Sand

UNIT - II

Chapter - 4 (C)

SAND

 

SAND

For building purposes, cement and lime are mixed with sand. Though the addition of sand weakens the mortar, yet it has the following advantages:

1. Sand is a hard ingredient. It gives greater strength to mortar against crushing.

2. It reduces the amount of cement necessary to make a mortar, thus decreasing the cost.

3. It prevents cracks and minimizes shrinkage of mortar in drying.

 

1. NATURAL SOURCES OF SAND

1. Pit Sand

Pit Sand is found as deposits in soil. It is obtained by digging pits into soils. It consists of silica, mica, clay, etc. These ingredients are easily decomposed by the atmospheric weathering actions. Pit sand is free from salts. The grains of pit sand may have sharp and rough angles or rounded without sharpness. When rubbed between the fingers, the pit sand should not leave any stain on the fingers. If there is any stain, it indicates the coating of iron oxide over the sand grains.

2. River Sand

River Sand is obtained from banks or beds of rivers. It is not sharp or angular in its grains, due to the action of water currents. River sand is preferable to pit sand, as it is free from particles of clay and dirt. But, it is lacking in adhesion due to round and smooth surfaces. It is largely used for plastering owing to its fineness.

3. Sea Sand

Sea Sand is found mixed with alkaline salts. These salts absorb moisture from air and cause permanent dampness, disintegration and efflorescence of work. [Efflorescence means white incrustation on exposed faces.] Sea sand must not be used for building purposes.

 

2. CLASSIFICATION OF SAND ACCORDING TO THE SIZE OF GRAINS

1. Fine Sand

The sand passing through a screen with clear meshes of 1.58 mm is known as Fine Sand. This is ordinarily used for mortar for plastering work.

2. Coarse Sand

The sand passing through a screen with clear meshes of 3.17 mm is known as Coarse Sand. It is ordinarily used for mortar for masonry work.

3. Gravelly Sand

The sand passing through a screen with clear meshes of 7.62 mm is known as Gravelly Sand. This is generally used for concrete work.

 

3. CHARACTERISTICS OF GOOD SAND

1. Coarse: Sand should be chemically inert, coarse and granular to the touch.

2. Voids: Sizes of sand grains should be so graded as to get minimum of voids. Sand should pass BIS No. 480 mesh sieve and should not pass BIS No. 15 sieve.

3. Sharpness: Sharpness of sand ensures adhesion of cement. Sharpness is tested by the cutting action of the grains of sand when rubbed in the hand.

4. Cleanness: Sand should not contain impurities like leaves, etc., which prevent the adherence of cement. It should not contain salts which attract moisture from the atmosphere.

5. Strong Mortar: Coarse sand makes stronger mortar, provided voids are filled with cement.

 

4. FUNCTIONS OF SAND

1. Mechanical Strength

Sand imparts mechanical strength to the mortar or concrete.

2. Effective Setting

Sand prevents shrinkage and cracking of mortar while setting. Thus, it helps setting of the mortar effectively.

3. Cost Reduction

Sand forms the bulk of the mortar. Hence, it reduces the cost of construction.

4. Workability

Water makes the mortar a paste-like substance and renders it workable.

 

5. TESTS FOR SAND

1. Test for Silt Content: The content of silt in the sand should be limited to 6 - 10%.

2. Sedimentation Test: Sedimentation shall not exceed 4% in natural sand, 10% in crushed sand and 1% in coarse aggregate.

3. Test for Organic Impurities: Shake the sample of sand with an equal volume of solution of NaOH and allow to settle for 24 hours. Then examine the colour of the liquid above the sand. Clear or pale yellow shows that the sample is free from organic impurities. Dark yellow shows that the sand should be rejected.

4. Size of Grains: BIS specifies the grading of sand in terms of the percentage pass through the sieve meshes. Sand should contain particles of various sizes in suitable pro-portions. It should pass BIS No. 480 mesh sieve and should not pass BIS No. 15 sieve.

5. Soundness Test: For concrete liable to be exposed to the action of frost, sand shall pass a Sodium Sulphate (Na2SO4) Soundness Test.

6. Optional Tests: The moisture content, specific gravity, absorption and bulk density are specified as Optional Tests.