CEMENT
Cement is the basic and most important ingredient of cement concrete. Cement used for construction work should be fresh and it should meet the standard specifications.
FINE AGGREGATE
Sand is used as fine aggregate in cement concrete. Sand particles should have coarse, sharp and angular edges. Size of sand particles should be such that it passes through 4.75mm sieve. Sand should be clean, free from dust and organic matters. Sea sand is prohibited for construction work, because of high salt contents.
Stone dust can also be used as fine aggregate in cement concrete, but before use make sure that it is as per specifications.
COARSE AGGREGATE
Generally, pieces of igneous rocks are used as coarse aggregates. These stones should be hard, tough, durable and clean. Aggregates shape should be cubic or closed to cubic shape. Shape of coarse aggregates should not be laminated and elongated. It should be clean and free from any irrelevant organic matters.
Size of coarse aggregates should meet the approved construction work requirements. It should not pass through the sieve size of 5mm and coarse aggregates should be graded. Voids should not exceed 42%.
PROPORTIONING
Cement, sand and coarse aggregates should be measure according to their fixed proportions. Make a standard measuring box according to the volume of one cement bag. Volume of one cement bag is 1.25 cubic foot.
Bulking of sand should be under consideration while measuring sand proportion. Consider dry sand while calculation of proportioning. Measure the moisture content in sand and add extra volume of sand. Continuously measure moisture content during construction work and add extra volume of sand according to the amount of moisture. Don’t compact coarse aggregates while proportioning.
CONCRETE MIXING
Mixing machine is used for large scale construction works. While, hand mixing is preferred as it is economical for smaller concrete works.
CONCRETE CONSISTENCY
Concrete consistency depends on water to cement ratio. Excess amount of water decreases the strength of concrete and also concrete ingredients get separate very easily. Following amount of water should be used against 50kg cement bag.
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