The concrete floor becomes the finish, one grade at a time.

The grades a floor can be given

“Polished concrete” is a label rather than a product. The industry body Cement Concrete & Aggregates Australia, in its briefing on polished floors, describes the term as covering a family of decorative floor options in which the concrete itself stays visible as the finished surface.

Steel trowelled

The plain one. The body says steel trowelling is usually specified where a basic flat, smooth finish is what is wanted.

Burnished

A hard-wearing finish with a surface lustre, usually specified where a surface with no trowelling marks is wanted.

Honed

Grinding takes the surface to a smooth but matt finish and opens the aggregate. The body describes honing as a smooth, low-maintenance, durable finish, a level of grinding that leaves a matt rather than a gloss surface, and the grinding is what brings up the aggregate inside the concrete. The specification adds, of honing, that the same grinding is what gives the finish its good durability along with its smooth look.

Polished

Carrying the honing further is what makes it polished. The body says further grinding, with finer and finer abrasives, produces the polished finish, and that extended honing on the same progressively finer grits is how the surface takes its lustre.

The grinding, coarse to fine

The industry body’s specification for honed and polished finishes is a model specification, and it puts its grit numbers in square brackets for the person writing the specification to set: 30 to 60 grit for the first pass, to grind and level the surface, and finer abrasives from 80 up to 300 grit after it. The order is the method: level first, then refine.

What the grinding brings up

Grinding is what makes the look, because it decides what the eye meets. The industry body says the process exposes the aggregate, and the aggregate therefore has a large influence on how the finished floor looks. How much of it shows depends on depth: the deeper the grinding goes, the more of the aggregate inside the concrete comes up.

The depth of the honing is the dial, and the body puts it in steps. A light honing brings up the fine aggregate, the sand in the mix, and sometimes the tops of the coarse particles; the depth itself decides how much of the coarse aggregate shows; and for the fullest exposure it gives a figure, roughly half the nominal size of the largest coarse aggregate in the mix.

Among the factors it lists as affecting the final appearance are the colour and hardness of the coarse aggregate the grinding exposes, and the colour of the concrete matrix itself. Both parts play their part, the body adds, but the polish is held by the aggregate rather than the matrix. That is why the nature of the stone matters: hard igneous rock is the most suitable material for these finishes, since it resists wear and holds its smoothness.

Where the exposed aggregate comes out patchy, the remedy is another pass. The body says rectification is usually a matter of grinding a little deeper, to even out the exposure of the aggregate.

The concrete the finish sits in

Two more additions change how the surface comes out. In a dry-shake topping trowelled into fresh concrete, a hardener can be added to strengthen the surface layer, which is why these products are often called coloured surface hardeners. And integral colour, the pigment added to the mix, needs less of itself where the aggregate is on show: with a honed finish, 1 to 2 per cent is usually enough, because the colour of the exposed aggregate is what dominates the look.

The industry body’s documents also put figures on the slab’s own strength. Its specification recommends a minimum grade of S32, to the Australian Standard AS 1379. Its 2006 briefing recommends a higher strength concrete for polished work: N32 for steel trowelled and honed finishes, and N40 where the finish is burnished. The two documents give the slab’s strength in their own terms, and each stands with its own figure here.

Densifying, when the slab is laid

The densifying happens when the concrete is placed, not at the grinder. The industry body’s specification requires the surface layer to be properly densified, by steel trowelling worked by hand or by machine, so that small imperfections are held down and the floor’s resistance to abrasion is raised.

Grouting the small faults

Small faults are filled, not left. The industry body’s specification says minor imperfections are repaired with a coloured cement-based slurry, and then the surface is ground again, so that the floor reads as one surface.

The sealer at the end

The last stage is the sealer, and it is the stage a floor lives with. The industry body’s briefing describes the application of a surface sealer as the final step of the job, there to stop stains, to keep the floor looking well for years, and to make it easier to clean. Its data sheet on sealers puts the function simply: the sealer exists to stop the surface staining and to make it easier to clean.

Sealers come in two kinds, it says: penetrating ones, and ones that form a film on the surface. Most are solvent-based for durability, and water-based ones are available as well. Wax-based products, natural beeswax and petroleum-based wax among them, are film-forming, and the sheet says they can be used on internal surfaces. Oil-based sealers go on in coats until the concrete will take no more oil. Acrylics come in water-based or solvent-based form. Silicone sealers are solvent-based and of the penetrating type, giving a matt finish that leaves the surface’s texture and colour as they were. Epoxy sealers give a high gloss and take a tint readily. One more difference worth knowing: urethane sealers resist abrasion better than acrylic ones do. And some sealers change how the floor reads, since a sealer can give concrete a wet look that lifts its colour.

Where this goes next

This guide gives no rating and no figure for any particular floor: the documents describe how the finish is made, not how a given slab will come out. The machine’s pass is described in the grinding guide, the dust it raises is in the dust guide, and the water and the noise it leaves behind are in the last guide.

This guide is general information from the documents named with each point; it rates no floor and promises no result.