Grade is the one part of a concrete order that nobody can eyeball on delivery. Volume you can see. Slump you can see. Strength shows up weeks later, which is why picking the right number at the point of order matters more than anything else you decide on the phone.
C25 means 25 N/mm² at 28 days
The number is characteristic compressive strength in newtons per square millimetre, tested on a cylinder 28 days after the pour. Higher number, stronger concrete, more cement in the mix.
Why grades have two names
You will see the same mix written two ways. C20 is the strength class under BS 8500. GEN3 is the designated mix name for the same thing. GEN mixes cover general applications, ST mixes are standardised prescribed, PAV mixes are paving grades with air entrainment for frost resistance, and RC mixes are for reinforced concrete.
If your drawing says GEN3 and your merchant says C20, nobody is wrong. Quote whichever your paperwork uses when you order and we will match it.
Every grade and what it is for
Picking by job rather than by number
Most people ring up with a job, not a grade, which is fine. Three questions get you to the right mix: what is going to sit on it, will it be exposed to frost and rain, and is there steel in it.
A shed base carries a light timber building and nothing else, so C20 at 100mm is plenty. A driveway takes a car, then a van, then a skip lorry, and it freezes every winter, so C30 with air entrainment. An industrial floor taking forklifts and racking wants C35 or above. Anything with reinforcement in it needs a mix specified for reinforced concrete, because durability and cover matter as much as strength.
Going a grade stronger than you need costs a few pounds per cubic metre. Going a grade weaker costs you the slab.
Why grade is easier with a volumetric mixer
Slump, fibres and the bits people forget
Grade is strength. Slump is how wet and workable the mix is, quoted as S1 to S4 or as a target in millimetres. A stiff S2 holds a slope on a driveway; a wetter S3 or S4 flows into a narrow trench or around dense reinforcement. Asking for extra water on site to make the pour easier waters down the strength you paid for, so ask for the slump you want up front instead.
Fibre reinforcement is worth raising too. Polypropylene fibres reduce early shrinkage cracking in slabs and often replace light mesh in domestic work. Steel fibre is used in industrial floors. Both are easier to specify on a volumetric truck because they are added during batching.
Describe the job and we will specify the mix
Tell us what it is, what goes on it and the postcode. We will confirm the grade, the slump, the price and a two-hour delivery window on the call. Same and next-day delivery across Surrey and the surrounding areas.
Quick answers
What grade for a shed base?
C20 / GEN3 at 100mm on a compacted sub-base.
What grade for a driveway?
C30 / PAV1 at 150mm, air entrained. Go to C35 / PAV2 if vans or plant will use it.
What grade for blinding?
C8 / GEN0. It is also the usual mix for kerb bedding and backing, drainage and bedding, and mass and cavity fill.
Can I use a higher grade than specified?
Usually yes, but check with whoever specified it. On reinforced or exposed work the specification covers durability as well as strength, so a swap is not always a simple upgrade.
How long before I can walk on it?
Usually 24 to 48 hours for foot traffic depending on grade and weather. Vehicles need about a week on a domestic driveway, and full design strength arrives at 28 days.
Concrete supplied by specialists within the construction industry
Surex Concrete runs volumetric mixers out of Effingham, minutes from Leatherhead, covering Surrey and the surrounding areas. We came from site work, so grades, slumps and building control are familiar ground rather than something we look up.
Every load is batched to BS 8500 and BS EN 206 and comes with a delivery ticket recording the mix supplied. If your inspector wants that confirmed before the pour, say so when you order and we will put it in writing.