DEFECTS IN FLEXIBLE PAVEMENTS

Defects inflexible pavements can be placed into one of five classes. These classes are cracking, distortion, disintegration, slippery surfaces, and surface treatment problems.

Cracking

Cracking takes many forms. To make the proper repairs, first you should determine the type of crack and _{the cause.} 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 alligator's 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 pavement.

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 surrounding earth.

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.

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 overlays over portland concrete and 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.