Two forms of pavement distress were noted on recent asphalt concrete overlay projects. A general loss of wearing surface under traffic action, termed surface ravelling, was observed on four projects approximately the second season after completion.
Construction record reviews and field testing and sampling of these projects along with four undistressed projects were made to define the construction methods and equipment used and to obtain samples from areas of good and bad pavement condition. These samples were tested to determine quality of compacted mix, and of the aggregate and asphalt constituents.
Common factors from the projects showing surface ravelling were a thin lift pavement overlay placed late in the paving season with a tendency toward less than optimum asphalt content. The resulting pavement had high air void content and was readily abraded by traffic during ensuing severe winter weather. Recurrence of this type of problem can best be prevented by increased emphasis on proper compaction and mix design. Remedial work on the present projects should be directed toward a seal coat application.
Strip ravelled projects were found to have similar pavement properties in ravelled and non-ravelled sections and generally to be within desirable limits. Lower strength in resilient modulus tests and physically observed asphalt stripping in the ravelled sections indicate a localized failure of the asphalt aggregate bond. The failure occurrence in narrow longitudinal strips indicates a cause associated with paving operations--possibly differential compaction or hydrocarbon contamination. Preventive measures should ensure uniform compaction and more thorough checking of stripping tendencies. Corrective measures will require continued patching and possibly removal and replacement of affected pavement.