# Study on Permeability and Blocking Resistance of Composite Specimen with Double-Layer Permeable Asphalt Mixture

Authors: Xiaoge Tian; Huitong Yuan; Ren Zhang; Yichao Xu; Xiaofei Wang
Journal: Advances in Civil Engineering
 (2019)

---

## Abstract

In order to study the permeability and antiblocking performance of composite specimens with double-layer permeable asphalt mixture, three types of PAC-10 asphalt mixture with different target porosity (20%, 22%, and 24%) and PAC-16 asphalt mixture with a target porosity of 22% were designed, and the double-layer Marshall specimen was fabricated through “hot + hot” method. Their orthogonal vertical sections were scanned with X-ray CT. The pore distribution and its characteristics of the specimens were analyzed by digital image analysis technology. The permeability of composite specimens was studied through penetration test, and their blocking resistances were studied through using different particle sizes of fine machine-made sands as blocking materials. The results show that the permeable capacity increases linearly with the increase of porosity. The permeable capacity of PAC-16 with 22% porosity is greater than PAC-10 (20%, 22%, and 24%) porosity. The porosity of the upper layer increases, and the permeability of the double-layer composite samples increases linearly. The critical particle size causing blocking is 0.15 mm, followed by 0.3 mm. The vertical permeability coefficient decreases exponentially with the increasing of blocking times.


---

## Body

## 1. Introduction

Permeable asphalt pavement is a hot issue at home and abroad [1, 2]. Single-layer permeable asphalt pavement is widely used in research and application. Its porosity is generally 18%–25%, and its seepage ability is strong enough to quickly drain out the water on the pavement surface, reduce the thickness of water film, ensure sufficient tire friction, reduce water mist, water splash, and glare; so, it can improve driving safety and reduce accident rate by 85% [1, 3–6]. However, it is found that the pores in the single-layer permeable asphalt pavement are easily blocked during its service period by dust, chips, tire wear, grease, etc., and gradually lose its excellent permeable capacity [7–11].In order to solve this problem, a double-layer permeable asphalt pavement was proposed [12–14]. The upper layer is composed of porous asphalt mixture with more pores, and their size is smaller. The lower layer porous asphalt mixture has larger pores, strong permeable capacity, and antiblocking ability. So, the upper layer can prevent large-size dust particle from entering the lower layer, just like a sieve. While the lower layer can discharge the small particle size blocking material infiltrated from the upper layer pores under the action of water or air flow, so as to reduce the probability of blocking. At the same time, compared with the traditional single-layer permeable asphalt pavement, double-layer permeable asphalt pavement has better noise reduction, permeability, slip resistance, and durability [12, 14–16]. Kragh and Bendtsen investigated the life and acoustic characteristics of double-layer permeable asphalt pavement. It was found that the service life of double-layer permeable asphalt pavement was prolonged, the level of road noise reduction gradually decreased in the first three years after paving, and the noise reduction level tended to be stable after three years [14]. Hamzah et al. compared the blocking resistances of double-layer porous asphalt mixture and single-layer porous asphalt mixture with different particle sizes and different upper thicknesses through indoor simulated blocking tests. The results showed that a longer time should be taken to clog the composite samples with a thinner upper layer, and the double-layer porous asphalt mixture had better blocking resistance than the single-layer porous asphalt mixture [12].


## 2. Raw Materials

### 2.1. Coarse Aggregate

Coarse aggregates crushed with limestone were adopted in the paper, and their technical indexes are shown in Table1 [11].Table 1
Technical indexes of coarse aggregates.
Indexes
Unit
Actual measurement
Crushing value
%
13.5
Apparent relative density
—
—
Water absorption
%
1.2
Strength
%
9.4
Needle and plate particle content
%
8
Content of <0.075 mm material
%
0.43
Adhesion with SBS modified asphalt
Level
5


### 2.2. Fine Aggregate

Machine-made sand with limestone and their technical indexes are shown in Table2 [11].Table 2
Main technical indicators of fine aggregates.
Material particle size (mm)
Apparent relative density
Surface relative density
Gross volume relative density
Water absorption rate (%)
1.18
2.702
2.672
2.654
—
0.6
2.723
2.683
2.661
—
0.3
2.740
2.707
2.688
—
0.15
2.728
2.701
2.689
—
0.075
2.721
2.693
2.676
—


### 2.3. Filler

#### 2.3.1. Mineral Powder

Grind with limestone and its main indexes are shown in Table3.Table 3
Results of limestone ore powder test.
Project
Unit
Test results
Requirements
Apparent density
g/cm3
2.716
≥2.50
Size range
<0.6 mm
%
100
100
<0.15 mm
%
94.5
90∼100
<0.075 mm
%
90
75∼100
Water content
%
0.41
≤1
Plasticity index
—
2
<4
Hydrophilic coefficient
—
0.5
<1
Exterior
—
No agglomeration
No agglomeration


#### 2.3.2. Slaked Lime

In order to enhance the adhesion between asphalt binder and aggregate, slaked lime was used to replace part of mineral powder (the amount of slaked lime is 3% of the mass of the asphalt mixture) to improve the spalling resistance of the mixture.


#### 2.3.3. High-Viscosity Modified Asphalt

Compared with the traditional dense-graded asphalt mixture, the permeable asphalt mixture belongs to the skeleton-pore structure, and the coarse aggregates were in point-to-point contact state. The strength mainly depended on the bonding performance of asphalt binder, so high-viscosity asphalt binder was necessary to be used. The dynamic viscosity of asphalt at 60°C should be greater than 20,000 Pa·s in both Chinese and Japanese specifications.The high-viscosity asphalt binder produced by Taihe Asphalt Products Co. Ltd. was used. Its technical indexes are shown in Table4.Table 4
Technical indexes of high-viscosity asphalt binder.
Indexes
Unit
10% GH-1
12% GH-1
14% GH-1
Penetration (25°C, 5 s, 100 g)
0.1 mm
53.3
49.5
44.2
Softening point
°C
82.7
89.6
92.1
Dynamic viscosity at 60°C
Pa·s
54790
90147
184318
Brookfield viscosity at 135°C
Pa·s
4.746
4.562
5.671
Brookfield viscosity at 180°C
Pa·s
2.215
2.308
2.771
Elastic recovery
%
95.9
96.6
97.5


## 2.1. Coarse Aggregate

Coarse aggregates crushed with limestone were adopted in the paper, and their technical indexes are shown in Table1 [11].Table 1
Technical indexes of coarse aggregates.
Indexes
Unit
Actual measurement
Crushing value
%
13.5
Apparent relative density
—
—
Water absorption
%
1.2
Strength
%
9.4
Needle and plate particle content
%
8
Content of <0.075 mm material
%
0.43
Adhesion with SBS modified asphalt
Level
5


## 2.2. Fine Aggregate

Machine-made sand with limestone and their technical indexes are shown in Table2 [11].Table 2
Main technical indicators of fine aggregates.
Material particle size (mm)
Apparent relative density
Surface relative density
Gross volume relative density
Water absorption rate (%)
1.18
2.702
2.672
2.654
—
0.6
2.723
2.683
2.661
—
0.3
2.740
2.707
2.688
—
0.15
2.728
2.701
2.689
—
0.075
2.721
2.693
2.676
—


## 2.3. Filler

### 2.3.1. Mineral Powder

Grind with limestone and its main indexes are shown in Table3.Table 3
Results of limestone ore powder test.
Project
Unit
Test results
Requirements
Apparent density
g/cm3
2.716
≥2.50
Size range
<0.6 mm
%
100
100
<0.15 mm
%
94.5
90∼100
<0.075 mm
%
90
75∼100
Water content
%
0.41
≤1
Plasticity index
—
2
<4
Hydrophilic coefficient
—
0.5
<1
Exterior
—
No agglomeration
No agglomeration


### 2.3.2. Slaked Lime

In order to enhance the adhesion between asphalt binder and aggregate, slaked lime was used to replace part of mineral powder (the amount of slaked lime is 3% of the mass of the asphalt mixture) to improve the spalling resistance of the mixture.


### 2.3.3. High-Viscosity Modified Asphalt

Compared with the traditional dense-graded asphalt mixture, the permeable asphalt mixture belongs to the skeleton-pore structure, and the coarse aggregates were in point-to-point contact state. The strength mainly depended on the bonding performance of asphalt binder, so high-viscosity asphalt binder was necessary to be used. The dynamic viscosity of asphalt at 60°C should be greater than 20,000 Pa·s in both Chinese and Japanese specifications.The high-viscosity asphalt binder produced by Taihe Asphalt Products Co. Ltd. was used. Its technical indexes are shown in Table4.Table 4
Technical indexes of high-viscosity asphalt binder.
Indexes
Unit
10% GH-1
12% GH-1
14% GH-1
Penetration (25°C, 5 s, 100 g)
0.1 mm
53.3
49.5
44.2
Softening point
°C
82.7
89.6
92.1
Dynamic viscosity at 60°C
Pa·s
54790
90147
184318
Brookfield viscosity at 135°C
Pa·s
4.746
4.562
5.671
Brookfield viscosity at 180°C
Pa·s
2.215
2.308
2.771
Elastic recovery
%
95.9
96.6
97.5


## 2.3.1. Mineral Powder

Grind with limestone and its main indexes are shown in Table3.Table 3
Results of limestone ore powder test.
Project
Unit
Test results
Requirements
Apparent density
g/cm3
2.716
≥2.50
Size range
<0.6 mm
%
100
100
<0.15 mm
%
94.5
90∼100
<0.075 mm
%
90
75∼100
Water content
%
0.41
≤1
Plasticity index
—
2
<4
Hydrophilic coefficient
—
0.5
<1
Exterior
—
No agglomeration
No agglomeration


## 2.3.2. Slaked Lime

In order to enhance the adhesion between asphalt binder and aggregate, slaked lime was used to replace part of mineral powder (the amount of slaked lime is 3% of the mass of the asphalt mixture) to improve the spalling resistance of the mixture.


## 2.3.3. High-Viscosity Modified Asphalt

Compared with the traditional dense-graded asphalt mixture, the permeable asphalt mixture belongs to the skeleton-pore structure, and the coarse aggregates were in point-to-point contact state. The strength mainly depended on the bonding performance of asphalt binder, so high-viscosity asphalt binder was necessary to be used. The dynamic viscosity of asphalt at 60°C should be greater than 20,000 Pa·s in both Chinese and Japanese specifications.The high-viscosity asphalt binder produced by Taihe Asphalt Products Co. Ltd. was used. Its technical indexes are shown in Table4.Table 4
Technical indexes of high-viscosity asphalt binder.
Indexes
Unit
10% GH-1
12% GH-1
14% GH-1
Penetration (25°C, 5 s, 100 g)
0.1 mm
53.3
49.5
44.2
Softening point
°C
82.7
89.6
92.1
Dynamic viscosity at 60°C
Pa·s
54790
90147
184318
Brookfield viscosity at 135°C
Pa·s
4.746
4.562
5.671
Brookfield viscosity at 180°C
Pa·s
2.215
2.308
2.771
Elastic recovery
%
95.9
96.6
97.5


## 3. Mix Design of Permeable Asphalt Mixture

According to the common structure and material composition of the double-layer permeable asphalt pavement, asphalt mixture, PAC-10, was selected as the upper layer, and its designed porosity was 20%, 22%, and 24%, respectively. PAC-16 with a designed porosity of 22% was used for the lower layer.According to Chinese Technical Specifications for Construction of Highway Asphalt Pavement [17], their gradations of aggregates were prepared, as shown in Table 5.Table 5
Gradations of permeable asphalt mixture.
Number
Mix type
Target porosity (%)
The percentage of mass passing through the following sieve holes (%)
19
16
13.2
9.5
4.75
2.36
1.18
0.6
0.3
0.15
0.075
1
PAC-10
20
100
100
100
90
50
13
7.6
6.5
5.5
5
4
2
22
100
100
100
90
50
11.3
6
4.7
4.7
4.7
4
3
24
100
100
100
90
50
10
6
4.1
3.3
3.3
2.3
4
PAC-16
22
100
90
70
45
12
10
6
4.3
3.8
3.8
3


### 3.1. Determination of the Optimum Asphalt Content

According to Chinese Technical Specifications for Construction of Highway Asphalt Pavement [17], the leakage test and the scattering test are, respectively, used to control the maximum asphalt content and the minimum asphalt content of the mixture, and the requirements of designed porosity, interconnected porosity, and Marshall stability were adopted to determine the optimum asphalt content [18].Assume that the asphalt film thickness,h, was 14 μm, according to the gradations, the calculated initial asphalt contents, Pb, of gradations according to formulas (1) and (2) were 4.6%, 4.4%, 3.2%, and 3.4%, respectively. Marshall specimens were compacted through the compaction method, and the porosity was 19.8%, 21.7%, 24.1%, and 22.6%, respectively. The target porosity was close to the designed, so the gradations listed in Table 5 were the final gradation:(1)A=2+0.02a+0.04b+0.08c+0.14d+0.3e+0.6f+1.6g48.74,(2)Pb=h×A,where A is the total surface area of aggregates and a, b, c, d, e, f, and g are the passing rates of sieve size of 4.75 mm, 2.36 mm, 1.18 mm, 0.6 mm, 0.3 mm, 0.15 mm, and 0.075 mm, respectively.For each gradation, based on the calculated initial asphalt content, five groups of Marshall specimens were molded with different asphalt content. The interval of asphalt content was 0.4%. Then, Marshall test, leakage test, and Kentarou dispersion test were conducted. The variations of leakage of asphalt binder and dispersion of asphalt mixture with asphalt contents are shown in Figure1. The variation of porosity and interconnected porosity with asphalt content are shown in Figure 2.Figure 1
Relationship between leakage, dispersion, and asphalt content: (a) 20% PAC-10, (b) 22% PAC-10, (c) 24% PAC-10, and (d) PAC-16.
(a)
(b)
(c)
(d)Figure 2
Relationship between porosity and asphalt content: (a) 20% PAC-10, (b) 22% PAC-10, (c) 24% PAC-10, and (d) PAC-16.
(a)
(b)
(c)
(d)It can be seen from Figures1 and 2 that the leakage of permeable asphalt mixture increases with the increase of asphalt contents, and the dispersion loss decreases with the increase of asphalt contents. The porosities and interconnected porosities decrease with the increases of asphalt contents. So, according to Chinese Technical Specifications for Asphalt Pavement Construction, the optimum asphalt content for gradation 1 to gradation 4 is 4.4%, 4.2%, 3.2%, and 3.4%, respectively.The performance of the PAC mixtures at the optimum asphalt content is shown in Table6, which meets the requirements.Table 6
Performance of PAC mixtures with optimal asphalt content.
Mixtures
20% PAC-10
22% PAC-10
24% PAC-10
PAC-16 (22%)
Specification
Optimal asphalt content (%)
4.4
4.2
3.2
3.4
—
Porosity (%)
20.3
22.2
24.0
22.3
—
Interconnected porosity (%)
15.2
18.43
20.2
19.5
≥14
Leakage (%)
0.110
0.132
0.113
0.192
<0.3
Dispersion (%)
8.64
8.93
8.75
8.64
<20
Stability (kN)
7.06
5.72
5.17
5.51
≥5It can be seen from Table6 that the designed PAC-10 and PAC-16 mixtures meet the designed requirements and have high interconnected porosities, and all of the performances meet the specifications.


## 3.1. Determination of the Optimum Asphalt Content

According to Chinese Technical Specifications for Construction of Highway Asphalt Pavement [17], the leakage test and the scattering test are, respectively, used to control the maximum asphalt content and the minimum asphalt content of the mixture, and the requirements of designed porosity, interconnected porosity, and Marshall stability were adopted to determine the optimum asphalt content [18].Assume that the asphalt film thickness,h, was 14 μm, according to the gradations, the calculated initial asphalt contents, Pb, of gradations according to formulas (1) and (2) were 4.6%, 4.4%, 3.2%, and 3.4%, respectively. Marshall specimens were compacted through the compaction method, and the porosity was 19.8%, 21.7%, 24.1%, and 22.6%, respectively. The target porosity was close to the designed, so the gradations listed in Table 5 were the final gradation:(1)A=2+0.02a+0.04b+0.08c+0.14d+0.3e+0.6f+1.6g48.74,(2)Pb=h×A,where A is the total surface area of aggregates and a, b, c, d, e, f, and g are the passing rates of sieve size of 4.75 mm, 2.36 mm, 1.18 mm, 0.6 mm, 0.3 mm, 0.15 mm, and 0.075 mm, respectively.For each gradation, based on the calculated initial asphalt content, five groups of Marshall specimens were molded with different asphalt content. The interval of asphalt content was 0.4%. Then, Marshall test, leakage test, and Kentarou dispersion test were conducted. The variations of leakage of asphalt binder and dispersion of asphalt mixture with asphalt contents are shown in Figure1. The variation of porosity and interconnected porosity with asphalt content are shown in Figure 2.Figure 1
Relationship between leakage, dispersion, and asphalt content: (a) 20% PAC-10, (b) 22% PAC-10, (c) 24% PAC-10, and (d) PAC-16.
(a)
(b)
(c)
(d)Figure 2
Relationship between porosity and asphalt content: (a) 20% PAC-10, (b) 22% PAC-10, (c) 24% PAC-10, and (d) PAC-16.
(a)
(b)
(c)
(d)It can be seen from Figures1 and 2 that the leakage of permeable asphalt mixture increases with the increase of asphalt contents, and the dispersion loss decreases with the increase of asphalt contents. The porosities and interconnected porosities decrease with the increases of asphalt contents. So, according to Chinese Technical Specifications for Asphalt Pavement Construction, the optimum asphalt content for gradation 1 to gradation 4 is 4.4%, 4.2%, 3.2%, and 3.4%, respectively.The performance of the PAC mixtures at the optimum asphalt content is shown in Table6, which meets the requirements.Table 6
Performance of PAC mixtures with optimal asphalt content.
Mixtures
20% PAC-10
22% PAC-10
24% PAC-10
PAC-16 (22%)
Specification
Optimal asphalt content (%)
4.4
4.2
3.2
3.4
—
Porosity (%)
20.3
22.2
24.0
22.3
—
Interconnected porosity (%)
15.2
18.43
20.2
19.5
≥14
Leakage (%)
0.110
0.132
0.113
0.192
<0.3
Dispersion (%)
8.64
8.93
8.75
8.64
<20
Stability (kN)
7.06
5.72
5.17
5.51
≥5It can be seen from Table6 that the designed PAC-10 and PAC-16 mixtures meet the designed requirements and have high interconnected porosities, and all of the performances meet the specifications.


## 4. Permeability of Double-Layer Compound Specimen

### 4.1. Method for Preparing Double-Layer Compound Specimen

There is no specific preparation method for the double-layer Marshall specimen in the current specification. The “hot + hot” preparing method was utilized in this paper. The overall height of the double-layer specimen was 63.5 mm, and the height of the upper layer (PAC-10) and the lower layer (PAC-16) was determined to be 23.5 mm and 40 mm, respectively. The masses of the upper layer and lower layer were calculated according to the density of PAC-10 and PAC-16. Mixing and pouring the PAC-16 mixture into the mold, tamping and leveling its surface, immediately putting it into the oven at 175°C, then mixing PAC-13 mixture, pulling out the mold and filling PAC-13 mixture into it, tamping and leveling the surface, and compacting the surface for 50 times. So, a double-layer compound Marshall specimen was obtained.


### 4.2. Analysis of Pore Characteristics Based on CT Technology

The computed tomography (CT) scanning technique was used to detect nondestructively the internal structure of the compound specimen. The X-ray CT scanning system is shown in Figure3.Figure 3
X-ray CT scanning system.


#### 4.2.1. Scanning Sections

The double-layer specimens were X-ray CT scanned to obtain the characteristics and distribution of internal pores. Because the main purpose of this paper was to study the top-to-bottom pore characteristics relating to permeability and antiblocking, two orthogonal vertical sections of each specimen were scanned, as shown in Figure4.Figure 4
Scanned sections of the specimen.


#### 4.2.2. Analysis of Scanning Results

CT images of the two orthogonal vertical sections of each compound specimen are shown in Figure5.Figure 5
CT images of compound specimens: (a) (20% + 22%)-1#, (b) (20% + 22%)-2#, (c) (22% + 22%)-1#, (d) (22% + 22%)-2#, (e) (24% + 22%)-1#, and (f) (24% + 22%)-2#.
(a)
(b)
(c)
(d)
(e)
(f)The CT images were analyzed, and the pore characteristics were statisticised with software ImageJ-pro. The statistical pore characteristics for each section are shown in Table7.Table 7
Statistical pore characteristics in composite specimens.
Specimen type
Porosity (%)
Number of pores (a)
Total area of pores (mm2)
Average single pore area (mm2)
Equivalent diameter (mm)
Quantity ratio of pore area less than 0.09 mm2
(20% + 22%)-1
19.82
618
1290.0025
4.06
2.27
15.0
(20% + 22%)-2
12.26
526
782.1613
2.4
1.74
20.0
Average value
16.04
572
1036.0819
3.23
2.005
17.5
(22% + 22%)-1
16.64
619
1046.7975
1.69
1.46
48.7
(22% + 22%)-2
21.43
1049
1530.9850
1.46
1.36
30.3
Average value
19.035
818
1288.8913
1.575
1.41
39.5
(24% + 22%)-1
24.99
842
1785.3025
2.12
1.64
18.9
(24% + 22%)-2
21.59
1282
1537.8113
1.20
1.24
31.3
Average value
23.29
1001
1661.5569
1.66
1.44
25.1It can be found from Table7 that the statistical parameters of pores, such as the size, quantity, distribution, porosity, and connected porosity of the specimens, are related to the scanned section. There are large differences in different sections of the same specimen. The distribution of pores is random. The larger the designed void ratio is, the relatively larger the number, the area, the equivalent diameter, and the connected porosity are. Therefore, it is conducive to drainage.


## 4.1. Method for Preparing Double-Layer Compound Specimen

There is no specific preparation method for the double-layer Marshall specimen in the current specification. The “hot + hot” preparing method was utilized in this paper. The overall height of the double-layer specimen was 63.5 mm, and the height of the upper layer (PAC-10) and the lower layer (PAC-16) was determined to be 23.5 mm and 40 mm, respectively. The masses of the upper layer and lower layer were calculated according to the density of PAC-10 and PAC-16. Mixing and pouring the PAC-16 mixture into the mold, tamping and leveling its surface, immediately putting it into the oven at 175°C, then mixing PAC-13 mixture, pulling out the mold and filling PAC-13 mixture into it, tamping and leveling the surface, and compacting the surface for 50 times. So, a double-layer compound Marshall specimen was obtained.


## 4.2. Analysis of Pore Characteristics Based on CT Technology

The computed tomography (CT) scanning technique was used to detect nondestructively the internal structure of the compound specimen. The X-ray CT scanning system is shown in Figure3.Figure 3
X-ray CT scanning system.


### 4.2.1. Scanning Sections

The double-layer specimens were X-ray CT scanned to obtain the characteristics and distribution of internal pores. Because the main purpose of this paper was to study the top-to-bottom pore characteristics relating to permeability and antiblocking, two orthogonal vertical sections of each specimen were scanned, as shown in Figure4.Figure 4
Scanned sections of the specimen.


### 4.2.2. Analysis of Scanning Results

CT images of the two orthogonal vertical sections of each compound specimen are shown in Figure5.Figure 5
CT images of compound specimens: (a) (20% + 22%)-1#, (b) (20% + 22%)-2#, (c) (22% + 22%)-1#, (d) (22% + 22%)-2#, (e) (24% + 22%)-1#, and (f) (24% + 22%)-2#.
(a)
(b)
(c)
(d)
(e)
(f)The CT images were analyzed, and the pore characteristics were statisticised with software ImageJ-pro. The statistical pore characteristics for each section are shown in Table7.Table 7
Statistical pore characteristics in composite specimens.
Specimen type
Porosity (%)
Number of pores (a)
Total area of pores (mm2)
Average single pore area (mm2)
Equivalent diameter (mm)
Quantity ratio of pore area less than 0.09 mm2
(20% + 22%)-1
19.82
618
1290.0025
4.06
2.27
15.0
(20% + 22%)-2
12.26
526
782.1613
2.4
1.74
20.0
Average value
16.04
572
1036.0819
3.23
2.005
17.5
(22% + 22%)-1
16.64
619
1046.7975
1.69
1.46
48.7
(22% + 22%)-2
21.43
1049
1530.9850
1.46
1.36
30.3
Average value
19.035
818
1288.8913
1.575
1.41
39.5
(24% + 22%)-1
24.99
842
1785.3025
2.12
1.64
18.9
(24% + 22%)-2
21.59
1282
1537.8113
1.20
1.24
31.3
Average value
23.29
1001
1661.5569
1.66
1.44
25.1It can be found from Table7 that the statistical parameters of pores, such as the size, quantity, distribution, porosity, and connected porosity of the specimens, are related to the scanned section. There are large differences in different sections of the same specimen. The distribution of pores is random. The larger the designed void ratio is, the relatively larger the number, the area, the equivalent diameter, and the connected porosity are. Therefore, it is conducive to drainage.


## 4.2.1. Scanning Sections

The double-layer specimens were X-ray CT scanned to obtain the characteristics and distribution of internal pores. Because the main purpose of this paper was to study the top-to-bottom pore characteristics relating to permeability and antiblocking, two orthogonal vertical sections of each specimen were scanned, as shown in Figure4.Figure 4
Scanned sections of the specimen.


## 4.2.2. Analysis of Scanning Results

CT images of the two orthogonal vertical sections of each compound specimen are shown in Figure5.Figure 5
CT images of compound specimens: (a) (20% + 22%)-1#, (b) (20% + 22%)-2#, (c) (22% + 22%)-1#, (d) (22% + 22%)-2#, (e) (24% + 22%)-1#, and (f) (24% + 22%)-2#.
(a)
(b)
(c)
(d)
(e)
(f)The CT images were analyzed, and the pore characteristics were statisticised with software ImageJ-pro. The statistical pore characteristics for each section are shown in Table7.Table 7
Statistical pore characteristics in composite specimens.
Specimen type
Porosity (%)
Number of pores (a)
Total area of pores (mm2)
Average single pore area (mm2)
Equivalent diameter (mm)
Quantity ratio of pore area less than 0.09 mm2
(20% + 22%)-1
19.82
618
1290.0025
4.06
2.27
15.0
(20% + 22%)-2
12.26
526
782.1613
2.4
1.74
20.0
Average value
16.04
572
1036.0819
3.23
2.005
17.5
(22% + 22%)-1
16.64
619
1046.7975
1.69
1.46
48.7
(22% + 22%)-2
21.43
1049
1530.9850
1.46
1.36
30.3
Average value
19.035
818
1288.8913
1.575
1.41
39.5
(24% + 22%)-1
24.99
842
1785.3025
2.12
1.64
18.9
(24% + 22%)-2
21.59
1282
1537.8113
1.20
1.24
31.3
Average value
23.29
1001
1661.5569
1.66
1.44
25.1It can be found from Table7 that the statistical parameters of pores, such as the size, quantity, distribution, porosity, and connected porosity of the specimens, are related to the scanned section. There are large differences in different sections of the same specimen. The distribution of pores is random. The larger the designed void ratio is, the relatively larger the number, the area, the equivalent diameter, and the connected porosity are. Therefore, it is conducive to drainage.


## 5. Study on Permeability of Double-Layer Compound Specimen

For the single-layer specimen and the double-layer composite specimen (their heights are the same, 63.5 mm), the vertical permeability coefficient is tested on the specimen before being demolded. The test results are shown in Table8.Table 8
Vertical permeability coefficient of specimen.
Type of specimen
Permeability (ml/s)
20% PAC-10 + 22% PAC-16
77.318
22% PAC-10 + 22% PAC-16
88.082
24% PAC-10 + 22% PAC-16
106.763
20% PAC-10
46.926
22% PAC-10
54.146
24% PAC-10
61.289
22% PAC-16
111.896It can be seen from Table8 and Figure 6 that (1) when the porosity of the lower mixture is constant, the permeability of the double-layer compound specimen increases with the increase of the porosity of the upper layer mixture, showing a good linear relationship and (2) the drainage capacity of the double-layer compound specimen is larger than that of the single-layer specimen. This is because their total thicknesses are the same, and the pore characteristics of the whole single-layer specimen are the same, while the lower part of the double-layer specimen has a larger porosity, so that it is easier to drain.Figure 6
Relationship between permeability of compound specimen and porosity of the upper layer mixture.


## 6. Blocking Characteristics of Double-Layer Compound Specimen

### 6.1. Blocking Material

Dust, chips, tire abrasives, grease, etc., will be brought into the internal pores in permeable asphalt mixture under the action of driving and water flow, forming a blockage. In order to study the blocking characteristics of permeable asphalt mixtures, it is necessary to select suitable blocking materials to simulate the blockage at the site. Hamzah et al. used an aqueous solution of clay and sand as the blocking material. Both the clay and the sand were graded, respectively, and the concentration was 1.5 g/L [12]. Jung et al. composed blocking fine materials with machine-made sands with a particle size of 2.36 mm to 0.075 mm [13].In this paper, machine-made sands with different single particle sizes (0.075 mm∼1.18 mm) were selected as different blocking materials.


### 6.2. Blockage Test Scheme

The blocking materials were divided into 5 grams each in advance. The first blocking material was spreaded evenly on the surface of the specimen, and the specimen was gently vibrated for 3 s, then 200 ml of water was slowly and evenly poured, so that the blocking material can infiltrate into the pores. The seepage coefficient after the first blocking was tested. The above operations were repeated for five times.


### 6.3. Results of Blocking Test

The vertical permeability coefficients of different combinations of the double-layer compound specimen, (20%, 22%, and 24%) PAC-10 + 22% PAC-16, and their variations with the blocking times are shown in Figure7.Figure 7
Relationship between vertical permeability coefficient and blocking times: (a) 20% PAC-10 + 22% PAC-13, (b) 22% PAC-10 + 22% PAC-13, and (c) 24% PAC-10 + 22% PAC-13.
(a)
(b)
(c)These can be found from Figure7 the following:(1)
The vertical permeability coefficient of double-layer compound specimen has a good correlation with the blocking times. With the increase of the blocking times, the vertical permeability coefficient decreases continuously.(2)
The decrease of vertical permeability coefficient of double-layer specimens is the fastest when the blocking material with particle size of 0.15 mm is used.(3)
The key particle size blocking the pore in PAC-10 + PAC-16 double-layer compound specimen is 0.15 mm∼0.3 mm, followed by 0.3 mm∼0.6 mm. For blocking materials of smaller than 0.075 mm, most of them can flow out of the specimen through the interconnected pores under the pressure of water and/or air flow.(4)
The vertical permeability coefficients before and after blocking increase with the increase of the porosity of the upper layer mixture, and they have a good linear correlationship.


### 6.4. Comparison of Blocking Resistance between Double-Layer Compound Specimen (PAC-10 + PAC-16) and Single-Layer Specimen (PAC-10)

The blocking test was conducted on single-layer (PAC-10) specimens (thickness is 63.5 mm) with blocking material, and its particle size is 0.15 mm. The results are shown in Table9.Table 9
Blocking results of single-layer (PAC-10) specimen.
Particle size of blocking materials (mm)
Voidage of specimen (%)
Vertical seepage coefficient
Initial permeability coefficient
After blocked 1 time
After blocked 2 times
After blocked 3 times
After blocked 4 times
After blocked 5 times
After cleaning
0.15
20
0.579
0.251
0.151
0.090
0.067
0.051
0.143
22
0.668
0.314
0.207
0.129
0.100
0.070
0.217
24
0.756
0.430
0.299
0.220
0.158
0.107
0.278It can be found from Figure8 that the vertical permeability coefficient of the single-layer specimen also increases with the increase of the porosity, which is consistent with the variation trend of the double-layer compound specimen. Under the same number of blocking times, the vertical permeability coefficient of double-layer compound specimen, PAC-10 + PAC-16, is significantly larger than the vertical permeability coefficient of single-layer specimen, PAC-10, which is consistent with the porosity of the upper layer. This indicates that, within a certain range, the permeability of the compound specimen is greater than that of the single-layer specimen with the same thickness.Figure 8
Comparison of blocking effect of double-layer and single-layer specimens.


## 6.1. Blocking Material

Dust, chips, tire abrasives, grease, etc., will be brought into the internal pores in permeable asphalt mixture under the action of driving and water flow, forming a blockage. In order to study the blocking characteristics of permeable asphalt mixtures, it is necessary to select suitable blocking materials to simulate the blockage at the site. Hamzah et al. used an aqueous solution of clay and sand as the blocking material. Both the clay and the sand were graded, respectively, and the concentration was 1.5 g/L [12]. Jung et al. composed blocking fine materials with machine-made sands with a particle size of 2.36 mm to 0.075 mm [13].In this paper, machine-made sands with different single particle sizes (0.075 mm∼1.18 mm) were selected as different blocking materials.


## 6.2. Blockage Test Scheme

The blocking materials were divided into 5 grams each in advance. The first blocking material was spreaded evenly on the surface of the specimen, and the specimen was gently vibrated for 3 s, then 200 ml of water was slowly and evenly poured, so that the blocking material can infiltrate into the pores. The seepage coefficient after the first blocking was tested. The above operations were repeated for five times.


## 6.3. Results of Blocking Test

The vertical permeability coefficients of different combinations of the double-layer compound specimen, (20%, 22%, and 24%) PAC-10 + 22% PAC-16, and their variations with the blocking times are shown in Figure7.Figure 7
Relationship between vertical permeability coefficient and blocking times: (a) 20% PAC-10 + 22% PAC-13, (b) 22% PAC-10 + 22% PAC-13, and (c) 24% PAC-10 + 22% PAC-13.
(a)
(b)
(c)These can be found from Figure7 the following:(1)
The vertical permeability coefficient of double-layer compound specimen has a good correlation with the blocking times. With the increase of the blocking times, the vertical permeability coefficient decreases continuously.(2)
The decrease of vertical permeability coefficient of double-layer specimens is the fastest when the blocking material with particle size of 0.15 mm is used.(3)
The key particle size blocking the pore in PAC-10 + PAC-16 double-layer compound specimen is 0.15 mm∼0.3 mm, followed by 0.3 mm∼0.6 mm. For blocking materials of smaller than 0.075 mm, most of them can flow out of the specimen through the interconnected pores under the pressure of water and/or air flow.(4)
The vertical permeability coefficients before and after blocking increase with the increase of the porosity of the upper layer mixture, and they have a good linear correlationship.


## 6.4. Comparison of Blocking Resistance between Double-Layer Compound Specimen (PAC-10 + PAC-16) and Single-Layer Specimen (PAC-10)

The blocking test was conducted on single-layer (PAC-10) specimens (thickness is 63.5 mm) with blocking material, and its particle size is 0.15 mm. The results are shown in Table9.Table 9
Blocking results of single-layer (PAC-10) specimen.
Particle size of blocking materials (mm)
Voidage of specimen (%)
Vertical seepage coefficient
Initial permeability coefficient
After blocked 1 time
After blocked 2 times
After blocked 3 times
After blocked 4 times
After blocked 5 times
After cleaning
0.15
20
0.579
0.251
0.151
0.090
0.067
0.051
0.143
22
0.668
0.314
0.207
0.129
0.100
0.070
0.217
24
0.756
0.430
0.299
0.220
0.158
0.107
0.278It can be found from Figure8 that the vertical permeability coefficient of the single-layer specimen also increases with the increase of the porosity, which is consistent with the variation trend of the double-layer compound specimen. Under the same number of blocking times, the vertical permeability coefficient of double-layer compound specimen, PAC-10 + PAC-16, is significantly larger than the vertical permeability coefficient of single-layer specimen, PAC-10, which is consistent with the porosity of the upper layer. This indicates that, within a certain range, the permeability of the compound specimen is greater than that of the single-layer specimen with the same thickness.Figure 8
Comparison of blocking effect of double-layer and single-layer specimens.


## 7. Conclusions

(1)
Through the permeable test, it is found that the permeability coefficient of PAC-16 asphalt mixture with a designed porosity 22% is greater than that of PAC-10 asphalt mixture with the designed porosity 20% to 24%.
(2)
Under certain thickness, the permeability of the double-layer compound specimen increases linearly with the increase of the porosity of the upper layer mixture.
(3)
The vertical permeability coefficient of the double-layer compound specimen decreases with the increase of the number of blocking times. Both of the vertical permeability coefficients before and after blocking increase line with the increase of the porosity of the upper layer mixture.
(4)
The key size of blocking material particles is 0.15 mm, and the secondary particle size is 0.3 mm. It is not easy to be washed out after they enter into the pores of the specimens. For blockage materials which particle size is or smaller than 0.075 mm, most of them can flow out of the specimen through the connected porosity under the pressure of water and/or air flow.
(5)
Before and after blocking, the vertical permeability coefficient of the double-layer compound specimen (PAC-10 + PAC-16) is significantly larger than the single-layer specimen (PAC-10) whose porosity is the same as that of the upper layer mixture of the compound specimen. The permeable capacity and blocking resistance of the double-layer permeable asphalt mixture is better than that of the single-layer permeable asphalt mixture.



---

*Source: 7914816-2019-05-29.xml*