wet by a rainstorm, the proportion of water in the mix may have to be changed. Measuring Aggregate The  accuracy  of  aggregate  measurement  by volume  depends  upon  the  accuracy  with  which  the amount  of  “bulking,”   caused  by  moisture  in  the aggregate,  can  be  determined.  The  amount  of  bulking varies not only with different moisture contents but also   with   different   gradations.   Fine   sand,   for example, is bulked more than coarse sand by the same moisture   content.   Furthermore,   moisture   content itself varies from time to time, and a small variation causes a large change in the amount of bulking. For these   and   other   reasons,   aggregate   should   be measured  by  weight  rather  than  by  volume  whenever possible. To make grading easier, to keep segregation low, and to ensure that each batch is uniform, you should store  and  measure  coarse  aggregate  from  separate piles or hoppers. The ratio of maximum to minimum particle  size  should  not  exceed  2:1  for  a  maximum nominal size larger than 1 inch. The ratio should not exceed 3:1 for a maximum nominal size smaller than 1 inch. A mass of aggregate with a nominal size of 1  1/2  inches  to  1/4  inch,  for  example,  should  be separated  into  one  pile  or  hopper  containing 1 1/2-inch to 3/4-inch aggregate, and another pile or hopper  containing  3/4-inch  to  1/4-inch  aggregate.  A mass  with  a  nominal  size  of  3  inches  to  1/4  inch should be separated into one pile or hopper containing 3-inch  to  1  1/2-inch  aggregate,  another  containing 1   1/2-inch   to   3/4-inch   aggregate,   and   a   third containing 3/4-inch to 1/4-inch aggregate. Water-Cement  Ratio The major factor controlling strength, everything else being equal, is the amount of water used per bag of  cement.  Maximum  strength  is  obtained  by  using just the amount of water, and no more, required for the complete hydration of the cement. As previously mentioned, however, a mix of this type maybe too dry to be workable. Concrete mix always contains more water than the amount required to attain maximum strength. The point for you to remember is that the strength  of  concrete  decreases  as  the  amount  of  extra water  increases. The  specified  water-cement  ratio  is  the  happy medium between the maximum possible strength of the  concrete  and  the  necessary  minimum  workability requirements.  The  strength  of  building  concrete  is expressed  in  terms  of  the  compressive  strength  in pounds  per  square  inch  (psi)  reached  after  a  7-  or 28-day  set.  This  is  usually  referred  to  as  “probable average  7-day  strength”  and  “probable  average 28-day  strength.” SLUMP TESTING Slump   testing   is   a   means   of   measuring   the consistency  of  concrete  using  a  “slump  cone.”  The cone   is   made   of   galvanized   metal   with   an 8-inch-diameter  base,  a  4-inch-diameter  top,  and  a 12-inch height. The base and the top are open and parallel to each other and at right angles to the axis of the  cone  (figure  6-4).  A  tamping  rod  5/8  inch  in diameter  and  24  inches  long  is  also  needed.  The tamping  rod  should  be  smooth  and  bullet-pointed.  Do not use a piece of reinforcing bar (rebar). Samples of concrete for test specimens are taken at the mixer or, in the case of ready-mixed concrete, from  the  transportation  vehicle  during  discharge.  The sample  of  concrete  from  which  test  specimens  are made  should  be  representative  of  the  entire  batch. Such samples are obtained by repeatedly passing a scoop  or  pail  through  the  discharging  stream  of concrete,   starting   the   sampling   operation   at   the beginning of discharge, and repeating the operation until  the  entire  batch  is  discharged.  To  counteract segregation when a sample must be transported to a test site, the concrete should be remixed with a shovel until  it  is  uniform  in  appearance.  The  job  location from  which  the  sample  was  taken  should  be  noted  for future  reference.  In  the  case  of  paving  concrete, Figure  6-4.-Measurement  of  slump. 6-11