Solar heating reflective coating layer (SHRCL) to cool the asphalt pavement surface

Construction and Building Materials 139 (2017) 355–364

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Solar heating reflective coating layer (SHRCL) to cool the asphalt pavement surface Aimin Sha a,b,⇑, Zhuangzhuang Liu a,b,⇑, Kun Tang b, Pinyi Li c a

Key Laboratory for Special Area Highway Engineering, Ministry of Education, Chang’an University, Xi’an 710064, China School of Highway, Chang’an University, Xi’an 710064, China c School of Materials Science and Engineering, Chang’an University, Xi’an 710062, China b

h i g h l i g h t s
Solar heating reflective coating layer (SHRCL) was prepared to cool the asphalt pavement surface.
Heating process downwards was measured and analyzed, and was divided into four stages.
Durability of SHRCL was evaluated by Model Mobile Load Simulator.

a r t i c l e

i n f o

Article history: Received 15 October 2016 Received in revised form 16 January 2017 Accepted 16 February 2017

Keywords: Pavement engineering Cooling pavement Solar heating reflective coating layer Solar heating Accelerated pavement testing Model Mobile Load Simulator

a b s t r a c t The asphalt pavement heated by solar has attracted great attentions in heating urban cities, so-called urban heat island (UHI). In fact, the solar energy heating asphalt pavements also has potential ability to bring out geological disasters in permafrost, e.g., Qinghai-Tibet Plateau (QTP). This study designed a special solar heating reflective coating layer (SHRCL) in order to cool the asphalt pavement surface. The cooling performance of SHRCLs was studied by comparing the temperatures of normal pavements and SHRCLs. The result indicated that SHRCL could work as expected resulting in reducing around 10 °C compared to the normal pavement, no matter on the top or on the bottom. The temperature difference between the top and the bottom was about 5 °C, not only in SHRCLs but also in normal pavements, less linking with coating or un-coating. The field testing result agrees very well with the laboratory study. To enhance the skid resistance of SHRCLs, sands were added into coatings without influencing the cooling performance, but reducing the abrasive resistance. Accelerated pavement testing (APT) was employed to evaluate the engineering performance of SHRCL. It is suggested that inhomogeneous deformation, e.g., rutting, will bring cracking and spalling between asphalt pavement surfaces and coating layers. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Due to the black surface of asphalt pavements, and large amount covering area, the urban heat island (UHI) effects contributed by asphalt pavements have been studies for a couple of years [1,2], and lots of technologies have been developed to reduce the solar heating phenomenon [3]. In fact, the heat absorbing of asphalt pavement also heat the base and subbase, as well as the earth foundation. In some special regions, the heating downward brings high risks to the weak environmental system, e.g., in permafrost regions [4]. Thus, in transportation/pavement engineering,

⇑ Corresponding authors at: Nan Er Huan Road (mid-part), School of Highway, Chang’an University, Xi’an 710064, China (A. Sha and Z. Liu). E-mail addresses: [email protected] (A. Sha), [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.conbuildmat.2017.02.087 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.

lots of technologies have been applied on the pavement in permafrost regions to prevent the heat transfer downward [5]. Solar heat reflective coating layer (SHRCL) was thought to be an appropriate solution to prevent the heating on buildings [6] and asphalt pavements [7,8], as such to mitigate the urban heat island (UHI) effects in cities. Green roofs (low energy absorption or highly reflective) were confirmed to be effective and appropriate to fight with the UHI [9,10]. In some extreme conditions, however, e.g. in permafrost regions, the solar energy absorbed by the pavement, especially by the black surface (asphalt pavements), will transfer into frozen soil layers to damage the hydrothermal environment, reduce the active layer thickness (ALT) [11], and even bring out geologic hazards. The previous solutions were mainly focused on releasing or transferring the heat into other directions. For example, the energy stored in pavement structure could be brought out with the wind in ventiduct embankments [12]; due to the phase changing between Solid-Liquid, thermal pipes change

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the energy flow direction, from top-down to bottom-up [13]. And also, the energy dissipation was reported in crushed-rock embankment [14] due to the micro-convection. Besides, to prevent heating flow downwards, increase the thermal resistance of asphalt pavement was also reported. For example, a unilateral heat-transfer asphalt pavement was designed by control the thermal conductivity of pavement layers [15]. All solutions mentioned above are post-treatments, which are forced to prevent or release the absorbed heat in the pavement structure. Meanwhile, the SHRCL is a pre-treatment solution, which prevents the heating absorption from the first layer, and reflects energy back into the environment, as a result, reducing the heating source to the downwards in pavement structures. As shown in Fig. 1, the total solar energy could be divided into at least four parts: 1) reflected back into the environment; 2) stored in pavement structure permanently or long timely; 3) radiated outside by the black surface. This part was believed to be very little, and 4) transferred downwards into deeper location. To the frozen soil (permafrost), the most dangerous part is the fourth one, while the reflective, stored and radiated energy have few impacts. Thus, SHRCL could be seen as the first barrier to prevent heating downwards. In this study, a special coating layer was produced aiming to cool the pavement surface (reduce the temperature), and it could be potentially applied in permafrost regions. The engineering performances including cooling characteristics, skid resistance, and aging resistance were investigated. As well, the structure was also detected by an accelerated pavement testing (APT) in laboratory. Compared with the current coating layers, new application scenario (permafrost regions with cold climates and extreme ultraviolet radiation) and thermal conductivity phenomenon were reported in this paper.

ASTM D5M-15 [17], ASTM D36-14E1 [18], and ASTM D 113-07 [19]. The production process of asphalt mixtures was subjected to JTG F40-2004. The engineering performances of the selected concrete were pre-examined according to ASTM D6927-15 [20] and JTG E20-2011 [21]. Concerning that this paper focuses on the solar heating reflectivity, the technical indexes of asphalt binder, aggregates, and asphalt mixture are not presented here but detailed in appendixes A, B, and C, below. 2.1.2. Coating layer and requirements The coating layer mainly consists of films and pigments, in which the film herein is commercial condensation resins (polyvinyl alcohol and epoxy resin). The pigment in coating layers acts as the reflective element, thus there are special requirements for pigments [22]:1) large difference of refractive index (RI) between films and pigments; 2) suitable particle size of pigments because the particle sizes affect the reflective ratio; 3) appropriate volume concentration. Accordingly, the selected pigment was titanium dioxide (TiO2) [23]. The RI of polyvinyl alcohol and epoxy resin are much lower than TiO2 pigment (RI = 2.8). The refractive index of pigment particles and the wavelength is highly linked with each other according to Sellmeier Equation, as following.

n2 ðkÞ ¼ 1 þ

k  C1

þ

B 2 k2 2

k  C2

þ

B3 k2 2

k  C3

ð1Þ

where B1, B2, B3, C1, C2, and C3 were Sellmeier coefficients. To ideal pure transparent medium, the Sellmeier should be the same no matter what is the particle size. In the real case, the pigments could not keep that ideal, thus the Selleier coefficients of pigments are associated with the particle size. Additionally, the particle size also affects the scattering wavelength (k⁄) followed by Eqs. (2) and (3).

2. Materials, structure design and measurements

k ¼ 2.1. Materials and coating layer structure 2.1.1. Asphalt and mixtures It should be noted that hereby because there is no requirement of concrete performance during measuring cooling characteristics, the base sample (asphalt concrete) to be coated was a normal one without measuring the mechanical behaviors. The aggregates used in this study was basaltic crushed rocks. A conventional asphalt concrete (AC-13) following to the Chinese Specification of JTG F40-2004 [16] was produced with Styrene Butadiene Styrene (SBS) modified asphalt (base asphalt 70#). The technical properties of asphalt binder were checked according to

B1 k2 2

k¼

d k

ð2Þ

0:9ðm2 þ 2Þ  100% npðm2  1Þ

ð3Þ

where m is the scattering coefficient, n is the RI of resin, k is a constant determined by m and n. The pigment volume concentration (PVC) could be calculated by Eq. (4).

PVC ¼

Vp  100% Vp þ Vb

ð4Þ

where Vp is the pigment volume, and Vb is the film material volume. According to a previous study [24], the optimal volume concentration of TiO2 should be located in 10–20 vol%. Black pigment (Carbon Black, C) was also added into the coating to adjust the color of mixtures [25]. The usage of carbon black is arranged between 0.3–0.7%. Additionally, to produce stable and high performance coatings, some other additives, e.g., dispersing agent, antifoaming agents, flatting agent, ultraviolet ray absorbing agent (UV-531), etc. were also utilized at an optimal content. 2.2. Measurements

Fig. 1. Schematic diagram of the solar heating reflective coating layer (SHRCL) on asphalt pavements in permafrost regions.

Fig. 2 shows the device to simulate the solar heating by iodinetungsten lamps whose light is similar to the sunlight. Asphalt and/ or cement concrete plates (300  150 mm  50 mm) could be fixed in the container. The circumambience and bottom of the container were sealed by cystosepiment to keep the warmth from releasing. Thermocouple sensors were fixed on the surface and bottom of samples to collect the temperatures while the thickness of the samples was 50 mm. This device just could simulate the solar radi-

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357

Fig. 2. Samples and apparatus for laboratory experiment.

ation in the laboratory, but could not collect data outdoors. Thus, in field test, the temperature of the pavement surface was measured by an infrared camera by which the average temperature could be obtained, as Fig. 3 presented. The coating of this study is aimed to be applied in Qinghai-Tibet Plateau (QTP) where the ultraviolet rays (wavelength < 382 nm) are more serious than general regions. Thus the anti-aging performance of coatings was also evaluated, as Fig. 4 showing. The coating emulsions were exposed to ultraviolet rays for 12 h and then observed by a fluorescence microscope (LW300LFT, produced by CEWEI Shanghai). The accelerated pavement testing (APT) tool has been widely used to evaluate the asphalt concrete performance [26,27]. Thus, the coated asphalt samples were tested by a Model Mobile Load Simulator (MMLS3) in laboratory (see Fig. 5), with a wheel load-

ing = 0.7 MPa, frequency = 7200 cycle/h, speed = 26 km/h. The samples with 100 mm width, 150 mm long, and 50 mm thickness were fixed and tested. What should be declared is that the asphalt, aggregates and gradation here were slightly different with that used above, but this slight difference will not affect the mechanical behaviors of asphalt mixtures. 3. Cooling characteristic and engineering performance of the solar heating reflective coating layer 3.1. Cooling characteristics Fig. 6 presents the temperature of top and bottom of SHRCL after exposed 7 h. The result indicates that the temperatures, no matter of the top or of the bottom, were significantly reduced by

Fig. 3. Infrared camera (a) and collected image (b).

Fig. 4. Ultraviolet ray radiation and samples. Sample A, D: epoxy resin + 0.1 wt% UV-531, Sample B, E: epoxy resin + 14 vol% TiO2, Sample C, F: epoxy resin + 14 vol% TiO2 + 0.1 wt% UV-531. The A, B, and C samples were exposed, while D, E, and F samples were referred.

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The temperature differences between coated samples and control sample were calculated by Eq. (5), and the temperature differences between the top and bottom were calculated by Eq. (6).

DT CC ¼ TemperatureControl  TemperatureCoated

ð5Þ

DT TB ¼ TemperatureTop  TemperatureBottom

ð6Þ

where DTCC is the temperature difference between control samples and coated samples; DTTB is the temperature difference between top and bottom of measured samples. Figs. 7, 8 present the temperature differences of coated vs. control samples, and top vs. bottom samples, respectively. DTCC and DTTB herein are defined to describe the cooling performances of SHRCL. Some important phenomenon should be noted hereby according to Fig. 7: Fig. 5. Device of Model Mobile Load Simulator (MMLS3) used in this study.

coatings, meanwhile, the components of carbon black and TiO2 affected the temperature evolution. At the beginning, the start temperature was about 15 °C (top & bottom). After 1 h exposing, the temperature on top increased rapidly. As a result, the control sample (without coated) reached 50 °C, while the coated top raised to 40–45 °C. Since 1 h point, the temperature raised more slowly than that during the first hour. Differently, the temperature of the bottom raised equably, seeing Fig. 6(d, e, f), especially between the first hour which showed nearly a linear relationship in early 3 h.

(i): No matter on the top or the bottom, the coating compositions impacted the temperature differences, as stated above in Fig. 6. At the final stage (7 h), the coating layer containing 20% TiO2 and 0.5% Carbon black achieved the highest DTCC on Top and bottom surfaces. (ii): For the top surface, the DTCC increased rapidly between the first hour and then decreased reaching the bottom at 2 h. After a period of recovery, the DTCC stayed around 10 ± 2.5 °C finally. During the measurement, the highest DTCC reached 10 °C at 1 h point whose coating composition was 0.3%C + 15%TiO2.

(a) (top)

(b) (top)

(d) (bottom)

(e) (bottom)

(c) (top)

(f) (bottom)

Fig. 6. Temperatures of the top and bottom of coated asphalt concrete. The samples contained 10, 15, 20 vol% TiO2 and 0.3, 0.5, 0.7 vol% carbon black (marked as C), respectively.

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10

5

TiO2=10% C=0.3% C=0.5% C=0.7%

0

10

5

TiO2=15% C=0.3% C=0.5% C=0.7%

0

0

1

2

3

4

5

6

7

0

1

2

Time (hour)

(a) (top)

3

4

5

6

5 TiO2=10% C=0.3% C=0.5% C=0.7%

0

0

0

1

2

3

4

5

6

7

1

2

3

4

5

Time (hour)

(b) (top)

(c) (top)

6

7

15

10

5

TiO2=15% C=0.3% C=0.5% C=0.7%

0

10

5

TiO2=20% C=0.3% C=0.5% C=0.7%

0

-5

-5

-5

TiO2=20% C=0.3% C=0.5% C=0.7%

0

Time (hour)

ΔTemperature (Celcius)

ΔTemperature (Celcius)

10

5

7

15

15

10

-5

-5

-5

ΔTemperature (Celcius)

ΔTemperature (Celcius)

ΔTemperature (Celcius)

ΔTemperature (Celcius)

15

15

15

0

1

2

3

4

5

6

7

0

1

2

3

4

Time (hour)

Time (hour)

Time (hour)

(d) (bottom)

(e) (bottom)

(f) (bottom)

5

6

7

Fig. 7. Temperature reduction between the coated and uncoated samples. The samples contained 10, 15, 20 vol% TiO2 and 0.3, 0.5, 0.7 vol% carbon black (marked as C), respectively.

Fig. 8. Temperature differences between the top and bottoms. The samples contained 10, 15, 20 vol% TiO2 and 0.3, 0.5, 0.7 vol% carbon black (marked as C), respectively.

(iii): For the bottom surface, trends of DTCC curves were similar with each other. The DTCC was increased smoothly in the first 3 h and then reached a platform around 10 ± 3 °C. (iv): The increasing curve means that the heating rate on control sample’s surface is higher than that on the coated surfaces. Thus, the curves in Fig. 7(a), (b), and (c) could be divided into four stages based on this view: 1) the first stage between 0 and 1 h; 2) the second stage between 1 and 2 h; 3) the third

stage is located in 2 to 5 h, and 4) the fourth stage is between 5 and 7 h. In the first stage (0–1 h), the energy from sunlight is mainly absorbed by the normal surface, but is largely reflected by the SHRCL, thus the temperature of control is higher and rapidly increased than that of SHRCL. During the second stage (1-2 h), the heat breaks the barrier of up layers and transfers downward, resulting in the differences reduction between the control and SHRCL. The third stage

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ray), which could not be ignored in the working condition. However, the laboratory simulation did not consider this situation by heating the samples continuously. (ii): Stable difference between the control and SHRCL. The DTCC of field test kept stable at 10 °C linearly (Fig. 9a), while that of laboratory obtained a peak at 1 h point. As discussed in the last paragraph, the heating situations were different in laboratory and field. There was a clear dividing point between non-exposure and exposure in the laboratory. Meanwhile, there was no that clear dividing point during the field test, which, in fact, had been exposed to solar radiation far before 9:00 AM. Thus, the data collected in the field should belong to the later/final stage of the indoor experiments, when it achieving stable temperature differences between the normal/uncoated pavement surface and SHRCL.

(2–5 h) comes after the mid-layers absorbing energy fully. Finally, the temperature maintains stability in the fourth stage (5–7 h). No matter at any stage, energy continues to transfer downwards from up layers to the bottom, thus, the temperature of the bottom in Fig. 6 and the DTCC in Fig. 7(d), (e), and (f) raised, continuously. The result of the temperature difference between the bottom and top surfaces indicated that there was also a peak during sunlight heating process, which was located at 1 h point, as presented in Fig. 8. The peaks should be caused by the thermal resistance of the mid-layer between top and bottom. That is, the heat needs time to transfer from up downwards. Another phenomenon should be noted is that the DTTB keeps around 5 ± 1 °C in the final period (7 h) of the heating process. It is easily understood that this difference is related with the thickness and materials of mid-layers (between tops and bottoms), without regarding with the coating layers. This finding calls for more discussions and considerations of the function of the coating layers in the future. Fig. 9 shows the field evaluation of the SHRCL while the test section was located at the main campus of Chang’an University (Xi’an China). The test section was evaluated after construction (June 2010) and then was re-tested after 10 months using (April 2011). Because the camera could just scan the pavement surface, there was no data about the bottom of the test section. Result indicated that the highest temperatures of SHRCL and referred pavement (control) in field (Fig. 9a) greatly agreed with the measured data in laboratory (in Fig. 6a–c), e.g., the highest temperatures in field was 60 °C (control) and 50 °C (SHRCL), while that in laboratory was 65 °C (control) and 51 °C (SHRCL). This matching should thank the pre-calculation of the exposure time (7 h totally) which is expected to equal to the natural daily sunlight radiation in summer in Xi’an. As well, the DTCC (temperature difference between normal/un-coated and coated pavements), which was greenly marked in Fig. 9a, also agreed well with that in the laboratory experiment (DTCC = 10 °C). However, there were some differences and new findings between field test and laboratory experiments: (i): Temperature evaluation curves/trend. The temperature increased continuously in a laboratory test (seeing Fig. 6), but it showed a wavy trend in the field test. In fact, the evaluation trend of natural sunlight radiation followed with the cosine wave. Under the natural condition, the pavement not only absorbs energy but also radiates outwards (infrared 65

Compared with the data of Fig. 9a, after 10 months using, the DTCC in Fig. 9b remained stable but was reduced obviously than that surveyed after new construction (Fig. 9a). Potential results contributing to this reduction should be: 1) the abrasion of SHRCL; 2) the solid dusts and/or gel pollutions from natural conditions, which covers the area of SHRCL. As stated and discussed in Fig. 6, the cooling performances were highly related to the compositions of coating materials. The pollutions covering on the SHRCL acts as the carbon black or titanium dioxide, thus the absolute value measured might be affected/ reduced in Fig. 9b; 3) the influence of seasons should be considered. Fig. 9a was measured in June 2010 while the Fig. 9b was surveyed in April 2011. It is clear that the solar radiation situation was absolutely different between these two months in Xi’an. 3.2. Skid resistance Concerning the potential effects of coatings on the pavement surface, sands were added into the coatings to enhance the skid resistance of SHRCL. Two mixing technologies were evaluated in this study: 1) sand was mixed together with coatings in which the sand was locked inside, or 2) sand covers on the coated layers outside. Compared with the control sample (Fig. 10a), the sands significantly improved the roughness (seeing Fig. 10b, c) of the pavement surface. The British Pendulum Number (BPN) measurement (Fig. 11a) also confirmed that improvement by sands. Additionally, the BPN enhancement by sands mixed inside was a little weaker than that by sands covered outside. However, with the increasing of sand usage, the BPN decreased due to the roughness reduction. The 65

Control

60

Temperature (Celcius)

Temperature (Celcius)

60 55 50 45 40 SHRCL

35 30 25

Control

55 50 45 40

SHRCL

35 30

Data surveyed between 9:00 AM and 6:00 PM (June, 2010)

20

Data surveyed between 9:00 AM and 6:00 PM (April, 2011)

25 1

2

3

4

5

6

7

Time (hour)

(a)

8

9

10

1

2

3

4

5

6

7

8

9

10

Time (hour)

(b)

Fig. 9. Field test of the surface temperature of solar heating reflective coating layer (SHRCL): (a) new coating (b) after 10 months using. The test section was located at the main campus of Chang’an University.

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Control

Sand inside

(a)

Sand outside

(b)

(c)

Fig. 10. Surfaces of the coated asphalt pavement: (a) coating without sand (b) coating mixed with sand (c) covered with sand after coating.

80

10

ΔTemperature (Celcius)

70 60

BPN

50 40 30 20

Inside Outside

10 0

8 6 4 Practice TiO2 15%, C 0.5%, without sand TiO2 15%, C 0.5%, sand outside TiO2 15%, C 0.5%, sand inside

2 0

0

30

60

90

0

1

2

4

3

Content of standard sand (wt.%)

Time (hour)

(a)

(b)

5

6

7

Fig. 11. Effects of sands addition on the solar heating reflective coating layer (SHRCL) containing 15 vol% TiO2, 0.5 vol% carbon black: (a) skid resistance (b) cooling performance.

Fig. 12. Microscopic images (400) of coatings: (a, d) epoxy resin + 0.1 wt% UV-531 (b, e) epoxy resin + 14 vol% TiO2 (d, f) epoxy resin + 14 vol% TiO2 + 0.1 wt% UV-531. Figures (a, b, c) were exposed under ultraviolet radiation, while figures (d, e, f) were referred samples.

optimal sand content was achieved at 30 wt% based on the BPN value. Fig. 11b presents the DTCC (temperature difference between the control sample and SHRCL) measured in laboratory indicating

few effects on the trend. As well, the cooling performance of SHRCL was not reduced by the sands enhancement, seeming similar with each other no matter with or without sands.

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Table 1 Weight loss of coated samples after accelerated pavement testing. Samples

Before (g)

After (g)

Weight loss (g)

Weight loss,%

Ref. With 30% sand (inside) With 30% sand (outside) With 60% sand (outside)

1654.7 1641.7 1621.3 1652.3

1653.9 1640.6 1619.8 1651.0

0.8 1.1 1.5 1.3

0.05% 0.07% 0.09% 0.08%

Coating layer Rutting

Asphalt concrete base

Cracking and spalling

(a)

(b)

Fig. 13. Transection of the accelerated test samples: (a) fresh sample (b) failure sample.

3.3. Ultraviolet radiation aging Concerning that the SHRCL would be applied potentially in permafrost regions in China, i.e., Qinghai-Tibet Plateau (QTP) where it has strong ultraviolet radiation, the ultraviolet radiation aging performance was also evaluated in laboratory. The images in Fig. 4 showed that the UV-531 brought green color into the coatings (confirmed by Sample A, B, and C). However, after ultraviolet radiation, the green color changed to be light possibly caused by the UV-531 decomposing. After a period of ultraviolet radiation, coatings were also observed by a fluorescence microscope, as shown in Fig. 12. It could be seen that the fluorescent color of Sample C and F (Fig. 12c, f) remained the same due to the addition of UV-531. Meanwhile, the one without UV-531 (Sample B, Fig. 12b) changed to be a little bit darker (Fig. 12e). In conclusion, the inorganic materials (titanium dioxide and carbon black) can mostly keep stable under ultraviolet radiation, the organics (including resins, ultraviolet light absorber etc.) however will be aged exposed to ultraviolet radiation. 4. Accelerated pavement testing After 300,000 cycles APT test, the surface layers came to be a little bit smooth, and little sand particles bonded in coatings were gone. The weight loss during accelerated pavement testing is defined to evaluate the abrasive resistance of SHRCL in this study. As detailed in Table 1, the SHRCL, even enhanced by sand inside/ outside increasing the weight loss, obtained an expected abrasive resistance. It should be pointed that the sand outside will be more easily rubbed off than the sand inside. And also, the usage of sand addition should be considered carefully, because excess sand will absolutely reduce the abrasive resistance. Fig. 13 shows the transections of accelerated test samples. Asphalt concrete is a typical inelastic material, in which permanent deformation will appear under repeated loadings. Due to the differences in stiffness and modulus between asphalt and resin, rutting, cracking, and spalling would happen, as shown in Fig. 13b. At the same time, the one with excellent rutting resistance asphalt pavement did not have these damages between pavement layer and coating layer, as shown in Fig. 13a. Thus, it could be concluded that to achieve excellent engineering performance of SHRCL, two

potential solutions should be considered: 1) reduce the difference of stiffness and modulus between asphalt concrete bases and resinbased layers, achieving a resilient system to adapt the permanent deformation of structures; 2) improve the stiffness of pavement structures, by which to avoid early damages (spalling, cracking etc.) of asphalt mixtures. These two solutions above are also the main limitation of wide applications of SHRCL at present, as well, should be the focuses in the future studies. 5. Conclusions Solar heating reflective coating layers (SHRCLs) were proved working as expected to reduce the temperature of asphalt pavement surface and preventing the energy transferring downwards to bottoms. The temperatures reduced by SHRCL in this study were 10 ± 2.5 °C on the top and 10 ± 3 °C on the bottom. The temperature evolution during heating in the laboratory could be divided into four stages: 0–1 h, 1–2 h, 2–5 h, and 5–7 h. In the first stage (0–1 h), the solar energy is mainly absorbed by the normal surface, but largely reflected by the SHRCL. Then, the heat breaks the barrier of up layers and transfers downward in the second stage (1–2 h). After long time slow recovery (2–5 h), the temperature maintains stability (5–7 h). As a result, there is a peak of the temperature differences between the SHRCL and normal pavement, which is believed to be contributed by their different solar heating characteristic. The temperature differences between tops and bottoms measured in this study were always 5 ± 1 °C, which is not changed by SHRCLs, but just links with the layers between top and bottoms. The sand can improve the skid resistance of SHRCLs, without influencing the cooling performances. The addition of sands reduces the abrasive resistance performance. The ultraviolet absorbent (UV-531) in this study changed the color of SHRCL but improved their ultraviolet radiation aging resistance. Further research revealed that however, the ultraviolet absorbent seemed to be decomposed under ultraviolet exposure situation. The accelerated pavement testing by MMLS3 indicated that to some SHRCLs, early damages, e.g., cracking and spalling, will happen caused by permanent or inhomogeneous deformation (i.e., rutting). The main reason of this problem is the difference of stiffness/modulus between asphalt binders and resins. Thus, fur-

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ther studies might focus on the solutions to reduce the difference between asphalt and resin, or to improve the rigid of asphalt pavement structure to avoid inhomogeneous rutting problems. Acknowledgement This study was financially supported by "the 12th Five-Year Plan", National Key Technology R&D Program (2014BAG05B04). The main experiments of this study were finished at the Key Laboratory for Special Area Highway Engineering, MOE, China, at Chang’an University by undergraduate students Kun Tang and Pinyi Li. What should be noted here is that this manuscript just reflects the opinions and views of authors, but does not necessarily represent the views of material producers and funding providers. Appendix A Table 2 The physical properties of asphalt used in this study. Test item of asphalt binder

Unit

Measured value

Needle penetration (25 °C, 100 g, 5 s) Penetration index (PI) Ductility (5 cm/min, 5 °C) Soft point (Ring and ball) Kinematic viscosity at 135 °C Density (15 °C) After TFOT/RTFOT Mass change Needle penetration ratio (25 °C) Ductility (5 °C)

0.1 mm – cm °C Pas g/cm3 % % %

49 0.4 32.0 79.0 3.6 1.021 0.23 80.0 15.7

Table 3 The physical properties of aggregates used in this study. Test item of aggregate

Measured value

Crushing value (%) Polished stone value (PSV) Mass loss in Los Angeles Abrasion Test (%) Apparent relative density (g/cm3) Water absorption (%) Elongated particles (%)

9.45 46.2 8.65 2.983 1.15 8.5

Appendix B

References [1] R.D. Bornstein, Observations of the urban heat island effect in New York City, J. Appl. Meteorol. 7 (1968) 575–582. [2] M. Santamouris, Heat-island effect, Energy Clim. Urban Built Environ. (2001) 48–69. [3] K.P. Gallo, A.L. McNab, T.R. Karl, J.F. Brown, J.J. Hood, J.D. Tarpley, The use of a vegetation index for assessment of the urban heat island effect, Remote Sens. 14 (1993) 2223–2230. [4] R. Li, L. Zhao, Y. Ding, T. Wu, Y. Xiao, E. Du, G. Liu, Y. Qiao, Temporal and spatial variations of the active layer along the Qinghai-Tibet Highway in a permafrost region, Chin. Sci. Bull. 57 (2012) 4609–4616. [5] S. Wang, J. Chen, J. Zhang, Z. Li, Development of highway constructing technology in the permafrost region on the Qinghai-Tibet plateau, Sci. China Ser. E: Technol. Sci. 52 (2009) 497–506. [6] S. Bretz, H. Akbari, A. Rosenfeld, Practical issues for using solar-reflective materials to mitigate urban heat islands, Atmos. Environ. 32 (1998) 95–101. [7] H. Li, J. Harvey, T. Holland, M. Kayhanian, The use of reflective and permeable pavements as a potential practice for heat island mitigation and stormwater management – IOPscience, Environ. Res. Lett. 8 (n.d.) 15–23. [8] M. Santamouris, Using cool pavements as a mitigation strategy to fight urban heat island—a review of the actual developments, Renewable Sustainable Energy Rev. 26 (2013) 224–240. [9] Y. Gao, J. Xu, S. Yang, X. Tang, Q. Zhou, J. Ge, T. Xu, R. Levinson, Cool roofs in China: policy review, building simulations, and proof-of-concept experiments, Energy Policy 74 (2014) 190–214. [10] M. Santamouris, Cooling the cities – a review of reflective and green roof mitigation technologies to fight heat island and improve comfort in urban environments, Sol. Energy 103 (2014) 682–703. [11] Q. Wu, T. Zhang, Changes in active layer thickness over the Qinghai-Tibetan Plateau from 1995 to 2007, J. Geophys. Res. Atmos. 115 (2010) D09107. [12] W. Yu, Y. Lai, X. Zhang, F. Niu, Experimental study on the ventiduct embankment in permafrost regions of the Qinghai-Tibet Railroad, J. Cold Reg. Eng.: (ASCE) 19 (2005) 52–60. [13] L. Ran, X. Xue, L. Bao, Applications and technical characteristics of thermal pipe subgrade in Qinghai-Tibet railway design, J. Glaciol. Geocryol. 26 (2004) 151– 153. [14] Y. Dong, Y. Lai, J. Li, Y. Yang, Laboratory investigation on the cooling effect of crushed-rock interlayer embankment with ventilated ducts in permafrost regions, Cold Reg. Sci. Technol. 61 (2010) 136–142. [15] Q. Zhu, W. Wang, S. Wang, X. Zhou, G. Liao, S. Wang, J. Chen, Unilateral heattransfer asphalt pavement for permafrost protection, Cold Reg. Sci. Technol. 71 (2012) 129–138. [16] MOT, JTG F40-2004, Technical Specification for Construction of Highway Asphalt Pavements, Ministry of Transportation, Beijing, China, 2004. [17] ASTM, D5M-13, Standard Test Method for Penetration of Bituminous Materials, ASTM International West Conshohocken, Philadelphia, 2013. [18] ASTM, D36M-14E1, Standard Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus), ASTM International West Conshohocken, Philadelphia, 2014. [19] ASTM, D113-07, Standard Test Method for Ductility of Bituminous Materials, ASTM International West Conshohocken, Philadelphia, 2007. [20] ASTM, D6927-15, Standard Test Method for Marshall Stability and Flow of Asphalt Mixture, ASTM International West Conshohocken, Philadelphia, 2015. [21] MOT, JTG E20-2011, Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering, Ministry of Transportation, Beijing, China, 2011.

Table 4 The gradation of AC-13 mixtures used in this study. The mid-value of passing range in specification was utilized in this study. Particle size

16

13.2

9.5

4.75

2.36

1.18

0.6

0.3

0.15

0.075

Passing range (%) Mid-value (%)

100 100

90–100 95

68–85 76.5

38–68 53

24–50 37

15–38 26.5

10–28 19

7–20 13.5

5–15 10

4–8 6

Appendix C

Table 5 Marshall test results of AC-13 mixtures in measurement whose asphalt content falls in 3.5% and 5.5% by the weight of aggregates. The final asphalt binder usage is 4.6% according to the measurement. Asphalt binder-aggregate ratio (%)

Bulk specific gravity (g/cm3)

Void content (%)

Marshall stability (kN)

Flow value (mm)

Voids in mineral aggregate (%)

Asphalt saturation (%)

3.5 4.0 4.5 5.0 5.5

2.483 2.521 2.550 2.567 2.566

8.5 6.4 4.6 3.2 2.5

13.9 14.3 13.7 13.2 12.6

25.5 14.1 13.6 13.4 13.8

15.0 14.1 13.6 13.4 13.8

43.1 45.4 65.9 76.0 81.6

364

A. Sha et al. / Construction and Building Materials 139 (2017) 355–364

[22] A. Sha, Material and Structure of Eco-Friendly Pavements, Science Press, Beijing, 2012. [23] ASTM, D476-15, Standard Classification for Dry Pigmentary Titanium Dioxide Products, ASTM International West Conshohocken, Philadelphia, 2015. [24] X. Xu, X. Li, G. Wang, P. Wang, Effects of pigments on solar reflectance coating, Mod. Paint Finish. 4 (1998) 3–25.

[25] ASTM, D561-82/14R, Standard Specification for Carbon Black Pigment for Paint, ASTM International West Conshohocken, Philadelphia, 2014. [26] A.E. Martin, L.F. Walubita, F. Hugo, N.U. Bangera, Pavement response and rutting for full-scale and scaled APT, J. Trans. Eng. 129 (2003) 451–461. [27] E.M. Twagira, K.J. Jenkins, Application of MMLS3 in laboratory conditions for moisture damage classification of bitumen stabilised materials, Road Mater. Pavement Des. 13 (2012) 642–659.