Porous asphalt provides better water quality - Pt2
Posted by Jesse Willoughby on Wed, Apr 22, 2009 @ 06:25 AM
Construction and performance
Porous asphalt pavements are fast and easy to construct. With the proper information, most asphalt plants can easily prepare the mix and general paving contractors can install it.
The stone bed, often eighteen to thirtysix inches in depth, provides a tremendous subbase for the pavement. As a result, porous asphalt pavements tend not to exhibit cracking and pothole formation problems. The surface wears well. Under the stone bed is a geotextile which keeps fine particles from moving into the stone bed from below and filling in the spaces.
Porous asphalt has been proven to last for decades, even in extreme climates, and even in areas with many freeze-thaw cycles. The underlying stone bed can also provide stormwater management for adjacent impervious areas such as roofs and roads. To achieve this, stormwater is conveyed directly into the stone bed, where perforated pipes distribute the water evenly.
Economics
Porous pavement is a sound choice on economics alone. A porous asphalt pavement surface costs approximately the same as conventional asphalt. Because porous pavement is designed to "fit into" the topography of a site, there is generally less earthwork. The underlying stone bed is usually more expensive than a conventional compacted sub-base, but this cost difference is offset by eliminating the detention basin and other components of stormwater management systems. On projects where unit costs have been compared, the porous pavement has been the less expensive option. Porous pavements are therefore attractive on both environmental and economic grounds.
An installation at the University of North Carolina in Chapel Hill included parking lots where some sections were constructed from porous asphalt and others used porous concrete. The cost differential was approximately 4:1 - that is, the porous concrete pavement cost four times as much as the porous asphalt pavement.