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Transportation agencies have a variety of tools available for routine and preventive maintenance and need to use those in a cost-effective approach that benefits the agency’s total pavement system. Cold planning, or milling, is one of those tools that serve a variety of purposes. Milling is often used to remove surface distresses, maintain or correct elevation, restore roadway geometric properties such as cross-slope, and improve surface characteristics. Milling provides a supply of reclaimed asphalt pavement (RAP) that can be recycled into the new surface and generally improves the bond between layers as well.
The economic benefits of milling are important. Milling creates a supply of RAP material that may be reused in new hot-mix asphalt (HMA) pavements. This can amount to substantial savings when the contractor recycles the milled material to replace a portion of the virgin aggregate and asphalt binder in the overlay mix. Kevin Keith, chief engineer with the Missouri Department of Transportation (MoDOT), recently commented that it has only been during the last five years that MoDOT has begun incorporating RAP in its HMA mixes, but savings to the department are already estimated at $34 million.
Milling machines currently being used may cost up to $300,000, but they are more powerful and are capable of precisely milling to greater depths than ever before. The new milling machines’ precision can make some innovative techniques possible. One example is with open-graded friction courses, also called porous surface layers. When an aged porous surface layer begins to ravel, it is usually milled from the surface before placing a new overlay. Because the milling leaves small indentations that may trap water, a dense layer of HMA is usually applied before placing another permeable surface course.
The Georgia Department of Transportation (GDOT) recently experimented with the use of micro-milling, a relatively new milling process, on a 15.6-mile project south of Atlanta. The mandrel of a micro-milling head contains up to three times as many teeth as a conventional milling head and provides a much smoother plane surface, which allows the permeable layer to be placed directly on the milled surface. A comparison of milled surface textures is shown in Figure 1. The depth of the micro-milled surface texture was targeted at 1/16 inch (1.6 millimeters) and correction was required if the maximum ridge-to-valley measurement of the surface exceeded 1/8 inch (3.2 millimeters). GDOT estimates a saving of $58,000 per lane mile could be realized by using the micro-milling process to eliminate the need for an additional dense layer. On the 15.6-mile project, the total savings were $5.4 million. GDOT was so pleased with the results that another interstate project using the process will be contracted this year.
The Wisconsin Department of Transportation also evaluated several milling techniques for improving ride quality and surface texture of pavements that were beginning to rut and had potential for water ponding on the surface. It was concluded that micro-milling was the best available method for achieving the desired surface properties while removing ruts. Based on calculated Equivalent Uniform Annual Costs, Wisconsin determined that resurfacing would cost about fourteen times more than the micro-milling operation without crack treatment.
A literature review conducted for the Oregon Department of Transportation (ODOT) considered a variety of alternatives such as micro-surfacing, stone-matrix asphalt (SMA), both thin and conventional HMA overlays, micro-milling, roller-compacted concrete, and ultra-thin concrete overlays for removing ruts caused by studded tire wear. SMA was reported to offer the most resistance to rutting from studded tires, but micro-surfacing and micro-milling were also viable alternatives for pavements that the ODOT was planning to resurface within a couple of years.
One of the most beneficial aspects of milling is to remove surface distresses. Generally these distress mechanisms are in the form of rutting, raveling, or cracking of the existing pavement. Studies have shown, for instance, that most pavement rutting is confined to the top four inches (100 mm) of the pavement structure.
Mlling also can be used to maintain the reveal height along curb and gutter where an added overlay may create a significant drop-off at the pavement edge or require the overlay to extend into the gutter area, thereby reducing the curb height. In some cases, the milling depth at the face of the curb is set to the depth of the proposed overlay and the milling operation tapers out to existing grade. This allows the overlay to be placed full-depth transversely and still tie in at grade level with the top of the gutter.
Bridge clearance is also maintained on overlay projects by using milling. In those cases, milling begins to taper from the existing grade to the overlay depth about 200 to 500 feet (61 to 152 meters) before the bridge overpass, and extends about 200 to 500 feet beyond the overpass. This provides sufficient depth for the overlay without reducing vertical clearance and is much more economical than raising the bridge to maintain the clearance needed.
Milling is often used to restore cross-slope as it eliminates the need for wedge-shaped leveling courses that are, at best, difficult to compact. However, there have been cases where milling a standard 2 percent cross-slope resulted in exposing the base material underneath. This may particularly be a problem if milling for cross-slope is extended onto the shoulder. The agency/owner should core the pavement at several locations to verify that there is sufficient thickness available when milling to restore cross-slope is needed. The grade and crown point may be shifted on projects where widening is being planned. Milling becomes a very valuable option in such cases.
An evaluation of the Specific Pavement Studies 5 (SPS-5) data provided under the Long Term Pavement Performance program showed that milling was not a significant factor in smoothness values of projects with asphalt overlays. The study concluded there was no difference in International Roughness Index values where pavements were milled prior to overlay or not milled prior to overlay. A study by the Virginia DOT to consider factors that affect the achievable smoothness of HMA overlays found similar results. The study concluded that surface mix type, addition of structural layers, and use of milling were not significant factors. In contrast, a recent study by the National Center for Asphalt Technology (NCAT) of the latest LTPP database, SRD 232, showed that milling was a significant factor for improving IRI smoothness values. The NCAT study involved 18 projects from 16 states and two Canadian provinces. While other agencies may find different results, these studies do point out the need to avoid taking for granted that such factors would automatically contribute to improved smoothness.
When milling is to be used to improve smoothness and/or cross-slope, the machine should be capable of removing the pavement to a target depth within an accuracy of 1/8 inch (3 millimeters) and should be equipped with both grade and slope controls. If milling to a specific depth, watch out for delamination of layers. If the milling depth is near the interface of two layers, it is possible that portions of a layer may remain. These small, often isolated areas may debond during construction, or soon after traffic is applied, and result in isolated potholes soon after construction (Figure 3). In such cases it is recommended to either increase or decrease the milling depth to avoid the potential for delamination problems.
There are many applications where milling is a useful and economical option in a pavement management system. It can be used to remove badly distressed pavement, restore geometric properties, maintain vertical height restrictions, improve surface texture, and provide a valuable recyclable resource. It has been reported that the development and improvement of the milling machine is one of the paving
industry’s most notable equipment advances. v
Donald Watson is lead research engineer for the National Center for Asphalt Technology (NCAT). Watson may be reached at firstname.lastname@example.org.