The short answer
Exposed aggregate holds its stones in a thin cement matrix between them. High-pressure washing erodes that matrix and releases aggregate permanently — the loose stones underfoot are the warning sign. It needs moderate pressure with a surface cleaner, and a film-forming sealer that locks stones in place and deepens their color.
Why This Surface Needs Its Own Approach
Exposed aggregate is made by washing the cement paste off the top of a fresh pour to reveal the decorative stone underneath. It is common on driveways, pool decks and walkways across Macomb County — the textured, pebbled surface with visible river rock.
Structurally it is different from smooth concrete in one crucial way: the stones are held by a relatively thin matrix of cement paste between and beneath them. On a smooth slab, the surface is a continuous plane. Here it is an interlocking arrangement where each stone depends on the paste around it.
Erode that paste and the stones come out. They do not go back.
The Damage That Cannot Be Undone
Loose pebbles underfoot, or a collection of stones at the bottom of the driveway after rain, mean aggregate loss is already underway. Once a stone releases, it leaves a void, and the surrounding stones lose part of what was holding them. It accelerates.
Three things cause it:
- High-pressure washing. The most common and the most avoidable. A wand held close will visibly blow stones out — homeowners often describe seeing it happen and not connecting it to the equipment.
- Freeze-thaw. Water sits in the voids between stones, freezes, expands, and levers the paste apart. Our 30 to 40 cycles a winter do this relentlessly on unsealed aggregate.
- De-icing salt, which attacks the cement paste chemically and adds osmotic pressure to the freezing pressure.
Unsealed exposed aggregate in Michigan is on a clock, and the texture that makes it attractive is exactly what makes it vulnerable — all those voids hold water.
The early warning: run your hand over the surface. If loose grit and small stones come away, the matrix is already eroding. That is the point to clean and seal, not after a season of stones washing down the drive.
Cleaning It Correctly
The textured surface traps far more than smooth concrete does — dirt, algae, moss, leaf tannin and organic debris all settle into the voids between stones. It genuinely needs cleaning, and it needs the right method.
- Pre-treat with a cleaning solution and give it dwell time. On aggregate this matters more than anywhere else, because the chemistry has to do work that pressure is not allowed to do.
- Use a surface cleaner at moderate pressure, never a wand held close. The rotating jets under a shroud distribute energy evenly at a fixed height, which is precisely what prevents localised erosion.
- Treat efflorescence separately — the white mineral bloom common on aggregate needs a dedicated remover, not more pressure.
- Rinse thoroughly, flushing debris out of the voids rather than pushing it deeper.
The difference between a surface cleaner and a wand on this material is not subtle. It is the difference between a clean driveway and a driveway with a permanent light patch where someone lingered. Every concrete cleaning job we run on aggregate uses one.
Sealing: This Surface Genuinely Needs a Film
For most plain concrete we recommend a penetrating sealer. Exposed aggregate is the clear exception.
A film-forming sealer does three things here that a penetrator cannot:
- It physically locks the stones. The film bridges between aggregate and matrix, holding stones that are already slightly loose.
- It deepens the stone color dramatically. Dry aggregate looks washed out and gray; sealed aggregate shows the actual color of the river rock. This is the single biggest visual improvement available on the surface.
- It fills and bridges the voids where water would otherwise sit and freeze.
Two practical notes. Exposed aggregate is thirsty — the texture means considerably more surface area than a smooth slab of the same dimensions, so it absorbs more sealer and coverage rates are lower than the label suggests. And a grit additive is worth considering on pool decks and walkways, since a sealed aggregate surface is less slip-resistant than the bare texture.
Maintenance Schedule
| Task | Frequency | Notes |
|---|---|---|
| Clean | Annually | Texture traps organic debris quickly |
| Reseal | Every 2–3 years | Film wears under traffic and UV |
| Clear leaves | Every autumn | Tannin stains and holds moisture in the voids |
| Rinse off salt | Each spring | Salt attacks the cement matrix directly |
A resealed aggregate drive tends to look better than the owner remembers, because most have never seen it sealed — the stone color that appears is what was always there under the dry gray surface.
What Sealing Will Not Fix
Being straight about the limits: sealing locks down stones that are loose but still seated. It does not replace stones that have already gone, and it will not rebuild eroded matrix. Areas with significant aggregate loss will still read as smoother, lighter patches after sealing — better than before, but not restored.
That is the argument for doing it while the surface is intact rather than after a few winters of loss.
The Bottom Line
Exposed aggregate is one of the few surfaces where the wrong pressure washing does immediate, visible, permanent damage — you can watch the stones leave. It wants chemistry and a surface cleaner rather than force, and unlike plain concrete it genuinely benefits from a film-forming sealer that both protects the matrix and reveals the stone color underneath.
Want This Handled Properly?
Macomb Power Washing Solutions holds a 4.9-star rating across 160+ verified reviews, and free estimates go out within 24 hours. We carry $2M general liability and full workers' comp on every technician.