For certain types of works like water storage tanks, reservoirs, basement walls, roofs, swimming pools, sewage units, etc., the impermeability of concrete is absolutely essential. In general, it can be stated that if concrete is made dense and free from cracks, it is watertight. The dense concrete can be obtained by closely adhering the following essentials:
(1) Using a high class Portland cement of guaranteed quality.
(2) Taking extreme care to adopt correct grading and proportioning of the sand, aggregate and cement.
(3) Using clean and non-porous aggregates.
(4) Mixing thoroughly to the right consistency using the right amount of water.
(5) Placing, tamping and curing carefully.
(6) Making use of suitable water-proofing compound.
The cement concrete to a certain extent may be made impermeable to the water by using hydrophobic cement. All the flat roofs in the modern age are generally constructed of R.C.C. It becomes necessary to give some treatment of water-proofing to such roofs.
Following are the four methods adopted for water-proofing of R.C.C. flat roofs:
(2) Bedding concrete and flooring
(3) Mastic asphalt and jute cloth
(4) Use of water-proofing compounds.
(1) Finishing: For ordinary buildings of cheap construction, the finishing of roof surface is done at the time of laying cement concrete. The finishing of flat roof is carried out in cement mortar or proportion 1:4 i.e. one part of cement to four parts of sand by volume.
(2) Bedding concrete and flooring: In this method, the surface of R.C.C. slab is kept rough and on this surface, a layer of concrete is laid. The concrete may be brickbats lime concrete or brickbats cement concrete. The thickness of the concrete layer is about 100 mm. The surface of the bedding concrete is provided by a suitable flooring such as tiles, terrazzo, Indian patent stone
(i) It corrects a badly proportioned concrete mixture.
(ii) It cures immature green concrete.
(iii) It makes good concrete from the poor materials.
(iv) It permits less rigid supervision of the workmanship.
The concrete can be made coloured in the following ways: