Table 18-4.—Recommended Gradations for Bituminous-
Stabilized Subgrade Materials
tables 18-4 and 18-5, respectively. Mechanical
stabilization may be required to bring the soil to proper
gradation.
TYPES OF BITUMEN
Bituminous stabilization is generally accomplished
using asphalt cement, cutback asphalt, or asphalt
emulsion. The type of bitumen to be used depends upon
the type of soil to be stabilized, method of construction,
and weather conditions.
In frost areas, the use of tar as a binder should be
avoided because of its high-temperature susceptibility.
Asphalts are affected less by temperature changes, but
a grade of asphalt suitable to the prevailing climate
should be selected. As a general rule, the most
satisfactory results are obtained using the most viscous
liquid asphalt that can be readily mixed into the soil. For
higher quality mixes in which a central plant is used,
viscosity-grade asphalt cements should be used.
Most bituminous stabilization is performed in place.
The bitumen is applied directly on the soil or
soil-aggregate system, and the mixing and compaction
operations are conducted immediately thereafter. For
this type of construction, liquid asphalts, such as cut-
backs and emulsions, are used. Emulsions are preferred
over cutbacks because of energy constraints and
pollution control efforts.
The specific type and grade of bitumen will depend
on the characteristics of the aggregate, type of con-
struction equipment, and climate conditions. Generally,
the types of bituminous materials that will be used for
the soil gradation are indicated in table 18-6.
MIX DESIGN AND METHODS
OF TESTING MIXTURES
For guidance on the design of bituminous-stabilized
base and subbase courses, you should refer to
Bituminous Pavements—Standard Practice, TM5-
822-8, and to NAVFAC MO-330.
The Tentative Method of Testing Soil-Bituminous
Mixtures, ASTM D 915, provides for determination of
water absorption, expansion, and extrusion
characteristics of compacted soil or soil-aggregate
mixtures. The method maybe used for determining the
characteristics of a mixture of specified proportions
under specified conditions of curing or noncuring. Also,
it may be used for determining the effects on these
characteristics of varying the curing and the proportions
of the different ingredients. The test results are not
intended to determine thickness or to predict relative
field performance of the different bituminous materials.
Table 18-5.—Recommended Gradations for Bituminous-Stabilized Subbase Materials
18-8
