TM 5-822-14/AFJMAN 32-1019
percent of portland cement may be added at this
prevents or hinders the penetration of water and
time.
thereby reduces the tendency of the material to
(3) Step 3. Test three specimens using the
lose strength in the presence of water. The second
unconfined compression test. If frost design is a
mechanism has been identified as adhesion. The
consideration, subject three specimens to 12 cycles
aggregate particles adhere to the asphalt and the
of freeze-thaw durability tests (ASTM D 560) ex-
asphalt acts as a binder or cement. The cementing
cept wire brushing is omitted. The frost suscepti-
effect thus increases shear strength by increasing
bility of the treated material shall also be deter-
cohesion. Criteria for design of bituminous stabi-
mined as indicated in appropriate design manual.
lized soils and aggregates are based almost en-
(4) Compare the results of the unconfined
tirely on stability and gradation requirements.
compressive strength and durability tests with the
Freeze-thaw and wet-dry durability tests are not
requirements shown in tables 2-2 and 2-3. The
applicable for asphalt stabilized mixtures.
a. Types of bituminous stabilized soils.
lowest LF ratio content, i.e., ratio with the lowest
(1) Sand bitumen. A mixture of sand and
lime content which meets the required unconfined
bitumen in which the sand particles are cemented
compressive strength requirement and demon-
together to provide a material of increased stabil-
strates the required durability, is the design LF
ity.
content. The treated material must also meet frost
(2) Gravel or crushed aggregate bitumen. A
susceptibility requirements as indicated in the
mixture of bitumen and a well-graded gravel or
appropriate pavement design manuals. If the mix-
crushed aggregate that, after compaction, provides
ture should meet the durability requirements but
a highly stable waterproof mass of subbase or base
not the strength requirements, it is considered to
course quality.
be a modified soil. If the results of the specimens
(3) Bitumen lime. A mixture of soil, lime, and
tested do not meet both the strength and durabil-
bitumen that, after compaction, may exhibit the
ity requirements, a different LF content may be
characteristics of any of the bitumen-treated mate-
selected or additional portland cement used and
rials indicated above. Lime is used with material
steps 2 through 4 repeated.
that have a high PI, i.e. above 10.
e. Selection of cement content for LCF mixtures.
b. Types of bitumen. Bituminous stabilization is
Portland cement may also be used in combination
generally accomplished using asphalt cement, cut-
with LF for improved strength and durability. If it
back asphalt, or asphalt emulsions. The type of
is desired to incorporate cement into the mixture,
bitumen to be used depends upon the type of soil
the same procedures indicated for LF design
to be stabilized, method of construction, and
should be followed except that, beginning at step
weather conditions. In frost areas, the use of tar as
2, the cement shall be included. Generally, about 1
a binder should be avoided because of its high-
to 2 percent cement is used. Cement may be used
temperature susceptibility. Asphalts are affected
in place of or in addition to lime however, the total
to a lesser extent by temperature changes, but a
tines content should be maintained. Strength and
grade of asphalt suitable to the prevailing climate
durability tests must be conducted on samples at
should be selected. As a general rule, the most
various LCF ratios to determine the combination
satisfactory results are obtained when the most
that gives best results.
viscous liquid asphalt that can be readily mixed
3-4. Stabilization with Bitumen. Stabilization
into the soil is used. For higher quality mixes in
of soils and aggregates with asphalt differs greatly
which a central plant is used, viscosity-grade
from cement and lime stabilization. The basic
asphalt cements should be used. Much bituminous
mechanism involved in asphalt stabilization of
stabilization is performed in place with the bitu-
fine-grained soils is a waterproofing phenomenon.
men being applied directly on the soil or soil-
Soil particles or soil agglomerates are coated with
aggregate system and the mixing and compaction
asphalt that prevents or slows the penetration of
operations being conducted immediately thereaf-
water which could normally result in a decrease in
ter. For this type of construction, liquid asphalts,
soil strength. In addition, asphalt stabilization can
improve durability characteristics by making the
are preferred over cutbacks because of energy
soil resistant to the detrimental effects of water
constraints and pollution control efforts. The spe-
such as volume. In noncohesive materials, such as
cific type and grade of bitumen will depend on the
sands and gravel, crushed gravel, and crushed
characteristics of the aggregate, the type of con-
stone, two basic mechanisms are active: water-
struction equipment, and climatic conditions. Gen-
proofing and adhesion. The asphalt coating on the
erally, the following types of bituminous materials
cohesionless materials provides a membrane which
will be used for the soil gradation indicated:
3-6
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