Every municipality, county or state is facing the same pressures: do more with fewer tax dollars. So the key word among today’s materials specifiers is value....value that delivers more performance for every dollar spent. In road maintenance and repair, everyone recognizes the long life and superior performance characteristics of concrete paving. But budget constraints often have stopped specifiers from choosing concrete. Until now!

With the development of "whitetopping" and "ultra-thin whitetopping", these specifiers have been given the opportunity to choose a no-compromise material that delivers the looks and performance characteristics of concrete…at a competitive price.

Actually, whitetopping overlays of 4 inches or more, placed over asphalt for rehabilitation, have been constructed over the past 20 years in all pavement areas: highways, streets, airports, and parking areas. Some states claim as many as 300 miles of whitetopped county roads. And the process has performed exceptionally well.

Now, the new ultra-thin whitetopping (UTW) process is the new weapon. Essentially, the UTW is applicable for use where the deteriorated asphalt surface has sufficient structure, but poor or rutted surface. UTW gives the same advantages the regular whitetopping process has provided over the years, with the additional advantage of thinner sections for fewer physical limitations and greater overall applicability. The key to its success is the strong bond to the asphalt surface. With this bond to allow the concrete/asphalt section to act in unison, the compressive strength advantages of concrete combine with the tensile strength of the underlying asphalt to work like a concrete pavement. The resulting composite pavement delivers the long life and superior performance characteristics at a cost competitive with ordinary asphalt overlays.

Although there are many variables that should be considered on any UTW job, the general rule of thumb for thickness is this: The UTW should be 2 to 4 inches thick depending on traffic requirements. The asphalt pavement that the UTW bonds to should be at least 3 inches thick to provide sufficient base for the UTW.

The concrete mix selected for a particular project is matched to the traffic conditions and opened-for-traffic requirements. A normal mix design included cementitious materials (cement and fly ash) coarse aggregate, fine aggregate, air entraining agent, admixtures (water reducers and/or plasticizers), synthetic fibers (as specified), and a low water cement ratio. While NDDOT research has shown a fiber rich mix reaches strengths at a earlier time than regular mix, a Fast-Track Paving UTW project may consider using a high early strength cement, also. Goals for opening projects to traffic after less than 24 hours are certainly attainable with the right selection of materials and techniques.

A UTW project was sponsored by the NDDOT using 25 lbs. of 3M polypropylene fibers (2") per cubic yard at the Steele overpass on I-94. This level of fiber makes the mix a Fiber Reinforced Concrete (FRC) type. Construction of the first lane of the 3" to 4" UTW went off without a hitch. Finishing was unaffected by the fibers, however, forks replaced shovels, for the most part, when moving the mix around on the base. Joints, spaced from 9 to 25 feet, was soft-cut about 6 hours after placement. The longer joint spacing was used with the intent to push normally accepted joint spacing limits for UTW.

Other FRC projects have recently been constructed in neighboring states. The South Dakota Department of Transportation has recently become active with fiber reinforced concrete projects, installing bridge decks and barrier rails, concrete pavement and ultra-thin whitetopping. While previous UTW projects in Minnesota and Iowa reflected a short joint spacing, one of the goals of the South Dakota FRC whitetopping projects was to show how the joint spacing could be increased. On an unjointed section of 2.5 to 3.5 inch FRC UTW on their 1996 US Hwy. 14 project West of Pierre, the cracks were observed to average 54 feet on-center, which was similar to the 50 foot cut joints on other sections. Fewer joints mean less sawing and potential reduced future maintenance.

The South Dakota researchers found that none of the cracks had reflected from the asphalt, indicating no particular attention necessary for crack location or for filling prior to concrete placement.

General current recommendations for UTW is to keep joint spacing to a minimum. Research in other locations of the country, including the MINNROAD research facility in Minnesota and the Turner-Fairbanks facility out east, show that non-fiber reinforced mix at short joint spacing make extremely durable pavements

If you wish to try a design using UTW, check out the ACPA website, where software is available on-line to provide such. The software is set-up to impute the thickness information of the concrete and asphalt, and projects the ESALs the segment will be able to handle. The ACPA web-site is http:\\www.pavement.com.

Whitetopping technology will surely see further refinement and expanded use. Whitetopping is expected to save money, increase crack resistance, toughness, fatigue resistance, load capacity, expected pavement life, and resistance to rutting. Safety considerations on rutted bituminous roadways will likely become one of the prime considerations for all governments. Loss of sovereign immunity and general insurance protection conditions will trend toward provision of "best available surface" for skid resistance and hydro planing protection. Concrete will be there, ready to provide solutions to these persistent problems.