Accurate paving with no strings attached   

Peter Berton explains why 3D paving offers unmatched paving accuracy, with no strings attached.

The name ‘3D paving’ may seem a bit strange; after all, every form of asphalt paving is executed in three dimensions.

Yet 3D paving is something radically different: rather than relying on strings to define the project’s physical parameters, this system employs 3D engineering ‘job files’ (computer models) to define the work site in three dimensions.

When loaded into an asphalt paver’s machine control system, with the actual asphalt application being steered using real-time locational tracking, the resulting pavements are smooth, consistent, and tightly within specifications.

In fact, 3D paving is accurate to +/- 3 millimetres with respect to positioning specifications elevations, and pavement thicknesses. Such consistency has led to 3D paving being used to build airport runways, high-speed racing tracks, and sports surfaces such as the US Bank Stadium in Minneapolis (home of the US National Football League’s Super Bowl LII last year).

No attachments

There are no strings associated with 3D paving; at least not in a physical sense. They are not required to tell the paver operator where to pave.

“The stringlines become ‘virtual’ on a 3D paving job site,” says Spencer Wykes, Construction Technology Specialist with SITECH Mid-Canada.

He was quoted in the Canadian magazine Asphaltopics (Spring 2018 edition *) and the inspiration for this article.

“With the electronic referencing aids providing the necessary locational information for the paver’s machine control system, the actual strings are gone,” Spencer says, and adding that the time savings achieved by not running stringlines – and having to dodge them with heavy equipment – “is huge”.

3D paving explained

The three dimensions referred to in 3D paving are the desired depth, width, and direction of the road being paved; rather than the usual 3D elements of length, width, and height.

When combined with accurately-defined job files, constant positional tracking, and pavers capable of computer-controlled precision paving, the results are astounding. The surfaces produced are what the customer dreamed of: consistent, level, and end-to-end smooth.

“In contrast, a 2D paving job uses external physical references to manage the paving either through a paver’s 2D control system or by manual operation; using a string, the base, a curb, or a joint as reference markers,” says Laikram Narsingh, Wirtgen America’s manager of Commercial Support and Development.

“Because 2D paving relies on physical guides and human interaction, there is a higher probability for errors compared to 3D paving.”

Differing definitions

The quality achieved using 2D paving is defined by the quality of the roadbed at its base. If there are uneven transitions in the grading below, it will show up in the pavement laid above.

In contrast, 3D paving is defined by the design in the job file. If there are flaws in the actual roadbed, the 3D paving control system can compensate for them: the uneven transition below does not result in an uneven surface on top. (These corrections are usually achieved by gradual alterations on a layer by layer basis, rather than a suddenly thicker or thinner area in one specific lift).

3D positioning options

3D paving systems use one of two 3D positioning systems, to continually align the paver’s location in three physical dimensions to the paving job file.

The first 3D positioning system relies on a ‘millimetre GPS’ tracker fixed to the paver to track its location. To make this happen the unit’s radio receives locational data from GPS satellites to reference the paver’s latitude and longitude in real-time. Meanwhile, laser tracking keeps the paver’s asphalt application elevation accurate to the afore-mentioned +/- 3 millimetres.

The only downside is that the work site must not be obstructed, such that the GPS signals cannot be received.

In instances where constant access to GPS satellite signals may be blocked, ‘robotic total stations’ are used that are referenced to various onsite ‘control points’. (These stations are positioned along the area to be paved, from one end to the other.)

These control stations transmit locational data to the 3D paving control system, to verify where the paver is at all times.

The benefits of 3D Paving

This system of paving offers a level of accuracy and consistency that minimise problems for paving contractors, including the time and resources spent redoing sections of road that weren’t done right the first time.

“3D paving’s high precision can save contractors money by ensuring that the job is always being done right the first time,” says Spencer.

“You won’t find yourself going back after the fact having to mill down surfaces to achieve a targeted smoothness or having to redo work at your own expense.”

This accuracy can also reduce the amount of asphalt a contractor must buy to fulfill a contract, due to less waste and do-overs. With fewer mistakes to plan for during budgeting, quoting jobs to customers can be more accurate, and potentially less expensive.

Meanwhile, 3D paving provides customers with a more predictable and consistent product than 2D paving can deliver. With 3D paving, the design loaded in the job file is the result they see on the roadbed.

3D paving limits

The only downside to 3D paving is the time required to execute it. Compared to 2D paving, 3D paving takes longer.

This is why 3D paving would not be used to pave major stretches of roads when time is limited, such as the 400 series of superhighways in Canada.

“In these jobs, your paving time is generally restricted to between midnight and 5am, which is not a reasonable time margin to use 3D paving effectively,” says Andrew Simmonds, Paving Products Specialist at Toromont CAT (Toronto).

“This means you will want to use 2D paving on long stretches of highway lanes; although 3D paving can be used for on/off ramps and merge lanes that aren’t under such tight time constraints.”

Preparing a roadbed with true precision can allow a contractor to achieve 3D paving results using 2D paving techniques.

“If all of the road’s issues have been resolved by using 3D milling, for instance, then you don’t have to use 3D techniques to pave it,” Laikram says.

“You’ll simply be adding asphalt lifts to an already level and smooth base.”

Nevertheless, 3D paving is a superior way to pave roads, which is why it is gaining popularity in North America.

New Zealand contractors seeking an edge over their competition, might like to give 3D paving a try.

• All quotations are cited from the Asphaltopics article.

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