# A Fundamental Study on an SAP Mixed Asphalt Mixture for Reducing the Urban Heat Island Effect
## 3 Results and Discussion
### Strength Characteristics of the SAP Asphalt Mixtures
#### 3.1.3 Dynamic Stability Test Result  
The results of the dynamic stability tests of asphalt mixtures according to SAP type and content are shown in Figure 10. After mixing in the SAP, the dynamic stability met the requirement of more than 750 times/mm in all specimens except for the specimen with 7%  
Figure 8: Indirect tensile strength of SAP asphalt mixtures.  
Figure 9: Toughness of SAP asphalt mixtures.  
content. The results showed that adding the SAP to the asphalt mixture improves dynamic stability and flow resistance.  
The SAP results increased when the SAP content increased to 1091 times/mm at 3% and 1507 times/mm at 5%, and SAP-3500 showed the same trend, but the result was 429 times/mm for the 7% SAP content for both types, which did not meet the standard. For the 7% SAP mixture, it was determined that the excessive amount of SAP causes insufficient asphalt to bind in the aggregate, resulting in poor durability.  
Figure 11 shows the change in rutting over time in the wheel tracking test results. SAP-3500 showed less rutting than Non-SAP at all contents, while SAP-500 showed significant rutting at 3% and 5% contents. Another point to note in the experimental results is that although SAP-500 7% and SAP-3500 7% were similar in the dynamic stability test in Figure 10, the rutting changes over time in Figure 11 show that the two mixtures have very different tendencies. The dynamic stability of the two mixtures was the same at 429 times/mm, but the rutting depth at 60 min for SAP-500 7% was 10.8 mm and that for SAP-3500 7% was 8.3 mm, showing a large difference. This is because only the difference in a rutting depth of 45 to 60 min is considered when determining dynamic stability. This method was not an issue when evaluating similar existing asphalt mixtures, but it shows that evaluating only dynamic stability is problematic when evaluating new asphalt mixtures as in this study. This issue was reported in other previous studies that evaluated the correlation between dynamic stability and maximum rutting depth obtained from existing wheel tracking tests and showed a very low correlation (R2 = 0.28), which has many problems in practical application [24; 25].  
Figure 10: Dynamic stability results by SAP asphalt content.  
Looking at the maximum rutting depth and the rutting change in Figure 11, the maximum rutting depth is relatively large when SAP-500 is applied and the rutting change slope changes more rapidly compared to that of SAP-3500. Looking at the rutting change slope, it seems that the initial rutting increased quickly because of the smaller fineness of SAP-500 and its effect on the mixture is less than that of SAP-3500. The dynamic stability appears to have increased because mixing the SAP into the asphalt binder acts to limit the flowability of the binder. However, both types of SAPs show very low dynamic stability values when the SAP content is 7%, which is believed to be due to an excessive amount of SAP and is similar to that found in the Marshall stability and indirect tensile strength results. When the SAP content was excessively added, it caused the reduction in the adhesive force between the binder and the aggregate, thereby reducing the rigidity of the binder itself.