What is Pervious Concrete?; How does it work?; Why do we want it?
What Is Pervious Concrete?
Pervious concrete is a permeable material very similar to conventional concrete except that it contains little or no sand at all. This allows the hardscape area to have a porous structure with large enough voids for water to percolate down at a rate of 3 to 5 gallons per minute per square feet of surface area. Some of you might have seen the videos or taken classes at local horticulture schools where this material was demonstrated, showing how tap water just runs through if you place a sample under the faucet. It is quite remarkable to see it in person! You can find many YouTube videos for this demonstration.
What Are Typical Applications? How About Maintenance?
Most anywhere you would consider using concrete, you can instead use pervious concrete for many benefits outlined below.
Typical applications are driveways, sidewalks, courtyards, sports courts, and walkways.
The maintenance requirements are quite simple. You can vacuum once a year and pressure wash every five years. Percolation rates hold up over many years. If you notice clogging from sand or dirt, pressure washing should restore much of the original permeability.
Why Do We Care?
At a time when our groundwater reservoirs are shrinking, and cities are sinking (subsidence is accelerated by groundwater extraction), it is crucial that we keep water on our sites and let it sink slowly through the soil. With large surfaces such as driveways and walkways, a permeable surface will handle very heavy rains, and play a significant role in recharging the groundwater. This percolation also helps to keep pollutants on driveways from entering the waterways as the soil under the driveway will act as a filter. Otherwise, since the stormwater drains do not channel the runoff to treatment facilities, the runoff goes directly to local water bodies.
Considering that large percentage of hydrocarbons in urban runoff come from the materials in asphalt pavements, it is yet another great reason to replace old, degraded asphalt paths and driveways with pervious concrete.
Other benefits might be that builders get credit for permeable area in new construction builds, and clients can wash their surfaces easily without letting water go down to the storm drains and polluting our freshwater waters like creeks or nearby bays and the ocean.
Lastly, with the heavy rains we are getting in California, such a permeable surface will help to avoid flooding.
What I recommend and Why?
For my clients, I felt the main issue might be the rather Rice Krispies looking texture of the sand-free concrete. Luckily Ryan Marlinghaus of Pervious Products has a great product called PerkTop, which has a finer, more attractive texture that can be poured on top of the pervious concrete in a thin layer in various colors. This layer has a bit more sand, but it can still absorb a huge quantity of water (Ryan quotes 200” per hour) to keep the permeability. He offers a palette of greys and tans and custom colors for those more particular clients.
See the layers of pervious concrete and PerkTop on the photo on the left. The next two photos shows various colors and banding and texture detail.
Concerns and Benefits:
For those who are concerned about relative strength and cost of pervious concrete, Ryan quotes that when installed properly, the strength is the same as conventional concrete and will last as long, if not longer since pervious concrete tends to crack less. As for cost, PerkTop installations cost about the same as ordinary pavers. Because the surface is permeable, some of you might wonder about the clogging potential. Ryan also has an answer for this: "If no surrounding pavement or soil is draining onto the surface, typically no additional maintenance is required to maintain adequate permeability over time. However, if soil or other debris should erode onto or get dumped on the surface and clog it, then hosing off or pressure washing will restore the permeability."
To summarize the benefits of pervious concrete, a pervious pavement or driveway can:
· Reduce the amount of runoff discharging into storm drains
· Therefore, prevent pollutants from entering our water bodies (and causing things like algae blooms)
· Recharge the groundwater and prevent subsidence
· Keep water on site, potentially be a local source for on-site trees and plants
· Help the soil do what it does: act as a filter
· Help with avoiding puddles and hydroplaning on hardscapes
· And with new options like PerkTop, add pizzazz to the landscape design!
What is next?:
Wait for photos using white concrete base and acid wash PerkTop driveway installations for Blue Hibiscus Gardens! Sample photo of acid wash PerkTop among Arbutus and Carmel Quartzite samples here:
*Photos by Blue Hibiscus Gardens
*Find Ryan’s PerkTop product at https://perviousproducts.com/perktop/
*For pervious pre-made pavers, check out Pacific Interlock Pavers: https://pacinterlock.com/what-are-hydroflo-pavers/