Performance evaluation of uncrushed aggregates in unsurfaced road applications through accelerated pavement testing

The design of low-volume roads and unsurfaced pavements traditionally involves covering prepared subgrade with an aggregate layer of sufficient thickness such that traffic-induced loads are adequately distributed and stresses on the subgrade can be tolerated. Aggregate gradation and field density requirements are commonly the only considerations for constructing acceptable aggregate layers. Aggregate quality aspects and properties often are not considered in detail when aggregate sources with the lowest material hauling and transportation costs are selected. An approach based solely on economic considerations may result in the selection of locally available material for routine use as the primary load-bearing pavement layer. Full-scale test sections were constructed at the University of Illinois at Urbana-Champaign with aggregate materials of different types and qualities over a weak subgrade of controlled strength and tested to failure with the use of an accelerated transportation loading assembly. Pavement performance under near-optimum and flooded aggregate moisture conditions was monitored by measuring the surface profile. In addition, transverse trench sections were excavated to determine the mechanisms contributing to failure. Accelerated testing of a pavement test cell constructed with an uncrushed gravel with high amounts of nonplastic fines showed excessive rut accumulation through internal shear failure of the aggregate layer under near-optimum aggregate moisture conditions. Excavated trench sections clearly indicated the development of a shear surface within the gravel layer that caused a lateral offset in the subgrade deformation with respect to the wheel path. However, under flooded conditions, excessive subgrade movement was found to be the primary mechanism contributing to pavement failure.