By Matt Roberts
While the name Walden Pond, likely brings back thoughts of high school and reading the famous book by author Henry D. Thoreau, there is another notable historical feature of this park in Concord, Massachusetts that pertains directly to the asphalt industry.
Back in 1972, a group known as the Franklin Institute Research Laboratory did a study on the feasibility of permeable pavement. Permeable pavement is basically a technique where the asphalt is left more porous than a typical pavement. This is achieved by removing the smaller pieces of aggregate rock from the hot mix, thus leaving only large portions of aggregate rock and keeping the final mix more porous than conventional paving methods. In addition, the base course of this pavement system consists of crushed stone or gravel that must be able to not only withstand the load from the roadway and the vehicles but also must allow for water to permeate through this sub-base in a reasonable amount of time.
This technique, in theory, would allow rainwater and other surface liquids to permeate through the asphalt and soak back into the road subgrade. The benefits of such a practice are numerous. First, as a safety benefit, you are reducing or eliminating pooling and puddling of water on the road surface. This makes the roadway much safer for everyone who travels on it, preventing hydroplaning and ice buildup.
In addition to the safety benefit, it is believed that integrating a porous pavement would make less of an impact on the environment and leave the land more like it was prior to the road construction. Water can seep back into the earth, replenishing the water table and preventing excessive erosion from flash flood or extreme water runoff. Furthermore, road debris and other contaminants are not carried into streams and rivers. There are some drawbacks to this form of pavement. When used on high traffic or high-speed roadways, this form of pavement can cause excessive road noise.
Based on these ideas and factors, a group of engineers and the EPA got together in 1977 to work on an idea and perform a demonstration project, to combat problems of overbuilding of land. This project was proposed for the parking surface areas of the famous Walden Pond.
Several different specifications of porous asphalt were considered and four different mix designs were approved for use in this project. The base liquid asphalt binder of the mix always remained the same, essentially just the aggregate was varied between the four designs. The contractors used varying thicknesses of the different mixes to also determine whether the thickness of the pavement made an impact on its permeability. Finally, one area of the parking lot was left unpaved, which allowed the researchers to have this as their control group of the study.
After the project was completed, many tests were run and data was sampled at the area. Air temperature, soil temperature, precipitation, surface runoff and pavement infiltration by water were just some of the data tested by the researchers.
One year later, pieces of the parking lot were removed and different areas of the parking lot were tested. It was determined that several of the sampled areas did not meet the original specification and thus failed the permeability requirements. There was much discussion and argument over what factors caused the permeability failure. Eventually, failing areas of the parking lot were removed. New base was applied and the failing areas were repaved in August of 1978. The researchers decided that too much compaction could cause problems and decrease spacing between aggregate, thus impacting the permeability of the asphalt. This led the team to delay rolling the paved surface until it had cooled to about 180 degrees Fahrenheit.
The project was deemed a success and to this day has provided many benefits. The local water table has increased since the time of the project and there has been a noticeable reduction in erosion, pollution and the need for winter road salt. Many other porous pavement projects have been applied over the years and there is even a high likelihood that you have driven on or parked on one of these projects.
Roberts is vice president of sales at Asphalt & Fuel Supply in Oklahoma.