Cold Planing & Micro Milling Pavement Recycling

The equipment for a CP operation typically consists of a self-propelled cold planer (milling machine), haul trucks, water truck, sweeper or power broom and traffic control devices.

CP Size: CP equipment is available in a variety of sizes from mini-milling machines, for localized milling around manholes and valves, to half-lane width machines, to high capacity machines capable of milling up to 16 feet (4.9 m) wide and over 12 inches (300 mm) deep in one pass. The modern cold planer is sized so that the overall weight and available horsepower are related to the milling width and depth in order to maintain traction. 

CP Traction: Cold planers are equipped with either three or four tracks for load distribution, mobility and traction. Some cold planers are equipped with hard rubber tires for increased maneuverability. Typically, each track/wheel is driven by a separate hydraulic motor. Uniform traction is increased on slippery surfaces by various traction locking devices which divert power away from the track that is slipping to the ones that have traction. The use of rubber/polyurethane track pads increase traction and minimize damage to the roadway surface.

Cutting Teeth: Also referred to as cutting tools or bits, come in multiple configurations for a variety of applications depending on production, project and texture requirements. Cutting teeth wear and break requiring periodic replacement; therefore, they are designed for relatively quick and easy replacement. The most common cutting teeth used are conical with tungsten carbide tips. Recent developments include diamond tip teeth for longer wear and additional teeth shapes for changes in surface texture. 

Grade Control: Most cold planers are equipped with automatic grade control systems to accurately control grade and profile of the milled surface. One or more automatic leveling systems, operating independently, and a number of different sensors can be used to control the milled grade. This allows the machine to either match the existing profile or average the profile of the pavement and improve the smoothness of the pavement.  Cold planers can make geometric corrections to the profile of the roadway to improve drainage given that there is enough structure to allow for the correction.  Current sensors are typically a combination of mechanical and ultrasonic with newer developments occurring in GPS grade control systems. Laser sensors are also used with a laser transmitter for very tight grade control situations. More information on grade control and milling for smoothness can be found in ARRA's white paper on Smoothness.

Conventional Milling: The drum on a conventional cold planer has cutting teeth spaced 5/8 inch (12 to 19 mm) apart. There are variations on this such as profiling and excavating drums that have tighter or wider spacing respectively.  The cutting tool spacing produces a grid patterned textured surface with discontinuous longitudinal striations approximately the same distance apart as the cutting teeth.  There are other factors that can impact the resulting surface texture such as rotation speed of the drum and forward speed of the machine.  

Micro Milling: Micro milling is a cold planing process that uses a cutter drum equipped with cutting teeth spaced approximately 3/16 inch (5 mm) apart so the grid patterned textured surface with discontinuous longitudinal striations are much closer together, producing a much finer textured surface.  The same additional factors of rotation speed of the cutter drum and forward speed of the machine can still impact the resulting surface, making it courser or finer.  

Micro milling is used when the treatment that is going on the milled surface is very thin (an inch or less) or when the milled surface will be the final surface.  It's primary application is to improve rideability and/or uniformity of the surface prior to surface course application. Micro milling can also be used to correct some minor pavement distresses by removing a very thin layer of existing pavement or to remove pavement markings due to changing traffic flows or reconstruction activities. 

More information on micro milling can be found in ARRA's white paper on Micro Milling.

• CP + HMA Overlay: CP is often used to remove surface defects, improve smoothness and/or correct grade and cross-slope prior to placing an HMA overlay. This process is generally referred to as mill and fill.

• CP + CIR & CCPR: CP is used as a surface preparation or grade preparation, producing the RAP that is used in CIR and CCPR operations.

• CP + Single or Double Chip Seal: Where the pavement was milled for smoothness or grade and cross-slope corrections and the pavement structure is sufficient to carry the anticipated loads, the milled surface can receive a single or double chip seal as a very economical wearing surface.

• Micro milling + thin HMA overlay:  With the increased emphasis on pavement smoothness, micro milling prior to applying a thin HMA overlay can replace several lifts of HMA, economically restoring pavement smoothness.

• Micro milling + ultra-thin bonded wearing course: Micro milling with the placement off an ultra-thin bonded wearing course can be used to economically improve pavement smoothness, replacing several lifts of HMA pavement smoothness.

• Micro milling + Micro or Slurry Surfacing: With the increased emphasis on pavement smoothness, micro milling prior to applying micro or slurry surfacing can replace several lifts of HMA, economically restoring pavement smoothness.