C15/20 Grade Concrete for Foundations 2026 | UK Building Standards
C15/20 Grade Concrete for Foundations
Complete Guide to C15/20 Concrete Strength & Applications 2026
UK Building Standards Compliant Foundation Mix
C15/20 grade concrete (formerly known as GEN1) is a low-strength general purpose concrete mix commonly specified for non-structural applications including domestic foundation blinding, mass fill foundations, drainage works, and temporary works. With a characteristic compressive strength of 15 N/mm² (cylinder test) or 20 N/mm² (cube test), C15/20 concrete meets BS 8500 standards for designated concrete applications in UK construction.
While C15/20 concrete is NOT suitable for structural foundations supporting buildings, it plays a critical role in foundation preparation as blinding concrete - a thin 50-75mm protective layer poured onto excavated foundation trenches before placing main foundation concrete. This guide covers C15/20 specifications, applications, costs, and alternatives for 2026 UK construction projects.
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C15/20 concrete is classified under BS 8500-2:2015+A2:2019 as a designated concrete with minimum 28-day characteristic compressive strength of 15 N/mm² (measured on cylinder specimens) or 20 N/mm² (measured on cube specimens). The "C" denotes cylinder strength, while the second number represents equivalent cube strength.
Compressive Strength
Cylinder Test: 15 N/mm² (Newtons per square millimetre)
Cube Test: 20 N/mm² at 28 days curing
Equivalent Old Grade: GEN1 (General Purpose 1)
Minimum Cement Content: 220 kg/m³
Mix Designation
BS 8500 Code: GEN1 / C15/20
Exposure Class: X0 (No risk of corrosion)
Cement Type: CEM I, II, or IIIA
Aggregate: 20mm nominal maximum size
Workability & Slump
Slump Class: S3 (100-150mm) typical
Consistency: Medium workability
Placing Method: Manual/pump suitable
Finish: Tamped or floated surface
Density & Coverage
Wet Density: 2400 kg/m³ approximately
Coverage: 1 m³ covers 13.3 m² at 75mm thick
Weight: 2.4 tonnes per cubic meter
Truck Capacity: 6-8 m³ standard ready-mix truck
Approved Applications for C15/20 Concrete
C15/20 grade concrete is designated for non-structural applications only according to NHBC Standards 2026 and Building Regulations. Its primary use is foundation blinding - a protective layer that creates a clean, level working surface.
Primary Applications
Application
Typical Thickness
Purpose
Load Capacity
Foundation Blinding
50-75mm
Protective layer beneath strip/pad foundations
Non-load bearing
Trench Fill Foundations (Light)
600-1500mm
Mass fill for conservatories, single-storey outbuildings
Ground Beams: Structural beams need C25/30+ with designed reinforcement
Foundation Blinding with C15/20 Concrete
The primary application of C15/20 concrete is foundation blinding - a critical preparatory layer in foundation construction. Blinding concrete creates a clean, level, and dry working surface on which reinforcement cages and formwork can be accurately positioned before pouring structural foundation concrete.
✓ Purpose of Foundation Blinding:
Protection: Prevents contamination of structural concrete from soil/groundwater mixing into fresh concrete
Level Surface: Creates accurate datum for setting out foundation dimensions and reinforcement positioning
Clean Working: Provides clean surface for construction workers, preventing soil contamination on formwork
Reinforcement Support: Maintains correct cover to reinforcement bars (prevents direct soil contact)
DPM Support: Provides smooth substrate for placing damp-proof membrane without puncture risk
Curing Surface: Prevents moisture loss from structural concrete into dry ground
Blinding Concrete Thickness Standards
Standard Thickness: 50mm
Application: Firm, level ground conditions
Soil Type: Rock, hard chalk, compacted sand/gravel
Coverage: 1 m³ covers 20 m² at 50mm
Cost: Most economical option
Preferred Thickness: 75mm
Application: Standard construction practice
Soil Type: Clay, silt, typical UK ground
Coverage: 1 m³ covers 13.3 m² at 75mm
Cost: Standard specification
Increased Thickness: 100mm
Application: Poor ground or wet conditions
Soil Type: Soft clay, high water table, contaminated ground
Coverage: 1 m³ covers 10 m² at 100mm
Cost: Higher material usage
Maximum Thickness: 150mm
Application: Very poor/waterlogged ground
Soil Type: Peat, made ground, running sand
Coverage: 1 m³ covers 6.7 m² at 150mm
Cost: Consider upgrading to C20/25
C15/20 Concrete Specifications & Mix Design
C15/20 concrete is specified as a designated mix under BS 8500-2, meaning the concrete producer takes responsibility for meeting the performance requirements. The mix is designed to achieve minimum strength while maintaining adequate workability and durability for non-structural applications.
Standard Mix Composition
Component
Specification
Typical Content
Purpose
Cement (CEM I/II/IIIA)
BS EN 197-1
220-260 kg/m³ minimum
Binding agent, strength development
Fine Aggregate (Sand)
BS EN 12620
700-900 kg/m³
Workability, void filling
Coarse Aggregate (10-20mm)
BS EN 12620
1100-1300 kg/m³
Bulk, compressive strength
Water
Clean, potable
160-200 litres/m³
Cement hydration, workability
Water/Cement Ratio
Maximum 0.70
0.65-0.70 typical
Controls strength and durability
Admixtures (optional)
BS EN 934-2
Plasticiser if required
Workability enhancement
Cement Content
TypeCEM I/II/IIIA
Content220-260 kg/m³
StandardBS EN 197-1
Aggregates
Fine (Sand)700-900 kg/m³
Coarse (10-20mm)1100-1300 kg/m³
StandardBS EN 12620
Water Content
Volume160-200 L/m³
W/C Ratio0.65-0.70
QualityPotable water
C15/20 Concrete Costs 2026
C15/20 concrete is the most economical concrete grade available, making it cost-effective for blinding and mass fill applications. Prices vary by region, delivery distance, and order volume. Ready-mix concrete suppliers typically charge per cubic meter with minimum order quantities.
Suitable For: Larger projects, trench fill foundations
Total Installed Cost (per m³)
Material: £70-90 per m³
Labour: £15-25 per m³
Delivery: £10-20 per m³ (averaged)
Total: £95-135 per m³ installed
Alternative Concrete Grades for Foundations
While C15/20 concrete is suitable for blinding and light-duty applications, most foundation construction requires higher-strength grades. Understanding alternative grades ensures correct specification for structural safety and Building Regulations compliance.
Concrete Grade Comparison
Grade
Strength (N/mm²)
Applications
Cost (£/m³)
C15/20 (GEN1)
15 / 20
Blinding, drainage beds, mass fill (light structures only)
The most common question is whether C15/20 can substitute for C20/25 (GEN3) concrete in foundation applications. The answer is definitively NO for structural foundations, but the confusion arises from historic practices and terminology.
Resale Value: Building Control non-compliance affects saleability
Remedial Cost: Underpinning £10,000-50,000+
Placing & Finishing C15/20 Concrete
Proper placement techniques ensure C15/20 blinding concrete achieves its protective and leveling functions. While the grade has low strength requirements, correct installation is essential for subsequent structural foundation construction.
✓ Step-by-Step Blinding Concrete Placement:
Step 1: Foundation Trench Preparation - Excavate to design depth, remove all loose material, compact bottom if required by structural engineer
Step 2: Level Check - Verify trench bottom is level and to correct depth using laser level or string lines and spirit level
Step 3: Set Screed Rails - Install temporary timber screed rails at 50mm/75mm above trench bottom to control blinding thickness
Step 4: Pour Concrete - Discharge C15/20 concrete into trench, distribute evenly along length using shovels and rakes
Step 5: Level & Compact - Screed concrete to final level using straight edge across screed rails, tamp to compact and remove air voids
Step 6: Float Finish - Smooth surface with wooden float (not steel trowel) to create level but slightly textured surface
Step 7: Cure - Protect from rain, direct sun, and frost for minimum 24 hours (48 hours in cold weather)
Step 8: Inspection - Allow Building Control to inspect before proceeding with reinforcement/formwork installation
Cold Weather Concreting
❄️ Cold Weather Precautions (Below 5°C):
Temperature Monitoring: Do not pour concrete if air temperature is below 2°C or forecast to drop below 0°C within 48 hours
Ground Preparation: Remove any frost, ice, or snow from excavation before pouring - frozen ground prevents proper bond
Accelerated Cement: Request rapid-hardening cement or accelerating admixture from supplier for faster strength gain
Insulation: Cover freshly poured concrete with insulating blankets, straw, or polystyrene sheets immediately after finishing
Extended Protection: Maintain covering for minimum 3-7 days in cold weather (longer for C15/20 low-cement mix)
Avoid Heating: Do not use direct heat sources (heaters, fires) which cause rapid drying and cracking
Curing & Strength Development
C15/20 concrete achieves its characteristic strength through a chemical process called cement hydration which requires adequate moisture and temperature over time. The 28-day design strength represents full maturity, but practical construction can proceed earlier.
Strength Gain Timeline
Time Period
Strength Achieved
Percentage of Design
Construction Activity Permitted
24 Hours
3-5 N/mm²
20-30%
Light foot traffic for setting out only
3 Days
7-9 N/mm²
50-60%
Formwork installation, reinforcement placement
7 Days
10-13 N/mm²
70-85%
Structural concrete pour, workers + equipment
14 Days
12-15 N/mm²
85-95%
Brickwork/blockwork construction commences
28 Days
15 N/mm² (design)
100%
Full design strength achieved
3 Months
16-18 N/mm²
105-120%
Continued strength gain beyond design value
24 Hours
Strength3-5 N/mm²
Percentage20-30%
ActivityLight foot traffic
7 Days
Strength10-13 N/mm²
Percentage70-85%
ActivityStructural pour
28 Days
Strength15 N/mm² (design)
Percentage100%
ActivityFull design strength
C15/20 Concrete FAQs
Can I use C15/20 concrete for house foundations?
No. C15/20 concrete is NOT suitable for structural foundations supporting houses, extensions, or garages. Building Regulations Approved Document A requires minimum C20/25 (GEN3) concrete for all structural strip foundations. C15/20 is only approved for foundation blinding (50-75mm protective layer) beneath the structural concrete. Using C15/20 as structural foundation concrete will fail Building Control inspection and void structural warranties.
What is foundation blinding and why is C15/20 used?
Foundation blinding is a thin (typically 50-75mm) layer of weak concrete poured onto the excavated foundation trench bottom before structural foundations. C15/20 is ideal for blinding because it: (1) creates a clean, level surface for accurate setting out, (2) prevents soil contamination of structural concrete, (3) protects damp-proof membranes from puncture, (4) maintains correct reinforcement cover, (5) prevents moisture loss from structural concrete into dry ground. Blinding is non-structural so low strength is adequate.
How much does C15/20 concrete cost in 2026?
C15/20 ready-mix concrete costs £70-90 per cubic meter (ex-works price) in 2026, making it the most economical grade available. Add delivery charges of £60-120 per load (within 10 miles), potential small load surcharges of £30-50 (orders under 4m³), and waiting time fees of £20-30 per 15 minutes beyond 30 minutes on-site. Total installed cost including labour ranges from £95-135 per m³ for typical blinding applications.
What thickness should C15/20 blinding concrete be?
Standard blinding thickness is 75mm for most UK ground conditions. Use 50mm minimum only on firm, level ground (rock, hard chalk). Increase to 100mm on soft clay, high water table, or contaminated ground. Maximum 150mm on very poor/waterlogged ground (though consider upgrading to C20/25 if this thickness needed). Typical domestic extension with 10m foundation perimeter requires approximately 0.5m³ of C15/20 for 75mm blinding.
Can C15/20 be used for conservatory foundations?
C15/20 can be used for conservatory foundations ONLY if using mass concrete trench fill method for lightweight conservatories on good ground. However, most Building Control officers require minimum C20/25 even for conservatories due to glazing loads and thermal movement. C15/20 is definitively approved for conservatory foundation blinding. Always confirm concrete grade with Building Control before ordering - using C15/20 when C20/25 required will necessitate removal and replacement.
How long before I can walk on C15/20 blinding concrete?
Light foot traffic for setting out is permitted after 24 hours in normal conditions (15-20°C ambient temperature). Wait 3 days before heavier activity including formwork installation and reinforcement placement. Allow 7 days before pouring structural foundation concrete on the blinding. In cold weather (below 10°C), extend these periods by 50-100%. Protect freshly poured blinding from rain, direct sun, and foot traffic for at least 24 hours.
What's the difference between C15/20 and GEN1 concrete?
C15/20 and GEN1 are the same concrete grade using different naming conventions. GEN1 is the "designated concrete" name under BS 8500 (GEN = General purpose), while C15/20 is the "prescribed concrete" name indicating strength (C = cylinder strength 15 N/mm², cube strength 20 N/mm²). Modern specifications use C15/20 nomenclature, but suppliers may still refer to GEN1. Both names specify identical concrete meeting the same performance requirements.
Can I use C15/20 concrete for a garden path or driveway?
C15/20 is suitable for light-duty garden paths with pedestrian traffic only (no vehicles). Use 75-100mm thickness on well-compacted sub-base. However, C15/20 is NOT suitable for driveways or any vehicle traffic - cars and vans require minimum C25/30 concrete with A193 steel mesh reinforcement at 100-150mm thickness. Using C15/20 for driveways will result in cracking and failure within months due to insufficient strength.
Do I need Building Control approval for C15/20 blinding?
Building Control notification is required for ALL foundation work including blinding concrete, even though blinding is non-structural. The inspector will verify correct blinding thickness, level, and cleanliness before permitting structural foundation concrete placement. Foundation work cannot proceed without Building Control inspection and approval at each stage. Attempting to pour structural concrete without blinding inspection approval will result in enforcement action and potential foundation removal.
What happens if I accidentally ordered C15/20 instead of C20/25 for foundations?
If C15/20 is poured as structural foundation concrete instead of specified C20/25, you must: (1) Stop work immediately and inform Building Control, (2) Do NOT proceed with building construction, (3) Structural engineer assessment required to determine if foundation must be removed and replaced (likely outcome), (4) Expect costs of £3,000-10,000+ for foundation removal, disposal, and replacement with correct C20/25 concrete. Prevention: Always verify concrete grade on delivery ticket matches structural drawings before discharge.
