How do I get a good finish on a concrete slab?

Finishing concrete is entirely about timing, and the timing cannot be given as a number of minutes because it depends on the mix, the temperature, the wind and the humidity. The trigger is always what the surface is doing. Working it too early is the single biggest cause of a slab that dusts, scuffs and wears for the rest of its life. The timeline drawing shows the window, what opens and closes it, and which finish belongs where.

Safety

Step by step

1. Screed off level first.Strike the surplus off with a straight timber sawn side to side along the top of the formwork. Do it once, properly. Repeated screeding brings fines and water to the surface, which is exactly what you do not want.
2. Wait for the bleed water.Water rises to the surface after placing and lies there. Everything stops until it has gone. Working that water back into the surface is what makes the top few millimetres weak and dusty, and it is irreversible.
3. Test with a thumb.When the sheen has gone and a thumb pressed into the surface leaves a shallow mark without sinking in, the working window has opened. On a hot windy day this can be very soon; on a cold damp one it can be hours.
4. Choose the finish before you start.It depends entirely on where the slab is and what will happen on it. External and walked on: wood float or brushed. Internal, to be covered or painted: steel float. Sloped or a ramp: brushed, without exception.
5. Float with a wood float for external work.Sweeping arcs, keeping the leading edge slightly raised. It closes the surface enough to be durable while leaving an open, slightly sandy texture with real grip. This is the right default for most garden slabs.
6. Steel float only where you want it smooth.Follow the wood float with a steel float or trowel, in passes, letting the concrete stiffen between them. Each pass closes the surface further. Indoors this gives a hard, dense, dust-free floor. Outdoors it gives a skating rink in the rain.
7. Brush for a textured, non-slip finish.Float first, then draw a soft broom once across the surface at right angles to the direction of traffic, in a single steady pull without going back over it. The depth of texture depends on how soon after floating you do it.
8. Cut or form the movement joints.Either tooled in while the concrete is still workable with a jointing tool, or saw-cut a day or so later. Joints put in after the concrete has already cracked are decoration.
9. Finish the edges.Run an edging tool along the formwork before it stiffens. A rounded arris is far less likely to chip than a sharp one, and it makes a rough job look considered.
10. Cover and cure as soon as the surface will take it.Polythene or damp hessian, kept damp, held clear where it might mark the surface, left for several days. Concrete gains strength by reacting with water, so a slab left to dry out fast never reaches the strength you paid for.

Common mistakes

• Working the surface while bleed water is still on it — It works water back down into the top layer, giving a high water content exactly where you need strength. The result is a permanently weak, dusty, easily scuffed surface.
• Sprinkling dry cement on to dry the surface up — It creates a thin, over-rich, badly bonded skin that later flakes and dusts off. If the surface is too wet, wait.
• Splashing water on to make trowelling easier — Same problem in the other direction — it dilutes the surface paste and weakens the top layer.
• Steel floating an external slab — A polished concrete surface outdoors is dangerously slippery in the wet, and there is no easy way to give it grip afterwards short of acid etching or grinding.

If it doesn't work