For an overlay on an existing pavement, the degree of surface preparation is generally dependent on the condition and type of that existing pavement. Basically, the existing pavement should be structurally sound, level, clean and capable of bonding to the overlay. To meet these prerequisite conditions, the existing pavement is usually repaired, leveled, cleaned and then coated with a binding agent.
All failed sections of the existing pavements should be patched or replaced and existing pavement cracks should be filled. If an existing pavement is cracked or provides inadequate structural support, these defects will show up through even the best-constructed overlay and cause premature pavement failure in the form of still more and bigger cracks and deformations. Even small areas of localized structural failure in the existing pavement should be repaired or replaced to provide sufficient structural support. If the existing pavement contains areas of inadequate subgrade support, these areas should be excavated and removed, and the subgrade should be prepared as if it would be for a totally new pavement.
Which method to use in repairing the cracks in the existing pavement would depend on the type and severity of cracks. For instance, pavement sections with pattern or “fatigue” cracking should be patched or replaced because these distresses are often symptoms of more extensive pavement or subgrade structural failure. For cracks other than those symptomatic of structural failure, they can simply be filled with a crack-sealing material once cleaned and dried. Cracks less than about 0.375 inches in width may be too narrow for crack-sealing material to enter and hence should be widened with a mechanical router before sealing. If the existing pavement has an excessive amount of fine cracks but is still structurally adequate, it may be more economical to apply a general bituminous surface treatment or slurry seal instead of filling each individual crack.
Existing pavement should be made as smooth as possible before being finally overlaid. Before applying the final surface course, the existing pavement is thus usually further leveled, either by applying a leveling course of material, or by milling or grinding the existing pavement, or by some combination of the two methods. Finally, the leveled surface is uniformly covered with a tack coating material of bituminous liquid asphalt to promote bonding between the existing pavement and the new overlay.