Properties and Preparation of a New High-Permeability Emulsified Asphalt and Its Modification

Abstract

Emulsified asphalt is widely used as a prime coat in road engineering, especially in pavements with cement-stabilized semi-rigid bases. The objective of this paper is to develop a new emulsified asphalt with high permeability and pulling-off strength. A new type of high-permeability emulsified asphalt was prepared by using 55.2% diluted asphalt (44.16% matrix asphalt + 11.04% diluent) + 40% emulsion (2% emulsifier + 0.5% stabilizer + 1% osmotic agent + 1% op-10 + 35.5% water) + 4.8% aromatic oil. The storage stability was verified to be better, and the penetrating rate and depth were increased largely compared to the traditional products. To improve pulling-off strength, the high-permeability emulsified asphalt was further modified by mixing with 20% water-based epoxy resin and a curing agent. The penetration, pull-off, and shear strength tests were also conducted. It can be found that the permeability, adhesive, and water-resisting properties of the modified high-permeability emulsified asphalt can be further improved to a certain degree.

1. Introduction

As the most common form of pavement structure for highways in China, semi-rigid base layers and asphalt concrete surface layers are widely used for their excellent road performance with high load-bearing capacity, good water stability, etc. [1]. China’s current specification regards the pavement structure as a multi-layer elastic system with complete continuous layers but the pavement surface is not a completely continuous bonding state because the construction on site is a layered operation with different materials between the base and surface layers and poor co-ordination of deformation [2]. Under the action of high pressure, it is easy to produce interlayer slip, rutting, surface cracking, and other distress [3]. Due to the difference in material properties between the surface course and the base course, the bonding performance of asphalt mixture and chemically stabilized materials is poor, which will cause distress such as a shear slip or reflection cracks of asphalt mixture on the surface course [4]. To improve the continuity between the asphalt pavement structural layers, the interlayer bonding layer is usually set. It can improve the bonding performance between the base and surface course and make the whole pavement structure more complete and more continuous. Spreading prime coat asphalt on the semi-rigid base can significantly improve the bonding ability between layers; it can effectively prevent the occurrence of pavement distress.

Generally, the permeable layer is set between the surface course and the base in the asphalt pavement. At present, the most widely used permeable layer materials are kerosene-diluted asphalt and emulsified asphalt. Kerosene-diluted asphalt has the advantages of good permeability and low cost but is toxic and pollutes the environment. The environmental pollution caused by the volatile diluent and the safety concerns caused by low ignition point cannot be ignored; it is less used now [5]. While emulsified asphalt has the characteristics of environmental protection, safety, and low price, it also has good water stability [6]. It still has excellent adhesion under the action of immersion, ensuring the adhesion of the base course and surface course, so it is used more and more widely [7,8].

However, the penetration of ordinary cationic emulsified asphalt could be poor. Anionic emulsified asphalt can be penetrated into the semi-rigid base [9] but it often appears as a sticky skin in the actual construction and is easily carried by the construction machinery [10]. At present, prime coat asphalt is normally required as high-permeability emulsified asphalt in China. If the emulsified asphalt is modified, it can be a good solution to the disadvantages of poor permeability [11].

The development of emulsified asphalt began in the early 20th century. Emulsified asphalt was used for spraying to reduce dust. It was first applied in road construction in the 1920s [12]. Since the 1980s, some researchers have added anionic emulsifiers into the asphalt. In order to prepare prime coat emulsified asphalt, a certain proportion of mineral oil was also added [13]. At the beginning of the 21st century, the federal lands highway issued an investigation report called “guidelines for using prime and tack coats” which carried out an in-depth investigation and summary of the use of materials, methods, functions, and other aspects of the prime coat [14]. Wu added waste tire thermal asphalt in the matrix asphalt. A rubber emulsified asphalt with high adhesion and permeability was prepared [15]. Wu and Li used the concept of “selective permeability”, and developed high permeability and high-permeability materials [16]. Xu Yingchao et al. used environmentally friendly penetrants, compound modifiers, and other materials to improve the permeability and adhesion of emulsified asphalt and studied the construction and production control technology of high-permeability emulsified asphalt [17]. Bao et al. mixed a certain proportion of permeable solvent in asphalt to produce high-permeability emulsified asphalt [18]. The permeability on the semi-rigid base, the best amount of spraying time, and other factors were surveyed; it was concluded that the penetrating depth increased with the increase of spraying amount. Other factors such as temperature and the mixing ratio between emulsified asphalt and modifier have effects on the performance of the modified asphalt. Good high-temperature storage stability and suitable mixing ratios can help to prepare an effective modified emulsified asphalt [19].

Summarizing the research work and application of permeable materials, it was found that mixing diluted asphalt to emulsified asphalt was a new way to modify the emulsified asphalt [20]. Some experts and scholars had also studied emulsified asphalt that was used as a permeable material. However, these studies focused mainly on the performance of emulsified asphalt without modification, The unmodified emulsified asphalt has insufficient cohesion, low strength, and poor flexibility. How to prepare emulsified asphalt with good permeability through experiments is still a challenging work [21].

The objective of this paper is to develop a new emulsified asphalt with high permeability and pulling-off strength. The performance of the prepared high-permeability emulsified asphalt was evaluated by the penetration test, tensile test, shear test, and water resistance test. It is further modified because of the higher pull-off strength. With the utilization of the new high-permeability emulsified asphalt and its modification, the expectation could be that the service life of asphalt pavement can be better prolonged, the pavement performance can be improved, and the road maintenance cost can be reduced.

2. Materials and Methods

2.1. Materials

2.1.1. Emulsified Asphalt

Matrix asphalt: 70 matrix asphalt was selected for this test. The main technical indicators were measured according to the specification requirements [22], as shown in

Table 1. Technical indexes of matrix asphalt.

Technical Indicators	Value	Specification
Softening point/°C	49.5	≥46
Penetration (25 °C)/0.1mm	68.5	60–80
Penetration index	0.4	−1.5–1.0
Ductility (15 °C)/cm	>100	>100
60 °C dynamic viscosity/(Pa·S)	190	≥180

.

Cationic emulsifiers: the quick-breaking and slow-setting cationic emulsifier was selected to improve the permanent deformation resistance of emulsified asphalt [23].

Osmotic agent: A kind of surfactant was used as the osmotic agent. It can promote the permeability of liquid into solid.

Diluent: An environmental protection, low-viscosity aromatic hydrocarbon oil was selected as the diluent in this experiment.

Stabilizer: The addition of a non-ionic emulsifier chloride salt and of OP-10 was used to delay the natural demulsification time and increase storage time.

2.1.2. Other Materials

(1) Cement-stabilized crushed stone base

Portland cement class 42.5 was used as the semi-rigid base with 4.5% cement content. The main technical specifications of cement are shown in

Table 2. Cement technical indicators.

Indicators	Unit	Standard Values	Detection Values	Indicators	Unit	Standard Values	Detection Values
Specific surface area	%	≥300	340	Cl	%	≤0.060	0.041
Initial setting time	Minutes	≥45	189	MgO	%	≤5.00	3.39
Final set time	Minutes	≤600	286	SO3	%	≤3.5	2.86
3d flexural strength	MPa	≥3.5	5.3	3d compressive strength	MPa	≥17.0	28.1

.

Limestone: According to the specification [24],the main physical and mechanical index of limestone as coarse aggregate and fine aggregate are shown in

Table 3. Coarse aggregate technical indicators.

Technical Specifications	Measured Values	Technical Requirements
Needle flake content/%	4.1	<15
Crushing value/%	15.3	<20
Apparent relative density	2.78	≥2.5
Water absorption/%	0.87	≤3.0
Mud content/%	0.79	<1.0
Alkaline activity test	Inactive materials	Inactive materials

and

Table 4. Fine aggregate technical indicators.

Technical Specifications	Measured Values	Technical Requirements
Mud content/%	1.12	<3.0
Fineness modulus	3.28	>2.3
Apparent relative density	2.76	≥2.5
Porosity (%)	32	<47
Organic matter content	Lighter than standard color	Lighter than standard color

.

Gradation: The median gradation of cement-stabilized macadam base in specification [25] was used as the test gradation. The range and test gradation of cement-stabilized macadam are shown in

Table 5. Cement stabilized gravel base gradation.

Percentage by Mass (%) Passing the Following Square Hole Sieves (mm)
Size (mm)	31.5	19.0	9.5	4.75	2.36	0.6	0.075
Range	100	68–86	38–58	22–32	16–28	8–15	0–3
Gradation	100	77	48	27	22	12	2

.

2.1.3. Asphalt Concrete

In order to test the penetrating efficiency of the new high-permeability emulsified asphalt and its modification, a plate made of AC-25 asphalt concrete was prepared. The road petroleum asphalt of grade 70 was used in the AC-25 asphalt concrete. The grade and grading curve of AC-25 asphalt concrete are shown in

Table 6. Gradation of AC-25 asphalt concrete.

Percentage by Mass (%) Passing the Following Square Hole Sieves (mm)
Size (mm)	31.5	26.5	19	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Range	100	90–100	75–90	65–83	57–76	45–65	24–52	16–42	12–33	8–24	5–17	4–13	3–7
Gradation	100	95	82.5	74	66.5	55	38	29	17.5	16	11	8.5	5

. The optimal content of asphalt was 4.8%, and the void content was 5% after several relevant tests such as Marshall, rutting, specific density, etc.

2.2. Preparation Process

The colloidal mill was used as the preparation equipment for emulsified asphalt. A colloidal mill rotating at high speed produces huge shear force and friction force. According to the recommendation from the manufacture of emulsifier, the temperature of soap solution and asphalt should be set to 55–60 °C and 135–140 °C, and the initial temperature of emulsified asphalt must keep at 85–90 °C. The temperature of soap solution and asphalt in this test is set to 60 °C and 140 °C. Firstly, some additives were mixed at 60 °C, such as water, osmotic agent, stabilizer, and diluent, to form a homogeneous soap solution. Then, the asphalt was heated to 140 °C. Lastly, the soap solution and asphalt were mixed in the colloidal mill to crush, emulsify, and disperse the materials. The high-permeability emulsified asphalt was prepared. The preparation process is shown in Figure 1.

2.3. Test Methods

2.3.1. Filter Paper Titration Diffusion Test

The emulsified asphalt in 300 mesh, 400 mesh, and 500 mesh titration diffusion on the specific filter paper was tested every 1, 3, 4, 5, 6, 24, 48, and 72 h. The area of initial drop and spreading area at different times were calculated by the image pixel. Comparing the spreading area and the initial area, the permeability efficiency was analyzed.

2.3.2. Surface Morphology Test under Microscope

In this study, fluorescence microscopy (FM) was used to test the surface topography of high-permeability emulsified asphalt versus ordinary emulsified asphalt at intervals of 1, 7, 14, and 28 days. The magnification of the fluorescence microscope is 10 × eyepiece φ10 × eyepiece.

2.3.3. Penetration Test

According to the test method in the specification [26], the curing life of the cement-stabilized macadam base is taken as the test variable to measure the influence of permeable materials on the penetration depth of the base plate at different times. After the cement-stabilized macadam base plate formed, the plates were placed in the standard curing room for 0 h (i.e., spraying immediately after formation), 10 h, 3 d, 7 d, and 28 d. After reaching the curing age, 1.5 L/m² ordinary emulsified asphalt, high-permeability emulsified asphalt, and (modified) high-permeability emulsified asphalt were sprayed. After emulsified asphalt is completely broken, a core sample of φ10 × 10 cm was drilled by a core drill machine, and the penetration depth of the core sample was measured. Figure 2 below is the appearance of the specimen after spraying the high-permeability emulsified asphalt (or its modification).

2.3.4. Pull-Off Test

The bottom slab used in this test was made from a cement-stabilized macadam base and the top slab was AC-25 asphalt concrete. The structural model is shown in Figure 3.

The bonding strength between high-permeability emulsified asphalt and stone slab was tested by using LGZ-1 surface structure layer puller shown in Figure 4, among which ordinary emulsified asphalt, high-permeability emulsified asphalt, and modified high-permeability emulsified asphalt were used as bonding materials. Spray the base plate with ordinary emulsified asphalt, high-permeability emulsified asphalt, and modified high-permeability emulsified asphalt according to the ratio of 0.7 L/m², 0.9 L/m², 1.1 L/m², 1.3 L/m², 1.5 L/m², and move the base plate into the standard curing room for 7 days. Then, they were put into the test mold of 300 mm × 300 mm × 100 mm. After cooling for 12 h, the test mold was removed, and a φ10 drill bit was used to drill 5 cm from the top surface of the double-layer plate down to the interlayer bond interface (up to 4 drills per double-layer plate), and then the pull-off test was performed [27]. The test set is shown in Figure 5.

The specific test procedure is as follows: (1) the surface of the slab is cleaned, and the permeable material (traditional, high-permeability emulsified asphalt and its modification) is sprayed on the slab, and its strength will be formed after broken. (2) After the strength of emulsified asphalt is formed, the pull head and the permeable material on the slab are bonded together with AB glue; (3) after 10min, the strength of AB glue is formed, and the tensile strength of the material is calculated by measuring the pulling-off force.

Four groups of parallel experiments were carried out, and the average value was taken as the final experimental result. By comparing the pulling-off strength of various materials at different spreading rates, the optimal spreading rate of each permeable material was obtained.

2.3.5. Shear Test

In the linear shear test, the lower part of the sample is fixed on the base, and a horizontal force is applied to the lateral part of the sample to shear along the position of the permeable layer.

The shear strength is calculated as follows.

$$P=\frac{F}{A}$$

(1)

where:

• P—shear strength, MPa
• F—maximum load at the time of damage to the specimen, N
• A—bonded area of the specimen, mm²

The traditional emulsified asphalt, high-permeability emulsified asphalt, and modified high-permeability emulsified asphalt were sprayed on the formed bottom plate according to the best sprinkling amount. The double-layer plate was put into the standard curing room for 7 days after demolding. After 7 days, AC-25 single board with a thickness of 5 cm was formed on the bottom plate with a thickness of 300 mm × 300 mm × 100 mm notch plate on the test mold. After cooling for 12 h, the test mold was taken out, and then the core sample with a diameter of 10 × 10 cm was drilled by a core drill machine and the direct shear test was carried out at room temperature.

In the molding of the base plate according to the best sprinkling rate, according to the drawing test results, the best sprinkling rate of high permeability emulsified asphalt is 1.3 L/m² and the best sprinkling rate of ordinary emulsified asphalt and modified high permeability emulsified asphalt is 1.1 L/m² after the release of the double layer plate into the standard curing room curing for 7 days. After cooling for 12 h, the mold was removed and a core sample of φ10 × 10 cm was drilled with a core drill machine. The core sample is shown in Figure 6 and Figure 7.

2.3.6. Water Penetration Test

(1) Principle of the test

The calculation formula of water permeability coefficient is shown in the following formula:

$$C_w=\frac{V_2-V_1}{t_2-t_1}\times 60$$

(2)

where:

• C_w—coefficient of permeability (mL/min)
• V₁—volume of water at first timing (mL)
• V₂—volume of water at second timing (mL)
• t₁—time of first timing (s)
• t₂—time of second timing (s)

(2) Steps of the test

The double-layer plate sample was placed on a stable plane, and the seepage meter was placed on the surface of the sample, as shown in Figure 8. The base of the instrument was sealed. After placing the instrument, the test shall be carried out according to the operating procedure required by the specification [28]. Observe the water seepage of the specimen carefully. Under normal conditions, water will not leak out between the materials used for sealing on the base. The water penetration should come from the bottom and around the test piece. If it is not sealed well, the experiment needs to be retested.

For different penetration conditions, other test methods are adopted, as follows:

In the first case: when the seepage rate is very slow, only the amount of water seepage in 3 min should be recorded [28].

The second case: when the speed is relatively fast and the water reaches 500 mL but the time has not reached 3 min, then only the time it takes to get down to 500 mL needs to be recorded.

The third case: if the water surface in the measuring cylinder is unchanged, it indicates that the specimen material is impervious to water.

The water permeability coefficient of the sample was determined according to the above steps. Each material was tested in parallel three times, and the average value was taken as the test result.

3. Preparation of High-Permeability Emulsified Asphalt

3.1. High-Permeability Emulsified Asphalt Formulation

Five different high-permeability emulsified asphalt formulas in

Table 7. Formula scheme of high-permeability emulsified asphalt.

Number	Mix Proportion	The Emulsification Temperature
1	60% Diluted asphalt (48% matrix asphalt + 12% Diluent) + 40% Soap solution	Diluted asphalt 140 °C, soap solution 60 °C
2	55.2% Diluted asphalt (44.16% matrix asphalt + 11.04% Diluent) + 4.8% Aromatic oil + 40% Soap solution	Diluted asphalt 140 °C, soap solution 60 °C
3	48% Matrix asphalt + 12% Aromatic oil + 40% Soap solution	Diluted asphalt 140 °C, soap solution 60 °C
4	54% Matrix asphalt + 6% Aromatic oil + 40% Soap solution	Matrix asphalt 140 °C, soap solution 60 °C
5	Matrix asphalt 60% + 40% Soap solution	Matrix asphalt 140 °C, soap solution 60 °C

were preliminarily selected, among which the ratio of emulsion is: 5% cationic emulsifier + 1.25% stabilizer + 2.5% osmotic agent + 2.5% non-ionic emulsifier + 88.75% water.

3.2. Optimal Formula of High-Permeability Emulsified Asphalt

Five kinds of emulsified asphalt on filter paper of 300 mesh, 400 mesh, and 500 mesh; the photos had been taken at the interval of every 1, 3, 4, 5, 6, 24, 48 and 72 h. The diffusion area was calculated by pixel identification technology, and the diffusion area difference to the initial area (0 h) was analyzed. The test results are shown in

Table 8. The first diffusion area difference record.

Number	Filter Paper Specification	Diffusion Area Difference (mm2)
		1 h	2 h	3 h	4 h	5 h	6 h	24 h	48 h	72 h
1	300 mesh	79	64	55	74	81	87	174	225	254
	400 mesh	5	14	−5	−16	−20	8	18	65	122
	500 mesh	8	5	17	7	11	32	33	76	92
2	300 mesh	2	4	7	8	6	24	97	202	225
	400 mesh	41	55	42	41	57	91	169	246	240
	500 mesh	36	36	55	24	68	104	157	172	192
3	300 mesh	97	106	96	108	112	110	138	177	205
	400 mesh	62	129	79	121	130	98	122	178	206
	500 mesh	55	36	57	66	66	60	59	84	90
4	300 mesh	102	101	83	89	97	95	68	83	105
	400 mesh	19	23	17	22	24	21	10	24	17
	500 mesh	−9	−5	−1	2	−8	−3	−1	17	−2
5	300 mesh	71	90	81	86	70	72	74	76	67
	400 mesh	17	5	7	2	18	13	8	9	13
	500 mesh	9	7	5	7	6	5	7	10	10

and Figure 9.

As can be seen from

Table 9. D-value in final diffusion area after 30 days.

Filter Paper Specifications (mesh)	Diffusion Area Difference (mm2)
	Formula 1	Formula 2	Formula 3	Formula 4	Formula 5
300	55	227	49	6	3
400	43	94	16	4	6
500	11	30	27	9	2

and Figure 9, after 72 h on 300 mesh filter paper the area on 400 mesh and 500 mesh filter paper is the largest, which is 240 mm² and 192 mm² larger than the initial area of filter paper, respectively. When the diffusion area decreases, it indicates that the penetration effect of emulsified asphalt made from Formula 1 is not as high as Formula 2. Therefore, in general, emulsified asphalt made from Formula 2 has the best penetration effect, followed by Formula 1 and Formula 3. The final diffusion area of Formula 4 and Formula 5 is the smallest, which indicates that the penetration effect of these two emulsified asphalts is the worst. The above test steps were repeated to record the final diffusion area of the five kinds of emulsified asphalt on different filter papers after 30 days. The difference between the final diffusion area and the initial area of the five kinds of emulsified asphalt after 30 days is shown in Figure 10.

As can be seen from Table 9 and Figure 10, the emulsified asphalt made from Formula 2 after 30 days of storage has the best diffusion effect on the three levels of filter paper. It shows that it has the most long-term storage stability. Due to the increased content of diluent, the bond between the molecules of the bitumen can be more fully fused. The more diluent, the thinner the emulsified bitumen. The more permeable it is, the better penetration effect it has. However, this ultra-high dilution and penetration effect may have a negative impact on the bond after the emulsion is broken. Therefore, the new high-permeability emulsified asphalt from Formula 2 (55.2% diluted asphalt (44.16% matrix asphalt + 11.04% dilution agent) + 40% soap solution (2% emulsifier + 0.5% stabilizer + 1% penetrant + 1% op-10 + 35.5% water) should be further modified to improve the pull-off strength.

3.3. Properties of the High-Permeability Emulsified Asphalt

3.3.1. Microscopic Morphological Analysis

In this experiment, the microstructure of the newly developed high-permeability emulsified asphalt and the traditional emulsified asphalt in the laboratory were observed by fluorescence microscopy. The dispersion state of the asphalt particles and their changes with time under fluorescence microscopy of the two emulsified asphalts were observed and compared at intervals of 1, 7, 14, and 28 days. The results are shown in Figure 11 and Figure 12.

From Figure 11 and Figure 12, it can be concluded that the high-permeability emulsified asphalt in asphalt particles aggregation speed is slow on day 28 and that the asphalt particles aggregation group is still small. It was found that only a little asphalt precipitation at the bottom of the bottle appeared homogeneously, which indicated that the high-permeability emulsified asphalt had not yet been broken. The emulsified asphalt began to form an obvious asphalt mass on the 7th day, asphalt precipitate appeared at the bottom of the plastic bottle in which the conventional emulsified asphalt was stored. On the 14th day, the asphalt mass continued to grow, and the bottom of the bottle precipitated a very thick layer of asphalt. The asphalt precipitation began to group in a bigger area on images observed on the 28th day. Therefore, the storage stability of high-permeability emulsified asphalt is obviously better than that of traditional emulsified asphalt. This is because the preparation of high-permeability emulsified asphalt mixed with stabilizer and diluent agent. When the particle of asphalt is finer, it can be dispersed in the soap solution. It can increase the storage stability, which is one of the advantages of this material.

3.3.2. Pulling-Off Strength of High-Permeability Emulsified Asphalt

The pull-off strength of high-permeability emulsified asphalt and emulsified asphalt material was measured by the pulling test, and the bond effect of high-permeability emulsified asphalt was preliminarily determined. The test results are shown in

Table 10. The pull-off strength of two kinds of emulsified asphalt.

Test Material	Number	Pull-Off Force (N)	Pull-Off Strength (MPa)	Pull-Off Strength (MPa)
				Mean Value	Standard Deviation	Coefficient of Variation
High-permeability emulsified asphalt	1-1	241.57	0.215	0.220	0.003	0.014
	1-2	248.06	0.221
	1-3	250.72	0.223
	1-4	247.01	0.220
Traditional emulsified asphalt	2-1	488.07	0.468	0.451	0.021	0.047
	2-2	474.31	0.423
	2-3	536.32	0.478
	2-4	488.07	0.435

.

As we can see from Table 5, the tensile strength of emulsified asphalt is 0.451 MPa, and that of high-permeability emulsified asphalts is 0.22 MPa. It indicates the bonding strength of high-permeability emulsified asphalt is lower than the traditional emulsified asphalt. This is because the molecular chains of the high permeability emulsified asphalt composed of diluent are more broken, and the force between asphalt particles is weakened. The high-permeability emulsified asphalt should be modified to improve the pull-off strength.

3.4. Modification of High-Permeability Emulsified Asphalt

From the observation of the progress during the pull-off test, it can be found that the high-permeability emulsified asphalt on the stone slab has not been completely solidified, and the strength of the high-permeability emulsified asphalt has not been fully formed before it was pulled off. When the emulsified asphalt has been solidified, at this time the strength of emulsified asphalt has been fully formed. In addition, the high-permeability emulsified asphalt was applied to the stone slab and put into the oven for curing and observed every 24 h. The result was tested 72 h after high-permeability emulsified asphalt solidified completely. Therefore, the strength of highly emulsified asphalt could be further enhanced by lengthening the curing time.

The water-based epoxy resin with the corresponding curing agent could improve the tensile strength of composite-modified asphalt and enhance the bonding performance of emulsified asphalt [29]. The 20% aqueous epoxy resin and curing agent (water-based epoxy resin: curing agent = 2:1) were mixed into the high-permeability emulsified asphalt compound to prepare modified high-permeability emulsified asphalt.

The bonding strength of the modified high-permeability emulsified asphalt was preliminarily determined through the pull-off test. The test results are shown in

Table 11. The tensile strength of modified high-permeability emulsified asphalt.

Test Material	Number	Pull-Off Force (N)	Pull-Off Strength (MPa)	Pull-Off Strength (MPa)
				Mean Value	Standard Deviation	Coefficient of Variation
Modified high-permeability emulsified asphalt	1-1	629.45	0.561	0.550	0.025	0.045
	1-2	603.91	0.538
	1-3	649.65	0.579
	1-4	585.69	0.522

.

It can be seen from Table 11 that the tensile strength of the modified high-permeability emulsified asphalt is 0.55 MPa. Compared with the high-permeability emulsified asphalt, the tensile strength of the modified high-permeability emulsified asphalt is 150% higher, and the bond strength is also greater than emulsified asphalt. The experiment shows that the performance of the improved asphalt has been improved.

4. Results and Discussion

The properties of emulsified asphalt with high-permeability and its modification were prepared and studied. The permeability, tensile strength, shear strength, and water resistance of the permeable material were obtained through the penetration test, pull-off test, shear test, and water penetration test on a cement-stabilized macadam base, and the performance of the material was compared with that of modified high-permeability emulsified asphalt and traditional emulsified asphalt.

4.1. Permeability

The penetration test results of high-permeability emulsified asphalt and other materials are shown in

Table 12. Penetration depth of emulsified asphalt on bases in different conditioning periods.

Base Curing Time	Depth of Penetration (mm)
	Traditional Emulsified Asphalt		High-Permeability Emulsified Asphalt		Modified High-Permeability Emulsified Asphalt
	Average	Standard Deviation	Average	Standard Deviation	Average	Standard Deviation
0 h	0.7	0.034	4.8	0.275	2.3	0.101
10 h	2.4	0.105	15.0	0.344	8.8	0.297
1 d	3.4	0.031	23.1	0.281	14.6	0.251
3 d	4.2	0.033	27.7	0.234	18.1	0.132
7 d	4.7	0.021	30.0	0.104	19.9	0.091
14 d	4.9	0.011	31.0	0.042	20.7	0.023

. The table shows the influence of different curing periods on the penetration depth of cement-stabilized aggregate.

It can be seen that the maximum penetration depth is high-permeability emulsified asphalt, the second is modified high-permeability emulsified asphalt, and the last one is traditional emulsified asphalt, on the condition that the amount of spreading is constant. This is because of the diluents and osmotic agent added in the high-permeability emulsified asphalt and its modification. The long chain of asphalt molecules is broken into a smaller one. It can reduce the surface tension of the liquid, reducing the viscosity. The high-permeability emulsified asphalt and its modification have a better permeability effect [30]. Adding waterborne epoxy resin and curing agent into the high-permeability emulsified asphalt reduces the permeability effect to a certain extent. The penetration depth of high-permeability emulsified asphalt and modified high-permeability emulsified asphalt can meet the specification requirements. The maximum penetration depth is more than 5mm, but the traditional emulsified asphalt does not meet the standard.

When the amount of spreading is constant, the curing period of the cement-stabilized crushed stone base increases gradually, and the penetration depth of emulsified asphalt in the permeable layer shows a trend of increasing firstly and then decreasing. In order to meet the requirements of penetration depth, the spread of permeable layer material should not be too late or too early; it should be 10 h to 24 h after compaction of the base. Just after the base is compacted, there is still water on the surface of the base. The water will block the gap of the surface of the base, hindering the use of permeable materials, so it is not appropriate to spray permeable materials too early. After a curing period of the base, the water on the surface is evaporated, and the surface is slightly dry. It benefits the emulsified asphalt to penetrate the base course. With the longer curing time, the strength of the cement-stabilized crash tone base gradually forms, and its internal structure becomes more and more compact. Coupled with traffic and dust pollution, the tiny pores on the surface of the base will be blocked, making it difficult for the permeable material to penetrate downward [31].

4.2. Tensile Strength

When ordinary (traditional) emulsified asphalt, high-permeability emulsified asphalt, and modified high-permeability emulsified asphalt were used as bonding materials, the pull-off strength results under different spraying rates are shown in

Table 13. Tensile strength of different emulsified asphalts at different sprinkling rates.

Spraying Volume (L/m2)	Pull-Off Strength (MPa)
	Traditional Emulsified Asphalt	High-Permeability Emulsified Asphalt	Modified High-Permeability Emulsified Asphalt
0.7	0.20	0.17	0.45
0.9	0.23	0.19	0.48
1.1	0.32	0.20	0.53
1.3	0.28	0.27	0.50
1.5	0.24	0.23	0.46

.

As can be seen from Figure 13, the tensile strength of the three kinds of emulsified asphalt increased first and then decreased with the increase of spraying amount. The optimal spraying amount of traditional emulsified asphalt and modified high-permeability emulsified asphalt is 1.1 L/m², while the optimal spraying amount of high-permeability emulsified asphalt is 1.3 L/m².

It can be seen from Table 13 that at the optimal spreading rate, the tensile strength of modified high-permeability emulsified asphalt is the highest (0.53 Mpa), followed by traditional emulsified asphalt (0.32 Mpa), and the worst is high-permeability emulsified asphalt (0.27 Mpa). Obviously, traditional emulsified asphalt and modified high-permeability emulsified asphalt have higher viscosity and better tensile strength. From the bonding point of view of the base and surface, the increase of viscosity is beneficial to the bonding. However, the increase in viscosity inevitably leads to difficulty in penetration. The penetration depth of traditional emulsified asphalt is very small, only asphalt oil film is formed on the surface of the base, and the bonding force is poor. Compared with traditional emulsified asphalt and modified high-permeability emulsified asphalt, high-permeability emulsified asphalt has the best permeability. Although the modified high-permeability emulsified asphalt with waterborne epoxy resin and curing agent is not as good as the high-permeability emulsified asphalt in the penetration effect, it can also meet the specification requirements, and its tensile strength is better than that of the high-permeability emulsified asphalt.

4.3. Shear Resistance

The shear strength of traditional emulsified asphalt, high-permeability emulsified asphalt, and modified high-permeability emulsified asphalt was investigated. The effect of temperature on the interlayer bonding effect was also analyzed using a straight shear test. Nine double-layer composite specimens of each of the three types of emulsified asphalt were tested. The testing temperature was set to 5 °C, 25 °C, and 55 °C. The straight shear test results are shown in

Table 14. Shear strength of three types of emulsified asphalt at different temperatures.

Types of Asphalt	Test Temperature	Maximum Shear Force (kN)	Shear Strength (MPa)
Traditional emulsified asphalt	5 °C	6.731	0.857
	25 °C	6.087	0.775
	55 °C	2.097	0.267
Highly permeable emulsified asphalt	5 °C	9.142	1.164
	25 °C	8.215	1.046
	55 °C	3.636	0.463
Modified high-permeability emulsified asphalt	5 °C	9.880	1.258
	25 °C	9.055	1.153
	55 °C	4.068	0.518

.

It can be seen from Figure 14 that the shear strength of the adhesive layer is significantly influenced by temperature. The shear strength decreases with the increasing of temperature. However, the shear strength decreasing rate is different across the different temperature zones. When the temperature is low, the emulsification has a certain adhesion. When the temperature is high, the shear strength of the asphalt pavement interlayer decreases significantly because of the low viscidity. If the pavement is subjected to a large traffic load at this time, the shear strength of the binder layer material will not be sufficient to resist the shear stress generated between the layers, resulting in shear damage at the binder layer interface.

No matter how the temperature changes, the rank of the three materials does not change, i.e., modified high-permeability emulsified asphalt > high-permeability emulsified asphalt > traditional emulsified asphalt. The result indicates that the greater the viscosity of the material itself, the greater the shear strength it can provide. The reason for this is that the properties of the material itself affect the performance of the interlayer bond. With the addition of diluents and osmotic agents, the permeability of the highly permeable emulsified bitumen is greatly enhanced, but the highly permeable emulsified bitumen cannot form a bitumen film on the surface of the base that provides sufficient bond strength, resulting in a poor bonding effect. The addition of water-based epoxy resin and curing agents can make the modified emulsified asphalt fully form a certain strength of oil film on the surface of the base, obtaining stronger bonding strength.

As the temperature rose to 55 °C, the shear strength of all three adhesive layer materials decreased sharply as the asphalt mixture reached its softening point. The traditional emulsified asphalt decreased the most significantly. The modified high-permeability emulsified asphalt has the best shear strength at high temperatures because the epoxy resin has been completely solidified and is not affected by temperature.

4.4. Water Resistance

The results of the water resistance test after spraying different permeable materials are shown in

Table 15. Analysis of water penetration test results for different permeable layer materials.

Type of Permeable Material	Permeability Coefficient	Analysis of Water Seepage Phenomena
Nothing	545 mL/min	Severe water penetration
Traditional emulsified asphalt	20 mL/min	A little water runs around the face plate, almost no water seepage at the bottom
High-permeability emulsified asphalt	12 mL/min	A small amount of water runs out of the face plate and bottom
Modified high-permeability emulsified asphalt	8 mL/min	Almost no water penetration

.

The test results show that the double-layer plate without permeable material has a serious water seepage and a large water seepage coefficient. After spraying traditional emulsified asphalt, high-permeability emulsified asphalt, and its modification as permeable layers, the water seepage coefficient is significantly reduced, and there is almost no water seepage phenomenon at the bottom of the double-layer plate except a small amount of water flowing out from around the surface plate.

The permeability coefficient of the three permeable materials is small. The three kinds of emulsified asphalt have a good waterproof performance. The ranking of waterproof performance for three kinds of permeable materials is: modified high-permeability emulsified asphalt > high-permeability emulsified asphalt > traditional emulsified asphalt. The asphalt film from the emulsified asphalt modified by epoxy resin is more stable and rigid, and its tensile strength is also higher. So, it appears to be the best one. The high-permeability emulsified asphalt and its modification have great potential as good waterproof material in asphalt pavements.

5. Conclusions

In this study, a new high-permeability emulsified asphalt with good stability and bonding effect was prepared and modified. The properties of high-permeability emulsified asphalt and its modification were studied. The main conclusions can be drawn as follows:

(1) The optimal dosage of the high-permeability emulsified asphalt can be recommended as 55.2% diluted asphalt (44.16% matrix asphalt + 11.04% diluent) + 40% emulsion (2% emulsifier + 0.5% stabilizer + 1% osmotic agent + 1% op-10 + 35.5% water) + 4.8% aromatic oil. This high-permeability emulsified asphalt can be prepared with good stability and permeability.

(2) The adhesion of the modified high-permeability emulsified asphalt is greatly improved, and the tensile strength and shear strength of the modified high-permeability emulsified asphalt are about two times higher than that of the high-permeability emulsified asphalt, which meets the requirements of the specification, and the interlayer shear, stability, and waterproof performance are good. In general, modified high-permeability emulsified asphalt is the best surface material.

(3) The amount of spraying and the curing age of the base has a significant influence on the penetration depth of the prime coat material on the base. The best spraying time of the prime coat material prepared in this paper is between 10 h and 24 h after the compaction of the base. The optimal spraying amount of ordinary emulsified asphalt and modified high-permeability emulsified asphalt on a cement-stabilized macadam base is 1.1 L/m², and that of high-permeability emulsified asphalt is 1.3 L/m².

(4) Based on the analysis of permeability, cohesiveness, and waterproof performance, the modified high-permeability emulsified asphalt are the prime coat materials with the best performance.