The asphalt used in this study was conventional asphalt (90^#), and its properties are shown in Table 1. WEPS was obtained from recycled TV's buffer packaging liner, and its properties are given in Table 2.

Table 1.

Table 1.

Table 1. Properties of 90# asphalt Penetration (25 °C, 100 g)/0.1 mm Softening point (ring and ball method) (°C) Ductility (cm) 86.13 51.25 >200

Table 1. Properties of 90# asphalt

Table 2.

Table 2.

Table 2. Properties of recycled WEPS Main components Softening temperature (°C) Antifreeze temperature (°C) Compression degree (MPa) Density (g/cm2) EPS 75 -40 107 0.15

Table 2. Properties of recycled WEPS

The WEPS was dried at room temperature after being cleaned, and then shaved into small particles with Komon knife (as shown in Fig. 1).

Fig. 1.

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	Fig. 1. Optical images of recycled EPS (a) and (b) EPS small particles obtained by cutting.

The asphalt was heated at 150 °C until it was fully melted and then a certain amount of WEPS (listed in Table 3) was slowly added. The mixture of asphalt with WEPS was heated to 190 °C, followed by manual stirring for 30 min and automatic stirring for 1.5 h with a JRJ 300-S digital shear emulsifying mixer (Shanghai Specimen Model Factory, China). The mixture was then cooled to 120 °C and kept for 30 min to disperse WEPS evenly in the asphalt.

Table 3.

Table 3.

Table 3. Ratios of WEPS to based asphalt Modified asphalt samples WEPS (wt%) Asphalt (wt%) 1# 0 100 2# 2 98 3# 3 97 4# 4 96 5# 5 95

Table 3. Ratios of WEPS to based asphalt

The differential scanning calorimetry (DSC) curves were obtained on a 2910 differential scanning calorimeter (US Ta Model) in the temperature range from -40 °C to 180 °C at a heating rate of 10 °C/min.

Rheological characteristics were examined with an AR1500ex dynamic rheological shear apparatus provided by Ta company, US.

Complex shear modulus ( G^*), storage modulus ( G′), loss modulus ( G″), phase angle ( δ) were measured by a dynamic shear theometer. G^* and δ can be used to characterize the viscosity and elasticity of modified asphalt^{[ 7],[ 8]}.

Viscosity was measured by using a Brookfield DV-II type rotary viscometer made in Middleboro of USA.

The DSC peak area forms direct ratio to all heat of reaction. It provides a chemical method to study the endothermic behavior of asphalt quantitatively because of high-precision and good reproducibility^{[ 9]}. From a thermodynamic perspective, during the heating process three states are presented in asphalt, which are glassy, viscoelastic, and viscosity fluid state. The glass transition of asphalt, which is not a real thermodynamic second order transition, but a relaxation process of polymer or macromolecular chains, and viscous flow transition have a wide temperature range. As seen in Fig. 2, base asphalt has one absorption peak and the modified asphalt has two endothermic peaks. Zhang et al.^{[ 10] and [ 11]} demonstrated that the endothermic peak reflects the extent of transformation in the aggregation, the ratio of solid to liquid and the viscoelasticity of asphalt. As WEPS is added, the absorption firstly goes down and then slightly goes up. Weaker absorption usually suggests better plasticity and thermal stability of asphalt^{[ 12]}. The bigger absorption peak becomes, the microscopic properties of the asphalt components in this temperature range change more, and state interval occurs in more components, which include the crystalline component of the molten, the non-crystalline component of phase change and have an immense impact on macroscopic. The reason is that the high-speed mixing leads to the uniform disperse of WEPS in asphalt, as shown in Fig. 3. The WEPS can effectively absorb the wax in asphalt and remove the adverse effects of wax on the susceptibility of temperature and plasticity at low temperature.

Fig. 2.

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	Fig. 2. Thermal absorption of asphalt and WEPS-modified asphalt.

Fig. 3.

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	Fig. 3. Microstructure of modified asphalt with 2 wt% (a) and 4 wt% (b) of WEPS.

3.2.1. Viscoelasticity of modified asphalt

As shown in Fig. 4, Fig. 5 and Fig. 6, the G^*, G′, G″ of modified asphalt and asphalt decrease dramatically as the temperature increases from 64 °C to 70 °C, which is due to the sliding motion of chain molecules induced by the thermal absorption of asphalt. The G^* in viscoelastic material is bigger, the material gets harder, and the power required for overcoming the deformation of the material, which are caused by elastic deformation and unrecoverable elastic deformation, is bigger. The G^*, G′, G″ of modified asphalt is higher than asphalt, indicating that the flexibility and anti-deformation ability of modified asphalt are improved. Such improvement can be attributed to the swelling of WEPS by absorbing light oil in the asphalt.

Fig. 4.

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	Fig. 4. Change of G*by modification.

Fig. 5.

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	Fig. 5. Change of G′ by modification.

Fig. 6.

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	Fig. 6. Change of G″ by modification.

3.2.2. Variation of phase angle in modified asphalt

As shown in Fig. 7, the phase angle δ increases gradually when the temperature increases. The change is rapid in the temperature range of 64-76 °C and moderate in the range of 76-88 °C. Fig. 7 shows that the δ of modified asphalt is smaller than that of base asphalt at the same temperature except the asphalt modified with 3 wt% of WEPS. Under the same condition, the asphalt with smaller δ usually possesses more elastic composition and therefore shows stronger rutting resistance^{[ 13]}. So, the rutting resistance of modified asphalt is better than that of asphalt. The aromatic compound, which plays a leading role in glue-soluble of asphalt, has increased after being modified. Besides, the aromatic compound is easy to be absorbed. This process regulates the relative content of asphaltenes and solvent in the asphalt system, indicating the improvement of viscoelasticity. After adding the modifier, different degree of crosslinking between modifier and asphalt may make the molecular of modified asphalt become larger. Molecule motion after crosslinking gets harder and flow temperature rises, which cause good viscoelasticity.

Fig. 7.

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	Fig. 7. Change of δ by modification.

As shown in Fig. 8, at the same temperature, the tan δ of modified asphalt becomes smaller with increasing proportion of WEPS. Moreover, the tan δ value of asphalt modified with 2 wt% of WEPS is slightly greater than that of base asphalt. The asphalt modified with 4 wt% of WEPS has greater tan δ than all the other modified asphalt at temperatures ranging from 64 °C to 65 °C, but it decreases significantly and less than that of base asphalt in the temperature range of 65-80 °C, and changes gently after 80 °C. It demonstrates that the effect on asphalt modified by adding WEPS is not regular, which is related to its dispersion. There exist many chemical and physical differences (i.e. molecular weight, viscosity, density, polarity, difference of solubility parameter) between WEPS and asphalt. The WEPS and modified asphalt are mixed by mechanical thermal stirring. The WEPS particles do not dissolve in asphalt but can absorb soft matter in the asphalt.

Fig. 8.

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	Fig. 8. Change of tan δ by modification.

As shown in Fig. 9, for both asphalt and modified asphalt, the rut factor G^*/sin δ decreases with increasing temperature and a sharp decrease in G^*/sin δ occurs at 64-70 °C. Under the same thermal condition, the rutting resistance of WEPS-modified asphalt is correspondingly better than that of base asphalt. The base asphalt modified with 4 wt% of WEPS shows the best rutting resistance.

Fig. 9.

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	Fig. 9. Change of G*/sin δ by modification.

Asphalt binder mainly display viscous flow above the softening point, the most ideal flow pattern is Newtonian laminar fluid. However, most petroleum asphalt binder is non-Newtonian which is made up of molecules with different structure and weight. Flowability is determined by molecular structure and gravitational attraction with each other^{[ 14]}. As shown in Fig. 10, at the same temperature, the viscosity of modified asphalt is higher than that of the asphalt. The difference in viscosity between asphalt and modified asphalt increases with decreasing the temperature, which is due to the change of asphalt from viscous flow state to viscoelastic state. The significant difference exists in the asphalt modified with 4 wt% of WEPS at 165 °C. In addition, the modified asphalt has a high viscosity at low temperature and a low viscosity at high temperature. Li et al.^{[ 15]} demonstrated that such dependence of viscosity on temperature favors the pumping and mixing of asphalt during the construction process.

Fig. 10.

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	Fig. 10. Viscosity of asphalt at 135 °C and 165 °C.