Building Materials, Third Edition

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The recommendations for combined grading of coarse aggregates for use in concrete of
good workability are given in IS:383–1970. These are applicable for maximum nominal size of
40 mm, 20mm, 16 mm, 12.5 mm, aggregates, Table 11.6.

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The specific gravity of aggregates is also relevant to mix design. For heavy weight concrete of
density exceeding 3,200 kg/m^3 material such as crushed barytes, limonite, iron ore, steel
punching or small castings, with sp. gr. of 4.2, 4.75, 4 to 5 and 7.8 respectively are used as
aggregate. Natural aggregate usually has a specific gravity between 2.6 and 2.7. Hence, while
designing heavy weight concrete using such aggregates, allowance has to be made for the
specific gravity of these heavier materials. The aggregate-cement ratio by weight should be so
adjusted that the proportion of aggregate to cement by absolute volume remains constant.
Also, if the specific gravities of coarse and fine aggregates differ, the proportions should be so
worked out that the overall grading curve represents the absolute volume proportions instead
of the weight proportions of each grade of material.
If the specific gravity of the fine aggregate is higher than that of the coarse aggregate, the mix
will tend to segregate; however, it is better that both the aggregates should have nearly the
same specific gravity. The aggregate crushing value for barytes is about 40, i.e., its mechanical
strength is low; it should not, therefore, be used for concrete subjected to abrasion or for
concrete requiring high minimum strength without checking if it is suitable. Barytes concrete
has a lower ultimate compressive strength and it should not be used where high stresses are
involved. The flexural strength of barytes concrete may also be less than that to ordinary
concrete of the same compressive strength.

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Water is required primarily for hydration of cement and to give fluidity to the plastic mass.
Water should be clean and free from impurities. The permissible limits for solids in mixing and
curing water as specified in IS:456–2000 are: