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.
Faced with limited financial resources, pavement engineers constantly seek more durable and economical technologies for road preservations and rehabilitations. Consequently, there have been many efforts to study resurfacing strategies, including various types of sealing for local roads. Among different sealing methodologies, Otta seal is a technique that has not yet been sufficiently studied in the U.S.A. For this investigation, the first Otta seal site in the state of Iowa was constructed using a double-layer Otta seal design over 6.4 km of cracked asphalt pavement. Otta seal design and construction details are documented and discussed, and test sections using various aggregates are compared for performance. The key lesson learned was that proper aggregate selection within gradation limits and aggregate spread rates were critical factors for Otta seal performance. Otta seal capability for holding loose aggregate particles and for dust control were examined, and there were indications that excessive proportion of fine aggregate particles could lead to diminished performance associated with fugitive dust emissions and unbound aggregate particles. Although the Otta seal provided a smooth surface satisfying road user and agency requirements, it did not significantly add structural capacity to the existing asphalt pavement. The findings from this study will benefit road officials and other decision makers who need to consider alternatives for resurfacing distressed low-volume asphalt roads.
Natural gravel materials have traditionally been used for upgrading roads from earth to gravel or for regraveling existing gravel roads. However, serious concerns have arisen regarding the continuous use of vast amounts of gravel—a nonrenewable, finite resource—which not only is being rapidly depleted in many countries but also is unsustainable in the medium to long term. This problem has prompted road engineers to consider the use of low-cost bituminous surface treatments on these gravel roads as an alternative to regravelling. One type of surfacing that can provide an economic and practical alternative to traditional surfacings, such as the chip seal, is the Otta seal. Unfortunately, lack of information regarding this relatively new type of bituminous surfacing has suppressed its more widespread use despite its excellent performance in a number of countries. In view of this problem, state-of-the-art information on the provision of the Otta seal surfacing as an economic and practical alternative to the more traditional bituminous surface treatments is presented. The origin, properties, and design and construction of the Otta seal are outlined, and its implementation impacts and the various factors that contribute to its sustainability are discussed. Brief information is presented on the global use of the Otta seal.
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