Recycled asphalt can be obtained by milling or full-depth removal, after which it must be crushed. It is important to be selective in the removal of old pavements for use in recycling, to ensure that the material removed is of a sufficiently high grade for future use. Prior to removing the old pavement, material testing can provide substantial information. Core samples should be drilled and extracted to assess the material properties and the amount of material that will be removed. The mixture design and assessment of the suitability of recycled asphalt has the following successive steps:
In this article, Global Road Technology takes a closer look at the recycling process of recycled asphalt.
Milling entails the removal of the pavement surface using a road profiler, which can remove 25 to 50 mm for surface layers down to full depth pavements of up to 400mm. Deeper removal may involve heavier excavation techniques, although risks contamination with sub-base/subgrade layers. Profiling or milling is the preferred method, as it permits a more selective removal than does a full depth removal. In addition, using the profiling method makes it easier to allow traffic to access during the removal process. The type of re-use suitable for pavement material is largely determined by the size of the slabs, lumps, or blocks created when the pavement is removed – which is a product of the pavement type. If the material is to be processed through a crusher, the slabs removed from the road should not be larger than 1.0 x 0.7 x 0.4 m. The Renofalt asphalt regeneration process is a process where the lumps of recycled asphalt are crushed to a 0 to 40 mm grading, then steam treated prior to the rejuvenation process. Temperature and moisture penetration causes the lumps of recycled asphalt to disintegrate. The advantage of the Renofalt process over the crushing method is that the material is not shattered and so dust is formed as a filler. However, if the asphalt regeneration is done using the Renofalt process, the lumps should be smaller than 0.4 x 0.4 x 0.4 m.
The methods of asphalt removal and crushing are important in the production of well-graded, good quality recycled asphalt from brittle, aged, and hardened asphalt, New bituminous mixtures containing recycled asphalt can be modified by the addition of fresh aggregates to suit any pavement layer. However, it is recommended that recycled asphalt aggregate should not be used to manufacture bituminous surface course mixtures unless it possesses the high degree of uniformity required for such a mix. The uniformity and quality of the recycled asphalt coupled with the type of recycling plant will determine the percentage of recycled asphalt that can be used in new bituminous mixtures. The typical percentages of recycled asphalt to be included in new mixtures ranges from 10 to 30%, although inclusions as high as 80 to 100% are feasible. Once removed, recycled asphalt should be separated into its constituent layers. The higher-grade recycled material from the surface course can be used in the reconstruction of the new surface course and subsequent lower layers. Due to the low quality of material used in lower pavement layers, the material from the lower layers cannot be used in the construction of a new top layer.
Where asphalt material is removed from the road and not used immediately, stockpiling of recycled asphalt is a significant component of the recycling process. The full benefits of comprehensive material testing can be lost if great care is not taken when stockpiling the recycled asphalt material. Depending upon the variability found during testing, it might be necessary to build separate stockpiles of materials taken from different sections of a road. It is important to record the detail of each load as it arrives at storage, taking note of the type and amount of material, place of origin, and any undesirable contaminants. If any batch is contaminated, it must be kept separate while identification tests are undertaken. Stockpiled recycled asphalt pavement should not be piled higher than 3m, to prevent caking which can occur under the influence of generated heat. Asphalt destined for regeneration should either be covered with a layer of sand or kept under a roof to prevent water penetration. Some research shows that large piles of recycled asphalt do not agglomerate, although a 250 to 300mm crust may form at the surface of the stockpile and this should be removed prior to recycling. It is recommended that storage areas or collection points should not be accessible to the public, to prevent any unauthorized dumping of other types of waste.
There are two main methods of asphalt recycling: hot and cold, which can be further sub-divided into in-plant or on-site recycling. Recycling the old asphalt involves heating the material with or without the new mineral aggregates and the mix is laid hot. For hot processing, materials are either mixed on-site or in a mixing plant. Alternatively, cold generation of the old asphalt is processed in a crusher and can be used for the new binder course or sub-base. Virgin aggregate and bitumen can also be added to a new mix. Over the last decade, warm mix recycling has become popular. It is conducted in the plant, the process being very similar to the hot processing method but conducted at temperatures between 20 and 55 °C, lower than typical hot mix recycling. The choice of the recycling process will depend on several factors, such as:
In the recycling process, the environmental properties of recovered materials must be considered, to ensure that there are no potentially harmful constituents present in the mix. No leaching or toxic constituents, dust particles that might cause air emission concerns. The use of recycled asphalt is a brave move towards sustainability and all other factors being considered a noble one in the drive for alternative materials for road construction, provided there is good material quality control.
https://www.sciencedirect.com/science/article/pii/B9780857096821500166
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