Definition and Scope
Asphalt recycling is a mechanical process for processing reclaimed asphalt pavement (RAP) through crushing and screening operations to produce aggregates suitable for reuse in new asphalt mixtures, road base courses, or unbound applications. The process recovers both mineral aggregate and residual bituminous binder from existing asphalt pavements, reducing the demand for virgin materials.
Processing Methods
Recycling typically begins with milling or excavation of worn asphalt layers. Primary crushing reduces material to nominal sizes of 150–200 mm, followed by secondary crushing to 50–75 mm. Screening separates the crushed material into fractions: coarse aggregate (>4 mm), fine aggregate (0.063–4 mm), and filler (<0.063 mm). Modern plants employ vibrating screens with multiple deck configurations and air classification to achieve precise grading curves. Separation efficiency impacts the quality of recycled binder recovery and the suitability of aggregates for different applications.
Composition and Quality Parameters
RAP composition varies based on the original pavement design and age. Typical material from high-traffic pavements contains 4–6% residual bitumen by mass. The Los Angeles abrasion value (LA abrasion) for recycled aggregates typically ranges from 15 to 35, comparable to virgin materials. The Polish value or Micro-Deval abrasion test provides additional wear resistance data. Water absorption in RAP aggregates generally falls between 0.5% and 2.0% by mass. Bulk specific gravity of RAP is typically 2.20–2.35 g/cm³.
Applications and Incorporation Rates
Recycled asphalt is incorporated into hot mix asphalt (HMA) at rates of 15–50% by mass, depending on specification requirements and pavement performance demands. Higher RAP percentages reduce production costs and virgin material consumption but require careful binder selection to avoid age-hardening and embrittlement. Some specifications limit RAP in surface layers to 20–30% due to appearance and performance concerns, while base and binder courses may contain 40–50%. In granular base courses, recycled asphalt without additional binder provides structural support at minimal cost.
Environmental and Economic Benefits
Recycling reduces landfill demand and conserves finite aggregate reserves. Energy consumption for milling and processing is approximately 5–15 kWh per tonne, significantly lower than virgin material extraction and transportation. For every tonne of RAP used, virgin aggregate and bitumen consumption decreases proportionally. Market pricing for processed RAP typically ranges from 40–70% of equivalent virgin material costs, depending on local availability and quality specifications.
Regulatory Considerations
EN 13108-8 specifies requirements for asphalt mixtures containing reclaimed materials. Material must be homogeneous and free from contaminants such as concrete debris, clay, and excessive moisture. Testing according to EN 13043 for RAP aggregates ensures compliance with grading and durability requirements. Some jurisdictions require radioactivity screening in accordance with national building material regulations.