Aim of Asphalt Recycling

The general aim of asphalt recycling is to rehabilitate a worn asphalt surfaced pavement, by returning the mix to an "as new" binder quality and particle size distribution, in a cost-effective and environmentally sympathetic manner.

Asphalt recycling is both:

• a rehabilitation or maintenance treatment
• a means of conserving energy and road making material resources.

The selection of asphalt recycling as an appropriate rehabilitation treatment should involve a life cycle costing analysis.

Asphalt recycling can only correct the asphalt layer. The underlying pavement must be sound and likely to remain so for the life of the treatment. This is particularly the case with high percentage RAP or HIPAR mixes, where stiffening of the mix is likely to occur and lead to fatigue cracking in the recycled mix.

The recycling of asphalt has particular significance in urban situations where it is necessary to maintain fixed levels due to kerb, gutter and utility fixtures.

Asphalt recycling may be viewed as part of the wider issue of Ecologically Sustainable Development (ESD), which embodies the more effective use of (road-making material) resources.

ESD decisions in relation to roads are complex and require a longer term and wider view of events than is normally applied to individual projects.

In this context, clearly virtually all road materials can be recycled in one way or another. Asphalt may be recycled and reused:

• in the same pavement
• in another pavement
• for purposes other than road pavement maintenance and construction.

Asphalt recycling has the environmental benefits of:

• reducing the demand for scarce road making materials
• reducing the demand for landfill areas
• (for on-site recycling) reducing the traffic loading on the road network and reducing delays to motorists during construction work
• reducing the energy demands of the pavement rehabilitation process.

Evaluation of The Potential For Asphalt Recycling Work

To evaluate the potential to use asphalt recycling in any particular work, the following parameters need to be assessed:

• pavement site investigation
• materials characterization
• design of recycled materials
• pavement structural design
• material specification
• manufacture, placement and curing.

Criteria for each of these parameters are discussed for each of the following asphalt recycling treatments:

• use of reclaimed asphalt pavement (RAP) in plant mix hot asphalt
• hot-in-place Asphalt Recycling (HIPAR)
• cold asphalt recycling.

Use of Asphalt Recycling

At the end of its service life in many cases it is possible to recycle asphalt layers or use the materials as part of another asphalt treatment. The useful properties of bitumen may be restored with the appropriate treatment. This involves the rejuvenation (chemical refreshing) of the bituminous binder.

Generally, it is more difficult to achieve the same quality control over the recycling process compared to new asphalt production and laying. This is because there is less control over the source material that is being recycled, including localized repairs and wearing surfaces of different materials and ages.

However, with adequate quality control measures, a high quality recycled asphalt can be produced.

The selection of appropriate methods (and technologies) of recycling is dependent on specific site conditions and requirements.

Asphalt recycling is one of the many rehabilitation treatments available to road engineers. In most cases, it may be used when a conventional remove, replace or overlay solution is selected. Recycling can be particularly advantageous when there is a need to:

• restore riding quality of the surface, where there is no need for structural improvement.
• remove ruts from asphalt
• restore surface characteristics (for example, surface texture and skid resistance)
• change the properties of an asphalt (for example, convert dense graded asphalt to open graded asphalt where grading and circumstances permit)
• rejuvenate the oxidized binders in asphalt
• rehabilitate asphalt layers that fail due to:
  • faulty mix design
  • incorrect paving techniques
• add a thin layer of asphalt to an existing pavement
• repair nonstructural damage (for example, after utility patching)
• provide some improvement to structural capacity at minimal costs.

Economics of Recycling

An essential part of the selection of recycling as a rehabilitation option, and of a specific recycling method, is an assessment of the cost of the various options. Selection should be on a life cycle costing basis.

Significant saving may be achieved by recycling asphalt pavements. The savings depend on the selected recycling technology and the quality of the RAP. The major areas of savings are set out in the following table.

Some of the savings are indirect and not easy to measure. These may include:

• the longer life of adjacent road network due to lack of construction traffic
• the single pass operation of in-place recycling reduces construction time and delays to traffic
• existing surface levels can be maintained
• a hot bond is obtained between layers at a joint for HIPAR
• correction of mix deficiencies and restoration of functional characteristics of pavement.

Justification for plant recycling is usually related to the high cost of waste disposal at landfills. In-place recycling offers real savings in both construction and life cycle costs.