...Moving Forward While Slipforming in Reverse
The state of North Carolina is investing millions of dollars into the Highway 17 Bypass by Wilmington. The project includes the construction of a new bridge over the Cape Fear River, which is an inlet to the Atlantic Ocean. The bridge work portion of the contract included slipformed parapet over steel reinforcing. Areas of the bypass are also being reconstructed and the project has a portion of variable barrier wall. At its highest point, the new wall is 83 inches (2108 mm) tall.
General Concrete Construction Inc., working as a subcontractor on the project, is in charge of slipforming the wall on the project and they’re using their Commander III four-track for all of it.
They were responsible for every aspect of the wall, including tying the intricate steel reinforcing. The variable height portion of the barrier wall only added to the difficulty of the job.
“We’re setting all the steel reinforcing on this project,” Jon Clifford, vice president of General Concrete, said. “It has to be perfect and we don’t trust anyone else to set it for a wall like this one.”
The barrier is a state of North Carolina T2 wall that is 29.5 inches (749 mm) wide at the bottom and eight inches (203 mm) across the top. The height varies from 46 inches (1168 mm) to 83 inches (2108 mm) at the tallest point.
Job-site conditions were less than ideal along certain areas of the project. The grade around the wall was rough and rutted, so much so, that General Concrete hand-poured a four foot (1.2 m) wide footing next to the wall for a trackline for their Commander III to run on.
Conditions also added other complications. Obstacles gave General Concrete only one side to work and run their Commander III. They’d be starting their day’s pour at the highest point of the variable barrier and the wall would have to be poured with the Commander III slipforming backwards. The trick is to make the Commander III still think it’s slipforming forward.
“We don’t run the machine in reverse,” Clifford said. “We turn the tracks, spin them around, and the machine thinks it’s going forward. We just trick it a little bit.
“I was nervous. When you’re pouring wall that high, all it takes is one thing to go weird and it’s not pretty. This wall, though, everything went off without a hitch.”
Their day began with a dry run over the steel reinforcing to make sure it was in place and wouldn’t cause any problems. It was also a test for the stringline to make sure it was accurate and followed the variable height of the barrier.
“Our stringline runs underneath the machine, five feet (1.5 m) from the edge of the wall,” Clifford explained. “We set it off the high side of the finished pavement, measure from the stringline 46 inches (1168 mm) on this wall, and that gives us our variable height changes on the low side of the pavement.”
General Concrete started slipforming the variable barrier at 9 a.m. on a Saturday morning. By 6:30 p.m. that night, they were pulling the Commander III off the finished wall.
“There was 180 feet (54.9 m) of variable height barrier in that section of wall,” Clifford said. “We started at 83 inches (2108 mm) and then it just consistently dropped 12 inches (305 mm) every 100 feet (30.5 m) down to 46 inches (1168 mm) high. The pour went well. All the rebar stayed where it was supposed to and everything went really smooth.”
The concrete was a North Carolina Department of Transportation required Double-A barrier mix design. Slump averaged one inch (25 mm).
Finishing work behind the Commander III is kept to a minimum. The new wall is straight-edged and broomed. Joints are cut in every 20 feet (6.1 m) with expansion joints every 80 feet (24.4 m).
On the Bridge
Portions of the Northern Cape Fear bridge parapet were also poured with the Commander III traveling backwards. The standard parapet had a profile of 32 inches (813 mm) high, 17 inches (432 mm) wide at the bottom and eight inches (203 mm) wide across the top. All of it was slipformed over steel reinforcing.
“On this big bridge, we were averaging 700 to 800 feet (213 to 244 m) of production per day,” Clifford said. “The machine leaves a good finish on the wall, so there’s not much finishing work that has to be done by hand. We usually hit it once, broom it, and put the joints in.”
Commander IIIs and GT-3600s
General Concrete’s work radius stretches into both North and South Carolina. They’re accomplished slipformers specializing in barrier wall, bridge parapet, sidewalk and curb and gutter. Their slipforming machines of choice are the GOMACO Commander III and the GT-3600.
“We use Commander IIIs for the wall and all of our five foot (1.5 m) wide sidewalk,” Clifford said. “The GT-3600 is really good for curb and gutter. It’s very maneuverable and pours a tighter radius. We use it to pour as tight as a seven foot (2.1 m) radius.”
Jon and his brother David, president of General Concrete Construction, agree that the features on the GT-3600 and Commander III make the machines a great fit for their company. The Hook-and-Go mold mount on the GT-3600 make switching from a 30 inch (762 mm) standard mold to a 30 inch (762 mm) valley curb mold faster and easier on the job site. The side-shifting and vertical lifting trimmerhead and mold allow it to be lifted up or shifted out of the way of job-site obstacles, like fire hydrants and inlets. “Smart” cylinders, with the G21 controller, take less time to dial in the legs for machine set up. And the new, more powerful trimmerhead allowed them to trim through five inches (127 mm) of stone subgrade on a project in Durham, North Carolina.
“GOMACO equipment is all we’ll ever own,” Clifford said. “The ‘smart’ cylinders and the G21 really help us out and we can train our tracks to go where we want them to. It’s nice to have all the side-shifting features on the equipment, too, with the kind of work that we do. For us, we’re just better off owning GOMACO equipment.”