Stringless grade control can provide as good or better accuracy than conventional methods because it works from a digital model with a virtual stringline.
Using 3D grade control for both the milling and paving operations can ensure the paved surface meets specs at the lowest material costs.
The GOMACO GHP-2800 paver operates stringless with the aid of the Leica Geosystems 3D paving system.
John F. Kennedy International Airport's new Bay Runway was paved stringless. GOMACO pavers slipformed approximately 220,000 cubic yards of concrete on the massive runway project with the Leica Geosystems stringless paving system.
3D technology utilizing GPS augmented by total stations was used to slipform tight radii curb and gutter on a parking lot project. The contractor used their GOMACO GT-3600 curb and gutter machine with Leica Geosystems stringless guidance system.
A GOMACO PS-2600 placer/spreader with GPS receiver for steering control works in front of a GOMACO paving train at the Charlotte Douglas International Airport in Charlotte, North Carolina.
Technology currently exists to replace conventional stringline. This results in less setup time, eliminates the need to drive around stringlines and reduces the risk of damaging or moving the line accidently, which cuts rework. The technology can also lay more complex designs such as transitions, super-elevated curves and frequently changing cross slope.
Three-dimensional machine control is the enabler that makes this possible. Earthmoving contractors were the first to discover the possibilities of going 3D. “For years, grading contractors have benefitted from the superior accuracy and productivity this technology delivers,” says Tony Vanneman, Topcon Positioning Systems. Today, asphalt paving contractors are taking advantage of these same tools.
Advantages over stringline
One key advantage of 3D machine control is the savings in time and energy required to set up, maintain and tear down a conventional stringline system. “Another huge benefit is the increase in a crew’s daily production,” says Vanneman. “Stringless systems require a fraction of the space and are much easier to work around.”
The systems increase flexibility on the jobsite, allowing you to move into and out of projects quicker. And most paving applications can be achieved with 3D machine control.
“It is hard to find a case where 3D would not be able to replace stringline,” agrees Jeroen Snoeck, paving segment manager, Trimble. “It actually fulfills some applications that couldn’t be dealt with properly before.”
Savings come in multiple ways. “There is a savings in asphalt that can go into the hundreds of thousands of dollars on a project,” says Snoeck. Asphalt can now be laid with millimeter accuracy. “You can be sure you are paving to that design. It is not necessary to lay extra asphalt just to be safe.”
Methods to ensure accuracy
Digital models enhance accuracy. Stringless can provide as good or better accuracy because it’s working from a digital model with a virtual stringline, which is a constant and cannot be physically changed.
Conventional stringline is prone to displacement. During paving, trucks or workers can accidently hit the line, causing stringline movement and errors in paving. Relying on a physical string measured and “eye-balled” by workers for setup and accuracy can build error into the paving.
“When properly set up, stringless technology can provide greater accuracy than stringline,” states Scott Schmidtgall, marketing consultant, Caterpillar Connected Worksite Products and Services. “It eliminates many variables inherent in the use of a stringline. However, if the quality of the inputs is poor, the results will be poor, as well.” He notes that accuracies today may be consistently better than one quarter of an inch within target.
Companies use different approaches to enhance accuracy. The Trimble PCS900 Paving Control System uses a total station. “Using the Trimble SPS930 Universal Total Station, MT900 active target and a Trimble Control Box gives an increased level of accuracy and flexibility,” says Snoeck. “Using the PCS900 system, the height on one side of the screed can be controlled to the 3D design elevation. The other side can be controlled to the 3D design slope. It is a full stringless system.” Using the SPS930 Universal Total Station with the PCS900 Paving Control System yields asphalt mat accuracies of 3 mm to 6 mm.
Topcon has developed a system that augments the accuracy of conventional GPS machine control by adding a 33-foot-tall zone of laser light. “Millimeter GPS with Laser Zone provides the same level of horizontal and vertical accuracies as a conventional stringline,” says Vanneman. “The inherent weakness in stringlines are setup mistakes and sagging in the line. The key to success with GPS machine control for any application is the creation of the digital job file, initial equipment setup and checking the system as the job progresses.”
According to Schmidtgall, several factors influence accuracy. The first is design accuracy, since the paver will do what the design tells it. Next is the quality of the survey setup. Finally, manual interruption by operators can create problems. “Some manual operation may be required, but too much will interfere with the system’s ability to produce the design,” he says.
As with any technology, there is a learning curve. “For a contractor with no experience with stringless 3D technology, using 3D will be initially harder to set up,” says Snoeck. “We recommend that you invest in training for the paving crew, getting a high-quality design and ensuring accurate control points. Once the experience has been picked up, most contractors see a fast return on investment, usually on their first 3D job.”
Savings in the sub-base
Whether you mill asphalt prior to paving or build and grade the sub-base, the closer you can get to the intended design, the more you save.
The use of 3D makes sense when laying base material such as roller compacted concrete (RCC), Snoeck states, since there are often no reference points. Laying these layers accurately is critical to achieve smoothness specs and minimize asphalt consumption.
“Using 3D allows you to lay it very accurately,” he adds. “We often recommend if you have done the lower layers in 3D, that you can do the last layer using the 2D system. With the surface accuracy achieved in the lower layers, you can switch your focus to material yield and ultimate smoothness, and simplify the operation.”
But an accurate sub-base is crucial. “If you have dips in the milled surface and you pave over those dips, you are wasting asphalt,” says Snoeck. “If you can mill a surface in 3D and get it exactly to design, you don’t need 3D paving. You can use 2D paving, lay to a nominal thickness and use the least asphalt possible.
“If you can create a smooth surface with the mill and cut to design, that is the preferred option,” he continues. “It is much easier to create smoothness with the mill than the paver.”
This is due to differential compaction. In areas with thicker asphalt, such as over dips, the asphalt will compact more, so you have to compensate to create a smooth surface. “You create a design where the paver lets you lay a little more material over the low spots than above the high spots,” says Snoeck. “So when you roll that surface, it actually becomes a smooth surface.”
“Conventional grade control paving works well in most cases, particularly where the grade work is good or a mill-and-fill where the mill work is accurate,” says Schmidtgall. “If the base grade is accurate, it is easy for grade control paving to be accurate. However, in cases where the job size and complexity make even small efficiencies pay off, stringless paving should be considered.”
In some instances, it makes sense to have your entire fleet equipped with 3D grade control. “One of the benefits of having stringless solutions on your earthmoving equipment, as well as your mill and paver, is that they can all work off the same design plan, reducing the risk that one of the production phases is working off of bad information,” says Schmidtgall. “Ultimately, if the grade or milled surface is accurate, conventional grade control is more than adequate for a quality mat. So having stringless machine control solutions can really maximize a contractor’s ability to produce exceptional work, whether it is a conventional road or a complex pavement design.”
The use of 3D grade control on the paver really pays off on uneven surfaces. “If the base layer is on grade, there will be no difference in the accuracy of the traditional and stringless grade controls,” says Schmidtgall. “When the base is uneven, however, stringless technology has an accuracy advantage over traditional.”
In some cases, it may be impossible to mill out all of the dips and bumps. “In that case, following up with 3D paving makes sense,” says Snoeck.
Why make the investment?
It’s tempting to wait for just the right project before making the leap. “It’s easier to make the decision when a medium to large size job presents an excellent opportunity to realize the benefits the technology can provide,” says Schmidtgall.
Yet, the price of delaying investment in 3D machine control can be steep. “Not investing in stringless technology means you are not only leaving profits on the table, but you could also lose out on getting the project,” says Snoeck. “It’s not unusual today for a contractor to lose the project bid because the competition was using technology and they were not.”
Schmidtgall suggests asking yourself: What is the bidding environment? How can you control material costs? Do you want your business to be perceived as technological? What is your competition doing?
“It comes down to what do you need to do to grow your business,” he says. “If the advantages offered make you more competitive, why wouldn’t you do it?”
The technology will continue to generate revenue on each project. “You will be able to leverage the technology on many projects and more than realize payback on the investment,” Snoeck asserts. “As with any technology, yes, the systems will be improved in the future.” However, as he points out, “often, new component as well as software and firmware updates are available to those who own the technology.
“Despite the changing technology, contractors tell us they wish they had made the move to stringless paving earlier than they did,” he adds. “They understand the industry is changing and they don’t want to be left behind.”