Building Materials, Third Edition

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2ƒ—X2The functions of sand are to achieve economy by its use as adulterant in
mortar, prevent shrinkage and development of cracks in mortar, furnish strength to mortar
against crushing and allow carbon dioxide from the atmosphere to penetrate the fat lime
mortars necessary for its air hardening.

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2q—— X2The grading of fine aggregate has a great influence on workability of
mortar. Very fine sand and very coarse sand have been found to be unsatisfactory for making
mortar and concrete. Very fine sand results in a poor mortar and is uneconomical, whereas
very coarse sand produces a harsh mix affecting workability. When well graded (consisting of
particles of different sizes) the voids are minimised.

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2sX2The impurities such as clay, dust and organic materials are harmful for
mortar and concrete and in any case should not exceed 4 per cent. Of these clay is most harmful
since it coats individual sand particles and prevents their bonding with cement consequently
diminishing the strength of mortar which is further reduced by the enhanced water requirement
of mortar. The clay and dust impurities can be removed by careful washing. Addition of finely
ground clay to clean coarse sand may improve its grading and reduce voids. Hence, a lean
mortar deficient in fines may be improved both in density and workability by addition of small
percentages of such clays. The organic matters, shell and vegetables injure the hardening
properties of the cement reducing the strength and durability.

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2i—2e2 g ™X The quantity of fine aggregate required for making
concrete mix can be reduced by entraining air.

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These may be uncrushed, crushed or partially crushed gravel or stone most of which is
retained on 4.75 mm IS sieve. They should be hard, strong, dense, durable, clear and free from
veins and adherent coatings; and free from injurious amounts of disintegrated pieces, alkali,
organic matter and other deleterious substances. Flaky, scoriaceous and elongated aggregate
should be avoided.

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X2The functions of coarse aggregate are almost same as that of fine aggregate.