## 3 Results
### Comparative Analysis of Anti-Crack Performance  
To further investigate the anti-crack performance of the stress-absorption layers in the HVASAL and RASAL types, the crack propagation process of the three types of overlay with the same type of crack, and three types of crack with the same type of the overlay, were compared and analyzed. 2.1 Comparison and Analysis of the Crack Propagation Trends of the Three Types of Overlay with the Same Type of Crack  
Two types of crack in the overlay of the specimen with the stress-absorption layer, propagated upwards across layers I and II. The data of the _a-N_ curves with the stress-absorption overlay was graphed per each layer (_a_ is the scale value of cracks in the overlay, and \(N\) is the corresponding number of load cycles). The _a-N_ test curves of layer I and layer II for the three types of overlay with the middle crack are shown in Figure 5.  
It was observed through the experiment that the initial cracking points for the middle cracks of the HVASAL and RASAL type structures were located at the interface between the stress-absorption layer and the surface layer. The cracks slowly propagated upwards, then some new cracks began to appear at the bottom of the stress-absorption layer, and the two cracks propagated upwards at the same time. The cracks in the two layers met as the length of crack in the stress-absorption layer reached 20 mm. It could be clearly observed that the cracks bypass the aggregate. As an example, the fatigue crack propagation process of the RASAL overlay structure is shown in Figure 6.  
Figure 5: _a-N_ curves of three types of overlay structure with the middle crack in two layers. It was observed in the experiment with the No SAL type specimen with the middle crack, that the reflection cracks occurred along the interface of the cement concrete layer, as shown in Figure 7. The test results show that the number of load cycles for the initial-crack of the HVASAL and RASAL types is 2.34 and 3.25 times that of the No SAL type, respectively; and the fatigue life of the HVASAL and RASAL types is 1.79 and 2.01 times that of the No SAL type, respectively. The results show that the stress-absorption layer has the ability to retard the crack propagation, and the anti-crack performance of the RASAL type is better than that of the HVASAL type. Figure 6: Fatigue crack propagation of the middle crack of the rubber asphalt stress absorption layer (RASAL) type. The \(a\)-\(N\) test curves of the three types of overlay structure in layers I and II with side cracks are shown in Figure 8.  
It was observed that the experimental phenomena of the HVASAL and RASAL type overlay structures with side cracks were the same as those of the middle crack tests. However, the crack propagation direction was obviously deviated from the pressure head, and the propagation mode was open and shear composite. Furthermore, both mode cracks propagated at the same time, until the whole specimen was penetrated and broken. As an example, the fatigue crack propagation process of the HVASAL type overlay structure is shown in Figure 9.  
Figure 8: \(a\)-\(N\) curves of three types of overlay structure with the side crack in two layers. Figure 7: Fatigue crack propagation of the middle crack of the No SAL type. It was observed that the initial cracking point of the side cracks of the No SAL type were located at the interface between the concrete layer and the surface layer, the crack propagation direction deviated from the pressure head, and the propagation mode was open and shear composite. Then the specimen produced small cracks at the bottom of the middle span which developed vertically, showing an open mode of crack propagation. The cracks of both modes began propagating at the same time, and the speed of the crack propagation of the former was faster than the latter in the initial stage. The latter propagated rapidly towards the top when the latter was longer than the former, and then the former no longer propagated, until the whole specimen was penetrated by the mid-span crack and broken.