Concrete grades
Concrete grades in Ireland are strength classes under I.S. EN 206, written like C25/30: 25 N/mm² measured on a cylinder and 30 N/mm² on a cube, at 28 days. The old names count the cube figure: 30N is C25/30, 37N is C30/37.
The job sets the grade through its exposure class: a floor slab inside a house is XC1 (C25/30 minimum), many foundations are XC2 (C28/35), and outside surfaces that get wet and freeze need air-entrained C30/37 or C32/40.
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Old names and new classes
| Old name | Strength class (cylinder/cube, N/mm²) |
|---|---|
| 10N | C8/10 |
| 15N | C12/15 |
| 20N | C16/20 |
| 25N | C20/25 |
| 30N | C25/30 |
| 35N | C28/35 |
| 37N | C30/37 |
| 40N | C32/40 |
| 45N | C35/45 |
| 50N | C40/50 |
Exposure classes set the minimum
| Exposure | Where it applies (examples) | Minimum class | Max w/c | Min cement |
|---|---|---|---|---|
| X0 | Unreinforced concrete with no frost, abrasion or chemical attack | C12/15 | ||
| XC1 | Inside buildings with low humidity, e.g. floor slabs | C25/30 | 0.65 | 270 kg/m³ |
| XC2 | Long-term water contact: many foundations | C28/35 | 0.60 | 290 kg/m³ |
| XC3, XC4, XD1, XF1 | Inside with high humidity; outside, sheltered or wet and dry | C30/37 | 0.55 | 310 kg/m³ |
| XA1 | Slightly aggressive ground | C32/40 | 0.50 | 340 kg/m³ |
| XF3 (air entrained) | Horizontal surfaces exposed to rain and freezing | C30/37 AE |
Which grade for which job
| Job | Grade | Where it comes from |
|---|---|---|
| Plain strip foundations, non-aggressive ground | C12/15, 200 kg/m³ cement, w/c 0.85 | TGD A |
| Unreinforced ground-supported floor | 20 N/mm² (cube), 200 kg/m³ cement, w/c 0.85 | TGD C |
| Floor slab inside a building | C25/30 minimum (XC1) | I.S. EN 206 |
| Many foundations | C28/35 minimum (XC2) | I.S. EN 206 |
| Wet horizontal surfaces with freeze/thaw (drives, yards) | C30/37 air entrained, or C32/40 | Cold weather concreting |
| Farm structures, general | C30/37 (S.100 Mix B) | S.100 |
| Silage slabs, pits and aprons | C35/45 (S.100 Mix A) | S.100 |
Aggregate size and slump
A full order adds the aggregate size and the consistence. D20 means 20 mm maximum aggregate (D10 and D14 are the smaller ones). Slump runs from S1 (10 to 40 mm) to S5 (220 mm and over); S2 is 50 to 90 mm and S3 is 100 to 150 mm. So C25/30 D20 S3 is a C25/30 with 20 mm stone that flows a bit more easily.
ST mixes
The Irish National Annex lets a third-party accredited ready-mix producer supply an equivalent design mix in place of the old standard prescribed mixes: C12/15 for ST1, ST2 and ST3, and C16/20 for ST4.
Floors laid by the contractor who takes your call
Calls on this site ring JAM Floor Screeding: James Mulhall, based in Portlaoise, 25 years laying floors. Lines open Mon to Sat, 7am to 6pm. He pours concrete floors, slabs and foundations as well as screed. These are his jobs.
“JAM were punctual, professional and efficient and did an outstanding job on pouring our new floor as part of a renovation. Very highly recommend!”
“Excellent work, laser level finish, neat and tidy worker, highly recommend.”
“Top class work by james and his crew, highly recommend”
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Concrete grades questions
What does C25/30 mean?
A strength class under I.S. EN 206: 25 N/mm² characteristic cylinder strength and 30 N/mm² cube strength at 28 days. It used to be called 30N.
What grade of concrete for a house floor slab?
Under I.S. EN 206, internal floor slabs are XC1, C25/30 minimum, for reinforced work. TGD C allows 20 N/mm² concrete for an unreinforced ground-supported floor.
What grade of concrete for foundations?
Plain strip foundations in non-aggressive ground: C12/15 under TGD A. Reinforced foundations are often XC2, C28/35 minimum. Follow the engineer's spec.
What grade for a driveway?
For wet horizontal surfaces with significant freeze/thaw, I.S. EN 206 recommends C30/37 air entrained or C32/40.
Is 37N the same as C30/37?
Yes. The old N names use the cube strength: 37N is C30/37, 35N is C28/35 and 40N is C32/40.
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