Quantifying Effects of Particle Shape and Type and Amount of Fines on Unbound Aggregate Performance through Controlled Gradation

Construction of a pavement working platform often is needed on soft, unstable soils to provide sufficient stability and adequate, immediate support for equipment mobility and paving operations without excessive rutting. Standard specifications may allow the use of a wide range of aggregate materials for subgrade applications regardless of aggregate properties. The aggregate type and quality are important factors in determining the required treatment and replacement thicknesses. This paper describes laboratory findings from an ongoing research study at the University of Illinois at Urbana–Champaign to evaluate aggregate cover thickness requirements on soft subgrade. Evaluation was accomplished by characterizing strength and deformation behavior of crushed limestone and dolomite and uncrushed gravel, commonly used in Illinois for subgrade replacement and subbase. The initial laboratory phase consisted of moisture–density, unsoaked California bearing ratio, imaging-based aggregate shape characterization, and shear strength tests. These were based on a comprehensive experimental test matrix, which considered both plastic and nonplastic fines (passing No. 200 sieve or 0.075 mm) blended in the engineered gradations at 4%, 8%, 12%, and 16% target fines contents. From the test results, the most important property at low fines contents (less than 8%) was the aggregate type governed by the angularity (i.e., crushed or uncrushed) and the amount of voids in the aggregate matrix. The uncrushed gravel more quickly filled the voids at lower fines percentages, thus making gravel less tolerable to negative effects of increasing fines. When plastic fines (with a plasticity index of 10 or higher) were included, the amount of fines had a drastic effect on aggregate performance.