Florida DOT explores preservation treatment

by Greg Udelhofen
Created: September 24, 2008

In an effort to extend the service life of its road structures, Florida?s DOT uses a bonded wearing course on I-10 project near Jacksonville.

The compaction effort should start immediately after the ultrathin bonded wearing material is laid.
The compaction effort should start immediately after the ultrathin bonded wearing material is laid.
Ultrathin bonded wearing courses are "gap-graded" mixes that include a high proportion of "single-sized" crushed aggregate bound together with mastic of fine aggregate, filler and asphalt binder.
Ultrathin bonded wearing courses are "gap-graded" mixes that include a high proportion of "single-sized" crushed aggregate bound together with mastic of fine aggregate, filler and asphalt binder.
During a NovaChip application, the process of compaction is to merely "seat" - not crush - the aggregate in the mix.
During a NovaChip application, the process of compaction is to merely "seat" - not crush - the aggregate in the mix.
The ultrathin bonded wearing course uses a specially built paving machine. Breneman uses the Midland Machinery NovaChip paver.
The ultrathin bonded wearing course uses a specially built paving machine. Breneman uses the Midland Machinery NovaChip paver.
The Interstate 10 project west of Jacksonville called for milling 22 lane miles, then replacing it with two 1.5-inch lifts of structural hot-mix asphalt, with a final ¾-inch bonded open-graded...
The Interstate 10 project west of Jacksonville called for milling 22 lane miles, then replacing it with two 1.5-inch lifts of structural hot-mix asphalt, with a final ¾-inch bonded open-graded wearing course.
During a NovaChip project, the mix is applied at a temperature between 290 to 330 degrees F. Because the mix is placed very thin, it cools rapidly.
During a NovaChip project, the mix is applied at a temperature between 290 to 330 degrees F. Because the mix is placed very thin, it cools rapidly.

This technique provides a superior bond to the existing pavement. All material is placed by one single motorized machine. The surface improves skid resistance and visibility in wet weather, funneling water through the matrix of stone on stone contact and keeping surface water to a minimal. It also lowers the noise caused by tires on an asphalt surface and protects the investment below the surface.

The components of the bonded wearing course:

Polymer modified emulsion membrane 0.13 to 0.3 gallons per square yard
Ultrathin high performance hot mix using: high-quality aggregate; gap-graded; only ½ to 1 inch thickness; mix designed specially for process
NovaChip spray paver, which combines a distributor and paver in one unit and applies both emulsion and hot mix in a single pass lay down

The NovaChip process was a French process that used a unique paving machine manufactured in Germany. Many countries within the European Economic Community (EEC) have been using the system since 1986.

The process was brought to the United States in 1993 and has had great success in many states. During the construction seasons of 1995 and 1996, Pennsylvania laid some 1,000,000 square yards of NovaChip on state, county and city streets.

Pennsylvania continues to place ultrathin bonded wearing surfaces today. There are many reports by respected department of transportation research centers that have documented the outstanding performance of the durable surface. E.J. Breneman L.P. constructed the largest single NovaChip project in North America in 2001. The project was located in Burke County North Carolina on Interstate 40.

Project design ? material

Ultrathin bonded wearing courses are ?gap-graded? mixes that include a high proportion of ?single-sized? crushed aggregate bound together with mastic of fine aggregate, filler and asphalt binder.

The normal course aggregate consists of 100 percent single face (85 percent two face) crushed material with a maximum Micro-Deval loss of less than 18 percent (or a maximum Los Angeles Abrasion of less than 35 percent and maximum sodium sulfate soundness loss of 12 percent).

The aggregate should be cubical with a maximum of 25 percent 3:1 flat and elongated particles. The fine material should also be angular with a minimum uncompacted void content of 40 percent. During the mix design process, the mixture should be checked for moisture sensitivity.

A membrane of a polymer modified asphalt emulsion is sprayed prior to the application of the hot mix asphalt layer to both seal the existing surface and to bond the ultrathin bonded hot mix asphalt layer to the existing old pavement.

The membrane is applied at approximately 0.2 +/- 0.07 gallons per square yard. The rate will change in accordance to the surface condition of the existing pavement. The liquid bonding emulsion should fill any small voids and provide enough emulsion so that it makes up about one-third of the thickness of the ultrathin bonded wearing course.

Binder for the ultrathin bonded wearing surface course is chosen based on the climate, traffic speed and loading conditions for an individual project.

A PG 70-28 grade is typically used on northern projects and a PG 76-22 is used on southern projects. Each project undergoes a strict design procedure and the PG grade of asphalt is then specified in accordance with that design.

Mix design and construction

The goal of the mix design is to provide sufficient asphalt binder to ensure adequate film thickness on the aggregate to provide for a durable hot mix asphalt layer.

The design is accomplished by compacting the hot mix asphalt mixture with a Superpave gyratory compactor. The specimen is compacted, after compaction the specimen is removed and the bulk specific gravity is determined. The desired air voids in the mix is about 10 percent with a film thickness of about 10 microns.

After the design binder content has been determined, the mix is re-tested for moisture susceptibility using the procedures of ASSHTO T-283. The specimens should be conditioned in accordance with the standard agency procedures.

The mix is also tested for draindown, with a desired draindown of less than .10 percent. The binder content of the mix design ranges from 5.2 percent to 5.8 percent.

The ultrathin bonded wearing course uses a specially built paving machine. Breneman uses the Midland Machinery NovaChip paver. There are two other types of NovaChip pavers available through Vogele and Roadtec.