What Is Pervious Concrete?
Pervious concrete is a no-fines concrete mix — coarse aggregate with minimal sand, creating 15-25% interconnected voids that let stormwater drain directly through the pavement into the ground below. It is a stormwater-management pavement: parking lots, walkways, and low-traffic areas that infiltrate runoff instead of shedding it. The civil engineer designs it as part of the site's LID compliance; the concrete supplier batches it as a specialty mix.
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How it works
Rain falls on the pavement, drains through the voids (permeability of 100-700+ inches per hour when new), and infiltrates into a stone reservoir base below — which stores the water and releases it slowly to the soil or an underdrain. The system handles both water quality (filtering through the aggregate) and quantity (detention in the reservoir). Typical section: 6-8 inches of pervious concrete over 6-12 inches of open-graded stone reservoir.
Limitations
Pervious concrete suits parking lots, walkways, and low-speed areas — not highways or heavy truck traffic (the void structure has lower strength and abrasion resistance). Cold climates need design for freeze-thaw (air entrainment, adequate drainage so the reservoir does not freeze saturated). Contaminated soils and high groundwater limit infiltration applications. The surface clogs with sediment over time — vacuum sweeping 2-4 times yearly maintains permeability.
Design and construction
The civil engineer sizes the reservoir for the design storm per the local LID manual and confirms subgrade infiltration rates with testing. ACI 522R governs the concrete: strict water control at batching (too wet seals the voids, too dry crumbles), rapid placement (the mix stiffens fast), and proper curing (covered immediately — drying ruins pervious concrete). Use certified installers — pervious concrete placement is a specialty, and failed installations are expensive removals.
The bottom line
Pervious concrete infiltrates stormwater through 15-25% voids into a stone reservoir — for parking and walkways, not heavy traffic. Design per ACI 522R and local LID manuals; maintain with vacuum sweeping.
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