It seems to be a trend, especially in Florida and warmer States, where freeze/thaw cycles are rare or simply do not occur, that some designers feel one layer of asphalt is adequate for new running tracks. It may stem from the fact that many parking lots or minor roadways are designed this way and running tracks have long been considered their equal or less. Or it may simply be that designs have not kept up with the new trend of running tracks and their increasingly popular, sophisticated synthetic surfaces. Regardless, if you desire a running track facility that optimizes safety, performance and longevity, two layers of asphalt should always be specified and with the requirement that laser controls are utilized as part of the process.
First, let me refer you to an article written by Mary Ann Sprecher titled “A Tennis Court Is Not A Parking Lot And A Running Track Is Not A Roadway” that was published in AthleticBusiness.com. This article gives great insight into the differences of typical asphalt paving versus sports specific asphalt paving but in addition, here are some other important issues to consider:
The goal of any new running track facility should be, optimal safety, perform as designed and last as long as possible. For this to occur the asphalt must be installed as soundly and as precisely as possible. Theoretically, each new layer of asphalt improves the previous layer by approximately 70%. Regardless of how precise the stone base installation and fine grading of the stone base, there will always be deviations and imperfections of ½ inch or greater relating to perfect planarity and smoothness. In addition, the asphalt delivery traffic and the movement of the paver, etc. will also add to these imperfections. As a result, due to complications with installation, compaction, etc., one layer of asphalt has essentially no chance to achieve the 1/8”-1/4” surface smoothness and planarity requirement of most running track specifications today relating to the finished asphalt surface. This notion is clearly supported by the fact that all of these specifications are graduated relating to surface tolerances. More specifically, all running track smoothness specifications are written with tightening tolerances similar to the following example including allowable deviations: stone base approximately 1/2”, base asphalt approximately 5/16” and surface asphalt approximately 3/16”. What this should tell you is, that designers do not expect the paving contractor to achieve the same level of precision with the first layer of asphalt as with the second.
Why is this so important? In the days where an asphalt surface, with no synthetic surface was the standard, not so important. However, when you are spending a hundred or hundreds of thousands of dollars on a synthetic surface that you hope will be safe, perform optimally and will last 20 plus years, uniformity of the synthetic surface on a properly draining and constructed asphalt pavement is essential! A second layer of asphalt should result in greater precision of the asphalt surface and the greater the precision of your asphalt surface, the more uniform your synthetic surface should be. Uniformity in your synthetic surface results in a more consistent surface thickness with fewer thin and thick locales, thus the potential for optimal safety, performance and longevity.
In closing, it is the core challenge of all designers to create maximum value for a clients hard fought for funding. Relating to new running track construction, this must include a second layer of asphalt!