Building Materials, Third Edition

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The free water-cement ratio now obtained is checked against the requirements of durability
under various conditions of exposure expected during the life of the structure (Table 11.1), and
the lower of the two values should be adopted.

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In the IS method the quantity of mixing water per unit volume of concrete and the ratio of the
fine to total aggregate depends on the nominal size and type of aggregate. This is because
aggregate of different maximum size, grading, surface texture, shape, and other characteristics
produce concretes of different compressive strengths for the same free water-cement ratio.
Thus, for concrete of grade up to M 35 the maximum size of aggregate may be up to 40 mm
while for grades above M 35, the maximum size of aggregate of 20 mm is recommended (Table
11.9). As can be seen from the table the ratio of fine aggregate to total aggregate varies inversely
to the nominal maximum size aggregate.

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The quantity of water per m^3 of concrete and the sand content as a percentage of total
aggregates in the Table 11.9 are based on the assumption that the coarse aggregates are crushed
and angular conforming to IS: 383–1970; the fine aggregate consisting of natural sand conforms
to zone II grading of IS: 383–1970; workability correspond to a compacting factor of 0.80, and
the water-cement ratio (by mass) is 0.6 for grades up to M 35 and 0.35 (by mass) for grades
above M 35. The aggregates must be in a saturated surface dry (SSD) condition.
For other conditions of workability, water-cement ratio, grading of fine aggregate, and for
rounded aggregates certain adjustments have to be made in the quantity of mixing water and
the ratio of fine to total aggregate as given in Table 11.9. These adjustments are given in Table
11.10.

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The cement content per m^3 of concrete is calculated from the free water-cement ratio and the
quantity of water required per m^3 of concrete (Table 11.9). For example, the free water-cement
ratio for a target mean strength of 30 N/mm^2 is 0.45 for 28-days cement strength of 475–525
(curve d, from Fig 11.2). For maximum size aggregate of 20 mm, the quantity of water will be