Pouring a slab for polishing: what to decide before the concrete goes down

A polisher cannot add what the slab does not have. Stone that sits deep, a surface that undulates, a stain from curing compound - all of it is set on the day the concrete is placed. The cheapest time to get a polished floor right is at plans stage, while the slab is still a drawing.

If you are a homeowner, these are the questions to ask your builder. If you are an architect, they belong in the specification. If you are a builder, they belong in the brief to your concreter.

1. Decide the finish first

Choose how much stone you want to see and how much the floor should shine before anything is ordered. A full stone exposure needs a different mix and more depth than a salt-and-pepper finish, and a high-gloss floor needs a flatter slab than a matte one. Everything below follows from that choice.

2. Strength

Ask for 32 MPa as a minimum, and 40 MPa if you want a smooth cement look with no stone showing. Stronger concrete grinds cleaner, takes a better polish and wears better.

3. Flatness

The shinier the finish, the more a wavy slab shows. Write a flatness figure into the specification, and specify one step flatter than the finish needs, because flatness drifts between pour and grind. Our finish schedule lists the figure for each finish.

4. Set-downs and services

Grinding takes material off the top of the slab - from about 2 mm for a light exposure to 5-6 mm for full stone. Door clearances, thresholds and adjoining floor heights need to allow for it, and conduit or pipework needs enough cover that grinding will not reach it.

5. Placement and finishing

• Tell the concreter, before the pour, that the slab is for polishing. Finishing technique changes what we can achieve.
• For full stone exposure, the aggregate needs to be spread evenly. Where concrete rolls away from a form or wall, there will be bare patches.
• Avoid recycled water in the mix. It can leave brown discolouration that does not grind out.
• Boot prints, screed marks and trowel swirls survive a light grind. A careful finish matters most for smooth and salt-and-pepper looks.

6. Curing

Some curing compounds and membranes can stain the surface or stop densifiers and sealers from soaking in. Talk to us before a curing product is chosen. It is a five-minute conversation that can save a floor.

7. Saw cuts and joints

Control joints show in a polished floor, so they should be part of the design rather than an afterthought. Plan the layout with the architect so joints line up with walls, doorways and the room geometry.

8. Protect it during the build

Once the slab is down, every trade walks on it. Plaster, paint, oil, tile adhesive and steel all mark concrete permanently. Protection belongs in the build programme, not left to chance - see our guide to protecting a slab during the build.

When to bring us in

At plans stage. Send the drawings and the finish you have in mind. We will tell you what the slab needs, what to put in the specification, and what the floor will cost - before anyone orders concrete.