2.2. Preparation of Modified Asphalt Binder
In this study, three kinds of modified asphalt were prepared: diatomite modified asphalt (DA), crumb rubber modified asphalt (RA), and diatomite and crumb rubber compound modified asphalt (DRA). Wet process was widely adopted to prepare the modified asphalt binders [
11,
25,
26,
27], which can make crumb rubber, diatomite, and asphalt react for a period and take advantage of the benefits of all base ingredients [
4]. A high speed shear homogenizer [
20] was employed to stir the modified asphalt (shown in
Figure 1), the oil bath as a heating device can control the temperature of the modified asphalt when stirring. The preparation process mainly included three steps: heating of raw materials; shearing and mixing of ingredients; and storing of modified asphalt binder. Corresponding optimal or suggested preparation parameters of DA, RA, and DRA have been determined in previous research [
16,
28,
29]. The detailed preparation processes were as follows:
In the preparation of DA, the content of diatomite 12.8% (by the weight of asphalt) was adopted. The specific preparation steps were as follows: (1) Diatomite and asphalt were separately placed in the oven at 140 °C for 4 h to make sure that the asphalt flowed sufficiently and diatomite reached the blending temperature. (2) Diatomite and asphalt were mixed using the high-shear homogenizer with a speed of 4000 rpm, and the blending was conducted for 40 min at 150 °C in order to ensure that diatomite distributed uniformly in asphalt [
16].
In the preparation of RA, the content of crumb rubber 18% (by the weight of asphalt) was used. Specifically, (1) neat asphalt and crumb rubber were separately heated in the oven at 140 °C for 4 h, thus, asphalt was at a fluid state that can be easily stirred. (2) The crumb rubber was added into the asphalt, which was placed into the shear homogenizer after preliminary mixing. (3) Shear temperature and speed were set to 180 °C and 5000 rpm, respectively. The blend of asphalt and crumb rubber was mixed and sheared for 45 min after the temperature of the blend reached 180 °C. (4) The blended mix was contained in the shear homogenizer and allowed to swell for 60 min [
28].
In the preparation of DRA, the contents of diatomite and crumb rubber were 6.2% and 13.8% (by the weight of asphalt), respectively. The preparation steps of DRA were the same as those of the RA except the shear temperature, shear speed, shear time, and swelling time were 183 °C, 5300 rpm, 55 min, and 49 min, respectively [
29].
2.3. Characterization Method
Thin film oven test (TFOT) and rolling thin film oven test (RTFOT) are the most widely used methods for short term oxidative aging simulation of asphalt [
30,
31,
32]. Current research results indicate that the results from TFOT and RTFOT present strong correlation. The methods present similar severities for measurements with different parameters [
30]. In this study, TFOT was used for short term oxidative aging simulation in accordance with ASTM D 1754. Asphalt amount was 50 g in an iron pan, which was placed on the tray. The axis of revolution was vertical. In order to analyze the short term aging systematically and make the aging law of asphalt clear during the early age, the aging was conducted at 163 °C for 0, 1, 2, 3, 4, and 5 h.
Penetration is a useful property to evaluate the consistency of asphalt at intermediate service temperatures [
33]. It is the depth in units of 1/10 mm that a standard needle with a load of 100 g penetrates into the asphalt vertically in a time period of 5 s. In this study, penetration of modified asphalt was tested using penetration test instrument (DF-4, Beijing Zhonghangkegong Instrument Co., Ltd., Beijing, China) at 25 °C according to ASTM D5 before and after TFOT. Percent retained penetration (PRP) was calculated and used to evaluate the anti-aging effects on properties of DA, RA, and DRA, which can be obtained by
where
P1 and
P2 are the penetrations before and after TFOT, respectively.
Softening point is a widely used parameter to evaluate high temperature susceptibility of asphalt. It is the temperature at which a phase change occurs [
20]. The Ring and Ball Softening Point test (TR&B) (SYD-2806, Beijing Zhonghangkegong Instrument Co., Ltd., Beijing, China) was used to measure the temperature at which the asphalt could not support the steel ball with the weight 3.5 g at the heating rate 5 °C/min according to ASTM D36. Increment of softening point (
∆T) can be calculated and used to evaluate the aging degree [
14], which can be obtained by
where
T1 and
T2 are the softening points before and after TFOT, respectively.
Ductility is a method to indirectly evaluate the tensile cohesive property of asphalt. It is the distance in centimeters to which the sample can be elongated before breaking when two ends of a sample are pulled apart at a specified temperature and speed [
22]. In this study, ductility of modified asphalt was tested using a ductility test instrument (LYY-7, Beijing Zhonghangkegong Instrument Co., Ltd., Beijing, China) at 15 °C and a speed of 5 cm/min according to ASTM D113.
Viscosity reflects the flow characteristics of asphalt, which directly affects the workability of the asphalt mixture [
14,
22]. A Brookfield viscometer (SD-0625, Shanghai Geology Instrument Institute, China) was used to measure the rotational viscosity of crumb rubber and diatomite compound modified asphalt at 135 °C according to ASTM D4402. Viscosity aging index (
VAI) was applied to investigate the effect of aging on the shearing resistance of asphalt. A higher
VAI indicates a more serious aging pattern [
14].
VAI can be calculated by the following equation.
where
η1 and
η2 are the viscosities before and after TFOT, respectively.
Elastic recovery reflects the deformation recovery property of asphalt after being elongated. A degree of elastic recovery is desirable in pavement to avoid permanent deformation. The elastic recovery of asphalt is tested by use of a ductility test instrument (LYY-7, Beijing Zhonghangkegong Instrument Co., Ltd., Beijing, China), which is used to elongate an asphalt specimen at a constant rate. After a period of time, the elongated specimen is cut and then allowed to rest. After the period of rest is complete, the distance between the ends of the cut specimen is measured. The elastic recovery is the ratio of the difference in elongation between cutting and the end of the rest period to the total elongation [
28].
where
ER is elastic recovery of asphalt, %;
Eb is the elongation of asphalt specimen before cutting; and
Ea is the elongation after the period of rest.
The bending beam rheometer (BBR) test was used to investigate the low temperature creep property of modified asphalt. The size of beam sample was 125 mm × 12.5 mm × 6.25 mm, which was submerged in an ethyl alcohol bath at −12 °C for 60 min before tests. Creep tests were carried out at −18 °C by a bending beam rheometer (TE-BBR, Cannon Instrument Company, Harrisburg, PA, USA). Deflection at the mid-span point of the beam was measured continuously under constant loads, and creep stiffness (S) was determined according to ASTM D 6648.
The dynamic shear rheometer (DSR) test was used to investigate the rheological properties of modified asphalt at high and intermediate temperatures. Rheological parameters, including complex modulus () and phase angle (), can be measured. The DSR test (DSR, Malvern Instruments Ltd., Malvern, UK) was conducted at 70 °C. The rutting parameter () was used to evaluate the rutting susceptibility of asphalts, which can be determined by the measured and according to ASTM D 5801.