Since  maximum  density  varies  only  slightly  with variations in the cement content, only the median value is used in preparing specimens for the test. Additional information on selecting the cement content can be found in chapter 5 of NAVFAC MO-330. The  procedures  for  determining  the  OMC  are similar  to  those  described  in  chapter  13  of  this TRAMAN with the following exceptions: 1.  Compaction  is  performed  on  five  layers  of approximately  equal  thickness  to  result  in  a  total compacted depth of 5 inches. 2. Each layer is compacted by 25 uniformly spaced blows using a 10-pound tamper dropped from a height of 18 inches. . The wet-dry test (ASTM D 559) determines the cement  content  for  soil-cement  mixtures  used  in nonfrost  areas.  The  objective  is  to  determine  the minimum  amount  of  cement  that  will  enable  the soil-cement  mixture  to  pass  the  test.  For  the  test, specimens are molded using the OMC and the cement contents   described   above   for   different   soil classifications.  Use  the  procedure  for  the  OMC determination  to  mold  the  specimens,  and  take  a 750-gram sample from the second layer for a moisture determination.  Cure  the  specimens  for  7  days  in  high humidity. After curing, the specimens are weighed and submerged in tap water at room temperature for 5 hours. They are then oven-dried for 42 hours at 160°F. Material loosened by wetting and drying is then removed using two firm strokes of a wire brush. After this, you then reweigh the specimens and subtract the new weight from  the  old  weight  to  determine  the  amount  of disintegration  (soil-cement  loss)  that  occurred  during the cycle. The process is repeated for a total of 12 cycles. A passing grade ranges from 14-percent loss for sandy or gravelly soils down to 7 percent for clayey soil. Additional  information  about  the  wet-dry  test  and an example of determining the soil-cement loss can be found  in  NAVFAC  MO-330. . The freeze-thaw test (ASTM D 560) determines the cement content for soil-cement mixtures used in areas subject to frost action due to repeated freezing and thawing. As in the wet-dry test, the objective of the freeze-thaw test is to determine the minimum amount of cement that enables the mixture to pass the test. For the test,  specimens  are  molded  and  cured  in  the  same manner as the wet-dry test. After 7 days of curing, the specimens   are   placed   on   moist   blotters   and   are refrigerated for 24 hours at -10°F. They are then thawed in a moist atmosphere at 70°F for 23 hours. Then you brush   the   specimens   as   described   above   and,   if necessary,   remove   any   half-loose   scales   using   a sharp-pointed   instrument.   After   12   cycles,   the specimens  are  oven-dried  and  weighed.  The  soil- cement  loss  is  determined  the  same  way  as  in  the wet-dry   test.   Again,   passing   grades   range   from 14-percent  loss  for  sandy  or  gravelly  soils  down  to 7 percent for clayey soil. For   additional   information   regarding   the freeze-thaw  test,  you  should  refer  to  NAVFAC  MO-330. The   principal   requirement   of   a   hardened soil-cement mixture is to withstand exposure to the elements. Strength is a requirement also; however, most   soil-cement   mixtures   that   have   adequate resistance  to  the  elements  also  have  adequate strength.  In  the  ranges  of  cement  contents  producing results  meeting  the  requirements  above,  the  strength of  soil-cement  specimens  tested  in  compression  at various  ages  should  increase  with  age  and  with increases in cement. A sample that has an unconfined compressive  strength  of  approximately  300  pounds per square inch (psi) after curing 7 days and shows increasing   strength   with   age   can   be   considered adequately  stabilized.  NAVFAC  MO-330  has  the procedures that you should follow when performing unconfined compression tests. For  a  discussion  of  modified  mix  design  for  sandy soils and for approximate and rapid test procedures that you can use when complete testing is impracticable, you should   refer   to   NAVFAC   MO-330.   Construction methods using soil-cement can be found in  Military, Soils   Engineering,   FM5-541,  and  in  commercial publications, such as  Moving the Earth,  by Herbert L. Nichols, Jr., and various publications from the Portland Cement  Association. BITUMINOUS  STABILIZATION Bituminous soil stabilization refers to a process by which a controlled amount of bituminous material is thoroughly mixed with an existing soil or aggregate material  to  form  a  stable  base  or  wearing  surface. Bitumen  increases  the  cohesion  and  load-bearing capacity of the soil and renders it resistant to the action of  water. SOIL  GRADATION The  recommended  soil  gradations  for  subgrade  and base   or   subbase   course   materials   are   shown   in 18-7