Reflective Cracking Phase V
Phase V Test Summary
Full-scale tests started on March 3, 2017. After 1919 cycles, the overlay was completely separated and the test concluded on March 20, 2017.
The primary objective of Phase V test was to compare the performance of different overlay thicknesses and answer the question “Is increasing thickness an effective way to extend the overlay fatigue life?” Phase V test was also expected to further enrich reflective cracking database, particularly the crack propagation rates under controlled loading conditions to mimic the temperature cycles occurring in nature.
To maintain consistency of previous experiments, the design of Phase V overlay included two overlay strips that were 30- by 5-ft with a 2-ft gap in between. Both sections consisted of the same materials, the standard FAA P-401 PG 64-22 HMA, but different overlay thicknesses (i.e., 3-in. vs. 6-in.). The thin (north) and thick (south) overlays were designed to compare the crack development and avoid potential crack initiation from the surface. The thickness of the thin section represented the minimum requirement of HMA overlay on existing rigid pavements.
As shown in Figure 1, test overlay was paved in two lanes (i.e., north and south) using the same materials, the standard FAA P-401 PG 64-22 HMA, but different overlay thicknesses (i.e., 3-in. vs. 6-in.). After placing of each lift, the HMA materials were thoroughly and uniformly compacted by a power rollers without vibration due to the sensors underneath. Then the density atop each lift were measured using Nuclear Densometer. Nuclear density rolling pattern was established to get the density results of the two focus points on the mat.
Figure 1. Phase V Test Pavement
During the overlay paving, H-type asphalt strain gages (EG) were placed at the bottom of each lift (1.5 in.) of the overlay. After the overlay paving, surface strain gages (SG) were installed at various locations on the surface. Thermocouple trees were installed in each overlay section to acquire temperature gradients. Each tree consisted of multiple thermocouples (T) to measure temperature at the same depth as strain gages. Note that all strain gages are directly above and perpendicular to the concrete joint where the first reflection crack would most likely occur.
RC Phase V Drawing
Temperature variations were approximated by a haversine waveform describing the relationship between the joint opening and cycle time:
where t is the time of interest, D is the amplitude of joint opening, T is the cycle time, and R is rest period, which was included at the end of each loading cycle to allow the HMA materials to relax. In Phase V testing, the joint opening (D) was set at 12 mils which corresponded to a large temperature drop (17oF) at the overlay bottom in the field. Each haversine loading cycle began with a loading time of 75 sec, once the actuators reached the maximum horizontal displacement (joint opening), a 75-sec unloading was executed and then followed by a rest period of 600 sec to allow the overlay to relax.
Upon the completion of testing, cracks on the edges were full-depth through the overlay. On the thin section (Figure 2a), the crack initiated at the overlay bottom. At first, the crack propagated upward, indicating fracture Mode I dominance. Once this vertical crack reached the boundary of two 1.5-in lifts, it started to deviate at an angle toward the surface. Meanwhile, a second crack propagated from the top of the overlay, headed to the bottom, but was arrested at the lift interface. On the thick section (Figure 2b), despite of some wiggling, a bottom-up crack formed at the overlay bottom and progressed on its upward track. Figures 3 and 4 show that the resulting transverse crack was directly atop the underlying PCC joint inn the horizontal plane. The crack not only propagated in a vertical direction, but also tended to develop across the overlay (along the joint) as well.