# Analysis of Nanoindentation Test Results of Asphalt Mixture with Different Gradations
## 3 Results and Discussion
### Effect of Different Rates of Filler and Asphalt  
As given in Table 3, it can be seen that the modulus of the gap-graded asphalt mixture is about twice that of the open-graded mixture. Although both gap-gradation and open-gradation asphalt mixtures have less fine aggregate, there is still a significant difference in their nanoindentation test results due to the asphalt music composition between the two. It has also been shown that most SMA asphalt mixtures have an RFA of 1.6 to 1.8 [25], while the ratio of filler to the asphalt of OGFC-graded mixtures is around 1.0 [26; 27]. Therefore, in order to further investigate the differences in the results between the gap-graded and open-graded asphalt mixtures, asphalt music specimens with different RFA (0.8 to 1.6) were prepared for nanoindentation tests before and after polishing. The effects of the RFA and polishing on the nanoindentation test results were further examined. Figure 7 shows the asphalt music specimens before and after polishing. Since the music pieces were cast in the mold, the surface was inevitably convex or concave. Figure 8 shows the microscopic images of the test area before and after polishing the asphalt music specimen with the rate of filler and asphalt of 0.8. The creep curve of the loaded section is shown in Figure 9.  
The load-depth curves and modulus values obtained by the nanoindentation tests of asphalt music specimens with different RFA before and after polishing are shown in Figures 10 and 11, and Table 4. The fluctuation range of the maximum depth of the test curves for all specimens is within 15%, indicating the reliability of the test results. It is also  
Figure 8: Asphalt mastic nanoindentation specimens. Figure 9: Microscope image of asphalt music specimen. (a) before polishing, (b) after polishing. Figure 11: Cont. The red scale in the lower right corner of Figure 9a,b is 10 \(\upmu\)m, while the white area is the asphalt phase. It can be presumed that the black particles are mineral powder particles with particle sizes below 0.075 mm. By comparing Figure 9a,b, it can be deduced that the asphalt mastic surface was almost entirely covered by asphalt before polishing. After polishing, the surface asphalt was ground away, and some mineral powder particles were exposed and smoothed flat. From this perspective, the polishing process did cause damage and perturbation to the specimen surface. As shown in Figures 10 and 12 and Table 4, even though the polishing process exposed the mineral powder particles on the specimen surface (and there may even be mineral powder particles below or around the measurement points). However, the nanoindentation test curves and moduli results of the asphalt mastic specimens did not change significantly before and after polishing. This is possibly due to the densely packed and well-mixed mineral particles in the asphalt mastic, forming a stable adhesive mixture with the asphalt. So even if the polishing operation wipes out the surface layer covered with asphalt, the indentation probe acts on the mineral powder particles during the nanoindentation test. However, considering that the overall mechanical properties of the mixture do not change much under water-cooled polishing conditions. The larger indentation depth eliminates the disturbance effect, leading to the nanoindentation test results that are not significantly different from those before polishing. It is inferred that the polishing only changed the mastic surface grayness, and the overall mechanical properties of the mixture remain unaffected. The polishing process of the specimens did not affect the reliability of the nanoindentation test results. Figures 11 and 12 also show a significant difference in the nanoindentation modulus corresponding to different RFA. As the RFA of asphalt mastic increases, its corresponding nanoindentation modulus shows an increasing trend. In addition, the nanoindentation modulus of the RFA 1.6 was approximately twice the modulus of RFA 1.2.