Concrete Mix Ratios UK: Foundations, Slabs & Paths Guide

Quick Answer: The mix is specified by strength class to BS 8500-1:2023 and BS EN 206:2013+A2:2021. Use C20/25 (GEN 3) for unreinforced foundation strips, C25/30 (RC25/30) for reinforced foundations and ground slabs, C30/37 (PAV1/PAV2) for external paving and drives, and C16/20 (GEN 1) for kerb beds and binding layers. The corresponding nominal volume mixes are 1:2:4 (cement:sand:aggregate) for general structural and 1:3:6 for binding and oversite — but for any job inspected by Building Control, specify by class, not ratio.

Summary

UK concrete is no longer specified the way most older textbooks describe it. The shift from "1:2:4" volume ratios to "C25/30" strength class happened with BS 8500 and is now the basis of every Building Control approval, ready-mix delivery ticket and structural calculation. The volume ratios still get used on small sites where concrete is mixed on a barrow or in a small mixer, but for foundations, structural slabs and any reinforced work, you order designated or designed mixes by class.

Two numbers separated by a slash give the strength class: the first is the cylinder strength (e.g. C25), the second the cube strength (e.g. C30), both at 28 days in N/mm². UK practice has historically used cube strength, so the second number is what older specifications refer to.

For a tradesperson the practical issue is matching the specification to the application. Over-specifying is harmless but expensive (and slows curing on small pours). Under-specifying — particularly using GEN 1 binding mix for a structural slab — will fail inspection and may need cutting out.

Key Facts

• Strength classification — Cylinder/Cube in N/mm² (e.g. C25/30 = 25 cylinder, 30 cube at 28 days)
• BS 8500-1 governs UK concrete specification, conformity and production
• BS EN 206 is the European framework; BS 8500 is the UK companion
• Designated mixes — GEN 0–3, RC25/30 to RC50/60, PAV1, PAV2, FND2–FND4
• Slump classes — S1 (10–40mm), S2 (50–90mm), S3 (100–150mm), S4 (160–210mm)
• Workability — most foundation work uses S2 or S3; floor slabs S3
• Aggregate sizes — typically 20mm coarse aggregate (10mm where reinforcement is dense)
• Cement — CEM I (Portland), CEM II (Portland-composite), CEM III (Blastfurnace) — mix designs adjust replacement levels
• Air entrainment — required for external paving exposed to freeze-thaw (PAV1, PAV2)
• FND mixes — sulfate-resistant foundation concretes (FND2, FND3, FND4) for aggressive ground conditions
• Curing time — strikes formwork from 1 day (sides) to 7 days (soffits); full strength at 28 days
• Minimum cement content — varies by exposure class, typically 240–360 kg/m³
• Maximum water/cement ratio — typically 0.55–0.65 depending on exposure
• Standard ready-mix lorry — 6m³ or 8m³; minimum order usually 1m³ + per-m³ price
• Volumetric (mix-on-site) lorry — supplies exact volume, useful for small or split pours

Quick Reference Table

Detailed Guidance

Reading a delivery ticket

A ready-mix delivery ticket records the legally required information for the pour. Always check the ticket before discharge:

Customer: ..........................
Site: .............................
Order ref: ........................
Designation: RC25/30   (or GEN 3 / PAV1 / FND2)
Strength class: C25/30
Slump class: S3 (target 100-150mm)
Max aggregate: 20mm
Cement type: CEM II/A-LL 32,5R
Volume: 4.5 m³
Time mixed: 09:42
Time on site: 10:18    (max 90 min from mix)

If the lorry arrives more than 90 minutes after batching the load should normally be rejected — concrete loses workability and re-tempering with water on site is not allowed.

Volume ratio mixes (small mixer / barrow)

For tradespeople mixing on site without a structural specification — typically post bases, oversite blinding, small kerb beds — these ratios are the long-standing rule of thumb. Volumes are loose, not weighed.

• 1:3:6 (cement:sand:20mm) — binding mix, approx. C8/10 / GEN 1
• 1:2:4 — general-purpose, approx. C20/25 / GEN 3
• 1:1.5:3 — high-strength, approx. C30/37
• All-in ballast — typical mix 1:5 or 1:6 ballast:cement (replaces sand + stone with pre-mixed aggregate)

Water content is the variable that wrecks site mixes. A wet, sloppy mix loses 5–10 N/mm² of strength compared to the same mix at the correct water content. Aim for plastic but not flowing — the trowel mark should stand up cleanly.

Exposure classes — why specification matters

BS 8500-1 lists exposure classes (XC1, XC4, XS, XF1, XF4, XD, DS-1 to DS-5, etc.) that determine cover to reinforcement, minimum cement content and maximum water/cement ratio. The tradesperson does not normally pick these — the structural engineer or designer does — but understanding them helps explain why a particular specification has been given.

Common ones:

• XC1 — internal, dry concrete (suspended slabs under floor finish)
• XC2 / XC3 / XC4 — exterior or buried concrete subject to carbonation
• XF1–XF4 — freeze-thaw exposure (rising severity)
• XD / XS — chloride exposure (de-icing salts, marine)
• DS-1 to DS-5 — sulfate-bearing ground (drives selection of FND2/3/4 sulfate-resistant concretes)

Sulfate-bearing ground (FND mixes)

Before any new build foundation, a site investigation should identify sulfate concentration in soil and groundwater. Results are reported as DS-1 (no special protection) up to DS-5 (extreme). The most common case in domestic work is DS-2 — moderate sulfate — which requires an FND2 mix (sulfate-resisting cement, typically CEM IIIA or CEM IV/B-V). On DS-4 / DS-5 ground, additional protection (membrane wrap, increased cover) is required. Specifying GEN 3 in sulfate ground will lead to long-term concrete deterioration even if the pour passes a 28-day cube test.

Reinforcement and cover

For RC mixes, the cover to reinforcement (measured to the outermost steel) is specified by exposure class:

Reinforcement spacers (plastic chairs, concrete blocks, wire chairs) hold the cage at the correct cover. Cover that is too small lets carbonation reach the steel within 20–30 years, causing rust expansion and concrete spalling.

Curing

Strength gain depends on cement hydration, which needs water and moderate temperature. Practical curing rules:

• First 3 days — protect from drying wind, hot sun, frost
• Polythene cover — for slabs, lay polythene immediately after final tamp/float
• Hessian wetting — for hot weather, dampen hessian on the surface
• Cold weather — keep concrete above 5°C for 72h; insulating blankets or accelerator admixture may be required
• Frost — fresh concrete frozen within 24h is permanently damaged; never pour below 2°C and falling

Premature strength milestones:

• 24 hours — strip formwork sides, light foot traffic possible
• 3 days — partial loading possible at engineer's direction
• 7 days — approx. 65–75% of 28-day strength
• 28 days — design strength (used for cube tests)

Cube and slump testing

For larger pours, the structural engineer may require cube tests. The tradesperson's role is to allow the testing technician access and provide a clean, level location for slump and cube sampling. Slump testing follows BS EN 12350-2; cube testing BS EN 12390-3. Cubes are usually taken at 1 per 50m³ or 1 per day's pour, whichever is more frequent.

Volumetric vs barrelled (drum) ready-mix

• Drum / barrelled lorry — concrete batched at the plant, mixed in transit; loses workability after 90 minutes
• Volumetric / on-site mix lorry — raw materials in separate compartments, mixed at point of discharge; pay only for what is used, no waste or shortage

Volumetric is increasingly used for domestic extensions and small footings — minimum delivery as little as 0.5m³, no waste if the trench measures short, and unused capacity isn't paid for.

Frequently Asked Questions

Can I order "C20" concrete?

No — the full designation includes both cylinder and cube strength: C20/25, C25/30 etc. Saying just "C20" is ambiguous (cylinder or cube?). UK ready-mix suppliers will ask you for the BS 8500 designation (GEN 3, RC25/30, PAV1 etc.) — these designations have the strength class baked in.

What's the difference between GEN, RC and PAV mixes?

• GEN — designated mixes for general unreinforced work (GEN 0 to GEN 3)
• RC — designated mixes for reinforced concrete with controlled cement content and W/C ratio
• PAV — paving mixes with air entrainment for freeze-thaw resistance (PAV1 lighter, PAV2 heavier)
• FND — foundation mixes designed for sulfate-bearing ground (FND2, FND3, FND4)

How much concrete do I need for a 4m × 3m × 0.15m slab?

Volume = 4 × 3 × 0.15 = 1.8 m³. Order 5–10% extra for over-dig and waste — most ready-mix suppliers add a per-m³ minimum charge, so over-ordering by 0.2 m³ usually beats running short.

Can I add water on site to make it easier to work?

No — adding water on site (re-tempering) is not allowed under BS 8500. Water above the W/C ratio reduces strength sharply and voids the conformity of the load. The correct response to a stiff load is a superplasticiser admixture added at the plant before dispatch, or use of a workability retarder for hot weather.

What ratio of all-in ballast and cement do I use?

For general purpose concrete, 1 part cement to 5–6 parts all-in ballast (which is pre-mixed sharp sand + 20mm gravel). For higher strength, 1:4. Add water until the mix is plastic but not soupy — about 0.5 part water for an average mix.

Regulations & Standards

• BS EN 206:2013+A2:2021 — Concrete: Specification, performance, production and conformity

• BS 8500-1:2023 — Concrete: complementary British Standard to BS EN 206 — Part 1: Method of specifying and guidance for the specifier

• BS 8500-2:2023 — Part 2: Specification for constituent materials and concrete

• BS EN 12350 parts 1–10 — Testing fresh concrete (slump test, compactability)

• BS EN 12390 parts 1–8 — Testing hardened concrete (compressive cube strength)

• BS 8110 (superseded) and Eurocode 2 BS EN 1992 — Structural concrete design

• Building Regulations Part A — Structural safety; specifies foundation design

• BRE Special Digest 1 — Concrete in aggressive ground (sulfate classification DS-1 to DS-5)

• CDM Regulations 2015 — Manual handling of concrete materials, working with wet cement (Schedule 3 PPE)

• British Standards Institution — BS 8500-1 — UK concrete specification standard

• The Concrete Centre — UK industry guidance on mix design and applications

• BRE Special Digest 1 — Concrete in aggressive ground — Sulfate ground classification

• Mineral Products Association — Ready-mix industry technical bulletins

• Cement Industry Federation / Cembureau — Cement type classification (CEM I to CEM V)

• HSE — Cement and concrete safety — Wet cement skin and respiratory hazards

• concrete mix designs groundworks — Deeper dive into structural mix design parameters

• foundations — Foundation types and depths

• structural calculations guide — How structural mixes are specified by engineers

• soil types and bearing capacity — Sulfate classification and ground sampling

• site survey setting out — Setting out foundation pours