IMPLICATIONS OF LIGHT BITUMINOUS SURFACE TREATMENTS ON GRAVEL ROADS

Large portions of road networks in many countries consist of gravel or earth roads, which have many undesirable characteristics. They can be dusty in the dry season, slippery and muddy in the wet season, and often develop potholes. It is therefore necessary to find a method of upgrading their serviceability that will not require an extensive capital outlay. The use of dust palliatives and surface seals on gravel roads was investigated. The dust palliatives were tested in a pilot study and found to have a limited life span of some three years. After the first year, roads treated with dust palliatives begin to deteriorate rapidly even with a daily traffic volume of only approximately 90 vehicles and regular maintenance had to be carried out. The Maintenance and Design System program uas used to carry out an economic evaluation. Because of such poor performance and the construction cost, this type of treatment is not an economically sound solution. A bituminous surface seal has been used extensively in South Africa on gravel roads or light pavement structures. Roads treated in this way have generally performed satisfactorily. Tests with a heavy vehicle simulator have confirmed this observation. Under dry test conditions, some light pavement structures even carried more than a million equivalent 80-kN axle loads. However, the ingress of moisture through a cracked surfacing resulted in dramatic disintegration. Therefore, timely resealing is important since it prevents the structure from becoming soaked in the wet season. For construction, maintenance, and road user cost combinations commonly found in South Africa, the placement of a surface seal was found to be economically justified at traffic volumes of some 200 vehicles/day. However, sealing is only economically practical if funds can be made available as soon as resealing becomes necessary; otherwise, total disintegration could occur. In such cases it is better to keep the gravel surface. (Author)