TYPES OF BITUMEN

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