In this article we will discuss about:- 1. Natural Sources of Sand 2. Classification of Sand 3. Bulking Of Sand 4. Properties of Good Sand 5. Function of Sand in Mortar 6. Tests Conducted for Sand 7. Substitutes for Sand.
Natural Sources of Sand:
The sand particles consist of small grains of silica (SiO2). It is formed by the decomposition of sandstones due to various effects of weather.
According to the natural sources from which the sand is obtained, it is of the following three types:
(1) Pit sand
(2) River sand
(3) Sea sand.
(1) Pit Sand:
This sand is found as deposits in soil and it is obtained by forming pits into soils. It is excavated from a depth of about 1 m to 2 m from ground level. The pit sand consists of sharp angular grains which are free from salts and it proves to be excellent material for mortar or concrete work. For making mortar, the clean pit sand free from organic matter and clay should only be used.
When rubbed between the fingers, the fine pit sand should not leave any stain on the fingers. If there is any stain, it indicates the coating of oxide of iron over the sand grains.
(2) River Sand:
This sand is obtained from banks or beds of rivers. The river sand consists of fine rounded grains probably due to mutual attrition under the action of water current. The colour of river sand is almost white. As river sand is usually available in clean condition, it is widely used for all purposes.
(3) Sea Sand:
This sand is obtained from sea shores. The sea sand, like river sand, consists of fine rounded grains. The colour of sea sand is light brown. The sea sand contains salts. These salts attract moisture from the atmosphere. Such absorption causes dampness, efflorescence and disintegration of work. The sea sand also retards the setting action of cement.
Due to all such reasons, it is the general rule to avoid the use of sea sand for engineering purposes except for filling of basement, etc. It can however be used as a local material after being thoroughly washed to remove the salt.
Classification of Sand:
According to the size of grains, the sand is classified as fine, coarse and gravelly.
The sand passing through a screen with clear openings of 1.5875 mm is known as the fine sand. It is mainly used for plastering.
The sand passing through a screen with clear openings of 3.175 mm is known as the coarse sand. It is generally used for masonry work.
The sand passing through a screen with clear openings of 7.62 mm is known as the gravelly sand. It is generally used for concrete work.
Bulking Of Sand:
The presence of moisture in sand increases the volume of sand. This is due to the fact that moisture causes film of water around sand particles which results in the increase of volume of sand. For a moisture content of about 5 to 8 per cent, this increase of volume may be as much as 20 to 40 per cent, depending upon the grading of sand.
The finer the material, the more will be the increase in volume for a given moisture content. This phenomena is known as the bulking of sand and it can be expressed in a graphical way as shown in fig. 7-1.
When moisture content is increased by adding more water, the sand particles pack near each other and the amount of bulking of sand is decreased. Thus the dry sand and the sand completely flooded with water have practically the same volume.
The bulking of sand affects the volumetric proportioning of sand to a large extent. It is more with fine sand and less with coarse sand. If proper allowance is not made for the bulking of sand, the cost of concrete and mortar increases and it results into under-sanded mixes which are harsh and difficult for working and placing.
A very simple test, as shown in fig. 7-2, may be carried out to decide the percentage of bulking of sand.
Following procedure is adopted:
(i) A container is taken and it is filled two-third with the sample of sand to be tested.
(ii) The height is measured, say it is 200 mm.
(iii) The sand is taken out of container. Care should be taken to see that there is no loss of sand during this transaction.
(iv) The container is filled with water.
(v) The sand is then slowly dropped in the container and it is thoroughly stirred by means of a rod.
(vi) The height of sand is measured, say it is 160 mm.
Properties of Good Sand:
Following are the properties of good sand:
(i) It should be chemically inert.
(ii) It should be clean and coarse. It should be free from any organic or vegetable matter. Usually 3 to 4% clay is permitted.
(iii) It should contain sharp, angular, coarse and durable grains.
(iv) It should not contain salts which attract moisture from the atmosphere.
(v) It should be well graded i.e., should contain particles of various sizes in suitable proportions. It should pass BIS No. 480 mesh sieve and should not pass BIS No. 15 sieve. The fineness modulus of sand should be between 2 and 3.
Function of Sand in Mortar:
The sand is used in mortar and concrete for the following purposes:
It does not increase the strength of mortar. But it acts as adulterant. Hence the bulk or volume of mortar is increased which results in reduction of cost.
If building material is fat lime, the carbon dioxide is absorbed through the voids of sand and setting of fat lime occurs effectively.
It prevents excessive shrinkage of the mortar in the course of drying and hence the cracking of mortar during setting is avoided.
It helps in the adjustment of strength of mortar or concrete by variation of its proportion with cement or lime. It also increases the resistance of mortar against crushing.
(5) Surface Area:
It subdivides the paste of the binding material into a thin film and thus more surface area is offered for its spreading and adhering.
Tests Conducted for Sand:
Following tests may be carried out to ascertain the properties of sand:
(i) A glass of water is taken and some quantity of sand is placed in it. It is then vigorously shaken and allowed to settle. If clay is present in sand, its distinct layer is formed at top of sand.
(ii) For detecting the presence of organic impurities in sand, the solution of sodium hydroxide or caustic soda is added to the sand and it is stirred. If colour of solution changes to brown, it indicates the presence of organic matter.
(iii) The sand is actually tasted and from its taste, the presence of salts is known.
(iv) The sand is taken from a heap and it is rubbed against the fingers. If fingers are stained, it indicates that the sand contains earthy matter.
(v) The colour of sand will indicate the purity of sand. The size and sharpness of grains may be examined by touching and by observing with eye.
(vi) For knowing fineness, durability, void ratio, etc., the sand should be examined by the mechanical analysis.
Substitutes for Sand:
In place of sand, other materials such as stone screenings, burnt clay or surkhi, cinders or ashes from coal, coke dust, etc. may be used to prepare mortar. The stone screenings are obtained by screening crushed stones. They are sharp and impart more strength to the mortar.
They are generally used in big construction projects like concrete dams, bridges, etc. where sand in huge quantities is not available near the place of work. They should however be properly screened to remove the stone dust.
The surkhi is the popular substitute for sand. It is obtained by finely grinding burnt clay. It should be clean and free from any impurities. It plays the same functions as those of sand. But in addition, it gives strength and improves hydraulic property of the mortar. As it disintegrates under the action of air and humidity, the mortar with surkhi should not be used for external plaster or pointing work.