'Ultrathins' becoming popluar

Washington Aggregates and Concrete Association

Whitetopping, the placement of a concrete overlay on top of deteriorated asphalt pavement, has been an acceptable pavement restoration practice since 1977. Now there's ultrathin concrete whitetopping.

Much of whitetopping technology has been developed in Iowa, where more than 300 miles of asphalt pavement has been successfully rehabilitated since 1960. As this experience, pavement records and life cycle analysis have demonstrated, concrete whitetopping is the simplest concrete overlay technique available and actually provides some advantages over new construction and virgin subgrades. It has been so successful, that Iowa annually whitetops an average of 19 miles of county and state routes.

Concrete whitetopping technology is based on the outstanding historical performance of concrete pavement depths of 6 to 7 inches for typical new construction. It was reasoned that pavements could be built in 5-to 6-inch depths over the enhanced subgrade capacities of deteriorated asphalt pavements.

With the advent of new fast track concrete pavement technologies that allow the opening of a concrete pavement within 18 hours or less of initial paving, whitetop technology is advancing once again.

Ultrathin concrete whitetopping (UTW) is the next generation of concrete pavements. It involves the construction of a concrete pavement depth of 90 mm (three and a half inches) or less. With whitetopping pavements requiring little or no maintenance after 25 to 35 years, many state transportation departments and municipal engineers are re-examining the concept of a composite pavement of asphalt and concrete.

The advantages are simple to realize: the economics of a flexible subbase with the durability of a concrete wearing surface. The reduction of traditional concrete pavement depths results in a substantial reduction in concrete pavement economics.

The potential for ultrathin concrete pavements was discovered as a result of an innovative 1991 demonstration project in Louisville, Ky. The project involved reconstruction of a pavement at a waste disposal facility that carried 400 to 600 heavy trucks six days per week. Under accelerated test conditions, the project team had the opportunity to monitor the performance of a concrete pavement in depths of three and a half and two inches with the added advantage of a weigh master to weigh all trucks entering and leaving the site.

The concrete mix design used was similar to a three-day WSDOT mix design with the addition of high-range water reducers, polypropylene fibers and a maximum water cement ratio of 0.33. The concrete was slipformed and produced 27.6 MPa (4,000 psi) in 18 hours.

Traffic was applied to the roadway about 37 hours after paving was completed. The result was a thin concrete pavement section no taller than a credit card that carried over 200,000 ESALS over a one-year period (typical low-volume and residential roadways are designed to carry 50,000 to 100,000 ESALS over a period of 20 years).

The results of the Louisville project led to a new path of thinking regarding longitudinal and transverse panel and joint spacings and the ability of concrete to bond successfully to asphalt and form a composite pavement.

Since this experiment, the transportation departments of Kentucky, Tennessee, Georgia, New Jersey and the Carolinas have conducted additional tests at DOT weigh scales, exit ramps and intersections to determine new uses for this technology.

The process of a UTW pavement begins with the determination of pavement surface condition. Typical applications for ultrathin whitetopping include intersections and heavily traveled and channeled traffic roadways that experience rutting, shoving, traffic loop repairs and other surface deteriorations. Existing pavement and subgrade thickness is determined by coring.

The roadway is milled to a uniform depth throughout the surface layer of asphalt. Care is taken not to mill through to the subbase. The milled area is power broomed and cleaned and the initial and final milled areas are cut to a vertical plane. The edges of the milled areas serve as forms and significantly reduce the cost of forming.

Fast track concrete is then placed by slipform or hand to the finished grade of the existing roadway. Concrete placement is typical of traditional pavement methods with some key distinctions.

First, the pavement will reach initial set much sooner than a standard 14-day WSDOT mix. The finishing operations must be performed immediately behind the paving operations. Bull floating, tinning, curing and insulating should be completed within 90 minutes. "Soft cut" sawing should be timed to cut longitudinal and transverse joints while the concrete is still "green," and should be completed before initial stress of hydration begins and as soon as the joints can be placed without raveling.

If necessary, insulating blankets can be placed on the concrete pavement to maximize temperature and strength gain during the initial 12-to 24-hour hydration period.

The most recent state to evaluate this new trend in concrete paving is Kansas. The city of Leawood, Kan., recently completed a concrete resurfacing of 119th Street, which was originally constructed in 1987. Currently handling 22,000 vehicles per day and estimated to grow to 35,000 in 2014, the 9-inch asphalt pavement was fitted with a new 2-inch concrete wearing surface and was designed to be opened to traffic upon reaching 3,000 psi.

The Leawood project was funded by Section 6005 of the Intermodal Surface Transportation Efficiency Act (ISTEA), which established authority for construction of these bonded overlays with the intent to determine feasibility, costs, benefits and upgrading of highway bridge decks, pavements, with overlays or surface laminations.

The project will be monitored by the Kansas Department of Transportation for a period of five years using FWD measurements, condition surveys and tests for smoothness and bonding strength. Design features of the project include 3-foot panels in the eastbound lanes and 4-foot panels in the westbound lanes. The concrete pavement was un-reinforced and used fibrillated polypropylene fibers.

City officials throughout the country are increasingly confronted with insufficient funding to properly maintain their roadways. New technologies such as UTW paving are gaining popularity by minimizing regular and routine maintenance in heavily traveled roadways that experience rutting, shoving and traffic loop repairs. Concrete advantages are realized in: additional durability and service life over other wearing surfaces, additional reflectiveness at intersections, increased skid resistance, and less maintenance and inconvenience to the motoring public.

With too few dollars to repair so many miles of asphalt pavements, the time to review conventional wisdom may be upon us. New technologies that marry the best attributes of both pavement materials available are emerging. Ultra thin whitetopping composite concrete pavements may just be the "flexible solution" for many communities throughout Washington.

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