Quantcast Material Estimates - 14043_158

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concrete-mix   design   according   to   NAVFAC specifications. In  table  6-3,  one  of  the  formulas  for  3,000  psi concrete is 5.80 bags of cement per cubic yard, 233 pounds  of  sand  (per  bag  of  cement),  297  pounds  of coarse  aggregate  (per  bag  of  cement),  and  a water-cement ratio of 6.75 gallons of water to each bag of cement.    These proportions are based on the assumption that the inert ingredients are in a saturated surface-dry condition, meaning that they contain all the  water  they  are  capable  of  absorbing,  but  no additional free water over and above this amount. We  need  to  point  out  that  a  saturated  surface-dry condition  almost  never  exists  in  the  field.  The amount  of  free  water  in  the  coarse  aggregate  is usually small enough to be ignored, but the ingredient proportions  set  forth  in  the  specs  must  almost  always be adjusted to allow for the existence of free water in the fine aggregate. Furthermore, since free water in the fine aggregate increases its measured volume or weight  over  that  of  the  sand  itself,  the  specified volume or weight of sand must be increased to offset the  volume  or  weight  of  the  water  in  the  sand. Finally,  the  number  of  gallons  of  water  used  per  sack of  cement  must  be  reduced  to  allow  for  the  free  water in the sand. The amount of water actually added at the mixer must be the specified amount per sack, less the  amount  of  free  water  that  is  already  in  the ingredients in the mixer. Except   as   otherwise   specified   in   the   project specifications, concrete is proportioned by weighing and  must  conform  to  NAVFAC  specifications.  (See table 6-3 for normal concrete.) MATERIAL  ESTIMATES When tables, such as table 6-3, are not available for determining quantities of material required for 1 cubic yard of concrete, a rule of thumb, known as rule 41   or   42,   may   be   used   for   a   rough   estimation. According to this rule, it takes either 41 or 42 cubic feet of the combined dry amounts of cement, sand, and  aggregates  to  produce  1  cubic  yard  of  mixed concrete. Rule 41 is used to calculate the quantities of material  for  concrete  when  the  size  of  the  coarse aggregate is not over 1 inch. Rule 42 is used when the size of the coarse aggregate is not over 2 1/2 inches. Here is how it works. As  we  mentioned  earlier,  a  bag  of  cement contains 94 pounds by weight, or about 1 cubic foot by loose volume. A batch formula is usually based on the  number  of  bags  of  cement  used  in  the  mixing machine. For   estimating   the   amount   of   dry   materials needed to mix 1 cubic yard of concrete, rules 41 and 42 work in the same manner. The decision on which rule to use depends upon the size of the aggregate. Let’s say your specifications call for a 1:2:4 mix with 2-inch coarse aggregates, which means you use rule 42,   First,   add   1:2:4,   which   gives   you   7.   Then compute  your  material  requirements  as  follows: Adding your total dry materials, 6 + 12 + 24= 42, so  your  calculations  are  correct. Frequently, you will have to convert volumes in cubic   feet   to   weights   in   pounds.   In   converting, multiply the required cubic feet of cement by 94 since 1 cubic foot, or 1 standard bag of cement, weighs 94 pounds.  When  using  rule  41  for  coarse  aggregates, multiply the quantity of coarse gravel in cubic feet by 105 since the average weight of dry-compacted fine aggregate or gravel is 105 pounds per cubic feet. By rule  42,  however,  multiply  the  cubic  feet  of  rock (1-inch-size  coarse  aggregate)  by  100  since  the average  dry-compacted  weight  of  this  rock  is  100 pounds per cubic foot. A  handling-loss  factor  is  added  in  ordering materials   for   jobs. An   additional   5   percent   of materials  is  added  for  jobs  requiring  200  or  more cubic yards of concrete, and 10 percent is added for smaller jobs.    This loss factor is based on material estimates  after  the  requirements  have  been  calculated. Additional   loss   factors   may   be   added   where conditions   indicate   the   necessity   for   excessive handling  of  materials  before  batching. Measuring  Water The   water-measuring   controls   on   a   machine concrete  mixer  are  described  later  in  this  chapter. Water measurement for hand mixing can be done with a 14-quart bucket, marked off on the inside in gallons, half-gallons,  and  quarter-gallons. Never  add  water  to  the  mix  without  carefully measuring  the  water,  and  always  remember  that  the amount  of  water  actually  placed  in  the  mix  varies according  to  the  amount  of  free  water  that  is  already in the aggregate. This means that if the aggregate is 6-10

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