Experts advocate composite pavement for durable road construction

In view of persistent road failures in Nigeria, experts, after an exhaustive discussion last week, settled for composite pavement-admixture of the well known asphalt and concrete pavements as the reliable panacea to capable of arresting the menace. The event was the “CED Forum/Award 2016” ceremony, held at the Sheraton Hotel, Ikeja, with the theme: “Concrete Road in Nigeria: Durability, Acceptability and the Economic Benefit to a Developing Nation.” The Guest Speaker, Dr Isa Usman Emoabino, an engineer, in his lecture, noted that the primary structural difference between a rigid and flexible pavement is the manner in which each type of pavement distributes traffic loads over the sub grade. Explaining the concepts, Emoabino noted that rigid pavement has a very high stiffness and distributes loads over a relatively wide area of sub-grade, where a major portion of the structural capacity is contributed by the concrete slab itself. “For a flexible pavement (asphalt), the load carrying capacity is derived from the load-distributing characteristics of a layered system. “A flexible pavement structure is typically composed of several layers of material with better quality materials on top where the intensity of stress from traffic loads is high and lower quality materials at the bottom where the stress intensity is low. “Flexible pavements can be analysed as a multi-layer system under loading. A typical flexible pavement structure consists of the surface course and underlying base and sub-base courses. Each of these layers contributes to structural support and drainage. “When hot mix asphalt is used as the surface course, it is the stiffest (as measured by resilient modulus) and may contribute the most (depending upon thickness) to pavement strength,” he stated. The Speaker listed the advantages of Asphalt (Bitumen) as being economical, as it is still less costly compared to concrete. It also takes less time to build an asphalt road than a concrete road. Asphalt dries faster, freshly laid asphalt can carry load immediately. Asphalt is recyclable, it can be used again and again by melting it; it is also easy for maintenance; and repairing just a part of the asphalt road is easily possible. Asphalt roads can be relayed over the old layer, and they provide better traction and skid resistance for vehicles and tends to help keep roads free from ice and snow. However, its disadvantages are that in the context of durability, heavy rain and other extreme weather conditions damage the asphalt road, and they need frequent repairs. “Excess axle load causes deflection and deformation on the road including weather pollution. Melting asphalt produces lots of harmful greenhouse gases. Asphalt is a costly petroleum product,” said Emoabino. Conversely, rigid pavement structure is composed of a hydraulic cement concrete surface course, underlying base and sub-base courses. According to him, the surface course (concrete slab) is the stiffest layer and provides the majority of strength. “The base or sub-base layers are orders of magnitude less stiff than the concrete surface but still make important contributions to pavement drainage and frost protection and provide a working platform for construction equipment. “Rigid pavements are substantially ‘stiffer’ than flexible pavements due to the high modulus of elasticity of the concrete material, resulting in very low deflections under loading”. The rigid pavements can have reinforcing steel, which is generally used to handle thermal stresses to reduce or eliminate joints and maintain tight crack widths. The construction of a rigid pavement is a fairly complex process which involves; proper preparation of the sub grade and sub-base, placing reinforcing bars or dowels appropriately, Careful choice and handling of aggregates