Strength characteristics of crushed gravel and limestone aggregates with up to 12% plastic fines evaluated for pavement base/subbase applications

Abstract

Abstract Performance period or life span of a flexible pavement is dependent upon the load carrying capacity of its base and subbase layers. Pavement strength characteristics influenced by material properties of these foundation layers are of utmost importance in the pavement design. In this experimental study, the goal was to adequately define the limits of different unbound aggregate properties influencing the strength of unbound aggregate. Material type, gradation, maximum particle size, fines content, dust ratio and plasticity index were among the different properties studied. Dust ratio is defined as the ratio of material passing the No. 200 sieve (i.e. fines content) to material passing the No. 40 sieve. As for the aggregate materials, crushed limestone and crushed gravel, commonly used in Illinois in base/subbase applications, were considered. Illinois dense graded base specifications allow aggregate materials with a maximum particle size of 25 mm for the CA 6 specification and 50 mm maximum particle size for the CA 2 aggregate. Plasticity index, fines content, and dust ratio ranged from 5% to 13%, 5% to 12%, and 0.4 to 1.0, respectively. California Bearing Ratio (CBR) and staged triaxial tests were performed to characterize the material strength. Higher strength values were obtained for the CA 6 aggregates with 25-mm maximum particle size compared to the aggregates tested for CA 2 specification. Considering typical property ranges, a dust ratio of 1.0 was found to be a viable option in some cases for base/subbase applications providing an acceptable soaked strength for both crushed limestone and crushed gravel. However, for both material types, the combination of a dust ratio of 0.4 and a fines content of 12% posed a severe negative effect on aggregate strength.