Substrates & Preparation
Mechanical Preparation and Priming of a Power-Floated Slab
A power-floated surface is dense and low-porosity, so mechanical opening followed by a compatible primer is what makes bonded adhesion realistic.
Bringing a liquid screed to an old power-floated unit?
We'll assess the existing surface, recommend a preparation method and confirm a compatible primer before quoting your bonded or floating screed.
Or call our landline on 01392 237700
Power floating produces a dense, smooth, closed concrete surface — great for a finished industrial floor, but a difficult starting point if a bonded liquid screed is meant to adhere to it. The very process that makes the slab hard-wearing also seals off the porosity that a bonded screed relies on for mechanical key.
This page covers what changes when the substrate is an existing power-floated slab rather than a freshly cast, un-finished pour, and why the preparation method has to be more aggressive.
Power floating is a finishing process, not a preparation one, and it's worth remembering that distinction when a refurbishment brings a liquid screed into contact with a floor that was originally designed to be seen and walked on as-is. The steel trowel blades that produce the glassy sheen are the same thing that closes off the surface a bonded screed would otherwise key into.
Power-Floated Slabs — the points that decide it
- Power floating closes the surface pores of concrete, reducing the mechanical key available for a bonded screed
- Diamond grinding or shot-blasting is typically needed to open the surface and remove any curing compound or sealer
- A compatible bonding primer (often an SBR or resin-based product matched to the screed manufacturer's datasheet) is applied after preparation
- If the slab was designed as a finished floor, converting to a floating/unbonded screed system avoids the need for aggressive bond preparation altogether
- Existing joints and any surface coatings must be identified before grinding, as some coatings contain materials that need specific disposal routes
- Falls and levels on an existing floor are fixed, so any correction has to be built into the screed depth rather than the substrate
- Trial patch testing with a pull-off adhesion gauge after grinding gives an objective figure rather than relying on visual judgement of the opened surface
- Grinding dust from an existing industrial floor may need containment and specific disposal if the slab previously carried oils or chemicals
- A profile comparable to a CSP (concrete surface profile) of around 3 or above is a commonly referenced target for mechanical preparation ahead of a bonded screed
Why smooth concrete resists bonding
A power float works the surface with rotating steel blades, compacting cement paste into a dense, low-porosity skin. That's exactly what's wanted for an exposed concrete floor, but it leaves very little for a bonded screed to grip onto beyond a purely adhesive bond, which is far less reliable than mechanical key on a fractured, open surface.
This is why refurbishment projects that involve topping an existing power-floated slab with a bonded liquid screed almost always specify mechanical preparation first, rather than relying on primer alone to bridge the gap.
Choosing floating over bonded
In many refurbishments it's simpler to treat the existing slab as a stable base and float the new screed over a separating membrane rather than chase a bonded finish. This avoids grinding dust and disposal costs, sidesteps any uncertainty about what's in the existing surface, and gives more flexibility to correct minor level variation in the old floor.
The trade-off is a slightly greater overall build-up, which matters where door thresholds or existing skirting heights are tight.
Reading an existing industrial or commercial slab
Power-floated floors are common in warehouses, workshops and light industrial units, often with a history of forklift traffic, oil spills or chemical exposure that a straightforward domestic slab wouldn't have. Before grinding starts, it's worth checking that history against what's visible at the surface, since contamination changes both the preparation method and the disposal route for the resulting dust and arisings.
Existing floor joints, construction breaks and any areas of surface crazing from years of use should also be mapped, since these often need their own repair or detailing before insulation or a bonding primer goes on.
Matching the primer to the screed system
Bonding primers aren't interchangeable between screed manufacturers, and using the wrong one can undermine an otherwise well-prepared surface. An SBR-modified slurry primer suits many cement-based bonded screeds, while some proprietary systems specify their own resin-based product with a defined open time before the screed has to go on.
Following the specific datasheet for primer coverage rate, drying time and maximum overcoating window matters more on a bonded refurbishment job than on a straightforward new slab, since there's less margin for error once the existing surface has already been ground back.
Bringing a liquid screed to an old power-floated unit?
We'll assess the existing surface, recommend a preparation method and confirm a compatible primer before quoting your bonded or floating screed.
Or call our landline on 01392 237700
Power-Floated Slabs — your questions
In short
A power-floated surface is dense and low-porosity, so mechanical opening followed by a compatible primer is what makes bonded adhesion realistic.
Bringing a liquid screed to an old power-floated unit?
We'll assess the existing surface, recommend a preparation method and confirm a compatible primer before quoting your bonded or floating screed.