REFLECTION CRACKS.— Reflection cracksnormally occur in asphalt overlays. These cracks reflectthe crack pattern in the pavement structure underneath(fig. 16- 13). They are most frequently found in asphaltoverlaysover portland concrete andcement-treated bases. Reflection cracks are normallycaused by vertical or horizontal movements in thepavement beneath the overlay, resulting from trafficloads, temperature, and earth movements.SHRINKAGE CRACKS.— Shrinkage cracks areinterconnected cracks, forming a series of large blocksusually with sharp corners or angles (fig. 16-14). Oftenit is difficult to determine whether shrinkage cracks arecaused by volume change in the asphalt mix or in thebase or subgrade.Frequently, they are caused byFigure 16-13.-Reflection cracks.volume change of fine aggregate asphalt mixes that havea high content of high-viscosity asphalt. Lack of traffichastens shrinkage in these pavements.SLIPPAGE CRACKS.— Slippage cracks arecrescent-shaped cracks, resulting from horizontal forcesinduced by traffic (fig. 16-15). They are caused by alack of bond between the surface layer and the coursebeneath. Lack of bond maybe due to dust, dirt, oil, orthe absence of a tack coat.DistortionPavement distortion is any change in a flexiblepavement surface. It is the result of a subgrade surface.weakness where compaction or movement of thesubgrade soil has taken place or where base compactionhas occurred. It may or may not be accompanied bycracking, but in either instance, it creates a traffichazard, permits water to accumulate, and eventuallymakes matters worse. Distortion takes a number ofdifferent forms but is normally classed as channeling,corrugations and shoving, depressions, and upheaval.Figure 16-15.–Slippage cracks.Figure 16-14.—Shinkage cracks.16-18Figure 16-16.–Channeling.
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