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Example  Using  Absolute  Volume  Method - CONTINUED
Materials  Estimation - 14070_409

Engineering Aid 1 - Advanced Structural engineering guide book
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8. Determine the quantities for the first trial batch.  Let’s assume that the size of our laboratory trial batch is 1 cubic yard. For a batch of this size, you need the following quantities l l l l l of  the  ingredients: Cement Type IA = 610 pounds 94 pounds per sack = 6.49 sacks Water = 305 pounds 8.33 pounds per gallon = 36.6 gallons Coarse aggregate = 1,735 pounds Fine aggregate = 1,152 pounds Air content = 5.0 percent If needed, more trial batches should be mixed to obtain the desired slump and air content while  you  keep  the  water-cement  ratio  constant. Variation in Mixtures The proportions at which you arrive in determining mixtures will vary somewhat depending upon which method  you  use.  The  variation  is  the  result  of  the empirical nature of the methods and does not necessarily imply that one method is better than another. You start each method by assuming certain needs or requirements and then proceed to determine the other variables. Since the   methods   begin   differently   and   use   different procedures, the final proportions vary slightly. This is to be  expected  and  points  out  further  the  necessity  of  trial mixtures  in  determining  the  final  mixture  proportions. Adjustments for Moisture in Aggregates The initial mix design assumes that the aggregates are saturated, surface dry (SSD); that is, neither the fine aggregates nor the coarse aggregates have any free water on the surface that would be available as mixing water. This is a laboratory condition and seldom occurs in the field. The actual amount of water on the sand and gravel can be determined only from the material at the mixing  site.  Furthermore,  the  moisture  content  of  the aggregates will change over a short period of time; therefore,  their  condition  must  be  monitored  and appropriate  adjustments  made  as  required.  Coarse aggregates  are  free  draining  and  rarely  hold  more  than 2 percent (by weight) of free surface moisture (FSM) even after heavy rains. A good field test for estimating the FSM on fine aggregates is the squeeze test described below. l The squeeze test. 1. Take samples for the squeeze test from a depth of 6 to 8 inches below the surface of the piled sand. This negates  the  effect  of  evaporation  at  the  surface  of  the pile. 2. Squeeze a sample of the sand in your hand. Then open your hand and observe the sample. The amount of FSM can be estimated using the following criteria: a.  Damp  sand  (0-  to  2-percent  FSM).  The sample will tend to fall apart (fig. 17-1). The damper the sand, the more it tends to cling together. b. Wet sand (2- to 4-percent FSM). The sample clings together without excess water (fig. 17-2). c. Very wet sand (5- to 8-percent FSM). The sand will ball and glisten or sparkle with water (fig. 17-3). The hand will have moisture on it and may even drip. Figure 17-1.—Damp sand. 17-8







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