What concrete grade do I need for a house foundation?

For standard domestic strip foundations on non-aggressive ground, C25/30 (GEN 3 or ST 4) is the minimum grade. For trench fill foundations, C25/30 with S3 slump class is used for better flow. On aggressive ground with sulphates, FND designations with sulphate-resisting cement may be required — a site investigation report will specify this.

What is the difference between GEN, ST, RC and PAV concrete?

These are BS 8500 designated concrete types: GEN (General) is for non-structural use, ST (Standardised) is prescribed by strength class, RC (Reinforced Concrete) is for structural reinforced work, PAV (Paving) is designed for external hardstandings with freeze/thaw resistance, and FND (Foundation) is for foundations in ground with sulphates.

What does C25/30 mean?

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.

What slump should I order for ready-mix concrete?

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.

Do I need air-entrained concrete for a driveway?

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.

Concrete Grade Calculator — BS 8500 Specification

How We Calculate This

This calculator recommends a concrete grade and mix designation based on BS 8500-1:2023 (Concrete — Complementary British Standard to BS EN 206). It considers the application type, environmental exposure class, and expected loading.