Optimize Drainable Unbound Aggregate through Laboratory Tests

Unbound aggregate can lend itself to a cost-effective drainable base material when properly graded. A good performing drainable unbound aggregate should satisfy both permeability and structural stability criteria. However, there is a trade-off between unbound aggregates’ permeability and structural stability. This paper presents the results of a laboratory testing program to optimize unbound aggregate so that both permeability and structural stability requirements will be met. Mexican crushed limestone, a typical base course material used in Louisiana highways, was tested in this study. Its permeability was evaluated on the basis of hydraulic conductivity while its structural stability was assessed through several shear strength and stiffness testing procedures, including California bearing ratio, dynamic cone penetrometer, and repeated load triaxial tests. These testing results indicate that the fine branch of current Louisiana class II gradation has low permeability and yet may have stability problem. Also, an optimum gradation for the tested aggregate will be recommended for drainable base material.