Power Float Finish: Tolerances and How to Specify It

A power float finish is the dense, smooth surface produced by working a freshly placed concrete slab with a power float, a machine with rotating pans or blades, as the concrete stiffens. It is the standard finish for industrial floors because it turns the structural slab into a hard-wearing working surface in one operation.

People often ask what tolerance a power float finish achieves. The short answer is that the float decides the surface, not the flatness. Flatness is specified separately, usually to TR34 for industrial floors, and it is mostly set by how the concrete is placed and levelled before the float ever touches it. This guide explains how power floating is done, how the tolerances work and how to specify it.

How power floating is done

• Place and level. The concrete is placed and struck off to level, by laser screed on large floors or by hand in bays on smaller ones.
• Wait for the right moment. The surface has to stiffen enough to carry the machine. When that happens depends on the mix, the temperature and the wind, so it is judged on the slab, not by the clock.
• Float. The first passes use flat pans to close the surface and bring fines to the top.
• Trowel. Further passes with blades, at steadily increasing pitch, densify and polish the surface. Ride-on machines cover open areas and walk-behind machines work the edges and tight spots.
• Cure. The slab is protected so it keeps its moisture while it gains strength, or the surface that has just been closed will open up again.

Timing is everything. Float too early and bleed water is sealed under the surface, which can lead to blistering or delamination. Float too late and the concrete has gone off and will not close.

What tolerance does a power floated floor achieve?

For industrial floors the tolerance is a flatness class from the Concrete Society's Technical Report 34 (TR34), now in its 4th edition. TR34 controls levelness (Property E) and flatness (Property F) separately, and compliance is assessed on the 95th percentile value. For free movement floors, where trucks can travel anywhere, the classes are:

The racking and the trucks decide the class the floor needs. FM2 is typical on our laser screeded warehouse floors, and smaller floors laid by hand in large bays typically reach FM3 or FM4.

Screeds are a different standard. Surface regularity for screeds is specified to BS 8204 as SR1, SR2 or SR3, which is 3mm, 5mm or 10mm maximum deviation under a 2m straightedge. If the drawings show a levelling layer over a deck rather than a structural slab, see commercial floor screeds.

How to specify a power floated floor

A specification that prices cleanly names these things:

• The TR34 class, and whether the floor is free movement or defined movement. Don't specify tighter than the racking needs. Each class tighter costs money for no operational gain.
• The slab design. Thickness, reinforcement (steel mesh, structural fibres or both) and concrete grade, from the engineer. The slabs we lay are typically 150 to 250mm thick, sometimes much thicker.
• The joint layout and joint type. Joints are where floors fail, so they are a design decision, not a detail.
• Any surface treatment. For example a dry shake topping or a surface hardener where extra abrasion resistance is needed, and the curing method.
• The survey. Who carries out the flatness survey, when, and what happens if a reading fails. An independent survey at handover is normal.
• The conditions. Power floating is best done under a watertight roof. Outdoors, wind and sun change the finishing window entirely.

When a power float finish is the wrong choice

Power floating suits any floor where the slab is the working surface and takes wheeled traffic. It is the wrong choice where the floor is a levelling layer over a structural deck (you want a screed), where access or working hours make it impossible to have finishing crews on the slab at the right moment (a self-compacting system removes that constraint, see self-compacting versus power floated), and where the floor will be covered and only needs to be flat, not hard.

Power floated floors from Trent

Trent Construction Services lays power floated concrete floors for main contractors across the UK, finished to the TR34 class your specification names, with our own operatives, ride-on and walk-behind power floats and a Somero S22EZ laser screed. The finished floor is surveyed by an independent specialist, usually arranged and paid for by us. We work on commercial projects only.

See power floated concrete floors for the detail, or send us the drawings and the specification. Discuss your project.