Get the concrete mix ratios wrong and you will know about it. Maybe not on day one, but eventually a crumbling path, a sunken post or a cracked slab will tell the story. I’ve patched enough botched pours over the years to know that most mix mistakes come down to two things: guessing instead of specifying, and ignoring what the British weather is doing overhead. This guide covers the mixes that matter for the jobs you’re actually doing on site.
What do C20, C25 and C30 actually mean?
These are designated concrete strength classes, measured by compressive strength in Newtons per square millimetre (N/mm²). The number is the characteristic compressive strength of a 150mm cube tested at 28 days. So C20 has a target strength of 20 N/mm², C25 hits 25 N/mm², and C30 reaches 30 N/mm². The higher the class, the stronger and denser the mix, which also means less water, more cement, and generally more cost.
When you order ready-mix from a batching plant, you specify the strength class and they do the rest. When you’re site-batching by hand or with a mixer, you’re working with volumetric ratios of cement, sharp sand and aggregate. The two approaches need to align with the same end goal: the right strength for the application in front of you.
Concrete mix ratios for the most common jobs
Strip foundations and mass concrete footings
For most domestic strip foundations, C25 is the standard. The site-batch equivalent is roughly 1:2:3 by volume (cement, sharp sand, 20mm aggregate), with a water-cement ratio kept as low as you can get away with while still achieving workability. If you’re building on shrinkable clay or near trees, your structural engineer may push you to C30. For reference, the Building Regulations Approved Documents give guidance on minimum foundation requirements, though your building inspector will have the final say on site.
One thing I always tell lads on foundation pours: don’t add water to make it easier to place. A sloppy mix increases the water-cement ratio and drops the finished strength significantly. Use a plasticiser if you need better workability.
Floor slabs
Domestic floor slabs typically call for C25. A garage slab that’s going to take vehicle loads should be C30 with a minimum 150mm thickness. For industrial or workshop floors, C30 minimum is the starting point and you’d want a steel mesh throughout. The site-batch mix for C30 runs closer to 1:1.5:2.5, which is noticeably stiffer than a C25 pour. Plan your team and your timing accordingly, especially in warm weather when the clock is against you.
Paths, driveways and patio bases
C20 is the minimum for a pedestrian path, with a mix of around 1:2:4. For a driveway that’s going to take regular vehicle traffic, step up to C25 at minimum, laid at 100mm. Anything less and you’re likely to see cracking within a few winters once frost gets into surface cracks and starts doing its work. This is one area where the British climate is genuinely punishing, freeze-thaw cycling breaks down a weak surface mix fast.
A lot of tradesmen now specify a C25 air-entrained mix for external flatwork. The entrained air bubbles give the set concrete somewhere for water to expand into during a freeze, which drastically reduces surface spalling.
Fence posts and gate posts
C20 is more than adequate for setting fence posts in most cases, and the 1:2:4 site-batch mix works fine here. The key is getting the post plumb before the mix kicks and leaving it undisturbed for at least 24 hours, ideally 48 in cold weather. For heavy gate posts or anything structural, step up to C25 and pack the mix tightly around the post to eliminate voids.
In-situ concrete for small walls and mass fill
Backfill and mass concrete beneath steps or small retaining features: C20 is your benchmark. No need to overspec this stuff and burn through expensive cement. That said, if water is likely to sit against the pour, consider a C25 with a higher cement content for improved impermeability.
Ready-mix versus site-batched concrete: which should you use?
Ready-mix wins on consistency. Every load from a batching plant is mixed to a certified specification, and you get a delivery note showing the strength class, slump value and admixture details. For pours above half a cubic metre, I’d almost always recommend ready-mix. The logistics are simpler, the quality is reliable, and you’re not relying on someone accurately shovelling aggregate by eye at 7am.
Site-batching makes sense for small pours, a bag or two for a post, a small repair, filling a void. But you have to be honest about the accuracy of hand-batching. A volumetric mixer is better than a drum mixer for achieving a consistent ratio, but neither gives you the precision of a batching plant. If the job has structural implications, the accuracy of ready-mix matters.
For tradesmen who are building their profile and winning work through referrals and content, some even use professional guest posts to establish credibility in their trade, which is a reasonable modern approach to business development. But on site, credibility comes from getting the specification right the first time.
How UK weather affects concrete curing
This is where a lot of site pours go wrong, and it’s almost entirely avoidable. Concrete cures through a chemical reaction called hydration, not by drying out. Temperature has a big effect on how fast that reaction happens.
Below 5°C, hydration slows dramatically. Below 0°C, fresh concrete that hasn’t yet gained enough strength can freeze and suffer permanent damage to its internal structure. The Concrete Society recommends that fresh concrete be protected from freezing until it has reached at least 5 N/mm² compressive strength, which can take several days in cold conditions. In winter, cover pours with hessian and polythene sheeting, use heated water where possible, and avoid pouring when the ground temperature is below 2°C.
Hot weather causes the opposite problem. In a British summer heatwave (yes, they happen), rapid evaporation can cause plastic shrinkage cracking before the surface has cured. Cover the pour with damp hessian, mist the surface, and avoid pouring in direct sun during the hottest part of the day. Curing should continue for a minimum of seven days on most structural pours, and longer for C30 and above.
Admixtures worth knowing about
Most tradesmen use water and cement. A growing number are adding admixtures, and it’s worth understanding the main ones. Plasticisers improve workability without increasing water content. Accelerators speed up the early strength gain, useful in cold weather. Retarders slow the set, useful for large pours in warm conditions or where you need extended working time. Waterproofing admixtures (integral waterproofers) reduce the permeability of the set concrete, relevant to applications like basement walls or planters where ground moisture is a constant pressure.
None of these replace getting the base mix ratio right. They modify behaviour, they don’t rescue a bad mix.
A quick reference for common mix ratios on site
Strip foundations (domestic): C25, mix 1:2:3. Floor slabs (standard domestic): C25, mix 1:2:3. Floor slabs (vehicle loads): C30, mix 1:1.5:2.5. Paths and driveways: C20 to C25 depending on traffic, mix 1:2:4. Post setting: C20, mix 1:2:4. Mass fill and backfill: C20, mix 1:2:4.
These are starting points. If your building inspector or structural engineer specifies something different, go with their specification. The above covers the vast majority of domestic and light commercial work a UK tradesman encounters day to day.
If you’re also specifying pipework around your concrete pours, the sizing and connection rules covered in our guide on soil pipe sizing and drainage connections is worth having open at the same time. And for floor builds that go over a slab, understanding the difference between wet and dry systems is covered in the piece on underfloor heating options for UK homes.