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.

Distortion

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.

Figure 16-16.-Channeling.

CHANNELING.- Channeling, also referred to as "grooving" or "rutting," is channelized depressions that develop in the wheel tracks of flexible pavements (fig. 16-16). Channeling may result by consolidation or lateral movement under traffic in one or more of the underlying courses or by displacement in the bituminous surface itself. It may develop under traffic in new flexible pavements that had too little compaction during construction or from plastic movement in a mix that does not have enough stability to support traffic.

CORRUGATIONS AND SHOVING.- Corrugation, or washboarding, are a form of plastic movement typified by ripples across the flexible pavement surface (fig. 16-17). Shoving is the plastic movement of the pavement, resulting in localized bulging of the pavement (fig. 16-18). Both corrugations and shoving normally occur at points where traffic starts and stops or on hills where vehicles brake on the downgrade.

Corrugations and shoving usually occur in flexible pavement mixtures that lack stability. This may be the result of too much binder, too much fine aggregate, or round- or smooth-textured coarse aggregate. In the case

Figure 16-17.-Corrugations.

Figure 16-18.-Shoving.

of emulsified and cutback asphalt mixes, it maybe due to a lack of aeration.

DEPRESSIONS. - Depressions are localized areas of limited size that may or may not be accompanied by cracking (fig. 16-19). Water collects in depressions that then become not a source of pavement deterioration but a hazard to motorists. Depressions are caused by traffic heavier than that for which the pavement was designed, by poor construction methods, or by consolidation deep within the subgrade.

UPHEAVAL.- Upheaval is the localized upward displacement of the pavement due to swelling of the subgrade or some portion of the pavement structure (fig. 16-20). It is commonly caused by ice expansion in the granular courses beneath the pavement or in the

Figure 16-19.-Depression.

Figure 16-20.-Upheaval.

subgrade. Upheaval may also be caused by the swelling effect of moisture on expansive soils.