Cracking takes many forms. To make the proper
repairs, first you should determine the type of crack and
The most common types of cracks are
alligator, edge, edge joint, lane joint, reflection,
shrinkage, and slippage.
ALLIGATOR CRACKS. Alligator cracks are
interconnected cracks, forming a series of small blocks
resembling an alligators skin or chicken wire (fig.
16-9). In most cases, alligator cracking is caused by
excessive movement of the surface over unstable
subgrades or base courses. The unstable support is the
result of saturated granular bases or subgrade.
Normally, the affected area is not large. When it does
occur on a large scale, the cracking is most likely due to
repeated loads above the designed strength of the
EDGE CRACKS. Edge cracks are longitudinal
cracks approximately 1 foot from the edge of the
Figure 16-9.Alligator cracks.
Figure 16-10.-Edge cracks.
pavement (fig. 16-10). Edge cracks can have transverse
cracks, branching in towards the shoulder. Normally,
edge crack are caused by a lack of side or shoulder
support. They may also be caused by settlement or
yielding of the base material underlying the cracked
area. This, in turn, may be the result of poor drainage,
frost heave, or shrinkage from the drying out of the
EDGE JOINT CRACKS. Edge joint cracks
occur between the pavement and the shoulder (fig.
16-11). They are normally caused by alternate wetting
and drying beneath the shoulder surface. This can result
from poor drainage from a shoulder that is too high, or
it can result from depressions along the pavement edge.
The uneven pavement traps water on top, allowing it to
seep into the base. Another cause could be heavy trucks,
straddling the joint.
LANE JOINT CRACKS. Lane joint cracks are
longitudinal separations along the seam between two
paving lanes (fig. 16-12). This type of crack is usually
caused by a weak seam or poor bond between adjoining
spreads in the pavement.
Figure 16-11.-Edge joint cracks.
Figure 16-12.-Lane joint cracks.