What Is Exposed Aggregate Concrete?
- The stone topsThe gravel or crushed rock that was already in the concrete, now showing at the surface. This is what you see and what your tires and boots touch.
- The paste between themCement and sand, washed back just far enough to uncover the stone and still hold it in place. How far it is washed back decides how bold the finish looks and how well the stones stay put.
- The slab underneathThe same air-entrained concrete, gravel base and thickness as any other pour. The finish is the top skin of a normal slab, not a coating.
Get down close to an exposed aggregate patio and you are looking at the inside of the concrete. On a broom finish, a thin layer of cement paste covers everything and the broom drags a texture into it. On exposed aggregate, that top layer is taken off while the slab is still setting, so the stone in the mix shows through as a pebbled surface.
Nothing is glued or sprayed on to make the look. That is why it holds up well, and also why everything about it (how much stone shows, how even it looks, how well the stones grip) gets decided in a short window on pour day.
Two Ways to Take the Paste Off
A surface retarder. After the slab is placed and floated, the crew sprays a retarder on top. It slows the set of the thin surface layer while the concrete underneath hardens normally. Later, often the next day, the softened paste is washed and brushed off. Retarder makers sell their products in different reveal depths, from a light sandy etch to a deeper stone-showing finish, and recommend a mock-up before the real job because results shift with the mix, the weather and the timing.
A timed wash. Without a retarder, the surface is washed and brushed by hand while the concrete is still soft enough to give up its paste but firm enough to hold the stone. It needs no extra product, but the window is short and it moves with sun, wind and temperature. Wash too early and stones come loose. Wash too late and the paste will not come off.
Either way works in good hands. The method matters less than whether the crew is watching the slab instead of the clock.
Stone in the Mix vs. Seeded Stone
The standard version exposes the stone that comes in the ready-mix truck, so the look depends on what that plant uses. Seeded work goes a step further: extra decorative stone is scattered over the fresh surface and pressed in before it is exposed, which gives more control over color and density at more cost and labor. They are different jobs, so a bid should say which one it is. Our exposed aggregate pours expose the stone in the mix.
Choosing the Stone: Rounded Gravel or Crushed Rock
The stone decides how the finish looks, how it feels under bare feet, and how likely it is to throw a popout years later. Pick a tab, or read all three.
Rounded River Gravel and Pea Gravel
Smooth, water-worn stones in a mix of tans, grays and browns. This is the look most people picture: soft-edged, easy on bare feet, comfortable for kids sitting on the patio. The rounded tops give a little less bite than crushed stone, but still far more than a smooth trowel finish.
The catch is popouts. The National Ready Mixed Concrete Association names low-density chert in natural gravel deposits as the most common particle behind them. Not every gravel has the problem, which is why it is worth asking how the plant's gravel has held up on other local flatwork.
Crushed Limestone and Dolomite
Angular, fractured faces with a sharper, more uniform gray look. Crushed stone grips better, and the same NRMCA guidance notes that crushed aggregates are less likely to carry the light, absorbent particles that cause popouts. It is also rougher: harder on knees, bare feet and the bottoms of furniture legs.
Will County sits on dolomite bedrock (a limestone-like rock, per the Illinois State Water Survey), and Illinois aggregate comes both from quarried limestone and dolomite and from glacial and river sand and gravel. What actually arrives in your truck depends on the ready-mix plant, so ask to see the stone before you choose.
Small Stone or Large Stone
Smaller stone gives a tight, even, sandpaper-like surface that is easier underfoot and easier to shovel. Larger stone reads bolder from the street but stands up more and feels rougher.
Size also affects popouts. NRMCA's guidance is that a smaller maximum stone size produces lower freezing stresses, so fewer popouts form and the ones that do are smaller. On a driveway that sees salt and plows, that argues for the smaller end.
Exposed Aggregate vs. Broom Finish vs. Stamped
These are the three finishes most Plainfield homeowners weigh. Here is how exposed aggregate compares on the things people actually live with.
| Exposed aggregate | Broom finish | Stamped | |
|---|---|---|---|
| Look | Pebbled stone, hides dirt, leaves and tire marks | Plain, clean lines | Slate, stone, brick or plank pattern |
| Grip when wet or icy | Strong, especially with crushed stone | Good | Smoother; a sealer can make it slicker |
| Bare feet, knees, furniture | Roughest of the three | Fine | Smoothest |
| Shovels and snowblowers | Blades chatter over the stones | Easiest | Easy, but sealer and color wear under steel blades |
| Upkeep | Low; an occasional penetrating sealer is optional | Low | Sealer renewed every two to three years |
| Patio price on our list | $12 to $16 / sq ft | $10 to $13 / sq ft | $15 to $25 / sq ft |
| A patch or added section later | Hard to match | Easiest to blend | Hard to match |
Exposed aggregate sits in the middle on price and gives you most of a decorative look without the resealing schedule that stamped concrete asks for. It loses on comfort and on how easy it is to fix later. For a full side-by-side of every patio finish, including integral color, see concrete patios.
