Micro-Milling: Case Studies
In the latest edition of the RoadReady Newsletter, we explored the relatively new concept of micro-milling and how it’s changing asphalt rehabilitation. In this week’s blog, we’re going to take a look at a few case studies and see how micro-milling techniques have been incorporated.
Porous European Mix in Georgia
In 2009, the Georgia DOT decided to use micro-milling for the rehabilitation of a 15.6 mile stretch of I-75 near Macon, GA. In recent years, Georgia has been using a porous European mix (PEM) to overlay its interstate highways. However, concerns about poor bonding and water entrapment have prevented Georgia from placing this wearing course directly on traditionally milled surfaces. As a result, an overlay of stone matrix asphalt would typically be placed before the PEM. The improved smoothness provided by micro-milling and decreased ridge to valley depths presented Georgia with an opportunity to skip this step, saving significant amounts of time and money. By authoring special provisions for this unprecedented procedure and placing the PEM directly on the micro-milled surface, Georgia saved about $5.4 million, or $58,000 per lane mile.
To evaluate the effectiveness of the treatment, testing was conducted on cores of the finished surface. Tests were favorable regarding both moisture damage as well as rutting. When evaluating bond strength between the top layer and the micro-milled surface, the study determined that slippage risk was marginal in the short term, but that the bond was performing well after a period of seven months.
Micro-milling has shown money saving potential in Georgia
Performance Planing in Virginia
In 2004, Virginia conducted a series of pilot projects to evaluate their regulations for temporary lane drop-offs on temporary access highways. These drop-offs are created when a single lane is either milled or overlaid, and traffic is opened prior to leveling the adjacent surface. As part of these projects, VDOT also compared performance planing to typical mill-and-fill operations, as well as straight overlays without milling. Performance planing referred to creation of a smoother milled surface than traditional milling, and was accomplished by micro-milling in each case. The goal of the performance planing was to create a safer driving surface when the milled pavement was to be opened to traffic. In addition to comparing the milling treatments, this allowed VDOT to create a specification for performance planing.
The study, which included projects on two interstate highways, concluded that the micro-milling produced a smooth, uniform surface. In addition, the material left behind by micro-milling was more uniform, making it easier to collect from the roadway. Any remaining material was removed by traffic, leaving a clean roadway surface. The study noted that while bond strength on overlays has appeared to improve, the agency must wait to assess long-term performance.
To learn more about the projects above, as well milling basics, check out the links below: