subgrade. Upheaval may also be caused by the swellingeffect of moisture on expansive soils.DisintegrationDisintegration is the breaking up of a pavement intosmall, loose fragments. This includes the dislodging ofaggregate particles. If not stopped in its early stages,disintegration can progress until the pavement requirescomplete rebuilding. Potholes and raveling are two ofthe more common types of early stage disintegration.POTHOLES.— Potholes are bowl-shaped holesof various sizes in the pavement, resulting fromlocalized disintegration under traffic (fig. 16-21).They are usually caused by weakness in the pavement,resulting from too little binder, too thin a surface, toomany fines, or poor drainage.RAVELING.— Raveling is the progressive lossof surface material by weathering or traffic abrasion(fig. 16-22). Usually the fine aggregate wears awayfirst, leaving little pockmarks in the pavementsurface.As erosion continues, larger particleseventually break free, and the pavement soon has therough and jagged appearance, typical of surfaceerosion.Raveling is caused by poor constructionmethods, inferior aggregates, or poor mix design.Slippery SurfacesOne of the most common causes of a slipperyflexible pavement is a thin film of water over a smoothsurface. his can cause a vehicle to hydroplane. Othercauses of slippery surface in flexible pavements arebleeding and polished aggregates.Figure 16-21.-Pothole.Figure 16-22.—Raveling.BLEEDING AGGREGATES.— Bleeding is theupward movement of bituminous material in aflexible pavement, resulting in the formation of a filmof bituminous material on the surface (fig. 16-23).The most common cause of bleeding is too muchasphalt in one or more of the pavement layers. Thisis usually the result of a rich plant mix or a prime ortack coat that is too heavy. Bleeding normally occursin hot weather.POLISHED AGGREGATES.— Polished aggre-gates are those that have been worn smooth under traffic(fig. 16-24). Polished aggregates are caused by usingthe wrong type of aggregate in the pavement mix.Figure 16-23.-Bleeding asphalt.16-20
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