Vegetation Control for Safety

by Airton Kohls; Source: Vegetation Control for Safety - FHWA

How do we stretch the budget? That is a difficult question to answer. At the August 11 meeting of the Tennessee Chaper of the American Public Works Association (TCAPWA) Upper East Tennessee Branch, Shawn Lindsey, Public Works Director from the City of Athens, presented a proven alternative. The following is a synopsis of that presentation.

Athens is using a specific slurry seal product to extend the life of asphalt pavement in selected areas freeing up monies to be used in more expensive paving projects. The slurry seal is a blend of polymers, aggregate and reinforced asphalt emulsion, being comparable to a micro surface. Micro surfacing can cost up to 75% of the cost of doing a thin overlay while this slurry seal product can be done in house by municipal crews at about 1/3 the cost of paving. The product is applied to moderately traveled asphalt roadways that are 10 to 20 years old and are in moderate condition. The city of Athens has completed its application on several low volume streets between 11 years of age and over 30 years of age.

The advantages are:

• Extends the service life of pavement.
• Replaces the pavement binder that is lost through oxidation.
• Provides a rich black color that helps to melt snow and ice.
• Protects pavement from oxidation, UV rays, and moisture intrusion.

Some of the FHWA general recommendations for identifying candidates for slurry seal are:

• Verify the absence of any alligator cracking, indicative of subgrade failure or inadequate subsurface drainage.
• Verify that surface cracks wider than 1/8 in. (3 mm) are sealed.
• Verify that any surface rutting is 1/4 in. (6 mm) or less in depth.
• Verify that the bitumen application rate is reduced slightly for areas that are flushed or bleeding relative to areas that are not flushed or bleeding
• Verify that the type of slurry seal (Type I, II, or III) proposed is consistent with the severity of raveling present and the estimated traffic.

These and other recommendations may be found at http://www.fhwa.dot.gov/pavement/preservation/ppcl13.cfm.

Steps in preparing a road for the specific slurry seal product:

1. Identify the road at least a year in advance. Look for roads that show raveling, but do not exhibit severe alligator cracking or have base failures. Correct minor problems by excavating and patching and consider infrared patching to give a seamless repair.
2. Crack seal the areas. Again if the majority of the road is affected by alligator cracking with lots of base failures, the pavement is too far gone for this treatment.
3. Pick roads that are not heavily shaded. With sun exposure the product cures in 45 minutes to a few hours. A few shady spots are okay, but will need lengthen curing perhaps to overnight.
4. Don't pick a road that could be damaged by metal snow plows. Rubber snow plows are not a problem.

Steps to application:

1. Pick a sunny day with temperatures above 50 degrees, preferably about 70 degrees. Also avoid Fall days when leaves are falling, as they should be kept off the road.
2. Distribute fliers to give residents notice of the impending work and request that they avoid driving on closed roads.
3. Remove all dust, weeds, debris and oily spots.
4. Apply a light cold applied tack coat to the road. At a minimum, tack hills, intersections, oily areas, and around catch basins.
5. Apply the product in at least two thin layers. Intersections may need a third coat since turning movements cause more wear. The product may be applied with a spray/squeegee machine or hand squeegees. It is important that it is not applied too thickly or it will crack.
6. After the first coat dries, apply the second coat. This takes much less product and time. The first coat fills small voids and the second coat adds more thickness.
7. Remove loose grit not needed to fill the voids. As a best practice, the Street Maintenance crew will try to sweep the road after waiting a week for the product to cure and after traffic is allowed.
8. Use proper traffic control during application. Plan for each road and communicate with the residents.

The product will dry in about one to two hours on a sunny day. Athens tries to leave the roadway closed from 8:00 a.m. after most leave for work until about 3:00 p.m. when school is letting out. The crew performs a "door to door" reminder before starting the process to give residents the opportunity to move vehicles. It is best to avoid traffic on the uncured surface. Crews can put down up to 2,000 gallons between 8:00 a.m. and 1:00 p.m. even allowing for several breaks. The city uses six to eight workers when the street has a curb and gutter cross section as it requires more handwork. A crew of five employees can easily handle a non-curbed street. When working a curbed street, Athens combines the street maintenance crew and street construction crew to expand the work force. The Street Maintenance Crew handles noncurbed streets with minimal help.

It cost approximately $50,000 to purchase a spray/squeegee machine capable or spreading and keeping the slurry of aggregate and asphalt properly mixed. This amount can be saved in the first year of paving avoidance. A small 250 gallon tack tank is necessary to apply cold tacks. To speed up production, crews prefer that the aggregate be already mixed. Adding aggregate causes the crew to spend more time mixing and loading the tank than applying product. If the aggregate is already in the tank, the crew spends 5 to 10 minutes loading a tank and has more time for application.

Athens' current plan is to apply the product two to threee days per week in the hottest and driest months of August and September. May-July are possibilities, but crews that apply the product also do all of the other street, sidewalk, and storm water work. It helps to set aside months for various parts of the operation to concentrate on. Mr. Lindsey estimates that most municipal operations that have at least six to eight people in their street departments can work with this new product. Others may consider contracting it out.

For more information, contact Shawn Lindsey, City of Athens Public Works Director, publicworks@cityofathenstn.com.

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