Engineer permeable paver base assemblies using three strictly washed open-graded stone fractions conforming to ASTM C33: 1) Sub-Base: 6"–12" ASTM No. 2 (2"–3" fractured ballast, 40% void ratio); 2) Base Layer: 4" ASTM No. 57 (1/2"–1" crushed stone, 35% void ratio); 3) Bedding & Joint Layer: 2" ASTM No. 8 (1/4" washed chip grit). All stone must have less than 1% fine dust passing the #200 sieve.
At a Glance: Key Takeaways
- ASTM No. 2 Sub-Base Reservoir Stone (2" to 3" / 50–75mm): High storage capacity reservoir stone supporting heavy truck axles.
- ASTM No. 57 Base / Choker Stone (1/2" to 1" / 12.5–25mm): Interlocks over No. 2 stone to prevent bedding layer migration.
- ASTM No. 8 Bedding & Joint Chips (1/4" / 2.36–9.5mm): Double-washed angular chips provide flat screeding and joint friction interlock.
- Zero Fines Washing Standard (<1% Passing #200 Sieve): Prevents fine stone dust from washing into voids and choking permeability.
- Reversible Plate Compaction (5,000+ lbf): Vibrates angular fracture faces into rigid mechanical lock without breaking stones.
1. Aggregate Physics: Sieve Curves & Open-Graded Void Ratios
Traditional crushed road base (like Crusher Run or 3/4" Minus) contains fine rock dust and clay designed to compact into a solid, impervious layer. Under heavy rain, trapped water liquefies the road base, pumping mud through paver joints.
Permeable Interlocking Concrete Pavement (PICP) mandates Open-Graded Crushed Stone conforming to ASTM C33 / ASTM D448. Because all aggregate particles in a layer are uniform in size, they touch at angular contact points, leaving 35% to 40% open interconnected void space. Master hydrology math in stormwater infiltration hydrology.
2. The Choker Principle & 2" ASTM No. 8 Bedding Screeding
To prevent the fine ASTM No. 8 bedding chips from falling into the large gaps of the ASTM No. 2 sub-base, engineers apply the Terzaghi Filter Criteria.
The 4-inch layer of ASTM No. 57 acts as a 'Choker Layer', bridging the gap between large ballast and small bedding chips. Review edge restraints in edge restraints and joint maintenance and explore permeable pavers hub.
ASTM Open-Graded Aggregate Specifications for PICP
Use this reference table to calculate exact physical dimensions, ratios, and specifications before purchasing:
| Aggregate Fraction | Particle Size Range | Void Space Ratio | PICP Assembly Layer |
|---|---|---|---|
| ASTM No. 2 Stone (Optimal) | 2" to 3" (50mm to 75mm) | 40% Open Void Space | Sub-Base Reservoir (6"–12" thickness) |
| ASTM No. 57 Stone | 1/2" to 1" (12.5mm to 25mm) | 35% Open Void Space | Base / Choker Layer (4" thickness) |
| ASTM No. 8 Washed Chips | 1/8" to 3/8" (2.36mm to 9.5mm) | 30% Open Void Space | Setting Bed (2") & Joint Infill |
| ASTM No. 89 Micro-Chips | 1/16" to 1/4" (1.18mm to 4.75mm) | 30% Open Void Space | Tight Joint Infill (for narrow paver gaps) |
Unwashed Dirty Quarry Stone Void Clogging Constraints
Every successful decoration project must account for physical and environmental constraints:
Quarry Delivering Unwashed No. 57 Stone Containing Limestone Slurry Silt
Unwashed aggregate contains fine limestone quarry dust; during the first heavy rain, this silt washes downward, forming an impenetrable mud pan on the geotextile that stops drainage; strictly specify double-washed aggregate with certification showing <1% fines passing the #200 sieve.
How to Place, Grade, and Compact Open-Graded Aggregates
Execute this professional sequence to achieve flawless results without backtracking:
Verify Subgrade Excavation Depth and Slope
Ensure subgrade is graded flat with zero pitch or sloped <1% toward downhill collection pipes; line with geotextile.
Spread 6" to 12" of ASTM No. 2 Open-Graded Sub-Base
Place large No. 2 crushed ballast in 6" lifts; compact with 3 passes of 5,000 lb reversible plate compactor.
Install 4" Layer of ASTM No. 57 Base Choker Stone
Spread 3/4" No. 57 stone over sub-base; compact with 3 passes to choke and level the large aggregate voids.
Set 1" Galvanized Steel Screed Pipes
Place 1" outside-diameter steel pipes on compacted No. 57 base, setting finished bedding elevation with laser level.
Screed 2" Layer of ASTM No. 8 Washed Bedding Chips
Shovel No. 8 chips between pipes; drag aluminum screed bar across pipes to achieve smooth un-compacted bed.
Remove Screed Pipes and Fill Troughs with Chips
Carefully remove pipes; fill resulting indentations with No. 8 chips, smoothing flush with hand trowel.
Lay Pavers Directly onto Un-Compacted Bedding Bed
Place concrete pavers directly onto loose No. 8 bed, maintaining straight layout lines with laser guide.
Common Mistakes & Countermeasures
Avoid these costly errors that compromise aesthetics, ergonomics, and budget:
Using Sand or Stone Dust for the Bedding Layer
Project Execution Checklist
Track your physical progress. Checkboxes persist automatically on this device:
Frequently Asked Questions
Can crushed gravel with rounded river pebble edges be used for PICP bases?
NO. Rounded river gravel lacks angular friction planes and rolls under wheel loads like marbles. All PICP aggregates MUST be 100% crushed quarry stone with fractured angular faces.
