By: Ed Longville
The Ohio DOT (ODOT) began its move toward full implementation of Superpave with the completion of a 50,000-ton project on U.S. 33 outside Athens, Ohio. The Shelly Co., Thornville, Ohio, accepted the change order to convert the project to a complete Superpave project. The level 1 mix design and binder selection were based entirely on the traffic volumes and temperature ranges suggested by the TP-4 and MP-1 Superpave criteria.
The original pavement was a 9-in. reinforced concrete roadway with a 2.5-in. asphalt overlay. Shelly completed joint repairs and placed a 2-in. scratch course as prep work. A 2-in. Superpave leveling course with 19-mm size aggregate was placed. This was followed by a 1.25-in. Superpave course topping with an aggregate size of 12.5-mm. A performance grade 64-28 (PG64-28) was selected and utilized on all lifts.
An Astec portable drum mixer was used to produce the mix at slightly elevated temperatures (305 deg F-315 deg F) to match the temperature viscosity curve of the binder. It was placed by two Blaw-Knox PF 200 pavers with Omni III Screeds. The compaction of the mix was executed by a roller train including a
three-wheel breakdown roller followed by a 10-ton vibratory compactor and an 8-ton finish roller. The rolling pattern was established quickly and remained consistent throughout the job.
Dave Powers, bituminous engineer for ODOT, was involved at all stages and reports the department was pleased with how the project went. Interestingly, although the project was located in a primarily gravel aggregate area the sole limestone source was used in order to meet the Superpave mix design properties.
Powers also stated, "The coarseness of the mixes would have been a concern over the pavement life had a polymer not been used. The polymer binder PG64-28 is designed to inhibit the type of deterioration typically seen in coarse mixes. ODOT has a concern overall with the coarse side of the gradings from Superpave when used in surface courses, if the binder is not adequate to maintain good bonds at cracks and joints."
Manufacturing Process:
The PG64-28 binder was manufactured after reviewing a variety of supply options for the binder. Shelly has a wholly owned subsidiary, Middleport Terminal, Inc. (MTI), which supplies a portion of the company's asphalt requirement from a 9 million gal storage facility located in Gallipolis, Ohio. After consultation with Shelly's lab Gerry Denney, president of MTI, elected to manufacture a custom- blend liquid asphalt at MTI's facility and modify the liquid by injecting SBR Latex at the plant.
There were several considerations which factored into the decision to manufacture the PG64-28 at the plant including price, plant tank capacity, product storage life and the ability to produce mixes for other customers throughout the state contract.
The base asphalt binder was blended with a predetermined amount of SBR Latex through an in-line blender, which was placed in the asphalt line just ahead of the drum mixer. The in-line blender consists of a series of static baffles, which combine the binder and SBR Latex.
A sample valve located just after the mixer was used to provide daily sample for independent testing. The formulation for the blend of SBR Latex and binder was completed by Shelly's AMRL certified binder lab under the direction of Larry Shively in cooperation with the latex supplier, Ultrapave, Dalton, Ga. Ultrapave provided verification of the blend percentages and technical assistance throughout the job.
Although Ohio has utilized SBR Latex in hot-mix for over 25 years, this was the first attempt to meet the new PG standards with the product. Increased
quality controls, precise formulations and new blending equipment were the keys to reaching the binder requirements.
At the asphalt plant, a magnetic flow meter was utilized with the process controls of the plant to match the SBR Latex flow rate to the flow rate of the asphalt cement. The latex was stored in a portable stainless-steel tanker and pumped with a simple cavity pump. When the plant was required to produce a mix
for local customers, the latex pump was shut off and plant production of regular mix resumed.
Daily samples of the binder were tested by independent laboratories for verification of the performance grade. All samples met the specified PG64-28.
Additionally, mix samples were taken on lot basis and tested for both the Superpave gyratory compactor and Marshall mix design criteria for air voids, VMA and density. The contractor received a 100% pay level for the project and finished the project ahead of the completion date.
ODOT in cooperation with Shelly and Flexible Pavements, Inc. -- the Ohio State Asphalt Pavement Assn. -- held a highly successful one day open house during the project. Discussions regarding the mix design process and performance graded binders were followed by a visit to both the hot-mix plant and the job site.
Ohio's first Superpave project was completed in a highly successful fashion. Shelly utilized a widely accepted SBR Latex modifier with new application methods to manufacture the PG required on the project, and the plant production and laydown operations went smoothly. With one winter past, the roadway is performing well and from all perspectives the long term success of this project
should be as positive as the initial success.
