|THE BASICS OF ULTRA-THIN WHITETOPPING|
- Depending on required load-carrying capacity, overlays are 2 to 5 inches thick.
- Design is the same as conventional concrete mixes, though adding polypropylene fibers helps prevent shrinkage cracking.
- Pavement is placed in square panels of 3 to 8 feet. The National Concrete Pavement Technology Center at Iowa State University — which publishes Guide to Concrete Overlays in conjunction with the American Concrete Pavement Association (ACPA) — recommends limiting panel size to 1.5 times the overlay thickness.
- At least 3 inches of asphalt should remain after milling to maintain structural support. “You have to have the proper base, or else it's too dangerous,” says Jimie Wheeler, promotional director for ACPA's Illinois chapter. “Asphalt's a good product, but in many cases it's too thin.”
- Sometimes a full-depth concrete patch is needed to fix potholes and moderate to severe fatigue cracking.
- Concrete is placed using conventional fixed-form or slipform pavers. Within 30 minutes of placement apply the curing compound at twice the standard rate.
“One of our biggest roadblocks is conventional wisdom, and one reason it's been accepted in rural areas is that county engineers are much more autonomous than municipalities, so they're willing to try out new things,” says Jimie Wheeler, promotional director for the American Concrete Pavement Association's Illinois chapter.
Experience gave Schwab and his team more confidence in their decision than they may have had otherwise.
Although North Industrial Park is their first large-scale installation, they had data from a 2003 test project in which the first 4 inches of the 7- to 10-inch asphalt on a residential street were replaced. At the time, it was northern Illinois' largest installation.
“We're pleased with how the pavement has performed. We've had almost no maintenance in seven years,” says Schwab, whose team has monitored results each summer since then. The installation's shown almost no fatigue despite the fifth snowiest season on record and temperatures hovering in the low 20s for most of the 2010-2011 winter. “If the condition of Grace Street is any indication, the industrial park should hold up well through freeze-thaw cycles.”
According to Village Engineer Dave Dratnol, the technique is the equivalent of high early-strength concrete, which achieves compressive strength of 2,500 to 3,500 psi in 24 hours, with a 3- to 5-day cure in temperatures between the 40s and the 70s.
Schwab's previous experience with Grace Street also helped as he oversaw the milling of the industrial park's roads. He knew that curbs and gutters would first have to be raised 4 inches to ensure long-term performance. (The Illinois DOT recommends an asphalt subbase of at least 4 inches to ensure an overlay's long-term integrity.) Areas with significant failure received full-depth concrete replacement.
After elevating the curb face, crews ground ½ inch off the existing 3 to 5 inches of asphalt to create a rough surface to which the concrete could bond. Crews then cleaned the pavement to prepare for the concrete overlay. Aside from that, placement is no different from new concrete construction, Schwab explains.