# Initiation and Propagation of Top-Down Cracking in Asphalt Pavement
## 2 Field and Laboratory TDC Tests
Figure 1: Pavement temperature variations during testing. (**a**) Low temperature test for structure 1; (**b**) high temperature test for condition 1; (**c**) high temperature test for condition 2; (**d**) high temperature test for condition 3.  
that the deepest position in the either red circle is around 20 mm away from the tyre edge, where the longitudinal cracking is located. To further verify this observation, another test was conducted at high temperature conditions (Figure 1c) at 10 m away from the test site where longitudinal cracking was observed. Again, longitudinal cracks were found within 20 mm from the wheel path. Similarly, APT testing was performed on pavement structure 2 at the temperature conditions shown in Figure 1d. Similar deformations (Figure 2) and obvious longitudinal cracking were observed within 20 mm of the wheel path. It can be concluded from the APT test results that longitudinal cracking occurs more often at higher temperature conditions. Figure 3 shows the observed longitudinal cracking located within 20 mm from the tyre edge with a width of 3 mm. To identify whether or not the longitudinal cracking is TDC, cores were taken and cut in half. It was obvious that the longitudinal cracking observed outside the wheel path was TDC, and that TDC propagated downwards 9 mm from the pavement surface. This observation matches the FE analysis results, which indicates that the maximum shear stresses for pavement structures 1 and 2 at high temperature conditions are located at 10-12 mm below the pavement surface. Figure 3: Observed longitudinal cracks outside the wheel path. Figure 2: Deformation on pavement surface of high temperature test for structure 2. In addition to field APT testing, a simple laboratory test--uniaxial repeated penetration test [14]--was developed in Tongji University, as shown in Figure 4, that simulates TDC initiation and propagation. In Figure 4, the specimen diameter is 150 mm, the plunger diameter is 42 mm, test temperature is 60 \({}^{\circ}\)C, the loading rate is 1 mm/min, and the loading frequency is 10 Hz. Figure 5 shows a cracked specimen after the uniaxial repeated penetration test. There is downward deformation beneath the plunger in the specimen, and obvious cracks were observed at the edge of the plunger. Then, the cracked specimen was sliced two times with a diamond saw at Sections A and B, as shown in Figure 5. Cross-Section A was sliced perpendicular to the crack alongside the edge of the plunger; Cross-Section B was sliced around 20 mm away from Section A. The cross-sections of Sections A and B are shown in Figure 6. It can be seen that TDC of Cross-Section A was much more severe than that of Cross-Section B. Therefore, it can be excluded that TDC initiates from outside the specimen (Cross-Section B) and moves inside (Cross-Section A). Compared to the field TDC observed on cores taken from the APT site, the cracks observed in Cross-section A were very similar to the longitudinal cracking seen alongside the wheel path in terms of both the characteristic and the formation of cracks via top to downward propagation. Additionally, Figure 6 also shows the variations of shear stress (red line) and tangent tension stress (blue line) on the section along its depth. The most severe cracking area occurred at the top part of the specimen (6 mm down from the top surface of specimen)--where the largest shear stress was located--as shown in Figure 6. Meanwhile, tangent stress is compressive stress, rather than tensile stress in this area, and the greatest tangent stress is 40 mm from the top surface, located far away from the severe TDC area. Therefore, it can be concluded that the crack on Cross-Section A was due t shear stress at the edge of the plunger, and initiated at the maximum shear stress area. The uniaxial repeated penetration test result is consistent with the field APT test. Thus, the uniaxial repeated penetration test is a valid test for simulating TDC in the laboratory. Figure 4: Uniaxial penetration test.