PERVIOUS CONCRETE — WHAT, WHY & HOW?

WHAT IS PERVIOUS CONCRETE?

Pervious concrete is a special type of concrete with high porosity used in flatwork applications that allows water from precipitation and other sources to pass through, thereby reducing runoff and recharging groundwater levels. The high porosity is achieved through a highly interconnected void structure. Typically, pervious concrete contains little to no fine aggregate, with just enough cementitious paste to coat the coarse aggregate particles while maintaining the connectivity of the voids.

Pervious concrete is commonly used in parking lots, low-traffic areas, pedestrian walkways, and greenhouses. It is an important application for sustainable construction.

WHY USE PERVIOUS CONCRETE?

The proper use of pervious concrete is recognized as a Best Management Practice by the U.S. Environmental Protection Agency (EPA) for first-flush pollution control and stormwater management. As regulations further restrict stormwater runoff, the cost of real estate development increases due to the size and expense of drainage systems.

Pervious concrete reduces runoff from paved areas, decreasing the need for separate stormwater retention ponds and allowing for smaller-capacity storm sewers. This enables property owners to develop a larger portion of available land at a lower cost.

Additionally, pervious concrete naturally filters stormwater, reducing the pollutant load entering streams, ponds, and rivers. Acting as a stormwater retention basin, it allows water to infiltrate the soil over a large area, aiding in the local replenishment of groundwater supplies.

Pervious concrete can also help preserve trees in urban developments. By allowing both water and air to reach tree roots, it enables trees to thrive even in highly developed areas.

HOW TO CREATE PERVIOUS CONCRETE PAVEMENT?

Installing pervious concrete requires an experienced contractor to ensure success. As with any concrete pavement, proper subgrade preparation is crucial.

• Subgrade Preparation:
• The subgrade should be properly compacted to create a uniform and stable surface.
• When placed on sandy or gravelly soils, the subgrade should be compacted to 92-96% of maximum density (ASTM D 1557).
• For silty or clayey soils, a layer of open-graded stone may be needed, and geotextile fabric is often used to separate fine-grained soils from the stone layer.
• Care must be taken to avoid overcompacting swelling-prone soils.
• The subgrade should be moistened before concrete placement to prevent premature drying and setting.
• Mixing Pervious Concrete:
• The water-to-cementitious material (w/cm) ratio typically ranges from 0.35 to 0.45, with a void content of 15-25%.
• The mix consists of cementitious materials, coarse aggregate, and water, with little to no fine aggregate.
• Adding a small amount of fine aggregate can reduce void content and increase strength, which may be desirable in some cases.
• The correct water content is crucial: too much water can cause segregation, while too little can lead to balling in the mixer and slow unloading.
• A properly proportioned mix has a wet metallic sheen.

• Placing and Finishing:
• Pervious concrete can be placed using fixed forms or a slip-form paver.
• After placement, the concrete is struck off using vibratory or manual screeds.
• Rolling compaction is performed using a manually operated roller that bridges the forms, ensuring a strong bond between the paste and aggregate while creating a smoother surface.
• Excessive rolling pressure should be avoided to prevent void collapse.

• Jointing:
• Due to the lower water content, pervious concrete has significantly reduced shrinkage, allowing for wider joint spacing.
• However, the basic principles of jointing remain the same.
• Joints are typically created using a rolling jointing tool, allowing for faster cutting and uninterrupted curing.

• Curing:
• Proper curing is essential for the structural integrity of pervious concrete pavement.
• Curing should begin within 20 minutes of placement and continue for at least seven days.
• Plastic sheeting is commonly used to retain moisture during the curing process.

PERVIOUS CONCRETE IN FREEZE-THAW ENVIRONMENTS

While pervious concrete is primarily used in warm climates, successful applications have been observed in colder regions.

• Preventing Saturation and Freezing:
• Pervious concrete should not become fully saturated in freeze-thaw environments.
• This can be prevented by placing it on an 8 to 24-inch (200 to 600 mm) thick layer of open-graded stone.
• Enhancing Durability:
• Limited laboratory testing has shown that air entrainment may improve freeze-thaw durability even when pervious concrete is fully saturated.
• However, no standard ASTM test exists to verify air entrainment in pervious concrete.

MAINTENANCE & CLEANING OF PERVIOUS CONCRETE

The EPA recommends regular cleaning of pervious concrete to prevent clogging.

• Cleaning Methods:
• Vacuum sweeping or high-pressure washing can effectively remove debris and prevent clogging.

• Water Quality Monitoring:
• Although pervious concrete and underlying soil provide excellent filtration, they may not remove all contaminants.
• In critical areas where groundwater quality must be preserved, stormwater testing is advised.

Source: National Ready Mixed Concrete Association (NRMCA)

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