

where x represents the number of building floors, and S denotes the total building area.

In addition, the carbon emissions from formwork include emissions from material production, transportation, turnover, and recycling. Consequently, we have:

$$C_{\mathrm{cons}, \mathrm{f}}={\displaystyle \sum _{i=1}^n\left|\frac{M_{\mathrm{f}, i}}{{\phi }_i}\right|·\left[F_{\mathrm{f}, i}(1-R_i)+T_{\mathrm{f}, i}·D_{\mathrm{f}, i}\right]}$$

(6)

where M_{f, i} is the quality of the ith type of formwork (t), φ _i is the turnover times for formwork type i; F_{f, i} is the CEF for the formwork type i (kgCO₂e/t); T_{f, i} is the CEF for the transportation of the formwork type i per unit weight per unit distance (kgCO₂e/(t·km)); D_{f, i} is the average transportation distance for the formwork type i (km); R_i is the recycling and reuse rate for the formwork type i; and n is the number of types of formwork. The CEFs of different formwork types listed in [49] are applied as presented in

Table A3. Carbon emission factors for production of formwork.

Formwork Type	Unit	Carbon Emission Factor	Recycling Rate (%)
Steel	kgCO2e/t	2390.00	90
Aluminum	kgCO2e/t	2600.00	80
Wood	kgCO2e/m3	178.00	65

in Appendix A.

The recycling and reuse of construction waste are crucial for reducing carbon emissions throughout the life cycle of buildings. Different types of construction waste require distinct recycling and treatment methods, resulting in varying recycling rates. Here, the recycling rates from [49] are applied for further investigation as listed in

Table A4. Recycling rates and carbon emission factors of construction wastes.

Material	Recycled Material	Recycling Rate (%)	Carbon Emission Factor
Concrete	Recycled aggregate	70	295 kgCO2e/m3
Glass	Glass	70	1130 kgCO2e/t
Steel	Crude steel	90	2050 kgCO2e/t
Aluminum	Crude aluminum	80	2600 kgCO2e/t
Wood	Wood	65	178 kgCO2e/m3
Doors and windows	Doors and windows	80	189.5 kgCO2e/m2

in Appendix A.

The reduction in carbon emissions from the recycling and reuse of construction waste C_rec can be derived as:

$$C_{\mathrm{rec}}={\displaystyle \sum _{i=1}^n{W}_i{F}_i}$$

(7)

where W_i is the quality of construction waste i (t), F_i is the CEF of construction waste i, and n is the number of types of construction waste.

3. Results

3.1. Background Information of the Office Building

The case study in this work is a super high-rise office building with an RC core wall-steel frame structure in Nanjing, China. This structure is composed of three main components: the external steel frame, the concrete core tube, and the roof steel structure. The building has an above-ground floor area of approximately 31,400 m² and a footprint of 1227.5 m². It consists of 21 above-ground floors, one roof level, and one mezzanine for mechanical equipment. The height of the ground floor, the 21st floor, and the standard floor are 8.95 m, 4.55 m, and 4.2 m, respectively, resulting in a total height of 101.3 m. The steel used in the structure is of grade Q355B, and the concrete grades range from C30 to C60. The external steel frame structure is composed of frame beams and columns.

The structural design of the building is based on relevant codes including [54,55]. The design lifespan of the building is 50 years, with a structural safety level of Category II and a structural importance factor of 1.0. The seismic design category is C, with a basic snow load of 0.65 kN/m² (return period of 50 years) and a basic wind pressure of 0.40 kN/m² (return period of 50 years). The ground roughness is classified as Category B. The seismic fortification intensity is 7 degrees, with a design basic seismic acceleration value of 0.10 g. The seismic design grouping is Group I, and the site category is III. As mentioned, the structural system consists of an RC core-steel frame structure, with the cross-shaped steel columns and concrete core tube designed to seismic performance level II, and the steel beams designed to seismic performance level III.

The structural calculation of the building is conducted utilizing YJK4.1 software to ensure its safety and reliability. The model of the standard floor is shown in Figure 2, and the main computational results are presented in

Table 1. Results of structural calculation.

Structural Calculation Index	Results
Total mass (t)	34,000.31
Mass ratio	1.02 < [1.5]
Mode of vibration (s)	T1	2.6913
	T2	2.3040
	T3	2.0531
	T3/T1	0.7629 < [0.85]
Minimum shear weight ratio	X-direction	1.71% > [1.60%]
	Y-direction	1.75% > [1.60%]
Maximum displacement ratio	X-direction	1.38 < [1.50]
	Y-direction	1.49 < [1.50]
Maximum inter-story displacement Angle under earthquake load	X-direction	1/1808 < [1/800]
	Y-direction	1/1611 < [1/800]
Effective mass coefficient	X-direction	93.76% > [90%]
	Y-direction	95.13% > [90%]
Maximum inter-story displacement Angle under wind load	X-direction	1/2420 < [1/800]
	Y-direction	1/2256 < [1/800]
Maximum inter-story displacement ratio	X-direction	1.38 < [1.50]
	Y-direction	1.42 < [1.50]
Rigidity-to-weight ratio	X-direction	5.94 > [1.40]
	Y-direction	5.42 > [1.40]

. The wind load and seismic calculations are also included, with the detailed results presented in

Table A5. Results of wind load calculation.

Floor	X-Direction			Y-Direction
	Wind Load (kN)	Shear Force (kN)	Overturning Moment (kN·m)	Wind Load (kN)	Shear Force (kN)	Overturning Moment (kN·m)
1	202.1	3838.5	218,787.7	238.8	4546.6	259,009.3
2	102.7	3636.4	184,126.1	122.3	4307.9	217,953.1
3	113.9	3533.7	168,853.3	135.5	4185.5	199,860.1
4	123.7	3419.8	154,011.8	147.1	4050.0	182,280.8
5	132.6	3296.1	139,648.6	157.6	3902.9	165,270.8
6	140.9	3163.4	125,805.1	167.3	3745.3	148,878.7
7	148.6	3022.5	112,518.7	176.4	3577.9	133,148.6
8	155.9	2873.9	99,824.0	185.0	3401.5	118,121.3
9	163.0	2718.0	87,753.5	193.2	3216.5	103,834.8
10	169.7	2555.0	76,337.8	201.2	3023.3	90,325.4
11	176.3	2385.3	65,606.6	208.9	2822.1	77,627.6
12	182.7	2209.0	55,588.3	216.3	2613.3	65,774.7
13	189.0	2026.4	46,310.3	223.7	2396.9	54,798.9
14	195.2	1837.4	37,799.6	230.9	2173.2	44,731.8
15	201.3	1642.2	30,082.6	238.0	1942.3	35,604.2
16	207.3	1441.0	23,185.2	245.1	1704.3	27,446.4
17	213.4	1233.6	17,133.1	252.2	1459.2	20,288.5
18	219.4	1020.3	11,951.9	259.2	1207.0	14,160.0
19	225.5	800.9	7666.8	266.2	947.8	9090.6
20	231.5	575.4	4303.2	273.3	681.6	5109.9
21	258.0	343.9	1886.6	304.4	408.3	2247.2
22	85.9	85.9	322.0	103.9	103.9	389.5

,

Table A6. Seismic forces on each floor (CQC method).

Floor	X-Direction			Y-Direction
	Seismic Response Force (kN)	Floor Shear Force (kN)	Shear-Weight Ratio (%)	Seismic Response Force (kN)	Floor Shear Force (kN)	Shear-Weight Ratio (%)
1	308.75	5829.77	1.715	341.58	5953.30	1.751
2	360.03	5675.66	1.807	393.87	5819.08	1.853
3	507.13	5500.17	1.845	536.00	5667.48	1.901
4	611.95	5263.00	1.864	607.97	5466.09	1.936
5	659.79	4994.88	1.873	622.81	5242.11	1.965
6	665.18	4726.34	1.883	629.46	5010.35	1.996
7	650.06	4471.99	1.900	644.62	4774.64	2.028
8	650.81	4235.36	1.926	660.23	4545.59	2.067
9	662.82	4015.18	1.965	650.57	4335.51	2.122
10	668.54	3821.46	2.024	626.17	4151.67	2.199
11	651.31	3668.00	2.117	611.02	3987.91	2.301
12	611.60	3557.93	2.255	606.64	3834.50	2.430
13	569.07	3475.75	2.443	591.14	3688.67	2.593
14	545.69	3394.77	2.679	557.82	3547.38	2.799
15	542.36	3292.92	2.961	530.90	3395.53	3.053
16	538.82	3160.03	3.303	532.89	3210.24	3.355
17	513.74	2989.21	3.729	542.05	2975.75	3.712
18	502.35	2759.65	4.259	549.63	2684.20	4.143
19	562.34	2425.29	4.906	581.37	2313.93	4.681
20	708.74	1930.74	5.666	682.17	1821.72	5.346
21	956.71	1237.94	6.501	900.31	1163.84	6.112
22	282.30	282.30	7.522	265.32	265.32	7.069

and

Table A7. Seismic shear forces of frame.

Floor	X-Direction			Y-Direction
	Column Shear Force (kN)	Wall Shear Force (kN)	Total Shear Force (kN)	Column Shear Force (kN)	Wall Shear Force (kN)	Total Shear Force (kN)
1	24.1	5812.0	5829.8	19.8	5958.1	5953.3
2	287.3	5392.5	5675.7	323.9	5496.7	5819.1
3	328.4	5177.6	5500.2	449.9	5220.0	5667.5
4	340.1	4932.1	5263.0	417.7	5052.1	5466.1
5	369.6	4635.6	4994.9	446.4	4800.2	5242.1
6	408.5	4332.3	4726.3	478.4	4538.8	5010.4
7	381.0	4104.6	4472.0	442.0	4339.1	4774.6
8	405.4	3843.5	4235.4	464.1	4087.2	4545.6
9	408.2	3620.0	4015.2	462.8	3878.0	4335.5
10	413.5	3420.4	3821.5	463.3	3693.5	4151.7
11	414.6	3265.3	3668.0	465.3	3527.7	3987.9
12	418.1	3152.9	3557.9	465.0	3374.9	3834.5
13	418.1	3072.9	3475.8	460.5	3234.5	3688.7
14	416.9	2995.7	3394.8	454.1	3101.5	3547.4
15	414.6	2898.8	3292.9	448.4	2958.2	3395.5
16	406.4	2778.7	3160.0	426.5	2799.3	3210.2
17	406.6	2610.8	2989.2	432.4	2562.5	2975.7
18	393.5	2401.6	2759.6	407.4	2301.7	2684.2
19	377.0	2097.6	2425.3	381.5	1968.2	2313.9
20	346.2	1636.4	1930.7	333.4	1528.8	1821.7
21	424.3	932.7	1237.9	399.4	869.6	1163.8
22	0.0	282.3	282.3	0.0	265.3	265.3

in Appendix A. The results indicate that the safety and reliability of the structure meet the required standards [54,55].

3.2. Results of Carbon Emission Analysis

Based on the detailed building information, a BIM model is constructed in Revit, as depicted in Figure 3. Additionally, Chenxi Quantity Calculation software is utilized to generate the material list and quantities. Combined with the CEF database, the carbon emissions during the material production stage can be calculated, as listed in

Table 2. Carbon emissions for main materials during production stage.

Material Category	Material Name	Unit	Amount	Carbon Emission (tCO2e)
Steel	Large-scale steel	t	445.12	1059.386
	Medium and small-scale steel	t	2066.83	4857.050
	Steel bar	t	1289.6233	3017.719
Concrete	C25 concrete	m3	279.871	79.763
	C30 concrete	m3	4138.175	1220.761
	C40 concrete	m3	1280.100	411.424
	C50 concrete	m3	1564.172	602.206
	C60 concrete	m3	3732.958	1555.748
Gypsum	Waterproof putty for plastering	m2	74.89	34.300
Wood	Wood	m3	8.218	1.463
Glass, doors, windows, etc.	Flat glass	t	188.4	212.892
	Aluminum alloy window	m2	4.16	1.056
	Light steel joists	t	62.8	9.371
	Fire doors	m2	1158.45	144.806
Coatings and membranes	Modified asphalt waterproofing membranes	m2	18,414.2	5.894
	Epoxy resin paint for staircase treads	m2	1017.6	0.692

.

According to Table 2, the total carbon emissions during the material production stage amount to 13,214.531 tCO₂e. Figure 4 shows the carbon emission shares for each category of main materials during the production stage. The results illustrate that steel and concrete account for the largest shares, at 67.61% and 29.29%, respectively, during the production stage. In contrast, the carbon emissions from gypsum, wood, coatings, and membranes are relatively low, each contributing less than 1%.

The quality of the building materials during transportation is determined according to the quantities listed in Table 2. The transportation distances are based on the default values provided in [49] with concrete having a transportation distance of 40 km and other materials having a transportation distance of 500 km. Road transportation (gasoline) is utilized. The results are shown in

Table 3. Carbon emissions for main materials during transportation stage.

Material Category	Quality (t)	Distance (km)	Carbon Emission (tCO2e)
Steel	3801.573	500	197.682
Concrete	27,488.19	40	114.351
Gypsum	209.692	500	10.904
Wood	53.001	500	2.756
Glass, doors, windows, etc.	468.304	500	24.352
Coatings and membranes	164.272	500	8.542

.

The total carbon emissions during the transportation stage is 358.587 tCO₂e. As shown in Figure 5, the carbon emission shares during the transportation stage for various building materials are illustrated. It can be seen that steel and concrete have the highest carbon emission shares during the transportation stage, similar to the production stage. This indicates that these two types of building materials play a significant role in carbon emissions. Additionally, this reaffirms the rationale for primarily considering steel and concrete when evaluating the impact of building material types on carbon emissions.

For the construction stage, the carbon emissions from energy consumption during the construction process can be estimated with Equation (5). Given that the office building has a total floor area of 31,400 m² with 21 floors, the carbon emissions for energy consumption during the construction process amount to 721.886 tCO₂e.

For the carbon emissions from formwork, the average turnover times are considered to be 4 times for wooden formwork and 35 times for both steel and aluminum formwork. Additionally, it is assumed that the usage area for each of the three types of formwork is evenly divided. According to the calculation results from the Chenxi Quantity Calculation software for the model, the total formwork usage area is 101,447.614 m². Thus, the usage area for each type of formwork is 33,815.871 m². The carbon emissions from formwork during the construction phase are calculated as shown in

Table 4. Carbon emissions for production of formwork.

Formwork Type	Area (m2)	Thickness (mm)	Density (t/m3)	Turnover	Carbon Emission (tCO2e)
Steel	33,815.871	8	7.85	35	14.501
Aluminum	33,815.871	5	2.70	35	6.782
Wood	33,815.871	25	0.42	4	13.167

and

Table 5. Carbon emissions for transportation of formwork.

Formwork Type	Unit	Amount	Distance (km)	Turnover	Carbon Emission (tCO2e)
Steel	t	2123.637	500	35	3.155
Aluminum	t	456.514	500	35	0.678
Wood	m3	355.667	500	4	4.616

.

The carbon emission for formwork amounts to 42.899 tCO₂e. Given the same area, aluminum formwork has the lowest carbon emissions. The carbon emission shares of various types of formwork are shown in Figure 6.

Combining the calculation results from Table 4 and Table 5 with the carbon emissions for energy consumption during the construction stage, the total carbon emissions for the construction stage C_cons are calculated to be 764.785 tCO₂e.

Finally, the reduction in carbon emissions from the recycling and reuse of waste materials can be calculated with Equation (6). The results are shown in

Table 6. Reduction in carbon emissions during recycling stage.

Material	Amount	Unit	Recycled Material	Carbon Emission Reduction (tCO2e)
Concrete	10,995.276	m3	Recycled aggregate	2270.524
Glass	188.4	t	Glass	149.024
Steel	3801.573	t	Crude steel	7013.902
Aluminum	22.464	t	Crude aluminum	46.725
Wood	8.218	m3	Wood	0.951
Doors and windows	1162.61	m2	Doors and windows	176.252

.

The reduction in carbon emissions from the recycling of construction waste amounts to 9657.378 tCO₂e. Figure 7 illustrates the shares of carbon emission reductions for each category of recycled construction waste.

Consequently, the carbon emissions for the main stages in the life cycle of the building for the model can be summarized in

Table 7. Carbon emissions of main stages in life cycle of building.

Stage	Carbon Emission (tCO2e)
Production	13,214.531
Transportation	358.587
Construction	764.785
Recycling	−9657.378

.

The total carbon emissions for the materialization stage of the model amount to 14,337.903 tCO₂e. The carbon emissions per unit area of the building are 0.46 tCO₂e/m², and the annual carbon emissions per unit area of the building are 9.13 kgCO₂e/(m²·year).

As shown in Figure 8, a comparison of carbon emissions across different main stages is illustrated. The calculation results for the carbon emissions during the materialization stage indicate that the production stage accounts for the majority, with 92.4% of the total emissions. In contrast, the carbon emissions from the transportation stage and the construction stage are relatively low. Therefore, reducing carbon emissions during the production of the materials is crucial for overall carbon emission reduction. Additionally, the recycling stage demonstrates substantial reductions in carbon emissions according to the results, thereby playing an important role in overall carbon emission reduction efforts.

3.3. Impact of Material Manufacturing Processes

According to Section 3.2, it is evident that the production of the main building materials, specifically concrete and steel, constitutes the most significant portion of carbon emissions throughout the life cycle of super high-rise buildings with RC core wall-steel frame structures. The carbon emissions associated with these materials are significantly influenced by the specific manufacturing processes and raw materials utilized. Consequently, in this section, the impact of different manufacturing processes and raw materials on carbon emissions is discussed. Please note that in this section, only the material manufacturing process is altered without changing the material strength or structural form. Therefore, the structures discussed below all meet the safety requirements.

Mineral admixtures are often processed from waste products, making them a sustainable choice to replace cement and reduce the carbon emissions of concrete. Here, concrete mixed with fly ash and composite mineral admixtures is mainly discussed. The CEFs for these concretes are detailed in

Table 8. The carbon emission factors of concrete with different admixtures.

Concrete Grade	Admixture Type	Carbon Emission Factor (kgCO2e/m3)
C30	Ordinary concrete	295.00
	Single admixture of fly ash	208.20
	Composite mineral admixture	118.14
C40	Ordinary concrete	321.40
	Single admixture of fly ash	230.35
	Composite mineral admixture	185.25
C50	Ordinary concrete	385.00
	Single admixture of fly ash	286.26
	Composite mineral admixture	239.52
C60	Ordinary concrete	416.76
	Single admixture of fly ash	390.09
	Composite mineral admixture	319.73

.

In this section, only steel and concrete are considered for variations. Therefore, only the related carbon emissions of steel and concrete are calculated for comparative analysis.

Table 9. Carbon emission utilizing concrete with different admixtures.

Type of Admixture	Carbon Emission in Production Stage (tCO2e)	Carbon Emission in Transportation Stage (tCO2e)	Total (tCO2e)
Ordinary concrete	12,804.057	312.033	13,166.090
Single admixture of fly ash	12,073.478	312.033	12,385.511
Composite mineral admixture	11,308.130	312.033	11,620.163

shows the carbon emissions during the production and transportation stages for the models utilizing ordinary concrete, fly ash concrete, and concrete with composite mineral admixtures, respectively, with the same type of steel.

As shown in Figure 9, the carbon emissions of the model are reduced by 5.9% when utilizing fly ash concrete and by 11.7% when utilizing concrete with composite mineral admixtures. This indicates that when employing significant amounts of cement substitutes (e.g., fly ash, composite mineral admixtures, etc.) in buildings, the carbon emissions can be substantially reduced.

For steel, carbon emissions are correlated with manufacturing processes and the rate of utilizing recycled materials. Four scenarios are mainly investigated: blast furnace-basic oxygen furnace (BF-BOF), electric arc furnace (EAF), BF-BOF with 30% recycled scrap, and EAF with 80% recycled scrap. The CEFs for these four types of steel are presented in

Table 10. Carbon emission factors of steel with different manufacturing processes.

Production Process	Steel Type	Carbon Emission Factor (kgCO2e/t)
BF-BOF	Large-scale steel	2380.00
	Medium- and small-scale steel	2350.00
	Steel bar	2340.00
EAF	Large-scale steel	1650.00
	Medium- and small-scale steel	1620.00
	Steel bar	1610.00
BF-BOF with 30% recycled scrap	Large-scale steel	1707.00
	Medium- and small-scale steel	1677.00
	Steel bar	1667.00
EAF with 80% recycled scrap	Large-scale steel	522.00
	Medium- and small-scale steel	492.00
	Steel bar	482.00

. The chosen usage rates of scrap steel are achievable for each process.

The carbon emissions for each of the four scenarios mentioned are calculated and listed in

Table 11. Carbon emission utilizing steel with different manufacturing processes.

Manufacturing Process	Carbon Emission in Production Stage (tCO2e)	Carbon Emission in Transportation Stage (tCO2e)	Total (tCO2e)
BF-BOF	12,804.057	312.033	13,116.09
EAF	10,028.908	312.033	10,340.941
BF-BOF with 30% recycled scrap	10,245.598	312.033	10,557.631
EAF with 80% recycled scrap	5740.734	312.033	6052.767

.

As shown in Figure 10, the carbon emissions of the model decrease by 23.8% when employing the EAF process. This reduction is attributed to the EAF process mainly utilizing natural gas as fuel, which results in lower carbon emissions compared to the BF-BOF process which utilizes coal and petroleum. In the scenario where the BOF process incorporates 30% recycled scrap steel, the carbon emissions of the model decrease by 22.2%. Furthermore, when the EAF process utilizes 80% recycled scrap steel, the carbon emissions of the model decrease by 56.4%.

Compared to applying different admixtures in concrete, different steel manufacturing processes and recycled scrap steel significantly reduce carbon emissions in the construction of high-rise buildings. This is primarily due to the substantial contribution of carbon emissions of steel to the total carbon footprint. Therefore, in the production of construction materials, prioritizing more efficient manufacturing processes of steel can significantly reduce carbon emissions.

4. Discussion

This chapter discusses low-carbon construction strategies for super high-rise buildings with concrete core wall-steel frame structures based on the results presented above. According to the findings, carbon emissions generated during the production stage significantly dominate emissions throughout the entire materialization stage. Therefore, formulating targeted strategies to reduce carbon emissions during material production is particularly critical. Steel offers good seismic performance and low weight but has a high carbon emission factor, as shown in Table A1. In contrast, concrete features strong plasticity and can be adapted to various structural systems, with a lower carbon emission factor but a higher weight. The hybrid structure of steel and concrete can combine the advantages of both materials, thus reducing carbon emissions. Furthermore, utilizing 3R (reducible, recyclable, reusable) materials can decrease the frequency of resource extraction and reduce carbon emissions during the production process of building materials.

In addition, an optimized manufacturing process of building materials not only directly reduces carbon emissions but also enhances the strength and corrosion resistance of materials. This improvement in material efficiency decreases the amount of building materials needed, thereby indirectly reducing the carbon emissions associated with buildings. Based on the results in Section 3.3, incorporating fly ash alone in concrete and adding composite mineral admixtures results in carbon emission reduction rates of 5.9% and 11.7%, respectively. For steel, adopting the EAF results in a carbon emission reduction rate of 23.8%. When 30% of recycled scrap steel is utilized in the BF-BOF process, the carbon emission reduction rate is 22.2%. Moreover, when the EAF process utilizes 80% recycled scrap steel, the carbon emission reduction reaches 56.4%. Compared to different admixtures in concrete, optimizing the steel manufacturing process has a more significant effect in reducing carbon emissions in super high-rise buildings.

According to Equation (3), the carbon emissions during the transportation stage are influenced by the distance from the production site to the construction site, the weight of the building materials being transported, and the mode of transportation. Therefore, strategies for reducing carbon emissions at this stage include prioritizing locally sourced building materials to reduce transportation distances and lower carbon emissions. In addition, choosing transportation methods with lower carbon emission factors, including opting for gasoline-fueled vehicles over diesel-fueled ones for road transport, can also contribute to the reduction in carbon emissions.

During the construction stage, carbon emissions mainly originate from construction tools, including formwork, energy consumption of machinery, and related emissions from labor activities. Therefore, adopting prefabricated construction methods and selecting solutions with higher prefabrication rates can effectively reduce the utilization of formwork and improve construction efficiency. Additionally, introducing green construction technologies and prioritizing machinery with higher energy efficiency helps minimize energy consumption during equipment operation. Lastly, enhancing green construction management through optimized processes not only boosts construction efficiency but also reduces material waste throughout the construction process.

Regarding the waste recycling stage, the method of handling discarded materials is crucial. Implementing optimized demolition plans enhances the recycling rate of discarded materials and reduces pollution generated during demolition. Moreover, expanding methods for reusing discarded materials, including employing them for office area partitions, further enhances sustainable practices.

5. Conclusions

This work analyzes carbon emissions in super high-rise buildings with an RC core wall-steel frame structure. A BIM-based LCA carbon emission analysis framework is developed utilizing Revit software for model creation and Chenxi Quantity Calculation software for quantity analysis. Models for both the materialization and recycling stages of the building lifecycle are constructed. The impact of building material manufacturing processes on carbon emissions is further studied. Results show that incorporating fly ash and composite mineral admixtures in concrete lowers emissions. In addition, adopting the EAF method in steel manufacturing and utilizing recycled scrap steel significantly supports low-carbon objectives. Furthermore, based on the theoretical research and comparative analysis, green and sustainable construction strategies are proposed for the reduction in energy consumption and carbon emissions during production, transportation, construction, and waste recycling. These insights aim to guide sustainable practices in the industry.