# Effects of Continuous Laydown and Compaction on Interlayer Shear Bonding of Asphalt Layers

Authors: Haitao Zhang; Mingyang Gong; Jian Wu; Quansheng Sun
Journal: Advances in Materials Science and Engineering
 (2019)

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## Abstract

Traditional discontinuous paving technology of asphalt pavement is to pave and compact with two different gradation asphalt mixtures in the upper and lower layers, respectively, so the interlayer contact state of asphalt mixture is the major issue of pavement structure. Meanwhile, the continuous paving technology is to pave and compact with two different gradation asphalt mixtures in the upper and lower layers simultaneously, which can solve the issue of interlayer contact appropriately. In order to contrast the shear performance of the structural layer based on continuous and discontinuous paving technology, in this project, through simulating site construction, the double-deck Marshall and rut specimens are prepared based on two different gradation asphalt mixtures simultaneously, and the mechanical and interlayer shear performances of asphalt mixtures under continuous and discontinuous paving technology are tested at room temperature, low temperature, and freeze-thaw. The test results show that the mechanical and interlayer shear performance of continuous paving asphalt mixtures is better than that of discontinuous paving asphalt mixture. The findings can provide a certain technical basis for the design of continuous paving asphalt pavement.


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## Body

## 1. Introduction

Traditional asphalt pavement with discontinuous paving and compact technology, due to the fact that the asphalt mixture in each layer has long spaced time, the thickness of the layer is thinner, the temperature of the mixture is lost faster, and compaction time is short and effective, leads to the structure of asphalt mixture being mutually independent and lack of interlayer connectedness and integrity. Asphalt pavement based on continuous paving technology is a new technique for asphalt pavement construction, which is to pave and compact with two different gradation asphalt mixtures in the upper and lower layers simultaneously and make it not only become a coordinated integrity but also have high construction efficiency [1, 2]. At the same time, it can also save material costs, improve the interface state between layers, and improve the durability of asphalt pavement.The quality of interlayer bonding in asphalt pavement is one of the important factors that influence the service times of asphalt pavement, especially on the sections of large longitudinal, small radius, acceleration, and braking, where pavement structure will cause a larger horizontal load. When the loads are repeated more than a certain number of times, the shear stress generated in the intrinsic of asphalt pavement structure exceeds the shear strength of asphalt mixture, which can lead to fatigue damage and shearing damage in asphalt pavement [3–6]. In the study of the shear resistance of asphalt mixture, Wang [7] used the shear test to compare the shear strength of the double layer paving and the traditional paving core sampling specimens and the laboratory specimens. The results show that the double layer paving can significantly improve the bonding energy between the asphalt pavement layers. Raab [8] systematically summarized the interlaminar shear test method, systematically discussed the factors that may affect the bond strength between layers, and determined the standard evaluation method and the test idea of the interlayer bond strength. Wu [9] determined that the size of interlaminar shear strength of asphalt pavement is related to the test temperature, test speed, stress applicable to the interface, and the age of the sample. Zhang [10] holds that the interlayer shear strength increased with the decreased test temperature, increased traffic load (within design limit), and increased test confinement pressure. Santagata et al. [11] hold that the void fraction and contact roughness are the main factors that affect the pavement bond performance. Under the different vertical pressure, oil content, temperature, and different loading rates, Guan et al. [12] tested the shear modulus of three different material combinations by using the self-designed direct shear test. Under different temperatures and different levels of oil spraying, Fu et al. [13] used oblique shear apparatus to determine the shear strength of specimens. It is found that the interlaminar shear strength decreases with high temperature, and the appropriate layer of oil spray can increase the shear strength between layers.Therefore, in order to contrast the shear performance of the structural layer based on continuous and discontinuous paving technology, through the preparation of double-deck Marshall and rut specimens with two different gradation asphalt mixtures simultaneously, this study is to test the mechanical and interlayer shear performance of asphalt mixture based on continuous and discontinuous paving technology in three different test conditions (room temperature, low temperature, and freeze-thaw conditions). The findings can provide a certain technical basis for the design of continuous paving asphalt pavement.


## 2. Material Design

### 2.1. Asphalt

The virgin asphalt 90# (25°C penetration is about 80–100/0.1 mm) was selected in this test, and the indexes of the asphalt are shown in Table1 [14].Table 1
Performance index of virgin asphalt (90#).
Indexes
Unit
Standards
Measured value
Test methods
Penetration (25°C, 5 s, 100 g)
0.1 mm
80∼100
93
T0604
PI
−1.5∼+1.0
−0.77
T0604
Softening point (T&B)
°C
43
44.5
T0606
Ductility (10°C)
cm
45
90.7
T0605
Ductility (15°C)
cm
100
>100.0
T0605
Dynamic viscosity (60°C)
Pa.s
160
198
T0620


### 2.2. Aggregates

The three kinds of dense-graded asphalt concrete are applied in test, which are AC13, AC16, and AC20, respectively [15, 16]. Two types of specimens are designed, which are the structure of AC13/AC16 and AC16/AC20 (shown in Figure 1). Test results of the aggregates are shown in Table 2, and the aggregate gradation for three kinds of asphalt mixture is shown in Table 3.Figure 1
Structure of specimen.Table 2
Technical indexes of aggregate.
Indexes
Measured value
Standards
Test methods
Coarse aggregate
Los Angeles wear value (%)
24
≤28
T0317
Crushing value (%)
12.3
≤28
T0316
Asphalt adhesion
5
>grade 4
T0616
Percentage of lamellar carpolite (%)
14.7
≤15.0
T0312
Mud content (%)
0.4
≤1
T0333
Water absorption ability (%)
0.4
≤2.0
T0307
Fine aggregate
Mud content (%)
1.9
≤3
T0333
Sand equivalent (%)
68
≥60
T0334
Mineral powders
Water content (%)
0.5
≤1
T0332
Hydraulic coefficient
0.6
≤1
T0353Table 3
Gradation composition of aggregates.
Percentage of passing sieve (%)
Sieve size (mm)
0.075
0.15
0.3
0.6
1.18
2.36
4.75
9.5
13.2
16
19
26.5
AC13/
6
10
13.5
19
26.5
37
53
76.5
95
100
—
—
AC16/
6
9.5
12.5
17.5
24.5
34
48
70
84
95
100
—
AC20/
6
10
15
21
27
37
48
62
71
82.5
95
100


### 2.3. Proportion Design of Mixture

Through the proportion design of three kinds of asphalt mixtures according to the Marshall method, the optimum asphalt contents are, respectively, 5.4% for AC13, 5% for AC16, and 4.4% for AC20. The volumetric properties of the above asphalt specimens are shown in Table4.Table 4
Volumetric properties of single layer mixture specimens.
OAC (%)
Gmb (bulk density)/(g/cm3)
Gmm (maximum density)/(g/cm3)
V (%)
VMA (%)
VFA (%)
AC 13
5.4
2.364
2.530
6.576
13.914
52.739
AC 16
5.0
2.359
2.538
7.053
13.846
49.062
AC 20
4.4
2.358
2.547
7.432
13.422
44.630


## 2.1. Asphalt

The virgin asphalt 90# (25°C penetration is about 80–100/0.1 mm) was selected in this test, and the indexes of the asphalt are shown in Table1 [14].Table 1
Performance index of virgin asphalt (90#).
Indexes
Unit
Standards
Measured value
Test methods
Penetration (25°C, 5 s, 100 g)
0.1 mm
80∼100
93
T0604
PI
−1.5∼+1.0
−0.77
T0604
Softening point (T&B)
°C
43
44.5
T0606
Ductility (10°C)
cm
45
90.7
T0605
Ductility (15°C)
cm
100
>100.0
T0605
Dynamic viscosity (60°C)
Pa.s
160
198
T0620


## 2.2. Aggregates

The three kinds of dense-graded asphalt concrete are applied in test, which are AC13, AC16, and AC20, respectively [15, 16]. Two types of specimens are designed, which are the structure of AC13/AC16 and AC16/AC20 (shown in Figure 1). Test results of the aggregates are shown in Table 2, and the aggregate gradation for three kinds of asphalt mixture is shown in Table 3.Figure 1
Structure of specimen.Table 2
Technical indexes of aggregate.
Indexes
Measured value
Standards
Test methods
Coarse aggregate
Los Angeles wear value (%)
24
≤28
T0317
Crushing value (%)
12.3
≤28
T0316
Asphalt adhesion
5
>grade 4
T0616
Percentage of lamellar carpolite (%)
14.7
≤15.0
T0312
Mud content (%)
0.4
≤1
T0333
Water absorption ability (%)
0.4
≤2.0
T0307
Fine aggregate
Mud content (%)
1.9
≤3
T0333
Sand equivalent (%)
68
≥60
T0334
Mineral powders
Water content (%)
0.5
≤1
T0332
Hydraulic coefficient
0.6
≤1
T0353Table 3
Gradation composition of aggregates.
Percentage of passing sieve (%)
Sieve size (mm)
0.075
0.15
0.3
0.6
1.18
2.36
4.75
9.5
13.2
16
19
26.5
AC13/
6
10
13.5
19
26.5
37
53
76.5
95
100
—
—
AC16/
6
9.5
12.5
17.5
24.5
34
48
70
84
95
100
—
AC20/
6
10
15
21
27
37
48
62
71
82.5
95
100


## 2.3. Proportion Design of Mixture

Through the proportion design of three kinds of asphalt mixtures according to the Marshall method, the optimum asphalt contents are, respectively, 5.4% for AC13, 5% for AC16, and 4.4% for AC20. The volumetric properties of the above asphalt specimens are shown in Table4.Table 4
Volumetric properties of single layer mixture specimens.
OAC (%)
Gmb (bulk density)/(g/cm3)
Gmm (maximum density)/(g/cm3)
V (%)
VMA (%)
VFA (%)
AC 13
5.4
2.364
2.530
6.576
13.914
52.739
AC 16
5.0
2.359
2.538
7.053
13.846
49.062
AC 20
4.4
2.358
2.547
7.432
13.422
44.630


## 3. Preparation of Specimen and Test Methods

### 3.1. Preparation of Specimen

The diameter of Marshall specimen is 101.6 mm and the height is 63.5 mm. The diameter of the designed double-deck Marshall specimen is invariant and the height is 100 mm (upper layer is 50 mm; lower layer is 50 mm). The double-deck Marshall specimen is prepared based on Marshall compaction method. The process of making double-deck Marshall specimens based on continuous and discontinuous paving technology is shown in Figure2. The making method of discontinuous paving specimen is to simulate site construction, which is to compact the asphalt mixture in lower layer firstly and cool it down to room temperature, and then compact the asphalt mixture in upper layer. In order to simulate compaction method in construction site, the continuous paving specimen is compacted for 150 s at one side (do not flip the specimen in the compaction process). Meanwhile the discontinuous paving specimen is to be compacted for 75 s firstly at lower layer and then for 75 s at upper layer (do not flip the specimen in the compaction process). Therefore, both compaction numbers of the two type specimen are 2 × 75 s [17, 18].Figure 2
Mixture of single layer and double layer. (a) Single layer. (b) Double layer.
(a)
(b)The double-deck Marshall specimens based on continuous and discontinuous paving technology are shown in Figure3. The rut test specimen and beam specimen are shown in Figure 4.Figure 3
Discontinuous and continuous Marshall specimens. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)Figure 4
Rut and beam specimens. (a) Rut specimen (single-deck). (b) Rut specimen (double-deck). (c) Beam specimen.
(a)
(b)
(c)


### 3.2. Test Methods

#### 3.2.1. Tests of Mechanical Performance

(1) Rut Test. Rut test specimens (300 × 300 × 50 mm) are made by wheel rolling method. The load wheel pressure of the asphalt mixture rut test equipment is 0.7 ± 0.05 MPa, and the round rolling speed is 42 times/min. The dynamic stability of specimens is tested at 60°C. The specimen and test equipment are shown in Figure 5.Figure 5
Rut test.(2) Bending Test. The cutting technique is used to make the asphalt mixture beams based on the double-deck rut specimen which are sized at 250 × 60 × 70 mm. The loading rate of the test is at 50 mm/min and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 6.Figure 6
Bending test.(3) Split Test. The test adopted the double-deck Marshall specimen and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 7.Figure 7
Split test.


#### 3.2.2. Adhesion Tests

(1) Interlayer Adhesion Test. The adhesion strength of interlayer between two mixtures is mainly investigated in the test and not the adhesion strength in the mixture itself. Therefore, the project developed a modified interlayer adhesion test for evaluating the characteristics of the interlayer to carry out the comparative study on interlayer adhesion performance of asphalt pavement based on continuous and discontinuous paving technology. The interlayer adhesion test is to rotate the specimen for 90 degrees from the original splitting direction, the loading rate of the test is at 1 mm/min, and the test temperatures are at room temperature, low temperature, and freeze-thaw condition, respectively. Working state of the split shear test is shown in Figure 8.Figure 8
Working state of interlayer adhesion test. (a) Stress diagram of interlayer adhesion test. (b) Working state of interlayer adhesion test.
(a)
(b)According to the definition of shear strength, the modified split shear strength is calculated by equation (1).(1)τ=FsAs=4FmaxπD2,where τ is split shear strength (MPa); FS is shear force (N); AS is shear area (mm2); Fmax is maximum test force (N); and D is specimen diameter (mm), 101.6 mm.(2) Direct Shear Test. In order to reflect the actual stress conditions of the interlayer under vehicle load, the direct shear test was carried out on self-developed shear machine in laboratory (shown in Figure 9). The loading rate of the test is at 1 mm/min. The specimen is subjected to the stress which is perpendicular to interlayer and the shear strength is parallel to the interlayer during the load process. The test indexes are the same as the split shear test and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively.Figure 9
Working state of direct shear test. (a) Stress diagram of direct shear test. (b) Working state of direct shear test.
(a)
(b)The shear strength of direct shear test is calculated by equations (1) and (2).(2)Fs=Fsin45°,where F is failure load (N).


## 3.1. Preparation of Specimen

The diameter of Marshall specimen is 101.6 mm and the height is 63.5 mm. The diameter of the designed double-deck Marshall specimen is invariant and the height is 100 mm (upper layer is 50 mm; lower layer is 50 mm). The double-deck Marshall specimen is prepared based on Marshall compaction method. The process of making double-deck Marshall specimens based on continuous and discontinuous paving technology is shown in Figure2. The making method of discontinuous paving specimen is to simulate site construction, which is to compact the asphalt mixture in lower layer firstly and cool it down to room temperature, and then compact the asphalt mixture in upper layer. In order to simulate compaction method in construction site, the continuous paving specimen is compacted for 150 s at one side (do not flip the specimen in the compaction process). Meanwhile the discontinuous paving specimen is to be compacted for 75 s firstly at lower layer and then for 75 s at upper layer (do not flip the specimen in the compaction process). Therefore, both compaction numbers of the two type specimen are 2 × 75 s [17, 18].Figure 2
Mixture of single layer and double layer. (a) Single layer. (b) Double layer.
(a)
(b)The double-deck Marshall specimens based on continuous and discontinuous paving technology are shown in Figure3. The rut test specimen and beam specimen are shown in Figure 4.Figure 3
Discontinuous and continuous Marshall specimens. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)Figure 4
Rut and beam specimens. (a) Rut specimen (single-deck). (b) Rut specimen (double-deck). (c) Beam specimen.
(a)
(b)
(c)


## 3.2. Test Methods

### 3.2.1. Tests of Mechanical Performance

(1) Rut Test. Rut test specimens (300 × 300 × 50 mm) are made by wheel rolling method. The load wheel pressure of the asphalt mixture rut test equipment is 0.7 ± 0.05 MPa, and the round rolling speed is 42 times/min. The dynamic stability of specimens is tested at 60°C. The specimen and test equipment are shown in Figure 5.Figure 5
Rut test.(2) Bending Test. The cutting technique is used to make the asphalt mixture beams based on the double-deck rut specimen which are sized at 250 × 60 × 70 mm. The loading rate of the test is at 50 mm/min and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 6.Figure 6
Bending test.(3) Split Test. The test adopted the double-deck Marshall specimen and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 7.Figure 7
Split test.


### 3.2.2. Adhesion Tests

(1) Interlayer Adhesion Test. The adhesion strength of interlayer between two mixtures is mainly investigated in the test and not the adhesion strength in the mixture itself. Therefore, the project developed a modified interlayer adhesion test for evaluating the characteristics of the interlayer to carry out the comparative study on interlayer adhesion performance of asphalt pavement based on continuous and discontinuous paving technology. The interlayer adhesion test is to rotate the specimen for 90 degrees from the original splitting direction, the loading rate of the test is at 1 mm/min, and the test temperatures are at room temperature, low temperature, and freeze-thaw condition, respectively. Working state of the split shear test is shown in Figure 8.Figure 8
Working state of interlayer adhesion test. (a) Stress diagram of interlayer adhesion test. (b) Working state of interlayer adhesion test.
(a)
(b)According to the definition of shear strength, the modified split shear strength is calculated by equation (1).(1)τ=FsAs=4FmaxπD2,where τ is split shear strength (MPa); FS is shear force (N); AS is shear area (mm2); Fmax is maximum test force (N); and D is specimen diameter (mm), 101.6 mm.(2) Direct Shear Test. In order to reflect the actual stress conditions of the interlayer under vehicle load, the direct shear test was carried out on self-developed shear machine in laboratory (shown in Figure 9). The loading rate of the test is at 1 mm/min. The specimen is subjected to the stress which is perpendicular to interlayer and the shear strength is parallel to the interlayer during the load process. The test indexes are the same as the split shear test and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively.Figure 9
Working state of direct shear test. (a) Stress diagram of direct shear test. (b) Working state of direct shear test.
(a)
(b)The shear strength of direct shear test is calculated by equations (1) and (2).(2)Fs=Fsin45°,where F is failure load (N).


## 3.2.1. Tests of Mechanical Performance

(1) Rut Test. Rut test specimens (300 × 300 × 50 mm) are made by wheel rolling method. The load wheel pressure of the asphalt mixture rut test equipment is 0.7 ± 0.05 MPa, and the round rolling speed is 42 times/min. The dynamic stability of specimens is tested at 60°C. The specimen and test equipment are shown in Figure 5.Figure 5
Rut test.(2) Bending Test. The cutting technique is used to make the asphalt mixture beams based on the double-deck rut specimen which are sized at 250 × 60 × 70 mm. The loading rate of the test is at 50 mm/min and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 6.Figure 6
Bending test.(3) Split Test. The test adopted the double-deck Marshall specimen and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively. The specimen and test equipment are shown in Figure 7.Figure 7
Split test.


## 3.2.2. Adhesion Tests

(1) Interlayer Adhesion Test. The adhesion strength of interlayer between two mixtures is mainly investigated in the test and not the adhesion strength in the mixture itself. Therefore, the project developed a modified interlayer adhesion test for evaluating the characteristics of the interlayer to carry out the comparative study on interlayer adhesion performance of asphalt pavement based on continuous and discontinuous paving technology. The interlayer adhesion test is to rotate the specimen for 90 degrees from the original splitting direction, the loading rate of the test is at 1 mm/min, and the test temperatures are at room temperature, low temperature, and freeze-thaw condition, respectively. Working state of the split shear test is shown in Figure 8.Figure 8
Working state of interlayer adhesion test. (a) Stress diagram of interlayer adhesion test. (b) Working state of interlayer adhesion test.
(a)
(b)According to the definition of shear strength, the modified split shear strength is calculated by equation (1).(1)τ=FsAs=4FmaxπD2,where τ is split shear strength (MPa); FS is shear force (N); AS is shear area (mm2); Fmax is maximum test force (N); and D is specimen diameter (mm), 101.6 mm.(2) Direct Shear Test. In order to reflect the actual stress conditions of the interlayer under vehicle load, the direct shear test was carried out on self-developed shear machine in laboratory (shown in Figure 9). The loading rate of the test is at 1 mm/min. The specimen is subjected to the stress which is perpendicular to interlayer and the shear strength is parallel to the interlayer during the load process. The test indexes are the same as the split shear test and the test temperatures are room temperature, low temperature, and freeze-thaw, respectively.Figure 9
Working state of direct shear test. (a) Stress diagram of direct shear test. (b) Working state of direct shear test.
(a)
(b)The shear strength of direct shear test is calculated by equations (1) and (2).(2)Fs=Fsin45°,where F is failure load (N).


## 4. Test Results

### 4.1. Tests Results of Mechanical Performance

The test results of mechanical performance are shown in Table5. The standard deviations of test results are shown in Table 5. The sample number of measurements is three samples.Table 5
Results of test.
Types of specimen
AC13/AC16
AC16/AC20
Continuous paving
Discontinuous paving
Continuous paving
Discontinuous paving
Rut test (dynamic stability (kN))
60°C
2520
2258.8
2405.1
2122.9
Standard deviation
145
143.9
133.8
93.4
Bending test (bending strength (MPa))
Room temperature
5.628
4.846
5.415
4.752
Standard deviation
0.394
0.219
0.461
0.163
Low temperature
16.876
14.216
16.354
14.208
Standard deviation
0.773
0.976
0.664
0.763
Freeze-thaw
1.424
1.152
1.337
1.05
Standard deviation
0.112
0.056
0.054
0.068
Split test (split strength (MPa))
Room temperature
0.778
0.620
0.796
0.532
Standard deviation
0.026
0.039
0.054
0.041
Low temperature
2.007
1.672
1.771
1.793
Standard deviation
0.096
0.099
0.097
0.102
Freeze-thaw
0.414
0.155
0.279
0.285
Standard deviation
0.021
0.011
0.015
0.012
Note: AC13/AC16 (AC16/AC20): upper layer AC13 (AC16)/lower layer AC16 (AC20).Through analysis of the data in Table5, the conclusions can be obtained as follows:(1)
The results of rut test show that the specimen’s ability to resist high temperature rut based on continuous paving technology is better than that of specimen based on discontinuous paving technology. At the point of ability to resist high temperature rut, AC13/AC16 specimen is better than AC16/AC20 specimen(2)
The results of bending test show that the continuous paving specimens are better than discontinuous paving specimens at the point of flexural-tensile strength based on specimens with the same gradation combination, and AC13/AC16 specimen is better than AC16/AC20 specimen. The flexural-tensile strength of continuous paving specimens is better than that of discontinuous paving specimens at different temperatures. With the test temperatures of room temperature, low temperature, and freeze-thaw, the flexural-tensile strength is progressively lowering. The gap in flexural-tensile strength of specimens with two paving methods is the most obvious at low temperature(3)
The results of split test show that the shear strength of specimens based on continuous paving technology is better than that of specimens based on discontinuous paving technology at room temperature. The split shear strength of AC16/AC20 specimens based on continuous and discontinuous paving technology shows little change at low temperature


### 4.2. Tests Results of Shear Performance

#### 4.2.1. Results of Shear Test at Room Temperature

Results of shear test at room temperature are shown in Table6. The standard deviations of test results are shown in Table 6. The sample number of measurements is three samples. It can be seen from Table 6 that the shear strength of the specimen based on continuous paving technology is bigger about 1-2 times than that of specimen based on discontinuous paving technology. The influence of different gradation combinations on continuous paving specimen is small because the double layer of continuous paving specimen is paved and compacted simultaneously, which leads to the interlayer bonding strength between upper and lower layers become large. For discontinuous specimens, the result is opposite, which is because the bonding of asphalt in interlayer is subdued relatively and mainly depends on the interlock strength from aggregate in interlayer.Table 6
Results of shear test at room temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.252
0.222
0.121
0.089
Standard deviation
0.013
0.009
0.008
0.004
Direct shear
0.717
0.837
0.448
0.608
Standard deviation
0.041
0.041
0.018
0.021


#### 4.2.2. Results of Shear Test at Low Temperature

Results of shear test at −10°C are shown in Table7. The standard deviations of test results are shown in Table 8. The sample number of measurements is three samples. It can be seen from Table 8 that the shear strength at low temperature is better than that at room temperature. The reason is that water vapour forms ice crystals in the specimen pore at low temperature; the ice crystals provide a certain carrying capacity which leads specimens at low temperatures to be harder to break than specimens at room temperature [19, 20]. The gap among the results of shear test at low temperature by continuous and discontinuous paving specimens is larger because the ice crystals have the smooth style, and the interlayer of discontinuous paving specimens has a larger trend of slippage at low temperature, which makes the specimens more vulnerable.Table 7
Results of shear test at freeze-thaw condition.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.116
0.048
0.016
0.014
Standard deviation
0.006
0.003
0.001
0.001
Direct shear
0.409
0.349
0.124
0.156
Standard deviation
0.021
0.018
0.006
0.003Table 8
Results of shear test at low temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
1.828
1.897
1.230
0.772
Standard deviation
0.091
0.083
0.053
0.043
Direct shear
6.733
6.079
5.27
5.317
Standard deviation
0.241
0.304
0.271
0.199


#### 4.2.3. Results of Shear Test at Freeze-Thaw Condition

Results of shear test at freeze-thaw condition are shown in Table7. The standard deviations of test results are shown in Table 7. The sample number of measurements is three samples. It can be seen from Table 7 that the shear strength of specimens interlayer is reduced at high degree after a freeze-thaw cycle. On the aspect of the capacity resisting the water-induced damage and the temperature change, the continuous paved specimens are obviously better than that of the discontinuous paving specimens. That is because there are larger void space and poor condition of compaction on the interlayer of discontinuous paving specimens, which leads to water infiltrating into interlayer more easily. The ice crystals have greatly disintegrated the carrying capacity of the internal structure at low temperature so that the shear strength of discontinuous paving specimen is reduced at high degree after the freeze-thaw cycle.


#### 4.2.4. Results of Shear Test of Asphalt Mixture

Carrying out the shear test of asphalt mixture (not the interlayer) is to research the shear performance of the mixture itself. The method and test condition to make specimens are the same as double-deck specimens and the results of shear test of asphalt mixture are shown in Table9. It can be seen from Table 10 that, on the basis of comparison on different gradation combination, the test results of asphalt mixture itself in shear performance at different temperatures are shown as follows:(3)Freeze−thaw condition<room temperature<low temperature.Table 9
Summary of interlayer adhesion test results.
Specimen type
13/16
16/20
AC13
AC16
AC20
Bonding layer
Not bonding layer
C. pave
Disc. pave
C. pave
Disc. pave
τ (MPa)
Room T.
0.252
0.121
0.222
0.089
0.255
0.293
0.314
Standard deviation
0.028
0.014
0.026
0.011
0.021
0.016
0.024
Low T.
1.828
1.230
1.897
0.772
1.690
1.813
1.829
Standard deviation
0.13
0.09
0.148
0.062
0.11
0.16
0.15
Freeze-thaw
0.116
0.016
0.048
0.014
0.083
0.108
0.098
Standard deviation
0.014
0.003
0.009
0.0028
0.019
0.01
0.011Table 10
Results of shear test of asphalt mixture.
AC13
AC16
AC20
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
τ (MPa)
0.255
1.690
0.083
0.293
1.813
0.108
0.314
1.829
0.098
Standard deviationτ (MPa)
0.013
0.083
0.004
0.021
0.091
0.005
0.031
0.086
0.005The AC20 specimens have stronger shear capacity than that of the other two specimens at low temperature, which is because the coarser the grain size is, the bigger ice crystals in the specimen pore become, which can provide a stronger carrying capacity. The AC16 specimens do even better in the shear test at room temperature and freeze-thaw condition. On the ratio of the shear strength at freeze-thaw condition and room temperature, all are close to 30%.


## 4.1. Tests Results of Mechanical Performance

The test results of mechanical performance are shown in Table5. The standard deviations of test results are shown in Table 5. The sample number of measurements is three samples.Table 5
Results of test.
Types of specimen
AC13/AC16
AC16/AC20
Continuous paving
Discontinuous paving
Continuous paving
Discontinuous paving
Rut test (dynamic stability (kN))
60°C
2520
2258.8
2405.1
2122.9
Standard deviation
145
143.9
133.8
93.4
Bending test (bending strength (MPa))
Room temperature
5.628
4.846
5.415
4.752
Standard deviation
0.394
0.219
0.461
0.163
Low temperature
16.876
14.216
16.354
14.208
Standard deviation
0.773
0.976
0.664
0.763
Freeze-thaw
1.424
1.152
1.337
1.05
Standard deviation
0.112
0.056
0.054
0.068
Split test (split strength (MPa))
Room temperature
0.778
0.620
0.796
0.532
Standard deviation
0.026
0.039
0.054
0.041
Low temperature
2.007
1.672
1.771
1.793
Standard deviation
0.096
0.099
0.097
0.102
Freeze-thaw
0.414
0.155
0.279
0.285
Standard deviation
0.021
0.011
0.015
0.012
Note: AC13/AC16 (AC16/AC20): upper layer AC13 (AC16)/lower layer AC16 (AC20).Through analysis of the data in Table5, the conclusions can be obtained as follows:(1)
The results of rut test show that the specimen’s ability to resist high temperature rut based on continuous paving technology is better than that of specimen based on discontinuous paving technology. At the point of ability to resist high temperature rut, AC13/AC16 specimen is better than AC16/AC20 specimen(2)
The results of bending test show that the continuous paving specimens are better than discontinuous paving specimens at the point of flexural-tensile strength based on specimens with the same gradation combination, and AC13/AC16 specimen is better than AC16/AC20 specimen. The flexural-tensile strength of continuous paving specimens is better than that of discontinuous paving specimens at different temperatures. With the test temperatures of room temperature, low temperature, and freeze-thaw, the flexural-tensile strength is progressively lowering. The gap in flexural-tensile strength of specimens with two paving methods is the most obvious at low temperature(3)
The results of split test show that the shear strength of specimens based on continuous paving technology is better than that of specimens based on discontinuous paving technology at room temperature. The split shear strength of AC16/AC20 specimens based on continuous and discontinuous paving technology shows little change at low temperature


## 4.2. Tests Results of Shear Performance

### 4.2.1. Results of Shear Test at Room Temperature

Results of shear test at room temperature are shown in Table6. The standard deviations of test results are shown in Table 6. The sample number of measurements is three samples. It can be seen from Table 6 that the shear strength of the specimen based on continuous paving technology is bigger about 1-2 times than that of specimen based on discontinuous paving technology. The influence of different gradation combinations on continuous paving specimen is small because the double layer of continuous paving specimen is paved and compacted simultaneously, which leads to the interlayer bonding strength between upper and lower layers become large. For discontinuous specimens, the result is opposite, which is because the bonding of asphalt in interlayer is subdued relatively and mainly depends on the interlock strength from aggregate in interlayer.Table 6
Results of shear test at room temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.252
0.222
0.121
0.089
Standard deviation
0.013
0.009
0.008
0.004
Direct shear
0.717
0.837
0.448
0.608
Standard deviation
0.041
0.041
0.018
0.021


### 4.2.2. Results of Shear Test at Low Temperature

Results of shear test at −10°C are shown in Table7. The standard deviations of test results are shown in Table 8. The sample number of measurements is three samples. It can be seen from Table 8 that the shear strength at low temperature is better than that at room temperature. The reason is that water vapour forms ice crystals in the specimen pore at low temperature; the ice crystals provide a certain carrying capacity which leads specimens at low temperatures to be harder to break than specimens at room temperature [19, 20]. The gap among the results of shear test at low temperature by continuous and discontinuous paving specimens is larger because the ice crystals have the smooth style, and the interlayer of discontinuous paving specimens has a larger trend of slippage at low temperature, which makes the specimens more vulnerable.Table 7
Results of shear test at freeze-thaw condition.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.116
0.048
0.016
0.014
Standard deviation
0.006
0.003
0.001
0.001
Direct shear
0.409
0.349
0.124
0.156
Standard deviation
0.021
0.018
0.006
0.003Table 8
Results of shear test at low temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
1.828
1.897
1.230
0.772
Standard deviation
0.091
0.083
0.053
0.043
Direct shear
6.733
6.079
5.27
5.317
Standard deviation
0.241
0.304
0.271
0.199


### 4.2.3. Results of Shear Test at Freeze-Thaw Condition

Results of shear test at freeze-thaw condition are shown in Table7. The standard deviations of test results are shown in Table 7. The sample number of measurements is three samples. It can be seen from Table 7 that the shear strength of specimens interlayer is reduced at high degree after a freeze-thaw cycle. On the aspect of the capacity resisting the water-induced damage and the temperature change, the continuous paved specimens are obviously better than that of the discontinuous paving specimens. That is because there are larger void space and poor condition of compaction on the interlayer of discontinuous paving specimens, which leads to water infiltrating into interlayer more easily. The ice crystals have greatly disintegrated the carrying capacity of the internal structure at low temperature so that the shear strength of discontinuous paving specimen is reduced at high degree after the freeze-thaw cycle.


### 4.2.4. Results of Shear Test of Asphalt Mixture

Carrying out the shear test of asphalt mixture (not the interlayer) is to research the shear performance of the mixture itself. The method and test condition to make specimens are the same as double-deck specimens and the results of shear test of asphalt mixture are shown in Table9. It can be seen from Table 10 that, on the basis of comparison on different gradation combination, the test results of asphalt mixture itself in shear performance at different temperatures are shown as follows:(3)Freeze−thaw condition<room temperature<low temperature.Table 9
Summary of interlayer adhesion test results.
Specimen type
13/16
16/20
AC13
AC16
AC20
Bonding layer
Not bonding layer
C. pave
Disc. pave
C. pave
Disc. pave
τ (MPa)
Room T.
0.252
0.121
0.222
0.089
0.255
0.293
0.314
Standard deviation
0.028
0.014
0.026
0.011
0.021
0.016
0.024
Low T.
1.828
1.230
1.897
0.772
1.690
1.813
1.829
Standard deviation
0.13
0.09
0.148
0.062
0.11
0.16
0.15
Freeze-thaw
0.116
0.016
0.048
0.014
0.083
0.108
0.098
Standard deviation
0.014
0.003
0.009
0.0028
0.019
0.01
0.011Table 10
Results of shear test of asphalt mixture.
AC13
AC16
AC20
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
τ (MPa)
0.255
1.690
0.083
0.293
1.813
0.108
0.314
1.829
0.098
Standard deviationτ (MPa)
0.013
0.083
0.004
0.021
0.091
0.005
0.031
0.086
0.005The AC20 specimens have stronger shear capacity than that of the other two specimens at low temperature, which is because the coarser the grain size is, the bigger ice crystals in the specimen pore become, which can provide a stronger carrying capacity. The AC16 specimens do even better in the shear test at room temperature and freeze-thaw condition. On the ratio of the shear strength at freeze-thaw condition and room temperature, all are close to 30%.


## 4.2.1. Results of Shear Test at Room Temperature

Results of shear test at room temperature are shown in Table6. The standard deviations of test results are shown in Table 6. The sample number of measurements is three samples. It can be seen from Table 6 that the shear strength of the specimen based on continuous paving technology is bigger about 1-2 times than that of specimen based on discontinuous paving technology. The influence of different gradation combinations on continuous paving specimen is small because the double layer of continuous paving specimen is paved and compacted simultaneously, which leads to the interlayer bonding strength between upper and lower layers become large. For discontinuous specimens, the result is opposite, which is because the bonding of asphalt in interlayer is subdued relatively and mainly depends on the interlock strength from aggregate in interlayer.Table 6
Results of shear test at room temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.252
0.222
0.121
0.089
Standard deviation
0.013
0.009
0.008
0.004
Direct shear
0.717
0.837
0.448
0.608
Standard deviation
0.041
0.041
0.018
0.021


## 4.2.2. Results of Shear Test at Low Temperature

Results of shear test at −10°C are shown in Table7. The standard deviations of test results are shown in Table 8. The sample number of measurements is three samples. It can be seen from Table 8 that the shear strength at low temperature is better than that at room temperature. The reason is that water vapour forms ice crystals in the specimen pore at low temperature; the ice crystals provide a certain carrying capacity which leads specimens at low temperatures to be harder to break than specimens at room temperature [19, 20]. The gap among the results of shear test at low temperature by continuous and discontinuous paving specimens is larger because the ice crystals have the smooth style, and the interlayer of discontinuous paving specimens has a larger trend of slippage at low temperature, which makes the specimens more vulnerable.Table 7
Results of shear test at freeze-thaw condition.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
0.116
0.048
0.016
0.014
Standard deviation
0.006
0.003
0.001
0.001
Direct shear
0.409
0.349
0.124
0.156
Standard deviation
0.021
0.018
0.006
0.003Table 8
Results of shear test at low temperature.
Specimen type
Continuous paving
Discontinuous paving
13/16
16/20
13/16
16/20
τ (MPa)
Split shear
1.828
1.897
1.230
0.772
Standard deviation
0.091
0.083
0.053
0.043
Direct shear
6.733
6.079
5.27
5.317
Standard deviation
0.241
0.304
0.271
0.199


## 4.2.3. Results of Shear Test at Freeze-Thaw Condition

Results of shear test at freeze-thaw condition are shown in Table7. The standard deviations of test results are shown in Table 7. The sample number of measurements is three samples. It can be seen from Table 7 that the shear strength of specimens interlayer is reduced at high degree after a freeze-thaw cycle. On the aspect of the capacity resisting the water-induced damage and the temperature change, the continuous paved specimens are obviously better than that of the discontinuous paving specimens. That is because there are larger void space and poor condition of compaction on the interlayer of discontinuous paving specimens, which leads to water infiltrating into interlayer more easily. The ice crystals have greatly disintegrated the carrying capacity of the internal structure at low temperature so that the shear strength of discontinuous paving specimen is reduced at high degree after the freeze-thaw cycle.


## 4.2.4. Results of Shear Test of Asphalt Mixture

Carrying out the shear test of asphalt mixture (not the interlayer) is to research the shear performance of the mixture itself. The method and test condition to make specimens are the same as double-deck specimens and the results of shear test of asphalt mixture are shown in Table9. It can be seen from Table 10 that, on the basis of comparison on different gradation combination, the test results of asphalt mixture itself in shear performance at different temperatures are shown as follows:(3)Freeze−thaw condition<room temperature<low temperature.Table 9
Summary of interlayer adhesion test results.
Specimen type
13/16
16/20
AC13
AC16
AC20
Bonding layer
Not bonding layer
C. pave
Disc. pave
C. pave
Disc. pave
τ (MPa)
Room T.
0.252
0.121
0.222
0.089
0.255
0.293
0.314
Standard deviation
0.028
0.014
0.026
0.011
0.021
0.016
0.024
Low T.
1.828
1.230
1.897
0.772
1.690
1.813
1.829
Standard deviation
0.13
0.09
0.148
0.062
0.11
0.16
0.15
Freeze-thaw
0.116
0.016
0.048
0.014
0.083
0.108
0.098
Standard deviation
0.014
0.003
0.009
0.0028
0.019
0.01
0.011Table 10
Results of shear test of asphalt mixture.
AC13
AC16
AC20
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
Room T.
Low T.
Freeze-thaw
τ (MPa)
0.255
1.690
0.083
0.293
1.813
0.108
0.314
1.829
0.098
Standard deviationτ (MPa)
0.013
0.083
0.004
0.021
0.091
0.005
0.031
0.086
0.005The AC20 specimens have stronger shear capacity than that of the other two specimens at low temperature, which is because the coarser the grain size is, the bigger ice crystals in the specimen pore become, which can provide a stronger carrying capacity. The AC16 specimens do even better in the shear test at room temperature and freeze-thaw condition. On the ratio of the shear strength at freeze-thaw condition and room temperature, all are close to 30%.


## 5. Analysis of Shear Test Data

### 5.1. Analysis of Split Shear Test Data

Interlayer adhesion test results are shown in Table9. The standard deviations of test results are shown in Table 9. The sample number of measurements is three samples.


#### 5.1.1. Comparison of Test Results from Gradation

From Figure10, the split shear strength of continuous double-deck specimens based on the same gradation is larger than that of discontinuous double-deck specimens. The split shear strength of AC13/AC16 is larger than that of AC16/AC20.Figure 10
Comparison of test results from gradation.


#### 5.1.2. Comparison of Test Results from Test Conditions

From Figure10, the split shear strength of the specimens at low temperature is the highest, and that at freeze-thaw condition is the lowest. There is a wide gap in the split shear strength of two paving technologies at low temperature. By the comparison of the test results between room temperature and freeze-thaw conditions, the split shear strength of AC13/AC16 continuous double-deck specimen, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent) and the decline of the other specimen is over 50 percent. The ratio of split shear strength at room temperature and low temperature, respectively, is around 1 : 9.


#### 5.1.3. Comparison of Test Results from Load-Displacement Curves

From Figure11, after the shear stress at room temperature reaches the maximum, the shear stress decreases slowly with the displacement increasing (Figure 11(a)), while the shear stress plummets down rapidly after reaching the maximum at low temperature (Figure 11(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 11
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


#### 5.1.4. Comparison of Test Results from Modes of Specimen Damage

From Figure12, the fracture surface of continuous paving double-deck specimens is not the interlayer but the slant which is 10–15 degrees in the vertical direction. Meanwhile, the fracture surface of discontinuous paving double-deck specimens is the interlayer. That is, the interlayer bonding of continuous paving double-deck specimens is better.Figure 12
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


#### 5.1.5. Comparison of Test Results from Asphalt Mixture

From Figure13, the test results from asphalt mixture itself are regular; the shear strength ratio at different temperatures is the following:(4)Room temperature : low temperature : freeze−thaw condition=3:18:1.Figure 13
Comparison of test results from asphalt mixture.The ice crystals in the specimen pore have a certain resistance at low temperature, which leads the shear strength to become larger.


### 5.2. Analysis of Direct Shear Test Results

The direct shear test results are shown in Table11. The standard deviations of test results are shown in Table 11. The sample number of measurements is three samples.Table 11
Summary of direct shear test results.
Specimen type
AC13/AC16
AC16/AC20
C. pave
Disc. pave
C. pave
Disc. pave
τ (MPa)
Room T.
0.717
0.448
0.837
0.608
Standard deviation
0.061
0.042
0.075
0.059
Low T.
6.733
5.27
6.079
5.317
Standard deviation
0.44
0.33
0.45
0.35
Freeze-thaw
0.409
0.124
0.349
0.156
Standard deviation
0.043
0.012
0.033
0.021


#### 5.2.1. Comparison of Test Results from Gradation

Test results of different gradation are shown in Figure14. From Figure 14, for the same gradation specimens, the direct shear strength of the continuous paving specimens is greater than that of the discontinuous paving specimens because the continuous paving technology makes aggregate of upper and lower layers connected much better, which increases the interlayer shear performance. The direct shear strength of AC13/AC16 is smaller than that of AC16/AC20 because the skeleton structure of coarse aggregate plays a major role, which leads to the increase of damage load and shear forces. So the direct shear strength of AC16/AC20 is larger.Figure 14
Comparison of gradation.


#### 5.2.2. Comparison of Test Results from Temperature Conditions

Test results from different temperature conditions are shown in Figure14. From Figure 14, the direct shear strength at low temperature is the highest, and that at freeze-thaw condition is the lowest. By the comparison of the test results between room temperature and freeze-thaw conditions, the direct shear strength of AC13/AC16 continuous paving double-deck specimens at freeze-thaw conditions, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent), and the decline of the other specimens is over 50 percent. The ratio of direct shear strength between room temperature and low temperature is around 1 : 9; the reason is that moisture in the specimen pore forms ice crystals at low temperature and increases the interlayer connection performance. Meanwhile, after freeze-thaw treatment, the internal structure of the specimen is destroyed and the interlayer connection performance decreases naturally.


#### 5.2.3. Comparison of Test Results from Load-Displacement Curves

From Figure15, after the test force at room temperature reaches the maximum, the test force decreases slowly with the displacement increasing (Figure 15(a)), while the test force plummets down rapidly after reaching the maximum at low temperature (Figure 15(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 15
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


#### 5.2.4. Comparison of Test Results from Modes of Specimen Damage

From Figure16, there is no fracture surface produced when the damage of the continuous specimens happens, while the discontinuous specimens generate tiny fracture surface in that condition and the fracture surface is the connection interface of upper and lower layers, which shows that the interlayer bonding of the continuous paving asphalt pavement is better.Figure 16
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


### 5.3. Comparative Analysis on Two Shear Tests

#### 5.3.1. Comparative Analysis on Test Results

Both interlayer adhesion test results and direct shear test results show that the interlayer shear strength of the continuous paving double-deck specimens is larger than that of the discontinuous ones, but the range is different. Split shear test results show that the shear strength of AC13/AC16 is larger than that of AC16/AC20, while the direct shear test results show that the shear strength of AC13/AC16 is smaller than that of AC16/AC20, which is because the split shear test has no horizontal pressure. The dense structure based on fine aggregate of AC13/AC16 specimens plays a major role and the frame structure based on coarse aggregate plays a secondary role; that is, the cohesion (C) plays a major role and angle of internal friction (ф) plays a secondary role. During the direct shear test, the specimens are subjected to not only shear forces but also vertical pressure; at this moment, the angle of internal friction (ф) plays a major role, while the cohesion (C) and tightness play a secondary role.


#### 5.3.2. Comparative Analysis on Load-Displacement Curves

The comparison between Figures11 and 15 shows that, in the conditions of room temperature and low temperature, the test force in direct shear test decreases slower than that in split shear test while the displacement increasing after the test force reaches the maximum. This is because the compressive strength of the specimen continues to resist loads after the shear failure under the direct shear test, resulting in slower force’s decline.


#### 5.3.3. Comparative Analysis on Modes of Specimen Damage

The comparison between Figures12 and 16 shows that, in the case of continuous and discontinuous paving technology, the fracture surface of the specimen under the direct shear test is much smaller than that under the shear test and does not even have a fracture surface after the test force reaches the maximum during the test loading process. This is because the compressive strength of the specimen resists loads completely after the shear failure under the direct shear test; the specimens are not subjected to shear stress, resulting in slower stress’s decline. So the specimens have only smaller depredation and not even a depredation.


## 5.1. Analysis of Split Shear Test Data

Interlayer adhesion test results are shown in Table9. The standard deviations of test results are shown in Table 9. The sample number of measurements is three samples.


### 5.1.1. Comparison of Test Results from Gradation

From Figure10, the split shear strength of continuous double-deck specimens based on the same gradation is larger than that of discontinuous double-deck specimens. The split shear strength of AC13/AC16 is larger than that of AC16/AC20.Figure 10
Comparison of test results from gradation.


### 5.1.2. Comparison of Test Results from Test Conditions

From Figure10, the split shear strength of the specimens at low temperature is the highest, and that at freeze-thaw condition is the lowest. There is a wide gap in the split shear strength of two paving technologies at low temperature. By the comparison of the test results between room temperature and freeze-thaw conditions, the split shear strength of AC13/AC16 continuous double-deck specimen, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent) and the decline of the other specimen is over 50 percent. The ratio of split shear strength at room temperature and low temperature, respectively, is around 1 : 9.


### 5.1.3. Comparison of Test Results from Load-Displacement Curves

From Figure11, after the shear stress at room temperature reaches the maximum, the shear stress decreases slowly with the displacement increasing (Figure 11(a)), while the shear stress plummets down rapidly after reaching the maximum at low temperature (Figure 11(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 11
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


### 5.1.4. Comparison of Test Results from Modes of Specimen Damage

From Figure12, the fracture surface of continuous paving double-deck specimens is not the interlayer but the slant which is 10–15 degrees in the vertical direction. Meanwhile, the fracture surface of discontinuous paving double-deck specimens is the interlayer. That is, the interlayer bonding of continuous paving double-deck specimens is better.Figure 12
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


### 5.1.5. Comparison of Test Results from Asphalt Mixture

From Figure13, the test results from asphalt mixture itself are regular; the shear strength ratio at different temperatures is the following:(4)Room temperature : low temperature : freeze−thaw condition=3:18:1.Figure 13
Comparison of test results from asphalt mixture.The ice crystals in the specimen pore have a certain resistance at low temperature, which leads the shear strength to become larger.


## 5.1.1. Comparison of Test Results from Gradation

From Figure10, the split shear strength of continuous double-deck specimens based on the same gradation is larger than that of discontinuous double-deck specimens. The split shear strength of AC13/AC16 is larger than that of AC16/AC20.Figure 10
Comparison of test results from gradation.


## 5.1.2. Comparison of Test Results from Test Conditions

From Figure10, the split shear strength of the specimens at low temperature is the highest, and that at freeze-thaw condition is the lowest. There is a wide gap in the split shear strength of two paving technologies at low temperature. By the comparison of the test results between room temperature and freeze-thaw conditions, the split shear strength of AC13/AC16 continuous double-deck specimen, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent) and the decline of the other specimen is over 50 percent. The ratio of split shear strength at room temperature and low temperature, respectively, is around 1 : 9.


## 5.1.3. Comparison of Test Results from Load-Displacement Curves

From Figure11, after the shear stress at room temperature reaches the maximum, the shear stress decreases slowly with the displacement increasing (Figure 11(a)), while the shear stress plummets down rapidly after reaching the maximum at low temperature (Figure 11(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 11
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


## 5.1.4. Comparison of Test Results from Modes of Specimen Damage

From Figure12, the fracture surface of continuous paving double-deck specimens is not the interlayer but the slant which is 10–15 degrees in the vertical direction. Meanwhile, the fracture surface of discontinuous paving double-deck specimens is the interlayer. That is, the interlayer bonding of continuous paving double-deck specimens is better.Figure 12
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


## 5.1.5. Comparison of Test Results from Asphalt Mixture

From Figure13, the test results from asphalt mixture itself are regular; the shear strength ratio at different temperatures is the following:(4)Room temperature : low temperature : freeze−thaw condition=3:18:1.Figure 13
Comparison of test results from asphalt mixture.The ice crystals in the specimen pore have a certain resistance at low temperature, which leads the shear strength to become larger.


## 5.2. Analysis of Direct Shear Test Results

The direct shear test results are shown in Table11. The standard deviations of test results are shown in Table 11. The sample number of measurements is three samples.Table 11
Summary of direct shear test results.
Specimen type
AC13/AC16
AC16/AC20
C. pave
Disc. pave
C. pave
Disc. pave
τ (MPa)
Room T.
0.717
0.448
0.837
0.608
Standard deviation
0.061
0.042
0.075
0.059
Low T.
6.733
5.27
6.079
5.317
Standard deviation
0.44
0.33
0.45
0.35
Freeze-thaw
0.409
0.124
0.349
0.156
Standard deviation
0.043
0.012
0.033
0.021


### 5.2.1. Comparison of Test Results from Gradation

Test results of different gradation are shown in Figure14. From Figure 14, for the same gradation specimens, the direct shear strength of the continuous paving specimens is greater than that of the discontinuous paving specimens because the continuous paving technology makes aggregate of upper and lower layers connected much better, which increases the interlayer shear performance. The direct shear strength of AC13/AC16 is smaller than that of AC16/AC20 because the skeleton structure of coarse aggregate plays a major role, which leads to the increase of damage load and shear forces. So the direct shear strength of AC16/AC20 is larger.Figure 14
Comparison of gradation.


### 5.2.2. Comparison of Test Results from Temperature Conditions

Test results from different temperature conditions are shown in Figure14. From Figure 14, the direct shear strength at low temperature is the highest, and that at freeze-thaw condition is the lowest. By the comparison of the test results between room temperature and freeze-thaw conditions, the direct shear strength of AC13/AC16 continuous paving double-deck specimens at freeze-thaw conditions, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent), and the decline of the other specimens is over 50 percent. The ratio of direct shear strength between room temperature and low temperature is around 1 : 9; the reason is that moisture in the specimen pore forms ice crystals at low temperature and increases the interlayer connection performance. Meanwhile, after freeze-thaw treatment, the internal structure of the specimen is destroyed and the interlayer connection performance decreases naturally.


### 5.2.3. Comparison of Test Results from Load-Displacement Curves

From Figure15, after the test force at room temperature reaches the maximum, the test force decreases slowly with the displacement increasing (Figure 15(a)), while the test force plummets down rapidly after reaching the maximum at low temperature (Figure 15(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 15
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


### 5.2.4. Comparison of Test Results from Modes of Specimen Damage

From Figure16, there is no fracture surface produced when the damage of the continuous specimens happens, while the discontinuous specimens generate tiny fracture surface in that condition and the fracture surface is the connection interface of upper and lower layers, which shows that the interlayer bonding of the continuous paving asphalt pavement is better.Figure 16
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


## 5.2.1. Comparison of Test Results from Gradation

Test results of different gradation are shown in Figure14. From Figure 14, for the same gradation specimens, the direct shear strength of the continuous paving specimens is greater than that of the discontinuous paving specimens because the continuous paving technology makes aggregate of upper and lower layers connected much better, which increases the interlayer shear performance. The direct shear strength of AC13/AC16 is smaller than that of AC16/AC20 because the skeleton structure of coarse aggregate plays a major role, which leads to the increase of damage load and shear forces. So the direct shear strength of AC16/AC20 is larger.Figure 14
Comparison of gradation.


## 5.2.2. Comparison of Test Results from Temperature Conditions

Test results from different temperature conditions are shown in Figure14. From Figure 14, the direct shear strength at low temperature is the highest, and that at freeze-thaw condition is the lowest. By the comparison of the test results between room temperature and freeze-thaw conditions, the direct shear strength of AC13/AC16 continuous paving double-deck specimens at freeze-thaw conditions, relative to that at room temperature, has the lowest percentage decline (approaching 50 percent), and the decline of the other specimens is over 50 percent. The ratio of direct shear strength between room temperature and low temperature is around 1 : 9; the reason is that moisture in the specimen pore forms ice crystals at low temperature and increases the interlayer connection performance. Meanwhile, after freeze-thaw treatment, the internal structure of the specimen is destroyed and the interlayer connection performance decreases naturally.


## 5.2.3. Comparison of Test Results from Load-Displacement Curves

From Figure15, after the test force at room temperature reaches the maximum, the test force decreases slowly with the displacement increasing (Figure 15(a)), while the test force plummets down rapidly after reaching the maximum at low temperature (Figure 15(b)). In the shear test at low temperature, the form of damage to the specimen is brittle fracture.Figure 15
Comparison of test results from load-displacement curves. (a) Room temperature. (b) Low temperature.
(a)
(b)


## 5.2.4. Comparison of Test Results from Modes of Specimen Damage

From Figure16, there is no fracture surface produced when the damage of the continuous specimens happens, while the discontinuous specimens generate tiny fracture surface in that condition and the fracture surface is the connection interface of upper and lower layers, which shows that the interlayer bonding of the continuous paving asphalt pavement is better.Figure 16
Comparison of test results from modes of specimen damage. (a) Continuous paving. (b) Discontinuous paving.
(a)
(b)


## 5.3. Comparative Analysis on Two Shear Tests

### 5.3.1. Comparative Analysis on Test Results

Both interlayer adhesion test results and direct shear test results show that the interlayer shear strength of the continuous paving double-deck specimens is larger than that of the discontinuous ones, but the range is different. Split shear test results show that the shear strength of AC13/AC16 is larger than that of AC16/AC20, while the direct shear test results show that the shear strength of AC13/AC16 is smaller than that of AC16/AC20, which is because the split shear test has no horizontal pressure. The dense structure based on fine aggregate of AC13/AC16 specimens plays a major role and the frame structure based on coarse aggregate plays a secondary role; that is, the cohesion (C) plays a major role and angle of internal friction (ф) plays a secondary role. During the direct shear test, the specimens are subjected to not only shear forces but also vertical pressure; at this moment, the angle of internal friction (ф) plays a major role, while the cohesion (C) and tightness play a secondary role.


### 5.3.2. Comparative Analysis on Load-Displacement Curves

The comparison between Figures11 and 15 shows that, in the conditions of room temperature and low temperature, the test force in direct shear test decreases slower than that in split shear test while the displacement increasing after the test force reaches the maximum. This is because the compressive strength of the specimen continues to resist loads after the shear failure under the direct shear test, resulting in slower force’s decline.


### 5.3.3. Comparative Analysis on Modes of Specimen Damage

The comparison between Figures12 and 16 shows that, in the case of continuous and discontinuous paving technology, the fracture surface of the specimen under the direct shear test is much smaller than that under the shear test and does not even have a fracture surface after the test force reaches the maximum during the test loading process. This is because the compressive strength of the specimen resists loads completely after the shear failure under the direct shear test; the specimens are not subjected to shear stress, resulting in slower stress’s decline. So the specimens have only smaller depredation and not even a depredation.


## 5.3.1. Comparative Analysis on Test Results

Both interlayer adhesion test results and direct shear test results show that the interlayer shear strength of the continuous paving double-deck specimens is larger than that of the discontinuous ones, but the range is different. Split shear test results show that the shear strength of AC13/AC16 is larger than that of AC16/AC20, while the direct shear test results show that the shear strength of AC13/AC16 is smaller than that of AC16/AC20, which is because the split shear test has no horizontal pressure. The dense structure based on fine aggregate of AC13/AC16 specimens plays a major role and the frame structure based on coarse aggregate plays a secondary role; that is, the cohesion (C) plays a major role and angle of internal friction (ф) plays a secondary role. During the direct shear test, the specimens are subjected to not only shear forces but also vertical pressure; at this moment, the angle of internal friction (ф) plays a major role, while the cohesion (C) and tightness play a secondary role.


## 5.3.2. Comparative Analysis on Load-Displacement Curves

The comparison between Figures11 and 15 shows that, in the conditions of room temperature and low temperature, the test force in direct shear test decreases slower than that in split shear test while the displacement increasing after the test force reaches the maximum. This is because the compressive strength of the specimen continues to resist loads after the shear failure under the direct shear test, resulting in slower force’s decline.


## 5.3.3. Comparative Analysis on Modes of Specimen Damage

The comparison between Figures12 and 16 shows that, in the case of continuous and discontinuous paving technology, the fracture surface of the specimen under the direct shear test is much smaller than that under the shear test and does not even have a fracture surface after the test force reaches the maximum during the test loading process. This is because the compressive strength of the specimen resists loads completely after the shear failure under the direct shear test; the specimens are not subjected to shear stress, resulting in slower stress’s decline. So the specimens have only smaller depredation and not even a depredation.


## 6. Conclusions

(1)
The research results show that the interlayer shear strength of the continuous paving asphalt mixture can be improved to a certain extent compared with the discontinuous paving asphalt mixture
For the gradation combination of AC13/AC16 specimens at room temperature, the shear strength of the continuous paving specimens is two times bigger than that of the discontinuous paving specimens and the direct shear strength is 1.6 times bigger than that of the discontinuous paving specimens
For the gradation combination of AC16/AC20 specimens at room temperature, the shear strength of the continuous paving specimens is 2.5 times bigger than that of the discontinuous paving specimens and the direct shear strength is 1.4 times bigger than that of the discontinuous paving specimens
(2)
From the results of shear test for asphalt mixtures by different paving methods, the shear strength at different temperatures basically conforms to the following rules:freeze-thaw < normal temperature < low temperature
From the results of shear test for asphalt mixtures based on the same materials, the shear strength at different temperatures basically conforms to the following rules:normal temperature: low temperature: freeze-thaw = 3 : 18 : 1
(3)
The mechanical performance of asphalt mixture by different paving methods is sensitive to freeze-thaw shear. The freeze-thaw shear strength of AC13/AC16 continuous specimens is 50% lower than that at room temperature, while the other three specimens declined by more than 50%. This conclusion has some guiding significance for the gradation design of asphalt mixture in low temperature regions; for example, in low temperature regions, the AC13/AC16 continuous type is relatively superior to the AC13/AC16 discontinuous type and the AC16/AC20 continuous and discontinuous types. When designing the asphalt pavement structure, the most unfavorable freeze-thaw design method can be considered and the AC13/AC16 continuous type should be applied in asphalt pavement.



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*Source: 2184094-2019-12-28.xml*