3.3.4. Low-Temperature Creep Behavior

An asphalt pavement should be flexible enough to avoid cracking at low temperatures, which is dependent upon the characteristics of the asphalt binder. The Strategic Highway Research Program (SHRP) recommends the BBR test to assess the binder's low-temperature properties. In the SHRP specification, the creep stiffness (S) and rate of creep (m-value) obtained under a 60 s loading are the two main parameters to identify the low-temperature properties of the binder. The creep stiffness represents the shrinkage stress of an asphalt pavement caused by temperature change, and the creep rate reflects the degree of stress reduction. Lower creep stiffness means less shrinkage stress due to a temperature drop, indicating better low-temperature crack resistance. Higher m-values represent a better stress dissipation property. It is well known that asphalt pavements with lower stiffness and higher m-value have better low-temperature cracking resistance performance. To investigate the influence of APAO and PPA on the low-temperature properties of neat binder, variations in stiffness and m-value are presented in Figures 12 and 13.

As can be seen, the stiffness value increases and the m-value declines as the temperature drops from −6 ◦C to −18 ◦C. The increase in stiffness indicates that the binder becomes stiffer and has greater shrinkage stress, and the decrease in m-value suggests that the stress dissipation rate becomes smaller. The incorporation of APAO and PPA has a marked influence on the variations in stiffness and m-value. As observed, the stiffness of virgin binder presents an increasing trend and the m-value exhibits a decreasing trend as the concentration of APAO increases. This result indicates that APAO lowers the binder's low-temperature performance. However, the increasing concentration of PPA further improves the stiffness and reduces the m-value, indicating that PPA decreases the low-temperature property on the basis of APAO modified asphalt binder. The stiffness increases by 23.47% as the APAO content rises from 0 wt.% to 6 wt.% at −6 ◦C, and the stiffness increases by 58.76% as the PPA content increases from 0 wt.% to 2.0 wt.%. The results show that the deterioration in the low-temperature performance caused by APAO is less than that by PPA.

**Figure 12.** Creep stiffness variations with temperature for composite modified binders: (**a**) composite modified binders with various contents of APAO; (**b**) composite modified binders with various contents of PPA.

**Figure 13.** m-value variations with temperature for composite modified binders: (**a**) composite modified binders with various contents of APAO; (**b**) composite modified binders with various contents of PPA.

The stiffness of asphalt binder is less than 300 MPa and the m-value is higher than 0.3, which meet the requirements of the SHRP specification [52]. The neat binder can meet the requirement at −12 ◦C, but fails at −18 ◦C. After adding APAO and PPA, all composite modified asphalt binders fulfill the specifications at −12 ◦C. As expected, the composite modified binders fail to meet the requirement at −18 ◦C. It is clear that the composite modified binder has comparable low-temperature performance to the virgin binder. The aforementioned results indicate that the low-temperature grade is not reduced by the compound modification of APAO and PPA. Thus, the low-temperature PG grade of APAO/PPA modified binders can reach −22 ◦C, which meets the requirements of most areas in China. Therefore, it is clear that the compound modification of APAO/PPA is detrimental to the stiffness and m-value of asphalt binder, although it does not reduce the low-temperature PG grade.

#### 3.3.5. Fatigue Behavior

Fatigue cracking of asphalt pavement has become an important stress factor influencing the long-term service performance of pavements. There are many factors contributing to fatigue cracking, such as climate change, binder properties, pavement design and structure and traffic volume. Among these influencing factors, it is well established that the anti-fatigue performance of asphalt binder plays a significant role in controlling the fatigue properties of asphalt mixtures [53]. The Superpave fatigue index G \*sinδ, corresponding to the dissipated energy, is utilized to characterize the fatigue resistance of asphalt binder. Thus, the RTFOT-PAV aged specimens were subjected to a temperature sweep test to assess the binder's fatigue resistance, the G \*sinδ values of which are displayed in Figure 14.

As observed, G \*sinδ values decrease with increases in temperature. In the intermediate temperature range, a small G \*sinδ value indicates a better anti-fatigue performance of the binder. As observed from Figure 14a, it is obvious that the neat binder has a lower G \*sinδ value, which means the neat binder has better fatigue resistance. After adding 1.5 wt.% PPA, the G \*sinδ value becomes larger, suggesting that the PPA reduces the anti-fatigue property of asphalt binder. On the other hand, the G \*sinδ value declines with increasing APAO content, indicating that APAO strengthens the fatigue resistance of the asphalt binder. However, there is an intersection between the fitting curves of neat

binder and modified binder A6P1.5, which makes it difficult to determine the fatigue temperature. Moreover, Figure 14b shows that the modified asphalt binder with 2 wt.% APAO has a better anti-fatigue resistance performance than neat binder, and the G \*sinδ value increases with increases in PPA content. To quantitatively investigate the effects of compound modification of APAO and PPA on fatigue properties, the fatigue temperatures are determined.

**Figure 14.** G \*sinδ variations with temperature for composite modified binders: (**a**) composite modified binders with various contents of APAO; (**b**) composite modified binders with various contents of PPA.

In accordance with the SHRP specification, the fatigue temperatures of asphalt binder are calculated when the G \*sinδ value reaches the criterion of 5000 kPa [54]. The specific values of fatigue temperature are shown in Table 8. Obviously, A2P0 has the lowest fatigue temperature, followed by the neat binder. The increasing content of APAO leads to reductions in fatigue temperature, indicating that APAO can promote the anti-fatigue property of the binder. In contrast, increasing content of PPA increases the fatigue temperature, which indicates that PPA reduces the anti-fatigue property of the asphalt binder. The modified binder A6P1.5 has a lower fatigue temperature than A2P2.0. The combined effects of APAO and PPA on the fatigue resistance of the binder depend on their respective concentrations. Therefore, the APAO and PPA content should be optimized to guarantee the fatigue resistance of composite modified binder.

**Table 8.** Fatigue temperatures of all tested specimens.

