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CHAPTER  13 MATERIALS TESTING - 14071_269
Data  and  Calculations - 14071_271

Engineering Aid 2 - Intermediate Structural engineering guide book
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Figure 13-1.—Apparatus for soil compaction testing. fitted  with  a  detachable  base  plate  and  a  removable extension collar that is 2 1/2 inches high. The  larger  cylinder  is  the  CBR  mold.  It  is  6 inches in diameter, 7 inches high, and is fitted with a base  plate  and  a  2-inch-high  extension  collar.  When you are  compacting  a  soil  sample,  a  2  1/2-inch-thick spacer disk is placed inside the CBR mold to control the  thickness  of  the  compacted  sample.  With  the spacer disk in place, the volume of the mold is about 0.0735 cubic feet. The CBR mold is used for samples containing material retained on the No. 4 sieve. The compaction tamper consists of a drop tamper in a cylindrical guide. The tamper has a drop weight that weighs 10 pounds and has a striking face that is 2 inches in diameter. The guide sleeve regulates the height  of  drop  to  18  inches.  To  use  the  compaction tamper,  you  place  the  guide  on  top  of  the  specimen and then draw the tamper to the top of the guide and allow it to drop. Other items that you need to perform compaction testing   are   a   balance   or   scale   for   weighing   the material  in  grams,  a  3/4-inch  and  a  No.  4  sieve, moisture canisters, and tools,  such  as  a  mixing  pan, spoon,  trowel,  spatula,  and  a  steel  straightedge  for striking excess material from the top of the mold after compaction. Sample Preparation and Compaction Procedures About   five   specimens,   containing   successively increasing     moisture     contents,     are     needed     to determine  the  OMC  at  which  the  maximum  density for  a  given  compactive  effort  will   occur.   For   the Proctor   mold,   about   6   pounds   for   each   specimen (about 30 pounds total) is needed. For the CBR mold, you will need about 12 to 14 pounds per specimen, or about 60 to 70 pounds total. Before the compacting begins, the sample is air- dried and a moisture content of the air-dried material is determined. Airdrying is done by spreading out the material in the sun or in front of an electric fan. The water content of the air-dried material is determined as  a  basis  for   estimating  the  amount  of  water  you need   to   add   to   each   trial   specimen.   The   driest specimen   should   contain   just   enough   water   to produce  a  damp  mixture  that  crumbles  readily.  For each succeeding specimen, increase the water content by about 2 percent until the wettest specimen is quite wet and plastic. The  compaction  procedures  for  nongravelly  and gravelly soils are the same with two exceptions. First, the 4-inch Proctor mold is used for fine-grained  soil, and the CBR mold is used for gravelly soil. Second, 25 tamper blows per layer are used for the Proctor mold, and  55  blows  per  layer  are  used  in  the  CBR  mold. That  results  in  equal  compactive  efforts  for  the  two mold sizes and soil volumes. To  compact  the  soil,  you  first  attach  the  base plate and collar to the mold. Then you fill the mold to the top of the collar with the material placed in five equal    layers,    compacting    each    layer    with    the appropriate 25 or 55 equally distributed blows. After compacting the 13-2







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