A builder's guide to getting polished concrete right

By the time a grinder is called to site, most of the outcome is already fixed. The mix is in the ground, the flatness is what it is, the colour was decided by whoever placed the order, and whatever landed on the slab during the build is now part of it. We can control depth, sheen and sealer. Everything else was decided by people who were on site months before us.

That is the whole argument for bringing a grinder in early. Not to sell more work, but because early involvement is the only point at which the result can actually be controlled. This is written for site managers and project managers who want the floor to be a known quantity rather than a hopeful one at practical completion.

At design and pricing stage

Identify which areas are polished and confirm it in writing before the job is priced. That sounds obvious and it is the most common failure point on jobs that go wrong. A floor described as polished concrete on a drawing, with no exposure, no colour and no sheen nominated, is not a specification and will be priced five different ways by five different applicators.

Three things nail it down. The exposure, meaning how much aggregate shows, from cream through salt and pepper to full stone. The colour, which for anything other than natural grey has to be batched at the plant. The sheen, from matt through satin to gloss. Get those on the drawing set and every quote you receive is comparing the same floor.

This is also the stage to confirm whether the slab is designed for it. Slab edge insulation, saw cut layout, joint positions and the location of penetrations all affect how the finished floor reads, and all are cheaper to think about now than to work around later.

Before the pour

Send us this before the concrete is ordered. It takes ten minutes and it removes most of the risk from the job.

1. 01Floor plans marked with the polished areas, so we know what is in scope and where the boundaries fall.
2. 02Desired exposure, ideally confirmed against a sample or a previous job rather than a photograph found online.
3. 03Colour selection, including the oxide or pigment reference, because it is batched at the plant and cannot be changed afterwards.
4. 04Aggregate requirements, including whether decorative stone is to be seeded into the pour.
5. 05The sealer system, because it affects sheen, slip and the maintenance regime handed to the client.
6. 06Pour date and anticipated grinding dates, so the programme has the curing period built into it rather than discovered later.

The builders portal holds the full pre pour checklist and specification pack in a form you can attach to a subcontract. Use that as the reference document. This post is the reasoning behind it.

At placement

Flatness is the single largest control on how even a polished floor looks. A grinder cuts high points first and does not reach low points until the highs have come down. Three millimetres of variation across a room, which passes without comment under a floor covering, shows as a plainly different looking floor once it is cut.

Ask the placing crew for a tighter tolerance than they would normally work to, and tell them why. On larger slabs a laser screed pays for itself. Keep the depth consistent, keep the mix consistent, and avoid changes between loads. Two trucks with different water content, or a load that sat too long, will read as two different floors after grinding even though it was one pour.

Power trowelling matters as well. Where blades dwell or work harder, the paste densifies more than the surrounding area, and that difference is invisible on a raw slab and obvious once it is polished. Even, consistent trowelling across the whole floor is worth more than a hard closed finish in patches.

Protecting the slab through the build

Concrete is porous until it is sealed, and it is at its most vulnerable for the months when it is being used as a work platform. Most of the floors we get called to fix were not badly poured, they were badly protected.

Do keep the following in mind as site rules, and brief every trade on them rather than assuming they know.

• —Do not leave ferrous materials on damp concrete. Reo offcuts, banding, steel props, tools and swarf will bleed rust into the slab, and rust staining travels down into the surface rather than sitting on it.
• —Do not allow oil, diesel or hydraulic fluid anywhere on the floor. Plant leaks, compressors and generators go on boards with drip trays under them. Oil penetrates deeply and does not grind out at normal cut depths.
• —Do not let paint, plaster, adhesive or render dry on the slab. Clean spills immediately while they are wet. Cured adhesive is the most expensive contaminant on this list to remove.
• —Do not cut or grind other materials over the exposed slab. Brick, tile and block dust carries iron and pigment that stains, and cutting slurry runs into saw cuts and sits there.
• —Do not tape sheet protection directly to the concrete. Tape residue and some tape adhesives mark the surface and the mark shows after polishing.
• —Do cover the floor once the roof is on, with a breathable protective layer rather than plastic. Plastic traps moisture against the slab and can cause discolouration.
• —Do keep drainage clear so nothing ponds on the floor. Standing water leaves a rim and can drive efflorescence.
• —Do treat the polished areas as a finished surface from the day the slab is poured, because that is what they are.

Timing

Minimum 28 days from the pour before we grind. That is not a scheduling preference, it is the point at which the concrete has developed enough strength for the diamonds to cut it predictably and for the sealer to bond properly. Grinding green concrete pulls stone out instead of cutting it, and sealing a slab that is still releasing moisture is how you get a cloudy or blistered finish.

The best stage in the programme is once the building is watertight and the exterior joinery is installed, and before skirtings, kitchens and painting. Each half of that matters for a specific reason.

• —Watertight and joinery in, because grinding and sealing need a controlled environment. Rain blowing through an opening onto fresh sealer ruins it, and wind carries dust into a wet coat.
• —Before skirtings, because we can cut cleanly to the wall line instead of hand working an edge against a finished trim, and the skirting then covers the perimeter neatly.
• —Before kitchens and cabinetry, because a fixed island turns an open floor into a detail job, and the machine cannot reach where the cabinets sit.
• —Before painting, because grinding produces dust and painters do not want it, and because a painter can cut in to a finished floor far more easily than we can grind up to fresh paint.

What we need on site before we arrive

A crew turning up to a floor that is not ready costs everyone. This is the list worth checking the day before.

• —A watertight building, with exterior joinery installed or the openings properly closed.
• —A clear floor. No stacked materials, no scaffold standing on the polished area, no rubbish. We do not move other trades' gear.
• —Single phase power available at the floor. Three phase is useful on large commercial slabs but is not required for most jobs.
• —Water available on site for wet cutting and clean up.
• —Vehicle access close to the entry, and a clear path for equipment. Machines are heavy and carrying them costs hours.
• —No standing water on the slab, and no active leaks above it.
• —Uninterrupted access for the duration. Other trades working over a floor mid process is how contamination and boot marks happen.

Setting client expectations

This is the part that saves builders the most grief and it costs nothing. Polished concrete is not a manufactured tile or a sheet product. It is a natural material, poured on site, and variation is inherent to it. Aggregate distribution changes across a slab. Colour shifts subtly between loads. Trowel history shows. Saw cuts and joints are structural and stay visible.

A client told that at the start sees character. A client told it at handover hears an excuse. The conversation is the same conversation, and the only variable is when it happens. Show them a test grind or a sample tile, tell them plainly what is fixed and what is variable, and put it in writing.

It is also worth naming the maintenance regime early. A sealed floor needs a neutral cleaner and no acids, and it will want a maintenance coat somewhere in the five to ten year range. Handing that over as part of the documentation makes the floor look considered rather than fragile.

What to hand a grinder at first contact

If you want a real number and a real programme slot from the first phone call, send this. Floor plans with the polished areas marked. The pour date, or the anticipated one. The exposure, colour and sheen if they are decided, and a note if they are not. The sealer system if the specification names one. Site address and access notes. And the dates the building is expected to be watertight and ready for fitout.

That is enough to price the job properly, book it into a programme that respects the 28 day minimum, and flag anything that is going to be a problem while it is still cheap to change.