# Performance Evaluation of Recycled Asphalt
## 3 Results and Discussions for the RAP Materials
### Residual Cohesion of RAP Materials  
Figure 6 shows the triaxial test results of RAP materials.  
As can be seen from Figure 6, after the RAP materials are extracted, only the aggregates with different particle sizes are left in the mixture, as can be seen from the triaxial test results, the aggregate has the largest internal friction angle, due to the absence of asphalt film barrier, and the mixture at this time is closer to graded crushed stone.  
The triaxial test results of the specimen compacted by direct static compression of the RAP materials without heating show that the internal of the specimens has begun to form a certain cohesive force, this part of the cohesion is provided by the aged asphalt film on the surface of the old aggregate, because it has not been heated, the aged asphalt film cannot be re-wrapped during the compaction process. It is entirely dependent on the extrusion process between the particles during the static pressure compaction process to form a new asphalt film adhesion. Although this cohesive force is much smaller than that in the newly mixed asphalt mixture, it has fundamentally changed the strength characteristics of the specimens, changing the loose material similar to graded broken stones into an integral material. RAP is often referred to as \"black rock\" in the United States, Europe countries except for China, its properties are mainly determined by aggregate, the residual asphalt only changed the surface color of the aggregate, but has almost no effect on performance. It can be seen from the above test results that the argument of \"black rock\" is not sufficient, and the residual asphalt can play a certain bonding role even under cold recycling conditions. In other words, for the old RAP materials, if the compaction can be strengthened in the process of recycling, so that the particles can play a bonding role again by means of the residual aged asphalt film on the surface of the RAP materials, the strength of the recycled mixture will be improved [31].  
Comparing the results of the second group (unheated) and the third group (heating at 60 \({}^{\circ}\)C for 10 h) in Figure 6, it can be seen that the effects of low temperature heating on the strength of the RAP materials are not obvious. In the test results of the fourth group (heated at 60 \({}^{\circ}\)C for 24 h), the cohesive force was significantly increased, which may be relative to the viscosity increase caused by the aging of  
Figure 6: Triaxial test results.  
the asphalt film after long-term heating. In the fifth group (heating at 120 \({}^{\circ}\)C for 16 h), the heating temperature at 120 \({}^{\circ}\)C is close to the heating temperature of the RAP materials during warm recycling, it can be seen that the cohesive force inside the specimen is greatly increased at this time, it shows that the old asphalt has been redistributed and wrapped at this time, which has a better bonding effect and further improves the integrity of the specimen. Therefore, from the perspective of fully exerting and utilizing the adhesive effects of the old asphalt residue, the effects of warm recycling are obviously better than that of cold recycling.