UFC 3-250-03
15 May 2001
the mix to be too stiff to be compacted. The mixture should be compacted as soon as it will support the
roller without undue displacement.
7. ASPHALT ROAD MIX. Asphalt road mixes are normally mixed in place by the use of travel plants or
common types of road-building equipment, such as blade graders, disk harrows, drags, and pressure
distributors. The binders used in road-mix construction may be either cut-back asphalts or emulsified
asphalts. The percentage of asphalt required is generally the same as for cold-laid asphalt mixes and
depends upon the type and gradation of the aggregate used. Aggregates used in road mixes may be
existing subgrade materials, loosened existing subgrade materials blended with imported materials, or
properly processed imported materials placed on the existing base or subgrade. When the amount of
material passing the 75 Fm (No. 200) sieve exceeds 20 percent, stabilization with asphalt is difficult. A
wide range of aggregates may be used, and the gradation requirements are less strict than those for hot
or cold plant-mixed types. The bitumen is normally applied by a pressure distributor to the processed
aggregate on the base or subgrade and then thoroughly mixed with the aggregate. A travel-type mixing
plant combines the aggregates with an asphalt material and continuously discharges the mixture at the
rear of the machine as the plant travels along the strip being paved. The use of a travel plant permits
more accurate proportioning of the bitumen and aggregate and generally produces a more uniform and
higher quality mixture than a mixed-in-place method. Further, because more viscous types of cut-back
asphalt may be used, the curing time is reduced. Curing is usually required to reduce the volatiles in the
cut-back asphalt or water in the asphalt emulsion prior to spreading and compacting, because excessive
amounts of volatiles and water affect the compatibility of the mixture and the stability of the finished
pavement. Manipulation with blades or other road machines may speed up curing.
a. Advantages and disadvantages. Much less equipment is required for construction using asphalt
road mix than is required using asphalt concrete, thus resulting in cost savings. The use of locally
available materials also results in significant cost savings. Asphalt road mix, however, does not have
the strength or durability of hot-mix asphalt. The road-mix type of pavement provides an economical
means of obtaining a satisfactory surface for roads and streets when the required amount of pavement
is small, when the natural soil is suitable as aggregate, or when satisfactory aggregates are nearby.
Seal coats with aggregate cover should be applied as a part of road-mix construction, since road mixes
are often open graded or of relatively low density and are therefore susceptible to oxidation and
abrasion.
b. Uses. When properly designed and constructed on a suitable existing subgrade or using locally
available aggregates, the quality of road-mix construction approaches that of cold-laid plant mix. Road
mix is used for intermediate or surface courses, but because of the less accurate control, it is considered
inferior to plant mixtures. Road mix is not suitable for use above the base course for airfields. Road mix
may be used as a wearing course for temporary roads and streets, or as the first step in stage
construction for permanent roads and streets when these are to be supplemented by plant-mix surfaces
as the demands of traffic increase and warrant the increased thickness. Seal coats reduce infiltration of
air and water and thus improve the durability of road-mix pavements.
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