This paper was presented at the session titled `Materials testing - innovative methods and applied technology'. The nature of road foundation materials plays a significant role in defining pavement behaviour under traffic loading. Currently, however, this role and its impact on road performance are not well established. This calls for the need to develop adequate characterisation techniques for road materials. The requirement to consider relevant performance indicator(s) necessitates, in the beginning, reliance on a mechanical response. Mechanical responses are usually based on external loading conditions. In the case of a pavement structure, this translates into traffic and environmental conditions and constitutes an important input to the pavement design/rehabilitation process. This paper presents recent developments in the area of road foundation materials characterisation - an area that is currently being pursued at NRC's Institute for Research in Construction. A state-of-the-art laboratory facility was designed and built to develop effective measures to guard against road deterioration in the case of inadequate structural support provided to the asphalt surface. Using this facility, a silty sand material representing a typical subgrade and an unbound aggregate material commonly used in base and sub-base road layers were tested under various load and physical conditions. Results obtained in the form of resilient moduli of the tested materials were analysed using statistical techniques. The results highlighted the well-known need to control compaction quality of these road materials to effectively reduce the potential for premature failure. Furthermore, the results indicate that more emphasis should be placed on moisture effects than is currently the case. Current practice focuses on obtaining the highest possible percentage of relative field density with less emphasis placed on controlling field moisture content. The results obtained from the mechanical test on both materials investigated show a significant drop in strength for an increase in moisture within commonly accepted ranges. These findings demonstrate that there is a need to revisit the way road foundation materials are characterised and to examine the implication(s) that this may have on the design and rehabilitation of pavements. (A)
Abstract