Pulling the string

Oct. 18, 2004

Betonac nv has been slipforming for decades. The company is based out of Sint-Truiden, Belgium, and the general contractor specializes in tunnel, bridge and highway work. They’ve completed several challenging projects with a number of difficult build aspects during the last 25 years.

One of their recent projects may have proven to be the most difficult. The E40/A10 freeway runs from Brussels, the capital city of Belgium, to Ostend. It is one of the country’s busiest highways and carries up to 57,000 vehicles per day.

Betonac nv has been slipforming for decades. The company is based out of Sint-Truiden, Belgium, and the general contractor specializes in tunnel, bridge and highway work. They’ve completed several challenging projects with a number of difficult build aspects during the last 25 years.

One of their recent projects may have proven to be the most difficult. The E40/A10 freeway runs from Brussels, the capital city of Belgium, to Ostend. It is one of the country’s busiest highways and carries up to 57,000 vehicles per day.

Project specifications required the 10.3 miles of new roadway be completed in 150 calendar days. Betonac stepped forward and committed to completing the project in just 126 days and faced penalties of $50,000 per day for every day they might exceed the agreed-upon completion date. Adding to the overall difficulty of the project was the unheard of high temperatures in Belgium at the time.

They had to contend with tight-clearance working conditions to maintain a haul road for themselves, while keeping three lanes of traffic open to the public on both sides of the highway. With such tight working conditions, there was no room to set stringline. Betonac’s answer was utilizing a 3-D stringless guidance system to help them work within the project’s tight constraints and allow them to pave both night and day, 24-7.

The project also required the removal and replacement of the existing barrier wall. Betonac used a Gomaco three-track Commander III to slipform 25,098 ft of New Jersey-style barrier wall. The wall was 31.5 in. wide at the bottom, 15.7 in. across the top and 50.4 in. tall. It was slipformed with reinforcing steel cables and a very dry concrete mix design with a slump averaging only 0.4 to 0.8 in.

The low-slump concrete created problems with the ready-mix trucks when it came time to unload. The concrete would stick inside the mixer and because of the problem, production was slowed down and 12-hour days became 16-hour days. Production averaged 131 ft per hour.

With the barrier in place, work could be carried out on the roadway itself. The decision had already been made to slipform the project using the stringless guidance system.

Betonac hired a third-party surveying company to provide a survey crew to prepare the jobsite data, operate the Leica total stations (Circle 914) and do the as-built control checks during the paving operation. This allowed them to concentrate on the paving aspect of the project while also learning how to operate the 3-D system.

“Traditionally, when contractors are setting stringline, the surveyor has coordinates and levels and he stakes out points, drives the pins in and sets the levels and stringline. It’s a costly, time-consuming and error-prone process,” Karl Soar, service and support manager, machine automation for Leica Geosystems, said. “That exact same data can be used for the 3-D system. It’s just a matter of reformatting that information into a file that we can use on our controller on the paver.

“We then only need a list of the survey control or reference points around the site. These control points allow us to accurately position the total stations within the project.”

The paver is then equipped with front and rear slope sensors to measure the machine’s position and any cross slope in the slab. Prisms also are mounted to the paver and used by the total stations for tracking purposes. The computer, loaded with the project coordinates, is positioned next to the G21 controller on the paver and the two are interfaced.

“All we do is plug in one cable that lets our computer talk to the G21 controller and that’s all the interfacing to the paver that needs to be done,” Soar said. “Once the G21 is set to the Leica mode, it ignores any signals coming in from the wand sensors and listens for the messages coming over the CAN bus.”

Stations move the train

The new generation Gomaco GHP-2800, (Circle 915) with the Leica components installed, was ready to start paving. The paver was moved into position and the total stations were set up and oriented to their positions by taking measurements of the predetermined reference points. The total stations were then aimed at the prisms on the GHP-2800 and started the tracking process. The computer could immediately see whether the paver was positioned correctly. The G21 was set to the steering and elevation modes and the paver brought itself online and to level.

Betonac utilized three total stations on their project. Two of them measured the two prisms on the paver, six times per second, and sent those measurements to the computer.

The third total station was conducting as-built checks behind the paver. The as-built checks measure the line and level of the new concrete roadway and provide instant feedback regarding the accuracy of the new slab. The third instrument also is used for leapfrogging.

“Between 400 to 500 ft you need to leapfrog your total station,” Soar explained. “You take that third instrument, move it to where it’s suitable for the next part of paving, set it up level and take some shots to the reference points to bring it into the coordinate system again. When it’s time to make the switch, you press a button on the computer and it tells that third instrument to lock onto one of the prisms and start measuring.

“The total station that was previously looking at the machine now becomes your redundant instrument for doing your as-built checks and for leapfrogging again at another convenient time.”

Safety measures are built into the system, too. For instance, if someone parks a truck in front of one of the total stations and the instrument can’t see the prism on the paver it will instantly send an error signal to the computer and the measurement process stops. The computer receives the error signal and sends a stop signal to the G21. The G21 puts the paver into standby mode, shutting off the vibrators, tampers and augers until the problem is corrected.

“There’s no danger that the machine will pave incorrectly or pave out of tolerance,” Soar said. “It’s similar to a job using stringline. If the stringline is accidentally broken, the wand sensors go into the extremes of their measurement mode and the controller picks that up and puts the paver in standby.

“One of the myths surrounding 3-D control is that it’s a ‘black-box’—a new and difficult-to-grasp technology—when in fact it’s a very similar concept to running on stringline. There are checks and balances to stop the paver in case of errors or problems.”

One of the main advantages of the stringless system on this project was its ability to work within the tight clearances. Six live lanes to carry heavy traffic had to be left open, a haul road had to be maintained and the paver needed room to operate. Betonac had originally hoped to sensor off the new barrier wall, but decided that just wouldn’t work in this situation. Clearances were so tight even the tripods that held the total stations had to be modified to fit.

“We maximized the amount of available space around the paver without compromising safety and also maintained an access route for the trucks to bring the concrete in. The logistics were pretty nightmarish,” Soar said. “We had to develop this unusual-looking tripod that props itself up on the barrier. It was the perfect place for them. There were no line-of-sight interruptions and being perched on the barrier meant they had a rock-solid foundation.”

Ahead of time

Betonac brought in two mobile batch plants to produce concrete for the project. The Compactor-brand batch plants are each capable of producing 157 cu yd of concrete per hour. The plants are computer assisted, operated by remote control and capable of adding six different types of additives to the mix design. Each plant was equipped with a tank that holds up to 39,626 gal of water.

The plants operated in 12-hour rotating shifts producing the noise-reducing concrete. The special mix design included a minimum cement content of 25 lb/cu ft, entrained air, a high percentage of sand and aggregate no larger than 0.4 in. Slump averaged 0.8 to 1.18 in.

Concrete was delivered to the paver by semi-trucks with open beds hauling 10.5-cu-yd loads. There wasn’t enough room for a placer/spreader and the trucks couldn’t drive on the grade, so the concrete was dumped into a holding container. An excavator then placed it in front of the GHP-2800. The new concrete was slipformed on the milled-down existing asphalt with continuous steel reinforcing placed on top. The project was paved in four 10.3-mile passes. The width varied between 23.8 and 25.8 ft and had a thickness of 9.25 in.

During a 24-hour shift, Betonac’s production averaged 107,643 sq ft. Their highest production on the project was 9,150 sq ft in one hour.

Finishing the noise-reducing concrete was an extended process. First of all, an Auto-Float mounted to the back of the GHP-2800 helped seal and finish the new roadway. Behind the paver, another machine sprayed the concrete with a sugar and water mix retarder before it was covered with a special, biodegradable plastic. The plastic was left on for five to eight hours before it was removed. The concrete was then washed and brushed out with another machine to produce the exposed aggregate finish associated with noise-reducing roads.

Throughout the paving process, rideability and final smoothness of the finished roadway was a concern. The project specifications required a ride of only 0.24 in. per 9.8 ft. Betonac’s ride came in under the spec by half. They averaged only 0.12 in.

Even with the aggressive completion schedule Betonac set for themselves, they finished the E40/A10 project four days early and earned a $25,000 bonus for each day they finished ahead of schedule.

In just 122 days, Betonac placed 25,098 ft of safety barrier and 4,090,420 sq ft of noise-reducing concrete roadway. A total of 28,000 tons of cement and 4,310 tons of steel reinforcement was used on the project.