Aggregate
The review focused primarily on the use of aggregate material in the base course, sub-base, and bitumen-bound chips on the road surface. The limited literature available on recycling road aggregates often concentrates on the stabilization of these aggregates. Most research highlights the recycling of materials from other sectors for use as base and sub-base aggregate, including steel slag, concrete waste, reclaimed asphalt pavement (RAP), building rubble, and rubber tyres.
With increasing restrictions on the establishment of new quarries and a reduction in the quality of available aggregates in some areas, future aggregate supplies are expected to be limited. This scarcity has led to the use of marginal aggregates, which generally contain a higher proportion of fines or are weaker and thus more prone to failure. Recycling high-quality aggregates can provide direct benefits where these materials are scarce. Additionally, recycling aggregates helps conserve natural resources, extend landfill life, reduce transportation costs, and support local authorities in achieving recycling goals and meeting public expectations.
Instead of excavating and disposing of base and sub-base aggregates in landfills and replacing them with virgin materials, stabilization using lime or cement powder is a common practice in New Zealand. Research into using fly ash, fibres, and geotextiles for base course stabilization has increased over the past five years, although their use in New Zealand remains limited.
Recyclable Materials
Stabilizing aggregates can help rectify soil property deficiencies, making them suitable for road construction. Recycled aggregates have numerous potential uses, including as base course and sub-base in low traffic-density areas such as shoulders, gravel on unsealed roads, and backfill for bridge abutments. However, marginal or contaminated aggregates with a high proportion of fines cannot be reused in road pavements without the risk of structural issues. These materials are typically disposed of in landfills.
Research in New Zealand, such as by Dodds et al. (1999) and Hudson (1996), has examined the use of marginal aggregates in roads, with lime and cement stabilization becoming common practice since the mid-1960s. In northern and eastern areas of the North Island, marginal quality aggregates have caused problems and are usually unsuitable for recycling within the roading environment, often ending up in landfills. In other regions, however, aggregates usually have recycling potential.
Recycling pavement base material typically requires equipment to mix the necessary amount of stabilizing agent into the aggregate. Where aggregates are contaminated with fines, suitable washing equipment is needed to meet industry standards before reuse.
In Australia, RAP has been used to save aggregate resources. Crushed asphalt, left unbound without additional treatment, is used in the base course and sub-base. However, using crushed RAP is considered uneconomic from a lifecycle cost perspective, as the energy costs associated with crushing bitumen outweigh the benefits. Despite this, using RAP as aggregate is preferable to landfill disposal.
In the United States, various state transportation departments have implemented legislative requirements that roads contain a proportion of recycled materials or that a certain percentage of waste materials be recycled. In Australia, 75% of the base and sub-base course material is recycled.
References:
Recycling of materials for more sustainable road construction – Transfund Research Report No. 219
