Structural and base materials · Concrete & Cement
Recycled & Secondary Aggregate
Explains an aggregate whose identity is its provenance, and why what else came through the crusher is the subject that governs how it is documented and used.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What recycled aggregate is
Recycled and secondary aggregate is graded mineral material that has had a previous life. Recycled aggregate comes from construction and demolition waste that has been crushed and screened — most cleanly from concrete alone, less cleanly from mixed demolition arisings containing brick, mortar, tile and whatever else was in the building, and separately from asphalt planings taken off a road surface. Secondary aggregate comes from industrial processes rather than from buildings: slags, ashes, china clay and slate waste, and similar by-products of other operations.
The library's other aggregate entry describes the granular material itself — grading, shape, cleanliness, absorption — and those subjects apply here too. What makes this a separate entry rather than a note is that its defining characteristic is not a physical property at all. It is defined by provenance. The governing question about a load is not only what size the stone is but where it came from, what came with it, and whether the next load will be the same.
That produces a distinct set of subjects. Crushed concrete carries old mortar and cement paste on the particle faces, which absorbs more water than a clean quarried stone and changes how a mix behaves. Mixed demolition arisings may carry gypsum plaster, which brings sulfates, and sulfates in the wrong place are a known concern for anything cementitious placed against them. Asphalt planings carry bitumen, which behaves quite differently again. Any of them may carry contaminants that reflect what the source building was, and screening removes what it removes rather than everything. Variability between loads is normal in a way that it is not for quarried material from a single face.
So the documentation is the material. Where a recycled or secondary aggregate is used in anything structural or in anything cementitious, what the supplier can declare about the source, the processing and the testing is what the designer works from — and that is exactly the same shape as the library's other reclaimed entries, where a material's history is the subject rather than an aside.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Recycled aggregate
- Secondary aggregate
- Recycled concrete aggregate
- RCA
- Crushed demolition arisings
- Recycled asphalt planings
- Recycled crusher run
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Unbound fill and capping material in groundworks and level-raising.
- Sub-base material beneath paving, drives and hardstandings where the specification permits it.
- Pipe surround, trench backfill and general fill in external works.
- Coarse aggregate in concrete mixes where a designer has accepted recycled content.
- Piling mats and temporary haul routes during construction.
- Drainage and soakaway fill where the material is clean and open enough for the purpose.
- Landscaping bases, gravel surfaces and informal paths on domestic sites.
Consideration
Where it is commonly considered
- Where material is being crushed on site from the demolition of something already there.
- Where the project team is looking at reducing the quantity of material taken off and brought onto the site.
- Where the application is unbound fill and the demands on the material are about compaction rather than chemistry.
- Where a supplier can declare the source and the processing of the material being delivered.
- Where the designer has specifically permitted recycled content in a mix or a layer.
- Where the quantity needed is large and the environmental profile of the material is part of the discussion.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where the source of the material is unknown or undeclared.
- Where gypsum plaster may be present in the arisings and sulfate-sensitive construction is nearby.
- Where the material would be used in a structural concrete mix without the designer's agreement.
- Where the ground or the application is sensitive to contamination and no testing has been carried out.
- Where consistency between loads matters and the supply cannot provide it.
- Where the material will be in contact with a damp-proof membrane or a similar layer that sharp arisings could damage.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Documentation
- This is the entry's central subject. What the supplier can declare about source, processing, screening and testing is the material's identity, and a load with no provenance is a load nobody can reason about.
- Moisture behaviour
- Old mortar and cement paste on crushed concrete particles absorb water in a way that clean quarried stone does not, which affects both compaction on site and the behaviour of any mix the material goes into.
- Substrate dependency
- Where the material forms a layer, what sits above and below it — membranes, blinding, bedding — has to suit the sharpness and the grading of what has actually been delivered.
- Installation dependency
- Compaction behaviour varies with the mix of materials in the load, and a crew used to a consistent quarried product may find the same effort produces a different result from one delivery to the next.
- Weathering and exposure
- Sulfates, chlorides and other chemistry brought in with the arisings are the subject here rather than weather, and their relevance depends on what the material is placed against.
- Appearance
- Where the material is visible — a gravel surface, an exposed aggregate finish — brick fragments, mortar and other inclusions read as part of the look, and the look changes between loads.
- Repairability
- Matching a later load to one already placed is generally not achievable, so areas placed at different times can differ visibly and behave slightly differently.
Exposure
Exposure and environmental conditions
- Where did this material come from, and what was the source structure?
- Is there any possibility of gypsum plaster in the arisings, and what is the material being placed against?
- Is groundwater present, and could anything leach from the material into it?
- Is the site or the neighbouring land sensitive to contamination?
- Will the material be exposed to view, or covered?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- Any membrane, geotextile or blinding layer above or below has to suit the sharpness of the material delivered.
- Where the layer supports paving or a slab, compaction requirements are set by the designer and depend on the material actually supplied.
- Where the material goes into a concrete mix, the supplier's mix design accounts for its absorption rather than treating it as quarried stone.
- Drainage and the movement of water through or around the layer depend on grading and fines content.
- Adjacent buried construction — foundations, floor slabs, pipes — determines whether chemistry in the material matters.
Appearance
Appearance and finish considerations
- The material is usually buried, so appearance matters only where it is left exposed.
- Colour is mixed and irregular, showing brick, mortar and concrete fragments together.
- Loads differ visibly, so an exposed surface placed over several deliveries can be patchy.
- Asphalt planings read as a dark, distinctly different surface from a crushed concrete one.
- Where an exposed finish is wanted, a sample from the actual supply is the only meaningful guide.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What testing has been carried out on this material, and is it available in writing?
- How consistent is the supply, and what happens if a later load differs?
- Has the designer accepted this material for this particular application?
- Is anything cementitious being placed against it, and has sulfate content been considered?
- Is there a risk of settlement if the material compacts differently from what was assumed?
Upkeep
Maintenance and repair considerations
- What record exists of what was placed, where, and from what source?
- If additional material is needed later, how would a reasonable match be found?
- What would signs of settlement in a layer of this material look like at the surface?
- Does the presence of this material affect anyone digging on the site in the future?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Delivery records and source declarations are collected as the material arrives, not afterwards.
- Compaction is checked against what the designer has set rather than by appearance.
- Material is kept separate from anything it should not be mixed with once it is on site.
- Where crushing happens on site, what goes into the crusher determines what comes out of it.
- Oversize, reinforcement fragments and foreign material are screened out before placing.
- Placing against membranes and pipes may need a blinding or a protective layer.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- A declaration from the supplier of the source of the material and how it was processed.
- Any testing results available for the material being supplied.
- Delivery tickets kept as a record of what arrived and when.
- The designer's statement of what is permitted in each layer or mix.
- Where the material is crushed on site, a record of what the source structure was.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is this material, and where exactly did it come from?
- Has it been tested, and what was tested for?
- Is it permitted in this layer or this mix, and who decided that?
- Could there be gypsum in it, and does anything nearby care about sulfates?
- How consistent is the supply, and what happens if a load looks different?
- Does using this material change the compaction or the mix design in any way?
- Is any protection needed between this material and a membrane or a pipe?
- What should be recorded about this material for the building file?
Blind spots
Commonly overlooked points
- The material is identified by its history, so a load without a declared source is not really identified at all.
- Screening removes what it removes; it is not proof that nothing unwanted came through.
- Crushed concrete, mixed demolition arisings and asphalt planings are three different materials, not three names for one.
- Absorption from old mortar on the particles changes how the material behaves in a mix and on site.
- Consistency between loads is not a given, which matters more in a bound application than an unbound one.
- Gypsum from plasterboard and plaster is the contaminant most often overlooked in mixed arisings.
What this page does not do
- This page does not state that any recycled or secondary aggregate suits any application; that depends on the source, the testing, the design and local requirements.
- No environmental claim is made here; environmental information belongs to the supplier of the specific material.
- Contamination assessment, waste classification and site-waste handling are regulated matters for qualified professionals and the relevant authority.
- Build Design Hub does not supply, test, approve or endorse aggregates or any supplier of them.
Related entries
Used in
Entries this is commonly incorporated into as a constituent or layer.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Concrete & Cement Materials
Reference entries for cement-based materials — ready-mixed and precast concrete, screeds, levelling compounds, renders of the cementitious family and the admixtures that modify them.
Browse all concrete & cement entries →