Understanding Open Graded Base for Pavers
In modern landscape architecture, managing stormwater runoff is a critical design requirement. Permeable Interlocking Concrete Pavement (PICP) systems rely on an open graded base for pavers to facilitate rapid infiltration and temporary storage of water. Unlike traditional dense-graded bases that use a mix of fines and aggregates to create a water-tight foundation, an open graded system utilizes specific stone sizes with a high void ratio. This engineering approach allows water to pass through the joints of products like Aqua-Via I and into the sub-base, reducing the burden on municipal storm drains and improving local water quality.
Key Terminology for Permeable Systems
Open Graded Aggregate
Open graded aggregate refers to crushed stone that has been screened to remove small particles and fines. By using stones of relatively uniform size, the material maintains large interconnected voids (typically 30% to 40% of the total volume). These voids act as a reservoir for stormwater.
Sub-Base (Reservoir Layer)
This is the thickest layer of the system, typically consisting of ASTM No. 2 or No. 57 crushed stone. Its primary function is structural support and water storage. For heavy-duty applications or areas with poor soil infiltration, the depth of this layer is increased to accommodate larger volumes of runoff.
Setting Bed (Bedding Layer)
The bedding layer is the thin leveling course directly beneath the pavers. For permeable systems, a small, clean stone such as ASTM No. 8 is used. This allows for a flat laying surface while maintaining the vertical permeability required for products like Aqua-Via Trio or 4 x 12 AquaLiña.
Jointing Material
In a permeable system, the joints are not filled with sand. Instead, they are filled with the same small, open-graded aggregate used in the bedding layer (such as No. 8 or No. 9 stone). This ensures that water can enter the system at the surface level.
Structural and Functional Benefits
Using an open graded base for pavers provides several advantages for commercial and residential projects:
- Stormwater Management: It represents a best management practice (BMP) for controlling runoff at the source.
- Durability: Open graded bases are less susceptible to frost heave because the large voids allow for water expansion without displacing the aggregate.
- Environmental Compliance: Many municipalities require permeable solutions to meet strict environmental regulations regarding site coverage and drainage.
Comparison of Base Types
| Feature | Open Graded Base | Dense Graded Base |
|---|---|---|
| Aggregate Type | Clean, crushed stone (No fines) | Crushed stone with fines (3/4" minus) |
| Permeability | High (Rapid infiltration) | Low (Sheds water) |
| Primary Use | Permeable Pavers (e.g., Aqua-Via ML) | Standard Pavers (e.g., Holland I) |
| Void Space | 30% - 40% | < 5% |
Design Considerations for Professionals
When specifying an open graded base for pavers, architects must consider the subgrade soil type. In areas with low-permeability clay soils, sub-drains or perforated pipes may be necessary within the reservoir layer to prevent the system from becoming oversaturated. Additionally, the edge restraint must be robust enough to contain the open-graded material, which does not "lock" in the same way as dense-graded stone until it is fully compacted and the pavers are installed.
For pedestrian and light vehicular applications, products like the 4 x 12 can be adapted for specific drainage needs, though dedicated permeable series like Aqua-Via are specifically engineered with wider joints to maximize the efficiency of the open graded base.
Specify it
To ensure your project meets specific drainage and structural requirements, use the AckerStudio AI to generate custom patterns and technical specifications. You can also request physical samples of our permeable series to evaluate the finish and joint spacing in person.