Concrete Mix Ratios: Foundations, Posts, Floor Slabs and Structural Mixes Explained

Quick Answer: Standard UK concrete mixes are described by ratio (cement : sand : aggregate) or by characteristic strength class to BS 8500. C20/25 (1:2:4 by volume) is the workhorse for non-structural slabs and post-fixing; C25/30 (1:1.5:2.5) for foundations and floor slabs to Approved Document A; C30/37 for heavy-duty floors and reinforced foundations; ST2 (post mix or 1:6 with sharp sand) for fence posts. Always specify by strength class on structural work — ratios are for site mixing only.

Summary

Concrete is the construction material that goes wrong silently. A foundation poured at 1:3:6 instead of 1:2:4 looks identical on day one, takes weight at week one, and cracks under sustained load at year ten. By that point the cause is invisible and the remedy is a £15,000 underpinning job. The British Standards system (BS 8500-1, BS 8500-2, BS EN 206) replaces the old 1:2:4 ratio language with strength classes (e.g. C25/30) that are tested for compressive strength under controlled cubes — and most modern engineering specifications speak only in strength class.

This guide bridges both worlds: the volumetric ratios that are still useful on small site jobs (post-mixing, oversite, minor works) and the strength classes used in foundations, structural slabs, and any work requiring an engineer's certificate. It covers the common UK mixes, slump and workability for hand-place vs pump, durability classes for ground contact, and the worked examples for fence posts, garden walls, garage slabs, and small extension foundations.

The single biggest mistake in DIY mixes is the water-cement ratio. Adding too much water to make the mix easier to lay is the most common cause of weak concrete — every extra 10% water can drop strength by 30–50%. The professional mix specifier is more likely to add a plasticiser admixture than water; the DIY mix adds water from the hose. Knowing the right slump for the application is the difference between concrete that lasts a generation and concrete that powders out at year five.

Key Facts

Quick Reference Table

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Mix Class / Ratio Application Strength (MPa) Cement Content (kg/m³)
GEN 1 (1:3:6) Kerb bedding, blinding 7.5 180–220
GEN 2 (1:2.5:5) Garden paths, oversite 10 220–260
GEN 3 / C20/25 (1:2:4) Non-structural slab, post bedding 20–25 280–320
C25/30 (1:1.5:2.5) Foundations, floor slabs 30 320–360
C28/35 Reinforced strip foundations 35 340–380
C30/37 Garage slab, drive slab 37 360–400
C32/40 Suspended slab, structural 40 380–420
C40/50+ Heavy structural, basements 50+ 420–480

(Cement content figures are typical for plain concrete; reinforced concrete may use higher binder content per BS 8500-1.)

Detailed Guidance

Reading a structural specification

A structural engineer's drawing will specify concrete by strength class, exposure class, and consistency, e.g.:

Concrete: C25/30, XC2, S3, Dmax 20mm

This reads as:

Volumetric ratios from a textbook ("1:1.5:2.5") cannot be relied on to deliver the engineer's specification — order from a ready-mix supplier with the BS 8500 designation, get a delivery ticket showing strength and exposure class, and keep the ticket for your records and Building Control.

Site-mixed concrete — when ratios are appropriate

Volumetric mixing on site is fine for:

Site-mixed concrete is not appropriate for:

For these, ready-mix to the specified BS 8500 designation is the only compliant route.

Volumetric mixing — the practical method

For a 1:2:4 mix (GEN 3 / C20/25 equivalent):

Material Per 25kg cement bag Per m³
Cement (25kg bag) 1 7 bags (175kg total) — site shows 7–8 bags
Sharp sand 2 buckets (~50kg) 0.5m³ (~750kg)
20mm aggregate 4 buckets (~100kg) 1.0m³ (~1,500kg)
Water ~12L 90–110L

Mix order: dry materials first (sand + aggregate), then add cement and dry-mix until consistent grey, then add water gradually until target consistency. Test by squeezing a handful — should hold shape but not crumble.

Water-cement ratio — the strength killer

Adding water to make the mix easier kills the concrete. The relationship between water-cement ratio (W/C) and strength is approximately:

W/C Ratio Approx. % of design strength Workability
0.40 100% Very stiff, needs vibration
0.45 95% Stiff, vibrate or rod
0.50 85% Hand-place, tamp
0.55 70% Easy hand-place
0.60 55% Sloppy — too wet
0.65+ 35% or less Watery — failed

If concrete is unworkable at the design W/C, add a plasticiser admixture (£3–£8/L, dose 0.3–1.0% of cement weight) — never water. Plasticiser turns a 0.45 W/C mix into something that flows like a 0.55 W/C mix without the strength penalty.

Worked examples

Example 1 — Fence post bedding (4 posts at 600mm depth × 250mm dia)

Example 2 — Garden shed base (3.0m × 2.4m × 100mm slab)

Example 3 — Strip foundation for single-storey extension (12m × 0.6m × 0.3m)

Example 4 — Garage slab (6m × 3m × 150mm with reinforcement)

Curing — the missed half of the job

Concrete strength develops over 28 days. The first 7 days are critical — water loss through evaporation halts the hydration reaction and concrete that "sets" looks fine but never reaches design strength.

Cure by:

In hot weather (>20°C), accelerated drying can lose 20–30% of design strength if uncovered. In cold weather (<5°C), hydration stops and concrete can permanently freeze — protect with insulating blankets or use cold-weather admixtures.

For homeowners — when do I need ready-mix?

A 1m³ ready-mix delivery (the typical minimum) costs £140–£200 for non-structural; £180–£260 for C25/30 foundation mix; £200–£300 for C30/37 with admixtures or fibres. Pumping adds £350–£550 for a half-day boom pump.

Frequently Asked Questions

Can I use building sand for concrete?

No. Building sand (also called soft sand or plasterer's sand) has fine particle size and high silt content — it produces weak concrete that holds water and crumbles. Use sharp sand (concreting sand, BS EN 12620) for all concrete mixes. Building sand is for mortar only.

What's the difference between C20/25 and C25/30?

The first number is the cylinder strength (laboratory test on a cylindrical sample); the second is the cube strength (UK standard test on a 150mm cube). Both are characteristic strengths at 28 days under controlled curing — meaning 95% of test samples will exceed this value. C25/30 is one strength class above C20/25 and uses a slightly higher cement content.

Is post-mix really the same as concrete?

Post-mix is a pre-blended dry mix of cement, sand and aggregate plus an accelerator. It's a very lean, fast-setting mix designed only for fence posts. Strength at 28 days is roughly equivalent to GEN 1 (7.5 MPa) — adequate for a fence but inadequate for any structural use. Don't use post-mix for shed bases or garden walls.

Can I add an extra bag of cement to make the mix stronger?

Up to a point. Increasing cement content from 1:2:4 to 1.2:2:4 (a "rich" mix) does increase strength and durability, but only with the correct water-cement ratio. Adding cement and proportionally more water actually weakens the mix. Adding cement without admixture and reducing water makes the mix unworkable. Better: order a higher strength class from ready-mix.

What's the difference between sharp sand and ballast?

Ballast is a pre-blended mix of sharp sand and 20mm aggregate (typically 40:60 ratio), sold as one product. Mixing 1 part cement to 4 parts ballast gives roughly a 1:1.5:2.5 mix — close to C25/30 strength. Useful for site work where you want fewer materials to handle. Specify "all-in ballast 0/20" from the merchant.

Regulations & Standards