Nuts & Bolts: When It Rains, It's Porous

Article Tools

Image Gallery

Print

E-mail

Comments

RSS

More attractive, but also more costly, are concrete or ceramic permeable paving stones, or permeable interlocking concrete pavement (PICP). These blocks fit together in herringbone, basketweave or other patterns, leaving open space between. The openings, also called voids, are filled with gravel, which allows water to drain through and soaks up fairly heavy rainfall.

Interlocking permeable pavers cost about 25 percent more than conventional ones, and they require that you blow leaves and debris off them periodically to keep the voids from filling. Look for pavers containing a percentage of fly ash, a recycled byproduct from coal burning.

Water-permeable paving

It’s possible to build a driveway that looks essentially identical to everyday driveways but allows water through. Porous asphalt (blacktop) and pervious concrete are versions of traditional paving materials, but they’re made without fine sand. These mixes allow as much as 3 to 5 gallons of rainwater a minute to soak through each square foot. They’re slightly rougher looking than their conventional counterparts, but from a distance, it’s hard to tell the difference.

Porous asphalt is the less costly option, and it can be highly durable if installed correctly over a gravel bed. It also can be the trickiest material to work with. When heated, asphalt can become malleable—think of sticky blacktop on a hot summer day—and early versions of porous asphalt tended to clog as binders in the asphalt filled the open spaces. Today, porous asphalt includes polymer fibers that prevent binders from clogging. Those additives have to be applied at the right temperature; an experienced contractor is a must.

Pervious concrete is less complicated to pour—the National Ready Mixed Concrete Association certifies installers—but it’s more expensive than porous asphalt. Pervious concrete is faster to install than conventional because it doesn’t need to be smoothed with a trowel (finishing would seal the pavement surface, decreasing water penetration). Another benefit is that pervious concrete is lighter in color than asphalt, so it doesn’t absorb heat and radiate it back into the environment like blacktop does.

RSS | Comments | E-mail | Print