Iowa Overlay: Concrete on asphalt overlay upgrades farm-to-market road
There has been a great deal of whitetopping in Johnson County. Concrete overlays have become commonplace in Iowa as an economical means to rehabilitate the secondary road system.
But for this project, rather than beginning with the removal of existing concrete, the concrete overlay was placed over an asphalt roadway. It was also poured on the shoulders that were compacted and then built up to accommodate the new bike lines. “We tried to maintain about 6 inches on the old roadbed and 8 inches on the shoulder,” Kempf says. “Our ditches had to be filled, compacted and then brought up to grade.”
The rock laid after the shoulders were compacted includes crushed recycled concrete and modified sub base. Although this wasn’t a difficult process, at times it was challenging. “We’d have weeks where the dirt contractor could not work because it was too wet,” Kempf points out. “We had to deal with the weather. We had the shoulders built up and were ready to pour on a Saturday, but then we had 2 inches of rain the night before.”
Keeping it cool
The timing of the job also presented challenges.
The Oak Crest Road job was the first time Metro Pavers undertook a concrete overlay in summer. Previous overlays had been done in the fall. The summer presented challenges,
but by waiting until fall Metro ran the risk of the maturity curve taking longer. And as the day warmed up, the asphalt had to be kept cool. “We had to bring in a water truck to cool off the asphalt before we could pour on it,” Kempf explains. “We didn’t need to bring it in until between 10:30 a.m. or 11 a.m., but then we would need it all afternoon.”
According to the specs, the asphalt couldn’t be more than 90 degrees Fahrenheit because it would set on the overlay but not on the rock, Kempf points out. “We had to keep a consistent ground temperature,” he says. “We had to cool it down and stay just ahead of the paving machine. We had to have it cooled off, but it couldn’t be wet.”
Concrete overlays can be a good choice for the widening of an old pavement with narrow traffic lanes, the addition of new travel lanes – as was done with the addition of shoulders for bicyclists on Oak Crest Hill Road – or the extension of ramps, according to the University of Iowa National Concrete Pavement Technology Center’s (NCPTC) “Guide to Concrete Overlay Solutions.”
(For a downloadable PDF of the Innovationsguide, go to http://www.cptechcenter.org/publications/guide_concrete_overlays.pdf.) “Adequately designed and constructed
widening can improve both faulting and cracking performance
of the pavement,” according to the guide. “Widened slabs should be used with care with concrete overlays on stiff foundations (such as on concrete pavements) because of the increased risk of longitudinal cracking.”
NCPTC gives these rules of thumb for widening:
• Keep joints out of wheel paths, especially for bonded resurfacing.
• Tie longitudinal joints with #4 bars to prevent separation.
• Keep panels as square as possible (1.5:1 maximum)
• The width of widening rather than depth has more of a positive effect in reducing loads to the top of the existing
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