Learn : Fine Aggregates As per IS specification No. 1542-1960: Sand,Kinds of sand,Pit sand or quarry sand,River sand,Sea sand,Crushed stone,Flyash.
Fine Aggregates
Particles of fine aggregates pass through 4.75 mm mesh and are entirely retained on 0.15 mm mesh. Most commonly used fine aggregates are sand, crushed stone, ash or cinder and surkhi. Aggregates that do not pass through 4.75 mm mesh are termed as coarse aggregates.
1. Sand.
It consists of small grains of silica and is formed by the disintegration of rocks caused by weather. As per IS specification No. 1542-1960 sand should have the following qualities :
(i) Shall be hard, durable, clean and free from adherent coatings and organic matter and shall not contain appreciable amount of clay.
(ii) Shall not contain harmful impurities such as iron pyrites, alkalies, salts, coal, mica, shale or other materials which will affect hardening and attack reinforcement.
(iii) In natural sand or crushed gravel, the amount of clay, fine silt and fine dust should not be more than 4 per cent by weight and in crushed stone it should not be greater than 10 per cent.
Generally sea sand should not be used except in making precast piles and heavy stones for use in harbour works.
2. Kinds of sand.
Depending upon the source from which sand is obtained it is classified as (i) Pit sand or quarry sand: (ii) River sand: and (iii) Sea sand.
(i) Pit sand or quarry sand.
It is found as deposits in soil and has to be excavated out. Grains of it are generally sharp and angular. If free from organic matter and clay, it is extremely good for use in mortar and concrete.
(ii) River sand.
It is obtained from the banks and beds of rivers. It may be fine or coarse. There are chances of fine sand having silt and as such it should be washed before use. Coarse sand is generally clean and is excellent for all purposes.
(iii) Sea sand.
It consists of fine rounded grains of brown colour and is collected from sea-beach. It usually contains salt which attracts moisture from the atmosphere and causes disintegration of the work in which it is used. It could be used locally after it has been thoroughly washed to remove the salts.
3. Crushed stone.
It is obtained by crushing waste stone of quarries to the particle size of sand. Stone crushed from a good quality stone is an excellent fine aggregate. By using stone crush of the same stone, a mortar matching the colour of stone masonry can be easily had. Such mortars are usefully used in ashlar work.
4. Standard sand.
Requirements of Indian standard sand used in testing cement are :
(i) It shall be of quartz, light grey or whitish variety.
(ii) It shall be free from silt.
(iii) It shall be angular, shape of grains approximating to spherical form. Elongated and flattened grains being present in negligible quantities.
(iv) It shall be well graded.
(v) It shall be free from organic impurities. Loss of weight on extraction with hot hydrauchloric acid (sp. gravity 1.6) shall not exceed 0.25%.
The standard sand is obtained from Ennore, Tamil Nadu. Its supplies may be had from “Concrete and Soil Research Laboratory”, Chepauk, Madras.
5. Flyash.
It is obtained from the gases of ground or pulverized coal or lignite fired boilers by any suitable process such as cyclone separation or electrostatic precepitators.
It is used as a pozzolana for part replacement of cement for use with lime as an admixture and for manufacture of Portland-pozzolana cement conforming to IS : 1489-1976.
It is ground with lime to hae cheap yet strong mortar called black mortar.