Asphalt Installation & Application for Recreational Surfaces
Hot Mix Asphalt (HMA) pavements are a very important part of today’s recreational surfacing industry. Hot Mix Asphalt provides an affordable and durable all weather pavement that athletic surfaces are applied to throughout the world. The acrylic coatings used for tennis, basketball, volleyball and other sports were originally developed for application on asphalt.
No industry is static, the HMA and acrylic surfacing industries are no exception. Even though these two industries work side by side on many occasions, there can be a lack of understanding of some of the fundamental requirements of proper pavements for recreational surfaces.
The US Tennis Court and Track Builders Association, manufacturers of acrylic coatings (including California Products Corporation) and the National Asphalt Pavement Association recognize the need for well drained pavements with base of adequate thickness and stability for recreational surface to be successful. Moisture has a number of negative effects on asphalt pavements. It can cause frost heaves, distortion of the surface, cracking and deterioration of the mix.
By controlling moisture the pavement life is prolonged. Even when properly constructed asphalt pavements have a finite life expectancy. The mix is deteriorated by UV sunlight, moisture, traffic (or lack of) and temperature changes. It is generally accepted that well constructed asphalt pavements have a life expectancy of 15-20 years before they require significant reconstruction.
Most asphalt pavements are designed and installed with little thought about being used for a recreational use with an acrylic surface. Problems that might not be a problem at a parking lot or a road can be a long-term problem with a recreational pavement. Some of the problems that occur routinely are:
· Contamination of the aggregate filler – wood, clay, cigarettes. These materials tend to swell when they get wet. The result is a “pop out” at the surface. Some mix suppliers are known for dirty aggregate. They should be avoided for recreational surfaces. It is also important to review the individual State’s specification for contaminated aggregate and deleterious materials. Those specs that allow contaminated material should be questioned or modified.
· Ferrous Materials – Many aggregate sources contain iron. If stripping of the asphalt cement occurs, this will produce rust stains on the court surface. Additives such as anti-strip agents to fully encapsulate the iron in the mix can minimize this or once the stain is evident the surface can be touched up with Stain Blocker. Stain Blocker is not 100% effective, but is less expensive than options such as hot mix overlay.
· Petroleum Product Stains – Many pavers overuse diesel fuel to clean their trucks and equipment. These show up as light brown stains on the surface. Far more difficult to deal with is hydraulic oil that drips on the surface form poorly maintained equipment. This slow evaporating oil will soften the asphalt. These areas must be removed and patched. Close inspection of the asphalt equipment during installation can prevent both problems.
· Surface smoothness, Tolerance and Slope – There are the oldest of issues of this industry. Without an excellent base, well-established reference points and an attitude of attention to detail this problem will always be a conflict between the paving and surfacing contractor.
· Surface Cracking – There are some geographic regions where newer asphalt pavements, less than one year old exhibit 1-3 inch long surface cracks that are less than 1/8″ deep. This problem is a complex one that most owners and pavers attempt to simplify into a single statement; “It must be the fault of the coating”. Regardless of brand this is very seldom the case.
· Surface Blisters – This is a well known problem. It can be associated with poor surface preparation. In some cases they occur where there is a high moisture vapor pressure in the underlying asphalt pavement coupled with swelling and stripping of the mixes at the surface of the asphalt mixture.
Years of on-site testing have determined a number of common characteristics of the inadequate pavement. The characteristics that are normally seen:
· The hot mix pavement may be prone to stripping. This means the aggregate does not remain fully coated with Asphalt Cement due to moisture. When moisture penetrates to the top layer of the asphalt mix it acts to “strip” the asphalt cement away from the aggregate. The moisture is able to weaken the cohesive bond in the fine aggregate in the top 1/8″ of the HSMA. Uncoated pavements manifest the problem as raveling. Coated pavements manifest the problem as minor surface cracks because the surface is reinforced by the coating yet not strong enough to hold together the weakened pavements.
· Stripping occurs for a variety of reasons – these may occur individually or in combination:
· Type of aggregate
· High moisture content in and around the pavement
· High air void content (greater than 5%) in the in-place mix
· Late season installation when temperature is marginal. This leads to mix than may not be compacted properly because it is too cold
· Long distance trucking during installation may allow mix to cool well below the optimum temperature for compaction (normally 275°F)
· Thin overlays less than 1″ can cool rapidly, preventing proper compaction
· Inadequate compaction equipment may not allow the pavement to reach the specified density. Most mix designs assume at least a 5-ton roller for compaction. When a pavement is improperly compacted, the least compacted area is the top 1/8″ of the mix.
· Dust coated aggregate
Each contractor for paving and surfacing has their own skill levels. The best time to discuss construction procedures and quality expectations is before the project begins. Defining the demands on the finished product and tests that help insure compliance are very helpful.
A variety of ASTM tests will help to minimize the problems previously discussed. A few that are especially important are:
ASTM D-1559 – Marshall Stability Test: This is a method to verify mix design stability and optimum asphalt content. Since athletic surfaces are considered low traffic, the test sample should only be compacted with 35 blows of the hammer at 250°F.
ASTM 2950 – Nuclear Density Test: With a known Marshall density this field
test is a non-destructive method to measure relative density. In most cases the density should be 95% of the laboratory Marshall after correcting for moisture.
ASTM D3625 – Boiling Water Test: This test is an excellent method to test mixes and aggregates that are prone to stripping and determine the proper dosage of anti-strip agent if required.
Today’s competitive market place requires that each step of a construction process be examined to insure compliance with quality levels required for performance. The athletic surfacing industry has done a good job in identifying their problem areas. It is equally important for the asphalt industry to understand the unique demands of low traffic pavement used as recreational facilities.
Acknowledgement – The excellent practical experience of tennis court contractors throughout the world, Mr. Lansing Tuttle P.E. and Karl Mahonen, Senior Chemist at California Products made this article possible.
Resources: Hot Mix Asphalt Materials, Mixture Design and Construction
NAPA Research and Education Foundation
5100 Forbes Blvd.
Landham, MD 20706
W. Conshohochen, PA
USTC & TBA
Ellicott City, MD