FND is about sulfate, not strength
This is the same shape of misunderstanding as S355J2 in the steel grades: a designation everybody reads as "the strong one" that governs a completely different property.
FND stands for foundation. So people ordering concrete for a foundation reach for it, reasonably enough, assuming it is the grade specified for the job.
What FND2, FND3 and FND4 control is resistance to sulfate attack from aggressive ground. Which one you need is decided by a chemical analysis of the soil, and it has nothing to do with how much the building weighs.
That leaves two mistakes sitting next to each other:
- Ordering FND when the ground is fine: paying for a property you do not need.
- Ordering GEN when the ground is sulfate bearing: putting concrete in the ground that will be chemically attacked, slowly, where nobody can see it, for years.
This tool cannot tell you which. Not from a dropdown, not from a postcode, not from anything. It needs a ground analysis, and a website guessing at it would be worse than no website at all. What the tool does is say so, clearly, at the point where somebody is about to guess.
PAV is air entrained, and that is the whole point
The other one that costs real money, and it costs it visibly.
Air entrainment puts microscopic bubbles right through the concrete. When water in the surface freezes and expands, it has somewhere to go. Without them it has nowhere, so it pushes the surface apart from the inside.
That is spalling: the surface flaking off in patches over the first few winters. It is not a finish problem or a curing problem, and no sealer fixes it afterwards.
GEN3 is a floor mix. Perfectly good in a garage. Put it on a driveway because it is cheaper than PAV1 and you have bought the wrong property, and a stronger non-entrained mix does not substitute for it: it just costs more while doing the same thing.
Job first, designation second
Nobody arrives at a page like this wondering what GEN3 is. They arrive wondering what to order for a driveway.
So the tool asks what you are pouring and answers with the designation, and it puts what gets ordered by mistake right next to it. On several of these jobs, knowing what not to order is the more useful half.
What the strength class is telling you
C25/30 is two numbers: cylinder strength over cube strength, in newtons per square millimetre at 28 days. Cylinders test lower than cubes for the same concrete, so the same batch carries two numbers.
A designated mix like PAV1 carries a strength class with it, but it carries more than that: the designation also controls what the mix is suitable for. Order by strength alone and you can end up with something perfectly strong and completely unsuitable.
Your ready-mix supplier knows the ground and this page does not
It does not choose a mix for your job. Loading, exposure and ground chemistry decide that, and the last two need somebody to look at the site.
Your ready-mix supplier knows the local designations, what the plant produces, and what the ground round there is usually like. They are the right person to confirm an order with, and this page is for walking into that conversation knowing what the letters mean.
Next to it: how much you need, rebar weights and schedules, and aggregate for what goes underneath.
Common questions
What concrete mix do I need for a driveway?
PAV1, and the reason matters more than the name. PAV mixes are air entrained: they have microscopic bubbles through them that give freezing water somewhere to expand into. Use GEN3 because it is cheaper and the surface spalls and flakes over the first few winters. It is the commonest concrete mistake on a domestic job.
What does FND mean, and do I need it?
FND is a sulfate resistance specification, not a strength one. FND stands for foundation, so people order it assuming it is the strong one, but what it controls is resistance to chemical attack from aggressive ground. Which grade you need is decided by a chemical analysis of the soil, and nothing on a screen can tell you.
What is the difference between GEN3 and C20/25?
GEN3 is a designated mix under BS 8500, specified by what it is for. C16/20 is its strength class: cylinder strength over cube strength in newtons per square millimetre. The designation carries more than the strength: it also controls what the mix is suitable for, which is why ordering by strength alone can get you something unsuitable.
Can I use a stronger mix instead of an air entrained one?
No. Air entrainment is a different property, in the same way that the toughness suffix on structural steel is not a strength. Stronger concrete without entrained air still has nowhere for freezing water to go, and it still spalls. It just costs more while doing it.
What do I order for footings?
GEN1 or GEN2 in ordinary ground. If the ground is chemically aggressive, an FND grade, and which one comes from a ground report rather than a guess. The thing that decides a footing mix is usually the ground chemistry, not the load.
Will this tell me which mix to use?
It tells you what each designation means and what each is conventionally used for. It will not choose one, because loading, exposure and ground chemistry decide that and two of those need somebody to look at the site. Your ready-mix supplier knows the local designations and is the right person to confirm an order with.