The energy consumption and emission of polyurethane pavement construction based on life cycle assessment

Journal of Cleaner Production 256 (2020) 120395

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Short communication

The energy consumption and emission of polyurethane pavement construction based on life cycle assessment Lin Cong a, Guihong Guo a, *, Meng Yu b, **, Fan Yang a, Le Tan a, c a

Key Laboratory of Road and Traffic Engineering of the Ministry of Education, Tongji University, Shanghai, 201804, China State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co. Ltd, China c Highway Administration of Huzhou, Huzhou, 313000, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 July 2019 Received in revised form 28 December 2019 Accepted 2 February 2020 Available online 3 February 2020

Polyurethane-bounded pavement with good road performances is a new pavement form. Whether polyurethane (PU) pavement met the requirements of energy conservation, environmental protection and sustainable development or not, the energy consumption and emission of PU pavement in whole life cycle were analyzed systematically in this paper. Firstly, the life cycle of pavement was divided into six phases: material production, mixture production, transportation, construction, maintenance and recycling according to the construction technology of road in China. Based on the classification of the life cycle environmental impact assessment (LCIA), the effects of six main exhaust gases (CO2, CH4, N2O, CO, SO2, NOx) were divided into four categories: Global Warming Potential (GWP), Eutrophication Potential (EP), Photochemical Smog Potential (POCP) and Acidification Potential (AP). Secondly, the energy consumption and emission of each phase are studied, and the main phase with significant environmental impact was selected. Thirdly, the results of inventory would be normalized and weighted to analyze the environmental impact by the life cycle impact assessment (LCIA). The LCIA method was environmental design of industrial product 97 (EDIP 97). Results show that the construction of PU pavement is conducive to energy conservation. Besides, the energy consumption of mixture production phase is the largest in asphalt pavement, and it is material production phase in PU pavement. In terms of emission, the emissions mainly occur in material production and mixture production phases in asphalt pavement, and it is material production phase in PU pavement. Under the same structure, the construction of PU pavement is not conducive to reducing the emission, and will enlarge the GWP, EP and AP. For example, the CO2, N2O and NOx emissions of PU pavement are 1.98, 5.30 and 2.35 times higher than that of asphalt pavement respectively, but the CO emission of asphalt pavement is 3.86 times higher than that of PU pavement. Furthermore, the results of LCIA show that the score of all environmental impact of PU pavement is little smaller than that of asphalt pavement after normalization and weighting. However, the road performances of PU pavement are much better than that of asphalt pavement. With the same environmental impact, PU pavement has a longer service life. It is summarized that PU pavement meets the requirements of environmental protection and sustainable development in the long run. This is conducive to the promotion and application of PU pavement. © 2020 Elsevier Ltd. All rights reserved.

Handling Editor: Zhifu Mi Keywords: Polyurethane pavement Life cycle assessment (LCA) Energy consumption EDIP 97 Impact category

1. Background Polyurethane (PU) has been used as the pavement binder and crack repair material. Many studies show that the performances of PU mixture are much better than that of asphalt mixture. For

* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Guo). https://doi.org/10.1016/j.jclepro.2020.120395 0959-6526/© 2020 Elsevier Ltd. All rights reserved.

example, the stability and fatigue life of PU mixture are 3 times and 10 times higher than that of asphalt mixture (Cong et al., 2018). PU mixture had a good anti-rutting performance (Sun, 2016). And PU rubber mixture has a better noise absorption performance (Wang et al., 2017a,b and Wang et al., 2017a,b). Furthermore, the deformation resistance, skid resistance, fatigue resistance, corrosion resistance and photo-thermal aging resistance abilities of Porous PU mixture were good (Wang et al., 2014a,b and Wang et al., 2014a,b). The road performances of PU mixture could meet the requirements of the repeated traffic load and complex

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environment. PU mixture had been used for the pavement construction (Xu et al., 2015). The road construction has an adverse effect on the environment. The road construction needs a lot of stone, cement, asphalt and other materials. The exploitation of stone, the production of asphalt and cement also consume energy and release exhaust gases. In the United States, the construction and maintenance of highway consume an average of 350 million tons raw materials every year (Santero, 2009). According to the survey data, the road traffic industry will emit 1.108 billion tons of CO2 by 2030 in China (Shang et al., 2010). The energy conservation and environmental pollution reduction have become the common responsibility of the international community. Life cycle assessment (LCA) can evaluate the environmental impact of product in life cycle. The contents of LCA include the determination of purpose and scope, inventory analysis, impact assessment and interpretation. The energy consumption and emission in different phases can be considered comprehensively. It provides a theoretical support for promoting energy conservation and emission reduction in road construction. Therefore, LCA was used for analyzing the environmental impact of the new road materials, such as bio-modified asphalt binder (Samieadel et al., 2018), reflective coatings (Li et al., 2016), multiple-polymer modified asphalt (Wang et al., 2018), warm-mixed asphalt mixture (Wu, 2015a,b). Their research results showed that bio-modified asphalt binder and warm-mixed asphalt mixture were the environmentfriendly materials. For example, the production of conventional asphalt binder emitted over 5 times more of CO2 and approximately 3 times more of methane to the environment compared to that of bio-binder (Samieadel et al., 2018). Besides, the multiple-polymer modified binder had great high temperature stability, but it was not an environment-friendly material. With the polymer content increased, the energy consumption and greenhouse gas emission of multiple-polymer modified binder rose (Wang et al., 2018). Up to now, the energy consumption and emission of PU pavement in life cycle have not been analyzed systematically. The environmental benefits of PU pavement are still unclear, which hinder the promotion and application of PU pavement seriously. Therefore, the environmental impact of PU pavement will be studied by LCA. In order to identify the key phases of high energy consumption and emission, and fully excavate the energy conservation potential of PU pavement, the energy consumption and emission in the life cycle of PU pavement are studied systematically. The life cycle is divided into six phases: material production, mixture production, transportation, construction, maintenance and recycling. The energy consumption and emission of each phase are calculated and discussed in PU pavement and asphalt pavement. Then, the key phases of high energy consumption and emission will be selected. Finally, the comprehensive environmental impact assessments of asphalt pavement and PU pavement are conducted by Environmental design of industrial product 97 (EDIP 97). This is conducive to exploring the energy conservation and emission reduction potential.

2.2. PU mixture PU is used as a binder and mixed with aggregates according to standard practice for asphalt mixture (ASTMD). The production of PU mixture has two methods. One method is that PU and the dried aggregates are mixed in proportion at atmospheric temperature (Cong et al., 2018 and Wang et al., 2014a,b). Another is that PU is used as asphalt modifier. The PU modified asphalt and aggregates are mixed at the required temperature (Han, 2017; Ban, 2017). In this paper, the method discussed is the first one. 3. Method The technical framework of LCA includes four parts: the goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA) and life cycle assessment and interpretation (ISO14000, 2000). 3.1. The goal and scope definition The goal and scope definition include the determination of implementation objective and product system boundary. The road is a complex product system involving material, technology, transportation and so on. Due to the research scope, data availability and quality, it is necessary to make an appropriate adjustment for the system boundary. The range of this study includes the material production phase, mixture production phase, transportation phase, construction phase, maintenance phase and recycling phase. The research range is shown in Fig. 1. In material production phase, the major materials, which include PU, asphalt and stone, are considered. In mixture production phase, the energy consumption used for heating mixture and operating machine are mainly considered. Many research results (Pan, 2011; Zhang et al., 2018; Wu, 2015a,b; Li, 2016) showed that the quantities of energy consumption and emission were the larger in material production and mixture production phases. The energy consumption and emission in material production and mixture production phases are calculated by Comprehensive Pavement Management Evaluation System (CPMES). CPMES is used for calculating the energy consumption and gas emission of pavement structure in the whole life cycle. The parameters, which need to be input into system, include material inventory, pavement structure, construction technology and maintenance program. The output data of the system include energy consumption and emission. In transportation phase, the fuels consumed by vehicles are considered. In construction phase, the energy consumption and emission are calculated based on quota methods (Ministry of Transport of the People’s Republic of China, 2007a,b; Ministry of Transport of the People’s Republic of China, 2007a,b). In maintenance phase, the energy consumption and emission of the material, vehicle and machinery are mainly analyzed. In recycling phase, the energy consumption and emission will be calculated by CPMES. 3.2. Life cycle inventory (LCI)

2. Materials and application 2.1. Polyurethane Polyurethane (PU), a type of polymer, has been used in a myriad of industries. The high versatility of PU derives from various molecular structures. The main raw materials of PU include diphenylmethane diisocyanate (MDI), toluene diisocyanate (TDI) and polypropylene glycol (PPG). The TDI and MDI, made from toluene, niter and primary amines and carbon disulfide respectively, are the most widely used.

Life cycle inventory (LCI) is a process of compiling and quantifying the input and output data in the whole life cycle of the system. LCI is a quantitative analysis for energy consumption and emission of the product, process or activity. 3.3. Life cycle impact assessment (LCIA) LCIA could classify the results of LCI according to environmental impact. The results of LCI (energy consumption and emission) are correlated with environmental impact categories which include

L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395

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Fig. 1. Research range of LCA.

global warming potential (GWP), nutrition potential (EP), photochemical smog potential (POCP) and acidification potential (AP).

to GaBi (2013) and CLCD database (Pan, 2011; Zhang et al., 2018; Wu, 2015a,b; Li, 2016). The results are shown in Table 3.

3.4. Life cycle assessment and interpretation

4.2. LCI of mixture production

Interpretation can summarize the results of LCIA. Interpretation can assess opportunities to reduce energy consumption and emission, and provide decision makers with feasible measures.

The mixture includes aggregate, mineral powder and asphalt or PU. In asphalt mixture production, the required temperature is about 170
C. The minimum temperature of asphalt mixture transported to construction site is not less than 135
C (Ministry of Transport of the People’s Republic of China, 2004). The energy is consumed for heating asphalt and mixture. The heavy and diesel oils used for heating asphalt and aggregate generate a lot of gases in the process of mixture production. The process of mixture production also requires the cooperation of many machinery and equipment including asphalt mixing plant, wheeled loader, dump truck and other necessary equipment. The operations of conveyors, hoists, dust removal, drying drums and mixers also consume a lot of electricity.

4. Estimation of energy consumption and emission in life cycle The road includes subgrade, pavement, bridge, tunnel and traffic ancillary unites (Huang, 2014). The bridge, tunnel and traffic ancillary units are not considered in this paper. Two pavement structures are analyzed and shown in Table 1. In calculation, the asphalt mixture and PU mixture are considered, and cement stabilized macadam and graded aggregate are not considered. The length of selected road is 1 Km. The width consisting of four vehicle lanes is 15 m. 4.1. LCI of raw material production This study focuses on the energy consumption (fuel and electricity) and emission (CO2, CO, N2O, CH4, NOx and SO2) of the materials, their inventories are shown in Table 2. This study deems that fuel also consumes energy in the process of exploitation and refining, and produces gas emissions when burning. Based on CPMES, the LCI of each pavement structure is calculated according Table 1 The pavement structure. Structure type

Material

Thickness

Asphalt pavement

Middle-graded Asphalt mixture Coarse-graded Asphalt mixture Open graded cement stabilized macadam Graded aggregate

60 mm 80 mm 200 mm 200 mm

PU pavement

Middle-graded PU mixture Coarse-graded PU mixture Open graded cement stabilized macadam Graded aggregate

60 mm 80 mm 200 mm 200 mm

4.2.1. The production of asphalt mixture The energy consumption and emission are calculated based on quota methods (Wu, 2015a,b; Ministry of Transport of the People’s Republic of China, 2007a,b and Ministry of Transport of the People’s Republic of China, 2007a,b). Before compaction, the asphalt mixture is loose. In mixture production and transportation phases, the loose paving coefficient is considered. The loose paving coefficient of asphalt mixture, relating to materials, thickness and compacting technology, is the ratio of loosening thickness to the compacted thickness, and the value is 1.05e1.35 (Dai, 2006; Yang, 2013). The loose paving coefficients of PU mixture and asphalt mixture are same, it is 1.2 in this paper. Based on the pavement structure shown in Table 1, the requirements of middle-graded and coarse-grained asphalt mixture are 1080 m3 and 1440 m3 respectively. It is considered that the requirements of asphalt mixture are as same as that of PU mixture. The shift numbers of machinery and equipment used for mixing 1000 m3 asphalt mixture are provided by the budget quota. The calculated results are shown in Table 4. 4.2.2. The production of PU mixture PU mixture can be mixed at atmospheric temperature. Therefore, the process of PU mixture production does not consume heavy

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Table 2 The energy consumption and emission of materials and Fuels. Material (Kg)

Energy (MJ)

Crushed stone Asphalt Rubber particles Polyurethane Gasoline Diesel oil Heavy oil Electricity

2.95E-2 1.26Eþ0 4.03Eþ0 5.16Eþ1 4.31Eþ1 4.27Eþ1 4.18Eþ1 3.60Eþ0

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

3.30E-3 3.61E-1 2.06E-1 2.25Eþ0 3.07Eþ0 3.19Eþ0 2.99Eþ0 9.30E-1

9.85E-6 4.76E-3 8.92E-4 6.8E-3 0.13E-3 0.13E-3 0.12E-3 2.63E-3

1.93E-8 3.02E-6 2.03E-6 1.96E-4 0.3E-3 0.3E-3 0.2E-3 1.42E-5

1.80E-5 1.44E-2 1.32E-4 4.38E-3 0.35E-1 0.35E-1 0.42E-1 1.74E-4

7.72E-6 4.05E-3 3.79E-4 5.18E-3 0.3E-3 0.7E-3 0.7E-1 3.17E-3

2.59E-5 2.95E-3 2.644e4 3.99E-3 1.60E-3 2.44E-3 2.57E-3 2.58E-3

Table 3 The energy consumption and emission in material production phase. Pavement structure

Material

Consumption (t)

Energy (MJ)

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

Asphalt pavement

Crushed stone Asphalt Total

4700 235 e

1.38Eþ5 2.96Eþ5 4.34Eþ5

1.55Eþ4 8.49Eþ4 1.01Eþ5

4.63Eþ1 1.12E-3 4.64Eþ1

9.08E-2 7.10E-1 7.92E-1

8.48Eþ1 3.38Eþ3 3.47Eþ3

3.63Eþ1 9.51Eþ2 9.87Eþ2

1.22Eþ2 6.94Eþ2 8.16Eþ2

PU pavement

Crushed stone PU Total

4700 235 e

1.38Eþ5 7.80Eþ5 9.18Eþ5

1.55Eþ4 4.27Eþ5 4.43Eþ5

4.63Eþ1 1.60Eþ3 1.64Eþ3

9.08E-2 4.61Eþ1 4.61Eþ1

8.48Eþ1 1.03Eþ3 1.11Eþ3

3.63Eþ1 3.81Eþ3 3.83Eþ3

1.22Eþ2 2.61Eþ3 2.73Eþ3

Table 4 The consumption of fuel and electricity for the production. Asphalt mixture

Equipment

1440 m coarse-grained asphalt mixture

Asphalt mixing plant (320 t/h)

3

Machinery (shift)

Consumption (kg/one-shift)

Total consumption

1.95

Heavy oil: 9574.4 Electricity: 5917.6 Diesel oil: 92.86 Gasoline: 41.63

Heavy oil: 18670.1 Electricity: 11539.3 Diesel oil: 667.7 Gasoline: 156.6

Heavy oil: 9574.4 Electricity: 5917.6 Diesel oil: 92.86 Gasoline: 41.63

Heavy oil:13882.9 Electricity: 8580.5 Diesel oil: 314.8 Gasoline: 117.4

3

1080 m3 middle-grained asphalt mixture

wheeled loader (2m ) Dump truck (5t)

7.19 3.76

Asphalt mixing plant (320 t/h)

1.45

wheeled loader (2m3) Dump truck (5t)

3.39 2.82

and diesel oils for heating. It is considered that the quantity of electricity consumed in the process of PU mixture production is as same as that of asphalt mixture. Base on the results in Table 4, the electricity consumption of PU mixture is obtained and shown in Table 5(See. Table 6).

4.3. LCI of transportation phase After mixing, mixtures were transported to the paving site. The environmental impact of mixture transportation caused by the operation of vehicles. The quantity of energy consumption can be calculated basing on the document (Li, 2016). For two pavement structures, the transportation distances are same. It is assumed that the dump truck with 30t load is used as the transport vehicle, the transportation distance is 10 km. The energy consumption and emission of a dump truck are shown in Table 7.

The density of coarse-grained asphalt mixture is 2.44 t/m3, and the density of middle-grained asphalt mixture is 2.43 t/m3 (Tan, 2007; Zhang, 2011). The total weight of coarse-grained asphalt mixture is 3513.6t, and the total weight of middle-grained asphalt mixture is 2624.4t. The numbers of transport are 118 and 88 respectively. Therefore, the energy consumptions and emissions are shown in Table 8.

4.4. LCI of construction During construction, the paving and compaction process require the help of paver, road scraper and other related equipment. The operations of dump trucks, pavers and other equipment consume diesel, gasoline and other fuels, and emit exhaust gases. The energy consumptions are calculated by quota method and shown in Tables 9 and 10.

Table 5 The energy consumption and carbon emission in asphalt mixture production. Type

Gasoline Diesel oil Heavy oil Electricity Total

Consumption

274.0(kg) 982.5(kg) 32553.0(kg) 20119.8(KW$h) e

Energy (MJ)

1.18Eþ4 4.20Eþ4 1.36Eþ6 7.24Eþ4 1.49Eþ6

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

8.41Eþ2 3.13Eþ3 9.73Eþ4 1.87Eþ4 1.20Eþ5

3.56E-2 1.28E-1 3.91Eþ0 5.29Eþ1 5.70Eþ1

8.22E-2 2.95E-1 6.51Eþ0 2.86E-1 7.17Eþ0

9.59Eþ0 3.44Eþ1 1.37Eþ3 3.50Eþ0 1.41Eþ3

8.22E-2 6.88E-1 2.28Eþ3 6.38Eþ1 2.34Eþ3

4.38E-1 2.40Eþ0 8.37Eþ1 5.19Eþ1 1.38Eþ2

L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395

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Table 6 The energy consumption and carbon emission in asphalt mixture production. Type

Consumption (KW$h)

Electricity

Energy (MJ)

20119.8

7.24Eþ4

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

1.87Eþ4

5.29Eþ1

2.86E-1

3.50Eþ0

6.38Eþ1

5.19Eþ1

Table 7 The energy consumption and carbon emission of one dump truck. Project

Energy (MJ)

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

Dump truck (30 t/km) Distance (10 km) Total

2.42Eþ01

1.84Eþ00

6.54E-03

1.39E-02

1.65E-03

5.97E-02

2.42Eþ02

1.84Eþ01

6.54E-02

9.12E-05 10 9.12E-04

1.39E-01

1.65E-02

5.97E-01

Table 8 The energy consumption and carbon emission in transportation. Asphalt mixture

Energy (MJ)

Coarse-grained Middle-grained Total

2.83Eþ04 2.11Eþ04 4.94Eþ04

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

2.16Eþ03 1.61Eþ03 3.77Eþ03

7.66Eþ00 5.72Eþ00 1.34Eþ01

1.07E-01 7.98E-02 1.87E-01

1.63Eþ01 1.22Eþ01 2.85Eþ01

1.93Eþ00 1.44Eþ00 3.37Eþ00

6.99Eþ01 5.22Eþ01 1.22Eþ02

Table 9 The consumption of fuel for asphalt mixture production. Asphalt mixture 3

Middle-grained (1080 m )

Coarse-grained (1440 m3)

Equipment

Machinery one-shift

Diesel oil consumption (kg/one-shift)

Total consumption (kg)

smooth-wheel roller (6~8t) smooth-wheel roller (12e15t) Asphalt mixture paver (12.5 m) pneumatic tyre roller (16e20t) pneumatic tyre roller (20e25t) smooth-wheel roller (6~8t) smooth-wheel roller (12e15t) Asphalt mixture paver (12.5 m) pneumatic tyre roller (16e20t) pneumatic tyre roller (20e25t)

3.10 4.65 1.58 0.92 2.13 4.13 6.21 2.10 1.22 2.84

19.33 40.46 136.41 42.29 50.26 19.33 40.46 136.41 42.29 50.26

59.92 188.33 215.09 38.82 106.93 79.89 251.11 286.79 51.76 142.58

Table 10 The energy consumption and emission in construction phase. Type

Consumption (kg)

Energy (MJ)

Diesel oil

1421.2

6.07Eþ4

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

4.53Eþ3

1.85E-1

4.26E-1

4.97Eþ1

9.95E-1

3.47Eþ0

4.5. LCI of maintenance The performances of asphalt pavement will gradually decrease under the repeated traffic load and the external environment. Regular maintenance can maintain performances of the pavement. The maintenance frequency and area are the main factors. According to pavement condition index (PCI) and “Technical Specification for Maintenance of Highway Asphalt Pavement (JTJ 073.2e2001)”, the maintenance measures are selected (Wu, 2015a,b). PCI refers to the integrity of the pavement structure. It is the most direct reflection of the physical performances of the pavement damage. If the value of PCI is greater than 70, the maintenance measures are daily maintenance and light repair. If the value of PCI is smaller than 70, the maintenance measures are medium repair and overlay. In terms of the maintenance area, it is assumed that the maintenance area is 1/2 of the total area in asphalt pavement (Wu,

2015a,b). According to the statements at the beginning of the paper, the performances of PU pavement are much better than that of asphalt pavement. It is assumed that the attenuation speed of performances of asphalt pavement is 2 times higher than that of PU pavement. Therefore, the maintenance area of PU pavement is 50% less than that of asphalt pavement. In terms of the maintenance measure, it is assumed that there is only one maintenance during the analysis period, and the PCI of pavement is smaller than 70. Hence, the maintenance measure is thin overlay. In maintenance phase, the calculation parameters are shown in Table 11, the energy consumption and emission are calculated by CPMES and shown in Table 12. 4.6. LCI of recycling Due to the continuous influences of the load and environment, the performances of pavement cannot meet the minimum

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L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395 Table 11 The calculation parameters in maintenance phase. Project

Parameter

Project

Parameter

Maintenance measure Maintenance area Time Number of closed lanes

Thin overlay 50% 5 years later 1/2 Total

Depth of overlay Binder Asphalt-aggregate ratio Uncertainty of time

20 mm Asphalt or PU 5% 20%

Table 12 The energy consumption and emission of in maintenance phase. Pavement structure

Material

Consumption (t)

Energy (MJ)

Asphalt pavement

Crushed stone Asphalt Total Crushed stone PU Total

335.72 16.79 e 167.86 8.40 e

9.89Eþ3 2.12Eþ4 3.11Eþ4 4.95Eþ3 1.06Eþ4 1.56Eþ4

PU pavement

requirements, the pavement structure will be excavated. The discarded asphalt mixture is usually used for producing new asphalt mixture, but the discarded PU mixture are useless in present. In this phase, the application of the discarded materials is not considered. It is assumed that the energy consumptions and emissions of PU pavement and asphalt pavement are same. The calculation parameters in recycling phase are shown in Table 13. The energy consumption and emission are calculated by the CPMES and shown in Table 14. 5. Environmental impact assessment 5.1. Results of LCI Based on the previous results, the calculated data are arranged in Tables 15 and 16 and Fig. 2. The effects of energy consumption and emission on the environment in life cycle will be discussed in the following. According to Table 15 and Fig. 2, the energy consumption of asphalt pavement is 1.79 times higher than that of PU pavement in life cycle. It indicates that the construction of PU pavement can save energy. Furthermore, the energy consumption mainly occurs in the material production phase of PU pavement. They are the material production and mixture production phases in asphalt pavement. The energy consumed in the material production phase of PU pavement accounts for 76.5% of total energy. The energy consumed in material production and mixture production phases of asphalt pavement account for 20.18% and 69.30% of total energy. Because the heating of aggregate and asphalt consumes a lot of heavy and diesel oils in mixture production phase, and PU mixture can be produced at atmospheric temperature. According to Table 16, the emissions mainly occur in material production and mixture production phases in asphalt pavement, it is material production phase in PU pavement. The total emissions (CO2, N2O, NOx, CH4, CO and SO2) of PU pavement are 1.93 times higher than that of asphalt pavement in the whole life cycle. Among

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

1.11Eþ3 6.06Eþ3 7.17Eþ3 5.55Eþ2 3.03Eþ3 3.59Eþ3

3.31Eþ3 7.99Eþ1 3.39Eþ3 1.66Eþ3 4.00Eþ1 1.70Eþ3

6.49E-3 5.07E-2 5.72E-2 3.25E-3 2.54E-2 2.87E-2

6.05Eþ0 2.41Eþ2 2.47Eþ2 3.03Eþ0 1.21Eþ2 1.24Eþ2

2.59Eþ0 6.79Eþ1 7.05Eþ1 1.30Eþ0 3.40Eþ1 3.53Eþ1

8.71Eþ0 4.96Eþ1 5.83Eþ1 4.36Eþ0 2.48Eþ1 2.92Eþ1

Table 14 The energy consumption and emission in recycling phase. Energy (MJ)

8.84eþ4

Emission (Kg) CO2

CH4

N2O

CO

SO2

NOX

6.73eþ3

2.15eþ1

3.05e-1

4.58eþ1

5.44eþ0

1.96eþ2

them, the CO2 is the main emission, which accounts for 95.2% of total emissions in asphalt pavement, and 97.6% in PU pavement. the CO2, N2O and NOx emissions of PU pavement are 1.98, 5.3 and 2.35 times higher than that of asphalt pavement respectively. The CO emission of asphalt pavement is 3.86 times higher than that of PU pavement. And there is little difference in CH4 and SO2 emissions between PU pavement and asphalt pavement with the same pavement structure. The conclusion can be summarized that the construction of PU pavement is not conducive to reducing the emission of exhaust gas, especially global warming gases. 5.2. Life cycle impact assessment (LCIA) The LCIA method used in this paper is environmental design of industrial product 97 (EDIP 97). According to EDIP method, the global warming potential (GWP) is global impact. While the photochemical smog potential (POCP), eutrophication potential (EP) and acidification potential (AP) are regional impacts. The gases which can cause global warming include CO2, N2O, CH4. For nutrition, it is NOx. For photochemical smog, they are CO and CH4. For acidification, they are SO2 and NOx. Generally, there are 3 steps in LCIA: i) Classification and characterization, ii) Normalization and iii) Weighting. i) Classification and characterization The classification is an inventory collection process based on the several impact categories (GWP, EP, POCP and AP). The

Table 13 The calculation parameters in recycling phase. Pavement broken parameters Working efficiency of crushing machinery Vehicle fuel efficiency Type of fuel

Transportation parameters of recycled materials 3

1000m /one-shift 100L/one-shift Diesel oil

Type of vehicle Transportation distance e

30t Heavy Truck 20 km e

L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395

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Table 15 The total energy consumption and emission of two structures in the whole life cycle. Type

Energy (MJ)

Asphalt pavement PU pavement

Emission (Kg)

2.15Eþ6 1.20Eþ6

CO2

CH4

N2O

CO

SO2

NOX

2.43Eþ5 4.80Eþ5

3.53Eþ3 3.43Eþ3

8.94Eþ0 4.73Eþ1

5.25Eþ3 1.36Eþ3

3.41Eþ3 3.94Eþ3

1.33Eþ3 3.13Eþ3

Table 16 The emission of pavement at each stage in the life cycle. Pavement Type

Gas

Material Production

Mixture Production

Transportation

Construction

Maintenance

Recycling

Total

Asphalt Pavement

CO2 N2O NOX SO2 CH4 CO

1.01Eþ05 7.92E-01 4.61Eþ01 9.87Eþ02 4.64Eþ01 3.47Eþ03

1.20Eþ05 7.17Eþ00 2.86E-01 2.34Eþ03 5.70Eþ01 1.41Eþ03

3.77Eþ03 1.87E-01 1.87E-01 3.37Eþ00 1.34Eþ01 2.85Eþ01

4.53Eþ03 4.26E-01 4.26E-01 9.95E-01 1.85E-01 4.97Eþ01

7.17Eþ03 5.72E-02 2.87E-02 7.05Eþ01 3.39Eþ03 2.47Eþ02

6.73Eþ03 3.05E-01 3.05E-01 5.44Eþ00 2.15Eþ01 4.58Eþ01

2.43Eþ05 8.94Eþ00 4.73Eþ01 3.41Eþ03 3.53Eþ03 5.25Eþ03

PU Pavement

CO2 N2O NOX SO2 CH4 CO

4.43Eþ05 4.61Eþ01 2.73Eþ03 3.83Eþ03 1.64Eþ03 1.11Eþ03

1.87Eþ04 2.86E-01 5.19Eþ01 6.38Eþ01 5.29Eþ01 3.50Eþ00

3.77Eþ03 1.87E-01 1.22Eþ02 3.37Eþ00 1.34Eþ01 2.85Eþ01

4.53Eþ03 4.26E-01 3.47Eþ00 9.95E-01 1.85E-01 4.97Eþ01

3.59Eþ03 2.87E-02 2.92Eþ01 3.53Eþ01 1.70Eþ03 1.24Eþ02

6.73Eþ03 3.05E-01 1.96Eþ02 5.44Eþ00 2.15Eþ01 4.58Eþ01

4.80Eþ05 4.73Eþ01 3.13Eþ03 3.94Eþ03 3.43Eþ03 1.36Eþ03

Recycling Maintenance Construction Transporation Mixture production Material production

PU Pavement

Asphalt Pavement

500000

1000000

1500000

2000000

Table 17 The characteristic factors of gas. Type

GWP (100 years)

EP

POCP

AP

Gas

CO2

CH4

N2O

NOx

CH4

CO

SO2

NOx

Characteristic factor

1

25

298

1.35

0.007

0.03

1

0.7

equipment are the same in construction phase. However, the impact category scores of construction phase are different in PU pavement and asphalt pavement, they are 1.37% and 0.79% respectively. This is because the emissions generated during the life cycle of PU pavement are bigger. Furthermore, the POCP of asphalt pavement is 2.81 times higher than that of PU pavement, the GWP, EP and AP of PU pavement are 1.74, 2.35 and 1.41 times higher than that of asphalt pavement respectively. This is because that the emission produced in material production phase of PU pavement is larger.

Energy (MJ)

ii) Normalization Fig. 2. The energy consumption at each stage in the life cycle.

characterization is the sum of every element under same impact category. The characteristic factors of each impact category are showed in Table 17. The value of impact category is the sum of the products of inventory data under the same impact category and related characterization factors. The equation used for calculating the value of impact category is as below: S (Inventory data of gas emission under the same impact category  characterization factor) ¼ value of impact category. The calculated values of impact categories are shown in Table 18. According to Table 18, the environmental impacts caused in material production and mixture production phases contribute to the most. In terms of GWP, the material production and mixture production phases in asphalt pavement and PU pavement are the high-emission segments. In mixture production phase, the difference of emission between asphalt pavement and PU pavement is large, this is mainly because that the asphalt mixture production needs high temperature environment. No matter in asphalt pavement or PU pavement, the construction plan and mechanical

In order to quantify and compare the environmental impact of each category, normalization is conducted based on person equivalent. Normalization is a transformation, and establishes the potential environmental impact and resource consumption on the same benchmark. The normalized impact category is the ratio of the characteristic result to the normalization reference value. The normalization reference values of the impact category are shown in Table 19 (Yang, 2002). The calculation formula is shown in equation (1). The calculated results of normalized impact category are shown in Table 20.

Ni ¼

Ci Si

(1)

Where, i is impact category; N is the normalized impact category; C is the value of impact category (from characterization); S is the reference value. According to Table 20, the normalized impact categories are calculated at each phase. In terms of asphalt pavement, if arranged based on scores, the impact of POCP is the first, AP at the second

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L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395

Table 18 The environmental impact value of categories (kg/t). Pavement Type

Category

Material Production

Mixture Production

Transportation

Construction

Maintenance

Recycling

Total

Asphalt Pavement

GWP EP POCP AP

1.02Eþ5 1.10Eþ3 1.04Eþ2 1.56Eþ3

1.24Eþ5 1.86Eþ2 4.27Eþ1 2.44Eþ3

4.16Eþ3 1.65Eþ2 9.49E-1 8.88Eþ1

4.66Eþ3 4.68Eþ0 1.49Eþ0 3.42Eþ0

9.19Eþ4 7.87Eþ1 3.11Eþ1 1.11Eþ2

7.36Eþ3 2.65Eþ2 1.52Eþ0 1.43Eþ2

3.34Eþ5 1.80Eþ3 1.82Eþ2 4.34Eþ3

PU Pavement

GWP EP POCP AP

4.98Eþ5 3.69Eþ3 4.48Eþ1 5.74Eþ3

2.01Eþ4 7.01Eþ1 4.75E-1 1.00Eþ2

4.16Eþ3 1.65Eþ2 9.49E-1 8.88Eþ1

4.66Eþ3 4.68Eþ0 1.49Eþ0 3.42Eþ0

4.61Eþ4 3.94Eþ1 1.56Eþ1 5.57Eþ1

7.36Eþ3 2.65Eþ2 1.52Eþ0 1.43Eþ2

5.80Eþ5 4.23Eþ3 6.48Eþ1 6.13Eþ3

Table 19 The normalization reference value and weighting factor. Environmental impact type

Normalization reference value

Datum cell

Weight

GWP

8700

0.83

EP

62

POCP

0.65

AP

36

KgCO2eq./ (person*a) KgNO 3 eq./ (person*a) KgC2H4eq./ (person*a) KgSO2eq./ (person*a)

0.73 0.53 0.73

category; N is the normalized impact category; W is the weighting factor. According to Table 21, after normalization and weighting, the score of all environmental impact of PU pavement is little smaller than that of asphalt pavement. It can be considered that there is no difference. In asphalt pavement, there is no difference in the rank of impact categories before and after weighting. However, the rank of impact categories from big to small is AP, GWP, POCP and EP. The main two impact categories which can be highlighted are POCP and AP in asphalt pavement. They are AP and GWP in PU pavement.

5.3. Life cycle assessment and interpretation (LCAI) place and GWP is ranked at the third place. The material production phase contributes the highest score to most of the impacts after normalization in asphalt pavement, mixture production phase at the second place. In terms of PU pavement, if arranged based on scores, the impact of AP is the first, POCP at the second place and EP is ranked at the third place. The material production phase contributes the highest score to most of the impacts after normalization in PU pavement, maintenance phase at the second place. Furthermore, the score of POCP of asphalt pavement is 2.82 times higher than that of PU pavement. The scores of GWP, EP and AP of PU pavement are 1.73, 2.34 and 1.41 times higher than that of asphalt pavement respectively. iii) Weighting Weighting is conducted by multiplying the normalized impact category by weighting factor. Weighting is used for determining the relative degree of the environmental impact (Sharaai et al., 2011.). The calculation formula is shown in equation (2). Based on conditions of China, Normalization reference value of environmental impact is conducted, and it is showed in Table 19. The weighted environmental impacts are shown in Table 21.

LCIA ¼

X

Ni  Wi

(2)

Where, LCIA is the score of all environmental impact; i is impact

According to Table 20 and 21, after normalization and weighting, the score of GWP in PU pavement is higher than that of asphalt pavement, because the emission produced in material production phase is the biggest. And the emission produced in PU production process accounts for 96.41% of the total emission in material production phase. It can be concluded that the PU production is the key factor of environmental impact. For asphalt pavement and PU pavement, the CO2, N2O and CH4 emissions are mainly produced in materials production and mixture production phases, the emissions (CO2, N2O and CH4) of PU pavement are 4.4 times higher than that of asphalt pavement in material production phase, and the emissions (CO2, N2O and CH4) of asphalt pavement are 6.4 times higher than that of PU pavement in mixture production phase. Because the heating of aggregate and asphalt consumes a lot of heavy and diesel oils in mixture production phase, and PU mixture can be produced at atmospheric temperature. This is why the construction of PU pavement can reduce the emission of exhaust gas in mixture production phase. In terms of POCP, the CO and CH4 emissions produced in material production and maintenance phases account for 57.42% and 38.08% of total CO and CH4 emissions in asphalt pavement. The CO and CH4 emissions of asphalt pavement are 1.83 times higher than that of PU pavement in life cycle. This may be caused by inadequate combustion of the heavy oil in asphalt mixture production phase. In terms of AP and EP, the total SO2 and NOx emissions of PU pavement are 1.49 times higher than that of asphalt pavement in

Table 20 The normalized environmental impact value of impact category (Human equivalent/t). Pavement Type

Category

Material Production

Mixture Production

Transportation

Construction

Maintenance

Recycling

Total

Asphalt Pavement

GWP EP POCP AP

1.18E-2 1.78E-2 1.61E-1 4.33E-2

1.42E-2 3.00E-3 6.57E-2 6.77E-2

4.78E-4 2.66E-3 1.46E-3 2.47E-3

5.36E-4 7.56E-5 2.30E-3 9.51E-5

1.06E-2 1.27E-3 4.79E-2 3.09E-3

8.46E-4 4.27E-3 2.35E-3 3.96E-3

3.84E-2 2.91E-2 2.81E-1 1.21E-1

PU Pavement

GWP EP POCP AP

5.72E-2 5.94E-2 6.89E-2 1.59E-1

2.31E-3 1.13E-3 7.31E-4 2.78E-3

4.78E-4 2.66E-3 1.46E-3 2.47E-3

5.36E-4 7.56E-5 2.30E-3 9.51E-5

5.30E-3 6.36E-4 2.40E-2 1.55E-3

8.46E-4 4.27E-3 2.35E-3 3.96E-3

6.67E-2 6.82E-2 9.97E-2 1.70E-1

L. Cong et al. / Journal of Cleaner Production 256 (2020) 120395

9

Table 21 The weighted environmental impact value of impact category (Human equivalent/t). Pavement Type

Category

Material Production

Mixture Production

Transportation

Construction

Maintenance

Recycling

Asphalt Pavement

GWP EP POCP AP

9.77E-3 1.30E-2 8.51E-2 3.16E-2

1.18E-2 2.19E-3 3.48E-2 4.94E-2

3.97E-4 1.94E-3 7.74E-4 1.80E-3

4.45E-4 5.52E-5 1.22E-3 6.94E-5

8.77E-2 9.27E-4 2.54E-2 2.26E-3

7.02E-4 3.12E-3 1.24E-3 2.89E-3

3.19E-2 2.12E-2 1.48E-1 8.80E-2

2.90E-1

PU Pavement

GWP EP POCP AP

4.75E-2 4.34E-2 3.65E-2 1.16E-1

1.92E-3 8.25E-4 3.88E-4 2.03E-3

3.97E-4 1.94E-3 7.74E-4 1.80E-3

4.45E-4 5.52E-5 1.22E-3 6.94E-5

4.40E-3 4.64E-4 1.27E-2 1.13E-3

7.02E-4 3.12E-3 1.24E-3 2.89E-3

5.53E-2 4.98E-2 5.28E-2 1.24E-1

2.82E-1

whole life cycle. The total NOx emission of PU pavement is 2.35 times higher than that of asphalt pavement in whole life cycle. Hence, after normalization and weighting, the scores of AP and EP in PU pavement are 1.41 and 2.35 times higher than that in asphalt pavement. This may be because the raw material TDI or MDI contains a large number of eNCO groups. And TDI and MDI are made from nitric acid and amine. Based on the scores of all environmental impact, it can be considered that there is little difference between asphalt pavement and PU pavement. According to the road performances, the stability and fatigue life of PU mixture are 3 times and 10 times higher than that of asphalt mixture (Cong et al., 2018). Therefore, the service life of PU pavement is much longer than that of asphalt pavement. In a word, when the environmental impact is the same, it is a wise choice to choose the pavement with longer service life. But the production technologies of PU also need to be developed and improved to reduce the emissions in the production phase. 6. Conclusions and discussion Polyurethane-bounded mixture has great potential to overcome the pavement failures. In this paper, LCA is used for quantitatively analyzing the environmental impacts of asphalt pavement and PU pavement in the life cycle. Six exhaust gases (CO2, CH4, N2O, CO, SO2 and NOx) were selected as the research objects. The following conclusions are drawn. (1) The energy consumption of asphalt pavement is 1.79 times higher than that of PU pavement. The construction of PU pavement is conducive to energy conservation. In terms of asphalt pavement, the order of energy consumption from big to small is mixture production, materials production, recycling, construction, transportation, maintenance. In terms of PU pavement, the order of energy consumption from big to small is materials production, recycling, mixture production, construction, transportation, maintenance. The energy consumption can be further reduced by improving the production process or improving the utilization rate of energy in the production of PU. For example, the renewable resources can be used for producing biomass polyols whose raw materials are vegetable oil and cellulose. (2) The construction of PU pavement is not conducive to reducing the emission. The total emission of PU pavement in life cycle is 1.93 times higher than that of asphalt pavement. The CO2, N2O and NOx emissions of PU pavement are 1.98, 5.3 and 2.35 times higher than that of asphalt pavement. Therefore, under the same conditions, the construction of PU pavement will enlarge the GWP, EP and AP. This will reduce the quality of people’s living environment, affect the health of the ecological environment. The CO emission of asphalt pavement is 3.86 times higher than that of PU pavement. And

Total

there is little difference in CH4 and SO2 emissions between PU pavement and asphalt pavement with the same pavement structure. (3) The construction of PU pavement will increase the probability of acid rain and acid fog. The materials production phase contributes the highest score to most of the impacts after normalization in asphalt pavement. The main two impact categories which can be highlighted are POCP and AP in asphalt pavement. The main two impact categories which can be highlighted are AP and POCP in PU pavement. After normalization and weighting, the score of all environmental impact of PU pavement is little smaller than that of asphalt pavement. However, the road performances of PU pavement are much better than that of asphalt pavement. Therefore, the service life of PU pavement is also much longer than that of asphalt pavement. PU pavement can meet the requirements of energy conservation, environmental protection and sustainable development in the long run. (4) Although LCA is used to calculate the energy consumption and emission of PU pavement in this paper. Due to the limited capacity, there are still some problems (such as the improvement of PU production process which can reduce energy consumption and emission) that require further study and discussion. Furthermore, the inventory lists of pavement construction period did not consider the emission of fine particulate matter (PM2.5 and PM10) and asphalt fume.

Author contributions The authors confirm contribution to the paper as follows: study conception and design: Lin Cong: guidance teacher and data collection. Guihong Guo: draft manuscript: analysis and interpretation of results. Meng Yu: provide technology. Fan Yang, Le Tan: interpretation of results. All authors reviewed the results and approved the final version of the manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the State Key Laboratory of Special Functional Waterproof Materials (No. SKLW2019008). This support is gratefully acknowledged.

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