TECHNIQUES AND METHODS OF DAMP PREVENTION

The following precautions should be taken to prevent the dampness in buildings before applying the various techniques and methods described later:

1. The site should be located on a high ground and well-drained soil to safeguard against foundation dampness. It should be ensured that the water level is at least 3 m below the surface of the ground or lowest point even in the wet season. For better drainage, the ground surface surrounding the building should also slope away from the house or structure.
2. All the exposed walls should be of sufficient thickness to safeguard against rain penetration. If walls are of bricks, they should be made of at least 30 cm thickness.
3. Bricks of superior quality, which are free from defects such as cracks, flaws and lump of limestones, should be used. They should not absorb water more than one-eighth of their own weight when soaked in water for 24 hours.
4. Good quality cement mortar (1 cement:3 sand) should be used to produce a definite pattern and perfect bond in building units throughout the construction work. This is essential to prevent the formation of cavities and occurrence of differential settlement, due to inadequate bonding of units.
5. Cornices and string courses should be provided. Window sills, coping of plinth and string courses should be sloped on top and throated on the underside to throw the rainwater away from the walls.
6. All the exposed surfaces like tops of walls and compound walls should be covered with waterproofing cement plaster.
7. Hollow walls (i.e., cavity walls) are more reliable than solid walls in preventing dampness and, hence, the cavity wall construction should be adopted wherever possible.

23.3.1 Prevention of dampness

The various techniques and methods, generally adopted to prevent the defects of dampness, are as follows:

1. Use of damp-proofing courses (DPC) or damp-proofing membranes.
2. Waterproof or damp-proof surface treatments.
3. Integral damp-proofing treatments.
4. Cavity walls or hollow walls.
5. Guniting or shot concrete, or shotcrete.
6. Pressure grouting or cementation.

23.3.1.1 Use of damp-proofing courses

These are the layers or membranes of water repellent materials, such as bituminous felts, mastic asphalt, plastic sheets, cement concrete, mortar, metal sheets, slates and stones, which are interposed in the building structures at all locations wherever water entry is anticipated or suspected. These damp-proof courses of suitable materials should be provided at appropriate locations for their effective use. Basically, DPC is provided to prevent the water from rising from the sub-soil or ground and getting into the different parts of the building. The best location or position for DPC, in case of buildings without basements, lies at the plinth level or, in case of structures without plinth, it should be laid at least 1.5 cm above the ground level. These damp-proof courses may be provided horizontally or vertically in floors, walls, etc. In the case of basements, laying of DPC is known as ‘tanking’.

23.3.1.2 Waterproof (or damp proof) surface treatments

The surface treatment consists of filling up the pores of the material exposed to moisture by providing a thin film of water repellent material over the surface. The surface treatments can be either external or internal; the external treatment is effective in preventing dampness whereas internal only reduces it to a certain extent.

Many surface treatments like pointing, plastering, painting and distempering are given to the exposed surfaces and also to the internal surfaces. The most commonly used treatment to protect walls against dampness is lime cement plaster of (1 cement:1 lime:6 sand) mix proportions. A thin film of waterproofing material can be applied to the surface of concrete after it is laid. Some of the materials generally employed as waterproofing agents in surface treatments are sodium or potassium silicates, aluminium or zinc sulphate, barium hydroxide and magnesium sulphate in alternate applications, soft soap arid alum also in alternate applications, lime and linseed oil, coal tar, bitumen, waxes and fats, resins and gums, etc.

23.3.1.3 Integral damp-proofing treatment

The integral treatment consists of adding certain compounds to the concrete or mortar during the process of mixing, which when used in construction act as barriers to moisture penetration under different principles. Compounds like chalk, talc and fuller’s earth have a mechanical action principle, i.e., they fill the pores present in the concrete or mortar and make them denser and waterproof. The compounds like alkaline, silicates, aluminium sulphate and calcium chlorides work on a chemical action principle, i.e., they react chemically and fill in the pores to act water resistant. Similarly, some compounds like soaps, petroleum oils and fatty acid compounds such as stearates of calcium and sodium ammonium work on a repulsion principle, i.e., they are used as admixtures in concrete to react with it and become water repellent.

23.3.1.4 Cavity walls (or hollow walls)

A cavity wall consists of two parallel walls or leaves or skins of masonry separated by a continuous air space or cavity. Cavity walls consist of three main parts, namely,

1. The outer wall or leaf (10 cm thick) which is the exterior part of the wall.
2. The cavity or air space of 5–8 cm.
3. The inner wall or leaf (minimum 10 cm) which is the interior part of the wall.

The two leaves, forming a cavity in between, may be of equal thickness or the thickness of the inner leaf may be increased to take the greater proportion of the imposed loads transmitted by the floor and roof. The provision of continuous cavity in the wall efficiently prevents the transmission or percolation of dampness from the outer wall or leaf to the inner wall or leaf. Based on the climatic conditions in India, i.e., hot dry (hot humid), this cavity type of construction is most desirable as it offers many advantages such as better living and comfort conditions, construction economy and preservation of the building against dampness.

The cavity wall construction offers the following advantages over solid wall construction:

1. As there is no contact between the outer and inner walls of a cavity wall except at the wall ties, which are of impervious material, the possibility of moisture penetration is reduced to a minimum. It has been established that a cavity wall having 10 cm thick internal and external leaves with 5 cm cavity or air space in between is better and more reliable than a solid wall of 20 cm thickness, in respect of damp penetration.
2. As air in the cavity is a non-conductor of heat, it prevents the transmission of heat through the walls and maintains better consistency of temperature inside the building. In this regard, it has been established that cavity walls provide an improvement of 25 per cent in heat insulation over the solid walls of the same cross section less the cavity thickness. Therefore, cavity wall construction is best suited for a tropical country like India.
3. The cavity walls also offer good insulation against sound.
4. The cavity tends to reduce the nuisance of efflorescence.
5. This type of construction also offers many other benefits such as economy, better comfort and hygienic conditions in buildings.

23.3.1.5 Guniting (or shot concrete)

This consists in forming an impervious layer using a rich cement mortar (1 cement:3 sand or fine aggregate mix) for waterproofing over the exposed concrete surface or over the pipes, cisterns, etc., for resisting the water pressure.

Gunite is a mixture of cement and sand or well-graded fine aggregate, the usual proportions being 1:3 or 1:4 (i.e., 1 cement:(3 or 4) sand or fine aggregate). A machine known as cement gun, having a nozzle for spraying the mixture and a drum of compressed air for forcing this mixture under desired pressure, is used for this purpose. Any surface, which is to be treated, is first thoroughly cleaned of any dirt, grease or loose particles and then fully wetted. The mixture of cement and sand or aggregates is then shot under a pressure of 2-3 kg/cm² by holding the toe nozzle of the cement gun at a distance of 75–90 cm from the surface of the wall. The necessary quantity of water is added by means of a regulating valve soon after the mixture comes out from the cement gun. Thus, the mixture of desired consistency and thickness can be sprayed or deposited to get an impervious layer. This impervious surface should be watered for at least 10 days.

By this technique, an impervious layer of high compressive strength (560–700 kg/cm² after 28 days) is obtained and, hence, this is also very useful for reconditioning or repairing old concrete works and brick or masonry works, which have deteriorated either due to climatic effects or inferior workmanship.

23.3.1.6 Pressure grouting (or cementation)

Cementation is the process or technique of forcing the cement grout (i.e., mixture of cement, sand and water) under pressure into the cracks, voids or fissures present in the structural component or ground. That is, all the components of a structure in general and foundations in particular, which are liable to moisture penetration, are consolidated and, hence, made water resistant by this cementation process.

In this process, holes are drilled at selected points in the structure and the cement grout, of sufficiently thin consistency, is forced under pressure to ensure complete penetration into cracks or voids. This makes the structure watertight and restores its stability and strength to some extent.

Similarly, when the structure is resting on hard but loose textured ground, this process can increase its strength. For this, pipes are driven into the ground, cores within the pipes are removed by means of earth auger and finally the grout is forced (pumped) into the ground to fill the voids, loose pockets, etc.

This technique is also used for repairing structures, consolidating ground to improve bearing capacity, forming water cut-offs to prevent seepage, etc.