Grade tells you how strong concrete will be; slump tells you how it will behave while you are placing it. Order the wrong slump and a perfectly good C30 becomes a nightmare to lay — or a weak, crack-prone slab if someone “fixes” it with a hosepipe.
The slump classes
- S1 (10–40mm): very stiff — kerb bedding, haunching drainage runs
- S2 (50–90mm): firm — sloped slabs and driveways where the mix must not run downhill
- S3 (100–150mm): the standard order — flat slabs, footings, garage bases, easy to tamp and float
- S4 (160–210mm): flowing — congested reinforcement, pumped pours, trench-fill foundations
- S5 (220mm+): near self-levelling — specialist applications only
Matching slump to the job
- Driveway on a slope: S2 — see the driveway cost guide
- Shed or garage base: S3 — easy tamping to a level finish
- Trench-fill foundations: S4 — flows around the trench without vibrating — see foundations guide
- Fence posts: S2–S3 — stiff enough to hold the post braced
The water trap
The oldest mistake on site: the mix feels hard work, someone adds water, the pour gets easier — and the slab loses 10–15% of its strength and shrinks twice as much as it cures. If a mix needs to be more workable, the correct fix is a plasticiser admixture, which adds flow without adding water.
This is where volumetric mixing shines: slump is set at the point of pour and can be adjusted batch by batch. Pouring a stiff S2 apron first and a flowing S4 trench after it, from the same truck? No problem — say the word and the operator changes the mix.
Just tell us the job
You do not need to memorise slump classes — describe what you are pouring when you book online or call 01924 910 129, and we will arrive with the right grade at the right workability.
FAQs
What is concrete slump?+
Slump is the standard measure of workability — how easily fresh concrete flows. A cone of fresh concrete is lifted and the amount it sags (slumps) is measured in millimetres, grouped into classes S1 (stiff) to S5 (almost self-levelling).
What slump should I order for a slab?+
S3 (100–150mm slump) is the workhorse for domestic slabs, driveways and shed bases — wet enough to place and tamp easily, stiff enough to hold a level surface and full strength.
Does adding water to concrete weaken it?+
Yes — every extra 10 litres of water per m³ beyond the design mix can cut strength by around 5% and increases shrinkage cracking. Order the right slump instead of adding water on site.
What slump is needed for pumped concrete?+
S3–S4. Pump mixes also need a higher fines content to move smoothly through the line — tell us it is a pumped pour when booking and we will mix accordingly.