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22 Chapter 1:Cement&Concrete <br /> Proportioning the mix by absolute volume <br /> In practice,the proportions of each constituent of a <br /> concrete mix are determined by calculating their Mass (kg) <br /> absolute volumes. The unit volume of each Volume (ml)= <br /> component is calculated based on lm3 (10001) of Specific Gravity <br /> compacted concrete, and obtained by dividing the (kg/m3) <br /> mass of each component by the specific gravity <br /> Example: <br /> Specification:Cement dosage 325 kg/m3 <br /> Water/Cement ratio 0.48 <br /> Plasticizer 1%based on cement mass(_—3 kg) <br /> Assumption: Normal porosity 1.5%entrapped air(=15 1) <br /> Component Mass(kg) (kg/m3)Specific Gravity Unit volume(ms) <br /> Cement 325 3,100 0.105 <br /> Mixing water 156 1,000 0.156 <br /> Plasticizer 3 —1,000 0.003 <br /> Entrapped air - 0.015 <br /> Subtotal 484© 0.279 Os <br /> Dry aggregate 0.721 x 2,700 = 1947© 2,700 1-0.279=0.721(D <br /> Fresh concrete 484+1947= 24318 2,4310 1 <br /> 1) Mixing water = water added + moisture of aggregates. The number OO through ® indicate the sequence of the <br /> calculation. <br /> To calculate the actual amount of aggregate Influence of other factors on the workability & <br /> necessary, the water contained as moisture in the strength of concrete <br /> aggregate (generally 4 to 6%for sand and 1 to 3 <br /> for gravel) must be added for each fraction. Besides admixtures, many other factors influence <br /> Subtracting the moisture contained in all the concrete workability.Changing one or more of these <br /> aggregates from the total mixing water gives the factors changes not only the workability, but also <br /> necessary amount of water to be dispensed. other characteristics of concrete, for example <br /> strength. Table 1.4 shows how various changes in <br /> The unit volume of entrapped air bubbles(generally concrete constituents and mix affect the <br /> 1 to 2%)as well as the volume of entrained air must consistence and 28-day compressive strength of <br /> also be considered in proportioning the mix by concrete. <br /> absolute volume. The example shows a method of <br /> calculating the "dry" aggregate amount and the <br /> fresh concrete density. <br />