TM 5-822-10/AFM 88-6, Chap. 6
b. Cutting teeth.
The cutting teeth
on the milling machine are
a high maintenance item and must
be replaced often.
The teeth may last 1 or 2 days, depending on hardness of the material being milled and the number of operation hours per
day. The cutting teeth can be adjusted to provide a range of surface textures from smooth to very rough. When all the teeth
are in position, the finished surface texture is smooth. By removing some of the teeth, the surface texture can be made
rougher. When the milled surface is used for a riding surface, a rougher surface texture provides better skid resistance;
however, the rougher surface also causes more tire wear.
c. Milling versus heater-planing. The reasons for milling are similar to those for heater-planing. The milling can
remove bituminous or portland cement concrete pavement from bridges to avoid exceeding the maximum dead load. Also,
areas adjacent to curbs, manholes, and other structures can be milled before an overlay is applied so that the overlay
thickness can be maintained adjacent to these structures. One advantage the pavement milling machine has over the heater-
planer is its ability to remove portland cement concrete pavement. This is particularly advantageous when milling a
pavement that has some portland cement concrete along with the bituminous material that must be planed or removed,
such as in areas adjacent to manholes or patches. The most extensive use of the milling machine is in pavement recycling.
The removed materials can be mixed with new aggregate and new asphalt to produce recycled cold mix or recycled hot
d. Milled surface. Occasionally, when a pavement surface has been milled, the surface is used as the riding surface
for a period of time. For instance, when a pavement does not have adequate skid resistance but no immediate funds are
available to overlay this pavement, one alternative is to mill the surface to give it a rough surface texture and thereby
provide adequate skid resistance until it can be overlaid or otherwise repaired. An excessive amount of material should
not be removed because the pavement structure would be weakened. It is recommended that the pavement be overlaid as
soon after the milling as possible. Raveling may become a problem with asphalt concrete pavements after the milling
process. On airfields, raveling could result in foreign object damage (FOD) to the aircraft. Therefore, an overlay should
be applied immediately after the milling operation in most cases.
e. Milled material. The material obtained from milling operations can be used in pavement construction. The milled
material can be stockpiled, but care must be exercised not to stockpile it too high, especially in hot weather, since the
asphalt concrete material will have a tendency to bond thus making it difficult to use. In most cases, the material should
not be stockpiled over 10 feet. The milled material can be used for producing recycled cold mix, recycled hot mix, and
other mixes. Occasionally, this milled material can be used to surface secondary roads that otherwise would not be
surfaced. In this case some additional binder material, such as asphalt emulsion or rejuvenator, is usually added to
rejuvenate the old asphalt or improve binding qualities. This milled material, mixed with asphalt emulsion, can also be
used as a base course for high-quality pavements. The material can be mixed in place or removed and plant-mixed to
produce a satisfactory base course. For high-quality airfield pavements, this base course should be overlaid with the
minimum amount of asphalt concrete mixture required by design. The hot mix and cold mix prepared from materials
obtained by milling are discussed in chapters 3 and 4.
f. Gradation. The gradation
of the milled material obtained from the milling
operation is important
is to be used to produce recycled cold or hot mixes.
(1) When the material is to be used in recycled cold mix, the maximum size of the milled material, which is a
conglomeration, of aggregate and asphalt, should not exceed 1 inches. However, a small amount of material larger than
1 inches is acceptable if it can be removed by screening prior to mixing. Generally, the milled material, without
additional virgin aggregates, is used to produce recycled cold mix.
(2) When the milled material is to be used in recycled hot mix, the gradation of the milled material after extraction
of the asphalt cement is important. Very little breakdown of the aggregate should occur during the milling operation. It
is important that the maximum size of the material as milled does not exceed 1 to 2 inches to ensure that it will break
up and satisfactorily mix with the new materials in the production of recycled hot mix. Some filler material passing the
No.200 sieve will be manufactured during the milling operation. Depending on the aggregate type, 1 to 3 percent
additional filler may be manufactured. One of the problems in designing a recycled mixture is not to exceed the maximum
amount of filler allowed. Generally, new aggregates that are to be added to a recycled mixture are required to have little
or no filler. Therefore, washing of new aggregate is often required to remove the filler prior to producing the recycled
g. Base course. When the asphalt pavement material is to be removed down to the base course, care should be taken
to prevent damage to the base course. Any damage to the base course should be corrected prior to placing the recycled
mixture. Generally, approximately inch of asphalt mixture should be left in place to prevent damage to the base course
by the milling equipment or by rain.