The Mill and Overlay Process
A “mill & overlay” is a street maintenance technique that requires the removal of the top layer (2") of a street by the grinding action of a large milling machine. After the top layer is removed, a new layer of bituminous pavement is put in its place.
The City of Saint Paul has two different mill and overlay programs: one for arterial (main) streets and one for residential (neighborhood) streets. The residential street mill and overlay program is new in 2023. The city has dedicated $4 million each year to improve neighborhood streets.
The “milling” portion of the project typically takes one-to-two days. After the milling is completed, the “overlay” is placed in one-to-two days depending on the width of the roadway and traffic conditions. Before the new pavement is placed, the surface of the newly milled pavement is covered with a liquid asphalt tack coat to bond the old and new pavements.
Afterwards, a new asphalt overlay is placed at a depth of 1 ½” using a machine called a paver. Hot asphalt (350 degrees F) is brought in by trucks from the city’s Asphalt Plant. Each truck carries approximately 10 tons per load and overlay crews can quickly lay 400 to 600 tons of asphalt per day.
The city typically mills and overlays three-to-five miles of city and county arterial streets (primary roads) in Saint Paul each year, depending on funding availability. The projects are selected based on pavement condition and traffic volume. Parking is restricted in the immediate area of the work; restriction signs are posted 24 hours prior to work starting.