The production of construction and demolition waste has been increasing at a faster rate in recent years due to rapid urbanisation and thrust on infrastructure projects. Dumping of these wastes in landfill has adverse effect on environment and also results in increased use of natural resources. Also, the amount of landfill available to contain this material has been decreasing, and the need to find appropriate disposal locations has been of increasing concern. Recycling of construction and demolition waste offers a viable solution to the problem.
During rehabilitation, maintenance and reconstruction of roads, either a new layer is paved on top of the existing pavement thereby, resulting in burial of the limited non-renewal resources or the old pavement materials are being removed and dumped in a landfill. Rehabilitation of roads to overcome these problems by milling technology is the most cost-effective, suitable and eco-friendly solution. Asphalt pavements can be recycled using milling technology and the recovered material can be reused for constructing asphalt pavement.
Pavement milling is gradually becoming a routine activity in India for asphalt pavement maintenance, rehabilitation, and construction. This activity, which generally constitutes about considerable percentage of the pavement construction budget, is critical for ensuring sustainable pavements. It ensures the removal of the existing pavement in a safe and accurate manner and the procurement of the old materials for recycling.
Milling is the most widely used method for the recovery of Reclaimed Asphalt Pavement (RAP) from old pavements, prior to the placement of a new overlay. RAP is now one of the most recycled materials in road infrastructure construction. Recycling of RAP leads to significant savings in the use of virgin mineral aggregates and asphalt binder, and hence to a significant amount of conservation of our natural resources.
The upper layers of most pavements, either on roadways or airfields, are made up of asphalt mixes. The primary components of these mixes are asphalt binder (bitumen) and mineral aggregates. After construction, as a result of the combined action of traffic and the environment, a pavement deteriorates, and ultimately reaches a point where it needs maintenance or rehabilitation. When designed and constructed properly, this expected progressive degeneration is due to the generations of stresses and strains at different depths, resultant formation of fatigue cracks and/or permanent deformation (rutting), or deterioration of surface properties such as texture, which are related to roughness/smoothness, and friction.
Application of a new layer (overlay) of hot mix asphalt on the existing degenerated pavement increases the overall thickness of the pavement, which necessitates the relocation of drainage and other structures (such as guardrails) and can also reduce the overhead clearance under bridges. Furthermore, the existence of cracks and ruts underneath the new overlay causes reflection cracks or premature failure of the pavement. Therefore, one good option is to remove the existing deteriorated layer and place a new layer in its place. Before the advent of the milling machine, the only option to remove existing pavement layers was the use of scarifies, dozers, or earthmoving equipment fitted with ripper teeth. This process resulted in the formation of slabs of asphalt mix, which needed to be further crushed and then transported by haul trucks for disposal, along with a significant amount of dust and noise. Furthermore, the removed material was not suitable for recycling without significant additional processing. The ripper equipment used for breaking and removing the existing pavement causes a very uneven surface (on which the overlay needs to be placed). Also, because the material is obtained in unusable form, it is generally more economical to discard it in landfills – which have a significantly negative consequence on the environment. Landfill spaces are dwindling, and deposits of asphalt materials are undesirable as, over the long term, they may lead to environmental concerns. Hence, a new method was necessitated to remove deteriorated pavement layers in an accurate and uniform way and to procure the recycled materials in a usable way.
The process of pavement milling helps in avoiding the above problems. The basic idea of milling is to remove the deteriorated pavement layer to a desired depth using a controllable force, such that the existing damaged layer is removed completely, and the resulting surface is even. One advantage of milling, as opposed to the ripping and crushing operation, is that the removal and crushing of the material takes place simultaneously, resulting in materials in a granular form (RAP material). Generally, the resulting material can be utilized for recycling as is or with minimal processing and there is no extra step needed to reduce the size of the materials in this case.
For many years, the concept of traditional milling is adopted by most agencies in asphalt pavement rehabilitation. However, traditional milling leaves the pavement surface with a rough surface which might cause some limitations in some roadway rehabilitation treatments. Micromilling, on the other hand, is an alternative to traditional milling where it utilizes the same equipment but with additional teeth placed on the cutting drum. Its application results in a smoother pavement surface due to the reduced distance between the ridges and the valleys of the milled surface. Micromilling is used in some limited applications where a smoother milled pattern is desired such as thin surface treatments/overlays (i.e., chip seal, slurry seal, cape seal, micro surfacing, thin lift overlays, etc.), pavement marking removal, and some surface corrections like surface profiling, grade correction, friction restoration, and bump removal. It is important to mention the difference in the end product (RAP) between the traditional milling and micromilling. The latter generates finer materials close to the required project gradation which diminish the need for crushing before reclaiming the RAP into hot mix asphalt. Micromilling is mostly used in applications where the required cutting depth is two inches or fewer. However, a combination of traditional milling and micromilling can be utilized when the necessary cutting depth exceeds two inches. In this case, conventional milling will be employed to remove existing pavement material; afterward the surface is finished by micromilling a couple of inches.
Milling can be carried out to remove: (1) existing surface deformations and irregularities; (2) materials to a uniform depth and uniform cross slope; (3) an entire asphalt mix layer; and, (4) materials to variable depth along the project length.