Damp prevention is a chief requirement to ensure safety of building against dampness.
One of the basic requirements in all the buildings is that structure should be dry as far as possible.
If this is not satisfied it is likely that building may become inhabitable and unsafe from structural point of view.
In order to prevent entry of damp into a building the courses known as damp proofing courses are provided at various levels of entry of damp into a building.
Presently all buildings are given dpc treatment
So dpc prevent entry of moisture from walls floors and basements of a buildings.
The treatment given to roofs of buildings for some cause is called water proofing.
Cause Of Dampness
Responsible causes are one or more of the followings
1. Faulty design of structure
2. Faulty construction / poor workmanship
3. Use of poor quality of material in construction
These causes give rise to an easy access to moisture to the building from different points, such as rain penetration through walls, roofs and floors etc. The moisture entering into the buildings from foundation and roofs travels in different directions further under the effect of capillary action and gravity respectively. The entry of water and its movement in different parts of the buildings are positively due to one or more of the causes listed above
(1) Rising Of Moisture From The Ground
The subsoil or ground on which the building is constructed may be made of soils which easily give an access to water to create dampness in building. Generally the foundation dampness is caused when the building structures are constructed on low lying water logged areas where a sub soil of clay or peat is commonly found through which dampness will easily rise under capillary action unless properly treated.
This dampness further finds its way to the floors, walls etc. through the plinth.
(2) Action Of Rain Water
Whenever the faces of walls are not suitably protected from the exposer to heavy shower of rains, they become the sources of dampness in a structure. Similarly the poor mortar joints in walls and cracked roofs also allow dampness to enter the building structure. Sometimes due to faulty eave courses and eave gutters, the rain water may percolate through the roof coverings
(3) Rain Penetration From Top Of The Wall
All parapet walls and compound walls of the buildings which have not been protected from rain penetration by using dam proof courses or by such measures on their exposed tops are subjected to dampness. This dampness in the buildings is of serious nature and may results in unhealthy living condition or even in structurally unsafe conditions.
(4) Codensation Due To Atmospheric Misture
Whenever the warm air in the atmosphere is cooled it gives rise to process of condensation. On account of condensation the moisture is deposited on the whole area of walls, floors, and ceilings. However the sources of dampness is prevalent only in certain places in India, where very cold climate exist.
(5) Miscellaneous Sourses Or Causes
The various other sources responsible for dampness in buildings are mentioned bellow:-
(a) Poor Drainage Of Site
The structure if located on low lying site causes water logged conditions where impervious soil is present underneath the foundation.
So such structures which are not well drained cause dampness in buildings through the foundations.
(b) Imperfect Orientation
Whenever the orientation of the buildings is not proper or geographical conditions are such that the walls of buildings get less of direct sunrays and more of heavy showers of rains, then such walls become prone to dampness.
(c) Constructional Dampness
If more water has been introduced during construction or due to poor workmanship, the walls are observed to remain in damp condition for sufficient time.
(d) Dampness Due To Defective Construction
Dampness in buildings is also caused due to poor workmanship or methods of construction viz inadequate roof slopes, defective rain water pipe connection, defective joints in roofs in proper connection of walls etc.
Effect Of Dampness
The various effects (indirectly defects), caused due to dampness in buildings are mentioned below
All effects mainly result in poor functional performance, ugly appearance and structural weakness of the buildings.
(a) A damp building creates unhealthy living and working conditions for occupants.
(b) Presence of damp conditions causes efflorescence on building surface, which ultimately may result in the dis-integration of bricks, stones, tiles etc. and hence in the reduction of strength.
(c) It may cause bleaching and flaking of the paint which results in the formation of coloured patches on the wall surfaces and ceilings.
(d) It may result in corrosion of metals used in the construction of buildings.
(e) The material used as floor coverings, such as tiles, are damaged because they lose adhesion with the floor base.
(f) Timber, when in contact with damp conditions, gets deteriorated due to the effects of warping, buckilng and rolling of timber.
(g) All electrical fittings get deteriorated, causing leakage of electric current with the potential danger of a short circuit.
(h) Dampness promotes the growth of termites and hence creates unhygienic conditions in buildings.
(i) Dampness when accompanied by the warmth and darkness, breeds the germs of tuberculosis, neuralgia, acute and chronic rheumatism etc.which sometimes result in fatal diseases.
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 th 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 barrrier 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 radidius should be provided. the DPC should not be exposed on the wall surface, otherwise it is likely to be damaged by carpainters, 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, aluminium 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 abve 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, 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, aluminium 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 consist of two parallel walls/leaves/skins of masonry, separated by a continuous air space/cavity.
They consists 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.
1. 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.
2. 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.
3. The cavity wall offers good insulation against sound.
4. It reduces the nuisance of efflorescence.
5. 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 or1:4. 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, greese 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.