Fine and Coarse aggregate

Introduction 

Sand (> 0.07 mm) is used as a fine aggregate in mortar and concrete. It is a granular form of silica. Sand used for mix design is known as standard sand (IS: 650). In India Ennore Sand is standard sand and in U.K. it is Leighton-Burrard Sand. The Ennorestandard sand should be obtained from , Tamil Nadu.

 It should be quartz, light grey or whitish variety and should be free from silt.
It should (100%) pass through 2-mm IS sieve and should be (100%) retained on 90micron IS sieve with the following distribution

 1. Smaller than 2 mm and greater than 1 mm = 33. 33%

2.  Smaller than 1 mm and greater than 500 microns= 33. 33%

3. Smaller than 500 microns and greater than 90 microns =33. 33%

Sand used in mortars for construction purposes should posses at least 85 per cent of the strength of standard sand mortars of like proportions and consistency.

Classification. 

  Sand may be classified on the basis of source, mineralogical composition, size of the particles and particle size distribution. Depending upon the source sand may be classed as 

1. Natural sand—resulting from natural disintegration of rocks or deposited by streams

2. Crushed stone sand—produced by crushing hard stones

3. Crushed gravel sand—produced by crushing natural gravel. 

Based on mineralogical composition, sand is divided into 

1. Quartz

2. Feldspar and 

3. Carbonaceous varieties. 

Depending upon its size sand is classified as 

1. Coarse sand—fineness modulus (F.M.) 2.90-3.20; medium sand—F.M.: 2.60-2.90 and

2. Fine sand—F.M. : 2.20-2.60. 

Based on particle size distribution fine aggregate have been divided in four grades from grading zone I to grading zone IV

Function of sand. 

The 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.

Effect of gradation. 

The 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. 

Effect of impurties. 

The 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.

Effect of air entraining. 

 The quantity of fine aggregate required for making concrete mix can be reduced by entraining air

Coarse aggregates. 

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. 

Function . 

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

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