Wetting is the process where asphalt infiltrates into the aggregate surface, which is important for the bonding between asphalt and aggregates. In this paper, the aggregate surface textures were simplified to
Water damage is one of the most important failures for highway asphalt pavement, and many highway asphalt pavements face more and more serious water damage after serving for more than one year in South China. Water damage is mainly derived from the loss of the bonding between asphalt and aggregates, which is closely related to the viscosity and cohesion of asphalt, the alkalinity or acidity property, and the surface roughness of aggregates. Some authors demonstrated that moisture could damage the asphalt that lost in strength and durability due to the presence of water [
Surface energy theory could be used to analyze the bonding mechanism between asphalt and aggregates, but most of the current research studies focus on the properties evaluation after the asphalt had adhered on the aggregates and rarely involve in how the asphalt was adhered on the aggregates. The bonding of asphalt and aggregates was the result of asphalt wetting on the aggregates, which is important in the bonding or adherence between asphalt and aggregates [
Viscosity and surface tension of asphalt are the basic properties of asphalt and the important index of asphalt cohesion [
In this experiment, the 70# virgin asphalt is selected and the asphalt rotation viscometer is used to determine the viscosity. The experimental program is identical to the specification. Viscosity at different temperatures was measured as shown in Table
Viscosity of asphalt at different temperatures (MPa·s).
Temperature (°C) | 180 | 160 | 140 | 120 | 100 | 80 | 60 |
The matrix asphalt | 69 | 230 | 302.5 | 855 | 2800 | 39296 | 101772 |
According to the Saal formula, the viscosity and temperature of asphalt have the following relationship:
The results showed that the viscosity of asphalt decreased significantly with the increase of temperature and showed a double logarithmic linear relationship.
In this experiment, the 70# virgin asphalt is selected, and the hanging line method is used to determine the surface tension [
Surface tension of asphalt at different temperatures (mN/m).
Temperature (°C) | 60 | 80 | 100 | 120 | 140 | 160 | 180 |
Matrix asphalt | 18.54 | 17.39 | 16.12 | 14.90 | 13.64 | 12.45 | 11.27 |
According to the data in Table
With the increase of temperature, the surface tension decreases significantly and has a strong linear rule.
When we place a liquid drop on a clean, planar, solid surface, we can observe a contact angle
On a nonideal surface, the static contact angle turns out not to be unique. If, for instance, we inflate a drop, as shown in Figure
(a) Advancing angle when the drop is inflated. (b) Receding angle when the drop is deflated.
The contact angle between the 70# virgin asphalt and aggregates at different temperature was obtained as shown in Table
Contact angle between asphalt and aggregates at different temperatures (°).
Temperature (°C) | 60 | 80 | 100 | 120 | 140 | 160 | 180 |
Aggregates | 20.41 | 19.83 | 19.39 | 19.02 | 18.89 | 18.73 | 18.61 |
Through analysis, it can be seen that asphalt temperature has an effect on the contact angle of asphalt aggregates. When the temperature increases, the contact Angle decreases. But overall, the contact Angle varies little.
Wetting is the process of asphalt to maintain contact with aggregate surface and actually is a process of the asphalt to infiltrate into the aggregate surface texture [
Aggregate surface texture and the model of asphalt infiltrating into aggregates. (a) Aggregate surface; (b) simplification of aggregate surface texture.
According to the general wetting theory [
With the change of aggregate size and shape, its surface texture depth and width changes. According to the existing literature [
It is known that
In the basic wetting model, the parameters of asphalt are considered to be constant, not varying with temperature. So, the 70# virgin asphalt was selected, and the technical parameters were measured at 160°C. The surface tension of SK70# virgin asphalt is 12.45 mN/m, the viscosity is 230 MPa·s, and using the contact test, the obtained contact angle between asphalt and aggregates is 18.73°.
The above model parameters are substituted in equation (
Infiltration curves of asphalt on the aggregate surface.
The results show that the asphalt infiltrates into the aggregate surface texture faster in the preliminary stage and slower in the later stage, and the infiltrating asphalt could be only more and more closer to the bottom of surface texture but never reach to. But there is a moment at which the infiltration rate becomes very slow, and the infiltration process could be considered as approximately completed. This time is defined as infiltration ending time. The infiltration ending time is obtained by estimating the infiltration curves as in Figure
The above discussion is based on the assumption that the technical parameters of asphalt are not varied with temperature during the wetting process. But in fact, the asphalt surface tension, viscosity, and the contact angle between asphalt and aggregates are closely related to the temperature. With consideration of the influence of temperature on asphalt parameters, the basic wetting model should be modified.
The surface tension of the liquid is caused by its surface molecules suffering unbalanced force, and usually varied with the temperature [
According viscosity experiment, the variation of asphalt viscosity with temperature could be expressed as equation (
In order to obtain the variation law of temperature with the time, the asphalt was firstly heated to 160°C and then placed in air for cooling to 80°C, so as to simulate the cooling process of asphalt in the construction period. During the simulated cooling process, the asphalt temperatures over cooling time were recorded and analyzed. The relationship between asphalt temperature and cooling time is expressed as
In fact, the contact angle between asphalt and aggregates is changing with the asphalt temperature. While the experiment proved that the influence is very small, this paper takes no account of its effect and considers the contact angle is constant.
Rewrite equation (
Substitute equations (
The integral form of equation (
In equation (
Substitute the initial conditions in equation (
Comparison of infiltration curves deduced from the basic and modified models.
From the infiltration curves, the infiltration ending time could be determined, and the comparison of the infiltration ending times deduced from the basic and modified models is expressed in Table
Comparison of infiltration ending time deduced from basic and modified model.
Surface texture depth (mm) | Infiltration ending time from basic model (s) | Infiltration ending time from modified model (s) | Amplitude of variation (s) | Relative error (%) |
---|---|---|---|---|
0.8 | 0.53 | 0.83 | 0.30 | 56.6 |
1.0 | 0.83 | 1.54 | 0.71 | 85.5 |
1.8 | 2.75 | 3.80 | 1.05 | 38.2 |
Though the effects of cooling temperature could be ignored during mixing, the initial temperature affects the surface tension and viscosity significantly and may have an important influence on the infiltration time.
In order to analyze the abovementioned influence, this paper takes the constant value of
Infiltration curves of asphalt at different initial temperature.
According to Figure
Infiltration ending time at different initial temperatures.
The surface texture sizes are also the important parameters in the infiltration of asphalt on the aggregate surface [
Infiltration ending times with different
It could be found that the coarser the surface texture of aggregates, the better the bonding between asphalt and aggregates. The infiltration ending time increases with the depth of the surface texture, but decreases with the width; that is to say, the deeper and narrower the aggregate surface texture is, the longer the asphalt wetting process is. So we should choose aggregates with rough surfaces and large voids. The relationship between the infiltration ending time and surface texture size is expressed as
This paper analyzed the general wetting process of asphalt on the aggregate surface based on the surface energy theory and established a modified wetting model by considering the asphalt parameters variation on the temperature, according to which the effects of initial temperature and cooling rate of asphalt and aggregate surface texture size on the wetting process were evaluated. Some conclusions are obtained as following. According to the basic wetting model, the asphalt infiltrates into the aggregate surface texture faster in the preliminary stage and slower in the later stage, and the infiltrating asphalt could be only more and more closer to the bottom of surface texture but never reach to. The basic wetting model should be modified with consideration of the influence of temperature on asphalt properties, and the cooling and lower initial temperature result in a longer wetting process. However, the effects of cooling rate could be ignored in actual practice. The relationship between the infiltration ending time and surface texture size has been established with a shape-related correction coefficient and indicates that the infiltration ending time of asphalt on the aggregates is essentially proportional to the texture depth squared but inversely proportional to the texture width.
The data used to support the findings of this study are available from the corresponding author upon request.
All authors have no conflicts of interest to declare.
This work was supported by National Key R&D Program of China (no. 2018YFE0103800), China Postdoctoral Science Foundation (no. 2017M620434), Shaanxi Postdoctoral Grant Program (no. 2017BSHYDZZ17), and the Special Fund for Basic Scientific Research of Central College of Chang’an University (no. 310821173501). The authors gratefully acknowledge their financial support.