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In typical urban watersheds, pavements cover one third of the land; they produce two-thirds of the runoff and almost all the petroleum based pollution.  In the US, over a quarter of a million acres per year are being paved or repaved(Ferguson 23).  With all this land being paved, design must take into account where the moisture will go. 

ACI 330R specifies that the minimum pavement slope of a parking lot should be 1%, or 1/8 inch per foot, and 2% or 1/4 inch per foot is recommended whenever it is possible.  When placing a parking lot, it is extremely important to know where the water that runs off the parking lot goes.  T.R. Dion lists several reasons for wanting to remove water in his book Land Development for Civil Engineers, they are as follows:

  • Minimize flooding.

  • Excess moisture can weaken a soil's strength and even make it plastic if it is cohesive material.

  • Excess moisture can elevate the soil's moisture content to a level that makes it impossible to attain proper field compaction.

  • Excess moisture can result in seepage problems.

  • High groundwater can complicate excavation operations by destabilizing the soil.

  • High groundwater can cause pavement to fail as a result of hydrostatic pressure and loss of subgrade stability.

  • Excess water resulting from inadequate permanent drainage facilities can result in standing water in yards or water in basements and crawl spaces, provided an environment that invites mildew, rot, and possibly insect damage (Dion 324).

    Besides the traditional drainage systems used on roadways and parking lots, engineers have been recently using types of permeable pavements.  These pavements can take care of a large part of urban runoff where the rain falls without having to direct the water into basins at all.  One type of porous pavement used are concrete paver units.  These units sit on a sand setting bed with an aggregate base course.  The configuration of the pavers leaves drainage holes in 12% of the pavement area and the holes are filled with fine gravel.  The infiltration rate for this type of permeable pavement declines in the first four to six years after installation, and then after 10 hours it is nearly stable at a level of about 4 inches per hour (Ferguson 23).  Drawbacks to this system is that the blocks can misalign under certain traffic conditions.  Use of paving blocks in areas that are used for overflow parking make them ideal as well as in areas where they can be confined to resist movement.  

Concrete Paver Units (Courtesy of

    The other system uses concrete over a compacted granular base.  Runoff permeates through the pavement structure to an underlying stone reservoir or infiltration tank (Dion 372).  High quality control is required during installation because the amount and timing of the addition of the water to the mix is critical.  If done improperly, the mixture can be useless when it comes to permeability (Permeable Pavers..).  Larger pea gravel and a lower water to cement ratio is used to achieve a pebbled, open surface that is roller compacted (Permeable Pavement)A drawback to this type of permeable pavement is that freeze-thaw action can cause porous pavements to deteriorate prematurely in some cold-weather climates.  Also, there have been cases where the pavement has become clogged after a short period of service time because the surfaces were not swept(Dion372).  This alternative to the concrete paver units is that the concrete can support higher traffic loads.  A big advantage to using this porous concrete are the cost saving benefits.  Because the pavement absorbs some of the runoff during storms, a reduction is allowed in downstream conveyances and detention basins.(Ferguson 25).  A cross section of this type of pavement would be as follows.


Pervious Pavement (Source: Kara Construction)

Advantages of Pervious Pavement

Construction Cost Saving-Reduce or eliminate expenses for the construction of required stormwater drainage systems and retention areas   

Durability- pervious pavements becomes stronger when wet, and therefore does not deteriorate as fast from moisture as other paving materials

Less costly to maintain - As a result of its better longterm durability it requires less long term maintenance and repair

No standing water hazards- Standing water and slippery pavements from terrential downpours are almost all but eliminated. (Pervious Pavement)

Local Application at Penn State

Porous concrete was used locally here at Penn State at the Penn State and Centre County Visitors and Convention Bureau.  The bureau is located on the northeast side of Beaver Stadium.  The work was performed by Cahill Associates and the building was designed as a "sustainable" building.  Below is a picture from the site which shows a porous concrete sidewalk next to a bio-retention bed.



Dion, T.R.  Land Development for Civil Engineers.  Second Edition. New York:  John Wiley and Sons, Inc, 2002.

Ferguson, Bruce.  "Stormater management and stormwater restoration."  Handbook of Water Sensitive Planning and              Design.  New York: Lewis Publishers, 2002

"Guide for Design and Construction of Concrete Parking Lots."  ACI 330R.  American Concrete Institute, 2001.

"Permeable Pavement."  Toolbase Services.  3 April 2005.                                                                                              <>.

"Pervious Pavement."  Kara Construction.  3 April 2005. <>.



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