Porous Paving

Also called pervious or permeable, porous paving allows rainwater to infiltrate into the ground. Although some porous pavement types are not new, pavements that are engineered to allow infiltration while also withstanding regular vehicle use are a more recent innovation.

Porous concrete is similar to conventional concrete, but uniform aggregate is used, and fines (smaller particles) are left out. Porous concrete is cast in place over a deep sub-base, with care taken not to float the concrete too much during setting, as that can reduce permeability.

Porous asphalt, like porous concrete, is produced by eliminating the fines typically used in asphalt. Experience is required to maintain a porous surface during rolling.

Unit pavers such as concrete, brick, or stone can be installed with joints filled with uniform aggregate to allow drainage. Pavers are also available as open-celled blocks that are filled with permeable, uniform aggregate or planting media and turf.

Plastic geocell unit pavers, like open-celled masonry pavers, are filled with permeable, uniform aggregate or permeable soil planted with grass.

Loose aggregate such as uniformly sized crushed stone can provide porous paving, although it is only suitable in light-traffic applications where it won’t quickly be displaced, ground down, or mixed with organic matter. Soft materials such as chipped bark or crushed seashells may be used as porous paving in pathways. Conventional dirt roads use mixed aggregate, including fines, and are largely impermeable.

Porous pavement depends on a subsurface that can absorb water. This system of multiple layers, which is usually deeper and more carefully engineered than the average roadbed, supports loading and provides reservoir capacity for stormwater.

By stopping stormwater from pooling and flowing away, porous paving can help recharge underlying aquifers and reduces peak flows and flooding. That means that streams flow more consistently and at cooler temperatures, contributing to healthy ecosystems. Stormwater pollutants are broken down in the soil instead of being carried to surface waters.

Porous paving also contributes to healthy trees along streets and in parking lots. Those trees, along with turf planted in unit pavers, reduce the urban heat island effect. Porous paving is quieter than conventional paving and safer for drivers and pedestrians because less water pools on its surface. It can cost more than conventional paving on a unit basis, but projects can realize big cost savings and permitting benefits by reducing stormwater infrastructure such as retention ponds.

Working with experienced designers and contractors is key. Proper maintenance is also essential to keeping porous pavement functioning. Sand and salt applications should be avoided, for example, unfortunately limiting the suitability of this otherwise versatile strategy in many cold-climate applications.

April 1, 2009

DISCUSSIONS

Reader-contributed comments related to Porous Paving - EBN: 18:4. Comments are listed with newest at the top.

Where to (and not to) use pervious paving

Posted by Nellie Reid on Jul 10, 2009, 02:52 PM
While pervious paving may work great in some areas, there are certainly certain soil types where using the product could prove problematic. We've looked into using this material in north Texas, where the soils are highly expansive clays and visited a few projects that used pervious concrete, only to be told of the major maintenance issues and failures. Additionally, the annual maintenance (vacuuming and power washing) requires a commitment that some of our clients are simply incapable of committing to. Pervious concrete will only work if the open pathways are maintained, and silt and fine dust has got to be removed a few times a year.

pervious paving (concrete & asphalt)

Posted by Shelley Billik on Mar 31, 2009, 07:15 PM
what are the parameters for good applicability...in other words, are there areas in certain states where it is not a good idea to let water percolate through? is it dependent on the height of the water table? I would love to read some material on this. Also, what type of expertise is needed on a project to determine this? I would assume civil engineer...

Pervious Concrete !

Posted by Glenn Summers on Mar 31, 2009, 06:38 AM
It appears that permiting agencies and the skeptics are finally coming around to understand that this really does work. Can't recall the number of times people looked at me as if I just landed in a spaceship when I proposed using pervious for parking lots.
In the late 70's, water mangagement and plan review folks At Best "MIGHT" be willing to allow a 50% credit for drainage, IF they could be convinced this stuff was for real??
All they wanted, was to see you waste sq. ft. in constructing retention/detention ponds on a site. I particularly liked the signs around the work areas of these permiting Officials "What Don't You Understand About NO!". Some of them probably still have their Jobs!

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