Concrete Mix Grades Explained

Every ready-mix quote comes with a grade like C25/30 or C32/40. They look like tyre codes. They're actually the single most important line on the ticket, they tell you how much load the concrete will carry after it cures. Here's how to read them, and which one to order for the job you're about to pour.

What the two numbers mean

Every grade is two numbers separated by a slash, for example C28/35:

• The first number (28), minimum compressive strength of a cylinder of that concrete at 28 days, in N/mm² (MPa).
• The second number (35), minimum compressive strength of a cube of that concrete at 28 days, also in N/mm².

The cube number is always higher because cubes test stronger than cylinders for the same mix. UK engineers and BS EN 206 usually reference the cube figure, so C28/35 is often called "a 35 newton mix".

The standard grades, from weakest to strongest

"Gen" and "RC" are older UK names you'll still see on tickets, they map onto the same grades.

Which mix for which project?

Air entrainment, fibre, SRC, what the suffixes mean

The grade is only half the story. Extras on the ticket tell you how the concrete will behave in service:

Air-entrained

Microscopic air bubbles are introduced into the mix. They give water somewhere to expand into when it freezes, so the surface doesn't spall in winter. Essential for external slabs that see salt, frost or freeze-thaw cycles, driveways, yards, external car parks.

SRC, Sulphate-resisting cement

Replaces standard Portland cement with a version that resists sulphate attack. Needed wherever concrete sits in contact with sulphates: clay soils high in sulphate, slurry, silage effluent, some groundwater. Ignoring it in a silage clamp rots the concrete in a few years.

Fibre reinforced

Steel or polypropylene fibres are mixed in at the plant. They spread the crack-control job across the whole slab, no mesh fixing, better distribution. Common in commercial floors and large-volume slabs.

Slump (S1 / S2 / S3)

Slump measures how wet/workable the mix is. S2 (50-90mm) is the standard for most jobs. S3 (100-150mm) flows more easily, use it when you're pumping or pouring into a tricky form. Don't request a wetter mix just to make spreading easier; you lose strength.

Common mistakes when picking a grade

1. Copying your neighbour

"My mate ordered C20/25 for his drive and it's fine", maybe, but he probably has a thicker slab or parks a hatchback, not a Hilux. Order for your loading, not his.

2. Over-specifying "just to be safe"

Ordering C40/50 for a shed base wastes money and cracks more in shrinkage than C28/35 would. Stronger isn't always better, match the grade to the job.

3. Ignoring exposure class

A driveway that's near the sea or salted every winter needs air-entrainment even at the "right" strength grade. The grade tells you load; exposure class tells you environment. Both matter.

4. Using old "Gen" names without checking

A plan that says "Gen 3" should get C16/20. If the supplier interprets it differently, you can end up with the wrong strength. Clarify the BS EN 206 grade in writing.

Quick rules of thumb