Table 15-5.-Numbers of Layers Required for Specimens Specimen  Type  and  Size, Mode   of Number   of Approximate   Depth   of as  Depth,  in.  (mm) Compaction Layers Layer,  in  (mm) Cylinders: 12  (305) rodding 3 equal 4 (100) Over 12 (305) rodding as required 4 (100) Beams: 6  (125)  to  8  (200) rodding 2 equal half  depth  of  specimen Over  8  (200) rodding 3  or  more 4 (100) (table 15-5). As you fill a mold, rotate each scoop- ful  of  the  concrete  around  the  top  edge  of  the mold as the concrete slides from it. This ensures a symmetrical  concrete  distribution  within  the  mold. Tamp   each   layer   with   the   tamping   rod, distributing the strokes uniformly over the cross section of the mold and penetrating the underlying layer.   Tamp   the   bottom   layer   throughout   its depth. The number of roddings is determined by the  diameter  of  the  cylinder.  (See  table  15-6.) After  tamping  the  top  layer,  strike  off  the  sur- face with a trowel or rod so that the concrete fills the  mold  exactly.  Do  not  add  unrepresentative concrete to an underfilled mold. If voids are left by  the  tamping  rod,  tap  the  sides  of  the  mold lightly  with  your  open  hand  to  close  the  voids. If  desired,  cap  the  top  surface  of  freshly  made cylinders with a thin layer of stiff portland cement paste,  which  you  should  then  permit  to  harden and cure with the specimens. When finished, move the specimens to the storage place and leave them undisturbed  for  the  initial  curing  period. CURING  AND  STORING  CYLINDERS.– During  the  initial  curing  period  of  test  specimens, be sure to take precautions to prevent the evapora- tion  and  loss  of  water  in  the  specimens.  Cover  the specimens with a sheet of plastic. You may place wet  burlap  on  top  of  the  plastic  to  help  retard evaporation, but be sure that it does not come in contact with the concrete surface. The exterior of cardboard  molds  must  be  protected  against  the absorption of water or molds may expand, allow- ing  the  specimens  to  be  damaged.  The  test specimens now begin the initial curing period of 24 hr ± 8 hr. Test specimens maybe transported after  the  initial  curing  period,  providing  they remain   in   the   mold.   Upon   completion   of   the initial  curing  period,    remove  the  specimens from  the  molds  and  place  them  immediately  in a  moist  environment  with  water  maintained  on their  surface  at  a  temperature  of  73.4°F  ±  3° (23°C  ±  1.7°).  You  can  also  obtain  the  required condition by immersing the specimens in saturated limewater or by storing them in a moist room or cabinet.  Do  not  expose  test  specimens  to  a  flow of  running  or  dripping  water. CAPPING  CYLINDERS.—  The  ends  of  com- pression-test  specimens  must  be  planed  within 0.002  in.  and  within  0.5  degrees  of  being perpendicular  to  the  axis  of  the  cylinder. Specimens  formed  in  strong  metal  molds having accurately flat baseplates can be capped with  neat  cement  at  2  to  4  hr  after  molding.  A stiff paste of portland cement and water is made at  the  time  the  cylinder  is  molded  so  that  the capping mixture will have shrunk before applica- tion.  Any  free  water  or  laitance  (layer  of  fine particles on the surface) is removed from the end of the specimen. The paste is applied to the top of the concrete and worked with a flat plate until it is smooth and level with the top of the mold. Hardened concrete specimens may be ground to  place  ends  or  capped  with  a  material  having greater compressive strength than the concrete. Prepared mixtures of sulfur and granular mate- rials, special high-strength gypsum plasters, and neat high-early strength cement are satisfactory capping materials (ordinary low-strength plaster of  paris,  compressible  rubber,  or  fibrous  materials are not suitable for caps). You should apply these materials in a plastic state and finish them to the desired plane surface by applying glass or metal 15-38