Cold Planing (CP) is the controlled removal of the surface of an existing pavement to the desired depth or cross-slope with specially designed equipment capable of removing portions of the pavement surface to the specified grade and/or cross-slope. The modern cold planer has a large diameter rotary cutting drum housed in a cutter housing. The cutting drum is equipped with specially designed replaceable cutting teeth or bits that remove or mill the existing pavement. Water is sprayed on the teeth by a number of nozzles in the cutter housing to control the amount of dust generated and to cool and extend the life of the cutting teeth. CP results in a textured surface which can be immediately used as a driving surface, can be further treated with one of the other asphalt recycling methods, or once cleaned and tack coated, overlaid with a surface course. In addition, CP can be used to roughen or texture pavements to restore friction.
Cold planing machines are self-propelled and of sufficient size to provide the traction and stability needed to remove the pavement surface to the specified profile and cross-slope. Most are equipped with automated grade control systems to mill to specified elevations, grades or cross-slopes.
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.
Typically the fewer passes that are made the better the results but it is common to do a full lane with 2 passes of a half lane machine
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.
Cold planing is a smoothness opportunity and in most cases a better place to make geometric corrections than with the overlay that goes on top of it
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.