Construction of a working platform is often needed on soft, unstable soils to provide sufficient stability and adequate support for equipment mobility and paving operations without developing excessive rutting. Subgrade removal and aggregate replacement for cover is one of the most commonly used options for treating soft, unstable soils. Aggregate type and quality are important factors governing the required treatment/replacement thickness. This paper presents preliminary laboratory findings from an ongoing research project at the University of Illinois aimed to characterize behavior of three types of aggregates commonly used in Illinois for subgrade replacement and subbase. Samples were prepared with consistent engineered gradations, and their moisture-density and strength behavior were studied at different amount of fines, as well as with different types of fines. Plastic fines were found to reduce aggregate strength significantly, as compared to the same amount of non-plastic fines.