As in performing earthwork to grade, accuracy is the goal. Efficiencies, speed, and cost savings follow. Traditional milling, using stringlines and markings, has its limitations.

The operator needs to work to the design for the new surface, removing the right depths in the right places. Traditionally, this is done by following sprayed marks manually laid out by a surveyor. This prework is time-consuming and subject to error. To be carried out safely, the road needs to be closed and often the surveyor needs to work at night to minimize disruption.

Once the design has been marked, it can be difficult for operators to see—it gets hidden by other machinery onsite or might be difficult to read from their machine. Any inaccuracies lead to the machine operator either taking too much asphalt away, which subsequently means the area will need to be overfilled and the project will be more expensive than it should be, or not taking enough asphalt and the new surface layers being of poorer quality. Another risk, when milling deeper than required, is damaging the structural subbase layers.

Fortunately, 3D machine control systems for milling machines are widely applied nowadays. Using positioning data from a GNSS receiver on the machine, the milling pilot calculates the difference between the existing and the design surface as the cold planer moves over the area that is being milled.

Managers can then progress to the more accurate and faster 3D machine control solution. 3D systems can reproduce road grades and slopes far more accurately, making it easier to achieve complex designs, ultimately improving the smoothness of the final pavement.

The surveyor or the machine operator uploads the digital design file for the road into machine control software. The software compares the design model to the actual position of the machine’s cutting edge and assists the operator to position the machine to achieve the planned design. An active total station tracks the position of the cold planer in 3D, and the software on the machine verifies the elevation and slope against the design. The depth of asphalt to cut is sent to the machine controller. It is shown on the operator’s tablet interface that they have in the cab and controls the hydraulic response on the machine. This eliminates the need for a surveyor to make any marks or stringlines on site, or for the operator to follow man-made markers and control the adjustments.

The source of the as-built model could be from measurements taken using a total station, a 3D laser scanner, or a mobile mapping solution. A quick way to create the as-built model is to fit a mobile mapping solution to a vehicle and drive it along the road that needs to be resurfaced. Alternatively, a ground penetrating radar solution can be used, which provides data about the different layers with the most important information for the proper execution of the project, such as depth and thickness of reinforcement cover, concrete slab, and asphalt, detection of areas affected by corrosion, and detection of rebars.

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