What concrete grade for foundations?

Standard domestic foundations use C25 concrete (GEN 3 or ST 2 designation to BS 8500). In sulphate-bearing soils (common in clay areas), designated concrete DC-2 or higher is required. In contaminated ground, the structural engineer will specify a designed mix. Always check the soil investigation report for sulphate levels.

What is the difference between GEN and ST designations?

GEN (General) mixes are for non-structural use like blinding, fill and garden work. ST (Standard) mixes are for structural applications like foundations and floor slabs. RC (Reinforced Concrete) mixes are specifically designed for reinforced concrete elements. PAV mixes are for external paving and driveways with freeze-thaw resistance.

What concrete grade for a driveway?

External driveways and hardstanding should use C30 PAV 1 concrete with air entrainment for freeze-thaw resistance (exposure class XF1). Standard GEN or ST mixes without air entrainment will deteriorate when exposed to frost and de-icing salts. The PAV designation specifically provides the durability needed for external paving.

What is sulphate-resistant concrete?

Sulphate-resistant concrete uses cements that resist chemical attack from sulphates in the soil (common in clay soils). DS classes range from DS-1 (low, standard cement OK) to DS-4 (very high, requiring SRPC cement plus additional protection). A soil investigation determines the sulphate class. The BRE Special Digest 1 provides guidance.

What slump should I order?

Slump measures workability: S1 (10-40mm) = very stiff, S2 (50-90mm) = standard for slabs, S3 (100-150mm) = standard for foundations and pumped concrete, S4 (160-210mm) = flowing/self-compacting. For trench fill, S3 is standard. For floor slabs, S2 gives a stiffer mix that's easier to level. Never add water on site to increase slump.

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Concrete Grade Selector — Application Lookup & Specification

Select the correct concrete grade, designation, slump class and exposure class for your application.

Application

Select your intended use

Sulphate Class

From soil investigation

Volume Required

For cost estimate

Concrete Price

How We Calculate This

This tool selects the appropriate concrete grade, designation, exposure class and sulphate requirements based on the intended application.

Grade selection

Concrete grades are specified to BS 8500-1:2023.
The grade (C25, C30 etc.) indicates the characteristic compressive strength at 28 days in N/mm². The designation (GEN, ST, RC, PAV) indicates the application and durability requirements.

Sulphate class

Sulphate class is determined from soil investigation and BRE SD1. Classes range from DS-1 (low) to DS-4 (very high). Higher classes require sulphate-resistant cement types.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.