Grading of Aggregate

  Grading of agregates means particle size distribution of the aggregates . If all the particles of an aggregates are of one size , more voids will be left in the aggregates mass . On the other hand an aggregates having particle of varying sizes will exhibit smaller voids . Principle f grading is that the smaller size particles fill up the voids left in larger size particles . By adopting proper percentages of various sized aggregates , composite aggregate mix can be developed which will be thoughly graded . Properly graded aggregates produces dense concrte and needs smaller quantities of fine aggregates and cement .

  The grading of aggregates is expressed in terms of percentages by weight retained on a series of sieves , 80mm , 40mm , 20mm , 10mm and 4.75mm sieves are used for grading of coarse aggregates while 10mm , 4.75mm , 2.36mm , 1.18mm , 600mic , 300 mic , 150mic are used for grading the fine aggregates .

  Grading determines the workability of the mix , which controls segregation , bleeding water and cement requirements , handling , placing and other characteristics of the mix . These factors alsoaffect economy , strength , volume change and durability of hardened concrete .

  There is no universal ideal grading for the aggregates . Howwever , IS has specified certain limits , within which a grading must lie to produce a satisfactory concrete . But these limits depend upon the shape , surface texture , type of aggregates and amount of flaky or elongated material . Variation in grading of sand cause great variatiation in the grading  of coarse aggregates does not effect thse properties to the extent of fine agggregates  .

  Coarse aggregates are supplied in nominal sizes . Coarse aggregregates and their grading as suggested by ISI are given in Table 15.5.

  Coarse single sized aggregate nominal size and gradings as suggested by ISI are given in Table 15.6. Aggregates having all its particles predominantly of one uniform size , is known as single sized aggregates .