Generally, the warm mix asphalt (WMA) technology can reduce the mixing and paving temperature effectively as compared with that of hot mix asphalt (HMA), which is considered more environment-friendly. In this study, the environmental impacts and resource consumptions of WMA and HMA pavements were analyzed comparatively using the life cycle assessment (LCA) method. A LCA model of pavement was built; meanwhile, the relevant life cycle inventory (LCI) of WMA and HMA pavements was also collected and analyzed. The midpoint impact categories including Global Warming Potential (GWP), Chinese Abiotic Depletion Potential (CADP), and Particulate Matter Formation (PMF) were assessed for five cases. The assessment results showed that the resource consumptions of both WMA and HMA pavements in entire life were almost at the same level, while the environmental impacts of WMA pavement related to greenhouse gases and PM2.5 emissions were significantly less than that of HMA pavement, except for the case where the long-term performance of WMA pavement is much worse than that of HMA pavement. In final, it could be concluded that WMA pavement is more environment-friendly compared with HMA pavement although they have the same-level resource consumptions.
Nowadays, transport is vital for the well-functioning of economic activities and a key to ensuring social well-being and cohesion of populations. Transport ensures everyday mobility of people and is crucial to the production and distribution of goods. Transport infrastructure refers to the framework that supports our transport system and is a fundamental precondition for transport systems. However, the construction and maintenance of transportation infrastructures has become and will continue to be a significant contributor to the consumption of raw materials and greenhouse gases emissions worldwide. In America, more than 350 million tons of raw materials were consumed each year in highway construction and maintenance [
In recent years, the mileage of highway has been reached 108,000 kilometers in China. Asphalt pavement is the dominating pavement form in expressway due to the following advantages: smooth surface, comfortable driving, low noise, simple construction, and rapid open-to-traffic. Nevertheless, due to the high manufacturing temperature (150∼190°C) of typical hot mix asphalt (HMA) mixtures, the energy consumptions (fuel oil and electricity usage, etc.) and gases emissions (CO2 and other pollutants) are quite high during the construction process. In order to construct environment-friendly pavements, the warm mix asphalt (WMA) mixture, which has the relatively lower manufacturing temperature of 100∼140°C and has similar mechanical properties to that of HMA mixture, has drawn much attention in the past decade [
WMA mixture has a lower viscosity and remains in good workability at a relatively lower temperature by adding viscosity-reducing agents (e.g., Sasobit and Evotherm) or incorporating water (e.g., Asphalt-min and Double Barrel Green) during the mixing process [
Life cycle assessment (LCA) is a useful method to assess the environmental impacts of a product system throughout its entire life cycle, including extracting and processing of raw materials, manufacturing, transportation, utilization, maintenance, recycling, and final disposal during end-of-life stage [
In this study, it emphasizes on the comparative assessment associated with environmental impacts between WMA and HMA pavements using LCA method. Firstly, the LCA models of these two pavements were built and the research scope was identified, including the production of asphalt, aggregates and chemical additives, asphalt mixture manufacturing and transportation, pavement construction and operation, maintenance, and dismantling at the end of life. Furthermore, the inventory data including raw materials/energy consumptions and environmental emissions of WMA and HMA pavements in all stages were collected and analyzed. Finally, five cases were assumed for long-term performances of WMA pavement to comparatively analyze the environmental impacts for these two pavements.
According to ISO standards [
The scope of LCA has a direct impact on the collection of inventory data. Generally, the life cycle of road pavements included six major stages: pavement design, raw materials’ production, transportation, construction, use, maintenance, and final disposal at the end of life [
The scope of pavement LCA model in this study.
The life cycle of WMA and HMA pavements in this study started from the stage of raw materials’ production. The raw materials mainly included aggregates (sand, mineral powder, and macadam), asphalt binders (petroleum asphalt and modified asphalt), and warm-mixing agents. The resource consumptions at this stage mainly include heavy oil, diesel oil, gasoline, and electricity, which are consumed by the operation of relative machines used to produce and process raw materials, while the environmental impacts mainly come from mineral extraction, asphalt refinement, production of warm-mixing agents, and transportation of these raw materials to the asphalt-mixing plant.
As the raw materials were transported to the asphalt-mixing plant, the aggregates will be dried and screened; meanwhile, the asphalt would be heated. Then, the asphalt mixture was produced and transported to the construction site of pavement. The main resource consumptions are fuel and electricity used by machinery in the processes of drying and screening, heating and mixing, and transportation. The environmental impacts in this stage are mainly caused by the burning of fossil fuels.
The stage of pavement construction comprises the following processes: site cleaning and preparation, foundation compaction, construction of subbase and base layers, asphalt-mixture paving, levelling, and rolling. As the difference of environmental impacts between WMA and HMA pavement construction lies in the process of asphalt-mixture paving, only this process was assessed in this stage. The main resource consumption is fossil fuel used by paving machinery. The gas emissions in this stage are mainly from the burning of fossil fuels and the hot/warm asphalt mixture.
After a certain period of service, the pavement needs to be maintained due to its deterioration with the combination of environmental impacts and repeatable vehicle loading. In this study, for simplification, it was assumed that only medium repair with overlay will be adopted. The environmental impacts in this stage are mainly involving the demolition of damaged asphalt layers, cleaning the substrate, and paving a new layer of asphalt mixture.
At the end of service life, the pavement materials need to be properly disposed of. There are a number of options for end-of-life treatment, such as abandonment (together with pavement), landfill, or recycling. In this study, the recycling option was selected due to its high popularity. The resource consumptions and pollution emissions are mainly from the burning of fossil fuel by the equipment during demolition, transport, and landfill.
The functional unit was defined as a quantitative benchmark unit that should represent the function of the analyzed system. For a more accurate comparison, the same function unit for different road pavement systems is used in this paper. Herein, the function unit for road pavement LCA is defined based on the geometry, performances, and service life of the pavement. For the case study presented later, the section of road pavement with 6 lanes concerned has a length of 1 km and a width of 33 m. The entire service life of this road is assumed to be 15 years, which is the designed life of asphalt pavement. The average daily traffic volume is 20,000, of which 8% are heavy vehicles. The total thickness of the asphalt layer is 18 cm as the pavement structure consists of three layers from top to bottom, which is 4 cm stone mastic asphalt (SMA-13), 6 cm asphalt concrete (AC-20), and 8 cm AC-25, respectively, as shown in Figure
The pavement structure of asphalt pavement.
Composition of 1000 m3 asphalt mixtures.
Asphalt mixture | SBS-modified bitumen ( |
#70 asphalt ( |
Fiber stabilizer ( |
Sand (m3) | Mineral powder (m3) | Stone chip (m3) | Gravel (m3) |
---|---|---|---|---|---|---|---|
SMA-13 | 144.32 | — | 7.34 | 119.38 | 246.74 | 126.56 | 1111.35 |
AC-20 | — | 122.54 | — | 471.22 | 128.40 | 261.18 | 723.22 |
AC-25 | — | 113.47 | — | 389.79 | 117.72 | 226.75 | 854.79 |
In the stage of asphalt-mixture production, the initial temperature of the raw materials is assumed as 25°C, and the mixing temperature of HMA and WMA mixtures is 180°C and 140°C, respectively. All asphalt mixtures are transported from the asphalt plant to the construction site with an assumed average distance of 10 km. In the stage of end of life, the demolished pavement materials were recycled.
The pavement condition index (PCI) deterioration model was used to determine the moment of maintenance conduction in this study [
In this study, the maintenance will be conducted when the PCI reduces to 70, after which the PCI will be upgraded to a level equivalent to that of before five years ago, as shown in Figure
The change of PCI with age (maintenance event occurs at 10 years).
For WMA pavement, there were five assumed maintenance scenarios to account for the uncertainty of its long-term performance: (1) the PCI of WMA pavement deteriorated slower than that of HMA pavement by 20%, and the maintenance area was 20% less than HMA; (2) the PCI of WMA pavement deteriorated 10% slower than that of HMA pavement, and the maintenance area was 10% less than HMA; (3) the WMA pavement has the same maintenance condition with HMA pavement; (4) the PCI of WMA pavement deteriorated 10% faster than that of HMA pavement, and the maintenance area was 10% larger than HMA pavement; and (5) the PCI of WMA pavement deteriorated 20% faster than that of HMA pavement, and the maintenance area was 20% larger than HMA pavement.
The raw materials of asphalt mixture mainly include natural aggregates (sand, mineral powder, gravel, and stone chips), asphalt (petroleum asphalt and modified asphalt), and warm-mixing agents. The LCI of natural aggregates was from the Chinese Life Cycle Database (CLCD), which covers a large amount of LCI data for basic industry products in China, averaged over different scales of manufacturing and degree of technical sophistication. The LCI of asphalt comes from European Bitumen Association (EBA) because the CLCD lacks the environmental impact data for asphalt production, and the source of crude oil and refining process in China are the same as those in Europe. The Evotherm warm-mixing technology was used most widely in China for WMA mixture production, and hence, the LCI data were also from the Ecoinvent database from Europe.
The energy consumptions during the production of asphalt mixture mainly involve fuel and electricity consumptions. The fuel is consumed for asphalt heating and aggregate drying, while the construction machinery consumes the electricity. During this process, the aggregates drying and heating are likely to bring out plenty of dust, while the burning of fossil fuel leads to CO2 emission. Moreover, the asphalt heating during mixing releases a lot of harmful gases.
For the hot mixture asphalt (HMA), the energy consumptions are calculated using Chinese Highway Engineering Budget Quota and Machinery Quota (JTG/T B-06-02-2007) (JTG/T B-06-03-2007) (in short: Quota method). The number of machine team when producing every 1000 m3 HMA was surveyed from the Budget Quota, whereas the energy consumptions of machinery in unit machine team were surveyed from Machinery Quota. Then, the energy consumptions can be calculated through the product of these two sets of data. The energy consumptions of producing 1000 m3 coarse-graded asphalt (CGA) mixture are listed in Table
Energy consumptions of producing 1000 m3 coarse-graded asphalt (CGA) mixture.
Equipment | Machine team | Energy consumption per machine team | Total energy consumption | ||||
---|---|---|---|---|---|---|---|
Heavy oil (kg) | Electricity (kWh) | Diesel oil (kg) | Heavy oil (kg) | Electricity (kWh) | Diesel oil (kg) | ||
Mixer (320 |
1.35 | 9574.4 | 5917.61 | — | 12925.44 | 7988.77 | — |
Loader (3 m3) | 2.53 | — | — | 115.15 | — | — | 291.33 |
The LCIs of mixing every 1000 m3 HMA and WMA mixtures.
Type of consumption/emission | Amount | |
---|---|---|
HMA | WMA | |
Heavy fuel oil (kg) | 12955.44 | 9900.55 |
Electricity (kWh) | 7988.77 | 6105.02 |
Diesel (kg) | 291.33 | 222.63 |
CO2 (kg) | 45510 | 18204 |
NO |
147.6 | 40.44 |
SO2 (kg) | 108.24 | 26.84 |
PM2.5 (kg) | 10.33 | 5.38 |
For warm mix asphalt mixture, the energy consumptions were calculated using the thermodynamic equilibrium:
As listed in Table
The mixed asphalt mixture is transported by dumper truck from the asphalt-mixing plant to the construction site and poured into paving machinery. In this study, the carrying capacity of a dumper truck is assumed as 15 tons. The environmental impacts in this stage are from the burning of fuel by the vehicles. The energy consumptions also can be calculated using the Chinese Quotas mentioned before. 542.44 kg of diesel oil will be consumed when transporting 1000 m3 asphalt mixture for 1 km distance. The gaseous emissions are calculated based on the emission factors, including CO2, NO
During the pavement construction stage, it comprises different processes, whereas this study just focused on the asphalt-mixture paving process. The energy consumptions in this stage come from the fuel consumed by the paver and roller compaction machinery, which are calculated through the Quota method. The diesel oil consumptions of constructing 100 m2 stone mastic asphalt (SMA), medium-sized particle asphalt (MSPA), and coarse-graded asphalt (CGA) concrete pavement are 445.75 kg, 280.02 kg, and 279.71 kg, respectively. The gaseous emissions data are referenced from GB 20891-2007 and listed in Table
Gaseous emissions of constructing 100 m2 asphalt concrete pavement.
Pavement type | Gases emissions | ||||
---|---|---|---|---|---|
CO2 (kg) | NO |
CO (g) | PM (g) | HC (g) | |
SMA | 2817.11 | 89.25 | 60.95 | 3.57 | 14.87 |
MSPA | 1793.62 | 70.62 | 47.29 | 2.76 | 11.68 |
CGA | 1779.43 | 69.47 | 46.89 | 2.74 | 11.58 |
The maintenance was conducted periodically throughout the entire service life of the pavement. In this study, only the overlay technology was chosen to maintain the distressed pavement. The energy consumptions were calculated using the Quota method. Due to the similarity of the necessary processes for both new construction and maintenance (only asphalt-mixture paving process considered in new construction, whereas the rest is considered the same for both HMA and WMA pavements), the pollution emissions of maintenance can be calculated similarly (Refer to Section
The energy consumptions and pollution emissions of 1000 m2 overlay.
Energy consumptions (kg) | Pollution emissions (kg) | ||
---|---|---|---|
Heavy oil | 583.44 | CO2 | 2340.63 |
Electricity | 393.94 | SO2 | 4.73 |
Diesel oil | 144.09 | NO |
13.41 |
Gasoline | 1.03 | CO | 26.44 |
— | — | PM | 257.57 |
As mentioned previously, the environmental impacts were mainly from the use of fuel by demolition, transport, and landfill equipment at the end-of-life stage. The energy consumptions and pollution emissions can be calculated using the Quota method and emission factors from the European Environment Agency (EEA), respectively.
The impact assessments of WMA and HMA pavements were conducted using LCA-based software. These impacts were assessed in accordance with two sets of impact categories, which are midpoint impact categories and endpoint impact categories, respectively. The endpoint impact categories include damage to human health, damage to ecosystem diversity, and damage to resource availability. They may be affected by environmental conditions in different regions, such as atmosphere, water, soil, and ecological system. Due to a lack of such data in China, the endpoint impact categories were not assessed in this study.
Three midpoint impact categories, which are Global Warming Potential (GWP), Chinese Abiotic Depletion Potential-fossil fuel (CADP), and Particulate Matter Formation (PMF), were selected in this study to assess environment impacts and resource consumptions. GWP contains the impact factors of CO2, CH4, CO, and N2O, which are characterized as CO2 and expressed as GWP/kg. CADP is an exclusive midpoint impact category in China, which was obtained based on Abiotic Depletion Potential (ADP) of China applying CML method. The impact factors of CADP include coal, petroleum, and natural gas, of which the characteristic factor of CADP is coal, and expressed as CADP/kg. The impact factors of PMF contains of PM10 and PM2.5. The characteristic factor is PM2.5, which is expressed as PMF/kg.
The sensitivity analysis is used to quantitatively analyze the influence of inputs on the outputs for a mathematical model, which can be calculated using equation (
The credibility of LCA results is influenced by uncertainty during the LCA processes. The uncertainty of LCA consists of original data uncertainty and algorithm uncertainty. The original data uncertainty can be assessed in terms of data source reliability, sample integrity, technical, time, and geographical representativeness. In this study, original data uncertainty was obtained from the Ecoinvent database [
The algorithm uncertainty depends on the rationality of the algorithm. The uncertainties of algorithms: directly acquire algorithm, total algorithm, balancing algorithm, experience algorithm, and theory algorithm are 0, 0, 0.025, 0.05, and 0.1, respectively [
A commercial LCA-based software was used in this study to calculate the environmental impacts and resource consumptions of the WMA and HMA pavements. This software contains the key features of international popular LCA-based software, including data collection records and automatic generation of LCA reports. The inventory data can be obtained from Chinese life cycle database (CLCD), European life cycle database (ELCD), and Ecoinvent database, which are incorporated within this software. It also allows users to add new data and calculations to the database. The LCA modeling process of WMA and HMA pavements using this software is shown in Figure
The LCA modeling process of WMA and HMA pavements in LCA-based software.
Figure
The GWP impacts of HMA and WMA pavements for five cases: (a) case 1; (b) case 2; (c) case 3; (d) case 4; and (e) case 5.
Figure
The CADP impacts of HMA and WMA pavements for five cases: (a) case 1; (b) case 2; (c) case 3; (d) case 4; and (e) case 5.
Figure
The PMF impacts of HMA and WMA pavements for five cases: (a) case 1; (b) case 2; (c) case 3; (d) case 4; and (e) case 5.
Based on the above discussions, constructing WMA pavement has almost no advantage of saving resources. Nevertheless, the WMA pavement has an obvious effect to reduce the greenhouse gases and PM2.5 emissions. Therefore, popularizing WMA pavement is beneficial to the construction of environment-friendly society.
In this section, the sensitivity of impact category factors GWP, CADP, and PMF (relative change of factor induced by the change of unit process/inventory) was calculated. The pairs of unit process/factor that have sensitivity value larger than 10% are listed in Table
The sensitivity of GWP, CADP, and PMF with the corresponding unit process.
Unit process-inventory | GWP | CADP | PMF | |||
---|---|---|---|---|---|---|
HMA | WMA | HMA | WMA | HMA | WMA | |
Asphalt pavement-raw materials | 56.56 | 65.70 | 74.88 | 77.04 | 41.28 | 50.10 |
Raw materials’ production-petroleum asphalt | 36.50 | 41.22 | 50.57 | 51.27 | 20.95 | 23.93 |
Petroleum asphalt production-raw petroleum | 35.23 | 39.79 | 59.45 | 60.28 | 12.51 | 14.29 |
Asphalt pavement-asphalt-mixture production | 24.88 | 14.72 | — | — | 28.37 | 17.03 |
Asphalt-mixture production-CO2 emission | 18.18 | — | — | — | — | — |
Raw materials’ production-modified asphalt | 15.75 | 17.79 | 19.42 | 19.68 | 11.18 | 12.78 |
Modified asphalt production-raw petroleum | 11.33 | 12.80 | 19.13 | 19.39 | — | — |
Asphalt pavement-maintenance | 10.75 | 10.76 | 11.30 | 11.32 | — | — |
Asphalt pavement-end of life | — | — | — | — | 11.31 | 12.92 |
In this study, an uncertainty assessment was also conducted to determine the uncertainties of process inventories in the LCA results. The process inventories with sensitivity over 10% include asphalt pavement-raw materials, raw materials’ production-petroleum asphalt, raw materials production-modified asphalt, asphalt pavement-asphalt mixture production, and asphalt pavement-maintenance, of which the uncertainty of these process inventories is 5.59, 11.46, 11.46, 10.00, and 10.31, respectively. Due to the LCI data of asphalt collected from different databases, the productions of petroleum asphalt and modified asphalt have higher uncertainty than other unit process inventories. Therefore, to improve the accuracy of results in an LCA project, the database should be settled or collect the data from the field directly. In addition, the uncertainties of all processes are smaller than 15%. It indicates that the results in this study are relatively credible.
In this study, a comparative comprehensive life cycle assessment (LCA) was conducted for WMA and HMA pavements. The LCA of the pavement model was established, which includes the stages of raw materials’ production, asphalt-mixture production and transportation, maintenance, and disposal at the end of life. The inventories of every unit process were collected and analyzed. Since the long-term performance of WMA pavement has not been well understood, five maintenance scenarios were assumed to assess the environmental impacts of WMA and HMA pavements. The specific conclusions can be drawn as follows: The results suggest that, assuming comparable long-term performances with that of HMA pavement, WMA pavement produces less CO2 and PM2.5 emissions during their entire life cycle, which indicates that WMA pavement is friendlier for environment. The assessment reveals that the difference in Chinese Abiotic Depletion Potential (CADP) between WMA and HMA pavements could be negligible, which indicated that WMA pavement technique consumes almost the same resource as that of HMA pavement during the entire service life. The sensitivity assessment results indicated that improving the technology in raw material production is the most effective way to reduce the environmental impacts for both WMA and HMA pavements.
In this paper, all the data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
The authors would like to graciously thank the National Natural Science Foundation of China (Grant no. 51708061), 111 Project of China (Grant no. B18062), Science and Technology Research Program of Chongqing Municipal Education Commission (Grant no. KJQN201800126), State Education Ministry and Fundamental Research Funds for the Central Universities (no. 2019CDJSK04XK23), and the Fundamental and Frontier Research Project of Chongqing (no. cstc2018jcyjAX0535) for the financial support of this work.