Substrates & Preparation
New Concrete Slab Preparation Ahead of a Liquid Screed Pour
A freshly cast slab needs the right cure, a laitance-free surface for bonded work, and clean falls before liquid screed follows the groundworker.
A newly cast concrete slab is one of the more forgiving substrates for liquid screed, but 'forgiving' doesn't mean it can be poured on the day the shutters come off. The concrete needs to have taken its initial set, released its curing water at a sensible rate, and be free of the laitance and loose material that forms on the top surface as it cures.
How the slab is prepared changes depending on whether the screed is going down as a bonded system directly onto the concrete, or as an unbonded/floating system over a separating layer or insulation. Each route has a different set of checks before the pump arrives.
Groundworkers and screeders often work back-to-back on a programme, and the handover point between the two trades is where most avoidable problems start. A slab that's been rushed off the back of the lorry, still shedding bleed water, or covered in a curing membrane nobody mentioned, puts the screed contractor in the position of either delaying the pour or working around a substrate that isn't quite ready.
Bonded vs unbonded approaches
A bonded liquid screed relies on genuine mechanical and chemical adhesion to the concrete below, so the top few millimetres of laitance — the weak, cement-rich skin that forms as concrete cures — has to come off. This is usually done by diamond grinding, shot-blasting or scabbling, followed by thorough vacuuming and a compatible primer.
Where the screed sits on insulation, a DPM, or acoustic matting, the slab itself doesn't need this treatment because there's no direct bond to achieve. In that case attention shifts to flatness and cleanliness rather than surface texture, since sharp stones or debris under the membrane can puncture it during the pour.
Curing time and residual moisture
Concrete continues to give off moisture for weeks after it's cast, and an unbonded screed system with a functioning DPM largely isolates the screed from that moisture. Even so, most installers prefer the slab to be structurally sound and past its initial cure before loading it with plant and pump hose, and will check for any standing water in low spots that could compromise a DPM lap.
It's worth flagging any known concrete mix issues — retarders, admixtures, or a rushed pour — to the screed contractor in advance, as these can occasionally affect how quickly the slab is ready to be built on.
Reading the slab before committing to a method
A short site visit before the pour date lets the screeder check for the obvious tell-tale signs of a problem substrate — ponding in low spots, a shiny curing membrane still present, or a powdery, friable top surface that suggests the concrete hasn't gained enough strength yet. None of these are unusual on a live site, but spotting them early avoids a wasted pump visit.
Where the groundworker has used a spray-applied curing compound to control early moisture loss, it's worth confirming the product and when it was applied, since some compounds are formulated to break down over time while others need mechanical removal regardless of how long they've been left.
Coordinating with the groundworker's programme
Because the slab and the screed are usually delivered by different contractors, a short conversation about sequencing — when the pour happened, what the mix design was, and whether any additives were used — saves guesswork later. Retarding admixtures, in particular, can extend the period before the surface is firm enough to work on, and it's far easier to plan around this than to discover it on the day the pump is booked.
On larger slabs poured in bays, it's also worth checking whether all bays were cast to the same specification, since a mixed sequence of ready-mix deliveries occasionally produces slightly different surface characteristics from one bay to the next.
New Concrete Slabs — the points that decide it
- Bonded screeds need the laitance layer removed (scabbling or shot-blasting) to expose sound aggregate for mechanical key
- Floating and unbonded systems only need the slab to be reasonably flat and free of sharp protrusions before the separating layer goes down
- Curing compounds or plastic sheeting used to protect the pour must be fully removed before bonding — they act as a debonding agent
- Construction joints and any cracking should be assessed and, where needed, taped or detailed to control reflective cracking
- The slab's own moisture content is rarely tested unless it's being used as a bonded substrate, since the screed and DPM/insulation usually separate the two
- Falls to gullies or drainage points should be checked before the screed goes down, as liquid screed will follow the substrate's levels unless packed out
- Any oil, shuttering release agent or dust needs vacuuming or pressure washing off — self-levelling screed will find every contaminated patch
- A vapour check with a simple polythene sheet test can flag obvious surface moisture issues before committing to a bonded pour
- Shuttering marks, arris damage and construction joint arrises should be made good so they don't telegraph through a thin bonded screed
- Rebar cover and reinforcement mesh depth are a structural concern, not a screed one, but any exposed steel at the surface should be flagged before covering it
New Concrete Slabs — your questions
Got a slab going in soon and a screed to follow?
Send us the pour date and mix details and we'll advise on finish, curing and sequencing for your bonded or floating liquid screed across Devon, Somerset and Dorset.
Or call our landline on 01392 237700
In short
A freshly cast slab needs the right cure, a laitance-free surface for bonded work, and clean falls before liquid screed follows the groundworker.
Got a slab going in soon and a screed to follow?
Send us the pour date and mix details and we'll advise on finish, curing and sequencing for your bonded or floating liquid screed across Devon, Somerset and Dorset.