|dc.description.abstract||This report investigates the cause of asphalt concrete pavement distress along U.S. 331 between mile markers 88.480 and 91.070. The distress consisted of standalone bumpy sections of the asphalt concrete pavement, all less than about 50 feet in length, and bumps where the road crosses culverts or abuts bridges. At culverts and bridges, the bumpy sections were less than ten feet long. Because ALDOT personnel were not concerned about the causes of bumps at the bridges and culverts, those bumps were not investigated. The roadway was constructed on lime treated, clayey soils.
Swelling clays and ettringite formation were investigated as causes of the standalone distress. While swelling subgrade soils were present, the shape of the distressed surface did not coincide with the swelling soil distress. Swelling soil distress results in much longer wavelength distress over much longer distances than observed.
Ettringite formation was investigated as a cause of the standalone distress. Ettringite formation is accompanied by an increase in soil volume, leading to surface heave. This mineral forms in sulfate rich lime treated soils. Lime was added to the soils as part of the construction practice. High sulfate concentrations were found to exist in the soils.
The investigation had office, laboratory, and field components. Soil and drainage maps and construction records were examined. Field grab samples (all clayey soils) from U.S. 331 shoulders were evaluated for plasticity, mineral composition, and swell potential. Samples from under the asphalt concrete were taken by Alabama Department of Transportation personnel and evaluated for organic content, plasticity and mineral composition.
The pavement distress is most likely due to ettringite formation. The research implementation recommendation is to identify sulfate rich soils before construction of lime treated soil, and allow time for ettringite to form before placing asphalt concrete. Chapter 5 provides detailed recommendations.||en_US