cement) to fluid (as many as 10 gallons of water per sack  of  cement),  depending  upon  the  nature  of  the grouting job at hand. BATCHING When   bagged   cement   is   used,   the   field   mix proportions are usually given in terms of designated amounts of fine and coarse aggregate per bag (or per 94 pounds) of cement. The amount of material that is mixed at a time is called a batch. The size of a batch is  usual]  y  designated  by  the  number  of  bags  of cement it contains, such as a four-bag batch, a six-bag batch, and so forth. The process of weighing out or measuring out the ingredients for a batch of concrete is called batching. When  mixing  is  to  be  done  by  hand,  the  size  of  the batch depends upon the number of persons available to turn it with hand tools. When mixing is to be done by  machine,  the  size  of  the  batch  depends  upon  the rated  capacity  of  the  mixer.  The  rated  capacity  of  a mixer   is   given   in   terms   of   cubic   feet   of   mixed concrete, not of dry ingredients. On   large   jobs,   the   aggregate  is  weighed out in an aggregate   batching   plant   (usually shortened to   “batch plant”),   like   the   one shown   in    figure 6-5. Whenever possible, a batch plant is located near to and used in  conjunction  with    a      crushing      and screening   plant.   In   a crushing and screening 29.153 Figure 6-5.-Aggregate batching plant plant,  stone  is  crushed  into  various  particle  sizes, which  are  then  screened  into  separate  piles.  In  a screening plant, the aggregate in its natural state is screened by sizes into separate piles. The batch plant, which is usually portable and can be   taken   apart   and   moved   from   site   to   site,   is generally  set  up  adjacent  to  the  pile  of  screened aggregate.  The  plant  may  include  separate  hoppers for several sizes of fine and coarse aggregates, or only one hopper for fine aggregate and another for coarse aggregate. It may have one or more divided hoppers, each containing two or more separate  compartments for different sizes of aggregates. Each storage hopper or storage hopper compartment  can  be  discharged  into  a  weigh  box, which  can,  in  turn,  be  discharged  into  a  mixer  or  a batch  truck.  When  a  specific  weight  of  aggregate  is called  for,  the  operator  sets  the  weight  on  a  beam scale. The operator then opens the discharge chute on the   storage   hopper.   When   the   desired   weight   is reached  in  the  weigh  box,  the  scale  beam  rises  and the   operator   closes   the   storage   hopper   discharge chute.   The   operator   then   opens   the   weigh   box discharge  chute,  and  the  aggregate  discharges  into the  mixer   or   batch   truck.   Batch   plant   aggregate storage  hoppers  are  usually  loaded  with  clamshell- equipped cranes. The following guidelines apply to the operation of batch plants: All  personnel  working  in  the  batch  plant  area should wear hard hats at all times.   While  persons  are  working  in  conveyor  line areas,   the   switches   and   controls   should   be secured and tagged so that no one can  engage them until all personnel are clear.   When   hoppers   are   being   loaded,   personnel should   stay   away   from   the   area   of   falling aggregate.   The scale operator should be the only person on the scale platform during batching operations.   Housekeeping     of     the     charging     area     is important.   Personnel   should   do   everything possible  to  keep  the  area  clean  and  free  of spoiled material or overflow.   Debris in aggregate causes much of the damage to  conveyors.  Keep  the  material  clean  at  all times. 6-13