Batching Concrete

to get 600. Finally, multiply the 600 by 0.25 to determine the  volume  of  concrete  required  for  the  pad  which,  in this case, is 150 cubic feet. Concrete  is  ordered  and  produced  in  quantities  of cubic yards. To calculate the number of cubic yards required for the pad, divide the cubic feet of the pad by 27. This is required because there is 27 cubic feet in 1 cubic yard. Therefore, the concrete pad described in the previous  paragraph,  which  has  a  volume  of  150  cubic feet, requires 5.56 cubic yards of concrete: 150 cubic feet divided by 27 = 5.56 cubic yards. Concrete  projects  often  present  varying  degrees  of difficulty;  therefore,  extra  concrete  is  required  to compensate for these difficulties. Once the total number of cubic yards of concrete is computed, add a little extra, normally  10  percent,  to  compensate  for  waste.  To calculate the excess needed, multiply the cubic yards by .10 (10 percent). In the above case, multiply 5.56 cubic yards by .10 to get 0.556 cubic yards. Add the 0.556 cubic yards to the 5.56 cubic yards for a total of 6.116 or 6.12 cubic yards required for the concrete pad. BATCHING  CONCRETE Batching   is   the   process   of   weighing   or volumetricall y measuring and introducing into a mixer the ingredients for a batch of concrete. To produce a uniform quality concrete mix, measure the ingredients accurately for each batch. Most concrete specifications require that the batching be performed by weight, rathcr than by volume, because of inaccuracies in measuring aggregate,  especially  damp  aggregate.  Water  and  liquid air-entraining admixtures can be measured accurate]y by either weight or volume. Batching by using weight provides greater accuracy and avoids problems created by bulking of damp sand. Volumetric batching is used for  concrete  mixed  in  a  continuous  mixer,  and  the mobile  concrete  mixer  (crete  mobile)  where  weighing facilities are not at hand. Specifications  generally  require  that  materials  be measured in individual batches within the following percentages of accuracy: cement 1%, aggregate 2%, water  1%,  and  air-entraining  admixtures  3%. Equipment within the plant should be capable of measuring quantities within these tolerances for the smallest to the largest batch of concrete produced. The accuracy of the batching equipment must be checked and adjusted when neccessary. Mixing  Concrete Concrete should be mixed until it is uniform in appearance  and  all  the  ingredients  are  evenly distributed. Mixers should not be loaded above their rated  capacities  and  should  be  operated  at approximately the speeds for which they were designed. If the blades of the mixer become worn or coated with hardened  concrete,  the  mixing  action  will  be  less efficient.  Worn  blades  should  be  replaced  and  the hardened concrete removed periodically, preferably after each production of concrete. When a transit mixer (TM) (fig. 7-1) is used for mixing  concrete,  70  to  100  revolutions  of  the  drum  at the rate of rotation designated by the manufacturer as mixing  speed  are  usually  required  to  produce  the specified  uniformity.  No  more  than  100  revolutions  at mixing speed should be used. All revolutions after 100 should  be  at  a  rate  of  rotation  designated  by  the manufacturer as agitating speed. Agitating speed is usually about 2 to 6 revolutions per minute, and mixing speed is generally about 6 to 18 revolutions per minute. Mixing for long periods of time at high speeds, about 1 or more hours, can result in concrete strength loss, temperature  rise,  excessive  loss  of  entrained  air,  and accelerated  slump  loss. Concrete  mixed  in  a  transit  mixer  should  be delivered  and  discharged  within  1  1/2  hours  or  before the  drum  has  revolved  300  times  after  the  introduction of water to cement and aggregates or the cement to the aggregates. Mixers and agitators should always be operated  within  the  limits  of  the  volume  and  speed  of rotation designated by the equipment manufacturer. Overmixing  Concrete Overmixing concrete damages the quality of the concrete, tends to grind the aggregate into smaller pieces, increases the temperature of the mix, lowers the slump,  decreases  air  entrainment,  and  decreases  the strength of the concrete. Also, overmixing puts needless wear on the drum and blades of the transit mixer. To  select  the  best  mixing  speed  for  a  load  of concrete, estimate the travel time to the project (in minutes) and divide this into the minimum desired number of revolutions at mixing speed-70. The results will be the best drum speed; for instance, if the haul is 10 minutes, 70 divided by 10 equals 7. With this drum speed, the load will arrive on the jobsite with exactly 70 turns  at  mixing  speed,  with  no  overmixing  of  the concrete  mix  and  no  unnecessary  wear  on  the 7-4


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