The concrete was a state of Iowa quality management concrete (QM-C) mix design with fly ash added. Slump averaged 1.5 inches (38 mm). Trucks dumped the concrete directly on grade in front of the paver. The GHP-2800 was slipforming 16 feet (4.9 m) wide with two different pavement depths, 4.5 inches (114 mm) for the roadway and eight inches (203 mm) for the shoulder.
Manatts used four total stations for the 3D paving. Typically, one worker is in charge of moving them as part of the leap-frogging process. On this half-width paving project, two people were needed for leap frogging to keep ahead of the paver’s high-production rate.
“The total stations typically need to be less than 300 feet (91.4 m) away from the paver,” Tometich explained. “When we set up a stringless job, we’ll have the surveyor go out and set control points 500 feet (152.4 m) apart on each side of the road. Technically, they’re staggered so you’ll find a control point every 250 feet (76.2 m) on one side of the road or the other. We set up our total stations on those points. For the most part, we try to stay within that range to keep the accuracy that we’re trying to achieve.”
Paving accuracy is monitored by two paver-mounted GSI units on the back of the GHP-2800. The crew can monitor their paving results instantly on the GSI’s screen and make any necessary adjustments on-the-go. Iowa uses the zero-blanking band for their rideability specification. Profile indexes on roadways with speeds greater than 45 miles per hour (72.4 km/hr) require a measurement of 26.1 to 40 inches per mile (411 to 630 mm/km) for full pay. Anything under 26 inches per mile (410 mm/km) earns incentive pay.
“Our smoothness averaged 18 to 19 inches per mile (284 to 300 mm/km), with some days as low as 13 inches per mile (205 mm/km),” Tometich said. “The GSI is extremely helpful and I don’t know how I’d go back to not using it. It gives us up-to-date information allowing us to make adjustments on the fly and really helped our overall smoothness. We earned 94 percent of the smoothness incentive offered on the project because of it.”
Safety Wedge Required
A safety wedge was a unique requirement that had to be built into the project. Since live traffic was always running next to the first paved lane, IDOT wanted a safety wedge in place in case a vehicle should accidentally drop off or climb onto the new surface. The wedge eliminated the 4.5-inch (114 mm) drop-off between the existing lane and the new surface.
The requirement did present a challenge in how to accomplish it as part of the slipforming application. Manatts created a form box, attached it to the back of the paver, and used a skid loader to dump concrete from in front of the paver into the box. A vibrator placed inside the box vibrated the concrete before it was placed on top of a bond-breaking engineering fabric.
The safety wedge stayed in place until it was time to pave the second lane. Then, Manatts came through with a blade, scraped it off and loaded it into trucks to be hauled away.
Delivering a Cure
A T/C-600 texture/cure machine followed the paver. It applied a transverse tine to the 12-foot (3.7 m) driving lane and white spray cure. Wheels attached to the sensors on the T/C machine allow steering and grade to be referenced off the new slab during the texture/cure process on the stringless project.
The concrete curing rate was monitored by maturity method sensors placed into the edge of the new pavement. Those measurements determined when live traffic could be placed on the new lane.
“With a QM-C mix, we take time and temperature readings through a wire in the slab to determine maturity,” Tometich said. “From those measurements, we created a maturity curve that gave us the knowledge to know when the concrete was at the specified strength. Because we were paving in the fall with temperatures dropping fairly rapidly, we were looking at about 48 hours before we could have traffic on that lane. If we had paved it in the summer months, we could have had traffic on there in about 24 hours.”
Paving production averaged approximately 8,000 feet (2438 m) per day.
“For a project that’s never been done before in the state of Iowa, I think it went really well and this will be a viable option for other projects,” Tometich said. “This was the first half-width overlay we’ve done with stringless technology. It went well and we were able to achieve very, very good smoothness on the entire project.”