Ask most people what a road is made of and they will say tar or concrete. Both answers are wrong by volume. A road is overwhelmingly made of stone — the bitumen or cement is only the glue holding it together.
This post covers what aggregates are, the binders used with them, and the numbers that show just how dominant they are in any paving mixture.
What Aggregate Means
Aggregate is a collective term for the mineral materials — such as sand, gravel and crushed stone — that are used with a binding medium to form compound materials such as bituminous concrete and Portland cement concrete.
Three parts of that definition are worth separating out.
“A collective term”
Aggregate is not one material. It is a category covering everything mineral that goes into the mix — sand, gravel and crushed stone are all aggregates, and they differ enormously in size, shape and origin.
“Mineral materials”
Aggregates come from rock, whether taken naturally as gravel and sand or produced by crushing quarried stone. This matters because the properties of the parent rock — its hardness, its toughness, its resistance to weathering — carry straight through into the road.
“Used with a binding medium”
The binding mediums named are:
- Water
- Portland cement
- Lime
Bitumen is of course the other major binder, and it forms bituminous concrete with the same aggregates.
The Two Compound Materials
| Compound Material | Binder Used | Pavement Type |
|---|---|---|
| Bituminous concrete | Bitumen | Flexible pavement |
| Portland cement concrete | Portland cement and water | Rigid pavement |
Both are aggregate plus binder. What differs is the binder, and that single difference is what makes one pavement flexible and the other rigid.
How Much of a Pavement Is Aggregate
By volume, aggregate generally accounts for
92 to 96 percent of bituminous concrete
70 to 80 percent of Portland cement concrete
These two figures are among the most commonly asked facts in the chapter, and they repay a moment’s thought.
Why Bituminous Concrete Is So Heavily Aggregate
In a bituminous mix, the bitumen exists only as a thin film coating each particle and binding it to its neighbours. It does not fill the space between particles — the particles themselves are meant to touch and interlock. Only a few percent of binder is therefore needed, leaving 92 to 96 percent as stone.
This has a direct consequence for how the mix carries load. Because the aggregate particles are in contact, strength comes largely from interlocking and friction between adjacent particles, not from the binder. The bitumen holds things together and keeps water out; the stone carries the load.
Why Cement Concrete Needs More Binder
Cement paste works differently. It must fill the voids between the aggregate particles and then harden into a continuous solid mass. Filling voids takes far more material than coating surfaces, which is why the aggregate proportion drops to 70 to 80 percent.
In cement concrete, the hardened paste itself carries a substantial share of the load, and the aggregate is embedded within it. In bituminous concrete, the stone skeleton carries the load and the bitumen simply keeps that skeleton intact.
The Practical Consequence
If aggregate makes up more than nine-tenths of a bituminous pavement by volume, then the quality of a road is very largely the quality of its stone.
Use weak aggregate and no amount of good bitumen will produce a strong pavement. Use aggregate that polishes smooth under traffic and the road loses skid resistance. Use flaky particles and the mix loses stability. Use aggregate that strips its bitumen coating in the presence of water and the pavement disintegrates from within.
This is exactly why the next two topics of the chapter — desirable properties of aggregates and aggregate tests — receive so much attention. Seven separate tests exist because seven separate things can go wrong with the material that makes up most of the road.
Classification by Size
For mix design purposes, aggregates are classified into three groups:
| Class | Function in the Mix |
|---|---|
| Coarse aggregate | Contributes to stability, largely through interlocking and frictional resistance between adjacent particles |
| Fine aggregate (sand) | Contributes to stability, and fills the voids between coarse aggregates |
| Mineral filler | Largely a void filling agent |
The three sizes work as a system. Coarse particles form the load-bearing skeleton; fine particles pack into the gaps between them; filler occupies whatever small spaces remain. A well-graded mix contains all three in the right proportion, so that voids are minimised and the particles lock firmly together.
One further point worth noting: crushed aggregates and sharp sands produce higher stability in a mix than gravel and rounded sands. Angular faces grip one another; rounded ones roll past each other.
Quick Revision Notes
- Aggregate is a collective term for mineral materials such as sand, gravel and crushed stone.
- It is used with a binding medium — water, Portland cement, lime, or bitumen.
- The compound materials formed are bituminous concrete and Portland cement concrete.
- By volume, aggregate is 92 to 96 % of bituminous concrete.
- By volume, aggregate is 70 to 80 % of Portland cement concrete.
- Bituminous mixes need less binder because bitumen only coats particles, while cement paste must fill voids.
- Aggregates are classified as coarse, fine and filler.
- Coarse aggregate provides stability through interlocking and frictional resistance.
- Fine aggregate fills voids between coarse particles; mineral filler is mainly a void filling agent.
- Crushed aggregates and sharp sands give higher stability than gravel and rounded sands.
Mistakes Students Commonly Make
- Swapping the two percentage ranges. Bituminous concrete has more aggregate (92–96 %), cement concrete less (70–80 %).
- Thinking of aggregate as a single material rather than a collective term covering sand, gravel and crushed stone.
- Forgetting that water, cement and lime are all named as binding mediums, not only bitumen.
- Assuming the binder carries the load in a bituminous mix. Stability comes largely from aggregate interlocking.
- Treating mineral filler as merely an impurity. It has a definite function as a void filling agent.
- Assuming rounded gravel performs as well as crushed stone. Crushed and sharp materials give higher stability.
Conclusion
Aggregate is the material a road is mostly made of — over nine-tenths of a bituminous pavement and about three-quarters of a concrete one. The binder gets the attention, but it is the stone that carries the wheels. Understanding that the coarse fraction forms an interlocking skeleton, the fine fraction packs its voids, and the filler completes the packing explains both why gradation matters and why so many separate tests exist to check that the stone is fit for the job.
Frequently Asked Questions
What is aggregate?
A collective term for mineral materials such as sand, gravel and crushed stone, used with a binding medium to form compound materials like bituminous concrete and Portland cement concrete.
What binding mediums are used with aggregates?
Water, Portland cement and lime are named, and bitumen is the binder used in bituminous concrete.
What percentage of bituminous concrete is aggregate?
By volume, aggregate generally accounts for 92 to 96 percent of bituminous concrete.
What percentage of Portland cement concrete is aggregate?
About 70 to 80 percent by volume.
Why does bituminous concrete contain more aggregate than cement concrete?
Because bitumen forms only a thin film coating each particle, whereas cement paste must fill the voids between particles and harden into a continuous mass, which requires considerably more binder.
How are aggregates classified for mix design?
As coarse aggregate, fine aggregate and mineral filler.
What is the function of coarse aggregate?
It contributes to the stability of the mix, largely through interlocking and frictional resistance between adjacent particles.
What is the function of mineral filler?
It acts mainly as a void filling agent within the mix.
Do crushed aggregates perform better than gravel?
Yes. Crushed aggregates and sharp sands produce higher stability in the mix compared with gravel and rounded sands, because angular faces interlock rather than rolling past one another.