Concrete substrate preparation
Shot Blasting and Floor Grinding Before Screeding
Prepare the concrete beneath a bonded screed: remove the weak surface, deal with residue and agree the right profile for the new floor system.
Discuss preparation for your screed
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Shot blasting and floor grinding are mechanical ways of preparing an existing concrete substrate before a bonded screed. The aim is not a decorative finish: it is a sound surface that the specified primer and screed can grip. A dusty skin, old adhesive or loose coating can undermine that bond even when the new screed is correctly laid.
Floor preparation starts with the condition of the slab and the build-up planned above it. Tell us what is on the floor now, what will replace it and whether the screed is intended to bond directly to the concrete. The preparation scope should be agreed alongside the screed, rather than left as an assumption between trades.
What the preparation needs to achieve
A suitable substrate must be sound beneath the surface as well as clean on top. Mechanical preparation can expose concrete that is ready for the next layer; it cannot make a moving or failing slab sound. Cracks, weak patches and moisture concerns need assessing before a preparation method is selected.
The required surface profile comes from the floor system specification. More aggressive removal is not automatically better. The task is to remove material that would interrupt the bond without unnecessarily cutting into sound concrete or changing levels at doorways and thresholds.
Shot blasting or concrete grinding?
Shot blasting directs abrasive shot at the concrete surface to remove laitance and light contamination, leaving an open profile. It suits preparation across accessible slab areas where the following system requires that type of key. The surface condition and the equipment's access both influence whether it is the appropriate choice.
Concrete grinding uses abrasive heads in controlled passes. It can address coating residue and local high spots, with attention to the amount removed and the finish left behind. Grinding a high point is different from correcting an entire floor level: the available slab depth and intended screed thickness still matter.
For a comparison including heavier removal methods, read our shot blasting, grinding and scabbling guide. For the lighter preparation of a cured screed rather than a concrete slab, see screed sanding and laitance removal.
Planning the work before the pour
- 1
Identify the surface
Share photographs of bare concrete, remaining coatings, adhesive and any damaged areas. Include the planned floor build-up and primer specification if available.
- 2
Agree the scope
Set out which areas need preparation, any separate repairs, and who handles edges, corners and residue removal. Confirm whether preparation is included with screeding or arranged separately.
- 3
Arrange access and protection
Clear the working area and discuss power, equipment access, dust extraction and protection of adjoining rooms. Restricted spaces can need a different approach from open floor areas.
- 4
Check before bonding
After preparation, remove remaining dust and check the exposed substrate. Apply the specified primer only when the surface and conditions meet the system's requirements.
Programme and cost considerations
The floor area is only one part of the work. Thick residue, several existing coatings, awkward edges and repeated passes can affect both time and cost. Access for equipment, occupied rooms, waste handling and the need for separate slab repairs also belong in the enquiry.
Preparation time is separate from the primer's application window and the screed's curing and drying programme. Agree the sequence with the trades following the pour so a prepared surface is not contaminated again before bonding. Where a slab cannot provide a reliable bond, a different floor build-up may be more appropriate than further surface removal.
Frequently asked
Discuss preparation for your screed
Send photographs, the floor area, existing surface and planned build-up so the preparation scope can be agreed with the screeding work.