Development of acceptance specifications for tropical gravel paving materials

Abstract

The ideal standard grading specification for gravels was meant to ensure high density and stability, and specifications on maximum limits of content of fines, liquid limit and plasticity index were meant to control the magnitude of swell of materials in the pavement structure. Experience in Ghana and elsewhere in the tropics has shown that specifications based only on these index properties may be misleading. For example, the strength criterion based upon minimum soaked CBR of 80% for separating sub-base and base gravel materials has been found to be more critical and makes it possible to modify the allowable maximum fines content, liquid limit and plasticity index for the residual gravels studied. Material selection, field compaction (construction) control and long-term field performance studies on two trunk roads were analysed. Limiting values of index properties for separating these gravels into sub-base and base course materials for pavement construction in a moist sub-humid climatic environment was proposed. For example, the maximum fines content, liquid limit and plasticity index for accepting these gravels for base course construction were increased from 10, 25 and 6% to 12.5, 37.5 and 10%, respectively. The importance of using the products of fines content and plasticity parameters for selecting these gravels was also established.