15 March 2001
DISTRESS 43, BLOCK CRACKING
5.1. Description. Block cracks are interconnected cracks that divide the pavement into
approximately rectangular pieces. The blocks may range in size from approximately 1 by 1 feet
to 10 by 10 feet (0.3 by 0.3 meter to 3 by 3 meters). Block cracking is caused mainly by
shrinkage of the asphalt concrete (AC) and daily temperature cycling (which results in daily
stress/strain cycling). It is not load-associated. The occurrence of block cracking usually
indicates that the asphalt has hardened significantly. Block cracking normally occurs over a large
proportion of pavement area but sometimes will occur in nontraffic areas. This type of distress
differs from alligator cracking in that alligator cracks form smaller, many-sided pieces with sharp
angles. Also, unlike block cracks, alligator cracks are caused by repeated traffic loadings and,
therefore, are located only in traffic areas (i.e., wheel paths). Deduct curves for block cracking
are shown in Figure 5.1.
5.2. Severity Levels.
5.2.1. L. Blocks are defined by cracks that are nonspalled (sides of the crack are vertical) or only
lightly spalled, causing no FOD potential. Nonfilled cracks have 1/4 inch (6.4 millimeters) or less
mean width, and filled cracks have filler in satisfactory condition (Figures 5.2., 5.3., and 5.4.).
5.2.2. M. Blocks are defined by either: (1) filled or nonfilled cracks that are moderately spalled
(some FOD potential); (2) nonfilled cracks that are not spalled or have only minor spalling (some
FOD potential), but have a mean width greater than approximately 1/4 inch (6.4 millimeters); or
(3) filled cracks that are not spalled or have only minor spalling (some FOD potential), but have
filler in unsatisfactory condition (Figures 5.5. and 5.6.).