Quick answer: Coarse aggregate is the gravel or crushed stone in concrete. It makes up most of the volume and gives concrete its strength and durability. Good coarse aggregate is hard, clean, low in water absorption and roughly cubical in shape, with a specific gravity between 2.6 and 2.9. Flaky and elongated pieces weaken concrete and should be kept out.
Concrete is mostly stone. Cement is the costly ingredient and gets most of the attention, but the coarse aggregate fills the bulk of the mix, and a poor load of it can spoil an otherwise correct concrete. This guide explains what coarse aggregate is, its types, the qualities it must have and how to check a delivery on site.
What is coarse aggregate?
Coarse aggregate is gravel, crushed stone or a similar hard material used as the larger particles in concrete. Fine aggregate, which is sand, fills the gaps between the stones, and cement paste binds everything together.
The size, shape and nature of the aggregate decide two things: the properties of the hardened concrete, and how easily the fresh concrete can be placed and compacted.
What does coarse aggregate do in concrete?
- Carries load: the stones form the skeleton of the concrete and carry most of the compression.
- Gives volume: stone is far cheaper than cement, so it makes concrete economical.
- Controls shrinkage: cement paste shrinks as it dries. Stone does not, so a well-graded aggregate reduces shrinkage and cracking.
- Adds durability: hard, dense stone resists wear and weather.
Types of coarse aggregate
By source
- Crushed stone: rock broken in a crusher. The pieces are angular and rough, and bond well with cement. This is the standard aggregate for structural concrete.
- Gravel: naturally rounded stone from river beds and pits. It is smooth, so concrete made with it flows easily, but the bond is weaker.
- Recycled aggregate: crushed concrete or brick from demolition, used mainly in non-structural work.
By shape
Shape matters as much as source. The good shapes are rounded and angular (roughly cubical). The poor ones are these:
- Flaky aggregates: thin, flat pieces. An aggregate is flaky when its thickness is less than 60% of its mean dimension. Flaky pieces are generally not acceptable for concrete because they reduce both strength and workability.
- Elongated aggregates: pieces that are much longer than they are wide or thick. Like flaky pieces, they harm the strength and performance of concrete.
- Flaky and elongated aggregates: pieces that are both thin and long. These are the worst to work with. They badly impair workability and can compromise the structure.
Flat and long pieces tend to lie flat in the mix, trap water under them and snap under load. A small proportion is unavoidable in any crushed stone, but a load with a lot of them should be rejected.
What qualities should coarse aggregate have?
Correct grading and selection of coarse aggregate is essential for good concrete. The key requirements:
- Hardness and strength: the stone must be hard and strong so that the concrete is strong and lasts.
- Cleanliness: it must be free of clay, dust and organic matter, which weaken the bond between stone and cement.
- Low water absorption: stone that soaks up a lot of water takes it from the mix and harms the concrete.
- Durability: it must stand up to chemicals, weather and other outside forces over the life of the structure.
- Specific gravity: it should lie between 2.6 and 2.9. Denser stone gives denser, stronger concrete.
- No harmful substances: chlorides, sulphates and alkalis should not be present, because they damage concrete and reduce its durability.
- Good grading: a mix of sizes packs tightly and leaves fewer voids for cement paste to fill.
How is coarse aggregate tested?
For any sizeable project the aggregate is tested in a laboratory before the source is approved. The usual tests are:
- Sieve analysis: checks the grading, that is, the spread of particle sizes.
- Flakiness and elongation index: measures the share of flat and long pieces.
- Impact and crushing value: measure toughness and strength.
- Abrasion test: measures resistance to wear.
- Specific gravity and water absorption: show how dense and how porous the stone is.
The acceptance limits are laid down in the Indian Standards and in the project specification. Ask the supplier for a recent test report from the quarry.
How to check coarse aggregate on site
A homeowner cannot run laboratory tests, but a load can be judged quickly:
- Look at the shape: most pieces should be chunky and angular, not flat chips or long splinters.
- Check for dust: rub a handful. A heavy coat of powder or clay on your palm means the stone needs washing.
- Check the size: the stones should be of the size ordered and fairly uniform, not mixed with oversize lumps or fines.
- Look for soft or weathered pieces: stone that crumbles or breaks by hand has no place in concrete.
- Stack it properly: unload on a hard, clean platform, not on bare soil, and keep sizes in separate heaps.
- Keep it clean: do not let soil, leaves or site waste mix into the heap.
Common mistakes
- Buying on price alone from an untested source.
- Using dusty stone without washing it.
- Stacking aggregate on soil, so the bottom layer goes into the mixer with mud.
- Using whatever size is available instead of the size the structural drawing specifies.
- Ignoring a large share of flaky pieces because the stone looks hard.
Related guides
The other half of the aggregate in concrete is sand, covered in our guides to fine aggregates and coarse sand vs fine sand. To see how stone, sand and cement are proportioned, read about the M25 concrete ratio, and for checking the finished mix, the concrete testing methods used in civil engineering. Poor aggregate and poor compaction also show up as honeycombing in concrete. Material selection and testing are part of our construction service.
Frequently asked questions
What is coarse aggregate?
Coarse aggregate is the larger stone in a concrete mix: gravel, crushed stone or similar hard material. Together with sand, cement and water it forms concrete, and its size, shape and quality decide how strong and how workable the concrete is.
What is flaky aggregate?
A flaky aggregate is a thin, flat piece of stone. It is classed as flaky when its thickness is less than 60% of its mean dimension. Flaky pieces break easily and make concrete harder to place, so they are not acceptable in good concrete.
What should the specific gravity of coarse aggregate be?
The specific gravity of coarse aggregate should lie between 2.6 and 2.9. Stone within this range is dense and sound. Much lighter stone is usually porous or weathered and will not give durable concrete.
Which shape of coarse aggregate is best for concrete?
Roughly cubical, angular crushed stone is best for structural concrete because the pieces lock together and bond well with cement paste. Rounded gravel gives easier flow but a weaker bond. Flaky and elongated pieces should be avoided.
Why should coarse aggregate be clean?
Dust, clay and organic matter coat the stone and stop the cement paste from bonding to it. Chlorides, sulphates and alkalis are worse: they attack the concrete or the steel inside it over time. Dirty aggregate should be washed or rejected.
