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Sunday, October 3, 2010

What is Porous Pavements?

What is Porous Pavements?

Porous pavements, both asphalt and concrete have been around for years.  In most areas they haven’t really caught on.  Now, with the large focus on environmental issues and green building, are they worth looking at again?
Pavement design
Traditional pavement design
Typically when pavement mixes are designed, they include different sizes of aggregate.  They use a wide range from fine sand to coarse stone.  The largest size depends on the expected use of the material.  Then it is all bound together with binder or cement.  With asphalt pavement that top layer will go on a water proof layer then a base.  Concrete pavement may go on a base or directly on the ground.
This results in an impenetrable surface that blocks rain water from getting into ground water systems and increases runoff.
Porous pavement design
Porous pavement or pervious pavement is designed using medium and large sized aggregate without any smaller fines such as sand.  It is then held together using with cement or binder.  The lack of fines in the mix creates relatively large pore space in the pavement.  This large pore space allows water to pass through.
The top layer is placed either directly on the ground or on other porous base layers to allow water to drain completely through the system into the ground.
Pros and Cons
Here are some pros and cons as well as a few notes on them.
Pros
Increased water quality – Oils, heavy metals and other contaminates on the pavements are not carried downstream and into stormwater drainage systems.  Also, water is filtered as it passes through the pavement.
Lower initial construction costs – Construction costs may be lower because porous pavements lower the amount of stormwater drainage facilities that a site will need.  Fewer and smaller inlets, detention ponds and storm drain pipes means lower construction costs.
Lower long term costs – less maintenance needed for storm drain and filtration systems.
Fewer fees – Storm water impact fees may be lower since porous pavements are proven to reduce runoff.
Less runoff – Less runoff means less potential flooding and lower peak flows.
Increased safety – Since water drains through the pavement there is a lower chance of hydroplaning and an increase in traction.
LEED Points – It can indirectly help gain LEED Points.  It can contribute in the areas of Stormwater Design, Heat Island Effect, Water Efficient Landscaping, Recycled Content, and Regional Materials.  There may be other ways that using it can help LEED certification.
Cons
Higher initial construction cost – Yes, I know I listed construction cost as a pro also.  The cost of constructing the pavement itself tends to be higher than regular pavement.
Soil restrictions – The soil below the pavement must drain at least as well as the pavement.
Clogging – The pores in the pavement may clog.  Suppliers and other proponents say that regular cleaning and maintenance will nearly eliminate clogging.
Pavement strength – Porous pavements are structurally weaker than standard pavements.   That generally results in them being used only for low traffic roads and parking lots.  Extra care must be taken when designing a pavement for high traffic or heavy traffic.
New/Untested technology – That’s not entirely accurate.  The technology has been tested since at least 1971.  However, most contractors don’t have experience with it.  Proper training, clear instructions, material testing, and site investigations should be done to ensure that the pavement meets all applicable standards during construction.
Contamination – Pavement surfaces usually have a lot of contaminates on them.  Porous pavements can filter contaminants, but no system is 100%.  Since water drains directly into ground soil it is possible that it will take the contaminants with it.
Conclusions
There is a lot of potential for porous pavements in future projects.  Each project would have to be investigated independently to determine any cost or environmental savings that might be gained by using porous pavements.  However, the potential positives do seem to outweigh the potential negatives.  It would certainly be worth your time to investigate it and present your findings to your client.
What are your thoughts on Porous and Pervious Pavements?

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