C28/35 Grade Concrete 2026 | Structural Concrete Specifications

C28/35 Grade Concrete 2026

High-Strength Structural Concrete Specifications

Complete Guide to C28/35 Concrete Properties & Applications

C28/35 grade concrete is a high-strength structural concrete widely specified for demanding construction applications in the UK. Compliant with BS EN 206 and BS 8500, this concrete grade offers a characteristic compressive strength of 28 N/mm² (cylinder) or 35 N/mm² (cube) at 28 days, making it suitable for structural beams, columns, suspended slabs, and foundations in commercial and industrial buildings.

C28/35 concrete is stronger than standard domestic grades (C20/25) but more economical than ultra-high-strength options (C40/50). This guide covers complete specifications, applications, mix design, costs, and compliance requirements for 2026 UK construction projects. For structural design calculations, always consult Institution of Structural Engineers guidance and obtain Building Control approval.

C28/35 Concrete Key Specifications

C28/35 grade concrete provides enhanced strength and durability compared to general-purpose concrete grades. The designation "C28/35" indicates characteristic cylinder strength of 28 N/mm² and cube strength of 35 N/mm² respectively, measured at 28 days under standard testing conditions as defined in BS EN 12390.

Characteristic Strength

Cylinder Strength: 28 N/mm² (MPa)

Cube Strength: 35 N/mm² (MPa)

Testing Age: 28 days curing

Early Strength (7 days): ~20-25 N/mm²

Workability & Slump

Typical Slump: S2-S4 (50-150mm)

Consistency Class: Medium to high workability

Placing Method: Pump, skip, or direct discharge

Compaction: Vibration required

Durability Classification

Exposure Class: XC3/4, XD1, XF1

Min Cement Content: 280-340 kg/m³

Max W/C Ratio: 0.55-0.60

Cover to Reinforcement: 25-35mm minimum

Material Properties

Density: 2300-2400 kg/m³

Modulus of Elasticity: 31-33 GPa

Tensile Strength: 2.5-3.0 N/mm²

Fire Resistance: Up to R120 (2 hours)

C28/35 vs Other Concrete Grades

Understanding strength differences between concrete grades helps select appropriate specifications for structural applications. C28/35 offers a middle ground between standard domestic concrete (C20/25) and heavy commercial grades (C40/50+), providing enhanced structural capacity at moderate cost premium.

Concrete Grade Comparison 2026

Concrete Grade	Cube Strength (N/mm²)	Typical Applications	Cost per m³
C16/20 (GEN2)	20 N/mm²	Kerb bedding, drainage, non-structural	£95-£110
C20/25 (GEN3)	25 N/mm²	Domestic foundations, garage floors	£100-£115
C25/30 (RC25)	30 N/mm²	Lightly reinforced slabs, light foundations	£105-£120
C28/35 (RC28)	35 N/mm²	Structural beams, columns, commercial floors	£110-£130
C32/40 (RC32)	40 N/mm²	Pre-stressed concrete, heavy duty floors	£120-£140
C40/50 (RC40)	50 N/mm²	High-rise structures, bridges, marine works	£135-£160
C50/60 (RC50)	60 N/mm²	Specialist high-strength applications	£150-£180

C16/20 (GEN2)

Cube Strength 20 N/mm²

Applications Non-structural

Cost per m³ £95-£110

C20/25 (GEN3)

Cube Strength 25 N/mm²

Applications Domestic foundations

Cost per m³ £100-£115

C28/35 (RC28)

Cube Strength 35 N/mm²

Applications Structural elements

Cost per m³ £110-£130

C40/50 (RC40)

Cube Strength 50 N/mm²

Applications High-rise, bridges

Cost per m³ £135-£160

Applications of C28/35 Concrete

C28/35 concrete is specified for structural applications requiring enhanced load-bearing capacity and durability. Its 35 N/mm² cube strength makes it suitable for reinforced concrete elements in commercial, industrial, and multi-storey residential construction projects. The Concrete Centre provides comprehensive design guidance for structural concrete applications.

Structural Beams & Columns

Primary Use: Load-bearing frames in multi-storey buildings

Advantages: High compressive strength, excellent reinforcement bond

Typical Sizes: Columns 300x300mm+, beams 300x450mm+

Reinforcement: A393/A252 mesh, H16-H25 bars typical

Suspended Slabs & Floors

Primary Use: Commercial floors with heavy live loads

Advantages: Span capability 5-7m, high point load resistance

Typical Depths: 200-300mm depending on span

Applications: Offices, warehouses, retail units, car parks

Commercial Foundations

Primary Use: Heavy duty foundation systems

Advantages: Higher bearing capacity than C20/25

Typical Depths: 750-1500mm depending on soil

Applications: Industrial units, offices, retail developments

Precast Concrete Elements

Primary Use: Factory-manufactured structural components

Advantages: Quality control, rapid strength gain

Typical Products: Prestressed beams, hollow core slabs, columns

Benefits: Faster construction, consistent quality

Retaining Walls & Basements

Primary Use: Earth-retaining structures, below-ground works

Advantages: Enhanced durability in aggressive ground conditions

Typical Thickness: 250-400mm depending on height/surcharge

Waterproofing: Requires tanking system or waterproof admixtures

Industrial Ground Slabs

Primary Use: Heavy-duty warehouse floors

Advantages: Resists forklift traffic, racking loads

Typical Thickness: 150-250mm with steel fibre reinforcement

Applications: Distribution centers, manufacturing facilities

🏗️ When to Specify C28/35 Instead of C25/30:

Longer spans: Suspended slabs spanning over 5 meters
Heavy loads: Live loads exceeding 5 kN/m² (offices, retail)
Columns: Multi-storey buildings (3+ floors)
Structural frames: Commercial and industrial buildings
Durability: Aggressive exposure conditions (XD, XF classes)
Reduced section sizes: Where higher strength allows smaller members

C28/35 Mix Design & Constituents

C28/35 concrete requires careful mix design to achieve specified strength, workability, and durability characteristics. Typical mixes use Portland cement (CEM I 42.5N or CEM II), controlled aggregate grading, and water-reducing admixtures. Mix design must comply with BS 8500-2 guidance for designated concrete.

Typical C28/35 Mix Proportions

Component	Quantity per m³	Proportion	Notes
Cement (CEM I 42.5N)	340-380 kg	1.0	Portland cement or CEM II/A-V acceptable
Fine Aggregate (Sand)	650-750 kg	1.9-2.2	0-4mm grading, clean and washed
Coarse Aggregate (Gravel)	1100-1200 kg	3.2-3.5	10-20mm nominal size, crushed or rounded
Water	180-210 litres	0.53-0.62	W/C ratio 0.53-0.60 typical
Admixtures (Optional)	1-5 litres	0.3-1.5%	Plasticizers, water reducers, air entrainers
Total Yield	1.0 m³	2300-2400 kg	Fresh concrete density

Cement (CEM I 42.5N)

Quantity 340-380 kg/m³

Proportion 1.0

Fine Aggregate (Sand)

Quantity 650-750 kg/m³

Proportion 1.9-2.2

Coarse Aggregate (Gravel)

Quantity 1100-1200 kg/m³

Proportion 3.2-3.5

Water

Quantity 180-210 litres/m³

W/C Ratio 0.53-0.60

✓ Mix Design Best Practices for C28/35:

Cement content: Minimum 320 kg/m³ for structural use, 340+ kg/m³ for aggressive exposure
W/C ratio: Maximum 0.60 for XC exposure, 0.55 for XD/XF classes
Aggregate size: 20mm maximum for reinforced concrete, 10mm for congested reinforcement
Slump: S2 (50-90mm) for normal placement, S3/S4 (100-150mm) for pumping
Air content: 4-6% if air-entrained for frost resistance
Minimum cement: Use CEM I or CEM II/A for reliable early strength development

Placement, Curing & Quality Control

Proper placement and curing procedures are critical to achieving specified C28/35 strength and durability. Concrete Society Technical Reports provide detailed guidance on concrete placement, finishing, and curing for structural applications.

Concrete Placement

Discharge Height: Maximum 1.5m free-fall to prevent segregation

Layer Thickness: 300-500mm maximum per layer

Vibration: Poker vibrator, 50-75mm diameter, 15-30 second duration

Rate: 20-40 m³/hour depending on complexity

Curing Requirements

Minimum Period: 7 days for normal conditions (>5°C)

Method: Wet hessian, curing membranes, or polythene sheets

Temperature: Maintain >5°C, protect from frost in first 48 hours

Humidity: Keep surface moist to prevent plastic shrinkage cracking

Strength Development

1 Day: 10-15% of 28-day strength

3 Days: 40-50% of 28-day strength

7 Days: 65-75% of 28-day strength (formwork striking)

28 Days: 100% specified characteristic strength achieved

Testing & Compliance

Cube Testing: Minimum 1 sample per 50m³ or per pour

Sample Size: 3 cubes per sample (test at 7, 14, 28 days)

Acceptance: Average ≥ fck + 4 N/mm², individual ≥ fck - 4 N/mm²

Slump Testing: Check every load or every 50m³

C28/35 Concrete Costs 2026

C28/35 concrete typically costs £110-£130 per cubic metre delivered in the UK, representing a 10-20% premium over standard C20/25 concrete. Total project costs depend on volume, delivery distance, pumping requirements, and reinforcement specifications. Ready-mix suppliers offer competitive pricing for commercial-scale orders (10m³+).

Concrete Supply Costs

Item	Unit Cost 2026	Notes
C28/35 Concrete (Delivered)	£110-£130/m³	Within 20 miles radius, minimum 6m³ load
Small Load Surcharge	£60-£100	For loads under 4m³
Extra Delivery Distance	£3-£5 per mile	Beyond standard 20-mile radius
Concrete Pump Hire	£250-£400/day	Line pump for suspended slabs/columns
Boom Pump (Large Projects)	£500-£800/day	Required for high-level pours or long reach
Labour (Placement & Finishing)	£35-£50/m³	Skilled concreters, vibration, finishing
Reinforcement (A393 Mesh)	£8-£12/m²	Supply and fix in slabs
Bar Reinforcement (H16-H25)	£120-£180/tonne	Fabricated, delivered, fixed in columns/beams
Formwork (Suspended Slab)	£25-£40/m²	Temporary support, striking after 7 days
Cube Testing (Laboratory)	£60-£100 per set	3 cubes per sample, tested at 7/28 days

C28/35 Concrete (Delivered)

Cost £110-£130/m³

Concrete Pump Hire

Cost £250-£400/day

Labour (Placement)

Cost £35-£50/m³

Cube Testing

Cost £60-£100 per set

⚠️ Cost Optimization Strategies:

Order full loads: 6m³+ eliminates small load surcharges (save £60-£100)
Schedule efficiently: Coordinate multiple pours in same day to share pump hire
Local suppliers: Use nearest ready-mix plant to minimize delivery charges
Off-peak delivery: Some suppliers offer discounts for off-peak times
Specify appropriately: Don't over-specify strength - use C25/30 where C28/35 isn't structurally required
Avoid waiting time: Be ready for pour to avoid £100+/hour truck waiting charges

Durability & Exposure Classes

C28/35 concrete durability must be specified according to exposure conditions defined in BS 8500-1. Exposure classes determine minimum cement content, maximum water/cement ratio, and concrete cover to reinforcement to ensure design service life (typically 50-100 years for structural elements).

Exposure Class Requirements

Exposure Class	Environment	Min Cement (kg/m³)	Max W/C Ratio	Min Cover (mm)
XC1	Dry or permanently wet	260	0.65	15
XC3	Moderate humidity (internal)	280	0.60	25
XC4	Cyclic wet/dry (external)	300	0.55	30
XD1	Moderate humidity with chlorides	300	0.55	35
XF1	Moderate water saturation, no de-icing	300	0.55	30
XF3	High water saturation with de-icing salts	340	0.50	35
XA1	Slightly aggressive chemical environment	300	0.55	30

XC3 - Moderate Humidity

Min Cement 280 kg/m³

Max W/C 0.60

Min Cover 25mm

XC4 - Cyclic Wet/Dry

Min Cement 300 kg/m³

Max W/C 0.55

Min Cover 30mm

XD1 - Chlorides

Min Cement 300 kg/m³

Max W/C 0.55

Min Cover 35mm

Structural Design Considerations

C28/35 concrete is designed using Eurocode 2 (BS EN 1992-1-1) structural calculations. Design must account for characteristic strength, load combinations, reinforcement requirements, serviceability limits, and durability provisions. Always engage qualified structural engineers for commercial and multi-storey projects.

🔧 Design Parameters for C28/35:

fck (characteristic strength): 28 N/mm² (cylinder) for Eurocode calculations
fcd (design strength): 18.7 N/mm² (= 28 / 1.5 partial factor)
Ecm (modulus): 32 GPa for deflection calculations
fctm (tensile strength): 2.8 N/mm² mean value
Partial factor γc: 1.5 for ultimate limit state design
Cover allowance Δc: 10mm tolerance added to nominal cover
Crack width limit: 0.3mm for XC exposure classes

C28/35 Concrete FAQs

What is C28/35 grade concrete used for?

C28/35 concrete is a high-strength structural grade used for reinforced concrete beams, columns, suspended slabs, commercial foundations, retaining walls, and precast elements. Its 35 N/mm² cube strength makes it suitable for multi-storey buildings, warehouses, offices, and industrial structures where higher load capacity is required than standard domestic grades (C20/25).

What does C28/35 mean in concrete grading?

C28/35 indicates characteristic compressive strength measured on standard test specimens: 28 N/mm² on 150mm diameter x 300mm cylinders (European standard), and 35 N/mm² on 150mm cubes (UK standard). Both measurements are taken at 28 days. The "C" prefix denotes structural concrete designed to BS EN 206 and BS 8500 specifications.

How much does C28/35 concrete cost per cubic metre in 2026?

C28/35 ready-mix concrete costs £110-£130 per m³ delivered in the UK in 2026, compared to £100-£115 for standard C20/25. Additional costs include small load surcharges (£60-£100 for under 4m³), pump hire (£250-£400/day), labour for placement (£35-£50/m³), and reinforcement (£8-£12/m² for mesh). Total installed cost is typically £200-£280/m³ including all elements.

What's the difference between C25/30 and C28/35 concrete?

C28/35 is approximately 15% stronger than C25/30 (35 N/mm² vs 30 N/mm² cube strength). This allows longer spans, smaller structural sections, or higher load capacity. C28/35 costs about £5-10/m³ more but may reduce overall costs through reduced section sizes and reinforcement. Use C25/30 for light domestic slabs and foundations; specify C28/35 for commercial buildings, multi-storey structures, and heavy-duty applications.

Can I use C28/35 for house foundations?

Yes, but C28/35 is over-specification for most domestic foundations where C20/25 (GEN3) is adequate. Use C28/35 for house foundations only when: structural engineer specifies it for difficult ground conditions, building is 3+ storeys, heavy loads are involved, or aggressive ground conditions exist. The extra cost (£10-15/m³) is rarely justified for standard 2-storey houses on normal soil.

How long does C28/35 concrete take to cure and gain strength?

C28/35 concrete reaches 65-75% of design strength after 7 days (sufficient for formwork striking), and achieves full 28-day characteristic strength of 35 N/mm² after 28 days. Early loading requires calculations based on interim strength. Maintain curing for minimum 7 days by keeping surface moist with wet hessian, curing membranes, or polythene sheets. Cold weather (<5°C) significantly extends curing time and requires protection measures.

Do I need a structural engineer for C28/35 concrete?

Yes, for most applications. C28/35 concrete is specified for structural elements (beams, columns, suspended slabs) which require professional structural calculations under Eurocode 2. Building Control will require structural engineer certification for: all reinforced concrete elements, buildings over 2 storeys, commercial structures, retaining walls, and any complex loading scenarios. Simple ground slabs may use standard details without full calculations.

What reinforcement is needed with C28/35 concrete?

C28/35 structural elements typically require: A393 mesh (10mm bars @ 200mm centres) for slabs, H16-H25 deformed bars for beams and columns, with spacing calculated by structural engineer based on load and span. Minimum concrete cover is 25-35mm depending on exposure class. Reinforcement must be adequately lapped (40-50 bar diameters), tied securely, and supported on chairs/spacers to maintain correct position during pouring.

Can C28/35 concrete be pumped?

Yes, C28/35 pumps well with proper mix design. Specify S3 or S4 slump (100-150mm workability) for pumping, with plasticizing admixtures to improve flow without increasing water content. Maximum aggregate size 20mm for line pumps, 10mm for small-diameter pipes or congested reinforcement. Typical pumping rates: 20-40m³/hour for slabs, slower for columns/walls. Ensure pump operator inspects concrete slump before commencing.

What testing is required for C28/35 concrete?

BS EN 206 requires: cube testing minimum 1 sample per 50m³ or per pour (whichever is more frequent), with 3 cubes per sample tested at 7, 14, and 28 days. Slump testing on every delivery load. For structural acceptance, average strength must be ≥ 39 N/mm² (fck + 4), with no individual cube below 31 N/mm² (fck - 4). Failed tests trigger investigation and potential remedial work including core sampling or load testing.