C28/35 Grade (Structural)

Complete UK guide to C28/35 (RC28/35) — the structural concrete mix with 35 N/mm² characteristic cube strength. Covers BS 8500 specification, Eurocode 2 design properties, exposure class suitability, mix design, typical applications, and 2026 UK ready-mix pricing.

What is C28/35 Concrete?

C28/35is a structural concrete grade defined byBS EN 206and specified in the UK throughBS 8500. The notation C28/35 means: characteristiccylindercompressive strength f_ck=28 N/mm²and characteristiccubecompressive strength f_ck,cube=35 N/mm², both measured at 28 days. It is a mid-range structural grade widely used across UK residential and commercial construction for reinforced concrete elements.

In UK practice, C28/35 is ordered and tested using the BS 8500 designated mix referenceRC28/35the “RC” prefix denoting a reinforced concrete mix suitable for structural use. Cube specimens (150mm × 150mm × 150mm) are crushed at 28 days to verify the 35 N/mm² cube strength requirement under BS EN 12390-3.

C28/35 at a Glance — Key Identity Data

• BS EN 206 / BS 8500 designation:C28/35 (European notation) / RC28/35 (UK BS 8500 designated mix)
• Characteristic cylinder strength f_ck:28 N/mm²
• Characteristic cube strength f_ck,cube:35 N/mm² (tested at 28 days per BS EN 12390-3)
• Mean cylinder strength f_cm:36 N/mm² (= f_ck+ 8)
• Relationship to old BS 8110:Approximately equivalent to the old BS 8110 “Grade 35” (f_cu= 35 N/mm²)
• Position in the grade ladder:Sits between C25/30 (cube 30) and C30/37 (cube 37) — a structural upgrade from standard C25/30 with moderate cost premium
• Slump class:Typically S2 (50–90mm) or S3 (100–150mm) for pumped concrete — specified separately from strength class

Residential Structural Use

Commonly used in UK residential construction for reinforced concrete ground beams, suspended slabs, retaining walls, and pile caps where C25/30 is insufficiently strong but C30/37 would be over-specified. Popular with structural engineers for domestic basements and extensions.

Commercial & Industrial

Specified for structural frames, transfer beams, stair cores, and ground-bearing industrial floor slabs. Offers a useful performance step above C25/30 — particularly where the structural engineer needs to reduce section sizes or reinforcement congestion without moving to the higher cost of C30/37 or C35/45.

Eurocode 2 Structural Design

C28/35 is a fully supported concrete strength class in Eurocode 2 (BS EN 1992-1-1) — all material properties (f_ctm, E_cm, ε_cu) are defined by the standard formulae using f_ck= 28 N/mm². Design using the UK National Annex gives f_cd= 15.9 N/mm².

Eurocode 2 Design Properties — C28/35

All Eurocode 2 material properties for concrete are derived from the characteristic cylinder strength f_ck. For C28/35, f_ck= 28 N/mm². The following properties are determined using the formulae in BS EN 1992-1-1 Table 3.1 and the UK National Annex parameters.

Full EC2 Material Property Set — C28/35

• Property: Char. cylinder strength; Symbol: f ck; Value: 28; Unit: N/mm²; Notes / Formula: Design basis — use in all EC2 formulas
• Property: Char. cube strength; Symbol: f ck,cube; Value: 35; Unit: N/mm²; Notes / Formula: UK testing and specification (BS 8500)
• Property: Mean cylinder strength; Symbol: f cm; Value: 36; Unit: N/mm²; Notes / Formula: f cm = f ck + 8
• Property: Design comp. strength (UK); Symbol: f cd; Value: 15.9; Unit: N/mm²; Notes / Formula: 0.85 × 28 / 1.50 (UK NA α cc = 0.85)
• Property: Mean tensile strength; Symbol: f ctm; Value: 2.77; Unit: N/mm²; Notes / Formula: 0.30 × f ck 2/3 (f ck ≤ 50)
• Property: Lower char. tensile strength; Symbol: f ctk,0.05; Value: 1.94; Unit: N/mm²; Notes / Formula: 0.7 × f ctm
• Property: Mean elastic modulus; Symbol: E cm; Value: 32,000; Unit: N/mm²; Notes / Formula: 22 × (f cm /10) 0.3 × 1000
• Property: Strain at peak stress; Symbol: ε c1; Value: 2.0; Unit: ‰; Notes / Formula: Table 3.1 — constant for f ck ≤ 50
• Property: Ultimate compressive strain; Symbol: ε cu2; Value: 3.5; Unit: ‰; Notes / Formula: Table 3.1 — constant for f ck ≤ 50
• Property: Poisson’s ratio (uncracked); Symbol: ν; Value: 0.2; Unit: —; Notes / Formula: Clause 3.1.3
• Property: Coefficient of thermal expansion; Symbol: α T; Value: 10 × 10⁻⁶; Unit: /°C; Notes / Formula: Clause 3.1.3

Using f ctm for Minimum Reinforcement in EC2

The mean tensile strength f_ctm= 2.77 N/mm² for C28/35 is used in the minimum reinforcement formula for beams and slabs: A_s,min= 0.26 × (f_ctm/f_yk) × b_t× d. For B500B reinforcement: A_s,min= 0.26 × (2.77/500) × b_t× d =0.00144 × b_t× d. This gives a slightly higher minimum reinforcement than C25/30 (which gives 0.00135 × b_t× d) — a consequence of the higher tensile strength of the higher grade concrete.

Mix Design & BS 8500 Specification

In UK practice, C28/35 concrete is specified using theBS 8500framework, which translates the BS EN 206 strength class into a complete concrete specification including cement type, water-cement ratio, and minimum cement content. BS 8500 provides two specification routes: designated mixes and designed mixes.

Designated Mix — RC28/35

The BS 8500 designated mixRC28/35is the simplest specification route. The producer guarantees compliance with the full BS 8500 recipe — specifier only needs to state the designated mix reference and slump class. No mix proportions working required. Suitable for most standard reinforced concrete applications.

Designed Mix

A designed mix allows the specifier to define specific requirements beyond the designated mix — e.g. specific cement type (CEM I, CEM II, GGBS blend), alkali-silica reaction (ASR) mitigation, admixture requirements, or specific w/c ratio for a non-standard exposure condition. Requires a concrete technologist to design the mix to BS EN 206 / BS 8500.

Prescriptive Mix

For very small projects or site-mixed concrete, BS 8500 Table A.4 gives prescriptive mix proportions for various strength classes. For C28/35, this typically means a cement:aggregate ratio of approximately 1:4.5 with a maximum w/c ratio of 0.55. Site-mixed concrete is rare in structural applications and requires careful quality control.

Typical RC28/35 Concrete Specification Parameters

• Parameter: Strength class; Value / Requirement: C28/35 (f ck,cube = 35 N/mm²); Standard: BS EN 206; Notes: Tested at 28 days — 150mm cubes
• Parameter: Max w/c ratio (XC3/XC4); Value / Requirement: 0.55; Standard: BS 8500 Table A.4; Notes: Reduces to 0.50 for XD1/XS1
• Parameter: Min cement content (XC3/XC4); Value / Requirement: 280 kg/m³; Standard: BS 8500 Table A.4; Notes: Increases for more severe exposures
• Parameter: Typical cement types; Value / Requirement: CEM I 42.5; CEM II/A-L; GGBS 30–50% replacement; Standard: BS EN 197-1; Notes: GGBS reduces heat of hydration — good for mass pours
• Parameter: Max aggregate size; Value / Requirement: 20mm standard; 10mm for congested areas; Standard: BS EN 12620; Notes: ≤ min(cover; bar spacing – 5mm; 1/3 of slab depth)
• Parameter: Consistence (slump); Value / Requirement: S2 (50–90mm) or S3 (100–150mm); Standard: BS EN 206; Notes: S3/S4 for pumped concrete — specify flow class F for self-compacting
• Parameter: Admixtures; Value / Requirement: Plasticiser or superplasticiser (workability without excess water); Standard: BS EN 934-2; Notes: Do not add water on site — calls for superplasticiser
• Parameter: Air entrainment; Value / Requirement: Not required for XC/XD exposure; Standard: BS 8500; Notes: Required for XF3/XF4 — specify separately
• Parameter: Min curing period; Value / Requirement: 3 days (normal conditions); 7 days (slow-hardening cements); Standard: BS EN 13670; Notes: Extend for cold weather (≤5°C) or high wind

Never Add Water on Site

Adding water to concrete after batching reduces the water-cement ratio and lowers strength. For every 10 litres of water added per m³ of concrete, compressive strength falls by approximately 2–3 N/mm² — meaning a C28/35 can easily be reduced to C25/30 or below by on-site water addition. If workability is insufficient, the correct solution is to specify a higher slump class or request a plasticiser admixture at the batching plant — not to add water on site.

Exposure Class Suitability — C28/35

Exposure classes per BS EN 206 / BS 8500 define the environmental conditions the concrete will be exposed to throughout its design life. C28/35 (cube strength 35 N/mm²) is suitable for a specific range of exposure conditions — exceeding the minimum for some classes and falling short for others.

C28/35 is fully suitable for all XC classes (XC1–XC4).The minimum concrete grade for XC3/XC4 is C25/30 — C28/35 exceeds this with margin. Suitable for internal slabs (XC1), foundations in damp soil (XC2), sheltered external concrete (XC3), and fully exposed external elements such as soffits and external walls (XC4).

In BS 8500 terms: RC28/35 satisfies the carbonation durability requirements for all XC classes with appropriate cover. Minimum nominal cover for XC3/XC4 with C28/35 is c_nom= 35mm (c_min,dur25mm + Δc_dev10mm).

For XD1 (moderate humidity, airborne chlorides — e.g. car park soffits):BS 8500 requires minimum C30/37. C28/35 falls marginally short. In practice, engineers often specify C30/37 or above for all XD exposures. C28/35 should not be specified for XD exposure without specific mix design justification.

XD2/XD3 (cyclic wet/dry with de-icing salts — bridge decks, road kerbs):Require minimum C35/45. C28/35 is not suitable for XD2 or XD3 exposure.

C28/35 is not suitable for any XS (sea water chloride) exposure class.XS1–XS3 all require minimum C35/45 in BS 8500. Marine structures — coastal buildings, piers, sea walls, and offshore structures — require at minimum C35/45 with a cement combination offering good chloride resistance (e.g. GGBS blend or CEM III/B).

For any structure within the splash/tidal zone (XS3) or permanently submerged in sea water (XS2), C40/50 or higher is commonly specified in UK practice to extend durability towards the 100-year design life required for infrastructure.

XF1 (moderate saturation, no de-icing agent):C28/35 is suitable — minimum C25/30 required. Applies to vertical external concrete surfaces exposed to rain but not pooling water.

XF2–XF4 (high saturation, with/without de-icing agents):These classes requireair-entrained concretethe strength class alone is insufficient. For XF3 (high saturation, no de-icing agent — e.g. horizontal external paving), specify air-entrained C28/35 with 4–5% air content. For XF4 (de-icing salts + freeze-thaw — driveways, car parks), specify air-entrained C30/37 minimum — C28/35 marginal.

C28/35 may be used for XA1 (slightly aggressive — mild sulphate ground)provided a sulphate-resistant cement is used and the mix meets BS 8500 Table A.4 requirements. In UK practice, sulphate-bearing ground uses BS 8500 FND designated mixes (FND1–FND4) rather than RC mixes — FND mixes specify sulphate-resistant cements.

XA2/XA3 (moderate to highly aggressive chemical environments):Require FND3/FND4 mixes — C28/35 as an RC designated mix is not appropriate. Always commission a site investigation and NHBC/BRE sulphate class assessment before specifying concrete for foundations in brownfield or clay ground.

C28/35 is fully suitable for X0 exposurethis is the lowest risk class (plain concrete only in dry interior environments). X0 requires only C12/15 minimum — C28/35 significantly exceeds this. In practice, X0 exposure is rare for reinforced concrete as RC elements in any real building are exposed to at least XC1 conditions.

Note: X0 is specifically for plain (unreinforced) concrete only in BS EN 206. Reinforced concrete elements always require at minimum XC1 exposure classification.

Typical Applications of C28/35

C28/35 occupies a useful structural position in the UK concrete grade hierarchy — stronger than the basic structural grade C25/30, but less expensive than C30/37 or C35/45. It is widely used across residential, commercial, and civil engineering applications where moderate-to-high structural performance is required.

Domestic Basement Walls & Slabs

One of the most common applications in UK residential construction. Basement retaining walls, base slabs, and waterproof (tanked) construction frequently specify C28/35 or C30/37 — the higher strength reduces crack widths, aids waterproofing under EC2 Part 3 tightness class requirements, and provides durability against XC2 groundwater exposure. Often specified with a GGBS cement blend to reduce heat of hydration cracking in thicker walls.

Reinforced Concrete Ground Beams

Ground beams supporting the superstructure of timber frame or masonry buildings on strip or pad foundations are commonly C28/35 in UK residential practice. The structural engineer specifies C28/35 where the beam span, depth-to-width ratio, or loading requires higher performance than C25/30 can economically deliver, without the cost premium of C30/37.

Structural Frame — Beams, Columns & Slabs

In medium-rise commercial and residential frames (3–8 storeys), C28/35 is used for structural beams, flat slabs, and columns where the higher characteristic strength versus C25/30 allows reduced section sizes or reinforcement areas. The modest cost premium over C25/30 is offset by programme savings from thinner slabs and smaller sections reducing material usage and formwork requirements.

Pile Caps & Transfer Structures

Pile caps and transfer beams transferring heavy point loads from columns to pile groups are routinely specified at C28/35 to C35/45. The structural engineer’s choice depends on the strut-and-tie model geometry, the required anchorage lengths for large-diameter bars, and the pile head load. C28/35 offers a good balance of strength and durability for pile caps in XC2/XC3 ground conditions.

Retaining Walls & Earth-Retaining Structures

Reinforced concrete retaining walls — both free-standing L-shaped retaining walls and embedded walls — are frequently C28/35 in UK commercial and domestic construction. The mix provides adequate crack width control for the XC2 exposure of the retained side (permanently damp) and sufficient structural strength for the bending moments from retained earth pressures. Cruciform and counterfort walls may step up to C30/37 for taller retained heights.

Industrial Ground-Bearing Slabs

Heavy-duty ground-bearing industrial floor slabs — warehouses, distribution centres, factories — frequently specify C28/35 as a minimum to achieve adequate abrasion resistance and flexural strength for wheeled vehicle loading. The slab thickness (typically 150–250mm) and reinforcement (fabric or steel fibres) are designed using TR34 (4th edition) to carry the racking loads and vehicle wheel loads. C28/35 meets the TR34 minimum of C25/30 with margin.

Precast Concrete Elements

Many precast concrete products — including precast stairs, precast wall panels, precast beams, and hollowcore floor slabs — are manufactured at C28/35 to C35/45. The controlled factory environment allows higher strength to be reliably achieved with lower variability than site-batched concrete, often using CEM I cements with steam curing to accelerate strength development. The tighter quality control in precast production means Δc_devcan be reduced to 5mm (vs 10mm for site concrete) per the UK National Annex to EC2, reducing the nominal cover required.

C28/35 vs Adjacent Concrete Grades

Understanding where C28/35 sits in the grade ladder helps structural engineers and specifiers make informed decisions. The table below compares C28/35 with its neighbouring grades C25/30, C30/37, and C32/40 across the key design and specification parameters.

• Property: f ck (cylinder); C25/30: 25 N/mm²; C28/35 ★: 28 N/mm²; C30/37: 30 N/mm²; C32/40: 32 N/mm²
• Property: f ck,cube (cube); C25/30: 30 N/mm²; C28/35 ★: 35 N/mm²; C30/37: 37 N/mm²; C32/40: 40 N/mm²
• Property: f cd (UK NA); C25/30: 14.2 N/mm²; C28/35 ★: 15.9 N/mm²; C30/37: 17.0 N/mm²; C32/40: 18.1 N/mm²
• Property: f ctm; C25/30: 2.6 N/mm²; C28/35 ★: 2.77 N/mm²; C30/37: 2.9 N/mm²; C32/40: 3.0 N/mm²
• Property: E cm; C25/30: 31 GPa; C28/35 ★: 32 GPa; C30/37: 33 GPa; C32/40: 33.5 GPa
• Property: Typical max w/c (XC3); C25/30: 0.60; C28/35 ★: 0.55; C30/37: 0.55; C32/40: 0.50
• Property: Min cement content (XC3); C25/30: 260 kg/m³; C28/35 ★: 280 kg/m³; C30/37: 280 kg/m³; C32/40: 300 kg/m³
• Property: XD1 suitability; C25/30: No; C28/35 ★: Marginal; C30/37: Yes; C32/40: Yes
• Property: XC4 suitability; C25/30: Yes; C28/35 ★: Yes; C30/37: Yes; C32/40: Yes
• Property: Approx. ready-mix price; C25/30: ~£105–£120/m³; C28/35 ★: ~£110–£125/m³; C30/37: ~£115–£130/m³; C32/40: ~£120–£138/m³
• Property: BS 8500 designated mix; C25/30: RC25/30; C28/35 ★: RC28/35; C30/37: RC30/37; C32/40: RC32/40

★ = C28/35 (this grade). Ready-mix prices are indicative UK national averages for 2026 — actual prices vary significantly by region, supplier, volume, and market conditions. Always obtain supplier quotes.

When to Choose C28/35 Over C25/30 or C30/37

• Choose C28/35 over C25/30 when:The structural engineer needs a modest strength increase to reduce section sizes or reinforcement; when XC4 durability requirements are tight and a margin above the minimum C25/30 is prudent; when the specifier wants a step improvement for basement or retaining wall construction without the full premium of C30/37
• Choose C30/37 over C28/35 when:XD1 or XD2 exposure is present (C30/37 is the minimum); when the structural design shows a meaningful section size or reinforcement reduction from the higher f_cdthat justifies the marginal cost premium; when project specification or client requirements mandate C30/37 as standard
• C28/35 offers a useful “sweet spot”:Not all ready-mix suppliers stock every grade — in some regions C28/35 may be equally available and priced to C30/37. Always check availability with your local supplier before specifying uncommon grades

Ordering C28/35 Ready-Mix — UK 2026

When ordering C28/35 ready-mix concrete in the UK, the order must specify more than just the strength grade. A complete BS EN 206 / BS 8500 order instruction ensures the correct product is delivered and provides traceability for Building Control and structural warranty purposes.

What to Include in a Concrete Order — RC28/35

Strength Class & Designated Mix Reference

State:“RC28/35 to BS 8500-1”or equivalently“C28/35 to BS EN 206”. The RC prefix indicates a reinforced concrete designated mix. This alone specifies the cement content, w/c ratio, and strength class for the standard durability exposure class. If you have a specific exposure class beyond XC2/XC3, state it explicitly (e.g. “RC28/35 XC4”).

Consistence Class (Slump / Flow)

State the required workability:S2(50–90mm slump — standard for placed by skip or direct chute);S3(100–150mm — standard for pumped concrete);S4(160–210mm — highly workable, congested reinforcement); orF5/F6(flow class for self-compacting concrete). Most structural concrete on UK sites uses S3 for pump delivery. Never specify S1 for pumped concrete — the pump will struggle.

Maximum Aggregate Size

Standard is20mmmaximum aggregate size (suitable for most structural applications with adequate reinforcement spacing and cover). Specify10mmwhere reinforcement is congested (bar spacing <40mm clear, or sections thinner than 100mm). Using 10mm aggregate reduces workability — compensate by specifying a higher slump class or requesting additional plasticiser at batching.

Cement / Combination Type (If Required)

For standard RC28/35, the supplier selects the cement type. If you need a specific cement — e.g.GGBS blend (30–50%)for reduced heat of hydration in thick sections (≥500mm);CEM I onlyfor fast early strength (striking formwork); orsulphate-resistant cementfor XA exposure — state this explicitly. GGBS blends are increasingly common on UK sites for sustainability reasons (lower embodied carbon) and may be available at marginal or no cost premium.

Admixtures & Special Requirements

State any required admixtures:retarder(for pours in hot weather or long haul distances — delays initial set);accelerator(for cold weather working — check with structural engineer before use as can affect long-term strength);fibres(steel or polypropylene for industrial slabs or fire resistance);waterproofer(crystalline or integral for basement construction). All admixtures must comply with BS EN 934-2.

Indicative UK Ready-Mix Prices — C28/35 (2026)

• Order Size: Mini-mix / small order (<2m³); Indicative Price/m³: £140–£165/m³; Notes: Mini-mix trucks (1–2m³ capacity) — higher unit cost, short load surcharge applies
• Order Size: Standard load (3–5m³); Indicative Price/m³: £118–£138/m³; Notes: Full 6m³ truck may apply short load charge if <4m³ ordered
• Order Size: Full truck load (6–8m³); Indicative Price/m³: £110–£128/m³; Notes: Best unit price — maximise truck capacity to minimise per-m³ cost
• Order Size: Large project rate (>50m³/pour); Indicative Price/m³: £105–£120/m³; Notes: Negotiated rate for ongoing supply — discuss with supplier at project start
• Order Size: Weekend / bank holiday; Indicative Price/m³: +£8–£18/m³; Notes: Saturday surcharge standard; Sunday/bank holiday significantly higher
• Order Size: GGBS blend (30–50%); Indicative Price/m³: Nil to +£5/m³; Notes: Often same price or marginal premium — check with supplier
• Order Size: Fibres — polypropylene; Indicative Price/m³: +£3–£6/m³; Notes: Structural macro fibres at higher dose — +£15–£30/m³

Prices Are Indicative — Always Get Quotes

Ready-mix concrete prices vary significantly by region (London and South East are typically 15–25% above national average), by supplier, by proximity to batching plant, and by market conditions. Cement and aggregate prices have been volatile in recent years. Always obtain at least two supplier quotes for your specific location, grade, and volume. Prices above are indicative UK national averages for 2026 and do not include delivery beyond a standard distance from the batching plant (typically 5–10km) — distance surcharges apply beyond this range.

Cube Testing & Compliance — BS EN 12390

For C28/35, compliance is verified by crushing 150mm concrete cube specimens at28 daysunder BS EN 12390-3. The characteristic cube strength of 35 N/mm² means that in a population of cube results, 95% must achieve ≥35 N/mm² — only 5% are permitted to fall below this value (the characteristic value is a lower 5th percentile).

Cube Making — BS EN 12390-2

Cubes must be made from a representative sample taken at point of discharge from the mixer or truck (not from the first or last portion of the load). Use standard 150mm steel moulds, compact with a vibrating poker or vibrating table, store at 20°C ± 2°C, demould at 24 hours, and water-cure until testing. Cube making must follow BS EN 12390-2 exactly — deviations invalidate the result.

Conformity Criteria — BS EN 206

For C28/35, the concrete conforms if:(a)the mean of any 3 consecutive results ≥ f_ck,cube+ 4 N/mm² = 35 + 4 =39 N/mm²; and(b)every individual result ≥ f_ck,cube− 4 N/mm² = 35 − 4 =31 N/mm². A result below 31 N/mm² from any single cube triggers non-conformity investigation.

Early-Age Strength Testing

7-day cube results (approximately 75% of 28-day strength for CEM I concrete) give early indication of 28-day performance. If the 7-day result for C28/35 is below approximately 26 N/mm², investigate immediately — don’t wait until 28 days to find a failure. Early-age strength is critical where formwork is struck early (4–7 days) — confirm striking strength with your structural engineer.

What to Do If Cubes Fail Below 31 N/mm²

• Do not strike formwork or apply loaduntil the issue is investigated and resolved
• Notify the structural engineer immediatelythey will assess whether in-situ strength is adequate or whether remedial action is needed
• Core testing (BS EN 12504-1):In-situ cores drilled from the structure can demonstrate that the actual concrete in the structure has achieved adequate strength — cube results can underestimate in-situ strength due to curing differences. Core results are converted to equivalent cube strength per BS EN 13 390-3 or BS EN 13791
• CAPO / pull-out testing (BS EN 12504-3):Non-destructive in-situ test that estimates compressive strength from a cast-in or drilled insert pull-out force — can be used alongside core testing to build a picture of in-situ strength across the poured element
• Rebound hammer (Schmidt hammer — BS EN 12504-2):Rapid non-destructive surface hardness test — useful for identifying weak zones but not a substitute for core testing. Results must be correlated against known cube results for the specific mix
• If in-situ strength confirmed adequate:The structural engineer may accept the element with a note on the record. If in-situ strength is insufficient, options include: reinforcing with additional structural elements; reducing applied loads; partial demolition and reconstruction. All remediation must be agreed with the structural engineer and Building Control
• Root cause investigation:Identify why the cubes failed — common causes include: excess water added on site; poor cube making or curing; inadequate batching plant quality control; wrong mix delivered; cold weather affecting strength development. Address the root cause before continuing concreting

Cube Testing Frequency — BS EN 206 Guidance

• Initial production:Minimum 3 test specimens per 50m³ or per day’s production (whichever is more frequent) during initial production — the first 35 results from a concrete family
• Continuous production:Minimum 1 test per 200m³ or per week for established mixes — reduced frequency once a compliance record is established with the supplier
• Structural engineer’s requirements:For structural concrete, the structural engineer may specify more frequent testing — particularly for critical pours (raft foundations, transfer structures, post-tensioned slabs). Always follow the structural engineer’s specification, which may be more onerous than the BS EN 206 minimum
• NHBC warranty:NHBC Technical Standards require cube testing for all structural concrete pours — retain all test certificates for the warranty period (typically 10 years)
• Make pairs, not singles:Always make pairs (or sets of 3) of cubes from each sample — one for 7-day testing (early indication) and one or two for 28-day compliance testing. Store a spare cube until 28-day results are confirmed

Work out C28/35 Concrete Volumes with Our Free Mobile Guide

Use ourMobile Concrete Calculatorto work out exact volumes of C28/35 for slabs, foundations, columns, circular bases, and steps — with live UK 2026 cost estimates. Supports all BS 8500 grades including RC28/35, RC30/37, PAV1, PAV2, and FND mixes.

C28/35 Concrete — Frequently Asked Questions

Common questions about C28/35 (RC28/35) concrete grade for UK structural design and construction in 2026.

• Strength difference:C30/37 has a characteristic cylinder strength f_ck= 30 N/mm² vs 28 N/mm² for C28/35 — a difference of 2 N/mm² (7%). The cube strength difference is 2 N/mm² (35 vs 37 N/mm²). This is a small but meaningful step that affects the design compressive strength: f_cd= 17.0 N/mm² for C30/37 vs 15.9 N/mm² for C28/35 under the UK NA
• Durability:C30/37 meets the BS 8500 minimum for XD1 (chloride from de-icing agents, moderate humidity — e.g. car park soffits). C28/35 does not meet XD1 minimum without specific mix design justification. For XC1–XC4 carbonation exposure, both grades are fully adequate
• Cost:C30/37 typically costs approximately £5–£10/m³ more than C28/35 from the same supplier. On large pours this adds up — but on most domestic projects the difference is modest
• Availability:C30/37 (RC30/37) is more commonly stocked as a standard designated mix by UK ready-mix suppliers than C28/35 (RC28/35). In some areas C28/35 may need to be ordered as a designed mix, potentially at slightly higher cost or longer lead time. Always check availability locally
• Practical recommendation:Where the structural engineer specifies C28/35, use it. Where you have a choice and the application involves any chloride exposure (XD1) or you want a standard widely-available mix, C30/37 is often the more practical choice with minimal cost premium

• Strip foundations:The minimum grade for strip foundations in normal ground conditions per Approved Document A and BS 8103 is C16/20 (GEN3). C28/35 significantly exceeds this minimum — it is appropriate where the structural engineer has specified it for a particular reason (e.g. deep foundations, unusual loading, or tight reinforcement). For a standard domestic extension on normal ground, GEN3 or C20/25 is typically sufficient for unreinforced strip foundations
• Reinforced concrete ground beams:Where the extension uses a ground beam and suspended floor system (common on sloping sites or near trees), C28/35 is a common specification — the reinforcement requirements and structural integrity justify the higher grade
• Reinforced concrete slabs:A suspended reinforced concrete ground floor slab for an extension may be specified at C28/35 by the structural engineer — particularly where there are voids, variable ground, or tree root concerns below
• Retaining walls for split-level extensions:C28/35 is appropriate for reinforced concrete retaining walls integral to domestic extensions
• Exposure class for domestic extensions:Most domestic extension foundations and ground slabs are in XC2 (wet, occasionally dry — foundations in contact with soil) or XC1 (dry interior). C28/35 is suitable for both — though GEN3 or C20/25 would technically suffice for the minimum requirement
• Always follow the structural engineer’s specification:Do not substitute a lower grade to save cost without the structural engineer’s written approval — they have specified the grade for structural and durability reasons that may not be immediately apparent

BS 8110 Grade 35

• Direct equivalence:C28/35 (EC2 notation) = Grade 35 (BS 8110 notation). Both specify a 35 N/mm² characteristic cube strength at 28 days
• Why the different number?EC2 uses the cylinder strength as the primary identifier (28 N/mm² cylinder), while BS 8110 used only the cube strength (35 N/mm² cube). The relationship f_ck≈ 0.80 × f_ck,cubegives 0.80 × 35 = 28 N/mm² — hence C28/35
• Design strength comparison:BS 8110 design concrete compressive stress = 0.67 × f_cu/ γ_m= 0.67 × 35 / 1.5 = 15.6 N/mm². EC2 (UK NA) f_cd= 0.85 × 28 / 1.50 = 15.9 N/mm². The design compressive strength values are almost identical — confirming the equivalence for practical design purposes
• Tensile strength:BS 8110 did not use tensile strength explicitly in most design checks — it used empirical shear tables. EC2 uses f_ctmdirectly in minimum reinforcement and shear working, giving a more rational and slightly different result
• Ordering concrete:When ordering from a ready-mix supplier, always use the current BS EN 206 / BS 8500 notation (C28/35 or RC28/35) — not the old Grade 35 reference. Suppliers may still understand “Grade 35” informally but the formal specification must use current notation

• 24 hours:Initial set complete; concrete is hardened but very early-age. Cube strength typically 10–14 N/mm² for CEM I — enough to walk on carefully but not to apply structural load
• 3 days:Approximately 40–50% of 28-day strength. Cube strength ~14–18 N/mm². Formwork to vertical elements (walls, columns) can often be struck at 3 days in normal UK temperatures — confirm striking strengths with structural engineer
• 7 days:Approximately 65–75% of 28-day strength for CEM I. Cube strength ~23–26 N/mm². Soffit formwork to most beams can be struck at 7 days in warm weather (≥10°C) — always confirm with the structural engineer’s striking table. 7-day test results give good early indication of 28-day compliance
• 28 days:The standard age for compliance testing.Should achieve characteristic cube strength ≥35 N/mm² (mean target approximately 43–45 N/mm² to allow for variability). This is when Building Control and NHBC cube test certificates are assessed
• 56 days and beyond:Concrete continues to gain strength beyond 28 days — typically reaching approximately 110–115% of 28-day strength at 56 days and 120–125% at 1 year for CEM I. GGBS blend concretes gain strength more slowly initially but continue developing for longer
• Effect of temperature:Cold weather (≤5°C) significantly slows hydration — do not assume normal strength gain in winter without cold weather protection measures (insulated formwork, heated enclosures, or use of accelerating admixtures). Strength development can halt if concrete freezes before sufficient maturity is reached
• GGBS blends:GGBS replacement (30–50%) reduces early strength gain — at 7 days a GGBS blend C28/35 may only achieve 55–65% of 28-day strength. However, 28-day and 56-day strengths are comparable or better than CEM I mixes. Allow for slower early gain when planning formwork striking and loading

• Driveway without de-icing salts (XC4 + XF1):C28/35 is technically suitable — it exceeds the BS 8500 minimum of C25/30 for this exposure. However, the standard designated mix for driveways in UK practice isPAV1(C25/30, air-entrained) orPAV2(C30/37, air-entrained). PAV mixes are specifically designed for paving durability
• Driveway with de-icing salt (XC4 + XF4 + XD1):C28/35 as a standard RC mix isnot appropriate. De-icing salt exposure (XF4 + XD1) requires air-entrained concrete to resist freeze-thaw damage. The correct specification isPAV2(air-entrained, C30/37 minimum) — not RC28/35
• Air entrainment is key for driveways:The RC28/35 designated mix does not include air entrainment. External paving exposed to freeze-thaw cycles needs 4–5% entrained air to resist frost scaling. Strength alone does not protect against freeze-thaw damage — the air void system absorbs the expansive pressure of freezing water in the concrete pores
• Surface finish:PAV1 and PAV2 mixes are designed with a surface wear resistance in mind as well as freeze-thaw resistance. RC28/35 is designed for structural use, not surface wear. For driveways and paths, specify a PAV mix for best long-term performance
• Minimum slab thickness for driveways:Domestic car driveway — minimum 100mm (GEN3 or PAV1). Heavy vehicles, HGVs — minimum 150–200mm C28/35 or above, designed by a structural or geotechnical engineer

• XC1 (dry or permanently wet — interior slabs):c_min,dur= 15mm → c_nom=25mm
• XC2 (wet, rarely dry — foundations in soil):c_min,dur= 25mm → c_nom=35mm
• XC3 (moderate humidity — external sheltered concrete):c_min,dur= 25mm → c_nom=35mm
• XC4 (cyclic wet/dry — external exposed concrete):c_min,dur= 30mm → c_nom=40mm
• Cover to blinding:Where concrete is cast directly against soil without blinding, minimum cover =75mmregardless of exposure class. This is a very common situation for domestic strip foundations — always provide at least 50mm blinding and maintain 40–50mm cover above it
• Fire resistance cover:Cover must also satisfy fire resistance requirements from EC2 Part 1-2 (axis distance). For R60 (60-minute fire resistance) a beam bottom bar axis distance of 25mm is required — for C28/35 with 10mm links, this gives a nominal cover of approximately 25 + 10 + φ/2 to the link. Check both durability cover and fire axis distance and use the greater
• Bond cover (c_min,b):Minimum cover for bond = bar diameter φ. For T32 bars, c_min,b= 32mm → c_nom= 42mm — this may govern over durability cover for large-diameter bars in mild exposure classes

BS 8500 designated mix reference

• RC:Reinforced Concrete — indicates this is a designated mix formulated for use with reinforcement (as opposed to GEN for general-purpose plain concrete, PAV for paving, or FND for sulphate-resistant foundation mixes)
• 28:The characteristic cylinder compressive strength f_ck= 28 N/mm² in the BS EN 206 C28/35 notation
• 35:The characteristic cube compressive strength f_ck,cube= 35 N/mm² — this is the value tested on site with 150mm cube specimens
• What RC28/35 guarantees:When you order RC28/35 from a BS 8500 compliant ready-mix producer, the supplier takes responsibility for the complete mix design — cement content, w/c ratio, aggregate grading, and all other parameters needed to achieve C28/35 performance in XC2/XC3 exposure. You do not need to specify individual mix proportions
• What it does not specify:RC28/35 alone does not specify slump/consistence class, maximum aggregate size, cement type, admixtures, or special requirements for more severe exposure classes — you must add these to your order
• On a drawing:You will typically see a specification note such as: “All reinforced concrete to be RC28/35 to BS 8500-1, slump class S3, 20mm maximum aggregate size, nominal cover as shown.” The structural engineer may add additional requirements for specific elements
• Full specification string example:RC28/35, S3, 20mm, XC3, CEM II/A-L — this fully specifies the concrete for a typical external reinforced concrete element in moderate humidity conditions

2,350–2,450 kg/m³

• Normal-weight concrete (quartzite/limestone aggregate):Typically 2,400 kg/m³ — this is the standard value used in Eurocode 2 structural design checks for self-weight loading. In kN/m³ this is approximately 24 kN/m³ (using g = 9.81 m/s²)
• Reinforced concrete:Adding steel reinforcement increases density slightly. For structural design checks, EC2 Clause 4.1.1 gives the density of reinforced concrete as25 kN/m³(approximately 2,550 kg/m³) to account for the steel — use this value for all self-weight working in structural design
• Ready-mix truck capacity:A standard 8m³ ready-mix truck loaded with C28/35 carries approximately 8 × 2,400 =19,200 kg (19.2 tonnes)of concrete — plus the weight of the truck itself (typically 10–12 tonnes unladen). Total gross weight approximately 29–32 tonnes. Check access road load limits before allowing ready-mix trucks on site
• Practical examples:150mm ground floor slab, 6m × 4m = 3.6m³ × 2,400 =8,640 kg (8.6 tonnes)300mm × 600mm beam, 5m long = 0.9m³ × 2,400 =2,160 kg (2.2 tonnes)400mm × 400mm column, 3m high = 0.48m³ × 2,400 =1,152 kg (1.15 tonnes)
• Lightweight concrete:If lightweight aggregate is used (e.g. Lytag), density reduces to 1,600–2,000 kg/m³. Lightweight C28/35 is used to reduce self-weight in long-span structures. Specify separately as “lightweight concrete, density class D2.0” alongside the strength class

Related Concrete Grade Guides & Resources

Free companion resources for UK concrete specification, structural design, and ready-mix ordering in 2026.

Professional Disclaimer — Always Follow Your Structural Engineer’s Specification

This page is a reference guide for UK construction professionals, contractors, and self-builders. It does not constitute structural engineering or specification advice. Always use the concrete grade, cover, mix specification, and testing regime specified by your structural engineer and Building Control. Standards and prices change — verify current BS 8500 and BS EN 206 requirements atbsigroup.combefore specifying concrete for structural work.

For related concrete mix design and proportion working, see MixDesignCalc.xyz.