Aggregate characteristics affecting response and performance of unsurfaced pavements on weak subgrades

Design of low volume roads and unsurfaced pavements traditionally involves covering the prepared subgrade with an unbound aggregate layer of sufficient thickness such that traffic-induced wheel 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 cover layers. Aggregate quality aspects and properties are not considered in detail while selecting aggregate sources often with the lowest material hauling and transportation costs. This approach based solely on economic considerations may result in the selection of locally available poor quality material for routine use as the primary pavement load bearing layer. The primary objective of this PhD research was to evaluate individual effects of selected aggregate physical properties (test factors), such as particle shape, texture and angularity, type and amount of fines, and compaction (moisture-density) conditions on the response and performance of unsurfaced pavements.;Three different aggregate types, namely crushed limestone, crushed dolomite and uncrushed gravel were selected to first quantify the effects of individual test factors on aggregate shear strength, permanent deformation and directional modulus characteristics through a controlled laboratory test matrix. Laboratory test results showed that the effects of individual test factors on aggregate behavior changed significantly depending on the levels assigned to other test factors. Crushed aggregates showed consistently higher shear strength and modulus, and lower permanent deformations due to improved particle interlock when compared to the uncrushed gravel. The effect of type of fines on aggregate behavior was erratic at low fines contents, and was more pronounced when the fines fraction occupied a significant proportion of the voids in the aggregate matrix. Plastic fines when combined with excessive moisture, were found to destroy the inter-particle load transfer in the aggregate matrix, thus inducing excessive deformations under loading leading to specimen failure. The concept of anisotropic modulus ratio, an indicator of compaction induced anisotropy in aggregates, was successfully applied as a material quality indicator for predicting unbound aggregate behavior.;Important findings from laboratory testing of aggregates served as the basis for material selection and thickness design of full-scale unsurfaced pavement sections for accelerated testing. Five different test "cells" were constructed at different combinations of aggregate material quality and engineered subgrade strength, and were tested to failure using an Accelerated Transportation Loading Assembly (ATLAS) under near-optimum and flooded aggregate moisture conditions. Test section performances under simulated traffic loading were monitored through surface profile measurements and transverse scanning with Ground Penetrating Radar (GPR) for assessment of subsurface deformations. Transverse trenches were subsequently excavated for visual identification of different mechanisms contributing to pavement failure. The use of non-destructive field modulus measurement techniques was pursued to establish their applicability as means of identifying anomalies in construction quality. Earth pressure cells were installed on top of the subgrade at the aggregate-subgrade interface to evaluate the effects of aggregate material type and quality on the dissipation of traffic-induced stresses within the aggregate layer. Depending on the type and quality of aggregates used, internal shear movement of the aggregate layer or subgrade deformation was found to be the primary mechanism contributing to pavement failure. Recommendations regarding the material selection and construction practices for unsurfaced pavements were finally presented to ensure improved performance of such pavement systems.