August, 2006

Hot Mix Asphalt Maintenance Guide (HMA)


It is well known that the success of a sport surface is highly dependent on a well designed and constructed hot mix asphalt surface (HMA). Either by choice or contract restriction, the Plexipave applicator may not have significant control over the manufacture, design or installation of the HMA.

Although HMA is a very suitable and desirable base for the Plexipave System, it is important to recognize that the main use for HMA is for high traffic roads. All the research done by suppliers, equipment manufacturers and testing companies is focused on meeting the challenges of heavy loads imposed by car and truck traffic.

Asphalt tennis and basketball courts and running tracks are not required to carry heavy loads. They are essentially a zero traffic pavement. This puts less load stress of the pavement during its life but does not prevent it from being effected by environmental factors and the need for excellent construction procedures.

It is agreed by most designers of asphalt sport surface that the key elements for long-term performance of the HMA are:

1. Drainage – The sub grade and base material must be of adequate thickness, strength, density and slope to provide a well drained foundation for the HMA. Perimeter drainage must also be accounted for.

2. The asphalt mix should be of adequate thickness to withstand the temperature changes of the area.

3. The mix must be designed and placed to have low air voids content (less than 5% in place).

4. The finish surface should have excellent planarity.

The HMA installer has challenges to meet these design and performance needs.

1. The standard mixes available for installation are set up for highway use. 
2. The size of the recreational pavement normally requires the use of smaller equipment. 
3. Many recreation sites are in residential or remote areas that increase the time the mix must be transported. 
4. The site is normally confined by a fence that can effect compaction/rolling patterns.

The HMA contractor can achieve success if some critical elements are addressed during installation.

1. The hot mix must arrive at the site at a temperature hot enough to allow for proper compaction after lay down. Most paving experts agree the mix temperature should be 300°F when delivered to the site. An inexpensive temperature gun can verify the temperature.

2. It is best to use a lay down machine with a vibrating screed. It will help to establish initial compaction of the HMA. Larger pavers provide good preliminary compaction of the pavement.

3. Adequate and timely supply of mix at the job site to allow continuous installation at the correct temperature.

4. Thin lifts of HMA cool quickly after it is placed on the surface. The rate at which the HMA cools is affected by the temperature of the surface it is being placed on, the thickness of the mix, wind speed and ambient air temperature. These factors must be taken into account when the paving takes place. The compaction of the hot mix must take place when the mix is above 240°F. When the HMA is cooler than 240°F it is difficult to achieve proper results. Below 170°F, the HMA may be rolled to achieve smoothness. This rolling has minimal effect on the ultimate compaction, since large rollers are generally not practical on recreational pavements. The paving contractor must have a vibratory roller capable of achieving the compaction required.

5. If the ability of the paver, his equipment or the operator to achieve the proper density (compaction) is a concern, then on-site testing is an option. Nuclear density gauges such as the Troxler 4640-B, provide excellent information for the contractor and owner. They compare the theoretical density to the actual density. Troxler can be reached at www.troxlerlabs.com.

If core samples are desired, a method to obtain density should be through an instrument similar to the Instrotek Corelock in accordance with ASTM Standard D6752-2. Instrotek can be reached at www.InstroTek.com.

As with any test procedure, the testing company should be familiar with the equipment and the requirement of the site. It must be routinely noted to all parties that recreational pavements are zero traffic pavements. The need for density and low air voids is critical for long-term performance.

California Products is constantly working experts in the Asphalt industry. No industry is static. Equipment and materials do change over time. It is important for each interested party to stay informed about those changes. Asking questions of hot mix asphalt suppliers, knowledgeable pavers, consultants and test equipment suppliers is the best way to stay abreast of the latest developments. These resources can provide unique viewpoints to problems. It is important to evaluate the information from each source since each firm has a “point of view” that may not be fully aware of the needs of a recreational pavement. Carefully and thoughtful evaluation will provide a variety of solutions. Ultimately the goal is to do it right the first time with the best techniques and materials that are available.

Test Equipment:

• Troxler Electronic Laboratories, Inc. – www.troxlerlabs.com
Job site density measurement tools

• Instrotek, Inc. – www.InstroTek.com Laboratory density equipment

Industry Associations:
• National Asphalt Paving Association (NAPA) – www.hotmix.org 
Paving installers and suppliers association

• Asphalt Institute – www.asphaltinstitute.org
Asphalt and bitumen producers’ organization

Consultants:

• Benchmark, Inc. –mgroh@benchmark-inc.com