Aggregate size; well-graded from grading curve

Size of aggregate

Aggregates are classified by size (IS 383):

• Fine aggregate: mostly passes the 4.75 mm sieve (down to about 150 micron).
• Coarse aggregate: mostly retained on the 4.75 mm sieve.
• Standard coarse sizes: 40 mm, 20 mm, 12.5 mm and 10 mm, either graded or single size.
• The maximum size is chosen according to the thickness of the member, the spacing of the bars and the method of placing: 20 mm for ordinary RCC, 40 mm and above for mass concrete and foundations, and 10-12.5 mm for thin sections or pumped mixes.
• A larger size gives less surface area, so less water and cement are needed, but too large a size reduces strength because of weaker bond, and it is also limited by cover and bar spacing. For high-strength concrete, a smaller maximum size (10-20 mm) is better.
• Size is determined by sieve analysis using the IS sieves 80, 40, 20, 10, 4.75 mm, 2.36 mm ... 150 micron.

Judging well-graded from the grading curve

Plot the sieve size on a log x-axis against the percentage passing on the y-axis.

 % pass
 100|          ____.-----  upper limit
    |      _.-'  ..--- curve of sample
    |   .-' ..--'
  50| .' .-'      lower limit
    |.'-'
   0+-----------------------> sieve size (log)

• The aggregate is well graded if the curve is smooth and continuous (S-shaped) and lies within the upper and lower limits given in IS 383 (Table 7 for graded coarse aggregate and Zones I-IV for sand).
• A curve with a flat portion (step) shows gap grading; a steep curve shows a single-size (uniformly graded) material.
• A curve lying above the upper limit means the aggregate is too fine, and one below the lower limit means it is too coarse.