BS 8500 Quick Reference

Complete reference guide to BS 8500 — the UK complementary standard to BS EN 206 for concrete specification. All designated mix families (GEN, RC, PAV, FND, ST), full exposure class tables (XC, XF, XA, XD, XS), cement designations, minimum cement contents, max w/c ratios and free downloadable reference cards. Updated for BS 8500:2023.

What Is BS 8500? — Overview & Structure

BS 8500 is theComplementary British Standard to BS EN 206for the specification, production and conformity of concrete in the UK. It was first published in March 2002 and replaced BS 5328 in December 2003. The current edition isBS 8500:2023, which introduced provisions for multi-component low-carbon cements (CEM II/C-M and CEM VI per BS EN 197-5:2021) and updated the durability recommendations for carbonation and chloride exposure classes. [web:132][web:145]

BS 8500 is published in two parts:

• BS 8500-1:2023 — Method of specifying and guidance for the specifier:Covers how to specify concrete for a given application, exposure class selection, designation selection, cover to reinforcement, and contains the critical Annex A application tables used by contractors and designers. [web:139]
• BS 8500-2:2023 — Specification for constituent materials and concrete:Covers permitted cement and combination types, aggregate specifications, admixture requirements and production requirements for conformity certification. The 2023 edition added CEM II/C-M and CEM VI multi-component cements to the approved combinations table. [web:138][web:134]

Four Types of Concrete Under BS 8500

1. Designated concrete(most common for domestic work) — Ordered by name (e.g. PAV1, GEN3). Producer must hold third-party conformity certification.2. Designed concreteSpecified by the engineer to achieve defined properties; used for structural work.3. Standardised prescribed concrete (SPC)ST1–ST5 mixes; specified by prescriptive composition; no strength guarantee. Suitable only for small-scale non-structural work.4. Proprietary concreteProducer-designed mix for specific purpose (e.g. self-compacting, rapid-set, coloured). [web:140][web:141]

Designated Concrete Requires Third-Party Certification

A key feature of designated concrete (GEN, RC, PAV, FND families) is that the producer must hold an appropriate level ofthird-party product conformity certificationapproved by BSI. When you order PAV1, GEN3 or any designated mix, you are entitled to ask your supplier for evidence of their certification (typically a QSRMC or Britpave certificate). Suppliers without this certification cannot legitimately supply designated concrete — they can only supply standardised prescribed (ST) mixes. This is particularly important when specifying PAV1 or PAV2 for driveways where air entrainment and freeze-thaw performance are critical. [web:132][web:137]

GEN Concrete Mixes — General Purpose (GEN0–GEN3)

GEN mixes are general-purpose designated concretes fornon-structural, domestic and light commercial applications. They do not contain air entrainment and are not suitable for external surfaces exposed to freeze-thaw cycling. The GEN family replaces the old ST1–ST3 standardised prescribed mixes when a certified supplier is used. [web:141][web:144]

• Designation: GEN0; Strength Class: C8/10 (~7.5 N/mm²); Min Cement (kg/m³): 180; Max w/c: —; Consistence: S1; ST Equivalent: ST1; Typical Applications: Kerb bedding & backing, blinding, general fill
• Designation: GEN1; Strength Class: C12/15 (~10 N/mm²); Min Cement (kg/m³): 210; Max w/c: —; Consistence: S3–S4; ST Equivalent: ST2; Typical Applications: Strip/mass foundations, trench fill, oversite, drainage haunching, unreinforced floors (permanent finish)
• Designation: GEN2; Strength Class: C16/20 (~15 N/mm²); Min Cement (kg/m³): 240; Max w/c: —; Consistence: S2; ST Equivalent: ST3; Typical Applications: House floors (no permanent finish), internal slabs with no embedded metal
• Designation: GEN3; Strength Class: C20/25 (~20 N/mm²); Min Cement (kg/m³): 240; Max w/c: 0.65; Consistence: S2–S3; ST Equivalent: ST4; Typical Applications: Domestic garage floors, light-duty internal slabs, non-aggressive DC-1 ground conditions only. NOT suitable for external surfaces or freeze-thaw exposure.

Do NOT Use GEN3 for External Driveways or Patios

GEN3 lacks air entrainment and is not designed for XF (freeze-thaw) exposure. Using GEN3 for an external driveway or patio in the UK will almost certainly result in surface scaling, spalling and delamination within 2–5 years as repeated freeze-thaw cycles exploit the lack of air-void system in the hardened paste. The minimum correct mix isPAV1 (C30/37, XF3, air-entrained). This is one of the most common and costly specification errors in UK domestic construction. [web:141]

RC Concrete Mixes — Reinforced Concrete (RC20/25–RC40/50)

RC designated mixes are forreinforced concrete applicationswhere embedded steel is present. The designation includes both the cylinder/cube strength class (e.g. C28/35) and implies specific minimum cement content and maximum w/c ratio to protect reinforcement against corrosion. The 2023 edition of BS 8500-1 removed minimum cement content recommendations for XD and XS chloride-exposure classes, leaving minimum characteristic strength as the primary specification. [web:132][web:141]

• Designation: RC20/25; Strength Class: C20/25 (25 N/mm²); Min Cement (kg/m³): 280; Max w/c: 0.60; Exposure Class: XC1; ST Equiv.: —; Typical Applications: Lightly reinforced ground floors (insulated formwork, above ground), non-suspended
• Designation: RC25/30; Strength Class: C25/30 (30 N/mm²); Min Cement (kg/m³): 280; Max w/c: 0.60; Exposure Class: XC1–XC2; ST Equiv.: ST4; Typical Applications: Lightly reinforced concrete, internal wearing surface (light foot & trolley traffic), reinforced foundations in DC-1 soils
• Designation: RC28/35 ★; Strength Class: C28/35 (35 N/mm²); Min Cement (kg/m³): 280; Max w/c: 0.60; Exposure Class: XC2–XC3; ST Equiv.: ST5; Typical Applications: General and agricultural reinforced concrete, livestock/crop store floors, domestic garages used as workshops
• Designation: RC32/40; Strength Class: C32/40 (40 N/mm²); Min Cement (kg/m³): 300; Max w/c: 0.55; Exposure Class: XC3–XC4; ST Equiv.: —; Typical Applications: General, agricultural and industrial reinforced concrete, floors and walls for manure/slurry stores, industrial floors (small-wheeled forklifts)
• Designation: RC35/45; Strength Class: C35/45 (45 N/mm²); Min Cement (kg/m³): 320; Max w/c: 0.50; Exposure Class: XC4; ST Equiv.: —; Typical Applications: Agricultural and industrial RC, stable floors, silage/brewer’s grain stores, parlours and dairies
• Designation: RC40/50; Strength Class: C40/50 (50 N/mm²); Min Cement (kg/m³): 340; Max w/c: 0.45; Exposure Class: XC4; ST Equiv.: —; Typical Applications: Severe exposure conditions — silage stores, brewers’ grain floors, parlours, dairies under heavy duty cycles

★ RC28/35 is the most commonly specified reinforced concrete for general domestic and agricultural UK projects and is the standard specification for reinforced ground-bearing slabs, suspended slabs and beam-and-block floor systems.

PAV Concrete Mixes — Paving & External Surfaces (PAV1 & PAV2)

PAV mixes are the correct designation for anyexternal horizontal concrete surface in the UK that will be exposed to rain and freezing conditions. Their defining characteristic is mandatoryair entrainmenttypically 3.5–6% by volume of entrained microscopic air bubbles that accommodate the 9% volume expansion of freezing water, preventing surface scaling and spalling under UK freeze-thaw cycles. PAV mixes cannot be replicated by site-mixing. [web:27][web:141]

• Designation: PAV1 ★; Strength Class: C30/37 (35 N/mm²); Exposure Class: XF3; Air Entrainment: Mandatory (3.5–6%); Min Cement (kg/m³): 300; Max w/c: 0.55; Consistence: S2–S3; Applications: Domestic driveways, patios, footpaths, car parks, hardstandings — no regular de-icing salts
• Designation: PAV2; Strength Class: C35/45 (40 N/mm²); Exposure Class: XF4; Air Entrainment: Mandatory (enhanced); Min Cement (kg/m³): 320; Max w/c: 0.45; Consistence: S2–S3; Applications: Road-adjacent surfaces, areas regularly salted in winter, reinforced paving in de-icing exposure, commercial hardstandings

PAV1 vs PAV2 — Which Do I Need?

PAV1 (XF3)is correct for the vast majority of UK residential driveways, patios and hardstandings where the surface is exposed to rain and freezing but isnotregularly treated with de-icing salts.PAV2 (XF4)is required where the surface will be regularly salted or gritted in winter (e.g. driveways directly adjacent to a public road that is salted, or areas where de-icing salt is applied to prevent slipping). The key technical difference is that PAV2 has a higher air entrainment level and a lower maximum w/c ratio (0.45 vs 0.55) to resist the combined attack of freeze-thaw cycling and chloride penetration from de-icing salts. If in doubt for a residential driveway, PAV1 is appropriate. [web:141]

PAV Mixes Are Not Suitable for Power-Float Finishing

Neither PAV1 nor PAV2 is suitable for power-float (helicopter) finishing. The power-floating process densifies and closes the concrete surface, whichdestroys the air void systemcritical to freeze-thaw resistance. Power-floated PAV1/PAV2 will perform no better than GEN3 under freeze-thaw conditions. For external power-floated surfaces (e.g. commercial yard floors), a different specification approach is needed — discuss with your concrete supplier and structural engineer.

FND Concrete Mixes — Sulfate-Resistant Foundations (FND2–FND4)

FND mixes are designated concretes formulated to resistchemical attack from sulfates in aggressive ground conditions. They replace the previous SRPC (Sulfate Resisting Portland Cement) approach by specifying a performance requirement (DC class) rather than a specific cement type. The correct FND mix depends on the Design Chemical (DC) class of the ground, which is determined by soil/groundwater analysis as per BRE Special Digest 1. [web:32][web:135]

• Designation: FND2; DC Class: DC-2; Strength Class: C28/35; Min Cement (kg/m³): 300; Max w/c: 0.55; Exposure: XA1 / DS-2; Ground Conditions: Slightly aggressive sulfate ground — SO₄ 0.5–1.5 g/l in groundwater
• Designation: FND3; DC Class: DC-3; Strength Class: C28/35; Min Cement (kg/m³): 320; Max w/c: 0.50; Exposure: XA2 / DS-3; Ground Conditions: Moderately aggressive — SO₄ 1.5–3.0 g/l; acidic ground pH 5.5–6.5
• Designation: FND4; DC Class: DC-4; Strength Class: C28/35; Min Cement (kg/m³): 340; Max w/c: 0.45; Exposure: XA3 / DS-4; Ground Conditions: Highly aggressive — SO₄ >3.0 g/l; very acidic pH <5.5; colliery spoil; made ground
• Designation: FND4Z; DC Class: DC-4z; Strength Class: C28/35; Min Cement (kg/m³): 380; Max w/c: 0.40; Exposure: XA3 / DS-4z; Ground Conditions: Most aggressive — Thaumasite attack risk; brownfield and mining sites
• Designation: FND4M; DC Class: DC-4m; Strength Class: C28/35; Min Cement (kg/m³): 360; Max w/c: 0.40; Exposure: XA3 / DS-4m; Ground Conditions: High magnesium sulfate content sites

When Do I Need an FND Mix?

FND mixes are required whenever the site ground conditions have been assessed (or suspected) to contain elevated sulfate levels. Indicators include:(1)Made-up or filled ground, brownfield sites, former industrial land.(2)Clay soils — particularly in the Midlands, Yorkshire, London and South East where Lias, Oxford and London Clay formations are common and naturally contain elevated SO₃.(3)Any site where a preliminary soil investigation report specifies a DS (Design Sulfate) class above DS-1. On a typical greenfield site with no previous industrial use, GEN1 or RC28/35 is usually adequate — but BRE Special Digest 1 (2005) remains the definitive UK guidance for ground assessment and FND mix selection.

ST Standardised Prescribed Mixes (ST1–ST5)

Standardised prescribed concrete (SPC) mixes are specified byprescriptive composition rather than performance. They have no guaranteed strength, only a minimum cement content and prescribed proportions. They do not require third-party certification from the producer. Heidelberg Materials and other major suppliers note that ST mixes arenot recommendedwhere a designated mix is available from a certified supplier — designated GEN mixes offer lower carbon, better quality assurance and equivalent or better performance. [web:141]

• Designation: ST1; Assumed Strength: ~7.5 N/mm²; Min Cement (kg/m³): 180; GEN / RC Equivalent: GEN0; Typical Applications: Kerb bedding/backing, blinding, drainage haunching, general fill; Notes: Use GEN0 from certified supplier instead where available
• Designation: ST2; Assumed Strength: ~10 N/mm²; Min Cement (kg/m³): 210; GEN / RC Equivalent: GEN1; Typical Applications: Foundations (strip, trench-fill, mass), oversite, drainage, unreinforced floors with permanent finish; Notes: Most common ST mix for general domestic use
• Designation: ST3; Assumed Strength: ~15 N/mm²; Min Cement (kg/m³): 240; GEN / RC Equivalent: GEN2; Typical Applications: House floors (no permanent finish), unreinforced internal slabs; Notes: —
• Designation: ST4; Assumed Strength: ~20 N/mm²; Min Cement (kg/m³): 240; GEN / RC Equivalent: GEN3 / RC25/30; Typical Applications: Domestic garage floors, ground floor slabs (lightly reinforced); Notes: NOT for external freeze-thaw exposure
• Designation: ST5; Assumed Strength: ~25–30 N/mm²; Min Cement (kg/m³): 290; GEN / RC Equivalent: RC28/35; Typical Applications: House and garage ground floor slabs — fully nominally reinforced (ground-bearing or suspended), infill to ICF above ground; Notes: Strength only assumed, not guaranteed

BS EN 206 Exposure Classes — Full Reference

BS 8500-1:2023 uses the BS EN 206 exposure class system to define theenvironmental conditions that will influence concrete durability. Each concrete element must be assigned all relevant exposure classes — one for each type of attack it will face. The exposure classes directly drive the minimum cement content, maximum w/c ratio and minimum characteristic strength requirements. [web:27][web:28][web:133]

High water saturation, no de-icing agents. Horizontal surfaces exposed to rain and freezing — UK driveways and patios.

Cement & Combination Designations in BS 8500-2:2023

BS 8500-2:2023 permits a wide range of Portland cements, composite cements and factory-blended combinations. The 2023 edition addedCEM II/C-M and CEM VImulti-component cements (per BS EN 197-5:2021), enabling up to 65% Portland cement replacement with two or more low-carbon SCMs such as GGBS and fly ash — significantly reducing embodied carbon in concrete while maintaining durability performance. [web:132][web:134][web:138]

Pure Portland cement clinker with up to 5% minor additional constituents. The highest-strength, fastest-gaining cement type. Used where rapid strength gain is needed or where the purest cement is specified. Standard for sulfate-resistant variants (CEM I SR).

BS EN 197-1 · Classes: 32.5N, 42.5N, 42.5R, 52.5N, 52.5R

Portland cement blended with 6–35% of a single addition — typically limestone (LL), fly ash (V), slag (S) or silica fume (D).CEM II/A-LL(Portland-Limestone) is the standard GP cement sold in UK builders’ merchants by Blue Circle and Hanson. Lower embodied carbon than CEM I.

BS EN 197-1 · Most common UK bagged cement: CEM II/A-LL 32.5R

Introduced to BS 8500-2:2023 via BS EN 197-5:2021. Combines Portland clinker withtwo or more additions(e.g. GGBS + limestone fines). Enables lower embodied carbon than CEM II/B while maintaining performance. Gaining rapid adoption from UK ready-mix producers seeking lower carbon concretes.

BS EN 197-5:2021 · Added to BS 8500-2:2023

High GGBS content cement offering excellent sulfate resistance (CEM III/A+SR permitted for FND applications), very low permeability, good resistance to chloride ingress and significantly lower embodied carbon than CEM I. Slower strength gain — requires longer curing or heating in cold weather.

BS EN 197-1 · GGBS: BS EN 15167-1 · Widely used in UK ready-mix

Portland cement combined with pozzolanic additions — typically pulverised fuel ash (PFA/fly ash) from coal power stations. Good long-term durability, reduced heat of hydration (suitable for mass pours), lower embodied carbon. UK fly ash supply has reduced significantly as coal power generation has wound down.

BS EN 197-1 · PFA (fly ash): BS EN 450-1

The highest SCM replacement level cement now permitted in BS 8500-2:2023 via BS EN 197-5:2021. Up to 65% Portland clinker replaced with two or more SCMs (e.g. GGBS + fly ash, or GGBS + limestone fines). Lowest embodied carbon of all permitted cement types. Performance categories in BS 8500-2 track chloride diffusion and sulfate resistance for mix selection.

BS EN 197-5:2021 · Added to BS 8500-2:2023 · MPA-validated ternary systems

Low-Carbon Concrete & BS 8500:2023 — What Changed?

The 2023 revision to BS 8500 was driven primarily by the need to facilitate lower-carbon concrete. The key changes include:(1)Inclusion of CEM II/C-M and CEM VI multi-component cements (up to 65% clinker replacement).(2)New durability performance categories for ternary combinations (CEM I + GGBS + limestone fines, and CEM I + fly ash + limestone fines) based on chloride diffusion and sulfate resistance data.(3)Removal of minimum cement content limits for XC and XD/XS classes (leaving minimum characteristic strength as the primary requirement). These changes allow concrete producers to significantly reduce embodied carbon while maintaining BS 8500 compliance and without requiring separate design or specification work from the engineer. [web:132][web:134]

BS 8500 Reference Downloads & Resources

The following official and industry reference documents are available for download or purchase. The BSI standards themselves require purchase — however, several free industry guidance documents and quick-reference cards are available from The Concrete Centre, Mineral Products Association and BRMCA.

Current Edition: BS 8500:2023 — In Force Since November 2023

BS 8500-1:2023 and BS 8500-2:2023 are the current editions, published by BSI in November 2023. They supersede BS 8500:2015+A2:2019. The most significant changes are the inclusion of CEM II/C-M and CEM VI multi-component low-carbon cements, revised durability limit table structure, and removal of minimum cement content limits for certain exposure classes. Transition from the 2019 edition was immediate upon publication. Any project specifying concrete under BS 8500 should now reference the 2023 edition. [web:36][web:154]

Cover to Reinforcement — BS 8500-1:2023 Reference

Minimum nominal cover to reinforcement is a critical figure of the BS 8500 specification process. Cover must be sufficient to protect reinforcement against corrosion for the intended service life — typically50 years for residential buildingsand100 years for infrastructure. Nominal cover = minimum cover + an allowance for deviation (Δc_dev). For concrete cast against formwork, the standard minimum allowance isΔc_dev= 10mm(but 5mm is permitted where quality monitoring is enhanced). For concrete cast directly against the earth, the minimum allowance isΔc_dev= 50mm. [web:130]

• Exposure Class: XC1 (dry / perm. wet); Nominal Cover at 50yr Life: 15 + Δc dev mm; Nominal Cover at 100yr Life: 15 + Δc dev mm; Notes: Typically 25mm nominal cover in practice
• Exposure Class: XC2 (wet, rarely dry); Nominal Cover at 50yr Life: 25 + Δc dev mm; Nominal Cover at 100yr Life: 30 + Δc dev mm; Notes: Foundations in contact with soil
• Exposure Class: XC3 / XC4 (cyclic); Nominal Cover at 50yr Life: 30 + Δc dev mm; Nominal Cover at 100yr Life: 35 + Δc dev mm; Notes: External exposed RC slabs and columns
• Exposure Class: XD1 / XS1 (moderate); Nominal Cover at 50yr Life: 35 + Δc dev mm; Nominal Cover at 100yr Life: 40 + Δc dev mm; Notes: Salt-spray exposure — coastal structures
• Exposure Class: XD2 / XD3 / XS2 / XS3; Nominal Cover at 50yr Life: 40 + Δc dev mm; Nominal Cover at 100yr Life: 45 + Δc dev mm; Notes: Marine splash zone, car park slabs, road bridges
• Exposure Class: Cast against earth; Nominal Cover at 50yr Life: Min 75mm nominal; Nominal Cover at 100yr Life: Min 75mm nominal; Notes: Includes blinding allowance where used

Practical Cover Rule of Thumb for UK Domestic RC Slabs

For a typical domestic ground-bearing reinforced concrete slab (RC28/35, XC1–XC2, 50-year life, cast against blinding): minimum cover = 25mm + Δc_devof 15mm =40mm nominal cover. In practice, most UK domestic slabs use 50mm cover when cast against blinding, or 75mm when cast directly onto compacted hardcore without blinding — these round numbers provide comfortable compliance margins. Always check your structural engineer’s specification first — specific projects may require more onerous requirements than the BS 8500 minimums. [web:130]

How to Order BS 8500 Designated Concrete Correctly

When ordering ready-mix concrete, you must provide the producer with acomplete concrete specificationnot just the mix designation. The standard format used when ordering is:

Standard Order Format — Designated Concrete

Designated mix + Aggregate size + Consistence class + Volume + Any additional requirements

Example:“PAV1, 20mm aggregate, S3 consistence, 4.0 m³, BS 8500-1:2023, no additional requirements”
Example:“GEN1, 20mm aggregate, S4 consistence (trench fill), 6.5 m³, BS 8500-1:2023”
Example:“RC28/35, 20mm aggregate, S3 consistence, 8.0 m³, minimum cover 40mm, BS 8500-1:2023”

• Designated mix name:Always use the BS 8500 designation (PAV1, GEN1, RC28/35 etc.) — not old C-class names or mix ratios. This triggers the correct quality assurance system at the plant. [web:135]
• Aggregate size:20mm is standard for most applications. Use 10mm for thin sections (<150mm) or congested reinforcement. Pea gravel (10mm) or 14mm for screed toppings.
• Consistence class:S1 (stiff) for drainage haunching, S2–S3 for slabs and floors, S4 for trench fill and columns. Do not over-specify — wetter concrete is weaker.
• Volume:Always include determined volume plus your agreed waste allowance (typically 8–10%). Short-load surcharges apply when ordering less than 3–4 m³ — ask your supplier for their threshold.
• Any special requirements:Fibres (polypropylene or steel), coloured concrete, retarder (hot weather), accelerator (cold weather), foam agent. Specify at ordering stage — cannot be added on-site.
• Certification:You are entitled to ask for a delivery ticket (DC ticket) with every load confirming the mix designation, batch plant certificate number and time of batching. Keep all tickets for your records. [web:137]

BS 8500 FAQ

Frequently asked questions about BS 8500 designated concrete mixes, exposure classes, specification and the 2023 revision — answered for UK contractors, self-builders and specifiers. [web:149]

BS EN 206is the European standard for concrete — it defines general requirements for specification, production, performance testing and conformity across all EU/EEA member states and the UK. However, BS EN 206 contains numerous clauses that are left to “national provisions” — it sets the framework but allows each country to specify its own requirements for durability, cement types, exposure class limits and so on.

BS 8500is the UK’s “complementary standard” to BS EN 206 — it fills in all those national provision gaps for the United Kingdom specifically. It specifies designated mix families (GEN, RC, PAV, FND), the UK exposure class durability tables, permitted cement and combination types, and third-party certification requirements. You cannot fully specify concrete for a UK project from BS EN 206 alone — you must also reference BS 8500. [web:145][web:36]

In practice for UK contractors:if your delivery ticket references BS 8500-1:2023 and gives the correct designated mix name, you are compliant with both standards simultaneously.

BS 8500:2023 (published November 2023, superseding the 2015+A2:2019 edition) introduced the following principal changes: [web:36][web:130][web:154]

• New cement types permitted:CEM II/C-M (multi-component Portland composite, 50–64% clinker) and CEM VI (multi-component cement, 35–49% clinker) per BS EN 197-5:2021 are now included in BS 8500-2’s permitted cement and combination table. This was the primary driver of the revision — enabling significantly lower-carbon concrete while maintaining BS 8500 compliance.
• Revised durability table structure:The format of the durability limiting values tables was restructured, and some minimum cement content requirements were removed where characteristic strength was deemed the primary durability parameter. This reduces unnecessary cement content in some mix designs.
• New combined performance categories:The CPC (Combined Performance Category) system was refined to accommodate the new CEM II/C-M and CEM VI additions alongside existing GGBS and fly ash combinations, simplifying specification of ternary and quaternary cement systems.
• Carbonation and chloride class revisions:Updated recommendations for XC3/XC4 and XD/XS exposure classes incorporating new durability data from UK field and laboratory studies published since 2019.
• No change to designated mix names or principal requirements:GEN, RC, PAV and FND designations are unchanged. PAV1 remains C30/37, XF3, air-entrained. The transition was immediate on publication — no grace period was specified.

No — not for a compliant external driveway or patio.PAV1 and PAV2 both require air entrainment (3.5–6% entrained air), which cannot be achieved reliably by drum mixer or hand mixing. Air entrainment is introduced at the batch plant using highly controlled dosing of an air-entraining admixture (AEA) and is verified by a fresh concrete air content test. Even if you added an AEA to a site-mixed batch, you would have no way of verifying the resulting air content and no third-party certification to demonstrate compliance. [web:141]

Site-mixed concrete remains perfectly appropriate for the following applications where PAV mixes are not required:

• Foundations and trench fill (GEN1 / ST2 equivalent)
• Internal floor slabs without reinforcement (GEN1–GEN2 equivalent)
• Post holes, fence posts and gate posts
• Kerb bedding and backing (GEN0 equivalent)
• Garden walls and low masonry structures
• Repair and patching work where a small quantity is needed

For any application where site-mixing is appropriate, always order bagged cement asCEM II/A-LL or GP (General Purpose)this is the standard Blue Circle / Hanson bagged product. Mix to the correct ratio for the application (see the mix ratios table on our Materials Cost page), and use clean potable water at a water/cement ratio of approximately 0.5 for structural applications.

BS 8500 in its strictest sense applies tofactory-produced (ready-mix) concretesupplied by a certified producer to a certified product conformity scheme. Site-mixed concrete and bagged concrete products (bag-mix, post-mix, rapid-set concrete bags) are regulated by the component material standards (BS EN 197-1 for cement, BS EN 12620 for aggregates) rather than by BS 8500 directly.

However, BS 8500 provides theperformance benchmarksthat any concrete element should achieve regardless of how it is produced — and responsible designers and contractors use the BS 8500 exposure class and mix requirement tables to inform site-mix design even when ordering bags. Key practical points:

• Bagged concrete products (e.g. Postfix, Tarmac post-mix) are typically equivalent to GEN1–GEN3 depending on the product — always check the data sheet.
• Rapid-set bagged concrete products often sacrifice durability for speed. They are typically not appropriate for reinforced or exposed applications.
• For any application where your structural engineer has specified a BS 8500 designated mix, you must use ready-mix from a certified supplier — site-mixing is not acceptable as a substitute.
• For permitted development work (domestic extensions, garages) where no engineer has been engaged, BS 8500 provides the correct framework for selecting an appropriate mix strength and durability class. [web:145]

For a typical UK domestic reinforced ground-bearing concrete floor slab (e.g. house extension, new-build house), the correct minimum BS 8500 designated mix isRC28/35 (C28/35, XC2, S3 consistence). This provides:

• Characteristic compressive strength: 35 N/mm² (cube) at 28 days
• Minimum cement content: 280 kg/m³
• Maximum w/c ratio: 0.60
• Exposure class XC2: accounts for the cyclic wet conditions typical of ground-bearing slabs in UK soil moisture conditions

If the ground investigation has identified sulfate-bearing soils (DC-2 or above), you will need the appropriateFND mixinstead. If the slab is lightly reinforced (nominal mesh only, no structural reinforcement),GEN3 or ST4is acceptable for sheltered internal conditions — but RC28/35 is always the safer and more compliant choice for any slab with embedded steel. Always confirm with your structural engineer and Building Control. [web:132]

Prior to BS 8500, UK concrete was widely specified using old C-class (C10, C15, C20, C25, C30, C35, C40) or RC-class designations from BS 5328. These have been replaced by the BS 8500 designated mix system and the BS EN 206 characteristic strength class system (C12/15, C20/25 etc.). The approximate conversions are:

• Old C-Class: C10; BS EN 206 Strength: C8/10; Nearest BS 8500 Designated Mix: GEN0; Application: Blinding, kerb backing
• Old C-Class: C15; BS EN 206 Strength: C12/15; Nearest BS 8500 Designated Mix: GEN1; Application: Strip foundations, trench fill
• Old C-Class: C20; BS EN 206 Strength: C16/20; Nearest BS 8500 Designated Mix: GEN2–GEN3; Application: Internal slabs, garage floors
• Old C-Class: C25; BS EN 206 Strength: C20/25; Nearest BS 8500 Designated Mix: GEN3 / RC25/30; Application: Reinforced foundations, light RC
• Old C-Class: C30; BS EN 206 Strength: C25/30; Nearest BS 8500 Designated Mix: RC28/35 / PAV1; Application: General RC slabs; driveways / patios
• Old C-Class: C35; BS EN 206 Strength: C28/35; Nearest BS 8500 Designated Mix: RC32/40 / PAV2 / FND2; Application: Agricultural RC; road-adjacent driveways
• Old C-Class: C40; BS EN 206 Strength: C32/40; Nearest BS 8500 Designated Mix: RC35/45 / RC40/50; Application: Heavy duty industrial, agricultural severe

Note that the old C-class to new designation mapping is approximate — the exposure class and application are equally important. Do not simply order by strength class — always specify the full BS 8500 designation. [web:140]

For many simple domestic extensions, a structural engineer is not always legally required to specify the concrete mix — Building Regulations Approved Document A provides prescriptive foundation depth and type guidance for straightforward strip foundations on typical ground conditions. However, a structural engineer or competent designer should specify the concrete mix wherever:

• The building has more than two storeys or any unusual geometry or loading
• Ground conditions are uncertain, made up, or may contain sulfates (DC-2 or above)
• The extension includes a reinforced concrete slab, beam and block floor or any structural RC element
• Building Control or your mortgage lender requires an engineer’s sign-off
• Any element of the work is near or below the water table

For a simple single-storey extension with strip foundations on undisturbed clay or firm ground with no history of contamination, a competent contractor or experienced self-builder usingGEN1 at S4 consistence for trench fill(or GEN1 at S3 for strip foundations) is generally appropriate for the concrete specification — but always confirm with your Building Control officer before proceeding. Specifying a slightly stronger mix than the minimum (e.g. GEN1 where GEN0 might technically suffice) adds very little cost and provides a comfort margin against variability in ground conditions. [web:145]

Related Resources & Guides

More free UK concrete specification resources, guides and cost guides — all updated for BS 8500:2023 and 2026 material prices.