Recommended Concrete Grade
C30/37
Standard structural concrete
Calculate Concrete Grade & Strength Requirements
BS EN 206 Compliant Strength Selection Guide
Selecting the correct concrete strength is critical for structural performance, durability, and building regulation compliance. Our Mix Strength Calculator helps determine appropriate concrete grades based on application requirements, load conditions, and BS EN 206:2013+A2:2021 standards.
Concrete strength is measured in Newtons per square millimeter (N/mm²) at 28 days after placing. UK construction uses standardized strength classes from C8/10 to C90/105, with common grades including C16/20, C25/30, and C35/45. Understanding strength requirements ensures proper specification and compliance with Building Regulations 2010 throughout 2026 projects.
Determine required concrete strength for your application
British Standard BS EN 206:2013+A2:2021 defines concrete strength classes using a two-number system representing cylinder and cube strength at 28 days. Understanding this classification system ensures correct specification and compliance with UK building regulations.
| Strength Class | Cylinder (N/mm²) | Cube (N/mm²) | UK Designation | Typical Applications |
|---|---|---|---|---|
| C8/10 | 8 | 10 | GEN 0 | Mass concrete fill, non-structural |
| C12/15 | 12 | 15 | GEN 1 (ST1) | Blinding, kerb bedding, drainage |
| C16/20 | 16 | 20 | GEN 2 | Mass concrete, oversites, internal floors |
| C20/25 | 20 | 25 | GEN 3 (ST2) | Paths, patios, domestic driveways |
| C25/30 | 25 | 30 | RC25 | Light reinforced, lightly loaded slabs |
| C28/35 | 28 | 35 | RC28 | Commercial floors, reinforced foundations |
| C30/37 | 30 | 37 | RC30 (ST3) | Foundations, floor slabs, general structural |
| C32/40 | 32 | 40 | RC32 (ST4) | Commercial buildings, suspended slabs |
| C35/45 | 35 | 45 | RC35 | Heavy-duty commercial, beams, columns |
| C40/50 | 40 | 50 | RC40 (ST5) | Industrial floors, heavy-duty structures |
| C45/55 | 45 | 55 | RC45 | Specialized structural elements |
| C50/60 | 50 | 60 | RC50 | High-rise, bridges, specialized structures |
The BS EN 206 strength designation format C30/37 represents two different test methods measuring the same concrete. The first number is cylinder strength, the second is cube strength, both measured at 28 days in Newtons per square millimeter (N/mm²).
📊 Strength Designation Explained:
GEN 0 (C8/10): Mass concrete fill, non-structural applications
GEN 1 (C12/15): Blinding layers, drainage bedding
GEN 2 (C16/20): Strip footings without reinforcement
GEN 3 (C20/25): Light duty floors, paths, patios
Note: GEN grades used for unreinforced concrete only
RC25 (C25/30): Lightly reinforced elements, light foundations
RC28 (C28/35): Reinforced foundations, commercial slabs
RC30 (C30/37): Most common structural grade in UK
RC32 (C32/40): Commercial suspended slabs, beams
RC40 (C40/50): Heavy-duty industrial applications
ST1 (C12/15): Simple trade designation for blinding
ST2 (C20/25): Paths and domestic light use
ST3 (C30/37): Foundations and general structural
ST4 (C32/40): Commercial floors and beams
ST5 (C40/50): Industrial and heavy-duty
C7.5: Approximate to C8/10 or GEN 0
C10: Approximate to C12/15 or GEN 1
C15: Approximate to C16/20 or GEN 2
C20: Approximate to C20/25 or GEN 3
C25-C50: Similar to equivalent RC grades
Choosing appropriate concrete strength depends on structural requirements, loading conditions, exposure environment, and design life. Our calculator considers all these factors to recommend suitable grades compliant with NHBC standards and Building Regulations.
Strip Footings (unreinforced): C20/25 (GEN 3) minimum
Strip Footings (reinforced): C25/30 (RC25) typical
Pad Foundations: C30/37 (RC30) standard
Raft Foundations: C30/37 to C35/45 depending on design
Trench Fill: C20/25 (GEN 3) acceptable for simple designs
Poor Ground: Upgrade to C30/37 or engineer designed
Ground Bearing Slab (domestic): C25/30 minimum, C30/37 typical
Ground Bearing (commercial): C30/37 to C35/45
Suspended Slab (domestic): C30/37 (RC30) standard
Suspended Slab (commercial): C32/40 to C40/50
Industrial Heavy-Duty: C40/50 (RC40) minimum
With UFH: Same grade, consider fiber reinforcement
Paths/Patios (pedestrian): C20/25 (GEN 3/ST2)
Domestic Driveway (cars): C25/30 to C30/37
Heavy Vehicle Access: C32/40 minimum
Commercial Parking: C35/45 (RC35) typical
Roadways: C40/50 (ST5) or higher
Fiber Addition: Improves crack resistance significantly
Beams/Lintels (domestic): C30/37 (RC30) minimum
Beams (commercial): C32/40 to C40/50
Columns (low-rise): C30/37 typical
Columns (multi-story): C35/45 to C50/60
Retaining Walls: C30/37 minimum, often C35/45
Prestressed Elements: C40/50 minimum, often higher
Swimming Pools: C35/45 with waterproofing admixtures
Water Tanks: C35/45 minimum, watertight specification
Agricultural Buildings: C25/30 to C32/40 depending on use
Precast Elements: C40/50 to C60/75 for optimization
Marine Structures: C35/45 minimum with special cements
Chemical Exposure: C35/45+ with sulfate-resistant cement
General Repairs: Match existing strength or upgrade
Structural Repairs: Minimum C32/40 typically required
Overlays/Toppings: C32/40 to C40/50 for durability
Rapid Repairs: High early strength grades available
Compatibility: Match exposure class and cement type
Bonding: Substrate preparation more critical than strength
BS EN 206 defines exposure classes based on environmental conditions affecting concrete durability. Selecting appropriate strength and specification for exposure class ensures long-term performance and prevents premature deterioration.
| Exposure Class | Description | Min Strength Class | Max W/C Ratio |
|---|---|---|---|
| XC1 | Dry or permanently wet (interior, underwater) | C20/25 | 0.65 |
| XC2 | Wet, rarely dry (water tanks, foundations) | C25/30 | 0.60 |
| XC3 | Moderate humidity (sheltered external, interior) | C30/37 | 0.55 |
| XC4 | Cyclic wet and dry (external exposed surfaces) | C30/37 | 0.50 |
| XD1 | Moderate humidity with chlorides | C30/37 | 0.55 |
| XD2 | Wet, rarely dry with chlorides | C30/37 | 0.55 |
| XD3 | Cyclic wet/dry with chlorides (de-icing salts) | C35/45 | 0.45 |
| XF1 | Moderate water saturation, no de-icing | C30/37 | 0.55 |
| XF3 | High water saturation with frost/de-icing | C35/45 | 0.45 |
| XS1 | Airborne salt (coastal, not direct contact) | C30/37 | 0.50 |
| XS3 | Tidal, splash and spray zones (marine) | C35/45 | 0.45 |
| XA1 | Slightly aggressive chemical (natural soil) | C30/37 | 0.55 |
Multiple factors influence the appropriate concrete strength selection beyond basic application type. Understanding these considerations ensures optimal specification for long-term performance and regulatory compliance.
✅ Key Strength Selection Factors:
1 Day: 30-40% of 28-day strength (CEM I)
3 Days: 50-60% of 28-day strength
7 Days: 70-75% of 28-day strength
Rapid Hardening: Can achieve 70% at 1 day
Temperature Effect: Cold weather significantly delays strength gain
28 Days: Design strength achieved (100%)
56 Days: 105-115% of 28-day strength
90 Days: 110-120% of 28-day strength
1 Year: 115-125% of 28-day strength
Long Term: Continues gaining slowly for years
CEM I (Portland): Standard strength development profile
CEM II (Composite): Slightly slower early strength
CEM III (GGBS): Slower early, better long-term strength
CEM IV (PFA): Slower early, excellent long-term
Rapid Hardening: Very fast early strength gain
Temperature: 20°C optimal; cold/hot affects rate significantly
Curing: Proper curing essential for full strength development
Humidity: Drying prevents full hydration and strength
Frost Damage: Freezing before strength gain causes permanent damage
Hot Weather: Rapid drying reduces ultimate strength
Accurate strength specification ensures suppliers provide correct concrete grade while maintaining clarity in contractual and technical documentation. Follow Concrete Centre guidance for proper specification format.
⚠️ Complete Specification Requirements: