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coatings
Article
Development of Super Road Heat-Reflective Coating
and Its Field Application
Yong Yi 1, Yingjun Jiang 1,*, Qilong Li 1, Changqing Deng 1, Xiaoping Ji 1and Jinshun Xue 2
1Key Laboratory for Special Area Highway Engineering of Ministry of Education, Chang’an University,
South Erhuan Middle Section, Xi’an 710064, China; yyong@chd.edu.cn (Y.Y.); 2018221149@chd.edu.cn (Q.L.);
changqingdeng@chd.edu.cn (C.D.); jixp82@163.com (X.J.)
2School of Civil Engineering and Architecture, Hubei University of Arts and Science, No.296, Longzhong
Road, Xiangyang 441053, China; jinshunx@chd.edu.cn
*Correspondence: jyj@chd.edu.cn
Received: 11 November 2019; Accepted: 26 November 2019; Published: 29 November 2019
Abstract:
Heat-reflective coatings, used to reduce the asphalt pavement temperature and urban
heat island effect, have a good cooling effect; coating development, however, generally focuses on
cooling. This study aims to design a heat-reflective coating with both durability and cooling effect
by considering the functions of each component, improving the coating bond strength and abrasion
resistance, and conducting laboratory tests and test section verification. The coating developed
exhibits bond strength and abrasion resistance 20% and 49%, respectively, higher than those of
ordinary coatings. The experiments reveal a cooling effect of red coating up to 10.2
◦
C, a bond
strength of up to 1.20 MPa at 25
◦
C, and an abrasion rate of up to 25% after 60 min; the cooling effect is
basically the same as that for traditional heat-reflective coatings, but the bond strength and abrasion
resistance significantly improved. In the analysis of the test section, the cooling effect reaches 7.0
◦
C,
the performance of anti-skid decreases little, and the coating is still usable normally after 4 months.
Keywords:
road; heat-reflective coating; coating development; bond strength; abrasion resistance;
cooling effect
1. Introduction
Big cities have significantly higher temperature than rural areas, both day and night: during
the day, the heat reflection and emissions from buildings exacerbate the urban heat island effect [
1
],
while at night, the temperature increases due to the release of the heat stored in roads and buildings.
The phenomenon of heat island effect increases year by year, consequently increasing the energy
consumption and reducing the life quality and comfort of urban residents [2–6]. The heat absorption
rate of asphalt pavements is so high that their daytime temperature exceeds 60
◦
C and, thus, the
ambient temperature of the road is also significant [
7
]; the heat stored in the pavement is released at
night, contributing to increasing the ambient temperature. In turn, the high temperature of the road
surface is the main cause of rutting deformation [
8
]. Therefore, reducing the heat absorption of asphalt
pavements has become crucial for preventing the urban heat island effect and reducing the rutting
deformation of roads [9].
Coating an asphalt pavement with a heat-reflective layer can not only enhance the road surface’s
ability to reflect solar radiation, and reduce both its temperature and rutting deformation but also
decrease its heat storage and alleviate the urban heat island effect [
10
–
15
]. Heat-reflective coatings
can have an excellent cooling effect, reduce the temperature of asphalt pavements by 8–16
◦
C during
high-temperature periods in summer [
16
–
19
]. Cao et al. compared the internal temperature of asphalt
and heat-reflective pavements; At 2.5 cm depth, the temperature of the traditional pavement reached
Coatings 2019,9, 802; doi:10.3390/coatings9120802 www.mdpi.com/journal/coatings
Coatings 2019,9, 802 2 of 18
62.7
◦
C, while that of the heat-reflective one was reduced by 9
◦
C [
12
]. Outdoor tests and experiments
on test sections have also been conducted to further confirm the cooling effect of heat-reflective
coatings [
7
,
10
,
20
,
21
]. Furthermore, Jiang et al. reported that heat-reflective pavements have better
stability at high temperatures [
17
,
22
,
23
]. This excellent cooling effect is one of the main reasons for
the rapid development of heat-reflective coatings for asphalt pavements; in addition, the convenient
construction and low cost of heat-reflective coating [
5
] have made them one of the best solutions to
rutting damage and heat island effect caused by asphalt pavements in summer [4,17,22,23].
Road researchers have studied not only the cooling effect of these coatings, but also the skid
resistance, aging performance, permeability, microstructure, and abrasion resistance of the resulting
heat-reflective pavements, as these coatings are applied to asphalt pavement surfaces. Cao et al.
verified the reduced anti-skid performance of heat-reflective road surfaces [7,17,23–26], and reported
that a controlled dosage of heat-reflective coating within 0.8 kg/m
2
can still satisfy the Technical
Standards of the Chinese Technical Specifications for Construction of Highway Asphalt Pavements
(JTG F40-2004) [
27
]. Sha et al. demonstrated that inorganic materials can remain mostly stable under
ultraviolet irradiation, while this condition would age the organic ones [
13
,
23
]. Cao et al. also tested
the permeability of heat-reflective pavements, finding that they are basically impermeable because the
coating almost seals their surface, preventing water from entering the concrete interior and, hence,
avoiding asphalt and pavement damage [
12
]. The microstructures of the heat-reflective coatings
and their components have been investigated via scanning electron microscopy [
16
,
18
,
23
,
26
,
28
–
32
].
Moreover, Hu et al. performed accelerated wear tests to study the wear performance of these coatings;
the results showed quality loss after wearing, but an improved anti-skid granular stirring method
could enhance the wear performance [17,24,33].
The research on heat-reflective coatings is currently focused on their cooling effect and road
performance. Previous research has showed that the developed heat-reflective coating not only can
improve the road performance but also reduce the pavement temperature to some extent. However,
the bond strength of coating has rarely been considered in road engineering. Moreover, the abrasion
property of coating is rarely evaluated quantitatively in terms of quality loss and wear rate. Therefore,
the bond strength and abrasion property were used as the control index to develop a durable coating.
In addition, a coating abrasion meter of the abrasive heat-reflective coating was developed, which can
test the quality loss and wear rate of coating. In this study, the influence of each coating component
was comprehensively considered throughout the development process. In addition, the type and
dosage of the film-forming material, functional filler, pigment, and auxiliary agent were selected based
on four aspects: bond strength, cooling effect, color, and construction workability. Based on the bond
strength and abrasion property to improve the durability of coating. Temperature tests, bond strength
test and abrasion resistance test were carried out to verify the improved coating cooling effect, bond
strength, and abrasion resistance. The heat-reflective pavement test section was paved, and the cooling
effect, durability and anti-skid performance of heat-reflective pavement were tested.
2. Materials and Test Plan
Materials, coating development process, and test methods are all mentioned in this work.
Materials included heat-reflective coating materials, asphalt mixture gradation and asphalt content; the
coating development process includes road heat-reflective coating development, coating performance
improvement, property tests and field applications. The test methods included the temperature test
method, bond strength test method, abrasion resistance test method and skid-resistance test method.
2.1. Materials
An epoxy resin modified with high-viscosity polyurethane was selected as the heat-reflective
coating substrate and added with a modified amine agent to form a film. Epoxy resin (P0), two different
polyurethane modified epoxy resins (P1 and P2), and high-elasticity epoxy resin (P3) were selected as
Coatings 2019,9, 802 3 of 18
the binder. Testing methods for the material of coating are shown in Table 1. The technical properties
of modified amine curing agent and four epoxy resin binder are shown in Table 2.
Table 1. Properties test methods for heat-reflective coatings.
Items Testing Methods
Viscosity GB/T 1723-1993 [34]
Epoxide number GB/T 4612-2008 [35]
Volatile matter content GB/T 1725-2007 [36]
Density GB/T 15223-2008 [37]
Table 2. Technical properties of film formation.
Types Materials
Property Parameters
Viscosity
(60 ◦C, mPa·s)
Epoxide Number
(mol/100 g)
Volatile Matter
Content (%)
Binder
P0 355 0.50 1.8
P1 11270 0.37 0.4
P2 38005 0.39 0.4
P3 4731 0.25 0.5
Curing agent Density(20oC):1.08 g/cm3; Amine value:485 mg/g
Two different filler types were used: pigments and a reflective filler [
32
,
38
]. The coating colors
were enriched by adding pigments, while the reflective filler provided the ability to reflect sunlight.
Table 3summarizes their properties and parameters.
Table 3. Property parameters of the filler.
Items Iron Oxide Pigment Reflective Filler
Blue Red Green Yellow Orange Brown SiO2TiO2
Refraction coefficient 1.70 2.05 1.80 2.41 1.60 1.44 1.54 2.80
The additional agent is an additive used in the formulation of heat-reflective coatings. It can
improve the original properties of the coating formulation and provide new functions. In this
study, diluents, adhesion promoters, and wear-resisting agents were adopted as additional agents.
The properties and parameters of the resulting film formulation are summarized in Table 4.
The asphalt content was 4.8% and 4.5% in the AC-13 and AC-16 samples, respectively; Technical
properties of SBS modified asphalt are shown in Table 5. Table 6reports the asphalt mixture gradation.
Table 4. Property parameters of additional agent.
Materials Density (20oC, g/cm3)Non-volatile content (%) Purity (%)
Diluents 0.789 0.1 99
Adhesion promoters 0.946 100.0 100
Wear-resisting agents 1.014 70.0 80
Table 5. Technical properties of SBS modified asphalt.
Indexes
Penetration
(25 ◦C, 100 g, 5 s)
(0.1 mm)
Ductility (5 ◦C)
(cm)
Softening
Point (◦C)
Density (15 ◦C)
(g/cm3)
Penetration
Index
Tested value 68 50.4 86 1.028 0.06
Standards 60~80 ≥30 55 - ≥−0.4
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Table 6. The gradation of AC-13 and AC-16.
Gradation Passing Percentage (%)
16 13.2 9.5 4.75 2.36 1.18 0.6 0.3 0.15 0.075
AC-13 100.0 97.3 70.4 36.1 27.4 18.8 13.6 9.6 7.1 5.9
AC-16 99.4 85.4 62.8 41.2 27.7 19.1 13.3 8.2 6.0 4.6
2.2. Development Process
Figure 1illustrates the development process for a super road heat-reflective coating, as divided
into four parts: road heat-reflective coating (RHRC) development, super road heat-reflective coating
(SRHRC) development, performance verification, and engineering application. In this study, the type
and content of the film-forming material, reflective filler, pigment, and auxiliary agent were selected
based on the corresponding bond strength, thermal resistance, color, and construction workability,
respectively. After the RHRC development, the adhesion promoter and wear-resisting agent were
added to improve the adhesion strength and abrasion resistance, respectively, and thus, obtain an
SRHRC. The SRHRC performance was verified via laboratory tests and its service performance was
monitored in the paving test section.
Coatings 2019, 9, x FOR PEER REVIEW 4 of 19
Table 6. The gradation of AC-13 and AC-16.
Gradation
Passing Percentage (%)
16
13.2
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
AC-13
100.0
97.3
70.4
36.1
27.4
18.8
13.6
9.6
7.1
5.9
AC-16
99.4
85.4
62.8
41.2
27.7
19.1
13.3
8.2
6.0
4.6
2.2. Development Process
Figure 1 illustrates the development process for a super road heat-reflective coating, as divided
into four parts: road heat-reflective coating (RHRC) development, super road heat-reflective coating
(SRHRC) development, performance verification, and engineering application. In this study, the type
and content of the film-forming material, reflective filler, pigment, and auxiliary agent were selected
based on the corresponding bond strength, thermal resistance, color, and construction workability,
respectively. After the RHRC development, the adhesion promoter and wear-resisting agent were
added to improve the adhesion strength and abrasion resistance, respectively, and thus, obtain an
SRHRC. The SRHRC performance was verified via laboratory tests and its service performance was
monitored in the paving test section.
Figure 1. Super road heat-reflective coating (SRHRC) development process.
2.3. Testing Methods
2.3.1. Temperature
Indoor Temperature Test
The light simulation box (Figure 2) independently designed by our research group was used to
investigate the cooling effect of the as-developed heat-reflective coating pavements. Different coating
formulations were prepared and sprayed on 30 cm × 30 cm × 5 cm rutting boards made of an asphalt
mixture; an unsprayed sample was used as the control. The temperature difference between each
sprayed sample and the control was used as the index to evaluate the cooling effect of the coating.
Sunlight exposure was simulated using iodine–tungsten lamps [17,23]; the temperature data were
Figure 1. Super road heat-reflective coating (SRHRC) development process.
2.3. Testing Methods
2.3.1. Temperature
Indoor Temperature Test
The light simulation box (Figure 2) independently designed by our research group was used to
investigate the cooling effect of the as-developed heat-reflective coating pavements. Different coating
formulations were prepared and sprayed on 30 cm
×
30 cm
×
5 cm rutting boards made of an asphalt
mixture; an unsprayed sample was used as the control. The temperature difference between each
sprayed sample and the control was used as the index to evaluate the cooling effect of the coating.
Sunlight exposure was simulated using iodine–tungsten lamps [
17
,
23
]; the temperature data were
Coatings 2019,9, 802 5 of 18
measured and collected using temperature sensors and data storage containers. The accuracy of the
temperature sensor is around 0.1 ◦C. Meanwhile, the lighting time was set to 3 h.
Outdoor Temperature Test
The outdoor temperature test was to test the cooling effect of thermal reflective coating under
natural light. Infrared detection guns were used to detect surface temperature specimens, and infrared
imaging devices were used to take thermal-imaging images. Outdoor temperature test devices and
specimens are shown in Figure 3.
Coatings 2019, 9, x FOR PEER REVIEW 5 of 19
measured and collected using temperature sensors and data storage containers. The accuracy of the
temperature sensor is around 0.1 °C. Meanwhile, the lighting time was set to 3 h.
Outdoor Temperature Test
The outdoor temperature test was to test the cooling effect of thermal reflective coating under
natural light. Infrared detection guns were used to detect surface temperature specimens, and
infrared imaging devices were used to take thermal-imaging images. Outdoor temperature test
devices and specimens are shown in Figure 3.
(a)
(b)
Figure 2. Indoor temperature test device: (a) light simulation box; (b) temperature acquisition system.
(a)
(b)
(c)
Figure 2.
Indoor temperature test device: (
a
) light simulation box; (
b
) temperature acquisition system.
Coatings 2019, 9, x FOR PEER REVIEW 5 of 19
measured and collected using temperature sensors and data storage containers. The accuracy of the
temperature sensor is around 0.1 °C. Meanwhile, the lighting time was set to 3 h.
Outdoor Temperature Test
The outdoor temperature test was to test the cooling effect of thermal reflective coating under
natural light. Infrared detection guns were used to detect surface temperature specimens, and
infrared imaging devices were used to take thermal-imaging images. Outdoor temperature test
devices and specimens are shown in Figure 3.
(a)
(b)
Figure 2. Indoor temperature test device: (a) light simulation box; (b) temperature acquisition system.
(a)
(b)
(c)
Figure 3.
Outdoor temperature test: (
a
) infrared detection gun; (
b
) infrared imaging devices; (
c
) outdoor
temperature test specimens.
Coatings 2019,9, 802 6 of 18
2.3.2. Bond Strength
The bond strength between the cured heat-reflective coatings and the asphalt mixture was measured
with a digital display adhesion tester (Figure 4). Different heat-reflective coating formulations were
sprayed uniformly on the surface of asphalt samples. Once each coating was cured, the DP810 acrylate
AB adhesive was applied to bond the upper surface of the coating with the spindle; after 12 h, the
spindle was covered by a piston cylinder and the armrest was pressed evenly and slowly. Then, the
spindle was slowly pulled away from the sample. The strength of coating and asphalt mixture was
tested when bond failure occurred.
Coatings 2019, 9, x FOR PEER REVIEW 6 of 19
Figure 3. Outdoor temperature test: (a) infrared detection gun; (b) infrared imaging devices; (c)
outdoor temperature test specimens.
2.3.2. Bond Strength
The bond strength between the cured heat-reflective coatings and the asphalt mixture was
measured with a digital display adhesion tester (Figure 4). Different heat-reflective coating
formulations were sprayed uniformly on the surface of asphalt samples. Once each coating was
cured, the DP810 acrylate AB adhesive was applied to bond the upper surface of the coating with the
spindle; after 12 h, the spindle was covered by a piston cylinder and the armrest was pressed evenly
and slowly. Then, the spindle was slowly pulled away from the sample. The strength of coating and
asphalt mixture was tested when bond failure occurred.
(a)
(b)
Figure 4. Bond strength test: (a) digital display adhesion tester; (b) bond strength test specimen.
2.3.3. Abrasion Resistance
To simulate the real situation of single-lane vehicles driving in a constant direction on a road,
the coating abrasion meter of the abrasive heat-reflective coating (Figure 5) in a constant direction at
a set speed was also independently developed. A 30 cm × 30 cm heat-reflective coating sample was
placed on the friction belt in its downward direction. After fixing the sample and starting the fixture
loading of the oil pump, the friction belt travel speed and abrasion time could be set to begin the
abrasion test. The wear rate of the coating after a specific time was used as the index to evaluate its
wear performance.
Figure 5. Abrasion resistance test device.
Figure 4. Bond strength test: (a) digital display adhesion tester; (b) bond strength test specimen.
2.3.3. Abrasion Resistance
To simulate the real situation of single-lane vehicles driving in a constant direction on a road,
the coating abrasion meter of the abrasive heat-reflective coating (Figure 5) in a constant direction
at a set speed was also independently developed. A 30 cm
×
30 cm heat-reflective coating sample
was placed on the friction belt in its downward direction. After fixing the sample and starting the
fixture loading of the oil pump, the friction belt travel speed and abrasion time could be set to begin
the abrasion test. The wear rate of the coating after a specific time was used as the index to evaluate its
wear performance.
Coatings 2019, 9, x FOR PEER REVIEW 6 of 19
Figure 3. Outdoor temperature test: (a) infrared detection gun; (b) infrared imaging devices; (c)
outdoor temperature test specimens.
2.3.2. Bond Strength
The bond strength between the cured heat-reflective coatings and the asphalt mixture was
measured with a digital display adhesion tester (Figure 4). Different heat-reflective coating
formulations were sprayed uniformly on the surface of asphalt samples. Once each coating was
cured, the DP810 acrylate AB adhesive was applied to bond the upper surface of the coating with the
spindle; after 12 h, the spindle was covered by a piston cylinder and the armrest was pressed evenly
and slowly. Then, the spindle was slowly pulled away from the sample. The strength of coating and
asphalt mixture was tested when bond failure occurred.
(a)
(b)
Figure 4. Bond strength test: (a) digital display adhesion tester; (b) bond strength test specimen.
2.3.3. Abrasion Resistance
To simulate the real situation of single-lane vehicles driving in a constant direction on a road,
the coating abrasion meter of the abrasive heat-reflective coating (Figure 5) in a constant direction at
a set speed was also independently developed. A 30 cm × 30 cm heat-reflective coating sample was
placed on the friction belt in its downward direction. After fixing the sample and starting the fixture
loading of the oil pump, the friction belt travel speed and abrasion time could be set to begin the
abrasion test. The wear rate of the coating after a specific time was used as the index to evaluate its
wear performance.
Figure 5. Abrasion resistance test device.
Figure 5. Abrasion resistance test device.
2.3.4. Skid-Resistance
The British Pendulum Number value was used to evaluate the skid resistance of the pavement
covered with the developed coating. Slip resistance of test section are shown in Figure 6.
Coatings 2019,9, 802 7 of 18
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2.3.4. Skid-Resistance
The British Pendulum Number value was used to evaluate the skid resistance of the pavement
covered with the developed coating. Slip resistance of test section are shown in Figure 6.
Figure 6. Slip resistance of test section.
3. Results and Discussion
Coating development, coating performance improvement and property tests are conducted and
discussed in this section.
3.1. Coating Development
The RHFC development process included the selection of a film-forming material, fillers, and
additives, as well as the dosage determination.
3.1.1. Film-Forming Material Selection
The mechanical strength of the film-forming materials determines the overall strength of the
coating [38]. Therefore, its selection should be based on this parameter. In this study, the bond
strength was used as the index for choosing this component because heat-reflective coatings must
have a good bond strength with the asphalt pavement, to avoid easy wear and peeling. Different
contents of the curing agent (20%, 25%, 30%, 35%, 40%, 45%, and 50%) were added to the four base
materials, respectively, and sprayed on the surface of asphalt mixture samples (with a spraying
amount of 0.55 kg/m2). The coating bond strength test was carried out as described in Section 3.2.2
and the bond strength difference between the coating and asphalt mixture was used as the maximum
damage strength. Figure 7 displays the bond strength test results for these samples.
When using the P1 polyurethane-modified epoxy resin as the base material, the bond strength
was the best and its maximum bond strength with the asphalt mixture, with the optimal curing agent
content, was 1.05 MPa. The P2, P0, and P3 samples exhibited a maximum bond strength of 0.97, 0.88,
and 0.76 MPa, respectively. The dosage of the curing agent was optimal (Figure 7), namely, 35% for
the P1, P2, and P3 samples and 30% for P0. Based on the principle of optimal bond strength, the
sample formed with P1 polyurethane-modified epoxy resin and the curing agent dosage of 35% were
selected.
Figure 6. Slip resistance of test section.
3. Results and Discussion
Coating development, coating performance improvement and property tests are conducted and
discussed in this section.
3.1. Coating Development
The RHFC development process included the selection of a film-forming material, fillers, and
additives, as well as the dosage determination.
3.1.1. Film-Forming Material Selection
The mechanical strength of the film-forming materials determines the overall strength of the
coating [
38
]. Therefore, its selection should be based on this parameter. In this study, the bond
strength was used as the index for choosing this component because heat-reflective coatings must
have a good bond strength with the asphalt pavement, to avoid easy wear and peeling. Different
contents of the curing agent (20%, 25%, 30%, 35%, 40%, 45%, and 50%) were added to the four base
materials, respectively, and sprayed on the surface of asphalt mixture samples (with a spraying amount
of 0.55 kg/m
2
). The coating bond strength test was carried out as described in Section 3.2.2 and the
bond strength difference between the coating and asphalt mixture was used as the maximum damage
strength. Figure 7displays the bond strength test results for these samples.
Coatings 2019, 9, x FOR PEER REVIEW 8 of 19
20 25 30 35 40 45 50
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Bond strength (MPa)
Content of curing agent (%)
P0
P1
P2
P3
Figure 7. The bond strength of each resin with different proportions of curing agent.
3.1.2. Reflective Filler Selection
The functional filler comprises TiO2 and SiO2. High reflectivity guarantees the cooling effect of
heat-reflective coatings; the higher the coating reflectivity, the better its cooling effect [21,39,40].
Therefore, in this study, the functional filler content was optimized based on the resulting cooling
effect. Based on previous research, both TiO2 and SiO2 were used as reflective filler and the ratio of
the TiO2 and SiO2 is 2:1 [17]. According to the procedure described in Section 3.2.1, different
functional filler/film-forming material ratios (4:6, 5:5, and 6:4) were tested for the temperature of the
pavement heat-reflective coating and asphalt mixture samples. The test results are shown in Figure
8.
After 180 min of simulated illumination, the surface temperature of the uncoated asphalt
samples reached 75 °C. Under the same conditions, the maximum surface temperature of RHRC-1
with the functional filler was only 70.3 °C; the RHRC-1 formulation with the 5:5 functional filler/base
material ratio showed the lowest surface temperature, i.e., 67 °C. The cooling effect of RHRC-1
gradually increased with the illumination time; the sample with the 5:5 functional filler/base material
ratio exhibited the best cooling effect (up to 8 °C), followed by those with 4:6 (up to 5.5 °C) and 6:4
(only 4.7 °C) ratios. Based on these results, the optimal functional filler/base material ratio was
identified as 5:5.
30 60 90 120 150 180
45
50
55
60
65
70
75
80
Surface temperature (
o
C)
Illumination time (min)
4:6,Ratio of functional materials to base polymer
5:5,Ratio of functional materials to base polymer
6:4,Ratio of functional materials to base polymer
Unspraying
30 60 90 120 150 180
0
2
4
6
8
10
12
Cooling effect (
o
C)
Illumination time (min)
4:6,Ratio of functional materials to base polymer
5:5,Ratio of functional materials to base polymer
6:4,Ratio of functional materials to base polymer
(a)
(b)
Figure 8. Results of select functional packing: (a) surface temperature; (b) cooling effect.
Figure 7. The bond strength of each resin with different proportions of curing agent.
Coatings 2019,9, 802 8 of 18
When using the P1 polyurethane-modified epoxy resin as the base material, the bond strength
was the best and its maximum bond strength with the asphalt mixture, with the optimal curing agent
content, was 1.05 MPa. The P2, P0, and P3 samples exhibited a maximum bond strength of 0.97, 0.88,
and 0.76 MPa, respectively. The dosage of the curing agent was optimal (Figure 7), namely, 35% for the
P1, P2, and P3 samples and 30% for P0. Based on the principle of optimal bond strength, the sample
formed with P1 polyurethane-modified epoxy resin and the curing agent dosage of 35% were selected.
3.1.2. Reflective Filler Selection
The functional filler comprises TiO
2
and SiO
2
. High reflectivity guarantees the cooling effect
of heat-reflective coatings; the higher the coating reflectivity, the better its cooling effect [
21
,
39
,
40
].
Therefore, in this study, the functional filler content was optimized based on the resulting cooling
effect. Based on previous research, both TiO
2
and SiO
2
were used as reflective filler and the ratio of the
TiO
2
and SiO
2
is 2:1 [
17
]. According to the procedure described in Section 3.2.1, different functional
filler/film-forming material ratios (4:6, 5:5, and 6:4) were tested for the temperature of the pavement
heat-reflective coating and asphalt mixture samples. The test results are shown in Figure 8.
After 180 min of simulated illumination, the surface temperature of the uncoated asphalt samples
reached 75
◦
C. Under the same conditions, the maximum surface temperature of RHRC-1 with
the functional filler was only 70.3
◦
C; the RHRC-1 formulation with the 5:5 functional filler/base
material ratio showed the lowest surface temperature, i.e., 67
◦
C. The cooling effect of RHRC-1
gradually increased with the illumination time; the sample with the 5:5 functional filler/base material
ratio exhibited the best cooling effect (up to 8
◦
C), followed by those with 4:6 (up to 5.5
◦
C) and
6:4 (only 4.7 ◦C)
ratios. Based on these results, the optimal functional filler/base material ratio was
identified as 5:5.
Coatings 2019, 9, x FOR PEER REVIEW 8 of 19
20 25 30 35 40 45 50
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
Bond strength (MPa)
Content of curing agent (%)
P0
P1
P2
P3
Figure 7. The bond strength of each resin with different proportions of curing agent.
3.1.2. Reflective Filler Selection
The functional filler comprises TiO2 and SiO2. High reflectivity guarantees the cooling effect of
heat-reflective coatings; the higher the coating reflectivity, the better its cooling effect [21,39,40].
Therefore, in this study, the functional filler content was optimized based on the resulting cooling
effect. Based on previous research, both TiO2 and SiO2 were used as reflective filler and the ratio of
the TiO2 and SiO2 is 2:1 [17]. According to the procedure described in Section 3.2.1, different
functional filler/film-forming material ratios (4:6, 5:5, and 6:4) were tested for the temperature of the
pavement heat-reflective coating and asphalt mixture samples. The test results are shown in Figure
8.
After 180 min of simulated illumination, the surface temperature of the uncoated asphalt
samples reached 75 °C. Under the same conditions, the maximum surface temperature of RHRC-1
with the functional filler was only 70.3 °C; the RHRC-1 formulation with the 5:5 functional filler/base
material ratio showed the lowest surface temperature, i.e., 67 °C. The cooling effect of RHRC-1
gradually increased with the illumination time; the sample with the 5:5 functional filler/base material
ratio exhibited the best cooling effect (up to 8 °C), followed by those with 4:6 (up to 5.5 °C) and 6:4
(only 4.7 °C) ratios. Based on these results, the optimal functional filler/base material ratio was
identified as 5:5.
30 60 90 120 150 180
45
50
55
60
65
70
75
80
Surface temperature (
o
C)
Illumination time (min)
4:6,Ratio of functional materials to base polymer
5:5,Ratio of functional materials to base polymer
6:4,Ratio of functional materials to base polymer
Unspraying
30 60 90 120 150 180
0
2
4
6
8
10
12
Cooling effect (
o
C)
Illumination time (min)
4:6,Ratio of functional materials to base polymer
5:5,Ratio of functional materials to base polymer
6:4,Ratio of functional materials to base polymer
(a)
(b)
Figure 8. Results of select functional packing: (a) surface temperature; (b) cooling effect.
Figure 8. Results of select functional packing: (a) surface temperature; (b) cooling effect.
3.1.3. Pigment Selection
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in the
laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 9 of 19
Pigments were added to adjust the coating color for different applications. The choice of the
pigment content is positively correlated with its hiding power. Standard-color cards were used to
select the coating colors; six types of heat-reflective coatings with different colors were prepared in
the laboratory (Table 7). The content of the fillings was 4.2%–6.8%.
Table 7. Color coating samples.
Color Red Yellow Green Blue Orange White
Samples
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment.
Therefore, in this study, after the RHRC development, the coating durability was improved based on
the bond strength and abrasion resistance, and an SRHRC with better pavement performance was
obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel off from the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding an
adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples and
RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%, 0.8%,
and 1.0%) were prepared. Figure 9 shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by 12.4%,
but higher contents reduced the coating adhesion performance. The adhesion promoter is a silane-
coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019,9, 802 9 of 18
3.1.4. Additional Agent Selection
Direct mixing and stirring of RHRC may result in insufficient construction workability due to,
for example, air bubbles, filler precipitation, or insufficient fluidity. To improve the construction
workability of the coatings, defoamers, dispersants, and leveling agents can be added and the specific
mixing amount should be adjusted based on the real mixing conditions.
3.2. Coating Performance Improvement
When applied to asphalt pavements, commercial heat-reflective coatings are inevitably eroded
under the traffic load due to wheel action. This probably occurs because traditional heat-reflective
coatings are developed without considering their durability in this application environment. Therefore,
in this study, after the RHRC development, the coating durability was improved based on the bond
strength and abrasion resistance, and an SRHRC with better pavement performance was obtained.
3.2.1. Bond Strength Improvement
The adhesive strength of a coating indicates its adhesive performance with an asphalt mixture
layer. A coating with high adhesive strength is not easy to peel offfrom the road surface, even if
cracks form under the traffic load. Thus, increasing the adhesive strength, for example, by adding
an adhesion promoter, can improve the coating durability and service life. Ordinary RHRC samples
and RHRC formulations added with different contents of an adhesion promoter (0.2%, 0.4%, 0.6%,
0.8%, and 1.0%) were prepared. Figure 9shows the results of the test described in Section 3.2.2 and
conducted on these as-prepared samples.
The addition of 0.6% adhesion promoter improved the adhesive strength of the coating by
12.4%, but higher contents reduced the coating adhesion performance. The adhesion promoter is a
silane-coupling agent; therefore, when it is placed between inorganic and organic interfaces, an organic
matrix/silane-coupling agent/inorganic matrix bonding layer can be formed to improve the coating
adhesion. However, excessive content of the adhesion promoter can lead to coating that is too brittle,
and prone to brittle fracture, reducing the overall bond strength. Therefore, the optimal adhesion
promoter content was identified as 0.6%.
Coatings 2019, 9, x FOR PEER REVIEW 10 of 19
0.0 0.2 0.4 0.6 0.8 1.0
1.00
1.04
1.08
1.12
1.16
1.20
1.24
Bond strength (MPa)
Adhesion promoter dosage (%)
Figure 9. Results of improved bond strength.
3.2.2. Abrasion Resistance Improvement
The abrasion property of heat-reflective coatings indicates their ability to resist the friction action
of the traffic load on the asphalt pavement; it is an intuitive expression to predict their service life.
The abrasion properties of RHRC-1 samples, pure and added with different amounts of a wear-
resistant agent (0.5%, 1.0%, 1.5%, 2.0%, and 2.5%), were tested as described in Section 2.3.2. The
results are shown in Figure 10.
The wear time simulates the service life of a heat-reflective coating under vehicle friction. With
its increase, the coating wear rate and service life increase, but all properties of the remaining coating
decrease. For wear times of 0–5, 5–10, 10–25, and 25–30 min, the wear rate of the coating increased,
respectively, very fast, fast, slowly, and slowly. This is attributable to the fact that the initial coating
thickness was large, the same abrasion condition could wear away more quality coating, and as the
coating was worn, the exposed stone basically could not be worn, so the wear rate growth slowed
down.
With an increase in the wear-resistant agent as a type of coating resistant to abrasion ability,
Figure 10b shows that the mixing with this agent decreased the rate of coating abrasion, at 30 min
abrasion ratio, as the evaluation index of coating abrasion ability; without the wear-resisting agent,
the abrasion rate was as high as 79%, while the wear-resisting agent contents of 0.5%, 1.0%, 1.5%,
2.0%, and 2.5% resulted in abrasion rates of, respectively, 62%, 53%, 40%, 30%, and 29%. Under the
dual control of performance and control cost, the optimal content of the wear-resistant agent was
identified as 2.0%.
Figure 9. Results of improved bond strength.
3.2.2. Abrasion Resistance Improvement
The abrasion property of heat-reflective coatings indicates their ability to resist the friction action
of the traffic load on the asphalt pavement; it is an intuitive expression to predict their service life.
The abrasion properties of RHRC-1 samples, pure and added with different amounts of a wear-resistant
agent (0.5%, 1.0%, 1.5%, 2.0%, and 2.5%), were tested as described in Section 2.3.2. The results are
shown in Figure 10.
The wear time simulates the service life of a heat-reflective coating under vehicle friction. With its
increase, the coating wear rate and service life increase, but all properties of the remaining coating
Coatings 2019,9, 802 10 of 18
decrease. For wear times of 0–5, 5–10, 10–25, and 25–30 min, the wear rate of the coating increased,
respectively, very fast, fast, slowly, and slowly. This is attributable to the fact that the initial coating
thickness was large, the same abrasion condition could wear away more quality coating, and as the
coating was worn, the exposed stone basically could not be worn, so the wear rate growth slowed down.
With an increase in the wear-resistant agent as a type of coating resistant to abrasion ability,
Figure 10b shows that the mixing with this agent decreased the rate of coating abrasion, at 30 min
abrasion ratio, as the evaluation index of coating abrasion ability; without the wear-resisting agent, the
abrasion rate was as high as 79%, while the wear-resisting agent contents of 0.5%, 1.0%, 1.5%, 2.0%, and
2.5% resulted in abrasion rates of, respectively, 62%, 53%, 40%, 30%, and 29%. Under the dual control
of performance and control cost, the optimal content of the wear-resistant agent was identified as 2.0%.
Coatings 2019, 9, x FOR PEER REVIEW 11 of 19
010 20 30 40 50 60
0
20
40
60
80
100
120
Without wear-resisting agent
Wear-resisting agent=0.5%
Wear-resisting agent=1.0%
Wear-resisting agent=1.5%
Wear-resisting agent=2.0%
Wear-resisting agent=2.5%
Wear rate (%)
Wear time (min)
0.0 0.5 1.0 1.5 2.0 2.5
0
20
40
60
80
100
120
Wear rate (%)
Wear-resisting agent content (%)
10min
20min
30min
60min
(a)
(b)
Figure 10. Results of improved abrasion resistance: (a) the relationship between wear time and wear
rate; (b) the relationship between the content of wear-resisting agent and wear rate.
3.3. Property Tests
SRHRC, a coating with good durability and cooling performance, was obtained by improving
the bond strength and abrasion resistance of RHRC. To evaluate its cooling effect, durability, and
other road performance and to verify its applicability on asphalt pavements, tests on outdoor
temperature, bond strength, abrasion resistance, and slip resistance were conducted.
3.3.1. Super Road Heat-Reflective Coating (SRHRC) Cooling Effect
The cooling effect of SRHRC was investigated via outdoor temperature tests performed between
4 and 16 August. The molded SRHRC-1 samples of white, yellow, red, green, orange, blue, and
asphalt mixtures were placed outdoors (Figure 11); the surface temperatures of the samples were
recorded by an infrared detection gun and the corresponding thermal images were generated with
infrared imaging devices. Table 8 shows the daily surface temperature measured at the maximum air
temperature, revealing a proportionality between these two parameters. This might have occurred
because the temperature is directly proportional to the solar radiation intensity, which also
determines the sample temperature [5]; however, this can also be affected by wind, rain, and other
factors [1]. The air and sample surface temperatures measured on 10 August in the outdoor test are
shown in Figure 12, clearly revealing the effect of color on the cooling effect, in the following order:
white > yellow > red > green > orange > blue. The white coating, with the best cooling effect, reduced
the sample surface temperature by 11.7 °C at the maximum air temperature; under the same
condition, the blue coating decreased the surface temperature by only 3.5 °C. This may be because a
white coating reflects all colors of light, while those of other colors reflect preferentially the
corresponding color. Reflectivity is the key factor determining the cooling effect of a coating [18,29].
The white and yellow coatings exhibited the best cooling effect, but a too strong glare on the actual
road surfaces, which may affect the driver safety. Therefore, the red coating is recommended. The
thermal image of the sample surface recorded on 15 August (Figure 13) further demonstrates the
influence of the coating color on its cooling effect.
(a)
(b)
(c)
(d)
Figure 10.
Results of improved abrasion resistance: (
a
) the relationship between wear time and wear
rate; (b) the relationship between the content of wear-resisting agent and wear rate.
3.3. Property Tests
SRHRC, a coating with good durability and cooling performance, was obtained by improving the
bond strength and abrasion resistance of RHRC. To evaluate its cooling effect, durability, and other
road performance and to verify its applicability on asphalt pavements, tests on outdoor temperature,
bond strength, abrasion resistance, and slip resistance were conducted.
3.3.1. Super Road Heat-Reflective Coating (SRHRC) Cooling Effect
The cooling effect of SRHRC was investigated via outdoor temperature tests performed between
4 and 16 August. The molded SRHRC-1 samples of white, yellow, red, green, orange, blue, and asphalt
mixtures were placed outdoors (Figure 11); the surface temperatures of the samples were recorded by
an infrared detection gun and the corresponding thermal images were generated with infrared imaging
devices. Table 8shows the daily surface temperature measured at the maximum air temperature,
revealing a proportionality between these two parameters. This might have occurred because the
temperature is directly proportional to the solar radiation intensity, which also determines the sample
temperature [
5
]; however, this can also be affected by wind, rain, and other factors [
1
]. The air and
sample surface temperatures measured on 10 August in the outdoor test are shown in Figure 12, clearly
revealing the effect of color on the cooling effect, in the following order: white >yellow >red >green >
orange >blue. The white coating, with the best cooling effect, reduced the sample surface temperature
by 11.7
◦
C at the maximum air temperature; under the same condition, the blue coating decreased the
surface temperature by only 3.5
◦
C. This may be because a white coating reflects all colors of light,
while those of other colors reflect preferentially the corresponding color. Reflectivity is the key factor
determining the cooling effect of a coating [
18
,
29
]. The white and yellow coatings exhibited the best
cooling effect, but a too strong glare on the actual road surfaces, which may affect the driver safety.
Therefore, the red coating is recommended. The thermal image of the sample surface recorded on
15 August (Figure 13) further demonstrates the influence of the coating color on its cooling effect.
Coatings 2019,9, 802 11 of 18
Coatings 2019, 9, x FOR PEER REVIEW 11 of 19
010 20 30 40 50 60
0
20
40
60
80
100
120
Without wear-resisting agent
Wear-resisting agent=0.5%
Wear-resisting agent=1.0%
Wear-resisting agent=1.5%
Wear-resisting agent=2.0%
Wear-resisting agent=2.5%
Wear rate (%)
Wear time (min)
0.0 0.5 1.0 1.5 2.0 2.5
0
20
40
60
80
100
120
Wear rate (%)
Wear-resisting agent content (%)
10min
20min
30min
60min
(a)
(b)
Figure 10. Results of improved abrasion resistance: (a) the relationship between wear time and wear
rate; (b) the relationship between the content of wear-resisting agent and wear rate.
3.3. Property Tests
SRHRC, a coating with good durability and cooling performance, was obtained by improving
the bond strength and abrasion resistance of RHRC. To evaluate its cooling effect, durability, and
other road performance and to verify its applicability on asphalt pavements, tests on outdoor
temperature, bond strength, abrasion resistance, and slip resistance were conducted.
3.3.1. Super Road Heat-Reflective Coating (SRHRC) Cooling Effect
The cooling effect of SRHRC was investigated via outdoor temperature tests performed between
4 and 16 August. The molded SRHRC-1 samples of white, yellow, red, green, orange, blue, and
asphalt mixtures were placed outdoors (Figure 11); the surface temperatures of the samples were
recorded by an infrared detection gun and the corresponding thermal images were generated with
infrared imaging devices. Table 8 shows the daily surface temperature measured at the maximum air
temperature, revealing a proportionality between these two parameters. This might have occurred
because the temperature is directly proportional to the solar radiation intensity, which also
determines the sample temperature [5]; however, this can also be affected by wind, rain, and other
factors [1]. The air and sample surface temperatures measured on 10 August in the outdoor test are
shown in Figure 12, clearly revealing the effect of color on the cooling effect, in the following order:
white > yellow > red > green > orange > blue. The white coating, with the best cooling effect, reduced
the sample surface temperature by 11.7 °C at the maximum air temperature; under the same
condition, the blue coating decreased the surface temperature by only 3.5 °C. This may be because a
white coating reflects all colors of light, while those of other colors reflect preferentially the
corresponding color. Reflectivity is the key factor determining the cooling effect of a coating [18,29].
The white and yellow coatings exhibited the best cooling effect, but a too strong glare on the actual
road surfaces, which may affect the driver safety. Therefore, the red coating is recommended. The
thermal image of the sample surface recorded on 15 August (Figure 13) further demonstrates the
influence of the coating color on its cooling effect.
(a)
(b)
(c)
(d)
Coatings 2019, 9, x FOR PEER REVIEW 12 of 19
(e)
(f)
(g)
Figure 11. Outdoor temperature test specimen: (a) White; (b) Yellow; (c) Red; (d) Green; (e) Orange;
(f) Blue; (g) without coating.
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (
o
C)
Time (h)
Air temperature
White coating
Without coating
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (oC)
Time (h)
Air temperature
Yellow coating
Without coating
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (
o
C)
Time (h)
Air temperature
Red coating
Without coating
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (oC)
Time (h)
Air temperature
Green coating
Without coating
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (oC)
Time (h)
Air temperature
Orange coating
Without coating
3:00 7:00 11:00 15:00 19:00 23:00
20
25
30
35
40
45
50
55
60
Air temperature (oC)
Time (h)
Air temperature
Blue coating
Without coating
Figure 12. Outdoor temperature test results (10 August).
Figure 11.
Outdoor temperature test specimen: (
a
) White; (
b
) Yellow; (
c
) Red; (
d
) Green; (
e
) Orange;
(f) Blue; (g) without coating.
Coatings 2019, 9, x FOR PEER REVIEW 13 of 20
Figure 12. Outdoor temperature test results (10 August).
Coatings 2019,9, 802 12 of 18
Coatings 2019, 9, x FOR PEER REVIEW 13 of 19
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 13. Specimen thermal imaging diagram at the highest air temperature on 15 August: (a) White;
(b) Yellow; (c) Red; (d) Green; (e) Orange; (f) Blue; (g) without coating.
Table 8. The surface temperature of the specimen at the maximum daily air temperature (from 4 to
16 August).
Air temperature
Specimen Surface Temperature
White
Yellow
Red
Green
Orange
Blue
Without Coating
27.6
38.2
40.1
41.4
41.5
42.1
42.1
45.9
28.5
39.8
40.3
44.0
45.8
45.1
46.7
49.0
30.7
43.1
45.8
47.5
48.0
48.4
48.5
51.7
31.6
46.5
49.6
50.2
55.4
55.8
56.7
60.5
32.5
50.9
52.4
54.0
56.2
55.7
57.0
61.0
33.0
48.9
51.9
55.6
56.4
56.2
57.1
61.1
33.2
45.9
48.2
49.8
52.4
53.0
54.1
57.6
33.4
53.1
56.4
57.7
61.0
59.8
61.5
65.8
34.2
52.2
55.2
58.3
60.4
59.5
61.3
65.4
34.5
53.6
56.7
58.8
61.4
61.8
62.3
66.7
36.9
55.9
59.9
61.6
63.8
64.5
66.1
71.8
37.4
55.8
61
61.9
65.0
65.4
66.2
72.1
37.9
59.9
63.7
65.1
67.5
68.0
69.0
74.6
Figure 13.
Specimen thermal imaging diagram at the highest air temperature on 15 August: (
a
) White;
(b) Yellow; (c) Red; (d) Green; (e) Orange; (f) Blue; (g) without coating.
Table 8.
The surface temperature of the specimen at the maximum daily air temperature (from 4 to
16 August).
Air
Temperature
Specimen Surface Temperature
White Yellow Red Green Orange Blue Without
Coating
27.6 38.2 40.1 41.4 41.5 42.1 42.1 45.9
28.5 39.8 40.3 44.0 45.8 45.1 46.7 49.0
30.7 43.1 45.8 47.5 48.0 48.4 48.5 51.7
31.6 46.5 49.6 50.2 55.4 55.8 56.7 60.5
32.5 50.9 52.4 54.0 56.2 55.7 57.0 61.0
33.0 48.9 51.9 55.6 56.4 56.2 57.1 61.1
33.2 45.9 48.2 49.8 52.4 53.0 54.1 57.6
33.4 53.1 56.4 57.7 61.0 59.8 61.5 65.8
34.2 52.2 55.2 58.3 60.4 59.5 61.3 65.4
34.5 53.6 56.7 58.8 61.4 61.8 62.3 66.7
36.9 55.9 59.9 61.6 63.8 64.5 66.1 71.8
37.4 55.8 61 61.9 65.0 65.4 66.2 72.1
37.9 59.9 63.7 65.1 67.5 68.0 69.0 74.6
Coatings 2019,9, 802 13 of 18
3.3.2. SRHRC Bond Strength
The bond strengths of RHRC and SRHRC at different temperatures were compared (Figure 14).
At 25 ◦C,
SRHRC reached a bond strength of 1.20 MPa, about 20% higher than that of RHRC.
The increasing temperature decreased the bond strength difference between RHRC and SRHRC. This is
attributable to the asphalt softening caused by the rising temperature, which reduced the bond strength
between the coating and asphalt mixture. This result indicates that when the temperature of the asphalt
pavement is high, the coating is easy to peel and undergoes other forms of damage.
Coatings 2019, 9, x FOR PEER REVIEW 14 of 19
3.3.2. SRHRC Bond Strength
The bond strengths of RHRC and SRHRC at different temperatures were compared (Figure 14).
At 25 °C, SRHRC reached a bond strength of 1.20 MPa, about 20% higher than that of RHRC. The
increasing temperature decreased the bond strength difference between RHRC and SRHRC. This is
attributable to the asphalt softening caused by the rising temperature, which reduced the bond
strength between the coating and asphalt mixture. This result indicates that when the temperature of
the asphalt pavement is high, the coating is easy to peel and undergoes other forms of damage.
28 35 42 49 56
0.4
0.6
0.8
1.0
1.2
1.4
Bond strength (MPa)
Road surface temperature (
o
C)
SRHRC
RHRC
Figure 14. Bond strength of coating at different temperatures.
3.3.3. SRHRC Abrasion Resistance
The samples that underwent one spraying, multiple sprayings, multiple sprayings and
successive coating with a gel layer, and multiple sprayings after curing were named as OS, RS-X,
SAG-X, and SAC-X (X = spraying times), respectively. Eight groups of samples were obtained via
these different spraying methods and used for wear tests. The results are shown in Figure 15.
The wear resistance of the coating is related to the spraying method but not the spraying time;
choosing a suitable spraying method can significantly improve the wear resistance of the coating,
while optimizing the spraying time has little effect on it. However, increasing the spraying time can
improve the coating uniformity. The SAG-X and SAC-X samples were, respectively, the most
favorable and most unfavorable to enhance the coating wear resistance; this may be because curing
and spraying the next layer will appear to make two layers of coating adhesion insufficient and it
will be worn off. The difference between multiple sprayings and single spraying was small. The wear
rate of the coating sprayed with SAG-3 after wearing of 60 min is less than 30%. It is suggested the
coating is sprayed with SAG-3, which is good not only for the wear resistance but also for spraying
uniformity.
Figure 14. Bond strength of coating at different temperatures.
3.3.3. SRHRC Abrasion Resistance
The samples that underwent one spraying, multiple sprayings, multiple sprayings and successive
coating with a gel layer, and multiple sprayings after curing were named as OS, RS-X, SAG-X, and
SAC-X (X =spraying times), respectively. Eight groups of samples were obtained via these different
spraying methods and used for wear tests. The results are shown in Figure 15.
Coatings 2019, 9, x FOR PEER REVIEW 15 of 19
OS RS-2 RS-3 RS-4 SAG-2SAG-3 SAC-2 SAC-3
0
10
20
30
40
50
Wear rate (%)
Spraying process
10min
30min
60min
010 20 30 40 50 60
0
10
20
30
40
50
Wear rate (%)
Wear time (min)
OS
RS-2
RS-3
RS-4
SAG-2
SAG-3
SAC-2
SAC-3
(a)
(b)
Figure 15. Effect of spraying method on wear resistance of coating: (a) spraying process; (b) wear
time.
4. Field Applications
To verify the reliability of the abovementioned theoretical analysis and test results, a heat-
reflective coating test section was paved in Jinhua, Zhejiang province, China and the SRHRC-1
applicability and road performance were verified in combination with physical engineering and
laboratory tests.
4.1. Project Profile
The test section was located at the junction between the West Erhuanxi road and the Gold Orchid
center line, in the Wucheng District, Jinhua City Circle, and the provincial road intersection, west on
the outer ring of the Wucheng District. Large numbers of heavy vehicles, long-term congestion, deep
pavement rut deformation, and a large number of diseases, such as network cracks, affect not only
driving comfort but also traffic safety.
4.2. Construction Technology and Process
The on-site construction of the color heat-reflective coating in the test section followed these
steps: blocking the traffic, cleaning the road surface from dust (soil), applying adhesive tape marks,
treating the road grid, coating spraying (Figure 16) and curing, and health preservation. The test
section was re-opened to traffic after the color coating was fully solidified (2 h) and reached a certain
strength.
Figure 15.
Effect of spraying method on wear resistance of coating: (
a
) spraying process; (
b
) wear time.
The wear resistance of the coating is related to the spraying method but not the spraying time;
choosing a suitable spraying method can significantly improve the wear resistance of the coating, while
optimizing the spraying time has little effect on it. However, increasing the spraying time can improve
Coatings 2019,9, 802 14 of 18
the coating uniformity. The SAG-X and SAC-X samples were, respectively, the most favorable and
most unfavorable to enhance the coating wear resistance; this may be because curing and spraying
the next layer will appear to make two layers of coating adhesion insufficient and it will be worn off.
The difference between multiple sprayings and single spraying was small. The wear rate of the coating
sprayed with SAG-3 after wearing of 60 min is less than 30%. It is suggested the coating is sprayed
with SAG-3, which is good not only for the wear resistance but also for spraying uniformity.
4. Field Applications
To verify the reliability of the abovementioned theoretical analysis and test results, a heat-reflective
coating test section was paved in Jinhua, Zhejiang province, China and the SRHRC-1 applicability and
road performance were verified in combination with physical engineering and laboratory tests.
4.1. Project Profile
The test section was located at the junction between the West Erhuanxi road and the Gold Orchid
center line, in the Wucheng District, Jinhua City Circle, and the provincial road intersection, west on
the outer ring of the Wucheng District. Large numbers of heavy vehicles, long-term congestion, deep
pavement rut deformation, and a large number of diseases, such as network cracks, affect not only
driving comfort but also traffic safety.
4.2. Construction Technology and Process
The on-site construction of the color heat-reflective coating in the test section followed these steps:
blocking the traffic, cleaning the road surface from dust (soil), applying adhesive tape marks, treating
the road grid, coating spraying (Figure 16) and curing, and health preservation. The test section was
re-opened to traffic after the color coating was fully solidified (2 h) and reached a certain strength.
Coatings 2019, 9, x FOR PEER REVIEW 15 of 19
OS RS-2 RS-3 RS-4 SAG-2SAG-3 SAC-2 SAC-3
0
10
20
30
40
50
Wear rate (%)
Spraying process
10min
30min
60min
010 20 30 40 50 60
0
10
20
30
40
50
Wear rate (%)
Wear time (min)
OS
RS-2
RS-3
RS-4
SAG-2
SAG-3
SAC-2
SAC-3
(a)
(b)
Figure 15. Effect of spraying method on wear resistance of coating: (a) spraying process; (b) wear
time.
4. Field Applications
To verify the reliability of the abovementioned theoretical analysis and test results, a heat-
reflective coating test section was paved in Jinhua, Zhejiang province, China and the SRHRC-1
applicability and road performance were verified in combination with physical engineering and
laboratory tests.
4.1. Project Profile
The test section was located at the junction between the West Erhuanxi road and the Gold Orchid
center line, in the Wucheng District, Jinhua City Circle, and the provincial road intersection, west on
the outer ring of the Wucheng District. Large numbers of heavy vehicles, long-term congestion, deep
pavement rut deformation, and a large number of diseases, such as network cracks, affect not only
driving comfort but also traffic safety.
4.2. Construction Technology and Process
The on-site construction of the color heat-reflective coating in the test section followed these
steps: blocking the traffic, cleaning the road surface from dust (soil), applying adhesive tape marks,
treating the road grid, coating spraying (Figure 16) and curing, and health preservation. The test
section was re-opened to traffic after the color coating was fully solidified (2 h) and reached a certain
strength.
Figure 16. Test section spraying.
4.3. Effect of Inspection
After the preparation of the test section was completed, monitoring of the performance (i.e., cooling
effect, anti-skid performance (Table 9), and durability (Figure 17)) of the heat-reflective pavement
was initiated. Table 10 shows the maximum and average temperatures of asphalt pavement and
heat-reflective pavement measured using the infrared temperature gun with once an hour.
SRHRC could reduce the asphalt surface temperature by 7.0
◦
C when this reached 65.7
◦
C. On the
other hand, it also reduced the anti-skid performance of the road surface; nonetheless, the reduction
was small, and hence, the resulting anti-skid performance could still meet driving safety requirements.
After being opened to traffic, SRHRC-1 showed good durability, ensuring its normal use for 4 months.
Coatings 2019,9, 802 15 of 18
Coatings 2019, 9, x FOR PEER REVIEW 16 of 19
Figure 16. Test section spraying.
4.3. Effect of Inspection
After the preparation of the test section was completed, monitoring of the performance (i.e.,
cooling effect, anti-skid performance (Table 9), and durability (Figure 17)) of the heat-reflective
pavement was initiated. Table 10 shows the maximum and average temperatures of asphalt
pavement and heat-reflective pavement measured using the infrared temperature gun with once an
hour.
SRHRC could reduce the asphalt surface temperature by 7.0 °C when this reached 65.7 °C. On
the other hand, it also reduced the anti-skid performance of the road surface; nonetheless, the
reduction was small, and hence, the resulting anti-skid performance could still meet driving safety
requirements. After being opened to traffic, SRHRC-1 showed good durability, ensuring its normal
use for 4 months.
(a)
(b)
Figure 17. Endurance of test section: (a) When the paving is finished; (b) 4 months later.
Table 9. Slip-resistance of test section.
Test Point
British Pendulum Number (BPN)
Average (BPN)
Pavement
1
71
71
2
71
3
71
4
73
5
70
6
70
Heat-Reflective Pavement
1
62
62
2
61
3
63
4
61
5
62
6
63
Figure 17. Endurance of test section: (a) When the paving is finished; (b) 4 months later.
Table 9. Slip-resistance of test section.
Test Point British Pendulum
Number (BPN) Average (BPN)
Pavement
1 71
71
2 71
3 71
4 73
5 70
6 70
Heat-Reflective
Pavement
1 62
62
2 61
3 63
4 61
5 62
6 63
Table 10.
Daily (08:00–19:00) maximum and average surface temperatures during the experimental period.
Date
Maximum Daily Temperature Average Daily Temperature
Asphalt
pavement
Heat-Reflective
Pavement
Temperature
Difference
Asphalt
Pavement
Heat-Reflective
Pavement
Temperature
Difference
7.05 67.6 60.1 7.5 56.0 51.9 4.1
8.05 67.9 60.0 7.9 56.3 52.1 4.2
9.05 67.7 59.9 7.8 56.1 52.1 4.0
9.16 65.3 58.7 6.6 53.7 50.4 3.3
9.23 65.7 58.7 7.0 54.1 50.5 3.6
9.30 65.5 58.7 6.8 53.6 50.2 3.4
10.07 65.3 58.8 6.5 53.5 50.2 3.3
10.14 65.5 59.0 6.5 53.5 50.1 3.4
5. Conclusions
•
The base material and curing agent content were selected according to the resulting bond strength.
After this selection, RHRC-1 reached a bond strength of 1.05 MPa and, therefore, was selected as
the base material. The optimal curing agent dosage was identified as 35%. The functional filler
content was selected based on the resulting cooling effect; the functional filler/base material ratio
of 5:5 provided the best cooling effect (up to 8 ◦C) and, thus, was selected as the optimal setting.
Coatings 2019,9, 802 16 of 18
•
An adhesion promoter and a wear-resisting agent were used to improve, respectively, the
coating adhesive strength and wear resistance. An adhesion content of 0.6% enhanced the
adhesive strength by 12.4%. The wear resistance was improved by 49% by adding 2% of the
wear-resisting agent.
•
The coating color significantly influences its cooling effect; the white, yellow, and red coating
exhibited the best cooling effect, up to 16.3, 11.1, and 10.2
◦
C, respectively. The increasing
temperature reduced the bond strength difference between RHRC and SRHRC. SAG was the most
favorable for promoting the coating wear resistance; it is suggested the coating is sprayed with
SAG-3, which is good not only for wear resistance but also spraying uniformity.
•
The test section of the paving confirmed the reliability of the laboratory results and verified the
cooling effect, skid resistance, and durability. When the road temperature was 65.7
◦
C, the coating
provided a cooling effect of up to 7
◦
C. The coating also reduced the anti-skid performance of the
road surface, but this could still meet traffic safety requirements. After 4 months, the coating was
still in good condition and usable as normal.
Author Contributions:
Conceptualization: Y.J. and Y.Y.; Methodology: Y.J. and Y.Y.; Validation: C.D. and Y.J.;
Formal Analysis: C.D. and J.X.; Investigation: Y.Y. and Q.L.; Resources: Y.J.; Data Curation: Y.Y. and Q.L. and
X.J.; Writing—Original Draft Preparation: Y.Y. and J.X.; Writing—Review and Editing: Y.Y. and C.D. and X.J.;
Visualizations: J.X.; Supervision: Y.J.; Project Administration: Q.L. and Y.Y.; Funding Acquisition: Y.J.
Funding:
The research was funded by the Scientific Research of Central Colleges of China for Chang’an University
(Grant No. 300102218212), the scientific project from Zhejiang Provincial Communication (No. 2014H60)
Conflicts of Interest: The authors declare no conflict of interest.
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