Gradation Effects Influencing Mechanical Properties of Aggregate Base-Granular Subbase Materials in Minnesota

Aggregate gradation effects on strength and modulus characteristics of aggregate base granular subbase materials used in Minnesota are described. The importance of specifying proper aggregate grading or particle size distribution has long been recognized for achieving satisfactory performance in pavement applications. In the construction of dense-graded unbound aggregate base subbase layers, well-graded gradation bands were often established years ago on the basis of the experience of the state transportation agency and may not have a direct link to mechanical performance. To improve specifications for superior performance targeted in the mechanistic empirical pavement analysis and design framework, there is a need to understand how differences in aggregate gradations may affect unbound aggregate base subbase behavior for site-specific design conditions. Aggregates with different gradations and material properties were compiled in a statewide database established from a variety of sources in Minnesota. Analyses showed nonunique modulus and strength relationships for most aggregate base and especially subbase materials. Laboratory resilient modulus and shear strength results were analyzed for critical gradation parameters by common gradation characterization methods. The most significant correlations were between a gravel-to-sand ratio (proposed based on ASTM D2487-11) and aggregate shear strength properties. Aggregate compaction (AASHTO T99) and resilient modulus characteristics could also be linked to the gravel-to-sand ratio and verified with other databases in the literature. The gravel-to-sand ratio can be used to optimize aggregate gradations for improved base subbase performances primarily influenced by shear strength.