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:
(I) 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 3m. below the surface of ground or lowest point even in the wet season. For better drainage the ground surface surrounding the building should also slope away.
(II) All the exposed walls should be of sufficient thickness to safe guard against rain penetration. If walls are of bricks they should be at least 30 cm thickness
(III) Bricks of superior quality which are free from defects such as cracks, flaws, lump of lime stones should be used. They should not absorb water more than 1/8 of their own weight when soaked in water for 24 hours.
(IV) Good quality cement mortars 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 cavities and occurrence of differential settlement.
(V) Cornices and string courses should be provided. Window sills, coping of plinth and string courses should be slopped on top and throated on the undesirable to throw the rain water away from walls.
(VI) All the exposed surfaces should be covered with waterproofing cement plaster
(VII) Hollow walls are more reliable than solid walls in preventing dampness and hence the cavity wall construction should be adopted wherever possible.
Prevention of dampness
(1). Use of damp proofing courses or membranes-
These are the layers or membranes of water repellent material such as bituminous felts, mastic asphalts, plastic sheets, cement concrete, mortar, metal sheets which are interposed in the building structure at all location wherever water entry is anticipated. These damp proof courses of suitable materials should be provided at appropriate location for their effective use. Basically D.P.C is provided to prevent the water rising from the sub soil and getting into the different part of the buildings. The best location for D.P.C in case of buildings without basement lies at the plinth level or in case of structure without plinth should be laid at least 15 cm above the ground. These damp proof courses may be provide horizontally or vertically in floors, walls etc. in case of basement laying of D.P.C is known as tanking.
While providing damp-proof courses in buildings, the following general principles should be observed in practice.
• The DPC should cover the full thickness of the walls excluding rendering, in order to act as an effective barrier to moisture under all conditions.
- The mortar bed upon which the DPC is laid should be level, even and free from any projections.
- The DPC course should be placed in correct relation with other DPC courses so as to provide a complete course should be placed in correct relation with other DPC courses so as to provide a complete and continuous barrier to the passage of moisture from below, top or sides. Therefore, the junctions and corners, formed by walls, or walls and floors, should be laid continuous.
- Where a vertical DPC is to be laid continuous with a horizontal DPC(i.e., forming angle projection), a fillet 75mm in radii’s should be provided. The DPC should not be exposed on the wall surface; otherwise it is likely to be damaged by car painters, tile layers, etc.
(2) Waterproof (or damp proof) surface treatment
The surface treatment consists in filling of the pores of the material exposed to moisture by providing a thin film of water repellent material over the surface. These surface treatments can be either external or internal, the external treatment is effective in preventing dampness where as internal one only reduces it to a certain extent.
Many surface treatments like pointing, plastering. Painting, distempering, are given to the exposed surfaces and also to the internal surfaces. Most commonly used treatments, to protect the walls against dampness, is lime cement plaster of mix (one cement: one lime: six sand) proportions. A thin film of water proofing can be materials, generally employed as waterproofing agent in surface treatments are: sodium or potassium silicates, aluminum or zinc sulphates, barium hydroxide and magnesium sulphate in alternate applications, soft soap and alum also in alternate applications, lime and linseed oil, coal tar, bitumen, waxes and fats, resins, and gum, etc.
Some of the above mentioned materials, like the waxes and fats, are unsuitable in the tropics as they melt with rise in temperature, resins and gums and also not lasting materials are coal tar and bitumen disfigure the original surface.
(3) Integral damp-proofing treatment
The integral treatment consists 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, fuller’s earth, and etc.have mechanical action principle, i.e., they fill the pores present in the concrete or mortar and make them denser and water proof. The compounds, like alkaline, silicates, aluminum sulphates, calcium chlorides, etc.work on chemical action principle i.e., they react chemically and fill in the pores to act as water resistant. Similarly , some compounds like soaps, petroleum oils, fatty acid compounds such as stearates of calcium,sodium ammonium etc.work on repulsion principle i.e., they are used as admixtures in concrete to react with it and become water repellent. The synthetic compound prepared under this principles are available in commercial forms, like Pudlo, Sika, Novoid, Ironite, Dampro, Permo Rainers,etc.
(4) Cavity walls
A cavity wall consists of two parallel walls/leaves/skins of masonry, separated by a continuous air space/cavity. They consist of three parts.
- Out wall/leaf (exterior wall part 10cm thick)
- Cavity/air space (5cm-8cm)
- Inner wall/leaf (minimum 10cm thick)
The two leaves forming a cavity in between may be of equal thickness or may not be. The inner wall thickness may more to take larger properties of imposed loads transmitted by floor and roof.
Provision of continuous cavity in the wall efficiently prevents the transmission of dampness from outer to inner wall.
Under climatic conditions of India (hot-dry/hot-humid), cavity type construction is most desirable as it offers many advantages such as better living and comfort conditions, economic construction and preservation of buildings against dampness.
- As there is no contact between outer and inner walls of a cavity wall except at wall ties, which are of impervious material, so possibility of moisture penetration is reduced to a minimum.
- It has been verified a cavity wall of 10cm thick internal and external walls with 5cm cavity/air space in between is better or more reliable than solid wall of 20cm thickness w.r.t damp prevention.
- The cavity wall offers good insulation against sound.
- It reduces the nuisance of efflorescence.
- It offers other advantages like,
- Better comfort
• Hygienic conditions in buildings.
(5) Shot concrete (gunting)
This consists in forming an impervious layer of rich cement mortar(1:3) for water proofing over the exposed concrete surface or over the pipes, cisterns, etc. for resisting water pressure. Gunite is a mixture of cement and sand on well graded fine aggregate, the usual proportion being 1:3 or 1:4. A machine known as cement gun, having a nozzle for spraying the mixture and a drum of compressed air for forcing the 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 mix of cement and sand is then shot under a pressure of 2-3kg/sq.cm by holding the nozzle of cement gun at a distance of 75-99 cm from wall surface. The necessary quality of water is added by means of regulating valve soon after the mixture comes out from cement gun. So mix of desired consistency and thickness can be sprayed, to get an impervious layer, the impervious surface should be watered for about 10 days.
By this technique impervious layer of high compressive strength can be obtained (28 days strength) and so it is useful method for reconditioning/repairing old concrete works, bricks and masonry works, which have deteriorated.
(6) Pressure grouts (cementation)
Cementation is the process of forcing the cement grout (mix of cement, sand, water) under pressure into cracks, voids, fitters present in structural components/ground. All the components of a structure in general and foundation, which are liable to moisture penetration are consolidated and so made water resistant by this process.
Here heels are drilled at selected points in structure and cement grout of sufficiently thin consistency is forced under pressure to ensure complete penetration onto cracks. This makes structure water tight and restores stability and strength.
When structure is resting on hard but loose textured ground its strength can be increased, by this process. This technique is used for repairing structures, consolidator ground to improve bearing capacity forming water cut offs to prevent seepage.
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