Rutting and Fatigue Resistance of High-Modulus Asphalt Mixture Considering the Combined Effects of Moisture Content and Temperature

Abstract

High-modulus asphalt mixtures (HMAM) have been widely used in asphalt pavement in high-temperature areas of China, owing to their advantages in rutting and fatigue resistance. However, moisture and temperature interdependently determine the degradation of pavement performance of the HMAM, owing to the unique climatic conditions in summer in some high-temperature areas of China. There were few studies on the rutting and fatigue properties of the HMAM under the combined action of moisture contents and temperatures. Hence, the moisture absorption characteristics of the HMAM at different temperatures were analyzed. The rutting performance of the HMAM was investigated under different moisture contents and temperatures. The fatigue performance of the HMAM was investigated under different moisture contents, temperatures, and stress levels. Results show that: the rutting and fatigue resistance of the HMAM decrease with the increase in temperature and moisture content. The dynamic stability decreases by 8.9% at 40 °C and by 7.0% at 60 °C on average per 10% increase in moisture content and decreases by 22.7% on average per 10 °C increase in temperature. The fatigue life decreases by 4.1% at 15 °C and by 3.1% at 40 °C on average per 10% increase in moisture content and decreases by 31.3% on average per 10 °C increase in temperature. Finally, a prediction equation was established to predict the fatigue life under different moisture contents and temperatures.

1. Introduction

Asphalt pavement has been widely used in high-grade roads owing to its good driving comfort. However, with the substantial increase in traffic volume, traffic overload, and traffic channelization, rutting, and fatigue cracks have become the most serious diseases of asphalt pavement in China [1,2,3,4,5]. In response to this problem, a high-modulus asphalt mixture (HMAM), which was prepared by adding high-modulus admixtures into common asphalt mixtures, is used to adopted in the construction of asphalt pavement to improve its rutting and fatigue resistance.

Karami et al. [6], Wang et al. [7], and Lv et al. [8] compared the performance of Buton rock asphalt (BRA) admixture with ordinary asphalt mixture through dynamic creep, elastic modulus tests, and rutting tests. The results showed that BRA could improve the high-temperature performance of the mixture. Pan et al. [9] and Li et al. [10] indicated the performance of the HMAM containing BRA. They concluded that the HMAM containing BRA had better fatigue resistance than the conventional asphalt mixture. Yan et al. [11] investigated the fatigue resistance and high-temperature stability of asphalt mixtures with various high-modulus additives. The results indicated that the rubber asphalt mixture, Styrene butadiene styrene (SBS) and BRA-modified asphalt mixture, and SBS-modified asphalt containing PR MODULE (PR.M) admixture have excellent rutting resistance. Zou et al. [12] carried out the performance of two types of high-modulus admixtures (PR PLAST S and PR.M) with different contents of high-modulus admixture. They demonstrated that PRS and PR.M could improve the high-temperature stability of the HMAM. Zhang et al. [13] analyzed the effect of resin alloy (RA) on the high- and low-temperature performance, water stability, and fatigue resistance of the HMAM. Subsequently, Zhang et al. [14] investigated the influence of three high-modulus admixtures (PRS, RA, and Duroflex) on the permanent deformation characteristics of the asphalt mixture through the rutting test and Hamburg wheel tracking test. They considered that the addition of RA could significantly improve the high-temperature stability and water stability of the asphalt mixture. The existing studies proved that the HMAM could provide improved rutting and fatigue resistance, which was beneficial to reduce the occurrence of the disease of asphalt pavement in high-temperature areas.

However, owing to the rainy season and urban heat island, a special period, namely ‘plum rain season’, comes and lasts for 30–40 days in summer in some high-temperature areas of China, such as southeastern coastal provinces (e.g., Jiangsu, Zhejiang, etc.). The total precipitation during the plum rain season is noteworthy because the medium-intensity rainfall lasts an unusually long time while the air temperature stays high. In other words, the rutting and fatigue resistance of asphalt pavement in these areas is determined by a combination of high-temperature and moisture effects during the plum rain season. Owing to the combined effects of high temperature and accumulated water, the risk of rutting and fatigue diseases of asphalt pavement significantly increases [15]. Hence, it is crucial to investigate the effects of moisture contents and temperatures on the rutting and fatigue resistance of the HMAM. However, few studies had addressed this issue [16].

Hence, the objective of this study is to investigate the rutting and fatigue resistance of the HMAM under different combinations of moisture content, temperature, and loads. Moreover, a series of prediction equations were established to predict the dynamic stability and fatigue life of the HMAM considering the effects of moisture and temperature.

2. Materials and Methods

2.1. Raw Materials

2.1.1. Aggregates

(1)

Coarse aggregates

Coarse aggregate selection is implemented in accordance with the Chinese testing standard “test methods of aggregate for highway engineering (JTG E42-2005)” [17]. Limestone was selected in this study. Its performance test results are listed in

Table 1. Technical index of coarse aggregate.

Technical Index	Result	Specification Requirements
Crushing value of stone (%)	14.4	≤28
Los Angeles Abrasion Value (%)	16.2	≤30
Robustness (%)	8	≤12
Needle-like particle content (%)	7	≤15
Water washing method < 0.075 mm Particle content (%)	0.5	≤1
Soft stone content (%)	2	≤5
Moisture absorption (%)	0.81	≤3

.

(2)

Fine aggregate

Fine aggregate selection is implemented in accordance with the Chinese testing standard “test methods of aggregate for highway engineering (JTG E42-2005) [17]”. The performance test results of fine aggregate are shown in

Table 2. Technical index of fine aggregate.

Technical Index	Result	Specification Requirements
Mud content (%)	1	≤3
Angular [flow time (s)]	65	≥30
Robustness (>0.3 mm) (%)	5.6	≤12
Appearance	No agglomeration	No agglomeration
Hydrophilicity coefficient	0.61	<0.8
Heating stability	Uniformity	Measured record

.

(3)

Mineral powder

Fine aggregate selection is implemented in accordance with the Chinese testing standard “test methods of aggregate for highway engineering (JTG E42-2005) [17]”.The mineral powder used in this test is finely ground limestone, and its technical performance indexes are shown in

Table 3. Technical index of mineral powder.

Technical Index		Result	Specification Requirements
Apparent density (g/m3)		2.732	≥2.45
Moisture content (%)		0.7	≥1
Particle size range (%)	<0.6 mm	100	≤12
	<0.15 mm	94.8	90–100
	<0.075 mm	80.4	70–100
Appearance		No agglomeration	No agglomeration
Hydrophilicity coefficient		0.6	<1
Plasticity index (%)		3	<4
Heating stability		Uniformity	Measured record

.

2.1.2. Asphalt

Asphalt selection is implemented in accordance with the Chinese testing standard “standard test methods of bitumen and bituminous mixtures for highway engineering (JTG E20-2011) [18]”. 70# asphalt is used in this test, and the technical indicators are shown in

Table 4. Technical index of asphalt.

Technical Index		Result	Specification Requirements
Penetration [100 g, 25 °C, 5 s, (1/10 mm)]		67	60–80
Penetration index (PI)		−0.7	−1.5–1.0
10 °C Ductility (cm)		>20	≥20
15 °C Ductility (cm)		>100	≥100
Softening point (°C)		48	≥43
60 °C Dynamic viscosity		183	≥180
Wax content [Distillation (%)]		2	≤2.2
Flash point (°C)		276	≥260
Solubility (%)		99.8	≥99.5
Density [15 °C (g/cm3)]		1.0029	Measured record
TFOT	Quality change (%)	0.1	≤±0.8
	Residual penetration ratio (%)	64	≥61
	10 °C Residual ductility (cm)	10	≥6
	15 °C Residual ductility (cm)	17	≥15

.

2.1.3. Admixture

According to the previous studies [6,7,8,9,10,11,12,13,14], three widely used high-modulus admixtures (i.e., RA, PR.M, and BRA) were adopted to prepare the HMAM in this study. RA is provided by Sichuan Kelutai Transportation Technology Co., Ltd. (Chengdu, China) (Figure 1a). It is an alloy resin material, of which the density is 0.96 g/cm³. PR.M is provided by Sichuan Mingdelai Technology Co., Ltd. (Chengdu, China) (Figure 1b). It is a thermoplastic resin material, of which the density is 0.95 g/cm³. BRA is provided by Sichuan Shengxiang Chemical Technology Co., LTD (Chengdu, China) (Figure 1c). It is a kind of natural asphalt, of which the density is 0.96 g/cm³. According to the suppliers’ recommendation, the content ranges of RA, PRM, and BRA used in this study are 0.35–0.45%, 0.40–0.50%, and 3.0–4.0%, respectively.

2.2. Methods

2.2.1. Rutting Test

According to the Chinese testing standard “standard test methods of bitumen and bituminous mixtures for highway engineering (JTG E20-2011)” [18], the rutting specimens (300 mm × 300 mm × 50 mm) of the HMAMs were prepared. The rutting tests were carried out at 40 °C and 60 °C. Before the rutting tests, the rutting specimens should be immersed in water at a constant temperature that was equal to the temperature used in rutting tests. Three parallel tests were successfully implemented for each group.

2.2.2. Fatigue Test

At present, among many indoor small fatigue test methods, the four-point bending fatigue test standard test method, indirect tensile fatigue test and three-point bending fatigue test selected by the Strategic Highway Research program (SHRP) research plan are the most widely used. However, these methods are still controversial at present, and each has its own advantages and disadvantages. It was, unfortunately, challenging to provide enough stability for the strain-controlled fatigue test of asphalt mixture at high-temperature owing to the limitations of the test apparatus used in this study. As a result, the stress-controlled three-point fatigue test was chosen, considering the relatively lax requirements of the loading control system in this situation. According to pertinent studies [19,20], the three-point bending fatigue test is a well-respected and straightforward test method with a high degree of simulation, whose loading mode is similar to that of a wheel acting on a road surface.

It is possible to calculate the stress level used in stress-controlled fatigue tests using Equation (1). Therefore, in order to ensure the reliability and availability of the test results for evaluating the fatigue properties of asphalt mixture, and taking the simplicity of the test as the principle, this paper adopts the three-point bending fatigue test as the evaluation method.

$$\mathsf{\sigma }=\frac{{\mathsf{\sigma }}_d}{{\mathsf{\sigma }}_s}$$

(1)

where $\mathsf{\sigma }$ is the stress level, ${\mathsf{\sigma }}_d$ is the applied load in fatigue tests, and ${\mathsf{\sigma }}_s$ is the ultimate three-point bending strength of the asphalt mixture fatigue specimens.

Before the fatigue test, the maximum load of the bending test was obtained to determine the load applied in the fatigue test. The trabecular bending test was conducted with a prismatic trabecular beam 250 ± 2.0 mm in length, 30 ± 2.0 mm in width, and 35 ± 2.0 mm in height cut after forming by wheel rolling method. The bending test results are shown in

Table 5. Trabecular bending test results.

Type of Admixture	Moisture Content (%)	Temperature (°C)	Maximum Load Value (N)	Deflection (mm)
RA	0	15	821	4.569
		40	308	10.534
	50	15	846	7.136
		40	300	16.609
	100	15	857	8.236
		40	296	19.213
PR.M	0	15	749	6.765
		40	381	12.331
	50	15	783	8.633
		40	357	17.862
	100	15	797	9.433
		40	347	20.232
BRA	0	15	842	5.234
		40	216	11.476
	50	15	877	7.816
		40	199	16.312
	100	15	892	8.923
		40	192	18.384

.

The specimens were formed by wheel rolling method according to the specifications and cut into beam specimens with a length of 380 ± 5 mm, a thickness of 50 ± 5 mm, and a width of 63.5 ± 5 mm. The fatigue tests were carried out at 15 °C and 40 °C. Before the fatigue tests, the specimens should be immersed in water at a constant temperature (Figure 2a) which was equal to the temperature used in fatigue tests. Three parallel tests were successfully implemented for each group (Figure 2b,c). The loading frequency is 10 Hz under stress-controlled condition.

2.3. Aggregate Gradation of the HMAM

The asphalt-aggregate ratio adopted in this test is 5.0%. The aggregate gradation is shown in

Table 6. Aggregate gradation.

Passing size (mm)	19	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Mass ratio (%)	100	100	91.7	72.3	49.0	34.8	23.6	16.0	10.0	8.2	6.9

.

3. Moisture Saturated Characteristics of the HMAM

The formed Marshall specimen of the HMAMs was immersed in a constant temperature water tank, and the test temperature was controlled at 15 °C, 40 °C, and 60 °C. The moisture content of the asphalt mixture can be assessed using moisture absorption percent (Equation (2)).

$$S_a=\frac{\left(m_f-m_a\right)}{\left(m_f-m_w\right)}$$

(2)

where $S_a$ is the moisture absorption percent, $m_f$ is the mass of the surface-dry specimen in air, $m_a$ is the mass of the dry specimen in air, and $m_w$ is the mass of the specimen in water.

Curves of moisture content of RA, PR.M, and BRA at 15 °C, 40 °C, and 60 °C are plotted in Figure 3a, Figure 3b and Figure 3c respectively. For example, in the figures, “15 °C-PR.M” indicates the change of moisture content of PR.M with soaking time under the condition of 15 °C.

As can be seen from Figure 3, the moisture content curves of different asphalt mixtures are almost the same at 15 °C and 40 °C. At the temperature of 60 °C, the curve of each asphalt mixture changes greatly with that at 15 °C and 40 °C, and it takes a longer time for each asphalt mixture to reach 50%, 80%, and 100% moisture content. Therefore, 0%, 50%, 80%, and 100% moisture content are selected as test indexes when conducting subsequent rutting and fatigue tests, as shown in

Table 7. Moisture content index.

Moisture Content (%)	Temperature (°C)	Soaking Time (h)	Moisture Content (%)	Temperature (°C)	Soaking Time (h)
0	15	0	80	15	3.5
	40	0		40	5.5
	60	0		60	22.5
50	15	0.3	100	15	50
	40	0.5		40	44.5
	60	3.5		60	72.5

.

4. High-Temperature Stability of the HMAM under Moisture and Temperature Coupling Effect

According to the data of high-temperature stability test results, the dynamic stability curves of three kinds of HMAM with moisture content were drawn under the conditions of 40 °C and 60 °C, respectively.

As can be seen from Figure 4, the dynamic stability of the HMAM is not only related to temperature but also significantly decreases with the increase in moisture content. Moisture content has a great influence on the dynamic stability of the HMAM. Under the condition of 40 °C, the relative moisture content of RA, PR.M, and BRA is 0%, and when the moisture content is 50%, 80%, and 100%, respectively, the average dynamic stability decreases by 42.8%, 22.3%, and 21.1%, respectively. The dynamic stability does not decrease linearly with the increase in moisture content. When the temperature rises to 60 °C, there is an approximate change rule. At the same time, under any conditions, the dynamic stability of the RA asphalt mixture is higher than that of PR.M and BRA. The main reason may be that RA is mainly composed of rock asphalt and resin alloys, which can optimize the properties of different polymers and significantly improve the properties of materials. However, PR.M and BRA are primarily composed of high melting point materials that perform poorly at low temperatures.

The dynamic stability curve of the HMAM with the same type of control admixture and different content is shown in Figure 5. For example, in the figures, “40 °C–50%” indicates the change of dynamic stability of the asphalt mixture with the moisture content of 50% at 40 °C.

The dynamic stability of the asphalt mixture increases with the increase in admixture content. The dynamic stability data of the three HMAMs under the condition of non-immersion at 60 °C are selected for research, and the dynamic stability results are shown in Figure 6.

When the dynamic stabilities of the three HMAMs with different admixture contents are compared, it is discovered that when the content of RA is 0.4%, it is 12.6% higher than 0.35%, and when the content is increased to 0.45%, it is only 6.1% higher. When the content of PR.M is 0.45%, the dynamic stability is increased by 30.0% compared with 0.4%. When the content is increased to 0.5%, the dynamic stability is only increased by 5.0%. When the content of BRA is 3.5%, it is 34.5% higher than 3.0%, and when the content is increased to 4.0%, it is only 14.0% higher. Therefore, the optimal content of PR.M, RA, and BRA is 0.45%, 0.4%, and 3.5%, respectively.

5. Fatigue Resistance of the HMAM under Moisture and Temperature Coupling Effect

5.1. Effect of Different Factors on Fatigue Resistance of the HMAM

5.1.1. Effect of Moisture Content

According to the fatigue test data, the fatigue life curves of the three HMAMs with different moisture contents are shown in Figure 7. For example, in the figures, “15 °C–0.2” indicates the change of fatigue life of asphalt mixture with moisture content under the condition of 15 °C and 0.2 stress level.

The fatigue life of the above three HMAMs decreases after the moisture content increases, indicating that moisture can reduce the fatigue life of the HMAM. The analysis of relevant studies [21,22] may be because the moisture in the asphalt mixture enters between the asphalt film and aggregate, blocking the mutual bonding of asphalt and aggregate. The contact surface of asphalt and aggregate surface is reduced, so that the asphalt from the aggregate surface spalling, reducing the strength of the mixture.

At a temperature of 15 °C and a stress level of 0.2, the fatigue life of RA, PR.M, and BRA asphalt mixture with increased moisture content decreases by 29.6%, 32.9%, and 33.2%, respectively. At a temperature of 40 °C and a stress level of 0.2, the fatigue life decreases by 29.3%, 26.9%, and 20.7%, respectively. The fatigue lives of the three HMAMs decreased by almost the same degree. Meanwhile, by comparing the fatigue life at 15 °C and 40 °C, it can be found that temperature also has a certain influence on the fatigue life. Under the condition that the moisture content is 0% and the stress level is 0.2, the fatigue life of RA, PR.M, and BRA decreases by about 79.9%, 73.4%, and 78.5%, respectively. When the stress level is 0.4 and 0.5, the variation law of fatigue life with moisture content is similar to that when the stress level is 0.2, which will not be discussed in this paper.

5.1.2. Effect of Stress Level

Figure 8 illustrates the relationship between stress levels and fatigue life of the three HMAMs at different moisture contents and temperatures. For example, in the figures, “15 °C–0%” indicates the change of fatigue life of asphalt mixture with different stress levels under the condition of 15 °C and moisture content of 0%.

The fatigue life of the three HMAMs decreases with the increase in stress level. Compared with that of 0.2, the average fatigue life of RA, PR.M, and BRA asphalt mixtures decreases by 79.6% (67.3%), 76.5% (66.8%), and 77.7% (70.5%), respectively, at stress levels of 0.4 (0.5). The decline rate of fatigue life of the three HMAMs with high-modulus after stress level increases are lower than that of temperature, but it is still much higher than that caused by moisture content increases, indicating that the influence of stress on fatigue life is greater than that of moisture content on fatigue life but less than that of temperature. At the same time, the decline rate of fatigue life of the three HMAMs under different stress levels is similar, but the fatigue life of PR.M is lower than that of the other two asphalt mixtures.

5.2. Fatigue Equation

Previous studies have shown [23,24,25,26] that the relationship between stress and fatigue life can be expressed by the following Equation (3).

$$N_f=k{\left(\frac{1}{\delta }\right)}^n$$

(3)

The stress-fatigue curves of the three HMAMs with a moisture content of 0%, 50%, and 100% at different temperatures were drawn based on the above experimental results, as shown in Figure 9. For example, in the figures, “15 °C-stress-fatigue” indicates the fatigue life at 15 °C.

The stress-fatigue equations of the three HMAMs are summarized in

Table 8. Stress-fatigue equation.

Type of Admixture	Temperature (°C)	Moisture Content (%)	Regression Equation (${\mathit{N}}_{\mathit{f}}=\mathit{k}{\left(\frac{1}{\mathit{\delta }}\right)}^{\mathit{n}}$)
			k	n	R2
RA	15	0	773.9	2.24	0.980
		50	549.5	2.31	0.984
		100	454.6	2.35	0.986
	40	0	63.0	2.80	0.999
		50	48.6	2.81	0.999
		100	42.4	2.83	0.999
PR.M	15	0	590.4	2.31	0.982
		50	442.8	2.33	0.984
		100	379.5	2.34	0.985
	40	0	134.7	2.35	0.998
		50	115.6	2.32	0.999
		100	107.5	2.30	0.999
BRA	15	0	649.2	2.33	0.977
		50	449.3	2.33	0.980
		100	435.0	2.33	0.982
	40	0	87.4	2.51	0.999
		50	69.6	2.56	0.999
		100	62.1	2.58	0.999

.

In the equation, the k value represents the level of the fatigue curve line, and the larger the k value is, the better the fatigue resistance of the mixture is. The larger n, the greater the change in the fatigue life of the mixture caused by the change in stress. With the increase in moisture content, the k value of the HMAM decreases, which means that moisture content influences the fatigue performance of the HMAM. Compared with the k value of the three HMAMs at 15 °C, the k value decreases significantly at 40 °C, and the decreasing range is much larger than that caused by the increase in moisture content, which indicates that the influence of temperature on the fatigue properties of the HMAM is stronger than that of moisture.

6. Conclusions

Through high-temperature stability tests and three-point fatigue tests, the changes in rutting and fatigue properties of three types of the HMAM (RA, PR.M, and BRA) with moisture, temperature, and loads were systematically analyzed. In addition, a series of prediction equations are established to predict the dynamic stability and fatigue life of the HMAM under different moisture contents and temperatures. The following conclusions are drawn:

• Under the temperatures of 40 °C and 60 °C, three kinds of HMAM with three contents and three moisture content were rutted tested. With other conditions unchanged, the temperature was adjusted to 15 °C and 40 °C, and the three-point bending fatigue life test was carried out. It was found that the high-temperature stability and fatigue life of the three HMAMs showed a decreasing trend with the increase in moisture content, but the high-temperature stability was more sensitive to moisture content. The high-temperature stability and fatigue life of the asphalt mixture is also affected by temperature. With the increase in temperature, its performance will decline, but the sensitivity of fatigue life to temperature is higher than that of high-temperature stability. The fatigue life is affected not only by temperature and moisture content but also by stress level. With the increase in stress level, the fatigue life tends to decrease.
• The high-temperature stability of the HMAMs does not linearly decrease with the increase in moisture content but presents a trend of first fast and then slow. With the increase in moisture content or stress level, the decrease in fatigue life is not linear. When the moisture content or stress level exceeds 50% or 0.4, the decay rate of fatigue life decreases obviously. In addition, the type of high-modulus admixture also affects the performance of asphalt mixture to a certain extent, among which RA is mainly composed of alloy resin and rock asphalt, and its high-temperature performance is better than the other two kinds of high-modulus asphalt mixture. The fatigue life of the PR.M asphalt mixture is lower than that of the other two, and the fatigue life changes of RA and BRA are similar.
• The dynamic stability equation and fatigue life prediction equation proposed in this paper have a high correlation coefficient, which provides a high-precision prediction and characterization for the high-temperature stability and fatigue life of the asphalt mixture.