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Example  Using  Absolute  Volume  Method

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Example  Using  Absolute  Volume  Method Determine  the  mix  proportions  for  a  retaining  wall,  using  the  following  specifications and conditions: Required  28-day  compressive  strength  (f´c) Maximum size aggregate Exposure   condition Fineness modulus of fine aggregate Specific  gravity  of  portland  cement Specific  gravity  of  fine  aggregate Specific  gravity  of  coarse  aggregate Dry-rodded unit weight of coarse aggregate Dry-rodded unit weight of fine aggregate Slump Cement 3,000 psi 3/4 in. Moderate  freeze-thaw  exposure— exposure to air 2.70 3.15 2.65 2.60 102 lb/cu ft 100 lb/cu ft 3 in. Type  IA To determine the mix proportions, proceed as follows: 1. Estimate the air content. From table 17-3, the air content should be 5 percent (3/4-inch aggregate, air-entrained concrete, moderate exposure). 2. Estimate the mixing water content. From table 17-3, you should use 305 pounds of mixing water per cubic yard of concrete (3-inch slump, 3/4-inch aggregate, air-entrained concrete). 3. Determine the water-cement ratio.  From table 17-2, a water-cement ratio of 0.59 will satisfy the strength requirement for 3,000 psi concrete. From table 17-1, you find that a water-cement ratio of 0.50 will satisfy the exposure conditions. Since 0.50 is the smaller of the ratios, that is what you should use. 4. Calculate the cement content.  By using the weight of the mixing water content (Step 2) and the water-cement ratio (Step 3), you can determine the cement content as follows: 5. Calculate the coarse aggregate content. By using table 17-5 and interpolating between fineness moduli of 2.6 and 2.8, you find that for 3/4-inch aggregate having a fineness modulus of 2.7, the volume of dry-rodded aggregate per unit volume of concrete is 0.63. Therefore,  the  volume  of  coarse  aggregate  needed  for  1  cubic  yard  of  concrete  is 0.63 x 27 = 17.01 cubic feet. Since the dry-rodded weight of the coarse aggregate is 102 pounds per cubic foot, then the weight of the coarse aggregate for a cubic yard of the concrete is 17.01 x 102= 1,735 pounds. 17-6



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