Concrete Grades Explained — Which Mix Does Your Job Need?
C20 or C35? GEN1 or PAV1? Here's the plain-English version from people who batch concrete for a living: what the names mean, which mix suits which job, and what each grade costs in Birmingham.
What C25/30 Actually Means
Every concrete strength class is written as two numbers, like C25/30. The first is the cylinder strength, the second the cube strength — both in newtons per square millimetre (N/mm²), measured 28 days after the pour. The UK trade works off the cube number, which is why C25/30 gets called "30-newton concrete" on site.
Builders usually drop a number and just say "C20" or "C35". That shorthand is fine over the phone, but it's how the wrong mix ends up in the ground, so we quote the class in full on every quote and every batch ticket — and translate it back into plain newtons so everyone's talking about the same concrete.
Two things the numbers don't tell you: whether the mix is air-entrained (essential for driveways — more below) and its consistence, or slump. In plain English: S1 is stiff (kerbs), S2 is normal (floors and paving), S3 is the general-purpose default (foundations and slabs), and S4 is near-pourable for trench fill.
Match the Job to the Mix
Find your job, read across. Names in brackets are industry "designated mix" names — see the note under the table for how we supply them.
| The job | The mix to ask for | Consistence |
|---|---|---|
| Blinding & mass fill | C6/8–C8/10 (GEN0–GEN1-equivalent) | S3 |
| Strip foundations, normal ground | C8/10 (GEN1-equivalent) | S3 |
| Trench-fill foundations | C8/10 (GEN1-equivalent) | S4 |
| Reinforced foundations | C25/30 (RC25/30-equivalent) | S3 |
| Foundations in sulfate ground | FND-equivalent designed mix to the soil report's DC class | Per spec |
| Oversite | C8/10 | S3 |
| Floor to be screeded | C8/10 | S2 |
| Floor with a direct finish | C12/15 | S2 |
| Unreinforced garage or shed base | C16/20 (GEN3-equivalent) | S2 |
| Reinforced garage floor | C28/35 (RC28/35-equivalent) | S2 |
| Driveway | Air-entrained C25/30 (PAV1-equivalent) | S2 |
| Heavy-duty or salted yards | Air-entrained C32/40 (PAV2-equivalent) | S3 |
| Structural & suspended work | C28/35+ to the engineer's spec | Per spec |
| Heavy industrial floors | C40/50 | S3 |
Defaults worth knowing: 20mm aggregate is standard, S3 is the general-purpose consistence, and you should allow 5–10% extra volume. Work yours out with the concrete calculator — and because we batch on site, over-estimating costs you nothing. We bring spare material; you only pay for what's poured.
GEN, RC, PAV and FND are "designated mixes" — names that may only be supplied as such by producers holding third-party certification (QSRMC or BSI Kitemark). We don't hold that certification, and we won't pretend to. We sell by strength class — C20 to C40 — as designed mixes batched to the strength and durability requirements of BS 8500 / BS EN 206, and we use the designated names on this page educationally so you can translate a spec into an order. If your specification strictly requires certified designated concrete (NHBC and National Highways specs do), tell us — we'll say so up front and advise.
GEN, RC, PAV and FND in Plain English
GEN — general purpose, unreinforced only
The GEN range covers everyday unreinforced work. GEN0 (C6/8, "7.5-newton") is for blinding, mass fill and kerb bedding. GEN1 (C8/10) does unreinforced strip and trench-fill foundations in normal ground, oversite and floors that will be screeded. GEN2 (C12/15) suits house floors with no permanent finish, and GEN3 (C16/20) handles unreinforced garage, shed and workshop bases. The rule that matters: anything with embedded metal counts as reinforced — one layer of mesh and you've left GEN territory.
RC — when there's steel in the pour
Reinforced concrete classes climb with the duty: RC20/25 for reinforced house floors, RC25/30 for reinforced foundations, RC28/35 for reinforced garage floors, suspended floors and livestock floors, RC32/40 for industrial wearing surfaces, RC35/45 for parlours, dairies and stables, and RC40/50 for heavy industrial work.
PAV — paving, and why air matters
PAV1 (C25/30, S2) is the driveway and domestic parking mix; PAV2 (C32/40, S3) covers heavy-duty and salted paving. Both are air-entrained: a minimum 4.5% of microscopic air bubbles (at 20mm aggregate) worked into the mix. When trapped water freezes it expands into those bubbles instead of blowing the surface apart — which is exactly how plain-concrete driveways fail, scaling and flaking after a few winters. One caveat: air-entrained concrete must not be power-floated.
FND — for ground that fights back
FND mixes are all C25/30 strength; what changes is their chemistry, matched to the DC class in a ground investigation report (DC-2 calls for FND2, and so on, with z-suffix variants for acidic ground). They're triggered by soil reports on sulfate clays, pyrite or brownfield made ground — never chosen by eye. Former industrial land around Birmingham and the Black Country can be exactly this territory: if your report specifies a DC class, we batch to it.
ST Recipes — and Who Carries the Risk
You'll also see ST1–ST5: standardised prescribed recipes that any producer may lawfully supply by name without certification. Their strengths are indicative only — ST2 is roughly 10-newton territory and ST5 caps out around C20/25 — and the fixed recipes use generous cement contents, which means a higher carbon footprint than a designed mix doing the same job. Some trade sites treat ST names as synonyms for C-grades; they aren't, and we won't quote them that way.
Designated, designed, prescribed — the three ways to buy
Designated means ordering by application name (GEN1, PAV1) from a certified producer. Designed means the producer designs the mix to hit your performance spec and is responsible for it doing so — that's what we supply. Prescribed means you dictate the exact recipe and carry the risk yourself. For almost every domestic and light commercial job, a designed mix by strength class is the straightforward answer.
Our Grades, Prices and Guides
Every grade page carries its guide price, what the grade is for — and what it isn't for.
| Grade | What we batch | Guide price (per m³, +VAT) | Typical jobs |
|---|---|---|---|
| C20 concrete | C16/20 designed mix ("20-newton") | From £105 | Shed & workshop bases, unreinforced garage floors |
| C25 concrete | C25/30 designed mix ("30-newton") | From £110 | Reinforced foundations & footings |
| C30 concrete | C25/30 family, durability focus; air-entrained for paving | From £115 | External slabs, driveway mixes |
| C35 concrete | C28/35 designed mix ("35-newton") | From £120 | Reinforced garage floors, suspended slabs |
| C40 concrete | C32/40–C40/50 designed mixes ("40-newton and up") | From £128 | Industrial floors & heavy-duty yards |
Sharp-eyed? Yes — C25 and C30 orders both land in the C25/30 family: one name comes from the cylinder half of the designation, the other from the cube half. That's the trade for you, and it's why we quote classes in full. Below C20, our GEN1-equivalent C8/10 designed mix starts from £99/m³ +VAT. All prices are guide prices, confirmed when you book — the full picture is in our ready mix concrete prices guide.
Prefer to start from the job instead of the grade? Read concrete for foundations, concrete for driveways or concrete for shed & garage bases.
Concrete Grade FAQs
What is GEN 1 concrete?
GEN1 is a designated-mix name for general-purpose concrete in the C8/10 strength class — "10-newton" concrete in trade speak. It suits unreinforced strip and trench-fill foundations in normal ground, oversite, and floors that will be screeded. It's for unreinforced work only. Designated names like GEN1 may only be supplied as such by certified producers, so we batch an equivalent C8/10 designed mix to BS 8500 / BS EN 206 — same strength class, honestly named. Ours starts from £99/m³ +VAT (guide price).
What is PAV1 concrete?
PAV1 is the designated mix for driveways, domestic parking and external paving that won't see de-icing salts: C25/30 strength ("30-newton"), air-entrained, S2 consistence. The air is the point — a minimum 4.5% of microscopic bubbles (at 20mm aggregate) gives freezing water room to expand, so the surface doesn't scale off in frost. We batch an equivalent air-entrained C25/30 designed mix. One caveat: air-entrained concrete must not be power-floated.
What is GEN 3 concrete?
GEN3 is the designated-mix name for C16/20-strength concrete — "20-newton" — used for unreinforced garage, shed and workshop bases. We batch an equivalent C16/20 designed mix; see our C20 concrete page for uses and prices. Remember GEN territory is unreinforced only: add mesh or rebar and you need a reinforced mix instead.
What do the numbers in C25/30 mean?
The first number is the cylinder strength and the second the cube strength, both in newtons per square millimetre (N/mm²) measured 28 days after the pour. The UK trade quotes the cube figure, which is why C25/30 is called "30-newton" concrete on site.
Can you supply GEN, RC, PAV or FND mixes by name?
Not under those names — and we'd rather tell you that than fudge it. Designated mixes may only be supplied as such by producers holding third-party certification (QSRMC or BSI Kitemark). We supply designed mixes by strength class — C20, C25, C30, C35, C40 — batched to the strength and durability requirements of BS 8500 / BS EN 206, and we use the designated names educationally so you can translate a spec. If your specification strictly requires certified designated concrete (NHBC and National Highways specs do), tell us and we'll say so up front and advise.
Which concrete grade do I need for a driveway?
An air-entrained C25/30 designed mix (PAV1-equivalent) at S2 consistence, laid around 150mm deep. Plain C20 driveways scale in frost — the surface flakes off after a few winters — because they lack entrained air. See our concrete for driveways page for the full picture.
Still Not Sure Which Grade You Need?
Tell us the job — we'll tell you the mix, the volume and the price, usually in one call. Same-day slots often available, from just 0.5m³.