Development of an Economic and Practical Alternative to Traditional Bituminous Surface Treatments

In many developing countries, a substantial portion of the road network, typically more than 75%, consists of earth and gravel roads that carry relatively low volumes of traffic. Traditionally, natural gravel materials are used for providing, maintaining, and upgrading such roads. However, most natural gravels have practical limitations on their use—for example, steep gradients and high rainfall, dust generation in dry weather, and slippery surfaces in wet weather. In addition, there is an increasing scarcity of suitable materials, and, moreover, maintenance is seldom carried out effectively in most developing countries. The inherent problems of maintaining unpaved roads can be overcome by paving them. However, even when this solution is economically justifiable, often it is not adopted widely in developing countries because of limited funds. However, a type of surfacing that can provide an economic and practical alternative to traditional surfacings and, by so doing, make better use of limited funds is the graded aggregate (Otta) seal. Otta seal surfacing is presented as an economic and practical alternative to the more traditional bituminous surface treatments, such as chip seals and slurry seals. The origin, properties, design, and construction of the Otta seal are outlined. The effects of its implementation and the various factors that contribute to its sustainability are discussed, and information is provided on global use of the Otta seal, including a case history relating to its introduction in southern Africa.