REFLECTION CRACKS. Reflection cracks
normally occur in asphalt overlays. These cracks reflect
the crack pattern in the pavement structure underneath
(fig. 16- 13). They are most frequently found in asphalt
cement-treated bases. Reflection cracks are normally
caused by vertical or horizontal movements in the
pavement beneath the overlay, resulting from traffic
loads, temperature, and earth movements.
SHRINKAGE CRACKS. Shrinkage cracks are
interconnected cracks, forming a series of large blocks
usually with sharp corners or angles (fig. 16-14). Often
it is difficult to determine whether shrinkage cracks are
caused by volume change in the asphalt mix or in the
base or subgrade.
Frequently, they are caused by
Figure 16-13.-Reflection cracks.
volume change of fine aggregate asphalt mixes that have
a high content of high-viscosity asphalt. Lack of traffic
hastens shrinkage in these pavements.
SLIPPAGE CRACKS. Slippage cracks are
crescent-shaped cracks, resulting from horizontal forces
induced by traffic (fig. 16-15). They are caused by a
lack of bond between the surface layer and the course
beneath. Lack of bond maybe due to dust, dirt, oil, or
the absence of a tack coat.
Pavement distortion is any change in a flexible
pavement surface. It is the result of a subgrade surface.
weakness where compaction or movement of the
subgrade soil has taken place or where base compaction
has occurred. It may or may not be accompanied by
cracking, but in either instance, it creates a traffic
hazard, permits water to accumulate, and eventually
makes matters worse. Distortion takes a number of
different forms but is normally classed as channeling,
corrugations and shoving, depressions, and upheaval.
Figure 16-15.Slippage cracks.
Figure 16-14.Shinkage cracks.