How long after the pour before we can grind

Twenty eight days from the date of pour. That is the answer, and on almost every job the real answer is longer, because the right point in the programme to grind a floor usually arrives well after the concrete is ready for it.

The 28 days is not a rule of thumb we invented. It is the age at which concrete is tested for its specified compressive strength, and it is the point at which the slab behaves the way the grinding and densifying process assumes it will behave. Before that you are cutting a material that has not finished becoming itself.

What curing actually is

Concrete does not dry. It hydrates. Cement reacts chemically with water and the reaction products grow and interlock through the mix, which is what gives concrete its strength. Water is a reactant, not something to be got rid of, and the reaction continues for months. It just does most of its work in the first four weeks.

That is why keeping a slab wet or covered in the first week matters so much, and why a slab that dried out fast in wind and sun is weaker at the surface than one that was cured properly. It also means concrete gets harder over time. A slab at 28 days is significantly harder than the same slab at 10 days, and a five year old slab is harder again.

What happens if we grind it green

Green concrete cuts fast. That sounds like an advantage and it is not. The paste is soft, so the diamonds pull material out rather than cutting through it cleanly, and there are several consequences.

• —Aggregate pull out. Stone is levered out of the soft paste instead of being cut flat, leaving pits in the surface that stay there.
• —A cloudy finish. The polish will not build the way it should, because the paste has not developed enough hardness to take and hold a refined surface.
• —Densifier does not work properly. Densifiers react with free lime in the concrete and need the right chemistry and moisture state to work. Applied too early they can be consumed unevenly or fail to penetrate.
• —Sealer trouble. A young slab is still releasing a lot of moisture. Seal it and the moisture has nowhere to go, which causes blushing, cloudiness and adhesion failure.
• —Extra shrinkage cracking after the finish is on. The bulk of drying shrinkage happens early, and cracks that appear after polishing are cracks in a finished floor.

None of that is repairable by going back and doing another pass. Aggregate pull out is permanent. A hazy polish on soft paste does not sharpen up later. Grinding early costs a floor, and the floor is generally the most visible finished element in the building.

Give us the pour date the day it exists

The single most useful thing a builder can do for a polished floor is tell us the pour date as soon as it is set, not when the plasterers are finishing up.

With the pour date we can work backwards and forwards. We know the earliest legitimate grinding window. We can book the crew against the rest of your programme rather than into the first gap available. We can flag the trades that need to be finished before we start and the ones that must not start until we are off. And you get a fixed slot in the schedule rather than a phone call three weeks out asking when we can fit you in.

Where the pour date moves, and it usually moves, tell us the same day. Moving a booking is administration. Turning up to a slab that is nineteen days old is a wasted mobilisation for both of us.

The right stage of the build

Concrete age sets the earliest date. The build programme sets the right date, and the two are rarely the same.

The window we want is after the building is watertight and the exterior joinery is in, and before skirtings, kitchens, wardrobes and painting. That order exists for real reasons.

Watertight matters because rain on a slab being prepared and sealed ruins the work, and because sealing needs a controlled, dry environment. Exterior joinery matters because the openings need to be closed for the same reason, and because installing joinery after a floor is finished means people working over your new surface.

Before skirtings matters because grinding right into a wall junction is far cleaner when the skirting goes on afterwards and covers the edge. Before kitchens and wardrobes matters because we can run the floor through as one continuous surface rather than cutting around cabinetry, and because a joiner working on a finished floor is a risk you do not need. Before painting matters because paint drips and splatter on a finished polished floor are a genuine problem and a repaint over a protected floor is not.

When the programme cannot hit the window

Sometimes it just does not line up. A slab gets poured, the job stalls, and the floor sits under a roof for eight months. Sometimes the client decides on a polished floor late, after the building is nearly finished.

Neither is a problem in itself. An older slab is harder, and harder concrete needs a different approach. We adjust the grit sequence and tooling, starting with a more aggressive metal bond than we would use on a 28 day slab and allowing more time in the early cutting stages. It is slower work, and we would rather price it correctly than pretend it grinds the same.

What does cause a problem is a slab that spent those months unprotected. Late grinding on a clean slab is a scheduling matter. Late grinding on a slab that has been rained on, driven on and covered in mortar droppings is a different quote.

Protecting the slab in the meantime

Between the pour and the grind, the slab is the site floor. What happens to it in that period shows up in the finished surface, and almost all of it is avoidable.

Do cover the slab with a breathable protection product once initial curing is complete, tape the joins so nothing tracks underneath, and keep the cover down until we arrive. Do keep plant and vehicles off it. Do put drip trays under anything with a motor or a hydraulic line. Do clean up spills the day they happen rather than the week they happen. Do stack materials on packers and timber rather than directly on the concrete.

Do not use plastic sheet taped down tight, because it traps moisture and leaves staining patterns that can be permanent. Do not park a scissor lift or a digger on the slab. Do not cut steel over it, since hot fragments embed and rust. Do not mix mortar, render or plaster on it. Do not let acid wash, brick cleaner or unknown chemicals run across it. Do not use duct tape or high tack adhesive directly on the surface.