Quick Verdict
Polished aggregate is the worn-smooth surface you feel when a parking lot or driveway gets slick in the rain. Over years of traffic, the stone particles at the top of the asphalt grind down from rough and gritty to glassy and flat, and that loss of surface texture is what drops skid resistance. In Oregon, where pavement stays wet for months, polished aggregate is a real liability concern, not a cosmetic one. The fix depends on severity: light cases respond to a micro-surfacing or thin friction course, while badly polished and worn surfaces need an overlay. This guide shows you how to spot it, grade it, and choose the right repair.
What Polished Aggregate Actually Is
Asphalt gets its grip from the texture of the stone aggregate near the surface. When the mix is new, those stones have sharp edges and the surface is open and gritty. Tires bite into that texture, which is what gives you traction — especially when there is a film of water between the tire and the road.
Polished aggregate is what happens after years of tires, sand, and weather wear those stones smooth. The sharp microtexture is ground off, leaving a slick, almost shiny surface. The pavement looks fine from a distance, but in the rain it behaves like wet tile. This is a surface-texture problem, not a structural one, so the pavement underneath can be perfectly sound while the top loses grip.
Why It Shows Up Faster in Oregon
Oregon's climate makes low skid resistance a year-round concern instead of a summer afterthought:
- Wet season length. The Willamette Valley sees rain from roughly October through May. A polished surface that would only matter occasionally somewhere dry is slick for half the year here.
- Fine grit and traffic. High-turn areas — drive aisles, entrances, loading zones — polish fastest because tires scrub the same spots constantly.
- Soft or rounded aggregate. Some local mixes use rounded river rock that polishes faster than crushed, angular stone. The aggregate source matters.
- Algae and moss film. Shaded lots in western Oregon grow a thin biological film that compounds an already-slick polished surface.
How to Grade the Severity
Pavement engineers rate distress using the ASTM D6433 method, which sorts most defects into low, medium, and high severity. Polished aggregate is a little different — it is usually rated present or not present rather than by width — but you can still think about it in practical tiers:
| Severity | What you see and feel | Typical response |
|---|---|---|
| Low | Slight smoothness in wheel paths; grip noticeably reduced only in heavy rain | Monitor; plan a friction treatment at next maintenance cycle |
| Medium | Visibly shiny aggregate over drive aisles; cars feel loose braking when wet | Micro-surfacing or thin friction overlay |
| High | Broad glassy areas, documented slip complaints, surface also raveling or worn | Overlay; restore both texture and any lost surface |
What Causes It
- Traffic volume and turning. The number-one driver. The more tire scrub, the faster the polish.
- Aggregate type. Rounded or soft stone polishes faster than hard, angular crushed rock.
- Age and oxidation. As the binder hardens and ravels away, more of the polished stone is exposed.
- No surface maintenance. Surfaces that never get a friction-restoring treatment simply wear until they are slick.
Polished aggregate rarely travels alone. It often appears next to rutting in wheel paths and surface oxidation, since all three follow the same high-traffic lines.
The Repair Paths
Restoring skid resistance means putting texture back on the surface. Your options, roughly cheapest to most involved:
- Micro-surfacing. A thin, polymer-modified slurry with sharp aggregate laid over the existing surface. It restores friction and texture without the cost of a full overlay. Good fit for medium severity on a sound base.
- Thin friction course / open-graded overlay. A new asphalt lift designed for grip. More expensive, but it also fixes minor surface defects at the same time.
- Full overlay. When polishing comes with raveling, rutting, or general age, a standard overlay restores the surface and the texture together.
- High-friction surface treatment (HFST). A specialty option for severe-risk spots like a steep wet entrance — a hard-aggregate coating bonded to the pavement for maximum grip.
Industry Baseline Range: surface friction treatments commonly run in the range of roughly $1.50 to $4.00+ per square yard for micro-surfacing, while a friction overlay runs meaningfully more per square yard, scaling with thickness and lot size. These are industry baseline ranges for planning only — actual pricing depends on lot size, access, condition, and current market conditions. Get a site-specific quote.
Current Market Reality
Aggregate and binder prices move with the asphalt index, and Oregon's tight May-to-October paving window means friction work competes for the same crews as everything else. A slick lot is a documented liability — if you have had a wet-weather slip complaint, that is the signal to move it up the list rather than wait. To see how this fits with other surface distress, our identify your crack type guide helps you sort what you are actually looking at.
The Bottom Line
Polished aggregate is low skid resistance you can measure with your own feet on a wet day. It is a surface problem, so the fixes are surface treatments — micro-surfacing or a friction overlay — and they work as long as the base is sound. If you have noticed your lot getting slick in the rain, get it looked at before someone slips. Cojo provides asphalt repair services and friction-restoring treatments across the Willamette Valley and statewide Oregon. Request an assessment and we will grade the surface and tell you which treatment fits.