The highways industry has embraced the many advantages of reclaiming and reusing asphalt – local authorities have been leading on this for many years. Stephen Child, chair of ADEPT’s Soils and Materials Design and Specification Group, discusses the development of reclaimed asphalt pavement (RAP) and why it is such a good idea.
Asphalt is almost unique in the construction industry in that it can be 100% recycled and this process does not downgrade its functionality, making asphalt the most recycled construction material in the world. It can be reused, often directly in situ, or recycled as part of the ingredients for new asphalt.
Even when reprocessed from its former use in pavements, recycled asphalt still has many uses in the construction industry such as aggregates for railway ballast, pathways or as fill materials for civil engineering works.
This flexibility makes reclaimed asphalt an extremely valuable material with a huge potential to reduce the costs and environmental impacts of construction works in comparison with projects using virgin materials. In many countries, RAP is by far the most recycled construction waste product, with some European nations even exporting to the UK.
Reusing asphalt as an aggregate has been around for a while. As far back as 1997, the County Surveyors’ Society (CSS), which later became the Association of Directors of Environment, Economy, Planning and Transport (ADEPT), devoted their annual conference to sustainable road maintenance. With the title ‘Sustainable Road Maintenance – Reduce, Reduce, Recycle?’ the Society examined policy, process and best practice guidance for asphalt and other reusable materials.
Even earlier, in 1994, the CSS published guidance on the use of recycling resources for road pavements, while in 1996, the organisation partnered with construction engineering company Colas and the Highways Agency for the Linear Quarry project.
Comprising both research and a trial full-scale reprocessing site, the Linear Quarry project took three years to complete and resulted in a structural design guide for in situ road recycling. The linear quarry principle, which views roads and pavements as a resource that can be reclaimed, is now firmly embedded in the construction industry.
From 1999 onwards, local authorities produced design guides and implemented maintenance policies and strategies that included recycling asphalt. The CSS continued its research, producing guidance right up to 2009, when it became ADEPT. This work has carried on through ADEPT’s Soils and Materials Design and Specification Group.
The Group provides advice on policy and innovation, best practice guidance and research, often working with the Road Surface Treatments Association (RSTA) and the Mineral Products Association (MPA) amongst other key groups. Members (including Highways England) represent ADEPT on BSI and European committees.
ADEPT has produced very clear guidance on best practice for local authorities. This includes recommendations for sampling and testing strategies to determine the make up of reclaimed asphalt to enable as much reuse, rather than reprocessing, as possible.
The production of new asphalt is the highest value application of RAP with significant economic and environmental benefits. Using reclaimed asphalt over virgin aggregates reduces the amount of raw materials needed from quarrying, easing pressure on what is a finite supply.
For areas such as the South East that have no local quarries, new aggregates have to be brought in by train, road or boat. Therefore, in situ recycling in particular decreases transportation costs and the impact of heavy haulage on roads and air quality.
This situation may not be the same in areas such as Cumbria, with an abundance of quarries that support local economies. Costs, transportation and environment impacts are very different in these areas, which may lead to a difference in approach where there is little financial advantage to reprocessing.
The use of RAP material in the production of new hot mix asphalt has now become standard practice, with many suppliers preferring to use it where possible as it reduces the waste product at the plant. Although dependent on the criteria of each individual project, local authorities are proactive in using reclaimed asphalt where it is suitable for in situ recycling. In other cases, the use of RAP will depend on the allowable RAP content and the source of new asphalt.
In 2008 the UK Government adopted the BS EN 13108-8 specification, which permitted up to 30% RAP content in hot mix binder courses and bases and 10% in surface courses. It allows for a far higher RAP mix (usually up to 60%) with permission.
The specification includes guidance on performance level and suitability. This includes size, shape, type and levels to which any contaminants such as soils, ceramics and plastics need to be separated out of the feedstock. Contaminants can be an issue, but often such material can be reprocessed in a different way. Tar, for example, is a major stimulant to cold mix recycling as it can be encapsulated and made viable for use on site.
Obviously performance is everything, but depending on the contract requirements, it is not uncommon to see suppliers incorporating up to 60% RAP. This does need careful consideration during the mix design process and requires permission from the local authority to implement, but it is an indication of what can be achieved in the right circumstances.
Unfortunately, the local authority skills shortage affects this sector as much as any other and in some cases a lack of technical knowledge leads to concerns that performance is not comparable with an ‘all new’ asphalt product. For this reason, contractors can self-censor and will offer only what they think a client will accept.
The supply chain has fully embraced the reclamation of asphalt as an integral part of the construction industry. Larger suppliers have developed plants that can process asphalt using different techniques to provide high levels of quality control.
Companies have introduced specialist machinery to reuse asphalt in situ wherever practicable, saving on time and cost.
With pressure on local authorities to be both cost effective and reduce impacts on the environment, there is a strong case for using RAP as standard. With so many benefits to the private and public sectors, RAP has an ensured future.