Cement concrete is composed of cement, fine aggregates, coarse aggregates and water. While writing detailed specifications, explain the specifications of every single ingredient in detail.
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.
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.
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%.
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.
Mixing machine is used for large scale construction works. While, hand mixing is preferred as it is economical for smaller concrete works.
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.
Concrete Ratio Amount of Water
1:3:6 34 liters
1:2:4 30 liters
1:2:3 27 liters
1:1:2 25 liters
If vibrator is used for concrete compaction, then decrease the water amount as per engineer in charge orders.
Laying of Fresh Concrete
After batching of concrete, lay it down slowly on the farm work. Make sure concrete is not fallen from height more than two to three feet. This results in segregation of concrete. Concrete pouring should be done before the start of concrete final setting time. Generally, initial setting time of normal Portland cement is 30 minutes. Time starts when water is added to mixing machine.
Concrete compaction should be done within initial setting time of concrete. Vibrator should be used for the compaction of concrete. In case, vibrator is not possible to arrange then use Rod for compaction. Pouring of concrete should be done in layers whose thickness should not exceed 6 inches. Make sure the upper layer should be pour before the beginning of final setting of first concrete layer.
In winters, if temperature drops below 4.5 Degree Celsius than stop concrete work immediately. In summers, temperature of concrete should not exceed 38 Degree Celsius. In normal conditions, avoid concrete pouring under water. If it is important, then make possible arrangement of equipment’s that are used for concreting under water.
Concreting under water should be completed simultaneously and avoid joint between two concrete layers. Smooth the top layer, whenever concrete pouring is finished. If concreting is not completed in one go, then rough the top layer before it gets harder. This makes a strong joint with the new layer whenever it is casted. Clean the first layer before pouring the new layer of concrete.