Insights — 9 October 2026

Polished concrete: what aggregate exposure decides, and why the mock-up is not optional

Most disputes over a polished concrete floor start the same way: the owner chose the finish from a photograph. The photograph showed somebody else's slab, ground to somebody else's depth, under somebody else's lighting — and the quotation just said polished concrete. When the floor that arrives shows more stone than the photo, or less, or shows it unevenly, nobody has been dishonest. The specification was simply silent on the decision that changes everything: how deep the grind goes, and therefore how much aggregate it exposes.

This article explains what exposure is, why the slab you already own has the final say, how gloss is a separate decision, and why the mock-up — a full-sequence panel on your own floor — is the only specification worth signing.

One name, four floors

Aggregate exposure is the depth of the cut

Concrete is stone and sand in hardened cement paste; trowelling works the paste to the top, leaving a thin skin of cream with the sand just beneath and the coarse stone lower still. Polishing grinds that surface with diamond tooling, and the depth of the grind decides what the finished floor is made of visually. Stay in the skin and you keep a uniform cream; cut slightly deeper and the sand appears — the trade's salt and pepper. Go deeper again and the cut edges of the coarse stone show, until at full depth the floor reads almost like terrazzo.

Those are not four grades of one product at four prices — they are four different floors, with different tooling time, flatness demands and sensitivity to what is already in the slab. Industry bodies such as the Concrete Polishing Council publish formal exposure classes precisely because the bare phrase polished concrete commits a contractor to none of them; where a specification cites those classes, the class definitions govern.

The four common aggregate exposures and what each one demands
Exposure	What you see	What it demands of the slab
Cream	Uniform cement paste, the tightest and most even colour, no visible sand or stone	A flat, well-trowelled surface in good condition — the finish lives in the thin top skin, so every mark, patch and burn in that skin stays part of the floor
Salt and pepper	Fine sand grains speckled through the paste	The most forgiving choice — the light cut removes minor surface defects without depending on where the coarse stone sits
Medium aggregate	Sand plus the cut edges of some coarser stone	Reasonable flatness and reasonably even aggregate distribution, or the stone shows in patches rather than as a field
Full aggregate	A near-continuous field of cut stone, terrazzo-like	The deepest cut and the most grinding — needs a flat slab with well-distributed aggregate near the surface, and puts the slab's placement history on display

Exposure class definitions and acceptance criteria vary by specification body — where a project cites a published classification, that document governs, not this table.

The material has a memory

The slab you own has the final say

Here is the part the photograph never shows: a grinder cuts a level plane, but the slab underneath it is not a uniform material. The aggregate sits wherever the placing crew, the weather and the finishing schedule left it — years before anyone thought about polishing. Grind the whole floor to the same depth and those differences become visible: one bay shows stone, the next shows cream, and the transition follows the history of the pour rather than any line a designer drew.

Flatness compounds this: on a wavy slab the level plane cuts deeper through high areas and barely touches the hollows, so highs expose aggregate while lows keep their cream. The mottled result is not operator error — it is geometry. Chasing uniformity by grinding the whole floor to the deepest cut costs real tooling and hours, and on a thin or worn slab may not be available at all.

Existing floors add their biography on top: patched trenches from old services, adhesive residue, curing compound, trowel burn. Polishing does not erase any of this, because polishing removes material evenly rather than covering it. A patch will polish, but it polishes as a patch — its different mix, colour and aggregate stay legible in the finished floor. On the right project that reads as character — and honest contractors say so before grinding starts, not after. When a client needs the floor to hide its history instead of wear it, the answer is usually not polish but an overlay or a coating — covered in our comparison of epoxy, urethane and polyaspartic floor systems.

You do not order an aggregate exposure the way you order a paint colour. You choose what to reveal in a slab that already exists — and the slab votes.

Sheen is a separate decision

Gloss is the other axis, and it gets tangled with exposure

Exposure is how deep the grind goes. Gloss is how fine it finishes. They are set independently, and conflating them is the second most common specification failure after ignoring exposure. The sequence runs from coarse metal-bond diamonds, which do the cutting that sets exposure, through progressively finer resin-bond grits, which do the refining that sets sheen. Stop the refinement early and the floor is honed — a soft, low-sheen finish that reads matte and hides fine scratching well. Carry on up the grits and reflectivity builds until overhead lights render as sharp mirror images.

Both ends of that range are legitimate, and we have installed both. The Rogers' Chocolates floor was finished at 400 grit — a honed, satin surface that suits the room rather than performing at it. Our standard commercial finish goes to 1500 grit, and at the far end of the scale we have taken a cream polish up to 3,000 grit, where the slab's own mottled surface ends up with a mirror-like gloss. The supermarket floor we polished at Fresh St. Market in West Vancouver in 2012 shows why the choice belongs to the building: a floor under bright, continuous lighting and constant trolley traffic wants a finish chosen for how it works and cleans, not how it photographs on day one.

Between the cutting and the refining sits a step invisible in the finished floor and essential to it: densification. A reactive silicate — on our floors typically MAPEI's Mapecrete Hard LI, applied to its datasheet — reacts with the free lime in the concrete to harden and tighten the surface, so it resists abrasion, sheds dust and holds its polish under traffic. Skipping or starving this step is how a polished floor ends up dull and dusting early. The full picture of how our polishing work is scoped sits on the concrete polishing page.

The only honest sample

The mock-up is the specification

The look of a polished floor is decided by a chosen depth, a chosen gloss and a particular slab's hidden history — and only one of those three is visible in a brochure. The mock-up brings the other two into the open before the whole floor is committed. Done properly, it works like this.

Put it on the real floor
The panel goes in the actual space, on the actual slab, under the lighting the finished floor will live with. A hand-ground sample tile, or a panel polished in a back corner, tells you nothing about the showroom.
Let it include the blemishes
If the slab has patches, trowel burn or an old trench line, take some in deliberately. The point is to see how the floor's history reads once polished — not to produce the one square metre that avoids it.
Run the full sequence
The complete grit progression, the densifier, any grouting of pinholes and the burnish — exactly as the production floor will get them, with the production machinery. A shortcut sequence produces a sample the finished floor cannot match.
Judge it in the finished conditions
Sheen and exposure both change character with light. Assess the panel under the permanent lighting, at the times of day the space will be used, before anyone signs anything.
Sign it and keep it
Once agreed, the panel is recorded and becomes the acceptance benchmark for the whole floor. Every later conversation about whether the floor is right happens next to the mock-up, not a photograph.

We treat the mock-up as part of the scope, not a courtesy: it settles in advance, at small cost, every argument that would otherwise be had over a finished floor — when the only remedies left are grinding deeper, living with it, or covering the polish you just paid for.

If you are considering a polished floor, the useful questions for the first conversation are about the slab, not the catalogue: how old, how finished, what has been on it, what lighting it will live under. From there exposure and gloss follow, and the mock-up confirms them on your own concrete. Our polished work — including the Rogers' Chocolates, Fresh St. Market and 3,000-grit cream polish floors — is on the projects page, and the wider flooring context sits with our commercial floor coatings.

Questions we get asked

Aggregate exposure FAQs

Can I choose any aggregate exposure for an existing floor?

You can ask for any exposure, but the slab decides how uniformly you get it. Aggregate sits at a depth set by how the concrete was placed, compacted and finished years ago, and a grinder cuts a level plane through whatever that history left behind. On a flat slab with even aggregate a specified exposure comes up consistently; on a wavy slab, high areas cut deeper and show more stone while lows keep their cream, and no operator skill changes that geometry. A mock-up answers this before the whole floor is committed: it shows what your slab, not a brochure slab, will reveal.

What does a densifier do, and is it optional?

A densifier is a reactive silicate applied during the polishing sequence. It reacts with free lime in the concrete to harden and tighten the surface, improving abrasion resistance and helping the floor take and hold a polish. On our polished floors this step is typically done with MAPEI Mapecrete Hard LI, with coverage and timing governed by its datasheet. It is not optional: an undensified floor dusts, wears faster and loses its sheen early, defeating the point of polishing.

How big does a polishing mock-up need to be?

Big enough to be honest. That means large enough to run the full grinding sequence with the production machine rather than a hand tool, placed in the actual space under the actual lighting, and positioned to include the character of the real floor, patches and trowel burn included. A sample that avoids every blemish and sits under different lighting is a brochure, not a benchmark. The finished panel is then agreed, recorded and kept as the standard the rest of the floor is judged against.

Is polished concrete the right finish for every commercial floor?

No. Polished concrete earns its place in retail, showroom, office and dry warehouse space, where its durability and low upkeep are hard to beat. It is generally the wrong answer where the floor lives wet or under chemical attack — commercial kitchens, breweries, wash bays, food processing — because polishing is a refinement of the concrete itself, not a protective barrier over it. Those floors need a coating or membrane system selected for the exposure. When weighing the two families of finish, the honest starting point is what actually hits the floor in service, not the look.

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