Power floating a concrete floor

Concrete levelled with a screed board is flat, but its surface is porous and full of water. The power float brings fine cement paste to the top, closes the pores and compacts the upper few millimetres of the slab. The result wears less, cleans more easily and gives off less dust than concrete that has only been screeded or hand-trowelled.

Why not just finish it by hand? On a few square metres, hand trowelling works. On a larger area, one person with a hand float simply cannot keep up. The whole slab stiffens at roughly the same time, and the window for finishing lasts only so long. A machine can cover tens or even hundreds of square metres in one shift and leaves a more even surface.

Typical uses

• Garages and carports where the concrete stays as the final floor
• Workshops, storage buildings and agricultural buildings
• Basements and plant rooms
• Base slabs in houses where an accurate level is needed for the layers above
• Concrete floors that will later be coated with epoxy or sealed

In the living areas of a house, power floated concrete is less common as a finished floor. Where people want a concrete look indoors, they usually mean a ground and polished floor, which is a different technique with different requirements.

Power floating is not a separate task you can book for any day you like. It follows straight on from the pour, so the crew has to be on site the same day, often until late evening or into the night.

Step by step

1. The concrete is placed, compacted with a poker vibrator or vibrating screed and struck off to the finished height using the level points.
2. The surface gets a first pass with a wide hand float on a long handle (a bull float) to smooth out the marks left by the screed.
3. The crew waits for the concrete to stiffen. They test it by stepping on it: when a boot leaves a print usually only 2 to 3 mm deep and no water collects in it, the machine can go on.
4. The first pass is made with a float pan fitted. It "opens" the surface, takes out small irregularities and brings up the cement paste.
5. If a dry shake topping is used, it is spread over the damp surface, usually in two applications, and worked in with the pan.
6. The crew then makes passes with the blades, raising their pitch a little each time. Every pass compacts the surface further, and it starts to take on a sheen.
7. Edges, corners and areas around pipes and drains are finished with a smaller machine or by hand.
8. Straight after finishing, the surface gets a curing compound or is covered with polythene sheeting.
9. Control joints are sawn, usually within the next 24 hours, depending on temperature and how fast the concrete sets.

Why timing matters so much

If the machine goes on too early, it sinks in, pushes up too much water and leaves waves. If it goes on too late, the surface is already too hard and the blades only polish it without compacting it. The right moment depends on temperature, wind, sun, cement type and admixtures. In summer it can be roughly 2 to 4 hours after placing, in cold weather 8 hours or more. That is why, in practice, a pour often starts in the morning and the finishing ends after dark.

The finish is only as good as everything underneath it. Many problems blamed on the finishers actually start with the groundwork.

• A firm, compacted sub-base, for example hardcore compacted with a plate compactor, with no soft spots
• A separating membrane, damp-proof membrane or insulation with a membrane, as the design specifies
• Reinforcement sitting on spacers or chairs, not lying on the ground, and tied securely
• Drains, channels, pipe penetrations and cable ducts fixed at the correct height
• Edge isolation strip around the perimeter so the slab does not bear against the walls
• Floor height and any falls marked out, for example towards a garage drain
• Access for the ready-mix truck and pump, or space to set them up
• Power or fuel for the machines, and lighting for work after dark
• In a roofed building, openings closed against draughts, shade in summer and frost protection in winter

Ordering the right concrete matters too. Power floated floors usually use concrete of at least strength class C25/30 under EN 206, with a consistent workability and no admixtures that make finishing difficult. If a dry shake topping is planned, the concrete should not be air-entrained, because a topping on air-entrained concrete is prone to delaminating later. Agree the mix with both the concrete supplier and the finishers in advance.

If the floor is part of a new garage, an extension or another change to the building, check your local building regulations with the local building authority before work starts. Do not assume no approval is needed. In a block of flats, talk to the building management or the owners' association before any work in shared areas such as basements.

Power floating without a topping

The machine works only with the cement paste that rises from the concrete itself. The surface is smooth and harder than screeded concrete. This is enough for basements, lightly used storage areas, base slabs or floors that will be covered later.

Power floating with a dry shake topping

A dry shake topping (sometimes called a surface hardener) is a dry mix of cement and hard aggregate, such as quartz sand or corundum. It is spread over the fresh concrete and worked into the surface. It creates a harder, more wear-resistant top layer that copes better with car tyres, trolleys and heavy items being moved around. Toppings also come in colours, most often grey, anthracite or a reddish tone. With a coloured topping, expect the shade to vary slightly. A light marbled look is part of the character of this kind of floor.

Which one to choose

• Garage, workshop, store with a pallet truck: usually with a topping
• Basement, plant room: often fine without
• Base for tiles, glued flooring or a levelling compound: without a topping, and often the surface should not be closed too tightly
• Base for an epoxy coating: without a topping, or as the coating supplier requires, since they will usually grind or shot-blast the surface anyway

Walk-behind and ride-on machines

Smaller areas and house projects use walk-behind power floats with a single rotor, roughly 60 to 120 cm in diameter. Large warehouses use ride-on machines with two rotors, where the operator sits on top. For a house garage or workshop, a ride-on machine is rarely used, because it will not fit through the door and cannot work close to walls and columns.

Fibre-reinforced concrete

If the slab uses concrete with steel or synthetic fibres instead of mesh, the finishers need to know beforehand. Fibres can stick out of the surface, and the crew has to work differently. With fibre concrete, a topping is often used precisely to push the fibres below the surface.

The effort per square metre depends mainly on how quickly and smoothly the machine can work. A simple open area is far easier for the crew than a space broken up into small sections.

What makes it easier

• A large rectangular area with no columns or penetrations
• A level floor with no falls
• Good access for the truck and pump
• A roofed space protected from sun, wind and rain
• Mild weather, when the concrete sets predictably
• Concrete from a reliable supplier with the same workability in every truck

What makes it harder

• Small rooms and many corners that have to be finished by hand
• Columns, drains, channels and pipes within the floor area
• Falls to a drain, which are harder to finish by machine than a flat surface
• Working outdoors without a roof, with a risk of rain or strong sun
• Winter, when the concrete sets slowly and finishing drags on into the night
• Heat and wind, when the surface dries quickly and unevenly
• A thin slab on insulation, which is softer and more sensitive to the machine's movement
• Underfloor heating in the slab, where joint cutting needs great care

A power floated floor sits between several trades. Once something is cast in concrete, it cannot easily be moved, so it pays to coordinate everything beforehand.

• The plumber and electrician must finish all pipes, drains and ducts in the floor before the pour
• With underfloor heating, the system must be pressure-tested and its pipe routes recorded so joint cutting does not hit them
• The site manager or designer sets the floor height in relation to door thresholds, the garage door and the ground outside
• The joint cutter must come on time, not several days later
• The coating or tiling contractor should say in advance what surface they need

With tiles and glued floors, one thing is often forgotten: a very smooth, closed surface gives poor adhesion for tile adhesive. The surface then has to be ground or primed as the adhesive manufacturer specifies. If you already know tiles will go on top, tell the finishers. They then do not need to close the surface to a sheen.

With epoxy and polyurethane coatings, the moisture in the concrete is what counts. A new slab takes weeks to months to dry, depending on thickness and conditions, and a coating on a damp base can later peel or blister. The coating supplier decides when coating can start, based on a moisture reading.

Adding water to the concrete

It still happens on site that someone adds water to the truck so the concrete spreads more easily. The surface then tends to be weak and dusty and shrinks more. Over time such a floor gives off dust and develops cracks.

Floating too early or too late

Starting the machine too early causes waves and dips where water later collects. Starting too late gives a surface that has a shine but is not compacted. That kind of surface can crumble and flake.

Delaminating topping

If the topping is spread on a surface that is already too dry, or the surface is sealed before water and air have escaped, a weak layer forms just below the top. Months later, thin flakes peel off, most often where cars drive.

Burnt surface

Too much trowelling with the blades at a steep pitch on already hard concrete leaves dark, glossy patches. This is not just a cosmetic issue: such a surface is often brittle and can develop fine crazing.

No curing

This is the most common mistake of all. A freshly finished surface left in sun or a draught dries too fast. Hairline cracks appear, the surface dusts and loses strength. The concrete usually needs to be kept damp for at least several days, covered with sheeting or treated with a curing compound as the manufacturer specifies.

Late or shallow joint cutting

Concrete shrinks as it hardens and will crack somewhere. A sawn joint tells it where. If joints are cut late or too shallow, the crack forms elsewhere, often running crookedly across the whole floor. The cut depth is usually around one third of the slab thickness, and bay sizes are set according to slab thickness and the shape of the room.

Damaged edges and marks

Around walls, thresholds and penetrations the work is done by hand, and when people are tired or working in the dark, this is where quality slips. It typically shows as an uneven edge at the garage door or knee prints in a corner.

Finishing a typical garage or workshop takes one day including the pour, often with work into the evening. Larger areas are poured in sections, and each section is finished on the day it is poured.

• Joint cutting usually within 24 hours, sometimes just a few hours after finishing, depending on temperature
• Careful walking on the floor usually after 1 to 2 days
• Curing and protection from drying out for roughly the first week
• Normal loads, such as driving a car in, usually after 1 to 4 weeks, depending on the concrete and the supplier's advice
• Full concrete strength is usually quoted at 28 days
• Coating or glued flooring only once the slab has dried to the level the manufacturer specifies

In winter, concrete must be protected from frost until it has gained enough strength. Pouring in freezing weather without precautions is not recommended. In summer, protect the surface from direct sun and keep the floor damp for longer.

Some owners power float a garage or workshop floor of a few dozen square metres themselves. Power floats can be hired from tool hire shops. Be aware, though, that getting the timing right takes experience, and the concrete will not wait for you. Do not attempt it without a helper, and consider whether it makes more sense to book a crew at least for the finishing itself. Wear sturdy boots when using the machine and keep everyone away from the spinning blades.

1. Prepare and compact the sub-base, then lay the membrane, the reinforcement on spacers and the edge strip.
2. Mark the floor height on the walls and set up the screed board and guide rails.
3. Hire a power float with both a pan and blades, practise the controls on dry ground and arrange to return it the next day.
4. Order concrete suitable for power floating and do not add water to it.
5. Place the concrete, compact it, strike it off with the screed board and smooth it with a bull float.
6. Wait until a boot print is only around 2 to 3 mm deep and no water collects in it.
7. Go over the area with the pan fitted, in regular overlapping strips.
8. Fit the blades and raise their pitch gradually over several passes.
9. Finish edges and corners with a steel hand trowel.
10. Apply a curing compound or cover with sheeting straight away and keep the surface damp for several days.
11. By the next day, saw the joints with a diamond-blade floor saw to the depth decided in advance.

If there is underfloor heating or any services in the slab, leave the joint cutting to someone who knows exactly where they run.

Inspect the floor in daylight, ideally with low-angle light from a window or a torch held close to the floor. This shows waves and patches you cannot see in direct light. Check flatness with a 2 m straightedge laid in different directions and measure the gap beneath it. The allowable deviation depends on what you agreed and on what will go on top, so agree it before work starts.

• Flatness or falls match the agreement and water runs to the drain
• Floor height lines up with the thresholds and the garage door
• The surface is even, with no waves, dips or machine marks
• The surface does not dust or crumble when you run your hand over it
• Tapping with a small hammer reveals no hollow spots or loose topping
• No cracks outside the sawn joints
• Joints are straight, in the agreed layout and at a suitable depth
• Edges at walls, thresholds and penetrations are finished and undamaged
• Colour variation in a coloured topping is within what was agreed
• You have been told how to cure the floor, when it can take loads and how long it needs to dry