When you’re planning a concrete project, choosing the right aggregate makes all the difference between success and costly mistakes. Aggregates form 60-80% of concrete’s volume, directly affecting strength, durability, and cost. This guide breaks down every type of aggregate used in concrete, helping you select the perfect material for your specific needs.
What Are Aggregates in Concrete?
Aggregates are granular materials like sand, gravel, crushed stone, or recycled materials that bind with cement paste to form concrete. They provide bulk, reduce cost, and improve concrete’s structural properties.
Think of aggregates as concrete’s skeleton – cement paste is the muscle, but aggregates provide the framework that gives concrete its strength and stability.
Why Aggregate Selection Matters
The wrong aggregate choice can lead to:
- Reduced concrete strength
- Poor workability during mixing
- Increased shrinkage and cracking
- Higher material costs
- Shortened lifespan of your structure
Fine Aggregates: The Foundation Layer
Fine aggregates pass through a 4.75mm sieve and fill voids between coarse particles, creating smooth, workable concrete.
Natural Sand
River Sand
- Source: Naturally weathered from riverbeds
- Particle size: 0.15mm to 4.75mm
- Best for: High-strength concrete, smooth finishes
- Advantages: Rounded particles, excellent workability, minimal impurities
- Limitations: Increasingly scarce, environmental concerns
Sea Sand
- Source: Coastal deposits and seabeds
- Key concern: High salt content requires thorough washing
- Best for: Non-reinforced concrete only (after proper treatment)
- Cost: Often cheaper but processing adds expense
Pit Sand
- Source: Excavated from land deposits
- Characteristics: Angular particles, good bonding
- Best for: General construction, moderate strength requirements
- Advantage: Widely available, cost-effective
Manufactured Sand (M-Sand)
Created by crushing quarry stones, M-sand offers consistent quality and addresses natural sand shortages.
Benefits of M-Sand:
- Consistent gradation and quality
- Lower silt content than natural sand
- Environmentally sustainable
- Better bonding due to angular particles
Considerations:
- Higher water demand initially
- May require admixtures for workability
- Quality varies between manufacturers
Coarse Aggregates: The Structural Backbone
Coarse aggregates exceed 4.75mm and provide concrete’s primary structural strength.
Natural Stone Aggregates
Granite
- Compressive strength: 100-250 MPa
- Best for: High-strength concrete, heavy-duty applications
- Characteristics: Extremely durable, low absorption
- Common sizes: 10mm, 20mm, 40mm
- Cost: Higher but excellent value for critical structures
Limestone
- Compressive strength: 50-150 MPa
- Best for: General construction, moderate loads
- Advantages: Good workability, widely available
- Considerations: May react with acids, check local building codes
Basalt
- Compressive strength: 200-350 MPa
- Best for: Roads, airfield pavements, high-wear surfaces
- Key benefit: Exceptional durability and skid resistance
- Limitation: Limited availability in some regions
Quartzite
- Compressive strength: 150-300 MPa
- Best for: Decorative concrete, exposed aggregate finishes
- Advantages: Chemical resistance, attractive appearance
- Cost: Premium pricing for aesthetic applications
Gravel Aggregates
River Gravel
- Naturally rounded, excellent workability
- Lower cement requirements due to smooth surfaces
- Good for pumped concrete applications
- May require supplementary angular material for high-strength mixes
Crushed Gravel
- Angular particles provide better interlocking
- Higher strength potential than rounded gravel
- More predictable gradation
- Preferred for structural applications
Lightweight Aggregates: Reducing Dead Load
When structural weight matters, lightweight aggregates reduce concrete density while maintaining strength.
Expanded Clay (LECA)
- Density: 300-800 kg/m³
- Best for: Roof slabs, precast panels, insulating concrete
- Benefits: Excellent thermal insulation, fire resistance
- Concrete strength: 15-40 MPa typically
Pumice
- Source: Natural volcanic material
- Density: 400-900 kg/m³
- Advantages: Natural, good insulation properties
- Applications: Block manufacturing, lightweight fills
Expanded Shale
- Manufacturing: Heat-treated shale
- Benefits: Consistent quality, good durability
- Common use: Structural lightweight concrete
- Availability: Regional, mainly North America
Heavyweight Aggregates: Maximum Density Applications
For radiation shielding and counterweights, heavyweight aggregates increase concrete density.
Barite
- Density: 4,200-4,500 kg/m³
- Primary use: Radiation shielding in hospitals, nuclear facilities
- Benefits: Chemically stable, effective radiation absorption
Magnetite
- Density: 4,500-5,200 kg/m³
- Applications: Nuclear reactor shielding, counterweights
- Advantage: Magnetic properties for specialised applications
Steel Shots/Punchings
- Density: 6,000-7,000 kg/m³
- Best for: Maximum density requirements
- Considerations: Corrosion protection needed, specialist mixing required
Recycled and Alternative Aggregates
Sustainable construction drives innovation in aggregate materials.
Recycled Concrete Aggregate (RCA)
Processing: Crushed demolished concrete structures Quality factors:
- Original concrete strength
- Contamination levels
- Processing methods
Performance Guidelines:
- Up to 30% replacement in structural concrete
- 100% replacement possible in non-structural applications
- May increase water absorption by 10-50%
Recycled Asphalt Pavement (RAP)
- Best for: Road base applications, some concrete uses
- Benefits: Reduces landfill waste, cost-effective
- Limitations: Asphalt content affects bonding
Industrial By-products
Blast Furnace Slag
- High-quality aggregate from steel production
- Often superior to natural aggregates
- Excellent for marine environments
Bottom Ash
- Coal combustion residue
- Lightweight properties
- Requires quality testing for consistency
Aggregate Size and Gradation Guide
Proper size distribution ensures optimal concrete performance.
Standard Size Classifications
| Size Category | Typical Sizes (mm) | Primary Use |
|---|---|---|
| Fine aggregate | 0.15 – 4.75 | Workability, void filling |
| Small coarse | 4.75 – 12.5 | General concrete |
| Medium coarse | 12.5 – 25 | Structural applications |
| Large coarse | 25 – 50+ | Mass concrete, foundations |
Gradation Principles
Well-graded aggregates contain particles across the full size range, minimising voids and cement requirements.
Gap-graded aggregates deliberately omit certain sizes, useful for:
- Exposed aggregate finishes
- Specific drainage requirements
- Reduced bleeding in certain applications
Uniform aggregates contain particles of similar size, creating:
- High permeability
- Consistent appearance
- Specialised drainage applications
Testing and Quality Standards
Reliable concrete requires tested, certified aggregates meeting established standards.
Essential Tests
Sieve Analysis
- Determines particle size distribution
- Ensures compliance with specification gradations
- Frequency: Every 500 tonnes or daily for large projects
Los Angeles Abrasion Test
- Measures aggregate durability under mechanical stress
- Acceptable loss: <30% for most applications, <20% for wearing surfaces
Specific Gravity and Absorption
- Critical for mix design calculations
- High absorption aggregates need pre-wetting or mix adjustments
Alkali-Silica Reactivity (ASR)
- Prevents destructive expansion in concrete
- Essential test for long-term durability
- Use mitigation strategies if reactive aggregates necessary
UK Standards and Compliance
BS EN 12620: Primary standard for concrete aggregates BS 882: Traditional UK standard (being phased out) Key requirements:
- Grading envelopes for different applications
- Chloride limits for reinforced concrete
- Shell content limits for marine aggregates
Regional Considerations and Availability
Aggregate choice often depends on local geology and transport costs.
England
- Abundant: Limestone, sandstone, marine-dredged materials
- Scarce: High-quality hard rock in southern regions
- Transport: Rail and water transport crucial for cost control
Scotland
- Strengths: Excellent igneous rocks (granite, basalt)
- Considerations: Weather effects on quarry operations
- Specialty: High-quality road stone export
Wales
- Resources: Slate, sandstone, some limestone
- Challenges: Mountainous terrain affects transport costs
- Opportunities: Recycled materials from urban demolition
Northern Ireland
- Advantages: Basalt (excellent for roads), some granite
- Limitations: Limited variety compared to mainland UK
- Focus: Maximising local resources, minimising imports
Cost Considerations and Budget Planning
Aggregate costs significantly impact project budgets, but cheap isn’t always economical.
Price Factors
Base material cost: Varies by type and quality Transport distance: Often exceeds material cost beyond 50km Processing requirements: Washing, crushing, screening add cost Market conditions: Supply, demand, fuel prices affect pricing
Cost-Benefit Analysis
Premium aggregates may justify higher costs through:
- Reduced cement requirements
- Lower maintenance costs
- Extended service life
- Better performance in harsh conditions
Standard aggregates suit projects where:
- Initial cost is primary concern
- Performance requirements are moderate
- Local availability reduces transport costs
Budgeting Guidelines
- Transport costs: Budget 20-40% of total aggregate cost
- Quality testing: Allow £500-2000 per project depending on scale
- Wastage allowance: Add 5-10% for handling losses
- Price volatility: Include contingency for material cost fluctuations
Selection Criteria for Different Applications
Choose aggregates based on your specific project requirements.
High-Strength Concrete (>40 MPa)
Preferred aggregates:
- Granite, quartzite, high-quality limestone
- Angular particles for better interlocking
- Low porosity and absorption
- Maximum size typically 20mm
Mass Concrete (Dams, Large Foundations)
Optimal choices:
- Larger maximum sizes (up to 150mm) reduce cement needs
- Local materials prioritised for cost control
- Thermal properties important for temperature control
- Gradation crucial for pumpability and placement
Marine Environments
Essential requirements:
- Low chloride content (<0.02% for reinforced concrete)
- Dense, low-absorption aggregates
- Avoid reactive materials
- Consider recycled concrete aggregate with proven marine performance
Decorative and Architectural Concrete
Key considerations:
- Colour consistency and stability
- Surface texture and finish quality
- Uniform gradation for consistent appearance
- Premium materials justified for visual impact
Road and Pavement Applications
Performance priorities:
- High Los Angeles abrasion resistance (<20% loss)
- Adequate skid resistance
- Freeze-thaw durability
- Consistent gradation for workability
Common Problems and Solutions
Understanding potential issues prevents costly mistakes.
Poor Workability
Symptoms: Difficult mixing, harsh concrete, segregation Causes:
- Excessive angular aggregates
- Poor gradation with gaps
- High absorption aggregates
Solutions:
- Adjust aggregate proportions
- Use plasticising admixtures
- Pre-wet high-absorption aggregates
- Consider alternative aggregate sources
Segregation and Bleeding
Symptoms: Aggregate separation, surface water accumulation Causes:
- Excessive fine or coarse material
- Poor aggregate gradation
- Over-vibration during placement
Solutions:
- Optimise gradation through sieve analysis
- Adjust mix proportions
- Modify placement techniques
- Consider viscosity-modifying admixtures
Alkali-Silica Reaction (ASR)
Symptoms: Map cracking, expansion, structural damage Prevention:
- Test aggregates for reactivity
- Use low-alkali cement
- Add pozzolanic materials (fly ash, silica fume)
- Limit moisture access in service
Durability Issues
Common causes:
- Contaminated aggregates (clay, organic matter)
- Inappropriate aggregate for environment
- Inadequate quality control
Prevention strategies:
- Implement comprehensive testing programs
- Source from approved suppliers only
- Match aggregate properties to exposure conditions
- Regular monitoring during construction
Frequently Asked Questions
What’s the difference between 10mm and 20mm aggregate?
10mm aggregate provides:
- Better surface finish
- Easier pumping and placement
- Suitable for thin sections and complex shapes
- Higher cement requirement
20mm aggregate offers:
- More economical (lower cement needs)
- Better for mass concrete applications
- Requires careful mix design for workability
- Standard choice for most structural work
Can I mix different types of aggregate?
Yes, combining aggregates often improves performance:
- Angular + rounded: Balances strength and workability
- Different sizes: Optimises gradation and reduces voids
- Local + imported: Controls costs while meeting specifications
Always test combined aggregates to ensure compatibility and performance.
How do I know if my aggregate is good quality?
Visual inspection:
- Clean appearance, minimal dust
- Consistent size and colour
- No obvious contamination
Essential tests:
- Sieve analysis for gradation
- Los Angeles abrasion for durability
- Specific gravity and absorption
- Chloride and sulfate content
Supplier credentials:
- Third-party certification
- Established quality management systems
- Traceability documentation
What’s the environmental impact of different aggregates?
Natural aggregates:
- Habitat disruption from extraction
- Transport emissions significant factor
- Finite resource availability
Recycled aggregates:
- Reduced landfill waste
- Lower extraction requirements
- Generally lower carbon footprint
- Growing availability and acceptance
Manufactured aggregates:
- Energy-intensive production
- Consistent quality reduces waste
- Can utilise industrial by-products
How much aggregate do I need for my project?
Basic calculation: Concrete volume × aggregate percentage (typically 75-80%)
Example for 10m³ concrete:
- Total aggregate needed: 7.5-8.0m³
- Fine aggregate (35%): 2.6-2.8m³
- Coarse aggregate (40%): 3.2m³
- Add 10% wastage: Total ≈ 9m³
Always consult mix design calculations for precise requirements.
Can weather affect aggregate quality?
Winter considerations:
- Frozen aggregates affect concrete temperature
- Ice contamination disrupts hydration
- Heating may be necessary for cold weather concreting
Summer effects:
- Hot aggregates increase concrete temperature
- Rapid moisture evaporation affects workability
- Shading and sprinkling help maintain quality
Wet weather:
- Changes aggregate moisture content
- Affects mix water calculations
- Proper drainage essential for storage
Summary and Best Practices
Selecting the right aggregate is crucial for concrete success. Here’s your action plan:
Assessment Phase:
- Define your project requirements clearly
- Identify local aggregate sources and availability
- Obtain samples for testing and evaluation
- Consider environmental and sustainability factors
Selection Criteria:
- Match aggregate properties to exposure conditions
- Balance performance requirements with cost constraints
- Ensure adequate supply for project duration
- Verify compliance with relevant standards
Quality Assurance:
- Implement regular testing protocols
- Maintain supplier qualification programs
- Monitor aggregate consistency throughout construction
- Document all test results and compliance evidence
Cost Optimisation:
- Evaluate total lifecycle costs, not just initial price
- Consider local materials to minimise transport costs
- Plan procurement to avoid supply disruptions
- Build relationships with reliable suppliers
Future-Proofing:
- Consider recycled content requirements
- Plan for changing environmental regulations
- Evaluate new aggregate technologies
- Maintain flexibility in specifications where possible
The right aggregate choice sets the foundation for durable, cost-effective concrete. Take time to understand your options, test materials thoroughly, and work with experienced suppliers. Your concrete’s performance depends on these early decisions, so invest the effort to get them right.
Further Reading:
- The Concrete Society – Technical guidance and standards
- Mineral Products Association – Industry information and sustainability
- BSI Standards – Official UK standards for aggregates and concrete