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:
- Using a hight class Portland cement of guaranteed quality.
- Taking extreme care to adopt correct grading and proportioning of the sand, aggregate and cement.
- Using clean and non-porous aggregate.
- Mixing thoroughly to the right consistency using the right amount of water.
- Placing, tamping and curing carefully.
- 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 waterproofing to such roofs.
Methods of waterproofing concrete
Following are the four methods adopted for waterproofing of R.C.C. flat roofs:
- Bedding concrete and flooring
- Mastic asphalt and jute cloth
- Use of water-proofing compounds
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 of proportion 1:4 i.e. one part of cement to four parts of sand by volume.
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 (1:2:4) or brickbats cement concrete (1:8:14). The thickness of the concrete layer is about 100mm. The surface of the bedding concrete is provided by suitable flooring such as tiles, terrazzo, Indian patent stone, etc. A convex joint is provided at the junction of parapet wall and roof.
Mastic asphalt and jute cloth
In this method, a layer of hot mastic asphalt is laid on the roof surface. The jute cloth is spread over this layer. Then one more layer of mastic asphalt is applied so that the jute cloth is sandwiched between the two layers of mastic asphalt. The sand is then sprinkled over the entire surface of roof. For better grip, the lead sheets are inserted at the junction of parapet wall and roof.
Uses of waterproofing compounds
Some of the waterproofing compounds like Pudlo, Impermo, etc. are available in the market and when such a compound is added to the cement during construction, it prevents seepage, leakage and damp caused by the capillary absorption of the moisture in cement, mortar and concrete. The quantity of water-proofing compound to be added is also very small, say 2% and thus a bag of cement will require only about 10N of such compound.
The water-proof compounds are available in the powder form and they are to be mixed thoroughly with cement by hand before the cement is mixed with the aggregate.
Advantages of waterproofing concrete
The advantages claimed by using a waterproofing compound of good quality are as follows:
- It corrects a badly proportioned concrete mixture.
- It cures immature green concrete.
- It makes good concrete from the poor materials.
- It permits less rigid supervision of the workmanship.