Stringless 1In traditional Portland cement concrete paving, the contractor will set up a string-line on one or both sides of the paver. The string-line is set at a fixed elevation above the proposed top of pavement slab.  A sensor then rides along the underside of the string-line, transmitting horizontal and vertical course corrections to the paver.

String-less paving removes the string-line, guiding the paver by using robotic total station equipment set up on control points along the project corridor. The control points are set at roughly 250-foot intervals on alternating sides of the paving.  The position of the paver is controlled by observations from the total station equipment and a three-dimensional model of the roadway.  Twin masts (circled in red below) on either side of the paver have prisms mounted on them that are tracked by the total stations along the project.  The data is transmitted to a radio receiver on the paver (circled in green below) and the system controls the horizontal/vertical position of the front of the paver, while the back of the paver is controlled by the grades from the design model.Stringless 2

The advantage of string-less paving is that it allows for a theoretically-smoother pavement to be constructed, because vertical curves are not formed by chords at a fixed interval (usually 25 feet), but are a true curve.  This is especially true for rural pavements such as highways.  String-less paving also eliminates human error – in the setting and tensioning of a string-line that is no longer used.stringless 3

There is also no need to have a crew set a string line, so it saves some labor in preparing the project for pavement. String-less paving is also safer for the paving crew, because there are no strings to constantly step over as workers move in front of the paver.

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