Production using static or mobile plants
The processing of RAP commonly takes place in the vicinity of Hot Mix asphalt plants for inclusion n Hot Mix or selling on to low grade applications. Logistical issues often arise when Hot Mix asphalt plants are modified for dual Hot/Cold Mix production. As a consequence of this lack of production optimisation dual production plants are rare.
Generally plants developed specifically for emulsion Cold Mix tend to be simpler than conventional Hot Mix plants. Regardless of its origin, the 'aggregate' produced does not necessarily need to be dried prior to use. Although moisture content control is important and a water metering system is required, heating requirements tend to be limited to the emulsion storage tanks.
In general terms the use of ex-situ processes for emulsion bound recycled asphalt has been limited to lightly trafficked categories - urban or estate roads, shopping centres and car parks although these materials are also used extensively for medium performance applications where axle loading can be heavy but infrequent, such as forestry commission estate roads. With this type of application it is common to construct the whole pavement, i.e. surface and base layers with emulsion bound RAP. The ability to mobilise plant locally to the excavation/construction site represents a substantial environmental benefit by reducing haulage distances.
In the UK, emulsion RAP mixtures have also been developed for use as storage grade material for permanent reconstruction or overlays of carriageways and reinstatement of excavations.
Production using in-situ techniques
In-situ processes are currently the most common techniques for recycling using emulsion binders. To date this is where maximum benefits have been achieved, environmentally as well as commercially. Some of the obvious benefits of in-situ recycling are that it reduces the need for the extraction and use of virgin aggregate, reduces haulage distances and energy used in production, and also has shorter possession times.
In Europe there are two primary emulsion-based in-situ recycling techniques/treatments.
- Shallow Cold Mix in-situ recycling (the retread process) tends to be limited to rejuvenation of the surface course. This is involves scarification of the surface layers, typically 75mm depth. The scarified material is mixed in-situ with an appropriate grade of bitumen emulsion and, if appropriate, additional aggregate. This application is usually associated with lighter traffic category applications.
- Deep Cold - the mix in-situ recycling is a structural treatment typically carried out at depths of up to 250mm. This requires a heavy-duty rotovator. These machines are designed to pulverise the existing pavement to a suitable particle size/gradation in a single operation, whilst mixing the pulverised pavement with a metered amount of emulsion binder and in most cases a small quantity of cement.
Benefits of Cold Mix recycling
Today's developments in emulsion binder technology and recycling techniques will allow specifiers and contractors to select from a greater choice of options based on specific needs: local, logistical, environmental or economic.
The incorporation of RAP into an asphalt mixture will benefit the drive towards the sustainability of natural resources whichever recycling technique/process is used.
Energy use associated with Hot Mix asphalt production is relatively high. Aggregate drying, heating and storage normally take place at temperatures exceeding 140°C with typical asphalt production temperatures in excess of 160° C. Emulsion mixtures are typically produced at ambient temperatures, in some cases the emulsion is used at elevated temperature. In-situ processing reduces haulage requirements further resulting in a further reduction in overall fuel use over mobile and static plant operation.
Accepting energy conservation, the active recycling of old asphalt pavements has the potential to unearth latent problems from the past, namely tar based materials. Historically tar has been used extensively as a road binder throughout Europe. The use of a Cold Mix emulsion binder to encapsulate the tar based materials, negating the potential for degradation at elevated temperatures while effectively tackling the leaching issues of the future, is a growing market.
The use of cold emulsion bound asphalt can provide logistical advantages where stockpiling or transport over long distances may be necessary - they are not affected by temperature loss. There is no requirement for insulated trucks which adds transport flexibility. Reduced ageing of the binder compared to Hot Mix production may improve durability. The workers are no longer exposed to risks from handling hot products on site.
Research into Cold Mix recycling with bitument emulsions continues to seek answers to issues relating to early life curing of mixtures, an issue which has hindered the commercial development of ex-situ Cold Mix using recycled asphalt.
The Nyrec system researched by Nynas utilises a trigger mechanism on compaction that allows for early curing and immediate trafficking of Cold Mix materials.
Investigations into the 'real' performance contribution of the binder in recycled asphalt demonstrate the positive aspects of recycling with emulsion binders.
The choice of bitumen grade is a function of the traffic and climatic conditions and can vary between a 40-60 grade and a softer viscosity grade such as MB12000 in Scandinavia. One unique property currently being investigated is the ability of Cold Mix systems to self heal irrespective of pen grade used: this is being researched to gain a better understanding of the mechanisms involved.
As we learn more through research, the importance of the mechanics of manufacture, application and compaction are identified, and new plants are currently being developed to improve the end performance of recycled asphalt with emulsion.
As our understanding increases, the quality of these materials will continue to improve.
Performance Programme: focusing on the future
Through its Performance Programme Nynas has realigned its products to demonstrate how it is meeting clients' need for delivery of value and functional requirements. The company has isolated trends that it believes are (or soon will be) affecting the way the bitumen industry does business. More of its customers are now seeking long term, cost effective solutions that are based on functional needs, requiring bitumen that can match varying demands of performance.