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 construction equipment, and climate conditions. Generally, the types of bituminous materials that will be used for the soil gradation are indicated in table 18-6.