Making pervious concrete involves carefully proportioning cement, aggregates, water, and specialized admixtures to create a mixture with a high void content, allowing water to pass through it freely. Unlike conventional concrete, the key distinction in pervious concrete mix design lies in the proportion and size of its aggregates, intentionally minimizing or eliminating fine aggregates.
Understanding Pervious Concrete
Pervious concrete, also known as porous concrete or no-fines concrete, is a special type of concrete with a high porosity. Its primary purpose is to allow water to drain through it, reducing stormwater runoff and replenishing groundwater. This makes it an environmentally friendly solution for pavements, parking lots, and walkways.
Essential Components of Pervious Concrete
Just like any other type of cementitious conglomerate, pervious concrete is made from cement, aggregates, water, and admixtures. The principal difference between a mix design for conventional concrete and a mix design for pervious concrete is the proportion and size of the aggregates used in the mix.
1. Cement
- Type: Typically, standard Portland cement (ASTM C150 Type I or Type II) is used. Blended cements (e.g., with fly ash or slag) can also be incorporated to enhance durability, workability, and environmental benefits.
- Role: Acts as the binder, coating the coarse aggregates and forming the structural matrix.
2. Aggregates
This is where pervious concrete truly distinguishes itself.
- Size and Type: Primarily uses single-sized coarse aggregates (e.g., No. 8, No. 57, or No. 67 according to ASTM C33 standards), often between 3/8 to 3/4 inches (9.5 to 19 mm) in diameter. The use of uniform aggregate sizes creates interconnected voids.
- Fine Aggregates: Fine aggregates (sand) are either completely eliminated or used in very small, controlled quantities (typically less than 10% by weight of the coarse aggregate). This absence of fines is critical for creating the porous structure.
- Role: Forms the bulk of the concrete and defines the void structure for water permeability.
3. Water
- Water-to-Cement Ratio (w/c): Pervious concrete typically uses a lower water-to-cement ratio than conventional concrete, usually ranging from 0.28 to 0.35. This low ratio helps create a stiff, paste-coated aggregate without excess paste that would fill the voids.
- Role: Activates the cement for hydration and provides necessary workability.
4. Admixtures
Admixtures are often crucial for optimizing the properties of pervious concrete.
- Hydration Stabilizers/Set Retarders: Can be used to extend the working time, which is beneficial for placement in warmer conditions or for longer hauls.
- Viscosity Modifying Admixtures (VMAs): Help maintain the paste consistency and prevent aggregate segregation, ensuring a uniform mix.
- Air-Entraining Admixtures: Can improve freeze-thaw resistance, though their effect on permeability needs careful consideration.
- Cementitious Materials (e.g., fly ash, slag): Can improve workability, strength, and durability while reducing cement content.
- Role: Enhance workability, strength, durability, and setting time.
Key Differences in Mix Design
The fundamental difference in mix design for pervious concrete compared to conventional concrete is summarized below:
| Feature | Pervious Concrete | Conventional Concrete |
|---|---|---|
| Fine Aggregates | Minimal to zero | Significant proportion (sand) |
| Aggregate Size | Uniform, single-sized coarse aggregates | Graded, varying sizes (coarse & fine) |
| Void Content | High (15-35%) | Low (2-5%) |
| Water-to-Cement Ratio | Low (0.28-0.35) | Moderate (0.40-0.60) |
| Slump | Zero to very low (stiff, crumbly mix) | Measurable slump (workable) |
| Permeability | High (drains water rapidly) | Low (impermeable) |
The Mixing Process
The mixing process for pervious concrete is similar to conventional concrete but requires attention to detail due to the stiffer, low-slump mix.
- Batching: Accurately measure all components by weight.
- Mixing:
- Start by introducing the coarse aggregates into the mixer.
- Add approximately half of the water and the cement.
- Add admixtures as per manufacturer recommendations.
- Add the remaining water gradually until a consistent, crumbly, yet paste-coated aggregate mixture is achieved. The aggregates should be thoroughly coated with a thin layer of cement paste, and the mix should hold together when squeezed, but still be open and porous.
- Avoid overmixing, which can lead to paste separation and reduced permeability.
Placement and Compaction
Proper placement and compaction are critical for achieving the desired void structure and strength.
- Preparation: Ensure the subgrade is properly prepared, sloped, and permeable. A granular base course is typically used beneath the pervious concrete.
- Spreading: The mix is spread in lifts (layers) using conventional paving equipment or by hand.
- Compaction: Light compaction is essential.
- Use a roller (e.g., steel drum roller, vibratory roller on static mode) or a plate compactor.
- Compact just enough to achieve particle-to-particle contact and densification without closing the void structure. Over-compaction will reduce porosity.
- Typically, one to two passes of a roller are sufficient. The target density is usually around 80% of the solid unit weight.
- Finishing: Minimal finishing is required. A light rolling or tamping to create a uniform surface is generally sufficient. Avoid excessive troweling, which can close surface voids.
Curing
Curing is paramount for the strength and durability of pervious concrete, especially given its low water-to-cement ratio and high surface area.
- Immediate Curing: Due to its open structure, pervious concrete can dry out very quickly. Curing should begin almost immediately after placement and initial compaction.
- Methods:
- Plastic Sheeting: The most common and effective method is covering the concrete with clear plastic sheeting for at least 7 days (or longer in dry, hot conditions). This traps moisture and prevents rapid evaporation.
- Fogging: Mist the surface with water before covering with plastic, especially in hot weather.
- Curing Compounds: While some specialized curing compounds exist for pervious concrete, plastic sheeting is generally preferred for its effectiveness in retaining moisture within the open matrix.
- Duration: Maintain proper curing conditions for at least 7 days to allow the cement to hydrate sufficiently and gain strength.
Practical Tips for Success
- Test Panels: Always create test panels or small trial batches to fine-tune the mix design and placement techniques before large-scale application.
- Experienced Contractors: Work with contractors experienced in pervious concrete installation, as it requires specialized knowledge and techniques.
- Weather Conditions: Be mindful of weather. Hot, windy conditions can lead to rapid drying and premature stiffening. Cold weather requires protection against freezing.
- Quality Control: Regularly check the slump (zero slump in most cases), permeability, and density during placement. A simple "squeeze test" of the mix in your hand can indicate proper paste consistency.
By following these guidelines, you can successfully create pervious concrete that effectively manages stormwater while providing a durable and functional surface.
