Prof. Rajib B. Mallick and Prof. A. Veeraragavan
Roads are a necessity for the modern life – and more specifically, good roads are essential for an efficient transportation system. Quite often roads deteriorate not because of the poor quality of the surface layer alone, but because of problems in the underlying layers, which are typically referred to as base, subbase and subgrade, as one goes down the pavement structure. Such problems can be due to poor choice of materials in the lower layers, inadequate compaction of lower layers, moisture/inadequate sub-surface drainage in the layers, poor gradation, or simply layers that have deteriorated over the service life of the pavement. The problems can manifest themselves as rutting, cracking, potholes and, severe roughness and deformations. A road with any of such problems is very uncomfortable to ride, can be dangerous because of potential harmful effects on the stability of vehicles, and will have significantly reduced speed and capacity. Furthermore, a thin overlay of Hot Mix Asphalt (HMA) or cheaper layers such as chip seals (surface dressing) will definitely not solve this type of structural problems, and if done, will lead to waste of materials, money, labour and time. An option that might come in the mind is: why don’t we excavate the pavement upto the layer that is causing the problem, and get rid of the problematic materials and start the construction of the pavement from that layer upwards with virgin materials? Yes, that is an option, but a terribly costly, from the point of view of both men/materials and money, and time. Also, the materials that are excavated need to be transported to a landfill or a different area, which has to bear the burden of this construction waste, and then there is the additional energy (diesel) need for the trucks that are used for the removal of the materials.
Is there a better option? YES, fortunately, thanks to the advent of modern sophisticated road recycling machines, there is the technology of Full Depth Reclamation (FDR) that can be used in such cases to improve the road, by utilizing the existing material, and save money!
Figure 1. Schematic of a typical FDR operation (Courtesy: Wirtgen GmbH)
What is FDR? It is a recycling method where all of the asphalt pavement section and a predetermined amount of underlying base material is treated to produce a stabilized base course. It is a cold recycling process in which different types of additives such as asphalt emulsions and chemical agents such as calcium chloride, Portland cement, flyash and lime, are added to obtain an improved base. A relatively thin surface layer can then be used to cover up the base. The four main steps in the FDR process are pulverization, introduction of additive, compaction, and application of a surface or a wearing course. In cases where the in-place material is not sufficient to provide the desired depth or the properties (such as gradation) of the treated base, new materials, such as aggregates may be added. Depths of upto 300 mm can be treated with this method, although deeper layers can also be treated by milling off surface layers to an appropriate depth, if the total thickness is too high. Figure1 shows a schematic and Figure 2 shows photos of a typical FDR operation, with a train that consists of a recycling machine hooked to a water tanker and additive truck (foamed asphalt, for example) and steel drum roller with pad foot shell. Figure 3 shows photos of a road before FDR and the recycled layer after FDR.
Figure 2: A FDR operation using foamed asphalt
Figure 3: Photos of road before FDR and recycled layer after FDR
There are many advantages of FDR, such as the ability to treat most pavement distresses, distresses that occur in the underlying layers, minimization of transportation costs, elimination of material disposal problem, enhancement of ride quality and load carrying capacity, and the ability to widen existing pavements, reduced construction cost and emissions.
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