Porosity and absorption 2

  In field conditions nearly all the aggreate have some moisture . Fine aggregates increase in volume due to presence of moisture . Fine aggregates increase in volume due to presence of moisture .  and hence suitable allowance must be made while taking amount of it . Also free moisture must be considere as a part of the mixing water and must be subtracted from the required amount of water to water and must be subtracted from the required amount of waer to be added . WHile batching the materials the weights of aggregates must be increased to compensate for the moisture they contain . However , in field , effort should be made to bring the aggregates to saturated surface dry condition , by sprinking water on coarse aggregates heap few hours before using the aggregates .

  To maintain the w/c ratio , the knowledge of  moisture content of an aggregates is important . Total water required for a constant w/c ratio will depend upon the type of moisture content prsent in the aggregates and is computed as follows :

(1) In bone -dry aggregates - Total water to be added = water absorbed by aggregates in 24 hours + Effective water .
(ii)Air dry condition . Total watr to be added  = water absorbed by aggregates in 10 to 15 minutexs + effective water .
(iii)Saturated dry condition . Total water to be added = efective water
(iv) Moist aggregate - Total water to be added = Effective water - water available at aggreate surface .
(vi) Bulking of sand . Bulking means increase in volume . Fine aggregates bulk i volume when they possess some moistuee . Bulking is due to the formation of a thin film of water around the fine aggregate or sand particles . Tickness of water film goes on increasing with more and more moisture and consequently increase in volume continue . But after certain percentage of water , increae in volume completely vanishes and volume occupied by sand becomes equal to the volume of dry sand . Percentage moisture content and percentage bulking of sand is exhibited in Fig 15.2

  To compensate the bulking effect , extra sand should be added in the mix so that the ratio of coarse to fine aggregates is maintained at a specified value . if this effect is not taken into consideration , the amount of and in the mix will be less than the required amount and the resulting concrete becomes harsh . Finer sands exhibit greater percentage of bulking than the coarser ones . Increase in moisture content may vary 25 to 40 percent when moisture content is 5 to 8 percent by weight .