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Concrete Overlays
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Stringless Slipform Technology for Concrete Overlays
 Known
for its innovation in concrete pavements, Iowa took another leap
forward with its recent completion of two stringless slipform concrete
overlay projects. Stringless technology as the name implies replaces
the traditional stringline with an electronic tracking process that
controls the horizontal and vertical operation of the slipform paver.
The stringless process eliminates the installation and maintenance
of stringlines and can decrease surveying and increase smoothness.
Early in 2009, a 9.5 - mile long two-lane concrete overlay project
was completed on route V18 in Poweshiek County near Brooklyn, Iowa.
Manatts, Inc., was the contractor and completed the project using
a GOMACO slipform paver. Reports from the project indicated a high
degree of success and a potentially bright future for stringless
technology.
Building on the success of the Poweshiek County project, a second
project is currently under construction on US65 in Worth County
near Mason City, Iowa. It consists of a 10.5 mile, 32-ft wide, 5-in
thick unbonded overlay. Flynn Company, Inc., is contractor on the
project using a Guntert & Zimmerman slipform paver. Similar
to the Poweshiek County project the existing was initially milled
prior to concrete placement. This project also included an additional
4-ft widening in each lane and the pavement was placed in half-widths.
Within 24 hrs after placement joints are cut in approximately 4.5
– ft. squares using early entry sawcut technology.
 Stringless
technology holds special potential for increasing the application
of concrete overlays. Along with advancements in streamlined profiling
of the existing pavement, stringless paving could significantly
decrease the amount of survey that is required prior to placing
a concrete overlay. This would then lead to further decreasing the
appeal of asphalt overlays. Iowa, and other states, have seen a
dramatic increase recently in demand for concrete overlays due to
spiking asphalt prices.
In Worth County, Iowa, 24 miles of 4-in. thick, 22-ft wide concrete
overlay was selected over a 2.5 - in. asphalt overlay with 4 - in.
cold-in-place recycling through an alternate bidding process. Initial
cost differences were only 9% higher for concrete in spite of the
design inequity. Recognizing concrete’s expected longer life
and lower maintenance costs, the County Engineer and County Board
selected the concrete alternative. Cedar Valley Corp, was the successful
low bidder and completed the project earlier this summer.
On September 2, 2009, an open house was held in Worth County, Iowa,
to learn about stringless technology and concrete overlays. More
than 100 attendees representing state DOTs, FHWA, county engineers,
contractors, consulting engineers and equipment and material suppliers
attended the open house. The program consisted of several presentations
in the morning followed by a site tour of the two Worth County projects
in the afternoon. There was a lot of discussion and photo taking
throughout the day. The Worth County Secondary Roads Department
has not placed a concrete pavement in decades; however, they are
reporting great satisfaction with having chosen concrete for their
current overlay needs. Based on the reaction of county officials
and the attendees to the open house, the future of concrete overlays
and stringless technology looks very promising.
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| Stapling tiebar into existing asphalt pavement. |
Bull floating the concrete surface |
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| Finishing the concrete |
Applying curing compound to finished concrete |
 The
National Concrete Pavement Technology Center offers a Guide
to Concrete Overlays which provides the key elements of
the six major types of concrete overlays along with specifics on
materials, typical sections, and important construction elements.
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