Aggregate physical properties affecting modulus and deformation characteristics of unsurfaced pavements

This paper presents findings from a recently completed research study at the University of Illinois aimed at investigating the effects of aggregate type and quality on mechanistic pavement response and rutting performance with implications to over 2.6 million km (1.6 million miles) of unsurfaced roads in the United States. Three aggregate types, crushed limestone, crushed dolomite, and uncrushed gravel, were tested in the laboratory for resilient modulus (M_R) and permanent deformation characteristics at different factorial combinations of selected aggregate physical properties. Aggregate properties studied included particle shape and surface texture, type and amount of fines, and moisture and density in relation to required compaction conditions. Stress-dependent material characterization models determined from the laboratory M_Rtests were used in a nonlinear axisymmetric finite-element analysis program to compute the vertical compressive stress on top of the subgrade as a critical pavement response. The adequacy of the aggregate layer to carry wheel loads and prevent subgrade rutting was evaluated using the concept of the subgrade stress ratio (SSR), defined as the ratio between the vertical stress on top of the subgrade and the subgrade unconfined compressive strength. The laboratory testing and modeling showed that the aggregate physical properties had significant influences on both the modulus and permanent deformation behavior of unbound aggregates. In addition to protecting the subgrade from rutting by means of placing stiff (high M_R) aggregate layers on top, a properly designed unsurfaced pavement system should also evaluate the susceptibility of the aggregate cover itself to excessive rutting, preferably through permanent deformation testing of the aggregates. The findings clearly highlighted the importance of considering both the load spreading and rut resistance aspects of the unbound aggregate layer in the design of unsurfaced pavements. The significance of different aggregate properties affecting modulus and permanent deformation model parameters was identified through statistical analyses of variance (ANOVA) conducted on the laboratory test results.