Let’s be clear – paving a surface does not reduce storm water run-off, but if you must create a street, parking lot, driveway, or any other form of this detriment, an apples-to-apples comparison between pervious concrete and asphalt paving will show many exciting benefits from this innovative form of paving. First of all, those little air pockets mean less material overall, and with the cost of oil as unstable as the resource itself, petroleum-based asphalt is no longer the cheapest form of paving. For the first time in history, the cost of concrete and asphalt have reached comparable dollar values. Add in the cost of additional storm water management devices – grids of piping directed towards drains and retention ponds, and suddenly permeable concrete starts making a lot more sense. Then there is the environmental cost of leaching toxic chemicals, such as polycyclic aromatic hydrocarbons (PAHs) used as a sealcoat on asphalt, into what should be nature’s flow of water back to the ground table.
The key is to compare drainage ‘systems’ and not just the surface material itself. While concrete production is a major producer of green house gases, it is inert once it’s solid, and it does not require periodic resurfacing like asphalt. Pervious concrete is made of larger aggregate cementitiously bonded with a 12-20% void ratio and a flow ratio of about 3-8 gallons of water drainage per minute.
As with all design, site-appropriate material selection is key. The Concrete Network has Ten Steps to Ensuring a Successful Pervious Concrete Installation. Sites with high clay content may not be the best candidates for permeable paving because they are prone to clogging. However, one tactic being used on many projects in the Pacific Northwest, is to combine paving types, so that large drainage strips of permeable paving can replace the traditional underground piping and covered drains. Concrete Thinker has case studies of projects with permeable paving which help show how and where this intuitive paving type is best used, and the National Ready Mix Concrete Association has installation demos available. To find out more about permeable paving and where to find local ready mix providers and installers, visit www.ConcreteNetwork.com
+ Pervious Pavement
Via Smart2beGreen
I’ve seen this stuff on several projects and it indeed does work great….for about 6 month. If it is not pressure washed regularly it clogs and essentially becomes impervious.
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test comment
Porous pavement is an excellent alternative to pavement an it shows a move in the right direction in the construction industry. However, designers should be cautious of the use of porous pavement in the Northeast and Northern climates, where there are harsh winters, as there are concerns with effects of the freeze/thaw cycle on the porous pavement. There are applications in these areas, however they are still relatively new. Hopefully there will be more applications in these harsher climates and new alternatives will be created.
how does it hold up to the freeze/thaw cycle? looks as if it might just crack to pieces, no?
We just recently designed and over saw the construction of the State of Michigan’s first pervious asphalt trail in Flint, MI. It’s a demonstration project because, as jbooth2009 noted, these materials are a tricky matter where freeze/thaw is a problem. This increases in areas where soils are poorly drained. To reduce these potential problems it requires a significant amount of subbase, detention, and underdrains to make sure the water is drained as fast as it infiltrates (and it infiltrates really, really quickly). The additional amount of engineering and ‘liability insurance’ (aka over-engineering) being required to ensure a product that doesn’t fail is blowing away any whisp of economic benefit.
Additionally, we’re seeing problems with long-term maintenance of this particular facility. Even though it’s maintained through the an agreement with a local college, repeated letters and directions as to its care are continuously ignored. They’re driving their skid-steers, mowers, security/utility carts all over it and in the process tracking mud, grass, and debris all over the path just as if it was a typical asphalt. I’m already seeing points of degradation and the trail isn’t even 8 months old yet.
My concerns with porous concrete lie with the harmful substances (i.e. gasoline, oil, pollution) it could dissolve and carry into the soil and watershed. What (permeable vs. impermeable) concrete is more detrimental to the environment is worth discussing.