Basics of Building Construction

Basics in Building Construction 4(2+2)

Lesson 04 : Damp Proofing : Methods And Materials

Methods Of Damp Proofing

Methods generally adopted to make a building damp proof are

1. Membrane damp proofing

   In this method of damp proofing a water repellent membrane or damp proof course(D.P.C.) is introduced in between the source of dampness and the part of building adjacent to it. Damp proofing course may consist of flexible materials such as bitumen, mastic asphalt, bituminous felts, plastic or polythene sheets, metal sheets, cement concrete. Damp proofing course may be provided either horizontally or vertically in floors, walls etc. Provision of Damp Proofing Course in basement is normally termed as ‘Tanking’. The general principles to be followedwhile providing damp proof course are:

   1. The damp proofing course should cover the full thickness of walls, excluding rendering.
   2. The mortar bed supporting damp proof course should be leveled and even, and should be free from projections, so that damp proof course is not damaged.
   3. Damp proof course should be laid in such a way that a continuous projection is provided.
   4. At junctions and corners of walls, the horizontal damp proof course should be laid continuous.
   5. When a horizontal damp proof course (i.e. that of a floor) is continued to a vertical face, a cement concrete fillet of 7.5 cm radius should be provided at the junction.
   6. Each damp proof course should be placed in correct relation to other damp proof course, so as to ensure a complete and continuous barrier to the passage of water from floors, walls or roof.
   7. Damp proof course should not be kept exposed on the wall surface otherwise it may get damaged during finishing work.


2. Integral damp proofing

   In the integral damp proofing method certain water proofing compounds are added to the concrete mix, so that it becomes impermeable. The common water proofing compounds may be in the following three forms.

   1. Compounds made from chalk, talc, fullers earth, which may fill the voids of concrete under the mechanical action principle.
   2. Compounds like alkaline silicates, aluminum sulphate, calcium chlorides, etc. which react chemically with concrete to produce water proof concrete.
   3. Compounds like soap, petroleum, oils, fatty acid compounds such as stearates of calcium, sodium, ammonia etc. work on water repulsion principle. When these are mixed with concrete, the concrete becomes water repellent.
   4. Commercially available compounds like Publo, Permo, and Silka etc.

   The quantity of water proofing compound to be added to cement depends upon the manufacturer’s recommendations. In general one kilogram of water proofing compound is added with one bag of cement to render the mortar or concrete water proof.

3. Surface treatment

   Moisture finds its way through the pores of material used in finishing. In order to check the entry of the moisture into the pores, they must be filled up. In the surface treatment method a layer of water repellent substances or compounds are applied on these surfaces through which moisture enters. The use of water repellent metallic soaps such as calcium and aluminum oletes and stearates are much effective against rain water penetration. Pointing and plastering of the exposed surfaces must be done carefully, using water proofing agents like sodium or potassium silicates, aluminum or zinc sulphates, barium hydroxide and magnesium sulphates etc. Surface treatment is effective only when the moisture is superficial and is not under pressure. Sometimes, exposed stone or brick wall face may be sprayed with water repellent solutions. The walls plastered with cement, lime and sand mixed in proportions of 1:1:6 is found to serve the purpose of preventing dampness in wall due to rain effectively.

4. Cavity wall construction

   Cavity wall construction is an effective method of damp prevention. In this method the main wall of a building is shielded by an outer skin wall, leaving a cavity between the two. The cavity prevents the moisture from travelling from the outer to the inner wall.

5. Guniting

   In this method of damp proofing, an impervious layer of rich cement mortar is deposited under pressure over the exposed surfaces for water proofing or over pipes, cisterns etc. for resisting the water pressure. The operation is carried out by use of a machine known as cement gun. The cement gun consists of a machine having arrangements for mixing materials and a compressor for forcing the mixture under pressure through a 50 mm dia flexible hose pipe. The hose pipe has nozzle at its free end to which water is supplied under pressure through a separate connection. The surface to be treated is first thoroughly cleaned of dirt, dust, grease or loose particles and wetted properly. Cement mortar consists of 1: 3 cement sand mix, is shot on the cleaned surface with the help of a cement gun, under a pressure of 2 to 3 kg/cm2. The nozzle of the machine is kept at a distance about 75 to 90 cm from the surface to be gunited. The mortar mix of desired consistency and thickness can be deposited to get an impervious layer. The layer should be properly cured at least for 10 days. Since the material is applied under pressure,it ensures dense compaction and better adhesion of the rich cement mortar and hence the treated surface becomes water proof.

6. Pressure grouting

   This consists of forcing cement grout under pressure, into cracks, voids, fissures and so on present in the structural components of the building, or in the ground. Thus the structural components and the foundations which are liable to moisture penetration are consolidated and are thus made water-penetration-resistant. This method is quite effective in checking the seepage of raised ground water through foundations and sub-structure of a building: