When you order concrete, you will often be asked what slump you need. Slump describes the concrete’s workability, meaning how easily it flows and can be placed and compacted. Ordering the right slump class makes the pour easier and helps the concrete reach its intended strength.
What Is a Slump Test?
In a slump test, fresh concrete is placed in a cone-shaped mould, the cone is lifted, and the distance the concrete “slumps” is measured in millimetres. A small drop means a stiff mix; a large drop means a more fluid mix.
Slump Classes S1 to S5
| Class | Slump (mm) | Typical uses |
|---|---|---|
| S1 | 10–40 | Semi-dry: kerb bedding, haunching, pipe bedding |
| S2 | 50–90 | Moist: simple strip footings, hand-placed slabs |
| S3 | 100–150 | Wet: trench-fill foundations, general slabs, many pumped pours |
| S4 | 160–210 | High workability: congested reinforcement, some pumped applications |
| S5 | 220+ | Very high workability: specialist uses, usually with admixtures |
Slump ranges follow BS EN 206 consistence classes. Very fluid mixes (S4–S5) can segregate if not designed properly, so they are produced with suitable admixtures rather than extra water.
Why You Should Never Just Add Water
Adding water on site makes concrete easier to place but weakens it, increases shrinkage and can cause cracking. If you need a more workable mix, order the correct slump class or a mix with plasticisers.
Which Slump Do You Need?
- Foundations: S3 is common for trench fill because it flows and self-levels better.
- Slabs and driveways: S2–S3 depending on placement and finishing method.
- Pumped concrete: usually S3 or higher with a pumpable mix design. See concrete pump hire.
- Specialist pours: ask about our specialist mixes.
Slump vs Strength
Slump and strength are separate. Strength is set by the grade (for example C25 or C30), while slump describes workability. You will usually specify both. See concrete grades explained.
Frequently Asked Questions
What slump is best for a house foundation?
Trench-fill foundations commonly use S3, but follow your engineer’s or Building Control’s specification.
Does higher slump mean weaker concrete?
Not if the mix is designed for it with admixtures. It only weakens if extra water is added.
Need Concrete Delivered in London?
Call 020 8129 0605 or 07723 319888, email sales@metro-ready-mix.com or request a free quote.
Sources: Concrete slump test, Pavingexpert.