Detailed guide to tarmac specifications, layer depths, preparation requirements, and material types for groundworks projects in Exeter.
Complete Guide to Tarmac Specifications for Exeter Groundworks
When it comes to tarmac installations in Exeter, getting the specifications right is crucial. Whether you're a homeowner planning a new driveway or a contractor working on commercial groundworks, understanding proper tarmac construction can make the difference between a surface that lasts decades and one that fails within years.
Understanding Tarmac Construction Layers
Think of tarmac construction like building a house - you need solid foundations. Each layer has a specific purpose, and skipping or skimping on any layer compromises the entire structure.
The Complete Layer System
| Layer | Purpose | Typical Depth | Material Type |
|---|---|---|---|
| Sub-grade | Natural ground foundation | N/A | Compacted earth |
| Sub-base | Load distribution & drainage | 150-350mm | Type 1 MOT |
| Base Course | Primary structural layer | 50-100mm | AC 20 dense bin |
| Binder Course | Load bearing & leveling | 50-60mm | AC 20 dense bin |
| Surface Course | Wearing layer | 25-40mm | AC 6 dense surf |
Preparation Requirements: Getting It Right from the Start
1. Site Assessment
Before any material arrives on site, proper assessment is crucial:
• Drainage assessment
• Load requirements analysis
• Existing surface condition
• Service locations marked
• Plate bearing test
• Moisture content analysis
• Compaction testing
• Level surveys
2. Excavation Specifications
Commercial Access: 400-500mm total depth
Heavy Traffic Areas: 500-700mm total depth
Footpaths/Patios: 150-200mm total depth
3. Sub-grade Preparation
The foundation of your tarmac - get this wrong and everything else fails:
• Excavate to required depth plus 50mm
• Proof roll to identify soft spots
• Compact to 95% Modified Proctor
• Install geotextile membrane if needed
• Ensure 1:40 minimum crossfall
Sub-base Installation: The Critical Foundation
Type 1 MOT Specifications
| Property | Specification | Why It Matters |
|---|---|---|
| Grading | 0-40mm well graded | Ensures proper compaction |
| Fines content | 5-12% passing 0.063mm | Binds material together |
| Moisture | Optimum ±2% | Achieves max density |
| Compaction | 95% Modified Proctor | Prevents settlement |
Installation Process
• Compact: Use vibrating roller (minimum 2.5 tonnes)
• Test: Check levels and compaction
• Layer 2: Add second layer if needed
• Final prep: Ensure surface tolerance ±10mm
Tarmac Types and Their Applications
Surface Course Options
1. AC 6 Dense Surface Course (6mm)
Best for: Domestic driveways, footpaths
Thickness: 25-30mm
Benefits: Smooth finish, good appearance
Limitations: Less durable under heavy loads
2. AC 10 Close Surface Course (10mm)
Best for: Standard car parks, access roads
Thickness: 30-40mm
Benefits: Good durability, reasonable cost
Limitations: Rougher texture
3. AC 14 Close Surface Course (14mm)
Best for: Heavy traffic areas
Thickness: 40-50mm
Benefits: Maximum durability
Limitations: Coarser appearance
Binder Course Specifications
| Type | Aggregate Size | Application | Layer Thickness |
|---|---|---|---|
| AC 20 HDM | 20mm | Heavy duty | 60-100mm |
| AC 20 Dense | 20mm | Standard duty | 50-75mm |
| AC 32 HDM | 32mm | Extra heavy duty | 75-150mm |
Temperature Requirements for Installation
Critical Temperature Ranges
Surface Temperature: Minimum 2°C
Mix Temperature at Delivery: 140-160°C
Mix Temperature at Laying: Minimum 120°C
Compaction Temperature: Above 85°C
Compaction Specifications
Equipment Requirements by Layer
• Minimum 4 passes
• Plate compactor for edges
• Moisture control essential
• Pneumatic roller for finish
• Start at joints and edges
• Complete before cooling
Compaction Targets
| Layer | Target Density | Air Voids | Test Method |
|---|---|---|---|
| Surface course | 96-98% | 3-7% | Core sampling |
| Binder course | 94-96% | 4-8% | Nuclear gauge |
| Base course | 93-95% | 5-10% | Nuclear gauge |
Drainage Integration
Essential Drainage Elements
• Edge restraints to prevent water ingress
• Linear channels at low points
• Connection to surface water system
• Permeable sub-base where required
Quality Control Checklist
Pre-Installation Checks
• Compaction tests passed
• Weather conditions suitable
• Materials certificates received
• Edge restraints installed
During Installation
• Layer thickness verified
• Joint construction correct
• Compaction achieved before cooling
• Surface texture adequate
Post-Installation
• Falls and levels correct
• No ponding after rain
• Joints properly sealed
• Edges neatly finished
Common Specification Mistakes in Exeter
1. Insufficient Sub-base Depth
Problem: Exeter's clay soils expand/contract
Result: Cracking and surface failure
Solution: Minimum 200mm Type 1, preferably 250mm+
2. Poor Drainage Design
Problem: High rainfall + clay = water retention
Result: Sub-base failure, frost damage
Solution: Proper falls, edge drainage, possibly French drains
3. Wrong Surface Course
Problem: Using AC 6 for heavy vehicles
Result: Rutting, premature wear
Solution: Match specification to actual use
4. Temperature Violations
Problem: Laying in cold/wet conditions
Result: Poor compaction, early failure
Solution: Strict temperature monitoring
Cost Implications of Proper Specification
Typical Exeter Pricing (2024)
| Specification Level | Cost per m² | Expected Lifespan | Cost per Year |
|---|---|---|---|
| Budget (minimum spec) | £45-55 | 8-10 years | £5.50/year |
| Standard (proper spec) | £55-70 | 15-20 years | £3.50/year |
| Premium (heavy duty) | £70-90 | 20-25 years | £3.20/year |
Specification for Different Applications
Domestic Driveways
Sub-base: 150-200mm Type 1 MOT
Binder: 60mm AC 20 dense bin
Surface: 30mm AC 6 dense surf
Edge restraint: Block paving or concrete
Commercial Car Parks
Sub-base: 250mm Type 1 MOT
Base: 100mm AC 32 base
Binder: 60mm AC 20 HDM bin
Surface: 40mm AC 10 close surf
Drainage: ACO drains at 30m centers
Heavy Goods Areas
Sub-base: 350mm Type 1 MOT
Base: 150mm AC 32 HDM base
Binder: 100mm AC 20 HDM bin
Surface: 50mm AC 14 close surf
Special: Concrete pads at loading bays
Maintenance Specifications
Preventative Maintenance Schedule
• Clear drainage channels
• Check edge restraints
• Remove oil stains
• Sweep loose material
• Crack sealing program
• Drainage CCTV survey
• Remarking if required
• Edge repair assessment
Working with Exeter Groundworks Contractors
Key Questions to Ask
• What compaction equipment do you use?
• How do you verify temperatures?
• What guarantees do you offer?
• Can I see similar local projects?
Red Flags to Avoid
• Single layer construction proposed
• No temperature monitoring mentioned
• Unwilling to provide specifications
• Pushing for work in winter months
Conclusion
Proper tarmac specification isn't just about following rules - it's about creating surfaces that withstand Exeter's unique challenges. From our clay soils to our wet climate, every aspect of specification matters.
Remember: tarmac is only as good as what's underneath it. Invest in proper preparation and specification, and you'll have a surface that serves you well for decades.