Interstate Paving LLC – Milling
Pavement milling (also known as cold planing, asphalt milling, or profiling) is the process of removing at least part of the surface of a paved area such as a road, bridge, or parking lot. Milling removes anywhere from just enough thickness to level and smooth the surface to a full depth removal.
One of the main reasons for milling a paved area instead of simply repaving over the existing surface involves recycling. Milling removes and grinds up a layer of the existing surface and recycles it into the aggregate which will be used in the new pavement. For asphalt surfaces the product of milling is reclaimed asphalt pavement (RAP), which can be recycled in the asphalt hot mix, then combined with new aggregate and asphalt cement (binder). This reduces the impact that resurfacing has on the environment.
Asphalt milling – surface repair
Asphalt milling removes distresses from the road surface, providing a better driving experience and longer roadway life. Some of the issues that milling can remove include:
- Raveling: aggregate becoming separated from the binder and loose on the road
- Bleeding: the binder (asphalt) coming up to the surface of the road
- Rutting: formation of low spots in pavement along the direction of travel
- Shoving: a washboard like effect opposite the direction of travel
- Ride quality: uneven road surface such as swells, bumps and depressions
- Damage: resulting from accidents
Asphalt milling can also control or change the height of part or all of the road in order to control heights and clearances of curb reveals, manhole covers and catch basins, shoulder and guardrail heights, and overhead clearances of underpasses. It can also be done to change the slope or camber of the road or for grade adjustments which can help with drainage.
Specialty milling can be used to form rumble strips. Using milling instead of other methods, such as rolling them in, means that the rumble strips can be added at any time after the road surface has hardened.
The Asphalt Recycling and Reclaiming Association has defined five classes of cold planing that the Federal Highway Administration has recognized. The classes are:
- Class I – milling to remove surface irregularities
- Class II – milling to uniform depth as shown on plans and specifications
- Class III – same as class II with the addition of cross slope
- Class IV – milling to the base or subgrade (full depth)
- Class V – milling to different depths at different locations
Milling machines, or cold planers, use a large rotating drum to remove and grind the road surface. The drum consists of scrolls of tool holders which are positioned around the drum such that the ground pavement is moved toward the center and can be loaded onto the machine’s conveyor belt. The tool holders wear out over time and often break when encountering highway structures like manhole covers.
Modern machines generally use a front-loading conveyor system which picks up any material that falls off the conveyor as milling progresses. Water is applied to the drum as it spins, because of the heat generated during the milling process and to control the dust. In order to control the depth, slopes, and profile of the final milled surface many millers now have automatic depth control using lasers, string-lines, or other methods to maintain milled surfaces to ±5 mm (0.20 in) of the target height.
Micro milling, or carbide grinding, costs less than diamond grinding. A specialty drum with three to four times as many cutting teeth either finishes the road surface]or acts as a treatment before applying a thin overlay. It removes many of the same distresses but usually to a shallower depth. A micro milled surface has a uniform finish with reduced road noise compared to standard milling.