TYPES OF BITUMEN - 14070_430

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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

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