What Does Porous Asphalt Do?

Parking lot paving with HMA Paving and Contracting

When designing a parking structure for your business, institution, or municipal facility, there are several factors to consider. Immediate cost is always a major consideration, but what about the cost of maintenance years down the line?

Another factor is the impact that the construction and existence of the structure will have on the local ecosystem. With the environment high atop the list of public concerns these days, we should all look to leave a smaller eco-footprint in all we do, including construction. Citizens of a municipality, as well as potential customers for a business, enjoy frequenting places that boast environmentally friendly facilities.

Finally, if the existing grade of an area is not right for positive drainage from the surface, how can it be turned into a useable parking surface? For these reasons and more, porous asphalt systems have become an increasingly popular option for new and re-constructed parking surfaces and roadways.

The immediate cost can be up to 40% more than conventional asphalt, but the eventual savings and increased business will almost always offset this cost. We will discuss what these systems are best suited for, installation, and maintenance.

What Makes Porous Asphalt Different?

Porous asphalt is not installed as a surface aggregate, like regular asphalt. It is installed as a system from the sub-grade, or natural undisturbed ground, up to the asphalt surface. The system works with the specific stone subbase and the local groundwater table, for a usual full depth of 2.5 feet.

For this reason, porous asphalt is usually not a financially reasonable option for small residential projects such as 2-3 car driveways. We are seeing this type of system used more and more in municipal parking facilities, school and university lots, and private businesses. Parking areas that are being constructed for the first time are ideal, so the porous system subbase can be installed originally.

This eliminates the need to excavate existing non-porous subbase in order to replace it with the proper stone. Excavation of existing subbase can be costly, but certainly easily achievable. Porous system parking lot owners save themselves the cost of installing and maintaining underground drainage structures like catch basins, and the rigid drainpipe that runs stormwater into them.

This can save tens to hundreds of thousands of dollars on future repairs, depending on the size of the lot. Another ideal situation for porous systems is where positive drainage off site or to existing drainage structures is not attainable due to grade issues. With Porous systems, the storm water permeates through the asphalt into the open stone subbase, and slowly seeps into the groundwater table or drains to a natural water retention area at a sustainable pace.

How Is Porous Asphalt Designed?

There are several different ways to design a porous asphalt system. Some have two layers of asphaltic concrete, a binder and a top course. Some only have one course with increased thickness. There are also different types of liquid asphalt, and different types of stone that’s used. HMA Paving and Contracting can provide you with these engineering solutions to ensure the right system is installed depending on customer needs.

To install porous asphalt systems, we begin as we would with any other parking surface installation: the stripping and excavation. The existing topsoil and virgin ground must be removed to the desired depth, usually around 2.5 feet. If it is not a new parking surface, then the existing asphalt and subbase must be removed down to the sub grade. The removed materials are hauled off site and recycled in most cases.

After stripping and excavation, the bottom layer of stone is installed on top of geotextile fabric. The bottom layer is usually #2 or #2 and 3 mixed stone. This is a large, washed stone with no fine material which allows water to permeate through it. There are very few fines (sand, stone dust, etc.) in a porous system since they block the flow of water, eliminating permeability. This layer of stone is installed at a thickness of 12-18 inches, and is installed in several layers, being graded and de-compacted in between each layer.

As opposed to standard asphalt, where a thick subbase needs to be compacted in layers, porous systems require installers to make sure equipment use and stockpiling has not resulted in compaction as the stone is put down. In some cases, if there is a retention pond or bio-retention area for the water to flow to, there will be flexible perforated underdrain pipe installed in this layer. It is all dependent on the users’ needs.

The next step is to install a second layer of stone. This is a much thinner layer, and is usually #57 washed stone. It is a smaller size aggregate, still with no fine materials. This is referred to in many cases as a “choker” layer of stone, since it slows the storm water’s flow to the sub grade. It is usually installed from four to eight inches, and is de-compacted and graded as well.

Finally, the porous asphalt is installed. Depending on field conditions and traffic use (heavy/light vehicles, busy, etc.) the asphalt could be two layers, a binder and top layer, or one thicker layer. The final thickness of the asphalt is usually somewhere between three and a half to seven inches. This layer will be installed by a paver just like regular asphalt, and rolled and compacted likewise. Although the asphalt is rolled, it will still move quite a bit until the water in the asphalt has evaporated. It will then harden up permanently.

Once a porous system is installed, careful maintenance is required. A sweeper or vacuum truck is necessary to remove any dirt, dust, debris, or sediment from the surface that will hinder permeability. This should be done 3-4 times a year.

Right before and right after the winter season are important times for lot cleaning, with one or two during the summer usually necessary as well. This can cost on average $500-$600 per cleaning. When compared to the cost of replacing catch basins and rigid underground storm pipe however, the savings are substantial. Also, no salt or sand should be used in winter for ice removal—porous asphalt must be plowed.

There are other benefits of porous asphalt systems as well. The asphalt is made at a much lower temperature, decreasing emissions and pollution at the plant. For this reason, there are many state and federal grants available to municipalities, institutions, and businesses that want to install an eco-friendly porous system for their parking structure. Check with your local political representative and ask about the availability of government funds for “green” construction projects.

Contact HMA Paving and Contracting Today

If you are interested in learning more about porous asphalt, HMA Paving & Contracting is here to help! We’re happy to answer any questions you may have about what it takes to maintain your parking lot or roadway over time. Fill out our form or call (518) 664-1014 to get in touch today!