Reference Manual
Module 3-4. Cold Milling
MODULE 3-4. COLD MILLING
1.
INSTRUCTIONAL OBJECTIVES
This module describes the basic techniques for a process commonly referred to as cold milling.
Developed originally as an effective means for mass removal of HMA surface material, the cold milling
process has also evolved into an effective technique for surface leveling, rut removal and restoration the
frictional resistance of the existing pavement surface.
Upon completion, the participant will be able to accomplish the following:
1. Describe the cold milling process.
2. Describe the characteristics of typical milling equipment.
3. Describe typical applications of cold milling and where it is best applied.
4. Describe design and construction issues.
2.
INTRODUCTION
Modern cold milling equipment was developed as a replacement for earlier less productive methods of
HMA surface removal that included sheepsfoot rollers and front-end loaders. The equipment came into
routine use in the late 1970s and has become increasingly popular. Cold milling, along with diamond
grinding, are the most basic forms of pavement surface restoration that are used to remove a variety of
surface distresses. Each of these techniques addresses a specific pavement shortcoming and is commonly
used in conjunction with other pavement restoration techniques as part of a comprehensive pavement
restoration program. In certain circumstances, it may be justified to use one of these techniques as the
sole restoration technique in a pavement rehabilitation project. Only cold milling will be covered in any
detail in this module. Diamond grinding and grooving are covered in module 3-5 of NHI Course 131062,
PCC Pavement Evaluation and Rehabilitation.
3.
DEFINITIONS
Cold milling is generally performed only on HMA pavements, although it is being increasingly used on
PCC pavements for partial-depth repairs. Cold milling operations use drum-mounted carbide steel cutting
bits to chip off the surface of the pavement. This technique is often used to correct rutting or other
surface irregularities, improve ride, to maintain curb lines, and in preparing an existing pavement to
receive an overlay.
4.
PURPOSE AND PROJECT SELECTION
Cold milling uses carbide-tipped cutters or bits, attached to a revolving drum, to chip away the pavement
surface. Cold milling operations can be conducted over the entire lane width and project length, as in the
case of a mill and fill operation, or on small areas for use in partial-depth repairs.
For mass removal of the HMA surface, depths of 50 to 100 mm (2 to 4 in) are typical although equipment
exists that is capable of removing more than 300 mm (12 in) of HMA in a single pass. Mass HMA
removal is typical for operations such as removing a layer of HMA over a PCC pavement, removing a
layer of rutted (or raveled) HMA pavement for recycling or replacing the HMA surface with a new
overlay. Cold milling can also be used to level and profile a pavement surface to correct minor roughness
and rutting. Leveling and profiling existing pavement surfaces to provide a new wearing surface requires
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a finer surface texture than is required for mass removal. The finer surface texture is achieved using
specially made double or triple wrap cold milling drums that have two to three times as many carbide bits.
Single wrap milling heads can be modified to produce the finer textures using forked attachments with
two or three carbide steel bits.
Conventional cold milling has been used very successfully for removing HMA surfacing and is often used
to remove surface rutting (Van Deusen 1979). As previously indicated, it is possible to remove up to 100
mm (4 in) of HMA surface in a single pass. Cold milling has also been used successfully on PCC
pavements to provide a surface for bonding a concrete overlay, and for removing an HMA overlay that
has deteriorated (FHWA 1985). Special care and equipment are needed for cold milling to be used for
final surface texturing of PCC pavements. The reason for this is that standard equipment and processes
can produce too rough a surface and may cause excessive microcracking of the pavement surface and
excessive spalling of the transverse joints (Van Deusen 1979).
Major uses for cold milling include the following:

Removal of rutting on HMA pavement.

Removal of oxidized layer of HMA pavement.

Removal of layer with top-down cracking of HMA pavement.

Restoration of curb line on HMA pavements.

Restoration of cross slope for drainage or correction of drainage inlet cover problem on HMA
pavements.

Restoration of surface friction on HMA pavements.

Restoration of longitudinal profile before HMA overlay in lieu of a leveling course.

Removal of an HMA overlay on a PCC pavement.

Providing a roughened surface for bonding a PCC overlay to an existing HMA pavement.

Providing a transition for HMA overlays into bridge decks and at the end of the overlay project.

Removal of material under overhead structures to maintain vertical clearance when constructing
HMA overlays.

Providing a roughened, clean surface for bonding a PCC overlay to an existing PCC pavement,
although secondary cleaning of the surface is still required.

Removal of HMA material in conjunction with HMA recycling.

Removal of HMA and PCC material when conducting partial-depth repairs.
Cold milling is most often used to remove material prior to placement of an overlay. Cold milling creates
a clean, roughened surface for bonding and eliminates the need to raise drainage structures and other
utilities to the level of the new pavement. Cold milling can also be used to remove HMA material for use
in recycling (see module 3-8).
5.
LIMITATIONS AND EFFECTIVENESS
Cold-milling provides a surface with good frictional characteristics, with improved macrotexture, and a
better drainage profile. The use of milling machines, modified with the multihead blocks, offers the
potential to obtain this improvement with a much finer surface texture, resulting in far less tire noise and a
smoother ride. In time, traffic will wear away the milled pattern and skid resistance will depend solely on
the fractured aggregate surface. If the aggregate is susceptible to polishing, loss of skid resistance will
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Module 3-4. Cold Milling
occur in a relatively short time. An investigation of the aggregate should precede the milling operation if
the milled surface will be subjected to traffic for an extended period.
The surface texture produced by any milling operation is a function of the carbide bit spacing, the
rotational speed of the drum, the bit quality and wear, and the speed at which the milling machine is
advanced over the surface. Increasing the number of carbide bits, increasing the rotational speed of the
drum, and using slower advance speeds, produces a smoother surface texture.
Whenever cold milling is applied, the longitudinal and transverse profile must be controlled. Milling
equipment available today can be equipped with automatic grade control similar to that found on HMA
paving equipment. With the proper use of skis or stringlines and electronic grade control, the desired
profile can be obtained. Grade control combined with the proper milling head, rotational speed, and
forward speed of the milling machine will result in a smooth, free draining surface for the traveling
public.
6.
DESIGN CONSIDERATIONS
Assessing Resulting Surface Profile
The quality of cold milling (and diamond grinding) work can be assessed through measurement of the
pavement roughness after surface removal. The same ride quality standards are used for cold-milling or
grinding projects as for new HMA pavement construction. Profile traces obtained prior to cold-milling
can be compared to profiles obtained after cold-milling to determine and document improvements in the
ride quality.
The most commonly used profile measuring device for grinding operations is the California profilograph.
Many agencies currently use this device for acceptance testing on new PCC pavements and HMA
pavements, and these specifications can be applied on grinding projects, as well. Typical acceptance
values for the California profilograph are as follows:

0.16 km (0.1 mi) increments: 0.19 m/km (12 in/mi) maximum.

1.6 km (1 mi) increments: 0.16 m/km (10 in/mi) average for job.

1.6 km (1 mi) increments: 0.11 m/km (7 in/mi) maximum.
The higher numbers historically have been used more in the eastern part of North America, while the
lower numbers have been used in the western regions.
Other devices, such as the Rainhart profilograph, Mays Ridemeter, BPR Roughometer, and the PCA
Roadmeter, have also been used for acceptance testing. Recently, the use of true profile has become more
pronounced for construction control. The true profile of the roadway is obtained with full-size or
lightweight vehicles that use combination of a sensor (laser, optical, or acoustic) to measure the distance
from the vehicle to the roadway and an accelerometer on the vehicle to measure the relative position of
the vehicle. The profile is then usually reduced to an index such as the International Roughness Index
(IRI) or Ride Number (RN). Typical acceptance values using true profile and IRI values are 1.26 m/km
(80 in/mi) (Collura et al. 1997).
Transverse slope is also important, and should be specified to obtain proper drainage. Generally, a
transverse slope of 1.7 percent, or 64 mm (2.5 in) over 3.66 m (12 ft), is recommended. Grinding limits
and transitions or stop lines at bridges and ramps should be clearly marked on the plans.
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Each agency should carefully evaluate its pavements and equipment in arriving at reasonable acceptance
standards. It must be remembered that the level of smoothness required has a great effect on the cost of
the cold milling operation. Setting of unrealistic levels that require extensive removal or additional
grinding will cause a large increase in the bid cost for cold milling and/or diamond grinding.
Skid Resistance
Pavement skid resistance is improved through milling by the restoration of the cross-slope and the
increased surface macrotexture. Proper cross-slope facilitates transverse drainage and reduces the
potential for hydroplaning, especially in cases where studded tire wear has produced “ruts” in the
pavement surface. The increased macrotexture initially provides high skid numbers, but this
improvement may be temporary, particularly if the pavement contains aggregate susceptible to polishing
(Henry and Satio 1983). Skid resistance is generally measured using either a standard ribbed tire (ASTM
E 501) or a standard smooth tire (ASTM E 524).
Pavement Removal and Surface Profiling
Cold milling operations can be used as a method of removing existing pavement layers prior to an overlay
or to correct surface irregularities such as rutting or corrugations. In the first instance, milling should be
considered as a means of maintaining curb height requirements after placement of an overlay or to
provide material to be recycled and used as the overlay material. Information regarding overhead
clearances and existing curb heights should be reviewed prior to determining the extent of cold milling to
be done. Information on the material characteristics and its ability to be recycled would be needed if cold
milling were being considered as part of a recycling operation. More information on recycling can be
found in module 3-8.
When cold milling is used to correct rutting or corrugation, the amount of the material to be milled should
be estimated. In instances where rutting and corrugation recur in the same areas, the integrity of the
underlying support should be investigated so that the cause of the problem can be corrected, rather than
addressing only the visible distress.
7.
COST CONSIDERATIONS
Cold Milling Costs
The cost of cold milling is highly dependent on the material to be cold milled and the depth of removal.
Typical costs for cold milling of HMA pavements range from $0.60/m2 ($0.50/yd2) to $3.60/m2
($3.00/yd2), depending on the depth of cut. The costs of milling PCC pavements are somewhat higher
than those figures due to its hardness.
Diamond Grinding Costs
The cost of diamond grinding in HMA, which is seldom done, is a little higher than cold milling costs.
Diamond grinding of PCC pavements ranges from $3.00/m2 ($2.5/yd2) to $10.80/m2 ($9.00/yd2).
8.
CONSTRUCTION
This technology has been used for both HMA and PCC pavements. For HMA pavements, the use of
multi-head blocks may prove to be a cost-effective method for removing ruts and increasing skid
resistance without producing a surface prone to excessive tire noise. An example of multiple head teeth
can be found in figure 3-4.1.
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Module 3-4. Cold Milling
Figure 3-4.1. Multiple teeth application on a milling drum.
Cold milled surfaces have been opened to traffic directly without using an overlay. One milled HMA
surface has been in service for over eight years on I-57 in Illinois. This pavement, shown in figure 3-4.2
is an HMA/PCC composite. The pavement surface was milled to remove excessive rutting. There were
some complaints by motorists due to the high tire noise initially produced, but as the milled surface has
worn down and the noise level dropped, complaints have subsided. A similar project on SR 5 in western
Washington was also milled to remove excess rutting but the contract required controls on the surface
texture that resulted in the use of a triple tooth milling drum on the project. This project, which provided
a finer smoother surface texture than normal grinding, is over 6 years old and has generated no motorist
complaints.
Safety of motorists and especially motorcyclists may be a concern if the milled surface will be used by
traffic. In addition to the texture left by the milling machine, project personnel also need to be aware of
transitions at the beginning and end of the milling at bridge decks within the project. Vehicles changing
lanes need to be considered if different milling elevations are specified for the various lanes.
Cold milling can be conducted over small areas, as in the case of material removal for partial-depth
patching. Cold milling can also be used on pavement that passes beneath bridges, and near curbs and
drainage structures prior to overlaying, providing clearance and eliminating the need for expensive grade
changes.
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Figure 3-4.2. I-57 in Illinois after milling to remove rutting.
Cold milling has been gaining wider acceptance as a rehabilitation method for PCC pavements. One
problem that was encountered in both Oregon and Iowa was spalling of transverse joints and cracks. This
problem may possibly be eliminated by filling the joints and cracks with a cementitous material prior to
the milling operation (CTL 1990). Although a number of projects have been conducted in Iowa, Illinois,
Oregon, Puerto Rico, and Washington, the long-term effectiveness of this technique for PCC has not been
established.
9.
EQUIPMENT
Cold milling equipment comes in a variety of sizes. This allows the user to customize the equipment
selected to the project. Cold milling equipment uses carbide bits mounted on a revolving drum to break
up and remove the surface material. Drum widths vary from as little as 0.3 m (1 ft) to as great as 3.6 m
(12 ft). The carbide bits must be continually maintained and frequently replaced to provide a uniform
texture with no ridges or low spots. This is critical if the pavement is not going to receive an overlay.
Positive, definitive grade control is also an essential part of a cold milling operation (FHWA 1985).
Traditionally, a single carbide bit is mounted on a block, which is then bolted to the revolving drum. This
results in a conventional bit spacing of approximately 15 mm (0.6 in). As the drum revolves and
advances forward, the pavement material is impacted and is chipped away, producing a rough texture that
is adequate for a riding surface.
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Module 3-4. Cold Milling
Newer carbide cutting blocks are available that mount two to three carbide cutting bits on a single block
(2-head and 3-head blocks). When attached to a conventional cold milling drum, the number of cutting
bits is increased by a factor of two to three, and the spacing between bits is significantly reduced (to
approximately 5 mm [0.2 in] for a 3-head block). Drums modified in this manner produce a much
smoother texture, more suitable for use as a riding surface. Some newer cold milling drums are being
built that have a steeper angle to the wrap on which the bits are mounted which produces the same
increase in carbide bits using single bit blocks.
In Oregon, cold milling equipment modified with the three-head blocks was used to remove ruts created
in PCC pavement by studded tire wear (CTL 1990). The Oregon DOT evaluated microcracking in PCC
cores obtained from pavement sections that were cold milled and diamond ground. A petrographic
analysis conducted according to ASTM C 856-83 revealed that the limited amount of microcracking
observed was more prevalent in those cores subjected to diamond-grinding than those abraded by the
three-head carbide bits (CTL 1990). The resulting surface, although rougher than that produced by
diamond-grinding, was still considered an acceptable riding surface.
Small milling machines are currently available which are specifically designed for partial-depth patching.
These machines have cutting head widths from 0.3 to 0.9 m (1 to 3 ft), and are highly maneuverable,
making them ideal for use as a primary means of material removal. They have been used on both HMA
and PCC for partial-depth pavement repairs and have increased the productivity of these repair
operations.
Equipment must be inspected frequently to ensure that all cutting bits are functioning properly and that
worn bits are replaced. This is particularly critical if the pavement is not going to receive an overlay.
Worn cutting bits will produce a surface texture characterized by ridges and low spots. Cold milling
equipment can remove up to 75 to 100 mm (3 to 4 in) of HMA with adequate grade control.
10. CONSTRUCTION PROCEDURES
Cold milling is generally conducted longitudinally along the pavement profile. It is recommended that
failed pavement areas be patched prior to cold milling, as cracking becomes difficult to locate on the
milled surface. The forward speed of the machine, the rotational velocity of the rotating drum, the
spacing of the carbide bits, and the grade control of the cutting head should be closely controlled to
produce a uniform texture throughout the project. The longitudinal profile should be held to the same
tolerance as new base course construction if the pavement is to be overlaid. If the pavement is to be
turned over to traffic, the tolerance should be the same as a new HMA surface course.
11. SUMMARY
Cold milling and diamond grinding are two basic forms of surface removal that have been used
successfully to correct a variety of HMA pavement surface distresses. The appropriate application of
these techniques can result in a very cost-effective extension of pavement life.
Cold milling uses carbide teeth cutting bits mounted on a revolving drum to chip away the surface
material, producing a highly textured surface. Cold milling is used primarily on HMA pavements. Cold
milling provides an excellent tool for HMA surface removal prior to placement of an overlay and when
conducting partial-depth patching.
Diamond grinding uses closely spaced diamond saw blades to remove a thin layer of material from a
HMA surface, resulting in a very smooth surface. It is primarily used to remove pavement rutting caused
by studded tire wear and to improve ride. Diamond grinding can also improve skid resistance by
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increasing the macrotexture of the surface and correcting deficiencies in pavement drainage, although this
effect may be temporary if the aggregate is susceptible to polishing.
12. REFERENCES
Collura, J., T. El-Korchi, K. Black, M. Chase, and L. Jin. 1997. Guidelines for Ride Quality Acceptance
of Pavements, Final Report. New England Transportation Consortium, Worcester Polytechnic Institute,
Worcester, MA.
Construction Technologies Laboratory (CTL). 1990. Microscopical Examination of Pavement Cores for
Microcracks Conducted for Oregon Department of Transportation. Construction Technologies
Laboratory, Inc., Chicago, IL.
Federal Highway Administration (FHWA). 1985. Pavement Rehabilitation Manual. FHWA-ED-88025. Federal Highway Administration, Washington, DC. (Supplemented April 1986, July 1987, March
1988, February 1989, October 1990)
Henry, J.J., and K. Satio. 1983. “Skid-Resistance Measurements with Blank and Ribbed Test Tires and
Their Relationship to Pavement Texture.” Transportation Research Record 946. Transportation
Research Board, Washington, DC.
Van Deusen, C. 1979. “Cold Planing of Asphalt Pavements.” Proceedings, Association of Asphalt
Paving Technologists, Volume 48. Association of Asphalt Paving Technologists, St. Paul, MN.
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