Ready-Mix Concrete C30: When Your Structural Engineer Says You Need More Than Standard
C30 concrete costs £120 – £145 per m³ delivered. When your SE specifies it, what to order, and the PAV1 trap that catches homeowners.
Your structural engineer's calculations arrive and the foundation schedule says "RC30/37." You've been reading about C25 being the standard domestic grade, so this looks like a step up. It is. And if you order the wrong product, you'll have concrete in a trench that doesn't meet the specification, a building control officer who won't sign it off, and a very expensive day of breaking out hardened concrete.
The confusion isn't academic. Concrete suppliers list "C30/PAV1/ST3" as a single product on their websites. But PAV1 is a paving grade with air bubbles deliberately mixed in for frost resistance. Your structural engineer specified RC30/37, which is a completely different mix designed for reinforced structural elements. Same strength number. Different product. Order the wrong one and you've got a problem.
What C30 concrete is and why your SE specified it
C30 concrete has a characteristic compressive strength of 30 N/mm² (measured on a cylinder) or 37 N/mm² (measured on a cube) at 28 days. That's why you'll see it written as C30/37. That's about 50% stronger than the C25 grade used in standard domestic strip foundations, and strong enough to handle the concentrated point loads from steel beams or the reinforcement cages in pad foundations.
The strength comes from more cement and less water. A C30 mix uses a minimum of 280 kg of cement per cubic metre with a maximum water-to-cement ratio of 0.60. Compare that to C25's 260 kg/m³ minimum. More cement means more chemical reaction, tighter internal structure, lower permeability, and better durability.
Your structural engineer specifies C30 (or more precisely, an RC-designated mix at C30/37 strength) when the loads or conditions demand it. The most common triggers on domestic extension projects are:
- Pad foundations for steel beams (RSJs) - these concentrate heavy point loads into small footings, requiring higher-strength concrete
- Reinforced raft foundations - where ground conditions are poor or variable (soft clay, filled ground), the whole floor slab becomes a single structural raft
- Sulphate-bearing ground - if your soil investigation reveals high sulphate levels (classified DS-3 or DS-4 under BS 8500), the engineer specifies FND3 or FND4 mixes, which start at C30/37 minimum strength
- Suspended ground floor slabs - where the slab spans between supports rather than sitting directly on the ground
The designation system: why "C30" alone isn't enough
This is where homeowners get confused, and where concrete supplier websites make things worse. There are three overlapping systems for describing concrete in the UK, all governed by BS 8500.
C-class is just a strength target. C30 means "this concrete will reach 30 N/mm² cylinder strength at 28 days." It says nothing about the mix recipe, durability, or intended use.
Designated mixes are complete recipes with guaranteed minimum cement content, maximum water ratio, and specific properties. The families are:
| Designation | What it's for | Key feature | Typical domestic use |
|---|---|---|---|
| GEN (GEN1-GEN3) | General, non-structural work | Cheapest option | Drainage haunching, path foundations |
| RC (RC25-RC50) | Reinforced concrete structures | Guaranteed cement content and w/c ratio for structural durability | Reinforced foundations, pad footings, floor slabs |
| PAV (PAV1-PAV2) | Paving and external surfaces | Air-entrained for freeze-thaw resistance | Driveways, external slabs exposed to de-icing salt |
| FND (FND2-FND4) | Sulphate-resistant foundations | Sulphate-resistant cement, minimum C30/37 | Foundations in sulphate-bearing clay soils |
ST-class (standardised prescribed mixes) is an older system you'll still see on supplier websites. ST3 maps to PAV1, not to RC30. This is the trap.
When your structural engineer writes "C30" on a foundation schedule, they mean RC30/37 (a structural reinforced mix), not C30/PAV1/ST3 (a paving mix). PAV1 contains deliberately entrained air bubbles for frost resistance, which reduces its bond with reinforcement steel. If you order PAV1 for reinforced foundations, it won't meet the BS 8500 designation your building control officer expects to see on the delivery ticket.
When you phone the concrete supplier, say "I need RC30/37 for reinforced foundations" or "I need a designed C30/37 mix for structural reinforced work." Don't just say "C30." If the supplier offers you "C30/PAV1/ST3," ask specifically whether it contains air entrainment. If it does, it's the wrong product.
When C25 is enough and when you need C30
Not every extension foundation needs C30. Standard domestic strip foundations for a single-storey extension, where the soil is stable and there are no unusual loads, use C25 (or more precisely, RC25/30 or GEN3). This is what Approved Document A specifies as the minimum for typical domestic foundations in non-aggressive ground conditions.
Your structural engineer upgrades to C30 when any of these conditions apply:
Heavy point loads. If your extension design includes steel beams (RSJs) that bear down onto individual pad foundations rather than continuous strip footings, those pads carry concentrated loads. A 4m RSJ supporting a first floor and roof might deliver 3-5 tonnes onto a single 600mm x 600mm pad. The pad needs to transfer that load into the ground without cracking, and C30 gives a wider safety margin than C25.
Reinforced elements. When the foundation includes a rebar cage (steel reinforcement bars tied together in a grid or cage shape), the concrete needs to bond properly with the steel and provide adequate cover to prevent corrosion. RC-designated mixes are formulated for this purpose. For pad foundations, the structural engineer typically specifies H16 rebar (16mm diameter high-yield steel bars) in cages with links at 200mm spacing.
Sulphate ground conditions. Clay soils containing high sulphate levels attack ordinary Portland cement over time, causing the concrete to expand and crack. If your ground investigation report classifies the soil as DS-3 or DS-4 (design sulphate class), the engineer specifies FND3 or FND4 mixes. These use sulphate-resistant cement and start at C30/37 minimum strength. You can't substitute a standard C30 mix here; it must be the specific FND-designated product.
Aggressive exposure. External concrete exposed to de-icing salt, seawater, or heavy weathering may need C30 for durability. This rarely applies to domestic foundations but can affect external ground-level slabs.
How to work with C30 concrete on site
C30 is stiffer than C25. It has less water in the mix (lower water-to-cement ratio), which makes it thicker, heavier to move by hand, and harder to compact into corners and around reinforcement. Your builder's team will notice the difference immediately.
Pumping vs barrowing
For any pour over 4m³, or where the concrete truck can't reverse to within a few metres of the trench, hire a concrete pump. A line pump (the type used on most domestic extensions) pushes concrete through a flexible hose up to 150m from the truck. The pump operator controls the flow from the end of the hose, placing concrete exactly where it's needed.
The alternative is wheelbarrows. A cubic metre of concrete weighs about 2.4 tonnes. On a typical extension foundation pour of 5m³, that's 12 tonnes of concrete moved by hand. At roughly 70 litres per barrow load, you're looking at about 70 barrow runs. With a line pump, the same pour takes under an hour.
Line pump hire
£275 – £800
The pump company is booked separately from the concrete supplier. Ask the pump company to recommend a concrete supplier they work with regularly. This coordination matters because C30's stiffer consistency means the pump needs the right slump (a measure of how fluid the concrete is, tested by filling a cone-shaped mould and measuring how much the concrete slumps when the cone is removed). Too stiff and the pump blocks. Too wet and the concrete loses strength. The pump operator and the concrete plant need to agree on the target slump before delivery day.
The pour itself
Building control must inspect the reinforcement before you pour. This is a mandatory hold point. The building control officer checks that:
- Rebar is clean (no mud, oil, rust, or grease on the bars, which would stop the concrete bonding to the steel)
- Concrete cover is correct (minimum 75mm from the steel to any ground-contact surface, 40mm over a blinding layer)
- Mesh or rebar is supported on proprietary spacers (plastic chairs) at 1m centres, not sitting on bricks or rubble
- Lap lengths are correct (where two bars overlap, the overlap must be at least 40 times the bar diameter plus 50mm)
Don't schedule the concrete delivery until building control has confirmed they've passed the reinforcement inspection. If you pour without their sign-off, they can require you to break out the concrete and start again.
Order 10% more concrete than your calculated volume. Trench walls are never perfectly straight, formwork flexes slightly under pressure, and over-ordering by half a cubic metre is far cheaper than being caught short at the end of a pour. Running out mid-pour creates a "cold joint" (a weak point where fresh concrete meets partially-set concrete) that can compromise the structural integrity of the foundation.
Curing
C30 reaches its design strength at 28 days, but the first 24-48 hours are the most vulnerable period. The concrete needs moisture and warmth to cure properly.
Don't pour C30 concrete if the temperature is below 5°C or if frost is forecast within the first 24 hours. Freezing water inside fresh concrete expands and creates a network of tiny cracks that permanently reduce strength. If you must pour in cold weather, the concrete supplier can add an accelerator admixture, and you'll need to cover the pour with thermal insulation blankets immediately after placing.
In hot weather (above 25°C), cover the concrete with damp hessian or polythene sheeting to prevent it drying out too quickly. Rapid moisture loss causes surface cracking and reduces strength.
How much do you need
Volume calculation for concrete is straightforward geometry: length x width x depth in metres gives you cubic metres.
Strip foundations: Measure the total trench length, multiply by width and depth. A typical 600mm wide, 1m deep trench-fill foundation running 12 linear metres uses: 12 x 0.6 x 1.0 = 7.2m³.
Pad foundations: Each pad is length x width x depth. Four pads at 900mm x 900mm x 600mm: 4 x 0.9 x 0.9 x 0.6 = 1.94m³.
Ground floor slab (reinforced): Floor area x depth. A 24m² slab at 150mm thick: 24 x 0.15 = 3.6m³.
Add 10% for waste, uneven trench walls, and the concrete that stays in the pump hose. Round up to the nearest half cubic metre.
Cost and where to buy
C30 ready-mix costs £120 – £145 per cubic metre delivered nationally, rising to £130 – £155 in London and the South East. The premium over C25 is modest: £3-8 per cubic metre on the delivered price. On a 5m³ pour, the total material cost difference between C25 and C30 is around £15-40.
The bigger cost variables are:
- Pump hire at £275–800 for a line pump, booked separately
- Short-load surcharges of £40–150 if you order less than a full truck
- Waiting time at £50-80 per hour if the truck sits on site beyond the free unloading window (typically 20-30 minutes)
The three major UK ready-mix producers are Hanson (part of Heidelberg Materials), CEMEX, and Tarmac (part of CRH). All supply RC30/37 to BS 8500. Regional independents like Mister Concrete, Pro-Mix, and EasyMix often undercut the nationals by 8-12% and can be more flexible on minimum order quantities.
You don't buy structural ready-mix from builders' merchants or DIY stores. Phone the concrete supplier directly or ask your builder to arrange it. Most builders have a preferred supplier. If you're project-managing, get quotes from two or three plants and confirm: exact mix designation (RC30/37), delivery time slot, minimum order, short-load surcharge, free unloading time, and whether they can coordinate with your pump hire company.
Ask the concrete supplier for test cube documentation. On every delivery, the driver should offer to fill test cube moulds. These are crushed at 7 and 28 days to verify the concrete reached its target strength. Building control can request this evidence. If there's ever a dispute about concrete quality, these cubes are your proof.
Alternatives
C25 (RC25/30) is the standard domestic foundation grade. If your structural engineer hasn't specified C30, you don't need it. C25 handles standard strip foundations, trench-fill foundations, and unreinforced ground floor slabs on stable ground.
C20 (GEN3) is for non-structural applications: drainage haunching, path foundations, oversite concrete below a floor slab, fence post foundations. Never use C20 for structural work.
C35 or C40 grades exist for heavier commercial applications, multi-storey buildings, and specialist structural elements. If your domestic extension requires C35 or above, something unusual is going on with the ground conditions or loading, and your structural engineer will explain why.
Site-mixed concrete is sometimes attempted for small volumes where the ready-mix minimum charge creates cost pressure. A nominal C30 site-mix uses a 1:2:3 ratio (1 part cement, 2 parts sharp sand, 3 parts coarse aggregate by volume). But strength is difficult to guarantee without lab testing, and building control officers are increasingly reluctant to accept site-mixed concrete for structural elements. Forum threads show site-mixed "C30" batches testing at only C22 strength due to incorrect aggregate or too much water. For structural work, use ready-mix.
Where you'll need this
C30 concrete appears during groundwork and structural phases of any extension or renovation project that involves reinforced foundations or steel beams.
- Foundations and footings - if your SE specifies C30 for pad foundations or reinforced strips
- Drainage - not typically C30, but combined pours sometimes include structural and non-structural concrete on the same day
Common mistakes
Ordering C30/PAV1/ST3 instead of RC30/37. The most common mistake, and the most consequential. PAV1 is air-entrained paving concrete. It has the right strength number but the wrong mix design for reinforced structural work. Always specify "RC30/37" or "designed C30/37 for reinforced foundations."
Pouring without building control sign-off on reinforcement. If the BCO hasn't inspected the rebar positioning, they can require you to remove the concrete. Schedule the inspection for the morning of your planned pour, with the concrete delivery booked for the afternoon. This gives time for any corrections.
Under-ordering. Running out of concrete mid-pour is far worse than having a quarter-metre left over. Over-order by 10%. Excess concrete can be used for path foundations, manhole benching, or even poured into a makeshift mould for garden stepping stones.
Pouring in freezing conditions. Fresh C30 that freezes in the first 24 hours will never reach its design strength. Check the weather forecast for the pour day and the following night. If frost is predicted, postpone.
Adding water on site. If the concrete arriving on the truck seems stiff, don't let anyone add water from a hose to make it easier to work. Extra water reduces strength. The correct slump should be agreed with the plant before delivery. If it arrives too stiff, phone the plant; they may authorise a small admixture addition, but only the driver or pump operator should do this.
Skipping test cubes. Your builder may wave this off as unnecessary. It isn't. Test cubes cost nothing (the supplier provides the moulds and collects them) and they're your only objective proof that the concrete reached C30/37 strength. If there's a structural issue years later, those test results matter.