Aggregate is the sand, gravel, and crushed stone mixed into concrete. It makes up most of the concrete’s volume and gives it strength. Grinding the surface can expose aggregate for a decorative look. The type and color of aggregate vary by region and supplier.
What Aggregate in Concrete Does
Think of concrete like a fruitcake. The cement paste is the batter, and the aggregate is the fruit and nuts packed into it. In concrete, the “fruit” is most of the cake: aggregate makes up as much as 60% to 80% of a typical mix.
Aggregate comes in two sizes. Fine aggregate is sand. Coarse aggregate is gravel or crushed rock. Packing them together helps the mix compact better, cuts the cement and water needed, and adds strength. Crushed old concrete can also be recycled into new aggregate.
Why Aggregate Matters for Your Floor
On a plain gray slab, you never see the aggregate. That changes when a floor gets ground and polished. The grinder cuts into the top of the slab, and the deeper it goes, the more stone shows up. It’s like slicing the top off a chocolate chip cookie: dough first, then specks, then whole chips.
Contractors describe that depth as aggregate exposure classes, from a smooth “cream” finish with little stone, to a speckled “salt and pepper” look, to full exposure with large stones on display. Full-exposure floors are often compared to terrazzo. You can see how this plays out on finished floors in our polished concrete floors guide.
What You Can and Can’t Control About Aggregate
On an existing slab, the stone is already set. Its color, size, and spread across the floor depend on what the supplier used when the slab was poured, and that varies by region. Two houses on the same street can show very different stone once they’re ground.
Stone also isn’t always spread evenly. Some areas may show lots of large rock, while others show mostly sand. That’s normal, and it’s part of the natural look of a ground floor. On new pours where the look really matters, aggregate shape, texture, and color can be chosen on purpose.
Aggregate FAQ
What are concrete aggregates?
Concrete aggregates are the sand, gravel, and crushed rock that get mixed with cement and water to make concrete. They make up most of the mix by volume and give concrete its bulk and strength. Sand is the fine aggregate, and gravel or crushed stone is the coarse aggregate.
What are the four main types of aggregates?
The four common types are crushed rock, sand, gravel, and recycled concrete. Crushed rock is quarried and broken to size, sand and gravel form naturally, and recycled aggregate comes from crushed old concrete. Your slab most likely contains a mix of sand and either gravel or crushed stone.
What type of aggregate is best for concrete?
It depends on the job. Gravel is often favored for decorative work, while crushed stone bonds better with the cement paste and is preferred where pavement strength matters most. For an existing floor, you’re working with whatever went into the slab, so a test area matters more than the type.