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close this bookCarpentry for Vocational Schools - A Teachers Handbook
close this folder3. CONCRETE
View the document(introduction...)
View the document3.1. Composition of concrete
View the document3.2. Proportions for mixing concrete
View the document3.3. Reinforcing concrete
View the document3.4. Finishing concrete

3.1. Composition of concrete

The concrete consists of Cement, Fine aggregate. Rough aggregate and Water. Concrete is used to make concrete slabs, wall foundation, septic tanks etc...

a) Cement: The most commonly used cement is Portland Cement. Cement itself is a grey powder which is made up of limestone, clay and marl. In the cement factory the raw material (limestone, clay, marl) are crushed. Afterwards these go through the raw mill for simultaneous grinding and drying of the raw material. If this process is done the raw material becomes raw meal and the raw meal is ready now for the rotary kiln, where the raw meal is fired. From the rotary kiln, the next step is the clinker cooler, to cool down the cement. At this step the raw material is already a cement and is packed in 50 kg paper sacks for sale.


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b) Fine Aggregate: Fine aggregate or sand consists of small grain, small pebbles or particles of crushed stone that will pass through a 6.350 mm wire mesh screen. The sand should be clean, hard, sharp and well graded. Well graded aggregate means aggregate which is not uniform in size.


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c) Coarse Aggregate; Coarse aggregate can be stone, crashed stone or gravel. Its depend on the project which coarse aggregate is suitable. Coarse aggregate should be clean, hard and free from harmful amounts of vegetable matter, loam or clay.


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The ideal composition of coarse aggregate is as follow:


30% coarse = 25 - 32 mm in size


30% medium = 16 - 25 mm in size


20% fine = 8 - 16 mm in size


20% very fine = 4 - 8 mm in size

The largest particles should not be more than one quarter the thickness of the wall or slab.


Figure