Effect of Drainage in Unbound Aggregate Bases on Flexible Pavement Performance

It has been well demonstrated that a positive subsurface drainage is beneficial in enhancing pavement performance and thus extending pavement service life. Typical permeable base materials include asphalt/cement-treated, open-graded aggregates and unbound aggregates. Although asphalt/cement-treated, open-graded permeable bases perform well based on the past engineering practice, they are expensive solutions and less desirable for some roadways when compared to unbound aggregates, especially for low- to medium-volume roadways. In these situations, it is possible to use a properly graded unbound aggregate that is adequately drainable and structurally stable during the construction and service lifetime after the roadway is open to traffic. This study is to determine a proper/optimum gradation by conducting laboratory testing for unbound aggregates of Mexican limestone that are commonly used in Louisiana highways. However, there is trade-off between structural stability and permeability of unbound aggregates. The increase of permeability is often at the cost of structural stability or vice verse. Therefore, the criteria for selecting an optimum gradation are: (1) an adequate permeability to drain the infiltrated-water from the pavement as quickly as possible; and (2) a sufficient structural stability to support the traffic loading. The permeability of unbound aggregate is quantified by its saturated hydraulic conductivity while its structural stability is characterized by various laboratory tests on the strength, stiffness, and permanent deformation of the material. A series of laboratory tests, including constant-head permeability, California Bearing Ratio (CBR), Dynamic Cone Penetrometer (DCP), tube suction (TS), monotonic load traiaxial tests, and repeated load triaxial (RLT) tests, were conducted on Mexican limestone with different gradations. The gradations under investigation include coarse and fine branches of Louisiana class II gradation, New Jersey gradation medium, and an optimum gradation (fine and coarse branches). The optimum gradation is the result of a series of laboratory trial-error tests. The results from laboratory tests indicate that: (1) the coarse branch of Louisiana class II gradation outperform the fine counterpart in terms of permanent deformation and hydraulic conductivity; (2) CBR and DCP values may not be good properties to differentiate performance of unbound aggregate with different gradations; and (3) an optimum gradation is identified, which outperforms current Louisiana class II base gradation in terms of both structural stability and permeability.