Rhode Island Department of Transportation has developed a paver-placed elastomeric surface treatment or PPEST as a preventive maintenance pavement treatment.
It provides a sealed, smooth, durable, long lasting, thin, flexible hot mix pavement that extends the road’s service life delaying the need for more costly reconstruction efforts,” says Charles St Martin, acting chief of public affairs for department.
The treatment is a polymer/rubber modified thin overlay placed at 1-inch thick. Produced in a conventional hot mix plant, it is a mixture of coarse-graded 3/8-inch crushed aggregate and a chemically modified crumb rubber asphalt binder.
The binder is PG 70-40 and contains a minimum 5% modified crumb rubber. The mix has a binder content of 6.0 to 7.5%.
This surface treatment however is gap graded, as opposed to the usual dense graded HMA. “This allows more room in the aggregate structure for the highly modified binder that we use,” St Martin says.
PPEST is recommended for use on roadways with moderate cracking of any form (fatigue or thermal), except those with reflective cracking. It can also be used on roadways with light rutting and cracks less than ¼-inch wide. However, if rutting is more than ¼-inch and the cracking is more severe, the rutted area should be leveled and the existing cracks should be sealed before PPEST is applied.
PPEST HMA is applied directly to a tack-coated surface, placed to a 1-inch compacted thickness. A tack coat is needed between the existing surface and the new layer to ensure adequate bonding.
As opposed to some surface treatment options that require special equipment, microsurfacing for example, PPEST can be installed using conventional paving equipment that you use to place hot mix asphalt; a hauling truck, a hot mix paver and a steel drum roller.
“RIDOT wanted to develop a preventive maintenance treatment that could be placed by conventional paving methods in urban localities, PPEST is that method,” St. Martin says.
Because it is thin and highly modified, it can be difficult to place by hand, St. Martin says.
“PPEST can be difficult to place by hand and rolling must take place immediately since it cools quickly,” he adds.
However, since PPEST has a thick asphalt film and highly modified binder, it is very resistant to cracking. St. Martin says PPEST does costs more per ton than conventional HMA, approximately $86,280 per lane mile, but since it is placed thin, the cost to pave a road is approximately the same. The mix is designed to last 10-12 years.
Since 1995, Rhode Island Department of Transportation and its Highway Assessment Committee have researched ways to extend the life of their roads. The committee incorporates members from various engineering divisions in RIDOT. The team focuses on five year old roads to determine the impact of design and construction practice on highway maintenance.
In 1998, the HAC turned their focus to researching a pavement preservation program. They started with researching cracksealing initiatives on their roadways and then began incorporating preventative maintenance surface treatments with experimental test sections in following years.
RIDOT began using PPEST in 2001, the first project was worth $1 million. In 2015, RIDOT paved eight roads using this surface treatment for a total of 6.1 miles, totaling $2.2 million.