Building Materials, Third Edition

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against poor compaction. For instance using irregular gravel as coarse aggregate of 20 mm
maximum size, natural sand, and normal methods of transport, experience would suggest that
the overall grading should be somewhere near curves 2 or 3, Fig. 11.4, for ‘medium’ workability.
The amount of fine aggregate passing the 300 micron and 150-micron IS sieves influences the
workability, finish, surface texture, and water gain. For concrete to have a smooth finish and
proper cohesiveness the minimum amount of fine aggregate passing the 300-micron sieve
should be at least 15 per cent and that passing 150-micron sieve at least 3–4 per cent of the total
quantity of fine aggregate. The presence of adequate fines is more important in wetter mixes
than in stiffer mixes, and in leaner mixes than in richer mixes. The entrainment of air through
the use of air-entraining agents is mostly in the voids of 600 to 150 micron fraction of sand.
Generally, aggregates conforming to the requirements of IS: 383–1970, which do not have a
large deficiency of excess of any size and which give a smooth grading curve produce the most
satisfactory concrete. The recommended grading according to IS:383–1970, for single size
coarse aggregate of nominal size 40 mm, 20 mm, 16 mm and 12.5 mm and 10 mm are given in
Table 11.4. Similarly, IS:383–1970 has classified fine aggregates in four grading zones from
coarser sand (zone I) to finer sand (zone IV). Fine aggregate falling under any zone is suitable
for making concrete. The limit for any particular size may be accepted if it deviates from the
limits specified by not more than ±5 per cent. This tolerance is not, however, applicable to
limits specified for the 600-micron IS sieve, or the coarser limit of zone I (10-mm), or the finer
limit of zone IV (150-micron) IS sieves.
The grading for fine aggregates are reproduced in Table 11.5.

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