Concrete Grade Calculator — BS 8500 Specification

C25/30 (RC25/30) is a common, conservative default for domestic strip and trench-fill foundations, and is the durability-driven choice once a foundation is reinforced (exposure class XC2). BS 8500 itself permits lower designated mixes (GEN1–GEN3) for plain mass-concrete footings on non-aggressive ground, so C25/30 is a sensible default rather than an absolute code minimum. For trench fill, an S3 slump aids flow. On sulphate-bearing ground the FND series (FND2/FND3/FND4) is used — all are still strength class C25/30 but with sulphate-resisting cement chosen by the Design Chemical class in the site investigation report (BRE Special Digest 1:2005). Use our Concrete Mix Calculator for hand-mix proportions or Concrete Volume Calculator for ordering quantities.

These are BS 8500 designated and standardised concretes. GEN (General, GEN0–GEN4 ≈ C6/8 to C20/25) is for non-structural use. ST (Standardised prescribed, ST1–ST5) follows a fixed recipe for small/hand-mixed jobs; in BS 8500 each ST grade mirrors a GEN grade (ST1↔GEN0, ST2↔GEN1, ST3↔GEN2, ST4↔GEN3, ST5↔GEN4), with ST5 assumed no higher than C20/25. RC (Reinforced Concrete) names the strength class directly — RC25/30, RC28/35, RC32/40, RC40/50. PAV (Paving) is air-entrained external hardstanding concrete — PAV1 = C25/30 for domestic drives, PAV2 = C32/40 for heavy-duty paving. FND (Foundation) is for sulphate-bearing ground — FND2, FND3 and FND4 are all strength class C25/30 and differ only by the sulphate-resisting cement / Design Chemical class.

The first number (C25) is the characteristic cylinder strength at 28 days in N/mm² (tested on a 150×300mm cylinder). The second number (/30) is the characteristic cube strength at 28 days (tested on a 150mm cube). UK practice traditionally uses cube strengths, so C25/30 means 30 N/mm² cube strength. Ready-mix suppliers work to the cube strength.

Slump describes workability: S1 (10-40mm) is very stiff for road works, S2 (50-90mm) is standard for most slabs and foundations, S3 (100-150mm) is flowing for trench fill and pumping, and S4 (160-210mm) is very fluid for self-compacting mixes. Higher slump does not mean weaker concrete — it is controlled by the water/cement ratio, not slump alone.

If the driveway will be exposed to freeze/thaw cycles and/or de-icing salts, air-entrained concrete (4-6% air content) is strongly recommended. This provides freeze/thaw resistance by creating tiny air bubbles in the concrete matrix. Specify exposure class XF1 (moderate) or XF3 (severe with de-icers) per BS 8500.