Concrete is specified by its characteristic compressive strength at 28 days, expressed in N/mm². When a structural engineer or Building Control officer asks for “C25 concrete”, they mean a mix that will achieve at least 25 N/mm² when tested as a 150 mm cube at 28 days. That single number encodes the cement:aggregate:water ratio and drives cost — so choosing the right grade matters both for structural adequacy and for budget.
For the vast majority of domestic work — strip foundations, trench-fill, ground-bearing slabs, oversite — C25 or C30 is the correct starting point. Going lower risks long-term durability; going higher adds cost without structural benefit unless a structural engineer specifies otherwise.
What Concrete Grades Mean (N/mm²)
The “C” designation comes from BS EN 206:2013 + A2:2021, which classifies concrete by its characteristic cylinder/cube strength. UK practice uses the cube strength (the second number in the EN notation “C16/20”). In everyday UK ordering, the cube strength alone is quoted — so “C30” means a characteristic cube strength of 30 N/mm².
The cement content, water-cement ratio and aggregate type are selected by the ready-mix plant to hit that target. CEMI, CEMII or ground granulated blast-furnace slag (GGBS) blends are common; a higher cement content generally means higher strength but also higher carbon and cost.
Exposure class also matters under BS EN 206 and BS 8500-1:2015+A2:2019. Foundations in aggressive sulfate soils (class DS-3 or DS-4) need sulfate-resisting cement regardless of grade — the ready-mix supplier will ask for your soil classification.
Common Grades and Their Typical Uses
| Grade | Characteristic Strength | Typical UK Use | Cement Content (approx) |
|---|---|---|---|
| C20 | 20 N/mm² | Blinding layers, non-structural fill, kerb bedding | 280 kg/m³ |
| C25 | 25 N/mm² | Domestic strip and trench-fill foundations, lightly loaded slabs | 300 kg/m³ |
| C30 | 30 N/mm² | Ground-bearing slabs, garage floors, mass concrete retaining walls | 320 kg/m³ |
| C35 | 35 N/mm² | Suspended slabs, RC beams, columns, piled caps | 340 kg/m³ |
| C40 | 40 N/mm² | Post-tensioned slabs, heavily loaded columns, bridge decks | 360 kg/m³ |
C20 is rarely used structurally — it is most often specified for a 75–100 mm blinding layer beneath a foundation to give a clean working surface. C25 is the minimum most structural engineers specify for unreinforced domestic strip foundations under Building Regs Part A. C30 is increasingly the default for ground-bearing floor slabs to allow for plastic shrinkage and the abrasion of foot traffic.
C20 and C25 — Domestic Foundations
For a standard two-storey domestic extension with strip foundations in firm clay or gravel, C25 is almost always adequate. Trench-fill (filling the trench to within 150 mm of ground level rather than building up blockwork) is generally specified at C25 to C30 because the volume of concrete is high and the mix needs to flow easily to fill around any reinforcement.
Under NHBC Standards Chapter 4.2, concrete in foundations should have a minimum cement content of 240 kg/m³ and a maximum free water-cement ratio of 0.70 for class DC-1 soils (no aggressive sulfates). For the more common DC-2 designation seen in many suburban sites, a minimum of 280 kg/m³ and 0.65 w/c is required — which corresponds approximately to C25 or C28.
C30 — Ground-Bearing Slabs
Ground-bearing slabs in domestic construction are typically 100–150 mm thick, cast on a compacted hardcore sub-base with a 1200-gauge (0.3 mm) DPM above or below the slab. C30 provides adequate flexural strength and surface hardness for residential loads. Where underfloor heating pipes are embedded, the mix must be workable enough to flow without vibration — a slump class S3 (100–150 mm) or flow class F4 is typically specified.
For commercial or light-industrial floors, a C35 or C40 with steel fibre or reinforcing mesh is more common, accompanied by a power float finish.
C35 and C40 — Structural Reinforced Concrete
Reinforced concrete elements such as suspended ground-floor slabs, lintels, ring beams and retaining walls are typically specified at C32/40 to C40/50 (EN notation) in structural engineering. In UK ordering, this translates to C35 or C40. The increased cement content and lower water-cement ratio also improves durability and reduces chloride/carbonation ingress, which is why C40 is standard in exposed or marine environments.
Ready-Mix vs Site Mix
| Method | Typical Cost | Pros | Cons |
|---|---|---|---|
| Ready-mix (lorry) | £95–£130/m³ delivered | Consistent quality, fast, BS EN 206 certified | Minimum order 1 m³; surcharges for small loads |
| On-site drum mixer | £70–£90/m³ materials only | Flexible timing, no minimum load | Variable quality, labour intensive, not suitable for structural RC |
| Volumetric mixer | £110–£145/m³ | Exact quantity, on-site adjustment | Slower pour rate |
Ready-mix is specified for any structural element. On-site mixing using bagged cement, sharp sand and 20 mm aggregate can be adequate for non-structural fill or patching, but the water-cement ratio is difficult to control and the results should not be used where a specific grade is required.
Ordering: Slump and Workability
When ordering ready-mix, you specify:
- Grade (e.g. C25)
- Slump class or flow class — S2 (50–90 mm) for foundations, S3 (100–150 mm) for slabs with UFH pipes or congested reinforcement
- Aggregate size — 20 mm for most applications; 10 mm for narrow sections or congested rebar
- Exposure class — XC2 (wet/rarely dry, foundations) or XC1 (dry, internal slabs)
- Cement type if sulfate resistance is needed
Always confirm access with the ready-mix supplier in advance. A standard 6 m³ truck needs roughly 3 m of clearance height and 3 m width. Pumping adds approximately £200–£350 to the pour cost but can be essential for rear-of-plot pours.
Building Regulations and Standards
- BS EN 206:2013 + A2:2021 — concrete specification, performance and conformity
- BS 8500-1:2015 + A2:2019 — complementary standard for specifying concrete to BS EN 206
- BS 8500-2:2015 + A2:2019 — constituent materials and concrete making
- Building Regs Approved Document A — structural requirements; references to BS EN 1992 (Eurocode 2) for RC design
- NHBC Standards Chapter 4.2 — foundation design and concrete specification for new-build housing
Specify concrete by reference to BS 8500-1 on your drawings: for example, “RC25/30, exposure class XC2, maximum aggregate size 20 mm, target slump S3.” This gives the ready-mix plant an unambiguous specification and protects you if there is a dispute over quality.
