Constructing porous asphalt pavements is straightforward. Any qualified asphalt pavement contractor can construct such pavements and virtually any asphalt plant can produce the material. No certification is required.
Successful construction starts from the ground up – literally. Care must be taken to not compact the subgrade during excavation and construction, as excessive compaction would reduce infiltration. Using backhoes and positioning the equipment alongside the recharge bed is one technique that can be used.
Photo courtesy of CH2M HILL
Where this is not feasible, excavate using equipment with tracks or over-sized tires. Narrow rubber tires should be avoided. The site area for the porous pavement should also be protected from other heavy equipment running on the subgrade.
As soon as the bed has been excavated to the final grade, place the filter fabric. Overlap the filter fabric a minimum of 16 inches. It should extend at least four feet outside of the bed. This excess fabric will be folded over the stone bed to temporarily protect it from sediment until the porous asphalt surface is placed.
Install drainage pipes if required.
Place aggregate for the stone recharge bed, taking care not to damage the filter fabric. Aggregate should be dumped at the edge of the bed and placed in layers of 8 to 12 inches using track equipment. Compact each lift with a single pass of a light steel wheel roller.
Photo courtesy of CH2M HILL
The porous asphalt layer is placed in 2- to 4-inch-thick lifts using track pavers, following state or national guidelines for the construction of open-graded asphalt mixes.
Photo courtesy of CH2M HILL
Compact the asphalt with two to four passes of a ten-ton static roller. Normally, only a few passes are necessary. In many cases it will be necessary to let the mix cool before beginning compaction. Additional passes with a lighter roller may be required to remove roller marks at the surface; these are best done after the mix has cooled substantially. After final rolling, traffic should be restricted for the first 24 hours, as the pavement may be more tender during this time.
It is critical to protect the porous pavement during and after construction from sediment-laden water and construction debris that may clog it.
Protecting from sediment
Protecting the pavement from uncontrolled runoff from adjacent areas is critical. Temporary stormwater controls need to remain in place until the site is stabilized so that soil-laden stormwater does not wash onto the pavement, clogging the surface and filling the voids in the stone recharge bed. The porous pavement should be constructed late in the project schedule so that most of the dirty work such as grading and landscaping has already been completed.
With regulations often requiring a hard paving surface before structures are constructed, conventional dense-graded pavements can be constructed for driveways and some of the other pavement surfaces. In rare cases this may not be possible. One example of this is Pringle Creek, a residential subdivision in Salem, Oregon with porous pavement streets. In this case, the porous pavement needed to be constructed before the site work including utilities, sidewalks and landscaping was constructed. Here, the porous asphalt was constructed in two layers – three inches of asphalt-treated permeable base (ATPB) with 1.5 inches porous asphalt surface. The ATPB was placed in the late summer before the site work was complete, then covered with a geotextile fabric to protect it.
Truck running on geotextile
Photo courtesy of Jim Huddleston/Asphalt Pavement Association of Oregon
Once the site work was substantially complete the following spring, the geotextile was removed, the pavement surface was cleaned, and the porous asphalt surface was placed. Even under very difficult site conditions the contractor was able to maintain the cleanliness and porosity of the ATPB.
Removing the geotextile
Finished pavement at Pringle Creek
Using the area for sediment control
In some cases, the area where the porous pavement is to be constructed may be used for temporary sediment control during construction. In this scenario, the bed should be excavated at least one foot above the final elevation of the bed. In the later stages of the project, the sediment is removed, the bed is excavated to final grade, and the porous pavement system installed. This also avoids the need for a separate sediment basin during construction and limits the exposure of the porous pavement to clogging by construction debris.
Construct for success
Invariably, when an infiltration best management practice (BMP) fails it is due to difficulties and mistakes in the design and construction process. Carelessness in compacting the subgrade soils, poor control of erosion, and poor-quality materials are all potential causes of failure. For that reason, detailed specifications on site protection, soil protection and system installation are required.
A pre-construction meeting should be held to discuss the need to prevent heavy equipment from compacting soils, the need to prevent sediment-laden waters from washing on to the pavement, the need for clean stone, etc. Designers should review the installation process with the project foreman and routinely stop by the site to provide construction advice.
Successful installation of any infiltration BMP is a hands-on process that requires an active role for the designer. Often, the failure does not lie with the contractor or with poor soils, but instead is due to a lack of specific guidance for construction procedures.
Signs are often posted at porous pavement sites to alert grounds keeping and maintenance personnel to keep silt and debris from entering the site. These personnel should also be educated not to seal the pavement and not to use sand or other abrasives for snow or ice conditions. These signs can also include some information regarding the advantages of porous pavement.
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