## 4 Discussion and Conclusions  
(1) Fatigue crack propagation experiments with the cyclic loadings acting in the middle of the specimens were carried out for three types of the old cement concrete asphalt overlay. The results show that for the fatigue numbers of the pre-crack of the three types of crack, the HVASAL type is 2.34 times, 3.15 times, and 2.84 times than that of the No SAL type; and the RASAL type is 3.25 times, 4.05 times,  
Figure 19: Comparison of the fitting results and test results of three types of fracture of the HVASAL type structure with different reliability values. Figure 20: Comparison of the fitting results and test results of three types of fracture of the RASAL type structure with different reliability values. and 3.27 times that of the No SAL type. The results show that for the final fatigue lives of the three types of crack, the HVASAL type is 1.79 times, 1.61 times, and 1.55 times that of the No SAL type; and the RASAL type is 2.01 times, 1.87 times, and 1.72 times that of the No SAL type. After the stress-absorption layers were added, the fatigue lives of the structures increased significantly. The anti-crack performance of the RASAL is better than that of the HVASAL. (2) The propagation processes, and the directions of the overlay cracks of the composite specimens, were analyzed and recorded. It was found that the cracks in the stress-absorption layer always initiated from the interface of layers I and II, then once there was a crack in the stress-absorption layer, the two cracks expanded at the same time. The crack propagation direction of the No SAL type structure was the same as the structure with a stress-absorption layer, but the crack expansion area was relatively concentrated. This demonstrated that the stress absorption layer could disperse the concentrated stress and reduce the peak stress caused by cracks in cement concrete, hence the reflection cracks were delayed. (3) The three types of structure and three types of crack (the middle crack, the side crack, and the inclined crack) have different effects on the fatigue properties of the asphalt overlay on the cement concrete basement. The inclined crack has the maximum fatigue life, the side crack has the minimum life, and the middle crack has an in-between life. (4) These three test configurations provide three different mixed mode behaviors of cracks. As middle cracks occurred, there were two initial cracking points in the structure with the stress-absorption layer, with one located at the interface between the stress-absorption layer and the surface layer, and the other located at the bottom of the stress-absorption layer; subsequently, the stress-absorption layer and asphalt overlay would fracture. When there is no stress-absorption layer, the crack occurred along the interface of the cement concrete layer, passing through the overlay to all the specimens; the crack propagated in the direction of a certain lateral deviation and then had a straight upward expansion. For the side crack, the mechanism of the initial cracking points in the structure with stress-absorption layer were the same as that of the middle crack; one propagated vertically upward in mode I, the other propagated to the span center at a \(45^{\circ}\) angle in a mixed mode I and II. When there is no stress-absorption layer, the initiation of the crack was the same as that of the middle crack; that is, it propagated to the span center in a mixed mode I and II, extended directly below the pressure head, and then the specimen was destroyed. For the inclined crack, the mechanism of the crack propagation with the stress-absorption layer is the same as the former two types. When there is no stress-absorption layer, the crack occurred at the bottom of the asphalt overlay, and then some cracks appeared in the middle of the specimen due to the loading at the right of the crack, and propagated to the pressure head until the specimen failed.