Two ways to get the same look
The first approach seeds the surface. A decorative aggregate, often a smaller rounded stone chosen for color, gets broadcast onto the wet concrete and pressed in, then the top skin of paste gets washed away once the slab is firm enough to hold the stone but soft enough to let the cement go. What is left behind is a layer of stone sitting in the top half inch or so.
The second approach uses the aggregate that was already batched into the mix. The coarse stone in a well graded structural mix, picked for color and top size instead of ordinary crushed rock, goes through the same washing step and shows the stone that was there all along, all the way down. Which one a job uses is often decided by the look you want and what the finishing crew is set up to do, more than by anything structural.
Picking the aggregate that gets exposed
Color and size drive the choice more than strength does. River rock, pea gravel and quartz aggregates all get used for the look, and the same rules about grading that apply to any concrete still apply here: a full spread of sizes washes and sets better than a load heavy on one size.
- Rounded river rock or pea gravel for a smooth, worn look
- Crushed or quartz aggregate for a sharper, more angular finish
- Size picked for the depth of the wash, not just the color
Timing the wash
Washing too early pulls stone out along with the paste. Washing too late means the paste has hardened enough that nothing comes off but a light scrub, and the stone never really shows. The window between those two failures is short, and it is why a surface retarder is often used, a product that slows the set right at the top skin so the crew has time to wash it back properly once the rest of the slab is ready to be walked on.
Color and sealing after the wash
Integral color is a common pairing with exposed aggregate, tinting the paste that surrounds the stone so the whole surface reads as one finish instead of gray cement around colored rock. Once the wash is done and the slab has cured, a sealer is usually applied to bring out the color in the stone and give the surface some protection, though whether a particular slab needs one is worth asking about rather than assuming.
What this costs against a plain finish
Exposed aggregate falls into our stamped and decorative pricing band, $14 to $22 per square foot installed, above a plain broom finish. Where a specific job lands in that range depends on the aggregate chosen, how much hand work the wash takes, and the size of the pour, and that is worked out at the estimate rather than guessed at here.