All of the paving projects Tim Tometich, a project manager/estimator for Manatts Inc., has worked on have utilized 3D in place of strings.
“I have actually not paved or managed a project with stringline, and I do not think I’d want to,” Tometich says. “There are so many advantages to using a 3D system -- for example, the labor you save on setting the stringline and then you don’t have that stringline in the way. A lot of times you can save on concrete and actually provide a better product; you can grade and trim the subbase more accurately; and you’re able to create a better riding road."
Changes can also be executed faster by immediately implementing the 3D into the system. "You can check the model before you start paving or trimming, or even grading for that matter," Tometich explains. "And you can find mistakes a little sooner and potentially save having to shut down a crew because you were able to find the mistake early on. Then you can find a solution and be ahead of the game."
He admits some growing pains are inevitable. "And it’s hard to not be able to see that stringline, see the grade and see where you’re at, but you do have the tools with the 3D system to check grade and so forth," he points out. "It’s just a little different way of thinking, because you don’t have a hub and you don’t have the level to check. You do have the GPS rover or the total station rover. It gives you more information than what you had before with stringline, even though you can’t see it by driving up and looking at it. Like I said before, it’s just a little bit of a different way of thinking.”
A Cost-saving Proposal
Manatts Inc. performed its first half-width concrete overlay project last fall paving 18 miles of U.S. 18 near West Union, IA. The project was unique in the state and was designed as a test project for the Iowa Department of Transportation (IDOT).
“This is a pilot project that was in competition with the asphalt industry,” says Tometich. “It was trying to compete with basically leaving the traffic open at all times using flaggers and pilot cars, building the project half-width at a time using stringless technology. The DOT wanted to keep traffic open throughout the duration of the project.”
Manatts brought in its GOMACO four-track GHP-2800 paver with two paver-mounted GSI (GOMACO Smoothness Indicator) units and a T/C-600 texture/cure machine for the project. Paving passes were 16 feet (4.9 m) wide, which included a 12-foot (3.7 m) driving lane and a new 4-foot (1.2 m) shoulder.
The existing U.S. 18 was only 24 feet (7.3 m) wide, so the project required excavating shoulders on both sides for a new roadway 32 feet (9.8 m) wide with a 2% crown. The existing asphalt roadway had to be milled to create a more uniform surface. Specifications originally called for a 0.5-inch (13 mm) surface milling. Manatt’s approached the DOT and asked for a change.
“We offered a value engineering proposal to the DOT to save on concrete,” Tometich said. “The idea was to profile mill the existing surface to match the new design profile closer and not overrun on concrete because of the wheel rutting and existing inconsistencies in the slab. We were given a range of zero to 1.5 inches (0 to 38 mm) of milling depth parameter. With this latitude in milling depth, we were able to save the DOT hundreds of thousands of dollars in concrete overages. Since they’d be saving concrete by profile milling, they added an extra 0.5 inch (13 mm) to the thickness of the new overlay creating a 4.5-inch (114 mm) thick unbonded concrete overlay.”
Ensuring Paving Accuracy
The project was divided into 3.5-mile (5.6 km) paving sections, with the single-lane traffic controlled by stoplights, flag men and pilot cars. The surface was milled, swept and cleaned to prepare for the unbonded concrete overlay. The final step before concrete paving could begin was tacking on #5 steel rebar, 6 feet (1.8 m) long and placed on 30-inch (762 mm) centers, to the existing road to tie on the new 8-inch (203 mm) thick shoulders.