Reinforced Concrete Mix Design, Volume & Cost Estimator – Updated 2026 Prices
RC28/35 is the BS 8500 designated mix for reinforced concrete requiring a characteristic compressive strength of 28 N/mm² (cylinder) / 35 N/mm² (cube). It is widely used for structural slabs, beams, columns, retaining walls, and foundations exposed to XC2, XC3 or XC4 carbonation conditions. Enter your element dimensions and project details to calculate volume, BS 8500-compliant mix specification, and estimated cost. Always confirm mix design with your structural engineer and ready-mix supplier.
RC28/35 is a BS 8500 designated mix for reinforced concrete. The designation breaks down as follows: RC = Reinforced Concrete (indicating the mix is designed for use with steel reinforcement); 28 = the minimum characteristic compressive strength measured on standard cylinders (150mm × 300mm) at 28 days, in N/mm²; 35 = the minimum characteristic compressive strength measured on standard cubes (150mm) at 28 days, in N/mm². The cylinder strength is always lower than the cube strength for the same concrete because cylinders are less restrained at their ends during testing, giving a more conservative (lower) result — the ratio is approximately 0.8 for normal-strength concrete.
RC28/35 concrete corresponds to strength class C28/35 under BS EN 206 and BS EN 1992-1-1 (Eurocode 2). It is one of the most widely used structural concrete grades in UK construction in 2026, appropriate for: reinforced concrete floor slabs; beams and lintels; ground floor slabs; retaining walls; staircases; columns in moderate exposure; and bridge abutments and parapet bases. It is stronger than the domestic GEN 3 / C25 concrete used for plain foundations, and provides the durability performance required for reinforced elements in moderate to aggressive exposure conditions.
Under BS 8500-1:2015+A2:2019, the RC28/35 designated mix is defined for use in specific exposure classes. The full designated mix notation includes the exposure class — for example, RC28/35XC4 identifies the concrete as RC28/35 grade in XC4 (cyclic wet and dry) exposure. This tells the ready-mix plant all the requirements: strength class, maximum water/binder ratio, minimum binder content, and cover to reinforcement. Specifying concrete using the BS 8500 designated mix system is the simplest and most reliable approach for standard UK structural applications.
The choice between adjacent strength classes often causes confusion in UK structural design. The key practical distinctions are:
The table below summarises the minimum concrete specification requirements for RC28/35 across common exposure classes and design service lives, per BS 8500-1:2015+A2:2019 Table A.3.
| Exposure Class | Binder | Design Life | Min. Strength | Max. w/b | Min. Binder (kg/m³) | Nominal Cover (mm) | Notes |
|---|---|---|---|---|---|---|---|
| XC2 | CEM I | 50 yr | C25/30 | 0.55 | 260 | 35 | RC25/30 meets XC2; RC28/35 provides margin |
| XC3 / XC4 | CEM I | 50 yr | C28/35 | 0.55 | 280 | 35 | Most common RC28/35 application – above-ground RC structures |
| XC3 / XC4 | CEM I | 100 yr | C32/40 | 0.45 | 320 | 45 | Upgrade to RC32/40 for 100-year XC4 life |
| XC3 / XC4 | CEM I + 35% GGBS | 50 yr | C25/30 | 0.55 | 260 | 30 | GGBS allows relaxation; RC28/35 spec still valid |
| XC3 / XC4 | CEM I + 50% GGBS | 50 yr | C25/30 | 0.55 | 260 | 30 | Excellent durability; reduced carbon; slower strength gain |
| XC3 / XC4 | CEM I + 25% PFA | 50 yr | C28/35 | 0.55 | 280 | 35 | PFA does not allow significant cover reduction vs CEM I |
| XC2 | CEM I | 100 yr | C28/35 | 0.50 | 280 | 45 | RC28/35 is the minimum for XC2 at 100-year life |
| XC4 + XF2 | CEM I | 50 yr | C28/35 | 0.50 | 300 | 40 | XF2 requires air entrainment; governs over XC4 w/b and binder |
| Mix / Specification | Supply Method | Cost per m³ (excl. VAT, SE England 2026) | Notes |
|---|---|---|---|
| RC28/35XC4, CEM I, S3, 20mm agg | Ready-mix | £115–£158/m³ | Standard above-ground structural RC; most common RC28/35 specification |
| RC28/35XC2, CEM I, S3, 20mm agg | Ready-mix | £110–£150/m³ | Below-ground; XC2 slightly less demanding; same strength class |
| RC28/35XC4, CEM I + 35% GGBS, S3 | Ready-mix | £112–£155/m³ | Modest carbon saving; similar cost to CEM I |
| RC28/35XC4, CEM I + 50% GGBS, S3 | Ready-mix | £108–£150/m³ | Good carbon saving; slower strength gain; suited to thick elements |
| RC28/35XC4, CEM I, S4, 20mm agg (pumped) | Pumped ready-mix | £125–£172/m³ + pump hire | Higher consistency for pump; pump hire £350–£700/day extra |
| RC28/35XC4, CEM I, S3, 20mm agg | Mini-mix / volumetric | £138–£182/m³ + call-out | No minimum order; good for small structural pours under 3 m³ |
RC28/35 concrete resources UK 2026: The Concrete Centre | BSI – BS 8500 / BS EN 206 | Mineral Products Association | CEMEX UK – Structural Concrete | IStructE – Find a Structural Engineer
| Item | Typical Cost (2026) | Notes |
|---|---|---|
| Ready-mix delivery charge | £150–£300 per delivery | Included above minimum order (typically 3–6 m³); short-load surcharge below minimum |
| Concrete pump hire | £350–£700/day + per m³ | Required for elevated slabs, congested reinforcement, or restricted access; line pump or boom pump |
| Reinforcement (rebar) supply | £580–£850/tonne supply | B500B / B500C high yield bar; typical RC slab uses 80–150 kg/m³ concrete volume |
| Reinforcement fixing labour | £200–£500/tonne fix | Bar bending, tying, and spacer placement; complex layouts at higher end |
| Formwork / shuttering supply and strike | £15–£45/m² formed face | Plywood, props, and falsework; beam soffits and column forms at higher end |
| Concrete cube testing (per set of 6) | £80–£200 per set | Required for structural RC; 28-day cube strength; UKAS-accredited laboratory |
| Plasticiser / superplasticiser admixture | £3–£8/m³ (usually in RM price) | Required at lower w/b ratios to maintain workability; confirm included in ready-mix price |
| Structural engineer design fee | £500–£5,000+ depending on project | Required for all RC structural elements; Eurocode 2 design; Building Regulations submission |
| Cover / spacer chairs | £0.10–£0.50 per spacer | Plastic or concrete spacers to maintain nominal cover; critical for durability compliance |
| Curing compound | £0.50–£1.50/m² applied | Spray-applied membrane; prevents premature drying; especially important in warm or windy weather |
RC = Reinforced Concrete designated mix (BS 8500). 28 = characteristic cylinder strength in N/mm². 35 = characteristic cube strength in N/mm². The designation tells the ready-mix plant the required structural performance. The full mix notation also includes the exposure class — e.g. RC28/35XC4 — which specifies the durability requirements (maximum w/b, minimum binder content) alongside the strength requirement.
UK concrete is tested by both methods: the cube test (150mm cube, BS EN 12390-3) is more common on UK sites; the cylinder test (150×300mm, BS EN 12390-3) is used in Eurocode 2 design. For normal-strength concrete, the cylinder/cube ratio is approximately 0.8 — so C28/35 means 28 N/mm² on a cylinder and 35 N/mm² on a cube. When ordering, specify the cube strength (35 N/mm²) to your ready-mix supplier; the structural engineer's design is based on the cylinder strength (28 N/mm²).
Nominal cover to reinforcement for RC28/35 in XC3/XC4 exposure with a 50-year design life (CEM I) is 35mm — including the 10mm Δcdev deviation allowance. Insufficient cover allows carbonation to reach the reinforcement faster, initiating corrosion and eventually causing concrete spalling. Cover must be maintained using correctly sized plastic or concrete spacers — never improvised materials (stone chippings, pieces of bar, or timber wedges). Check cover with a cover meter after reinforcement is placed and before concrete is poured.
For all structural reinforced concrete elements subject to Building Regulations, concrete cube testing is required to confirm that the delivered concrete achieves the specified strength. A set of 6 cubes (or 3 pairs) is taken from each load or at agreed intervals — typically one set per 50 m³. Cubes are cured in water at 20°C and crushed at 7 and 28 days by a UKAS-accredited laboratory. If cubes fail to meet the 28-day target (characteristic mean minus standard deviation), the engineer must assess whether the structure is adequate or remedial action is required.
The water/binder (w/b) ratio is the single most important parameter governing concrete durability. A lower w/b ratio produces denser, less permeable concrete that resists carbonation, sulfate attack, and chloride ingress more effectively. For RC28/35 in XC3/XC4 exposure, the maximum w/b is 0.55. Never add water to ready-mix concrete on site to improve workability — this raises the w/b above the specified maximum, increasing porosity and reducing durability. Specify a superplasticiser or higher consistency class instead.
Replacing 50% of CEM I with GGBS in an RC28/35 mix reduces embodied carbon by approximately 43–47% — from approximately 280–300 kg CO₂e/m³ for 100% CEM I to approximately 150–170 kg CO₂e/m³. GGBS also improves sulfate resistance and reduces heat of hydration (important for thick slabs and walls). The trade-off is slower early strength gain, which may extend formwork striking times and cold weather concreting risks. Under BS 8500, CEM I + 50% GGBS can achieve the same XC3/XC4 durability class as CEM I, often with a slight relaxation of the minimum strength class.
RC28/35 is the standard specification for reinforced concrete elements in domestic extensions and new builds — including reinforced ground floor slabs, cantilever retaining walls, exposed stair flights, and structural columns. Building Control typically requires RC28/35 (or the equivalent C28/35 designed mix) for above-ground reinforced elements, with a structural engineer's design confirming section sizes, reinforcement layout, and cover requirements. For unreinforced domestic strip foundations, the lower GEN 3 / C25 specification is still accepted.
RC28/35 should not be mixed on site using a drum mixer and bagged materials for any structural application requiring compliance documentation. The water/binder ratio, binder content, and consistency cannot be reliably controlled with site-mixed concrete, and no conformity certificate can be issued. For structural reinforced concrete elements subject to Building Regulations, ready-mix concrete from a QSRMC-certified plant is strongly recommended, providing mix design certificates, delivery tickets, and cube test procedures that satisfy Building Control requirements.
RC28/35 mixes commonly include a plasticiser or superplasticiser to achieve the required consistency at the maximum w/b ratio. For pumped mixes (S4), a higher-range water reducer (HRWR) is typically specified. Retarders extend workability in warm weather or long haul distances. Accelerators speed up early strength gain in cold weather. Admixtures must be compatible with the binder — in particular, some superplasticisers can interact negatively with high GGBS or PFA content at certain dosages. Always confirm admixture compatibility and dosage with the ready-mix plant before the pour.
Structural design of RC28/35 elements must be carried out to BS EN 1992-1-1 (Eurocode 2) — the mandatory UK structural concrete design standard. Eurocode 2 uses the characteristic cylinder strength (fck = 28 N/mm²) as the design basis, with a material partial factor γC = 1.5 for concrete. The design compressive strength is fcd = αcc × fck / γC = 0.85 × 28 / 1.5 = 15.9 N/mm² (using the UK NA value αcc = 0.85). All structural RC elements — slabs, beams, columns, walls — must be designed by a chartered structural engineer.
Adequate curing is essential for RC28/35 concrete to achieve its target 28-day strength and long-term durability. For CEM I mixes, maintain moist curing conditions for a minimum of 3–5 days. For GGBS blends (35–50%), extend to 5–7 days. In hot or windy weather, apply a curing compound immediately after finishing the surface. In cold weather (below 5°C), protect with insulating blankets and ensure the concrete temperature stays above 5°C for at least 72 hours after placement. Never allow RC28/35 to freeze within 28 days of casting.
RC28/35 concrete supply for new residential dwellings is zero-rated for VAT under HMRC's new build relief — a significant saving for self-builders and housing developers. For extensions, commercial construction, and infrastructure works, standard rate VAT at 20% applies to concrete supply and placing. Confirm the VAT treatment with your contractor and ready-mix supplier — particularly on self-build and conversion projects where the zero-rating rules are complex and the VAT difference on a large pour can be substantial.
| Designated Mix | Strength Class | fck (cyl, N/mm²) | fck,cube (N/mm²) | Typical Exposure | Typical Application | Approx. Cost/m³ (SE England 2026) |
|---|---|---|---|---|---|---|
| RC20/25 | C20/25 | 20 | 25 | XC1 | Internal RC elements; protected from weather | £95–£130 |
| RC25/30 | C25/30 | 25 | 30 | XC2 | Buried foundations, pile caps, below-ground walls | £100–£140 |
| RC28/35 | C28/35 | 28 | 35 | XC3 / XC4 | Above-ground RC slabs, beams, columns, retaining walls | £115–£158 |
| RC30/37 | C30/37 | 30 | 37 | XC4 / XD1 | Exposed RC, car parks, bridge elements, some foundations | £120–£165 |
| RC32/40 | C32/40 | 32 | 40 | XC4 (100yr) / XF2 | Infrastructure, highway structures, freeze-thaw exposed RC | £128–£175 |
| RC35/45 | C35/45 | 35 | 45 | XD2 / XS1 | Marine, coastal, chloride-exposed RC; prestressed concrete | £135–£185 |
| RC40/50 | C40/50 | 40 | 50 | XD3 / XS3 / XF4 | Highly aggressive environments; offshore; tidal zone | £148–£200 |
fck = characteristic cylinder compressive strength. fck,cube = characteristic cube compressive strength. Design by structural engineer to BS EN 1992-1-1 (Eurocode 2) required for all RC elements. Costs are for CEM I S3 20mm aggregate ready-mix, excl. delivery, SE England 2026. Add 10% for pumped supply, 15–20% for London.
RC28/35 is a BS 8500 designated mix for reinforced concrete. RC stands for Reinforced Concrete, identifying it as a mix designed for use with steel reinforcement. 28 is the characteristic compressive strength in N/mm² measured on standard cylinders (150×300mm) at 28 days. 35 is the characteristic compressive strength in N/mm² measured on standard cubes (150mm) at 28 days. The cylinder strength is lower than the cube strength for the same concrete because the test geometry gives a less restrained failure. The ratio is approximately 0.8 for normal-strength concretes. RC28/35 corresponds to strength class C28/35 under BS EN 206 and Eurocode 2.
RC28/35 is the standard structural concrete grade for above-ground reinforced concrete elements in UK building and infrastructure construction. Common applications include: reinforced concrete ground floor slabs and upper floor slabs; RC beams, lintels, and ring beams; structural columns and walls; cantilever and gravity retaining walls; stair flights; RC roof structures; domestic extension structures; and general infrastructure elements exposed to moderate weather (XC3 or XC4 carbonation exposure class). It is also used for below-ground elements with a 100-year design life (XC2, 100-year = C28/35 minimum). For elements in chloride or freeze-thaw environments, a higher strength class (RC30/37 to RC35/45) is typically required.
They refer to the same concrete strength class, but with different notations. C28/35 is the BS EN 206 / Eurocode 2 notation for the strength class — characteristic cylinder / cube strengths of 28 / 35 N/mm². RC28/35 is the BS 8500 designated mix notation, where the RC prefix identifies the mix as being designed for reinforced concrete applications and associated with specific durability requirements per BS 8500 Table A.3. In practice, when ordering from a ready-mix plant, you specify RC28/35 plus the exposure class (e.g. RC28/35XC4), which gives the plant all the information needed to produce a BS 8500-compliant mix. The structural engineer's Eurocode 2 design is based on C28/35 (the strength class notation).
RC28/35 ready-mix concrete costs approximately £115–£158 per m³ for a standard CEM I, S3 consistency, 20mm aggregate mix in South East England in 2026, plus a delivery charge of £150–£300 per truck. For North England, Wales, and Scotland, deduct approximately 10%; for Greater London, add approximately 15–20%. With a 50% GGBS binder, the cost is slightly lower (£108–£150/m³). Pumped supply adds approximately £10–£20/m³ plus pump hire of £350–£700/day. For small pours (under 3 m³), mini-mix volumetric concrete at £138–£182/m³ plus a call-out fee of £100–£160 is often the most practical option.
The minimum nominal cover to reinforcement for RC28/35 concrete depends on the exposure class: for XC2 (below ground, 50-year life) with CEM I, nominal cover is 35mm; for XC3/XC4 (above ground, 50-year life) with CEM I, nominal cover is 35mm; for XC3/XC4 with CEM I + 35%+ GGBS, cover reduces to 30mm; for XC2/XC3/XC4 with 100-year design life, cover increases to 45mm (CEM I). These values include the 10mm construction deviation allowance (Δcdev) and are taken from BS EN 1992-1-1 UK NA Table 4.4N. Always maintain cover using correctly sized plastic spacers — insufficient cover is the most common cause of premature RC durability failure.
Yes — RC28/35 is appropriate and commonly used for reinforced concrete slabs in domestic extensions. For a reinforced ground floor slab (as required for extensions with suspended or engineered floor systems), RC28/35 in XC2 or XC3 exposure class is the standard specification. For an upper floor slab in a domestic extension, RC28/35 in XC3 exposure is appropriate. The slab thickness, reinforcement layout, and cover requirements must be designed by a structural engineer to Eurocode 2 and submitted for Building Control approval. For unreinforced domestic strip and trench-fill foundations, the lower GEN 3 / C25 designation is still accepted by Building Control.
The most straightforward way to specify RC28/35 is using the BS 8500 designated mix system. Provide the following to the ready-mix plant: the designated mix (e.g. RC28/35XC4 for above-ground reinforced concrete in cyclic wet/dry exposure); the binder combination (e.g. CEM I, or CEM I + 35% GGBS); the maximum aggregate size (typically 20mm); and the consistency class (e.g. S3). The plant will then produce a mix meeting all BS 8500 Table A.3 requirements for those parameters. Request a mix design certificate and confirm the water/binder ratio target and admixture system. Also confirm cube testing arrangements — at minimum one set of 6 cubes per 50 m³ for structural RC.
Under BS 8500-1:2015+A2:2019 Table A.3, the minimum binder (cementitious) content for RC28/35 in XC3/XC4 exposure with CEM I is 280 kg/m³ for a 50-year design life — rising to 320 kg/m³ for a 100-year design life. In practice, most RC28/35 ready-mix contains 290–320 kg/m³ of binder to provide a margin above the minimum and to ensure consistent cube strength performance. The minimum binder content ensures adequate paste volume for workability and durability, and limits the w/b ratio at the specified cement content and water demand. With GGBS at 35–50%, the minimum binder content reduces to 260 kg/m³ (same total cementitious, but with better durability characteristics due to the finer pore structure produced by GGBS hydration).
Yes — for any structural reinforced concrete element subject to Building Regulations, cube testing is required to confirm that the delivered concrete meets the specified 28-day characteristic strength. The standard approach is to take sets of 6 cubes (or 3 pairs) at regular intervals — typically one set per 50 m³ poured, or per discrete structural element. Cubes are cured in water at 20°C for 28 days and crushed by a UKAS-accredited laboratory. Results must meet the BS EN 206 conformity criteria (mean of 3 results ≥ fck + 4 N/mm²; individual result ≥ fck − 4 N/mm²). Keep delivery tickets for all loads and cube test certificates for Building Control and future reference.
RC28/35 concrete with a CEM I binder typically achieves approximately 60–75% of its 28-day strength at 7 days — around 21–26 N/mm² cube strength. This means a target 35 N/mm² cube strength at 28 days corresponds to approximately 21–26 N/mm² at 7 days. GGBS-blended mixes gain strength more slowly: at 7 days, CEM I + 50% GGBS may have achieved only 40–55% of 28-day strength (approximately 14–19 N/mm²), though it continues to gain strength beyond 28 days and typically surpasses the CEM I equivalent at 90+ days. For formwork striking decisions, always use the actual 7-day cube result (or a maturity estimate) rather than a generic percentage — cold weather significantly slows early strength gain.
Additional RC28/35 resources UK 2026: The Concrete Centre | BSI – BS 8500 / BS EN 206 / Eurocode 2 | MPA – Mineral Products Association | IStructE – Structural Engineers | GOV.UK – Building Regulations | CEMEX UK – Structural Concrete