Unearthing the types of recycled aggregates in construction

Recycled aggregates are construction materials made by crushing and screening construction and demolition waste such as concrete, brick and asphalt. The main types are recycled concrete aggregate (RCA), recycled masonry aggregate and recycled mixed aggregate (RMA). They are used in road sub-bases, fill, drainage and some concrete mixes, and they reduce landfill and quarrying.

The construction industry is one of the largest users of raw materials in the UK. Traditional construction methods rely heavily on virgin aggregates, such as crushed rock, gravel and sand, quarried from natural deposits. A more sustainable option is gaining ground, with environmentally conscious sites choosing recycled aggregates sourced through a specialist aggregates supply partner.

In this guide we explain what recycled aggregates are, the main types available, how they are used (including in recycled aggregate concrete), and what to check so the materials deliver the performance your project needs.

What are recycled aggregates?

Recycled aggregates are granular materials produced from construction and demolition waste that has been processed into usable products for new sites and projects. Processing usually involves sorting, crushing, screening into size fractions and removing contaminants such as timber, plastics, metal and plasterboard waste.

Demand for recycled aggregates continues to increase as the industry looks for lower-carbon materials and ways to cut construction waste. According to the Mineral Products Association (MPA), recycled and secondary aggregates in Great Britain reached a record 73.5 million tonnes in 2022.

Recycled aggregates sit within the wider family of construction aggregates, and in many specifications they can be used alongside or in place of virgin materials, provided they meet the same requirements.

Where do recycled aggregates come from?

Most recycled aggregates come from construction and demolition waste. The main waste streams are:

• Concrete waste: slabs, foundations, kerbs and precast units, which become recycled concrete aggregate.
• Brick and masonry waste: walls, blocks and tiles, which become recycled masonry aggregate.
• Asphalt waste: road planings from resurfacing, which become reclaimed asphalt.
• Mixed demolition waste: mixed rubble and hardcore, which becomes recycled mixed aggregate.

Precast concrete factories and ready-mixed concrete plants are another source: returned and rejected concrete is a clean, single-source waste that makes high-quality RCA.

Excavated soils are a separate construction waste stream. They are usually removed through muck away services and may need WAC testing before disposal or reuse.

Because concrete, brick and asphalt are largely inert waste, they can be crushed into graded fractions instead of being sent to landfill.

Good segregation on a construction site makes a big difference to quality and can increase the value of the recycled product. Keeping concrete and hardcore separate from mixed waste in dedicated commercial skips produces cleaner feedstock and a more consistent recycled product. A site waste management plan is the best place to set out how each waste stream will be separated and where it will go.

By crushing these materials, you give them a second life, keep waste out of landfill and contribute to a more circular economy in construction.

Types of recycled aggregates

Several types of recycled aggregates can replace virgin materials in construction. The main categories are recycled concrete aggregate (RCA), recycled masonry aggregate and recycled mixed aggregate (RMA). Other materials, such as reclaimed asphalt, glass or certain slags, may also be processed for specific uses, subject to quality and project requirements.

Recycled concrete aggregate (RCA)

Recycled concrete aggregate (RCA) is made from crushed concrete recovered from demolished buildings, slabs, kerbs and pavements. It is one of the most adaptable recycled materials and is widely used in sub-base, fill and drainage layers.

To make RCA, concrete waste is broken out, reinforcing steel is removed with magnets, and the concrete is crushed in jaw or impact crushers before being screened into graded sizes. Clean, single-source concrete such as slabs, precast concrete units or returned concrete produces higher-grade RCA with more consistent properties than concrete mixed with other demolition waste.

RCA particles are made up of the original stone with some residual cement paste and mortar still attached. This adhered cement mortar gives RCA slightly different properties from virgin aggregate, which matter most when RCA is used in new concrete (see recycled aggregate concrete below). The quality of the original concrete has a direct effect on the properties and performance of the RCA made from it.

Recycled masonry aggregate

Recycled masonry aggregate is produced mainly from crushed brick, block, tile and similar materials. Its properties, particularly strength and water absorption, vary more than those of RCA, and higher absorption can increase the risk of frost damage in exposed layers, so it is generally better suited to fill, capping layers and landscaping than to load-bearing layers or concrete.

Recycled mixed aggregate (RMA)

Recycled mixed aggregate (RMA) combines materials from different construction and demolition sources, such as concrete, brick, asphalt and other masonry. Unlike RCA, which is made primarily from crushed concrete, or recycled masonry aggregate, which is mainly crushed brick, RMA has a more varied composition. Its properties can therefore vary between batches, affecting strength, performance and suitability. RMA is commonly used in non-structural applications, such as fill and sub-base, when it meets the project specification.

Reclaimed asphalt and other recycled materials

Reclaimed asphalt pavement (RAP) comes from planing and resurfacing existing roads. It can be processed and returned to new asphalt mixes, where the properties of the aged bitumen it contains need to be taken into account in the mix design. Some glass and industrial slags can also be processed into aggregate for specific applications, provided testing shows their properties are suitable.

Uses of recycled aggregates in construction

Recycled aggregates offer a sustainable and cost-effective alternative for several construction applications throughout a project. Here are the key uses.

Road construction and pavements

RCA is commonly used in road base and sub-base layers, where its performance must meet the project specification and expected traffic loads. Reclaimed asphalt can be incorporated into new asphalt mixes for surfacing and other pavement applications, and depending on its quality and the design, may also suit specified base or sub-base uses. Not every asphalt-derived material is appropriate for every layer. In the UK, aggregates and asphalt mixes should comply with the project requirements and applicable standards, including BS EN 13043 for aggregates for bituminous mixtures and BS EN 13108 for bituminous mixtures.

Two common specifications for unbound layers are:

• Type 1: a graded sub-base material defined in the Specification for Highway Works and used wherever the project specification calls for it. It may include recycled materials, including crushed concrete, if they meet the required grading, properties and specification. Reconomy supplies MOT Type 1 for site works.
• 6F2, 6F5 and 6F4: designations for selected granular fill classes used in specified construction applications. They describe material requirements, not the material’s source, so recycled content and grading can vary. See our 6F2 supply page for more detail.

RCA can also be used in cement-bound granular mixtures (CBGM) and other hydraulically bound layers, where a small amount of cement binds the aggregate into a stronger base.

Always check the project specification to confirm which recycled materials and gradings are permitted for each use.

Concrete production

Crushed concrete can be used as a partial replacement for virgin coarse aggregate in new concrete mixes, turning concrete waste back into concrete. Cement remains the main source of embodied carbon in any concrete mix, so recycled aggregate works best alongside lower-carbon cement choices. This is most common in non-structural and lower-strength applications such as pavements, blinding and some foundations, and it is covered in more detail in the next section.

Backfill and site development

RCA is well suited to backfilling trenches, utility runs and around structures. Once compacted, it provides a stable working platform for later construction activities. Using RCA as backfill on the same site where the concrete waste was produced avoids waste haulage altogether.

Recycled aggregate concrete: properties, mix design and performance

Recycled aggregate concrete is concrete in which some of the virgin coarse (and occasionally fine) aggregate is replaced with recycled aggregate, usually RCA. Recycled aggregate concrete can perform well, but the mix needs to account for how RCA differs from natural stone.

Key properties of RCA in concrete

• Water absorption: the adhered cement mortar on RCA absorbs more water than natural aggregate, which can increase the water demand of the concrete and reduce workability if the mix is not adjusted.
• Density: RCA is usually lighter than virgin aggregate for the same reason.
• Sulfate and chloride content: old plaster, gypsum and de-icing salts can raise sulfate and chloride levels, which can react with cement or affect steel reinforcement, so these properties should be tested before RCA goes into structural concrete.
• Consistency: the properties of RCA depend on the original concrete it came from, so batch-to-batch testing of these properties is important.
• Contaminants: materials such as gypsum from plasterboard, wood or glass can affect concrete performance, which is why plasterboard waste should always be kept separate from concrete waste on site.

Mix design and cement content

To use RCA in a concrete mix, designers typically adjust the water content or pre-wet the aggregate to account for higher absorption, and may use admixtures to maintain workability without having to increase the water content. Some mixes need slightly more cement to reach the same strength class. Because cement is the most carbon-intensive ingredient in concrete, any increase in cement content can offset part of the carbon saving, so the replacement level should be chosen with both strength and carbon in mind. Cement replacements such as ground granulated blast-furnace slag (GGBS) can help balance the two.

In the UK, BS 8500-2 sets out requirements for using RCA and other recycled aggregates in concrete, including where they can be used and in what proportions. BS EN 12620 covers aggregates for concrete. Check the current editions and the project specification before finalising a mix.

Trial mixes and testing

Before using a recycled aggregate concrete mix on a project, run trial mixes with the actual RCA batch. Test workability, compressive strength and, where relevant, durability properties, then use the results to fine-tune water, cement and admixture content. This is the most reliable way to confirm the mix will meet the specified performance.

Performance in use

At moderate replacement levels of coarse RCA, recycled aggregate concrete can deliver strength and durability comparable to conventional concrete. Higher replacement levels, and the use of fine recycled aggregate, can reduce strength and increase shrinkage and creep, so they are generally limited to non-structural uses unless testing shows otherwise. Recycled aggregate concrete is therefore most widely used today in lower-strength concrete, including:

• concrete paving, kerbs and edgings
• blinding concrete beneath slabs and foundations
• mass concrete foundations and fill
• precast concrete blocks and landscaping units

Testing the mechanical and durability properties of each recycled aggregate concrete mix is what allows the replacement level to increase safely over time. Asking your supplier for test data on the specific RCA batch is the most reliable way to predict performance.

Benefits of recycled aggregates in construction

Using recycled aggregates brings clear environmental and commercial advantages for sustainability-minded builders.

Sustainability benefits

Minimise waste sent to landfill. Construction and demolition waste is one of the UK’s largest waste streams, and concrete waste makes up a large share of it. Recycling that waste into aggregates helps the construction sector increase recovery rates and reduce the amount of waste sent to landfill. Recycled aggregates divert this material from landfill and support a more circular waste management approach. For more ideas, read our guide on how to reduce construction waste.

Conserve natural resources. Quarrying virgin aggregates such as limestone disturbs landscapes and wildlife habitats. Using recycled materials reduces the need for new extraction, keeps valuable materials in use and helps preserve natural landscapes.

Reduce carbon footprint. Producing virgin aggregates takes significant energy for extraction, processing and haulage. Recycling existing materials, often close to where they are needed, reduces the environmental impact of both production and transport. Using RCA in place of virgin aggregate in concrete and sub-base also avoids the emissions from quarrying new materials. Specifying recycled aggregates can also increase a project’s recycled content, which supports sustainability ratings such as BREEAM.

Economic benefits

Reduced material costs. Recycled aggregates are often cheaper than virgin aggregates because there is no extraction cost and less processing. As the cost of disposing of concrete waste continues to increase, recycling it makes more financial sense. Some sites still question the quality of recycled alternatives, but with the right specification and testing they can deliver significant savings.

Reduced transport costs. Recycled aggregates are typically processed locally, sometimes on the same site, which cuts haulage distances, fuel use and carbon emissions as well as cost.

Lower disposal costs. Recycling concrete and hardcore avoids landfill tax and gate fees that would otherwise apply to this waste (and landfill tax rates increase every year), and on-site crushing can turn a waste cost into recycled hardcore you can use straight away.

How to specify and quality-check recycled aggregates

Specification checklist

Follow these steps to get the right recycled material for each part of a construction project:

1. Define the use. Sub-base, fill, drainage, asphalt and concrete all have different requirements, so start with where the aggregate will go.
2. Choose the type. RCA suits most sub-base and concrete uses; recycled masonry aggregate and RMA suit fill and lower-strength layers.
3. Match the standard. Confirm which product standard and specification clause apply (see the standards listed below).
4. Check the key properties. Ask for test data on grading, density, water absorption, strength, sulfate and chloride content, and contaminants. Higher water absorption can increase the water and cement demand of a concrete mix, so these properties matter most for concrete.
5. Plan waste segregation. Keep concrete, brick and asphalt waste separate on site so you can increase the quality and quantity of material that can be recycled.
6. Record the data. Track tonnages and destinations for every waste stream, including how much concrete waste was recycled, to support your site waste management plan and reporting.

Check material quality, strength and durability

Studies have shown that well-processed recycled aggregates can perform comparably to virgin aggregates in many applications. A good example is a road maintenance project in Oxfordshire in 2023 that used a low-carbon recycling technique, recycling the top layer of the old road surface before a new surface was added.

When choosing recycled aggregates, match the material to its intended use and check that its composition, grading and physical and chemical properties meet the project specification. Ask the supplier for test evidence covering the relevant performance requirements, including strength and durability, and confirm the material is free from contaminants that could affect its use. A supplier’s reputation is useful, but it is not a substitute for documented evidence that the materials are suitable and compliant.

The relevant UK product standard depends on how you plan to use the aggregates:

• BS EN 13242: aggregates for unbound and hydraulically bound materials in civil engineering work and road construction
• BS EN 12620: aggregates for concrete
• BS EN 13043: aggregates for bituminous mixtures and surface treatments

Check the current editions, project specification and any national requirements before selecting materials, as meeting a product standard alone does not confirm suitability for every use.

Data challenges

The MPA’s report on recycled and secondary aggregates supply in Great Britain found that 60.3 million tonnes of recycled aggregates came from construction and demolition waste, one of the highest figures in Europe. The report also highlights gaps in industry data.

The UK’s move to mandatory digital waste tracking should improve the availability and quality of construction and demolition waste data, helping the industry find opportunities to increase an already strong recycling rate. Specialist construction waste management services can help you capture this data now.