Life Cycle Carbon Emissions and an Uncertainty Analysis of Recycled Asphalt Mixtures

Abstract

The efficient recycling of road materials is an effective way to reduce carbon emissions. Approximately 400 million tons of waste asphalt pavement materials is generated annually in China, but the utilization rate is less than 50%. In this paper, a whole life cycle CO₂ emission calculation model for recycled asphalt mixtures is established, and an uncertainty analysis is carried out on the basis of this model. The steps in the recycled asphalt mixture stage with the greatest impact on emissions are identified. The life cycle of asphalt mixtures is divided into raw material production, mixture preparation, material transportation and construction phases based on the carbon emission trends in the four phases. First, recycled asphalt mixtures are considered, and the carbon emissions along a two-way four-lane highway are analyzed based on the relevant carbon emission factor and other known data. The carbon dioxide emissions of each stage are also compared to identify the key links among stages. Then, the carbon emissions of hot-recycled asphalt mixtures are compared with those of ordinary asphalt mixtures and cold-recycled asphalt mixtures in each stage. Finally, an analytical uncertainty study of recycled asphalt mixtures is conducted using uncertainty modeling in different scenarios. Based on the quantitative results of the carbon emission calculations through life cycle assessment (LCA), future measures for reducing carbon emissions from recycled asphalt mixtures are proposed.

1. Introduction

Road construction produces approximately 100 million tons of carbon emissions per year. The State Council of China issued the “Carbon Peak Action Plan before 2030” [1] to promote the resourceful use of construction waste and in situ recycling of waste pavement materials, with the goal of utilizing approximately 4.5 billion tons of these materials by 2030. China’s annual output of waste asphalt mixtures is approximately 400 million tons, but the utilization rate is less than 50%.

At present, the international community is facing two major problems: energy constraints and atmospheric pollution. Concern for the environment and the promotion of low-carbon energy savings have become major social concerns. In recent years, the global temperature has been rising gradually, the environment has been deteriorating and greenhouse gases such as carbon dioxide have shown a clear growth trend compared with levels in the past. In 2022, the global carbon dioxide produced with fossil fuels reached 36.6 billion tons, an increase of approximately 1% compared with the level in the previous year and a new record high. If the current levels of CO₂ emissions are maintained, the global carbon budget will be disrupted in 9 years [2]. Globally, approximately 25% of CO₂ emissions come from the transportation sector [3]. By the end of 2022, the total mileage of highways in China was approximately 5.35 million kilometers, with highways accounting for approximately one-fifteenth of this total, ranking first in the world in terms of mileage [4]. Asphalt is the main type of pavement used in graded roads in China. Asphalt pavement construction requires many aggregates, asphalt, stone, modifiers and various other materials, and the production, transportation and construction stages of these materials and the use of various types of machinery and equipment produce large amounts of carbon dioxide.

Whether it is to respond to the policies regarding a low-carbon economy, to construct an increasingly optimal environmental carbon emission system, to analyze the carbon emissions of asphalt roads, to identify the links among carbon emissions from different sources, to propose energy-saving and emission reduction measures with practical and effective data or to design low-carbon roads, the impact of the carbon footprint of recycled asphalt on climate change is being studied. Consequently, various methods are being adopted to reduce emissions and improve the efficiency of the corresponding value chain.

In this paper, based on research on recycled asphalt mixtures in China and abroad and considering the material production stage, construction stage and milling regeneration stage of the life cycle of asphalt mixtures in China, we focus on the following tasks: (1) the carbon emissions of the whole life cycle of recycled modified asphalt, which encompasses raw material screening, pretreatment, production and transportation, energy production and transportation, recycled-asphalt-based construction, service and milling and regeneration, are analyzed, and an evaluation system for carbon emissions from all asphalt stages is established; (2) the carbon emission stages of hot-recycled asphalt, common asphalt and cold-recycled asphalt mixtures are compared; (3) the uncertainty of energy consumption and emissions due to cumulative variations in the process of resource utilization of waste polymer-modified asphalt is assessed; and (4) based on the quantitative results of the carbon emission calculations through life cycle assessment (LCA), future measures for reducing carbon emissions from recycled asphalt mixtures are proposed.

2. Literature Review

2.1. Research on Carbon Emissions of Ordinary Asphalt Mixtures

Most of the early life cycle assessment (LCA) studies of pavement focused on comparisons regarding concrete pavements and asphalt pavements [5]. Horvath et al. published the earliest LCA pavement study in 1998 in which Continuously Reinforced Concrete Pavements (CRCPs) and hot-mixed asphalt (HMA) pavements were compared; the results indicated the disadvantage of asphalt pavements in terms of the high consumption of raw materials, but on most of the environmental impact indicators, concrete pavements have a higher environmental impact than asphalt pavements. Moreover, end-of-life (EOL) recycling studies of both materials have shown the green sustainability associated with the effective recycling of asphalt pavements, and on the contrary, the recycling of CRCP may result in a greater environmental burden [6]. Santos et al. developed an LCA model for pavement management to assess the effects of flexible pavement structures during their life cycle, as defined in the Portuguese Pavement Design Handbook (PPDH). The study concluded that the material and use phases have the highest environmental impacts throughout the life cycle [7].

In China, Huang et al. [8] studied a section of asphalt pavement at London Airport, where the pavement aggregate was replaced with waste glass, incinerator ash and waste asphalt mixes, and showed that the production process of hot-mix asphalt mixtures was the most energy-intensive. Peng et al. [9] built an appropriate calculation model of CO₂ emission, and analyzed the proportion of carbon emissions to the asphalt pavement construction of aggregate stacking, aggregate supply, and other stages, based on the carbon emission calculation model.

Meng [10] of Northeast Forestry University explored the impact of asphalt mixture thickness and construction methods on carbon emissions by reviewing related literature and specifications and analyzing the life cycle energy consumption and carbon emission results for asphalt pavements in Heilongjiang Province. Giani et al. [11] analyzed the environmental sustainability of asphalt pavement from a material production and maintenance strategy point of view. Fernandes et al. [12] assessed the carbon dioxide emissions of stone mastic asphalt mixtures produced with high rates of different waste materials for binder modification or material recycling, and carried out an estimation of the carbon dioxide emissions associated with their production and transportation under different scenarios.

2.2. Research on Carbon Emissions of Recycled Asphalt Mixtures

Polymer-modified asphalt is projected to be the dominant product used in asphalt pavement construction in the future. At present, foreign researchers have systematically modified asphalt technology and modified polymers to provide a variety of asphalt recycling strategies; in recent years, they have promoted the recycling of waste asphalt mixtures, thus effectively avoiding the negative environmental impacts caused by the waste of asphalt mixtures.

Reclaimed asphalt pavement (RAP) is a kind of recycled asphalt pavement material. From its direct translation in English, RAP can be simply understood as follows: a type of material reused following a previous use process. The definition of RAP material can be fully understood based on its origin, main components, function, etc. RAP arises from the milling of old pavements and is the residue produced with milling-damaged pavements during maintenance or rehabilitation work [13]. This residue still contains old asphalt and aggregates and has some recycled material potential, so it can be recycled in the production of new asphalt mixtures [14]. RAP is obtained by milling various structural layers of pavement, and it can be used in the production of new asphalt mixtures after it has been crushed and sieved at an asphalt production plant to remove impurities [15]. Additionally, RAP generated through milling operations involving existing flexible pavements can be salvaged, and the use of recycled RAP reduces the need for asphalt binders and aggregates, which in turn saves natural resources. Several studies have shown that hot-mix asphalt mixtures containing RAP can achieve the same quality as asphalt mixtures produced from raw materials if asphalt recycling is performed correctly [16].

In recent years, scholars in China have studied the recycling technology of modified asphalt pavement to various degrees and have achieved various research results. In 2019, Peng et al. [17] simulated SBS-modified aging at high temperatures, and with the degradation of softeners, the softening point remained almost unchanged. Although applications involving modified asphalt in China have provided valuable information, overall, modification technology is still lacking, and therefore, applications and detailed studies involving modified asphalt are limited. At present, the main modified asphalt types used in China are SBR, PE and EVA, and especially SBS.

The environmental potential and sustainability benefits of using RAP with recycled asphalt pavements can be investigated based on LCA methods [18,19,20]. According to previous LCA studies, the hotspots of full life cycle research on recycled asphalt pavements include (1) comparative LCA studies of asphalt mixtures with different RAP contents [21] and (2) comparative LCA studies of asphalt mixtures that use different recycled materials in combination with different construction processes [22,23]. In a hybrid LCA study (hybrid LCA), Aurangzeb et al. found that recycled asphalt mixtures can reduce environmental impacts by up to approximately 28% compared to virgin asphalt mixtures [24].

Based on the above literature review, in summary, the domestic and international LCA research on recycled asphalt pavement has been nonuniform, with different definitions and research scopes. Most studies have focused on a single life cycle stage, and there have been few studies on the LCA of recycled asphalt mixtures over the complete life cycle. Moreover, assessments of the results of LCA research, uncertainty analyses and other methods need to be enriched and supplemented. Considering different asphalt production stages, different steps in these stages and different reasons for the emission of carbon dioxide due to various factors, the effects of various factors on carbon emissions are analyzed to quantify carbon emissions and identify key links to develop targeted energy-saving and emission reduction measures.

3. Methodology

3.1. Goal and Scope Definition

Recycled asphalt mixtures contain old materials, new stone, new asphalt and other materials after remixing and adding modifiers, providing certain road value for road construction.

Based on life cycle evaluation theory, the life cycle of recycled asphalt mixtures is divided into four stages (Figure 1): raw material production, mixture acquisition, the transportation of different materials and construction. For example, for a recycled asphalt mixture, the full life cycle of the mixture is decomposed and analyzed, and material production is divided into raw material production and mixture production; additionally, the construction stage includes mainly paving and rolling [25].

The CO₂ emissions from raw material production and mixed production are considered. The system boundaries used for each stage in this study are as follows:

• Raw material acquisition stage: Asphalt, cement, aggregate, mineral powder and old material are the raw material inputs of the system, and their transportation is considered. Electricity and diesel fuel are used as the energy inputs of the system. The raw inventory data are established based on the available information. The most important phase is the milling and reclaiming of old materials. This phase refers to the removal of old damaged pavement surfaces by milling and then reclamation of the materials. The main source of carbon emissions in the milling and reclamation phase is the carbon emissions from the milling machine [26,27].
• Mix production stage: This stage includes procedures such as asphalt conveying and heating; loading and transportation; and mixing and discharging. Carbon emissions from these procedures are mainly associated with the use of electricity and equipment [28,29].
• Raw material transportation stage: This stage mainly involves the carbon dioxide generated with the transportation of raw materials to the mixing plant and the transportation of the mix to the construction site. Carbon emissions in this phase are mainly from transportation equipment.
• Construction stage: This stage considers such activities as the paving and compaction of the asphalt layers; the main system input for this phase is the fuel consumption of construction machinery.

The functional unit in this paper is defined based on the production of hot-mix recycled asphalt pavement; the basic parameters are shown in

Table 1. Basic parameters of hot-mix recycled asphalt pavement.

Parameter Type Description		Unit	Value
Basic functional unit	Pavement length	km	1
	Lane width	m	3.75
	Number of lanes	/	4
Pavement structural thickness	Upper layer, SMA-13	cm	4
	Middle layer, AC-20	cm	6
	Regeneration layer	cm	15

. In addition, the difference between hot-recycling, cold-recycling and ordinary asphalt mixtures in terms of carbon emissions and the surface structure of different production materials is considered (Figure 2).

3.2. Modeling of the Carbon Emissions of Asphalt Mixtures

In this paper, a carbon emission calculation model based on the quota method is established to calculate the carbon emissions associated with regenerated asphalt mixture pavements. The main data collected for various machinery types and machinery fuel consumption are used to estimate the total energy consumption of various types of equipment. The different types of energy consumption are uniformly converted into carbon dioxide emissions, including carbon dioxide emissions in the raw material acquisition stage, mixture acquisition stage, transportation of raw materials and mixtures and construction stage [30]. The whole life cycle carbon emission model is expressed as

$$W_{{CO}_2}=W_1+W_2+W_3+W_4$$

(1)

where $W_{{CO}_2}$ is the total carbon dioxide emissions, kg; $W_1$ is the carbon dioxide emissions in the raw material acquisition phase, kg; $W_2$ is the carbon dioxide emissions during the acquisition phase of the mix materials, kg; $W_3$ is the carbon dioxide emissions from the transportation of different materials, kg; and $W_4$ is the carbon dioxide emissions from the construction stage, kg.

3.2.1. Calculation of Carbon Emissions from Raw Material Acquisition

The formula for calculating carbon emissions in the raw material acquisition stage is as follows:

$$W_1=\sum {LCE}_{ic}=\sum (M_i\times {EF}_{ic})$$

(2)

where ${LCE}_{ic}$ is the carbon dioxide emissions corresponding to raw materials in category i, kg; $W_1$ is the CO₂ emissions corresponding to raw materials, kg; $M_i$ is the mass of work corresponding to raw materials from category i, kg; and ${EF}_{ic}$ is the CO₂ emission factor corresponding to raw material type i, kg.

Table 2. Mass of different raw asphalt pavement materials.

Raw Material Type	Raw Material Mass (t)
	Hot-Recycled Asphalt Pavement	Ordinary Asphalt Pavement	Cold-Recycled Asphalt Pavement
Modified asphalt	423.93	473.67	140.19
Emulsified asphalt	0	0	178.99
Talcum powder	207.75	409.17	166.21
Aggregate	7284.8	8090.56	7286.2
RAP	1063.52	0	1063.52

shows the masses of different raw asphalt pavement materials.

Research from the European Asphalt Association, various studies of SBS-modified bitumen carbon emissions, the IPCC national gas guidelines and related literature was reviewed to derive the carbon emission factors for different materials, as shown in

Table 3. Carbon emission factors for different materials.

Material	Unit	Carbon Emission Factor
Modified asphalt	kg/t	365.8
Asphalt emulsion	kg/t	205.98
Aggregate	kg/t	9.02
Talcum powder	kg/t	7.14
Electric power	kg/t	0.7921
Diesel fuel	kg/kg	3.340
Heavy fuel oil	kg/kg	3.270

. Among them, the carbon factor of the used material regeneration stage can be defined, considering that the energy consumption and emissions in this stage are mainly associated with diesel consumption and the emissions of milling and transportation equipment. According to the specifications of the “Highway Engineering Budget Quota” and the “Highway Engineering Machinery Bench Cost Quota”, the energy consumption conversion for the machinery required for milling 1000 ${\mathrm{m}}^2$ of asphalt pavement is 57.14 kg, assuming that the thickness of the product is 5 cm; thus, the carbon emission factor of RAP can be calculated as 1.577 × 10⁻³ kg/kg, which can be ignored, when compared with that of other materials.

3.2.2. Calculation of Carbon Emissions from Mixed Material Production

The carbon emission factors for electricity and fuel oil are calculated as follows:

$$f_e=\frac{\sum _i^n{E}_j\times f_j}{E_e}$$

(3)

In Equation (3), $f_e$ is the carbon emission factor of electricity, and the unit is kg/kWh; $E_j$ refers to the input of various types of fossil fuels, such as raw coal, diesel and fuel oil, and the unit is kg or m³; and $E_e$ refers to the output of electricity, and the unit is kWh.

$$f_j=\frac{44}{12}\times J_j\times C_j\times O_j$$

(4)

In Equation (4), $f_j$ refers to the direct carbon emission factor of different energy sources. The unit is kg/kg or kg/m³, i.e., how many kilograms of CO₂ are generated with a certain energy source j in kg or m³; $J_j$ refers to the average low calorific value in kJ/kg; $C_j$ refers to the carbon content per unit of calorific value in kg/kJ; and $O_j$ refers to the rate of carbon oxidation in %. Energy consumption of equipment for producing different recycled asphalt mixtures is shown in

Table 4. Energy consumption of equipment for producing different recycled asphalt mixtures.

Structural Layer		Engineering Quantity (m3)	Material Mass (t)	Electricity and Fuel Consumption by Equipment
				Diesel Fuel (kg)	Heavy Oil (kg)	Electricity (kWh)
Upper layer	SMA-13	600	1470.00	290.28	11,725.96	4417.03
Mid-surface layer	AC-20	900	2169.00	435.42	14,983.17	5643.96
Lower layer	AC-25 (Recycled)	2250	5314.02	1049.36	35,806.83	13,488.18
	AC-25 (Ordinary)	2250	5334.42	1045.34	35,669.89	13,436.60
	HiRM	2250	5271.32	412.11	0	0

.

3.2.3. Calculation of Carbon Emissions from Material Transportation

Carbon emissions from material transportation are mainly generated from fuel consumption at the mixing plant and during the transportation of the mix to the construction site.

Transportation occurs by road, waterway and rail, with waterways and railroads being one-way transportation flows. Highway shipments include a full load that is delivered and an empty return trip. This paper focuses on road transportation.

$$E_j=\sum \sum m_i{q}_{ij}+\sum \sum m_i\frac{L_i}{100}{q}_{ij}(1+n)$$

(5)

$$W_1=\sum \sum E_j{P}_{jk}{GWP}_k$$

(6)

where E is energy consumption (fuel, electricity, etc.), MJ; P is the emission factor, kg/MJ or kg/(kW·h); GWP is the global warming potential (GWP); m is the material mass, kg; q is the energy consumption per unit mass, MJ/kg; L is the transportation distance, km; n is the return transportation coefficient, which is set to 0.7 for highways; i is the material type; j is the energy type; and k is the gas type. Calculation of carbon emissions during the transportation phase of thermally recycled materials is shown in

Table 5. Calculation of carbon emissions during the transportation phase of thermally recycled materials.

	Transportation Materials	Modified Asphalt	Aggregate	Talcum Powder	RAP	Emulsified Asphalt	Mixed Material
Hot-recycled asphalt	Mass (t)	423.93	7284.8	207.75	1063.52	/	8953.02
	Load capacity (t)	20	20	20	20	/	20
	One-way distance (km)	10	10	10	10	/	35
	Fuel consumption (kg)	289.89	4981.61	142.07	727.27	/	17,781.61
Ordinary asphalt	Mass (t)	473.67	8090.56	409.17	/	/	8973.4
	Load capacity (t)	20	20	20	/	/	20
	One-way distance (km)	10	10	10	/	/	35
	Fuel consumption (kg)	323.89	5531.55	279.80	/	/	17,822.08
Cold-recycled asphalt	Mass (t)	140.19	7286.2	166.21	1063.52	178.99	8835.11
	Load capacity (t)	20	20	20	20	20	100
	One-way distance (km)	10	10	10	10	10	50
	Fuel consumption (kg)	95.86	4981.61	113.66	727.27	122.39	6040.79

.

3.2.4. Calculation of Carbon Emissions during the Construction Stage

Carbon emissions in this phase are mainly associated with the fuel consumption of mechanical equipment during construction processes. However, the fuel consumption of dump trucks at the construction site is not included in the construction phase. According to the “Estimated Quotas for Highway Engineering” and “Quotas for Machinery Bench Costs in Highway Engineering”, the mechanical benchmarks and fuel consumption involve the paving and rolling processes for each layer.

In the recycled asphalt mix, SMA-13 is used in the top layer, AC-20 is used in the middle layer and thermally recycled AC-25 is used in the bottom layer. The carbon dioxide emissions mainly come from the paver, roller and spreader.

According to the construction scheme selected and the type of machinery needed and related equipment parameters, the carbon dioxide emissions generated during the construction process mainly come from fuel consumption. Carbon dioxide emissions are quantitatively calculated based on the use of machinery benches, and the quota method is applied to estimate carbon emissions. The carbon dioxide emissions generated in the paving and compaction processes during the construction phase are determined according to the budget quota and the bench cost quota. Energy consumption from surface construction is shown in

Table 6. Energy consumption from surface construction.

Mechanical Equipment (240 t/h Mixing Capacity)	Upper Layer (Bench)	Middle Layer (Bench)	Lower Layer (Bench)	Diesel Consumption (kg)	Carbon Dioxide Emissions (kg)
Asphalt paver	7.24	7.2	7.16	2942.59	5420.7
<15 t vibratory roller	20.25	20.18	20.03	4484.38	9001.2
16–20 t tire roller	10.12	10.05	10.05	1281.53	2361.68
20–25 t tire roller	9.75	9.68	9.83	965.33	1761.26

.

3.3. Uncertainty Analyses of Carbon Emissions

There are many uncertainties in the calculation of the environmental impacts of recycled asphalt mixtures, including data uncertainty, scenario uncertainty and modeling uncertainty. Since the model for carbon emissions is relatively accurate, only data uncertainty and scenario uncertainty are considered in this paper.

3.3.1. Data Uncertainty Analysis Model

The uncertainty calculation is carried out based on the DQI method. In the DQI method, multiple aspects and indicators of data quality are considered in a comprehensive way, and a comprehensive data quality indicator is obtained through the weighted summation of the comprehensive assessment indicators. This comprehensive assessment approach can reflect the degree of uncertainty of the data in detail. Each qualitative data quality indicator is assessed and assigned a weight to finally obtain a numerical data quality indicator. This allows the degree of data uncertainty to be expressed and comparisons to be performed based on specific numerical values to support a quantitative data analysis and decision making. The DQI method can be customized according to specific needs and domain characteristics. According to different application scenarios and data types, appropriate assessment indicators, definitions of metrics and schemes for setting and assigning weights can be selected to adapt to situations with different data quality needs. Moreover, a comprehensive data quality indicator is established, which can help users understand the degree of uncertainty of the data [31].

In the DQI method, each variable with available raw data is analyzed to determine the different factors affecting the data quality, and an indicator rating is obtained for each factor. The DQI value for each variable is obtained by sorting the ratings, and the standard deviation of the corresponding statistical distribution is used to express the uncertainty of the raw data, as shown in Figure 3.

The uncertainty formula for the data is

$$U_i=\sqrt{U_1^2+U_2^2+U_3^2+U_4^2+U_5^2}$$

(7)

$$U_{d,i}=\sqrt{U_b^2+U_i^2}$$

(8)

where U₁–U₅ are the uncertainties of the five data quality indicators, U_i is the uncertainty of data quality indicator i, $U_{d,i}$ is the data-related uncertainty of i and $U_b$ is the fundamental uncertainty of i.

3.3.2. Scenario Uncertainty Analysis Model

A scenario uncertainty analysis is the process of systematically analyzing and evaluating the uncertainties in a decision problem or problem scenario. The goals are to understand and quantify the potential impact of uncertainty on decision outcomes and to help cope with uncertainty.

Key uncertainties that may affect the outcome of the decision should first be identified and characterized, and then scenarios, i.e., different scenarios or development paths based on the identified uncertainties, should be constructed. Each scenario represents a possible uncertainty development trend or outcome. Scenarios can be constructed based on probability distributions, a trend analysis or simulation methods. Finally, the uncertainty is quantified, and each scenario is quantitatively explored to assess the probability of occurrence and the likely level of impact. The following scenarios with deterministic carbon emissions are considered.

• Types of energy consumption

In the production and construction phases of recycled asphalt mixtures, the main types of energy consumed using machinery are fuel oil and electrical energy. Considering the different types of power generation, the environmental emission factors are set according to the different power structures reported in references. An energy resource-based power generation structure is used in Scenario 1, a hydroelectric power generation structure is used in Scenario 2 and other power generation structures are considered in Scenario 3.

2.

Transportation distance

For different selected materials and manufacturers, the transportation distance varies; therefore, different transportation scenarios must be considered. In this paper, the transportation distance for mixed asphalt is set to a base value of 35 km, and the transportation distance of the other materials to the mixing plant is set to 10 km. A transportation distance of 35 km is used in Scenario 1, and a transportation distance of 20 km is used in Scenario 2.

Assuming that the scenarios are independent of each other, the uncertainty in each scenario is calculated as follows:

$$U_{Si}=\sqrt{U_{i1}^2+U_{i2}^2+\cdots }$$

(9)

where $U_{Si}$ is the uncertainty of scenario i and $U_{i1}$ and $U_{i2}$ are the uncertainties of different factors in scenario i.

4. Results and Discussion

4.1. Analysis of the Quantitative Results

Figure 4 shows a comparison of the life cycle carbon emissions of hot-recycled asphalt mixtures during different stages.

• Raw material production stage

In the modified asphalt production process, as more modified asphalt is used, more carbon dioxide is produced. In the traditional production process, fewer steps, which include crushing, division and sieving, are needed. Machinery is operated via the consumption of electricity or fuel oil, and the production of modified asphalt requires the collection of raw materials, refining, processing and heating to the appropriate temperature for processing. In addition, modifiers and other agents, all of which produce large amounts of carbon dioxide, are needed. As shown in the figure, modified asphalt production has the largest proportion of carbon dioxide emissions, and aggregate production has the second largest. Although aggregates have the highest demand of any asphalt product, modified bitumen production has the highest rate of carbon dioxide emission. Therefore, this stage of the asphalt production process should be specifically regulated.

2.

Mix production stage

Based on the preparation of the mixture, which mainly includes the mechanical dumping of raw materials, heating, mixing and other steps, the carbon emissions and selected thickness of the surface layer must be considered in relation to the preparation of the subsequent layer and the overall carbon dioxide produced. For a middle layer thickness of 15 cm, carbon dioxide emissions are high, and in the production process, more heavy oil is used than diesel fuel; additionally, although the use of electricity is high in this case, if electricity is produced from clean energy, the carbon dioxide generated will be far less than that if heavy oil is used. Therefore, clean energy should be used in the production of mixed asphalt to reduce carbon dioxide emissions.

3.

Material transportation stage

In the material transportation stage, the carbon dioxide emitted during the transportation of mixes is the largest among that for all materials used in recycled asphalt mixtures, and the carbon dioxide emissions from the equipment used to transport mixes are much larger than those from other transportation equipment. Therefore, the amount of carbon dioxide emitted is sensitive to the transportation distance, which should be controlled to mitigate the environmental impact of aggregate transportation. Notably, large quantities of materials must be transported, and the equipment emissions of carbon dioxide should be monitored. The principle of maximizing proximity should be adopted in transportation. In the material transportation stage, carbon emissions are related to the transportation distance, vehicle load, vehicle energy consumption and weight of the material.

4.

Comparative analysis of different stages

The results show that CO₂ emissions in the raw material production stage account for 41% of the total carbon emissions; additionally, those in the mix production stage account for approximately 42%, and those in the transportation stage account for only 13%.

The results for the mix production stage indicate that the maximum CO₂ emitted during the production of asphalt mixtures is approximately 42%. According to the literature, recycled asphalt mixtures are blended with old material to provide the basis of asphalt pavement. Therefore, the percentage of old material used has a greater impact on carbon emissions than does the percentage of other materials used in mixes, and carbon dioxide emissions can be effectively reduced through the use of cold-mix recycling techniques that increase the percentage of reusable old material.

4.2. Comparison of Hot-Recycled, Cold-Recycled and Ordinary Asphalt Mixtures

Figure 5 shows a comparison of the life cycle carbon emissions of hot-recycled, cold-recycled and ordinary asphalt mixtures.

• Raw material production stage

Ordinary recycled asphalt mixes do not use RAP. Notably, the modified asphalt, mineral powder, aggregates and emulsified asphalt used in ordinary asphalt mixtures are different. Additionally, as a result of the use of old RAP material, hot-recycled asphalt mixtures used in modified asphalt, aggregate and mineral powder are associated with different ranges of emissions. Moreover, according to the carbon emission factor calculation, ordinary asphalt mixtures are associated with higher carbon dioxide emissions than other mixtures due to the use of modified asphalt and other components. This high-emission trend is not observed for other raw materials. Thus, the use of recycled asphalt mixtures can effectively reduce carbon emissions.

The hot regeneration and cold regeneration of asphalt mixtures due to differences in construction technology and the use of raw materials also vary. In this paper, a 20% RAP recycling ratio is used as an example to compare the hot regeneration and cold regeneration of mixtures and the differences when different raw materials are used for the pavement. As shown in the table, for the same old RAP proportion in cold-recycled and hot-recycled asphalt mixtures, the differences between mixtures with modified asphalt and emulsified asphalt are clear. At the same time, because cold-recycled asphalt mixture production technology is different from hot-recycled asphalt mixture technology, the aggregates also differ. Carbon emissions in this stage are mainly due to the different raw materials used in production. Compared to hot regeneration mixes, cold regeneration mixes that include emulsified asphalt have lower contents of modified asphalt, and the contents of other raw materials are also lower, which leads to lower carbon dioxide emissions.

2.

Mixing phase of the asphalt

Due to the high temperatures required for thermal recycling, the higher the energy consumption required to raise the temperature is, the greater the amount of carbon dioxide produced. Compared to ordinary asphalt mixtures, thermally recycled asphalt mixtures produce more carbon dioxide [32].

In the mixing of asphalt, the methods used, e.g., cold-mix recycling, mixing equipment and load-bearing equipment, are the main sources of carbon emissions, as diesel and other fuels are consumed. Since cold-mixing methods are implemented at lower temperatures than hot-mixing methods, they require less heating energy and produce less carbon dioxide. Overall, cold-recycling technology is more effective than hot-recycling technology in reducing CO₂ in the mix preparation stage.

3.

Transportation phase of mixed asphalt

The materials required for mixing are first transported from the raw material production site to the mixing plant, and the mixed material is transported from the mixing plant to the construction site. The carbon emissions in this process mainly come from the use of dump trucks. Additionally, according to transportation optimization, the fuel consumption of a 20 t dump truck is smaller than that of other possible trucks. It is assumed that the transportation distance of raw materials is 10 km and that the transportation distance of the mix is 35 km.

Due to the differences in the production of raw materials, the transportation times of ordinary and hot-recycled asphalt mixtures are different, which in turn leads to different levels of fuel consumption. The main difference in this stage is related to the use and transport of different raw materials. Cold regeneration methods are associated with lower carbon emissions in the transportation phase than hot-mix regeneration methods, but the reduction is not significant.

4.

Total carbon emissions

In general, the quantitative analysis clearly indicates that the CO₂ emissions from all stages of cold recycling are low, while the CO₂ emissions from ordinary asphalt mixtures are high and those from hot recycling are in the middle of the observed range. Cold recycling is more efficient than hot recycling in reducing carbon emissions, which is mainly due to the use of binders and modifiers and the lower mixing temperatures. These factors result in a lower heat demand for the heating apparatus and lower fuel consumption, and thus lower carbon emissions.

For cold and hot mixes, the difference in carbon emissions during the production phase of raw materials is mainly due to differences in these materials. Additionally, due to the poor mechanical properties of plant-mixed cold reclaimed asphalt mixtures, their use may be limited in some cases based on the relevant pavement requirements. Therefore, the use of cold reclaimed asphalt mixtures may not reduce carbon emissions in all cases. Moreover, due to the addition of old materials that may be reused multiple times, recycled asphalt mixtures may perform poorly; therefore, the proportion and use of recycled asphalt mixtures should be controlled.

4.3. Results of the Uncertainty Analyses of Energy Consumption and Carbon Emissions

4.3.1. Results of the Data Uncertainty Analysis

As an example, the carbon dioxide produced in the production of 1 t of mineral powder is explored, and the data were obtained from the database used to establish the relevant standards. Specifically, the uncertainty in the amount of carbon dioxide produced from the production of 1 t of mineral powder is 0.0371.

As another example, the carbon dioxide produced in the production of 1 t of modified bitumen is explored, and the uncertainty in the amount of carbon dioxide produced from the production of 1 t of modified asphalt is 0.0292.

The uncertainty data in

Table 7. Uncertainty quantification results for mineral powder.

	DQI Score	Uncertainty	Total
Type of uncertainty	Mineral powder uncertainty
Source reliability	2	0.025	$U_i$ = 0.0371
Sample integrity	3	0.025
Technical representativeness	2	0.01
Temporal representativeness	1	0
Geographic representativeness	2	0.005
Fundamental uncertainty		$U_b$ = 0.0006	$U_{d,i}$ = 0.0371
Type of uncertainty	Modified asphalt uncertainty
Source reliability	2	0.025
Sample integrity	2	0.01
Technical representativeness	2	0.01	$U_i$ = 0.0292
Temporal representativeness	1	0
Geographic representativeness	2	0.005
Fundamental uncertainty		$U_b$ = 0.0006	$U_{d,i}$ = 0.0292

and the CO₂ emission data in Section 4.1 are combined, as shown in Figure 6. Notably, the loss of modified bitumen has a larger impact on CO₂ emissions than other factors do, but the corresponding uncertainty of the data is lower. Additionally, the loss of RAP has a larger impact on CO₂ emissions than other factors, though the uncertainty of the data is high. The uncertainties related to the use of aggregates and mineral powders are similar, but the impact of aggregate loss on CO₂ emissions is high. In summary, the uncertainty of factors in the material production phase is negligible as long as the data are chosen appropriately.

4.3.2. Results of the Scenario Uncertainty Analysis

As an example of the uncertainty in the transportation distance, the probability of selecting the transportation distance in scenario 1 is set to 70%, and that for selecting the transportation distance in scenario 2 is 30%. The uncertainty of $U_{i1}$ and $U_{i2}$ is calculated according to the formula presented above; the results are 0.35 and 0.347, respectively. The scenario uncertainty was then calculated to be 0.266.

The probability of selecting the thermal power scenario, hydropower scenario or wind power scenario is set to 70%, 20% and 10%, respectively. The probability of using regular diesel is 80%, and the probability of using biodiesel is 20%. The uncertainties of using different energy sources are calculated according to the previously presented equations, and $U_{i3}$, $U_{i4}$, etc., are calculated. Energy uncertainty quantification results is shown in

Table 8. Energy uncertainty quantification results.

Type of Uncertainty	DQI Score	Uncertainties	Total
Source reliability	3	0.05	$U_i$ = 0.0
Sample integrity	2	0.01
Technical representation	2	0.01
Temporal representation	2	0.015
Geographical representation	2	0.005
Basic uncertainty		$U_b$ = 0.0006	$U_{d,i}$ = 0.0543

.

In this case, $U_{i3}$ = 0.0543, which yields $U_{i4}$ = 0.0543, and the final scenario uncertainty is 0.0543.

In the transportation phase, for example, in the scenario with the largest reduction by category, when the transportation distance is changed to “20 km for mixes + 10 km for other materials”, the RAP proportion is associated with the largest reduction in carbon emissions, which is 32% of CO₂ emissions, i.e., a reduction of 1.5 kg of CO₂ eq. An adjustment to the RAP proportion also has the greatest effect on reducing CO₂ emissions, a change of 310 mg of CO₂ eq, as shown in Figure 7. When thermal power generation is replaced with hydroelectric power generation, the carbon emission reduction achieved by adjusting the RAP proportion is the largest, and CO₂ emissions are reduced by 32%, or 131 mg of CO₂ eq. Changes in the transportation distance and the type of power generation clearly promote emission reductions. Therefore, in the subsequent production process, it is necessary to consider the rational selection of manufacturers; switch to biodiesel and other types of energy in the transportation process to promote environmental protection and emission reductions; and collect the corresponding emission data for various energy use scenarios.

5. Conclusions

In this paper, a carbon emission model for recycled asphalt mixture pavements is established, carbon dioxide emissions are used as an environmental impact indicator and sensitivity analyses of the corresponding data and model parameters are quantitatively performed to assess carbon emissions during the material production phase and the construction and building phase. The following conclusions are drawn from the results: (1) The life cycle of highway asphalt pavements is divided into two phases, namely, the material production phase and the construction and building phase, and the system boundaries of carbon emissions are determined for each phase of use of recycled asphalt mixtures. (2) By investigating the production and construction processes for common recycled asphalt mixtures, the sources of carbon emissions are identified in different stages, and the corresponding quantitative analysis methods for carbon emissions are established. (3) A quantitative analysis and uncertainty analysis are combined, and a model of carbon dioxide emissions based on the quota method is developed. Carbon emission calculations are performed for recycled asphalt mixtures. (4) Through a graphical analysis of carbon emissions in each stage of asphalt production and use based on different technologies, it is clear that carbon emissions are most sensitive to the processes and parameters in the stage of material production and mixture preparation, which accounts for 83% of the whole life cycle. To implement low-carbon construction in road construction projects, technological innovation must be achieved in the material production stage, the research and development of materials must be promoted and suitable recycled asphalt mix ratios must be determined. Moreover, the proportion of recycled asphalt used in actual applications must be increased.

The focus of this study is on the carbon emissions associated with recycled asphalt mixtures throughout their life cycle, and some new issues are identified. (1) The material production and mix preparation stage is the source of the majority of carbon emissions in project engineering and must be adjusted to control carbon emissions and achieve energy savings and emission reductions. The process of project construction should be based on ensuring the quality and safety of the project, and green building materials should be used to reduce carbon emissions. Moreover, it is necessary to carry out research on the relevance of green building materials and technology to assess carbon emission reductions for different types of materials. (2) The factors included in the indicator system for carbon emissions from asphalt mixtures should be further optimized according to the actual project design and conditions, and more complete grading quality standards should be established. (3) A sound carbon emission auditing and management system for recycled asphalt mixtures should be established to conduct regular carbon emission audits and determine the sources and volume of carbon emissions to identify ways to reduce carbon dioxide emissions. Moreover, a complete carbon emissions management system should be implemented to ensure that in every process, especially the key processes, carbon emissions are minimized.