Evaluation of gravel aggregate for asphalt concrete

Mineral aggregate is the most widely used construction material for asphalt concrete pavements. Therefore, aggregate quality is a major factor for the performance of an asphalt concrete pavement. Gravel is a readily available natural deposit in Alabama for use in pavement construction. Crushing is performed in order to improve the structural quality of gravel, which provides the resistance against rutting. Unavailability of adequate feed size results in partial crushing. Therefore, rutting resistance is one of the important concerns in using gravel for asphalt concrete. Another concern is stripping of Chert type gravel. Chert gravel is highly absorptive and residual moisture causes most of the stripping problems. This research was undertaken to investigate the methods of measuring aggregate particle shape, angularity, and surface texture and also to develop a scheme to reduce stripping problems and at the same time improve rutting resistance.; Four test methods were evaluated to measure aggregate structural quality. Fractured particle count, flat and/or elongated particle count, particle index and uncompacted voids were evaluated. Interrelationships among the tests were investigated and suitable methods identified. Particle index and uncompacted voids tests are more objective and precise than other two tests. Both of them measure the same basic aggregate properties. Therefore, either one could be used for evaluation of aggregate quality but supplemental tests for particle shape are required.; Use of crushed gravel in the fine fraction of asphalt concrete mix was tried in order to reduce the effect of residual moisture. Retained tensile strength as measured by tensile strength ratio (TSR) was used to evaluate the potential for stripping. Since all the aggregate was oven-dry in the laboratory mix, the effect of residual moisture was not included in the investigation. But fine aggregate with its larger surface area dries faster and eliminates part of the residual moisture effect. TSR values did not show any difference in stripping potential by using oven-dry crushed gravel in fine fractions instead of coarse fractions. But use of liquid anti-strip improved the stripping, resistance for mixes with crushed gravel in both fine and coarse fractions.; Both repeated load deformation test and Georgia loaded wheel rut test were used to evaluate Rutting. Repeated load deformation test did not identify the differences in rutting performance of various mix. But, Georgia loaded wheel rut test (GLWRT) showed that aggregates with higher structural quality are more rut resistant. One of the most important factors in rut resistance was found to be the voids in the mineral aggregate (VMA). Rut susceptibility increases with the increase of VMA. Uncompacted voids of the aggregate blend and VMA explained about 83% of the variation in rut depths measured by GLWRT.