EN 206 Consistence Classes Explained
Workability — how easily fresh concrete can be placed and compacted — is specified under EN 206 as a consistence class, most commonly measured by the slump test. This page explains the S1–S4 slump classes and the alternative flow classes, and how the calculator’s consistence field uses them.
The slump classes, S1–S4
| Class | Slump (mm) | Typical description |
|---|---|---|
| S1 | 10–40 | Low workability — stiff mix, used for vibrated paving and some precast work |
| S2 | 50–90 | Medium workability — general strip footings, mass concrete |
| S3 | 100–150 | High workability — common for reinforced sections placed by chute or pump |
| S4 | 160–210 | Very high workability — congested reinforcement, pumped concrete over long distances |
EN 206 also defines an S5 class for slumps above 210 mm and a very dry, effectively no-slump condition below S1; S1–S4 cover the great majority of normal concrete work.
Flow classes, an alternative to slump
For very fluid or self-compacting concrete, slump becomes hard to measure precisely once the mix flows rather than merely subsides. EN 206 provides flow classes (F1–F6) instead, based on the spread diameter measured in a flow table test. Most conventionally placed and vibrated concrete is specified by slump class; flow classes appear mainly on self-compacting or highly flowable mixes.
Why consistence class matters for the mix
The target consistence class is one of the inputs the calculator uses, alongside maximum aggregate size and aggregate shape, to estimate the free water content needed — see water content. A higher consistence class generally needs more water for the same aggregates, which pushes up the water-cement ratio unless it is offset by more cement or a water-reducing admixture (see admixtures).
How consistence is specified and tested on UK sites
UK specifications use the same slump classes (S1–S4) and flow classes (F2–F6) as EN 206, and the tests are carried out to BS EN 12350-2 (slump) and BS EN 12350-5 (flow table). Many UK ready-mixed concrete orders simply state the consistence class alongside the designated or designed concrete, for example “RC32/40, S3, 20 mm”. It is also possible to specify a target slump value rather than a class; the specifier should make clear which approach is intended so that the acceptance tolerances on site are unambiguous.
Typical UK choices by element
| Element / method | Commonly ordered | Why |
|---|---|---|
| Strip and trench-fill foundations | S3–S4 | Trench fill needs to self-level around the trench without much vibration. |
| Ground-bearing slabs (power-floated) | S3 | Enough workability to place and level; bleed and setting time matter for finishing. |
| Walls, columns (vibrated) | S3–S4 | Placement through reinforcement cages and formwork ties. |
| Pumped concrete | S3–S4 or flow class | Pump line friction and blockage risk; confirm with the pump operator. |
| Congested reinforcement / self-compacting | Flow classes or SCC classes | SCC is specified with slump-flow classes (SF) rather than S-classes. |
These are starting points only. The specifier and the concrete producer should agree the class, because it affects the mix design, the admixture strategy and the site acceptance tests.
When to test, and what to do with a failing result
On UK sites, consistence is normally checked on arrival, from a sample taken after the first part of the load has been discharged. If the result is outside the specified class tolerance, the load should be referred back to the producer rather than “fixed” with water. Record the test result, the ticket number and the time — if strength results are later disputed, that record is what links the cubes to the concrete actually placed.
Frequently asked questions
What is the difference between the slump test and a flow test?
The slump test measures how far a cone of fresh concrete subsides under its own weight, in millimetres, and suits stiffer to fluid mixes. The flow test measures the diameter fresh concrete spreads to on a flow table, and is used for very fluid or self-compacting mixes where slump alone would just read as full collapse.
Which consistence class should I choose?
It depends on the section being cast and how it will be compacted — thin, heavily reinforced sections or pumped concrete generally need a higher class (S3–S4) than mass concrete placed and vibrated in open sections (S1–S2). The project specification will usually state the required class.
Does choosing a higher consistence class always mean adding water on site?
No — it should be built into the mix design from the start, normally with a water-reducing admixture rather than extra water, which would raise the water-cement ratio and reduce strength and durability. See common mix design mistakes.