Technical information

2-30
2.5.2 Cracking and Rutting Performance
Sections N1 and N2 experienced cracking in a very similar manner (Priest and Timm,
2006; Timm et al., 2006). Both failed in fatigue within two months of each other (see
Figure 2.5.5. Section N1 (modified HMA) failed prior to section N2 (unmodified HMA)
after six months of traffic. First, small transverse cracks appeared in the wheel path.
Then the cracks progressed to the edge of the wheel path and often curled in the
direction of traffic. Later, the individual transverse cracks became interconnected into a
classical alligator pattern. Pumping of the fines from the unbound aggregate base
through the cracks was also observed in the individual transverse cracks as well as the
alligator cracked areas. The progression of fatigue failure was fairly rapid once the first
cracks appeared and especially once pumping began. Subsequently, the responses
quickly over ranged the embedded instrumentation. More detailed crack mapping can
be found in Priest and Timm (2006).
Figure 2.5.5: Fatigued sections N1 (left photo) and N2 (right photo).
With respect to rutting, sections N1 and N2 did not rut much by the time they failed in
fatigue. The progression of average rut depth is shown in Table 2.5.2 (personal
communication, B. Powell, 2007); this data is also presented graphically in Figure 2.5.6.
As can be seen, after the application of about 4.5 million equivalent single axle loads
(ESALs), the final average rut depth was only about 8 mm.