Concrete Mix Designs for Groundworks: ST, GEN, RC and Designated Mixes Explained

Quick Answer: Concrete for groundworks is specified using BS 8500 designated mixes: ST (Standard) for non-structural fill and blinding; GEN (General) for lightly-loaded foundations and mass fill; RC (Reinforced Concrete) for structural elements. For domestic strip foundations, ST2 or GEN1 is typical for blinding; GEN3 or RC25/30 for the foundation concrete itself. Sulphate-resisting cement is required in DC-2 and above ground conditions.

Summary

Specifying the correct concrete mix for groundworks is not just about strength — it's about durability and resistance to the ground environment. Standard concrete mixes are attacked by sulphates naturally present in some UK soils and groundwater, which progressively expand and break down the concrete over years. Selecting the wrong mix means foundations that look fine at handover but deteriorate within a decade.

BS 8500 (Concrete — Method of specifying and guidance for the specifier) is the UK standard for concrete specification. It replaced BS 5328 in 2003 and provides a systematic approach to selecting concrete based on the exposure class (DC class for foundations in ground contact) and the structural requirements.

For most domestic groundworkers ordering ready-mix, the practical guidance is: confirm the DC class from the site investigation or engineer's spec, then order the correct designated mix. This article covers the key mix types and how to choose between them.

Key Facts

• BS 8500-1:2015+A2:2019 — UK standard for concrete specification; replaces BS 5328
• ST mixes — Standard mixes; not strength-specified; suitable for blinding, oversite fill, non-structural applications
• GEN mixes — General-purpose mixes; strength-specified; for lightly-loaded domestic foundations and floor slabs
• RC mixes — Reinforced Concrete mixes; higher strength and durability; for structural elements
• PAV mixes — Pavement mixes; surface-finished concrete for pavings and drives
• DC class — Design Chemical class; determines exposure to sulphates and other aggressive ground chemicals
• DC-1 — minimal sulphate risk (most UK greenfield sites); standard Portland cement acceptable
• DC-2 — low to moderate sulphate (some sites); SRPC (sulphate-resisting Portland cement) or enhanced mix required
• DC-3/DC-4 — higher sulphate; specialist mixes; uncommon in domestic groundworks
• Water:cement ratio — lower w/c ratio = stronger, more durable concrete; BS 8500 specifies maximum w/c for each exposure class
• GGBS/PFA additions — ground granulated blast-furnace slag (GGBS) or pulverised fuel ash (PFA) improve sulphate resistance and reduce heat of hydration; common in RC mixes
• Slump class — workability; S3 (80–150mm) typical for foundations; S4/S5 for pumped or congested reinforcement sections

Quick Reference Table: Common Groundworks Concrete Mixes

Detailed Guidance

ST Mixes (Standard)

ST mixes are the simplest and cheapest. They have no guaranteed characteristic strength — they are specifiable by mix proportions rather than performance. Used where structural performance is not required:

• ST1 — the leanest mix; suitable for pipe bedding in non-structural applications, mass fill in deep trenches, and temporary works concrete
• ST2 — standard blinding concrete; placed on the sub-base before reinforcement; 50mm thick; provides a clean, level working surface
• ST3–ST5 — progressively richer; used for non-structural mass fill applications where some strength is required

ST mixes do NOT have a minimum strength guarantee. For any foundation concrete, GEN or RC mixes are required.

GEN Mixes (General)

GEN mixes have a guaranteed characteristic compressive strength at 28 days. The number after GEN indicates the strength class (similar to the old C-designation):

• GEN1 (10 N/mm²) — mass concrete non-structural; lightly-loaded strip foundations in DC-1 ground
• GEN2 (15 N/mm²) — light strip foundations; oversite slabs with minimal loading
• GEN3 (20 N/mm²) — standard for domestic strip foundations in DC-1 ground; most common mix for domestic trench fill

GEN3 is the workhorse of domestic groundworks. It is what most groundworkers mean when they say "foundation concrete" for a standard domestic strip in clean clay or granular ground.

RC Mixes (Reinforced Concrete)

RC mixes are specified where reinforcement is present or where higher durability is required. The designation uses two numbers: characteristic strength and maximum aggregate size (e.g., RC25/30):

• RC25/30 — minimum for reinforced concrete; pad foundations, reinforced strips
• RC30/37 — standard for reinforced concrete in DC-2 ground; basement slabs, retaining walls with ground contact
• RC35/45 — higher durability; aggressive exposure classes

RC mixes also specify a maximum free water:cement ratio (w/c) and minimum cement content, which directly control durability. Increasing the cement content and reducing w/c makes the concrete less permeable and more resistant to sulphate attack.

DC Classes and Sulphate Resistance

The DC (Design Chemical) class system from BS 8500 and BRE Special Digest 1 categorises the ground's aggressiveness based on sulphate content and other chemical factors:

Sulphate-resisting Portland cement (SRPC): SRPC (BS EN 197 CEM I with restricted C₃A content) is specified for DC-2 and above. Alternatively, ordinary CEM I with a GGBS or PFA addition achieves equivalent sulphate resistance by reducing permeability.

Confirming DC class: The Phase 2 site investigation includes pH, sulphate, and chloride testing of soil and groundwater samples. The geoenvironmental consultant's report will state the DC class. If no investigation has been done, assume DC-2 for brownfield sites and any made ground as a precaution; confirm before specifying.

Specifying Ready-Mix Concrete

When ordering ready-mix, provide the following to the batching plant:

1. Designated mix name (e.g., GEN3, RC25/30) — the supplier will match their mix design to this
2. DC class (e.g., DC-1) — the supplier will use the correct cement type
3. Maximum aggregate size (20mm is standard for most pours; 10mm for tight reinforcement sections)
4. Slump class (S3 for most foundations; S4 for pumped; S5 for congested reinforcement)
5. Special requirements (e.g., accelerated mix for cold weather; retarder for hot weather; fibres for enhanced crack resistance)

The supplier will issue a delivery ticket confirming the mix design, cement content, and target slump. Retain all delivery tickets for the building control file.

Trench Fill vs Traditional Strip

Traditional strip foundation: Excavate to formation level, compact sub-base, pour 100–150mm thick strip to the engineer's width. The wall is then built down from the surface to foundation level using brickwork or blockwork.

Trench fill: Excavate a narrow trench and fill it entirely with concrete from formation level to within approximately 150mm of the surface. Faster and cheaper than traditional strip, as no brickwork in the ground. Requires more concrete but less skilled labour. Standard for most new-build housing.

Trench fill uses GEN3 in DC-1 ground. The concrete must be poured continuously without cold joints; a 10m run of trench fill needs to be poured in one operation or in planned sections.

Frequently Asked Questions

Can I use site-mixed concrete for strip foundations?

Yes, but it is not recommended for structural foundations. Site mixing produces inconsistent quality. The mix proportions for ST2 or GEN3 can be approximated (1:3:6 or 1:2.5:5 cement:sand:aggregate by volume), but there is no certified strength guarantee. For building control files and warranty purposes (NHBC, Premier Guarantee), certified ready-mix with a delivery ticket is strongly preferred.

What is the difference between C25/30 and RC25/30?

C25/30 and RC25/30 refer to the same characteristic compressive strength (25 N/mm² at 28 days). "C" is the Eurocode designation (BS EN 206); "RC" is the BS 8500 designated mix name for reinforced concrete of that strength. They are effectively the same mix. In practice, ordering "RC25/30" from a UK ready-mix supplier gives you concrete with a certified strength suitable for reinforced domestic groundworks.

Does cold weather affect concrete quality?

Yes significantly. Below 5°C, the hydration reaction that gives concrete its strength slows dramatically. Below 0°C, fresh concrete can freeze, permanently damaging the crystal structure and preventing it from reaching its design strength. In cold weather: do not pour below 5°C ambient without protection; cover with insulating blankets; use warm water in the mix; consider an accelerated admixture.

What is the minimum curing time for foundations before loading?

Foundation concrete (GEN3, RC25/30) reaches approximately 75% of its 28-day strength after 7 days. For domestic strip foundations, loading (starting brickwork) after 24–48 hours is common practice in mild weather. However, for structural concrete (basement slabs, RC foundations), waiting 5–7 days before significant loading is prudent, and 28 days before full design loads.

Regulations & Standards

• BS 8500-1:2015+A2:2019 — Method of specifying and guidance for the specifier; designated mixes

• BS EN 206:2013+A2:2021 — Concrete — Specification, performance, production and conformity

• BRE Special Digest 1 (4th edition) — Concrete in aggressive ground; sulphate resistance guidance

• Building Regulations Approved Document A — foundation concrete grade requirements

• BS 8500-1:2015 via BSI — UK concrete specification standard

• BRE Special Digest 1 — sulphate attack on concrete in ground

• Concrete Centre — Specifying Concrete — practical guidance on BS 8500 mixes

• strip foundation design — foundation design and concrete volume calculations

• raft foundation guide — reinforced concrete raft mix requirements

• oversite concrete slab — floor slab concrete grade and specification

• contaminated land groundworks — how ground contamination affects concrete specification