Spring 2012 Vol. 20 No. 2
Creative concrete pavements
Greg Felt is walking proof that every improvement in paving doesn’t require massive amounts of research and/or big piles of dollars. At the 2012 TERRA Pavement Conference on February 9, Felt, who has extensive pavement design experience with MnDOT and Washington and Scott Counties, described two concrete paving projects. Both included inventive solutions that cost little—or actually reduced cost.
Customized gradation
The first project was three miles of new road on Scott County CSAH 21. Because of extensive clay in the soils, Felt used a 2-foot-thick granular sub-base with a customized Class 6 gradation: “We required 50 percent crushed material, and we reduced the amount of fines for better drainage.” He also specified 4-inch drain tiles under both shoulders.
Integrant curb-and-gutter
Extruded integrant curb being laid by the paver
Perhaps the project’s most innovative element is a 1-foot-high integrant curb-and-gutter (right) that, in some places, becomes a MnDOT B-448 curb. “The integrant curb-and-gutter made the difference between this being an asphalt project and a concrete project,” Felt said. “By making the 12 miles of curb-and-gutter integrant, we got it for $1.86 per foot in first costs.” Since the integrant curb-and-gutter effectively contains water within the roadway, Felt also designed an inward-sloping median with center drop inlets, which functions two ways. First, it helps to keep water off the roadway, which, Felt pointed out, is especially important on super-elevated sections. Second, by including a grassy median, it provides some degree of filtration.
The wearing course is a standard 8-inch-thick jointed, non-reinforced pavement. However, the design includes a 12-foot-wide, full-depth shoulder; this was required on the full length of the project because scheduled transit buses need to travel on the shoulders at times.
Phased intersection construction
Another challenge was that the road needed to carry 19,000 ADT every day during construction. In response, Felt specified that the road’s intersection with Scott County Road 18 be built in stages—seven of them as it turned out. Five of these stages required repositioning of temporary signals. “Building concrete in stages is trickier than doing it with asphalt,” Felt said, “and we actually considered doing the intersection in asphalt. But the turning volume between the two roads is almost 10,000 vehicles a day, and lot of that is heavy truck traffic because of the businesses that face the road. So we were concerned about the survivability of asphalt with all the rutting and shoving there would be. Ultimately, we decided to go with concrete.” A big part of the solution was careful planning by the contractor.
Unbonded overlay on an 85-year-old concrete pavement
The other project was 5.2 miles of unbonded concrete overlay on Scott County 66. The existing pavement was composed of an 18-foot-wide concrete slab, built in 1926, with 4 to 6 inches of asphalt overlay. “At the beginning of the design process,” Felt said, “we focused on cost. We considered doing it as an asphalt project, but if we had done that we definitely would have had to remove the original slab and regrade. That would have cost about $850,000 per mile. But we thought, if we did a concrete overlay, we could do the project for $500,000 per mile; actually, the bid came in at $576,000 per mile. So we thought it was worth it to try the concrete.” Felt said he sought out advice from MnDOT’s Concrete Office and from the Concrete Paving Association of Minnesota before initiating the design. He urged anyone considering concrete pavement at the city or county level to do the same. He said he received important advice from both sources.
Highly inconsistent surface after milling old overlay
Felt said the old concrete slab “was nothing like the original plans; the drawing showed a lip curb, but there was no lip curb.” He added that there was tremendous variability in the quality of the old concrete. “There were places where you could break it apart with your hand, and other places where it was as hard as the day they put it in. You have to expect that on an 80-year-old pavement. It shows the importance of a consistent mix design.” In addition to all that, Felt said there was no uniformity to the edge; the edge shown in the second photo (right) was one of the best sections.
Filling an irregular surface with millings
Because he had heard there might be some rebar sticking up out of the old concrete, he decided the milling machine should leave the last inch of asphalt overlay intact. Nonetheless, he said, “We were left with a highly inconsistent surface, so getting a consistent profile was going to be hard. At first, we tried laying the concrete on that surface, but we were getting terrible yields—sometimes 15 to 20 percent over the projection. Our thickness was sometimes 12 inches, sometimes 10 inches, and sometimes 51⁄2 inches.” His creative solution was to fill the surface irregularities with millings. The geotextile used as a bond breaker was then placed, and the overlay was done on top of that. “That layer of millings gave us a more consistent depth for the concrete overlay,” Felt said, “and we actually ended up below our projections on the concrete yield.”
At first the overlay was attempted in a single pass. But because the right-of-way was very narrow, that did not allow enough room to bypass traffic during construction. So they went to a two-pass overlay.
First pass over the fabric bond-breaker; note the bent rebars at the edge.
In the photo (right), note that the rebar at the edge of the first slab is bent. “They had a special shoe on the paver that placed those bent bars on the first pass,” Felt explained. “Then, after the concrete had cured, they came along with pipes and bent the bars out—and then laid the second pass.”
“One thing we learned,” Felt continued, “is that you can’t have tandem dump trucks backing up on top of the fabric. When the trucks drove forward, there wasn’t a problem. But when you back up, you tend to make small course corrections. When the dump trucks did that, the tandem wheels were pulling the fabric out of shape. That was a problem when we were trying to lay the concrete in a single pass. But when we went to the two-pass approach, it wasn’t a problem because the trucks never got on the fabric.”
Striping groove made by the paver
Simple shoe used to create a groove for edge and center striping
As shown in the photo (right), the paver was equipped with a simple shoe that created grooves for the edge and center striping. ”They got the right depth on that shoe just by putting weights from the high school weight set on it,” Felt said. “Then, depending on how the mix was coming out and how deep they wanted the groove, they could vary the weight on the shoe. It worked great! The striping company told us we will get 10 to 20 years out of the striping because of the grooves. The plows can’t hit them. The only problem I’ve noticed is that, with all the salt they’ve put down this year, that groove is now full of salt.”
Felt concluded by saying there was considerable traffic during construction, but that “the contractor did an excellent job of informing the people along the road, and their signage did the job. We had no problems at all with traffic. So, on narrow roads where you have no ditches to work with, I think this two-pass method should be considered.”
—Richard Kronick, LTAP freelancer
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