Top Down Cracking

Top Down Cracking

Top down cracking appears to be a common mode of HMA pavement distress in at least several states and countries.  Traditionally, pavement cracking is thought to initiate at the bottom of the HMA layer where the tensile bending stresses are the greatest and then progress up to the surface (a bottom-up crack).  Most traditional transfer functions used in mechanistic-empirical structural design are based on this concept.  However, the late 1990s saw a substantial focus on a second mode of crack initiation and propagation: top-down cracking.

Water Bleeding and Pumping

Water Bleeding and Pumping

Water bleeding (left two photos) occurs when water seeps out of joints or cracks or through an excessively porous HMA layer.  Pumping (right-most photo) occurs when water and fine material is ejected from underlying layers through cracks in the HMA layer or out the sides of the HMA layer under moving loads.

Transverse (Thermal) Cracking

Transverse (Thermal) Cracking

Cracks perpendicular to the pavement's centerline or laydown direction.  Usually a type of thermal cracking.

Stripping

Stripping

The loss of bond between aggregates and asphalt binder that typically begins at the bottom of the HMA layer and progresses upward.  When stripping begins at the surface and progresses downward it is usually called raveling.

Slippage Cracking

Slippage Cracking

Crescent or half-moon shaped cracks generally having two ends pointed into the direction of traffic.

Rutting

Rutting

Surface depression in the wheelpath.  Pavement uplift (shearing) may occur along the sides of the rut.  Ruts are particularly evident after a rain when they are filled with water.  There are two basic types of rutting: mix rutting and subgrade rutting.  Mix rutting occurs when the subgrade does not rut yet the pavement surface exhibits wheelpath depressions as a result of compaction/mix design problems.  Subgrade rutting occurs when the subgrade exhibits wheelpath depressions due to loading.  In this case, the pavement settles into the subgrade ruts causing surface depressions in the wheelpath.