Structural and base materials · Aggregates & Groundworks
Concrete and Mortar Aggregate
This entry is about the stone and sand themselves rather than about what they are mixed into: what makes one aggregate different from another, and why those differences reach all the way to the finished surface.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What concrete aggregate is
Aggregate is the crushed rock, gravel and sand that forms the greater part of a concrete or mortar mix by volume. The binder is what holds it together; the aggregate is what is being held. The library already publishes the binder as an entry in its own right, and this is the other constituent — the one that most of the material actually consists of, and the one whose variation explains a great deal of the variation in the finished work.
It is described in two fractions. The coarse fraction is the stone, and the fine fraction is the sand. A mix needs both, and needs them distributed across a range of sizes rather than concentrated at one, because the smaller particles occupy the spaces between the larger ones and the paste then only has to fill what is left. That distribution is what the trade means by grading, and it is the first thing that distinguishes one aggregate from another.
Four further characteristics do most of the remaining work. Shape and surface texture — whether the particles are rounded river gravel or angular crushed rock — change how the wet mix moves and how the hardened material grips the stone. Cleanliness matters because clay, silt and organic matter coat the particles and interfere with the bond between paste and stone. Absorption matters because a porous, dry aggregate takes water out of the mix and a saturated one adds it back. And the source rock itself decides colour, hardness and how the aggregate behaves when it is exposed at a surface.
That last point is the reason this entry reaches beyond structural work. Any finish that exposes the aggregate — a ground and polished floor, an exposed-aggregate paving slab, a terrazzo, a granolithic topping — is showing the stone, and the stone was chosen when the mix was designed. The colour, size and distribution seen in the finished surface were decided at the batching plant, not by the grinding. Recycled and secondary aggregate, produced from crushed demolition material or from industrial by-products, is a distinct supply question with its own variability and its own documentation, and it is established rather than assumed.
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.
- Aggregate
- Construction aggregate
- Coarse and fine aggregate
- Sand and gravel
- Ballast (UK trade usage for mixed coarse and fine aggregate)
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- The body of concrete of every kind, from foundations and slabs to precast units and structural frames.
- Mortars and renders, where the fine fraction governs how the material works under the trowel and how it looks when set.
- Screeds and floor toppings, including those that will be ground back to show the stone.
- Terrazzo, granolithic and exposed-aggregate finishes, where the aggregate is chosen for appearance as much as for the mix.
- Cast stone and precast units, where a particular sand or stone is used to obtain a particular colour and texture.
- Bedding and jointing materials, where the fine fraction and its cleanliness decide how the material compacts and sets.
Consideration
Where it is commonly considered
- Wherever a cement-bound material is being made, since the aggregate is most of what the material is.
- Where a finish will expose the stone and its colour, size and distribution have to be chosen deliberately.
- Where a mix is being matched to existing work and the sand or the stone in the original is part of the match.
- Where recycled or secondary material is being considered and its source and consistency are being established.
- Where a lightweight or a particularly dense material is wanted and the aggregate is what changes.
- Where local supply determines what is realistically available and the mix is being designed around it.
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 a structural mix is involved and the aggregate is being chosen on site rather than within a designed mix.
- Where the source of the material is unknown and no documentation accompanies it.
- Where recycled or secondary material is proposed and its composition and consistency have not been established.
- Where an exposed-aggregate finish is intended and nobody has agreed which stone will appear in it.
- Where the aggregate would be used with a binder it has not been used with before and no one has raised it.
- Where a batch is visibly variable, dirty or contaminated and it is being used anyway.
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
- Aggregate is supplied against a description of its source, its fractions and its grading, and that description is the only way of knowing what has arrived. For recycled and secondary material it is also the record of what the material was before.
- Appearance
- Wherever a finish exposes the stone, the aggregate is the finish. Colour, size, distribution and the proportion of any decorative stone are decided when the mix is designed, and nothing done afterwards adds a stone that was never there.
- Moisture behaviour
- Aggregate carries water and takes water up. A dry, absorbent stone draws water out of a mix and a saturated one carries water into it, which is why the moisture state of a stockpile is a live concern rather than a technicality.
- Substrate dependency
- Cleanliness is a bond question. Clay, silt, dust and organic matter coating the particles sit between the paste and the stone, and that interface is where a poorly cleaned aggregate shows itself.
- Surface wear
- In any surface that is walked or driven on and that exposes the stone, it is the hardness of the aggregate rather than the paste around it that meets the wear.
- Thermal behaviour
- The stone is most of the material's body, so the density and behaviour of the aggregate are what make a lightweight mix light and a dense mix heavy. Those are properties of the material as designed rather than claims about any product.
- Installation dependency
- Grading and particle shape change how a wet mix moves, how it can be placed and how readily it segregates, which is why an aggregate change is a mix change rather than a substitution.
- Weathering and exposure
- Where the stone is exposed at an external surface, it is the aggregate that meets frost, salt and rain. How a given source rock behaves in that position is established from its documentation rather than from its appearance.
Exposure
Exposure and environmental conditions
- Will the aggregate be exposed at the finished surface, indoors or outdoors?
- Is the finished material going into a position that is wetted, frozen or salted?
- Is the stone going to be walked on or driven over once it is exposed?
- Is the material being made on site or delivered ready-mixed, and does that change what aggregate is available?
- Is there a local source whose colour and character are part of what the project wants?
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.
- Aggregate is a constituent of a designed mix, so it is never changed in isolation from the binder, the water and any admixtures.
- Storage on site matters: stockpiles segregate, take up water and pick up contamination from the ground beneath them.
- Where a finish will expose the stone, the aggregate decision is made at the same time as the finish decision, not afterwards.
- Matching existing work means matching the aggregate as well as the binder, and sometimes the aggregate more than the binder.
- Recycled and secondary material carries its own supply and consistency questions, which belong in the mix design rather than being handled on delivery.
- The fine fraction in a mortar or render interacts with the binder and with the background being worked onto.
Appearance
Appearance and finish considerations
- Exposed-aggregate and polished finishes show the stone, so the sample that matters is a sample of the finished surface rather than of the mix.
- Sand colour carries straight through into mortar and render, and two sands from different sources will not match.
- Stone size and distribution decide how busy or how calm a ground surface reads once it is opened up.
- Batch-to-batch variation in a natural material is normal, and continuity of source matters where appearance does.
- Decorative aggregates change appearance markedly when wet, which is worth seeing before the mix is settled.
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 is the source of this aggregate, and what documentation comes with it?
- How clean is it, and what has been done to establish that?
- Is it being exposed at a surface, and what will that surface meet?
- Is the material recycled or secondary, and what is known about what it was before?
- How is it being stored on site before use?
Upkeep
Maintenance and repair considerations
- If a patch repair is needed later, can the same aggregate still be obtained?
- Has the source and the description of the aggregate been recorded anywhere?
- For an exposed finish, how closely would a repair be expected to match, and has anyone said so?
- Where a decorative aggregate was used, was a sample kept?
- Who would be asked to match an existing mix, and what would they need to know?
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.
- The moisture state of the aggregate at the time of batching affects the mix, so stockpile condition is part of the work.
- Stockpiles segregate as they are built and drawn from, and how material is handled affects what actually goes into the mixer.
- Contamination from the ground beneath a stockpile is a common and avoidable source of variability.
- Where a finish exposes the stone, placing and finishing methods interact with the aggregate chosen.
- Changing supplier mid-project changes the aggregate, and that is a mix decision rather than a purchasing one.
- Sampling and agreeing the aggregate before the mix is fixed is what makes an exposed finish predictable.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- A description of the aggregate supplied, including its source and its fractions.
- For recycled or secondary material, the record of its origin and its composition.
- Whatever the supplier states about grading, cleanliness and absorption for the material offered.
- For exposed finishes, a made-up sample panel of the finished surface rather than a bag of stone.
- The mix design in which the aggregate is being used, held by whoever designed it.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What aggregate is in this mix, where does it come from and what documentation covers it?
- Is the material recycled or secondary, and what is known about its consistency?
- Will the stone be exposed anywhere in the finished work, and has that been agreed?
- How is the aggregate being stored and handled on site before it is used?
- What happens to this mix if the aggregate source changes partway through?
- How does the grading and shape of this aggregate affect placing and finishing?
- Is the sand in the mortar or render matched to anything existing, and by whom?
- Has a sample of the finished surface been made and agreed before the mix was fixed?
Blind spots
Commonly overlooked points
- Most of a concrete or mortar is aggregate, so most of what varies between two batches is the stone and the sand.
- An exposed-aggregate or polished finish is decided at the mix, not at the grinder — the stone that appears is the stone that was put in.
- Cleanliness is not cosmetic: material coating the particles sits between the paste and the stone.
- The moisture state of a stockpile changes the mix, which is why aggregate delivered wet is not the same as aggregate delivered dry.
- Sand colour carries straight into mortar and render, and a change of sand shows in the wall long after anyone remembers making it.
- Recycled and secondary material is a distinct supply question, and what it was previously is something to establish rather than assume.
What this page does not do
- This page states no grading, no particle size, no proportion and no mix design: those are determinations for the designer of the mix and the parties responsible for the work.
- Aggregate is one constituent of a designed material, and nothing here should be read as describing how to make or alter a mix.
- No aggregate is described here as suitable for any structural or exposed use; that depends on the mix, the position and qualified professional review.
- Local requirements and the documentation for the specific material govern what may be used in a given place, and they are confirmed rather than assumed.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
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.
Aggregates & Groundworks Materials
Reference entries for the unbound and geosynthetic materials that make up the ground beneath a building — crushed rock, fills, bedding, drainage media, geotextiles and the cellular products laid with them.
Browse all aggregates & groundworks entries →