Low temperature properties

Low temperature thermal cracking of pavements occurs due to the thermal stress accumulation in the asphalt mixture [9,28,48]. Asphalt binders with low stiffness and the ability to relax under high stress are preferred as they are less susceptible to thermal cracking at low temperature [32,48]. Low temperature properties of the asphalt binders, creep stiffness (S) and creep rate (m-value), were measured and obtained by creep tests over 240s using a BBR. The creep stiffness reflects the flexibility of the material and the m-value indicates the stress relaxation ability of the material at a certain test temperature [46,48]. The requirements for asphalt binder to prevent thermal cracking are the creep stiffness must be less than or equal to 300 MPa and the m-value must be greater than or equal to 0.300 according to the Superpave specification. The creep stiffness and m-value of the asphalt blends were calculated at a loading time of 60s at temperatures of -12°C and -18°C as shown in Figure 4.7.

The effect of LabAESO on the stiffness of neat asphalt binder can be observed in Figure 4.7 (a). The lower and intermediate concentrations of LabAESO (3% and 9%) had similar effect on reducing the stiffness of the neat asphalt binder as they decreased the neat binder’s stiffness by about 70 MPa at a temperature of -12°C. However, there was no reduction of stiffness observed on 3% and 9%LabAESO blends at a test temperature of -18°C, which indicated the effect of LabAESO on low temperature performed differently at different temperatures when the addition of LabAESO in the neat asphalt binder was at a lower or intermediate concentration level. This can also be inferred that 3% and 9% LabAESO were only able to increase the flexibility of neat asphalt binder at testing temperature of -12°C with a slightly lower critical low temperature than that of the neat asphalt binder. To further reduce the creep stiffness at a lower testing temperature of -18°C, higher concentration of LabAESO was necessary in the modification. As expected, the

15%LabAESO considerably reduced the stiffness of the neat asphalt binder at both -12°C and - 18°C around 90 MPa. The results indicated sufficient concentration of LabAESO was needed for enhancing low temperature performance by decreasing one grade of the neat asphalt binder’s low temperature performance grade, and thereby reducing the possibility of thermal cracking at a lower testing temperature of -18°C. In Figure 4.7 (a) and Figure 4.8 (a), LabAESO modified blends were found to meet the stiffness requirement at both test temperatures. For m-value, it can be seen that the m-value increased when the concentration level of LabAESO increased as shown in Figure 4.7 (c). The m-value decreased as test temperatures went down from -12°C to -18°C as expected. However, only the 15%LabAESO modified binder was able to relax under stress and pass the m- value criteria at -18°C, which agreed with the rheology analysis in the master curves that the 15%LabAESO had the most viscous behavior at low temperatures. It can be concluded that LabAESO improved the flexibility of the neat asphalt binder and decreased the neat asphalt binder’s susceptibility to thermal cracking at low temperature. Furthermore, high concentration level of LabAESO was needed to significantly enhance the low temperature properties of the neat asphalt binder. However, the ComAESO was found not to be effective on low temperature properties of the neat asphalt binder at lower temperatures. It can be seen in Figure 4.7 (b) that lower, intermediate, and higher ComAESO concentration levels (3%, 9%, and 15%) had similar softening effects on the neat asphalt binder’s stiffness at -12°C by reducing stiffness value about 40 MPa. Whereas, increase of stiffness was noticed at temperature of -18°C on ComAESO modified blends, which failed to meet the stiffness requirement at that test temperature. The m- value of all ComAESO modified blends was found to be followed the same trend of the neat asphalt binder when test temperature changed (Figure 4.7 (d)), which met the requirement at -

12°C, but failed at -18°C and thereby they were graded the same as the neat asphalt binders at low PG.

The changes of stiffness and m-value with increasing of AESO concentration level at test temperatures of -12°C and -18°C can be observed in Figure 4.8 (a). At test temperature of -12°C, the stiffness of the neat asphalt binder decreased when the concentration levels of AESO increased, while LabAESO had more softening effect on the neat asphalt binder compared to that of the ComAESO as the line of LabAESO was below the ComAESO’s in Figure 4.8 (a). However, when the test temperature went down to -18°C, it was noticed that the intermediate concentration level (9%) for both AESO was the threshold at which the neat asphalt binder needed to break through to get benefit from AESO on the low temperature properties. It was clear that concentration level above 9% significantly dropped the stiffness of neat asphalt binder as seen from the pronounced stiffness reduction observed for the 15%LabAESO modified binder. The effect of AESO at different concentration levels on m-value can be easily observed in Figure 4.8 (b). For both test temperatures, LabAESO was found to considerably increase the m-value as the concentration level increases, while the effect of ComAESO on m-value was negligible as the results shown in Figure 4.8 (b). Overall, the 15%LabAESO modified binder had the lowest stiffness with the highest m- value. These results indicated that the LabAESO helped with stress relaxation and was better able to dissipate stresses when a sufficient concentration of LabAESO was added, thus reducing the susceptibility of the neat asphalt binder to thermal cracking at low temperatures.

Figure 4.7. Effect of AESO on asphalt binder creep stiffness (S) and creep rate (m-value) versus temperature.

Figure 4.8. Effect of AESO concentration levels on asphalt binder low temperature properties.