Damp proofing
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This document discusses causes, effects, and methods of preventing dampness in buildings. It outlines several precautions that should be taken such as proper site drainage and wall thickness. Common causes of dampness include rising moisture, rain penetration, and poor drainage. Effects include breeding mosquitoes and damage to building materials. Methods of damp proofing discussed are damp proof courses, waterproof surface treatments, integral treatments during construction, cavity walls, and cement grouting of cracks. Specific materials used for damp proof courses like bitumen and mastic asphalt are also outlined.
Damp proofing
- 1. Dampness • Presence of hygroscopic or gravitational moisture • Reduce strength and lead to unhygienic conditions • Damp prevention is therefore one of the important factors for building design.
- 2. Causes of dampness • Moisture rising up the walls from ground • Rain travel from wall tops • Rain beating against external walls • Poor drainage at the building site • Defective construction
- 3. Effects of dampness • Breeding of mosquitoes • Moisture cause unsighty patches, softening of plaster • May cause efflorescence; disintegration of stones, bricks • Cause rusting and corrosion of metal fittings • Floor coverings are damaged
- 4. Precautions • 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
- 5. 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 should be provided. Window sills, coping of plinth should be slopped on top and throated on the undesirable to throw the rain water away from walls.
- 6. (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.
- 7. Methods of damp proofing 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 • 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
- 10. Mastic asphalt
- 11. 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. • 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 materials, generally employed as waterproofing agent in surface treatments are : sodium or potassium silicates, aluminium or zinc sulphates.
- 12. 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
- 13. 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 10 cm thick) Cavity/air space(5cm-8cm) Inner wall/leaf(minimum 10 cm thick) • The two leaves forming a cavity in between may be of equal thickness or may not be. • 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.
- 14. 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 Economy and Better comfort
- 16. Shot concrete(gunting) • This consists in forming an impervious layer of rich cement mortar(1:3) for water proofing over the exposed concrete surface for resisting water pressure. • Gunite is a mixture of cement and sand, 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. • 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 2-3 kg/sq.cm by holding the nozzle of cement gun at a distance of 75-99 cm from wall surface. • The impervious surface should be watered for about 10 days.
- 18. 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.
- 19. Material Used for DPC: • Hot bitumen: – This is highly flexible material, which can be applied with a minimum thickness of 3 mm. It is placed on the bedding of concrete or mortar, while in hot condition. • Mastic asphalt: – Mastic-asphalt is semi-rigid material which is quite durable and completely impervious. It is obtained by heating asphalt with sand and mineral fillers. However, it should be laid very carefully by experienced persons.
- 20. • Bituminous felts: – This is a very flexible material which is available in rolls of various wall thickness. It is laid on a leveled flat layer of CM. An overlap of 10 cm is provided at joints and full width overlap is provided at angles, junctions and crossings.
- 21. • Metal sheets: – sheets of lead, copper, aluminium can be used as DPC. Lead sheets are quite flexible and thickness should be such that its weight is not less than 20 kg/m2. They are laid similar as bituminous felts. – Copper sheets are of 3 mm thick and embedded in lime or cement mortar. It has high durability, resistance to dampness, sliding, etc. – Aluminium sheets are used with protective layer of bitumen and not as good as other two.
- 22. • Combination of sheets and bituminous felts: – Lead foil sandwiched between asphaltic or bituminous felts can effectively used as DPC and combination is known as lead core.
- 23. • Bricks: – Special bricks, having water absorption not less than 4.5 % of their weight may be used as DPC in locations where dampness is not excessive. • Stones: – Dense and sound stones, such as granite, trap, slates, etc. are laid in CM in two courses to form effective DPC • Mortar: – CM (1:3) is used as bedding layer for housing other DPC materials.
- 24. • Cement concrete: – Cement concrete 1:2:4 or 1:1.5:3 is generally provided at plinth level to work as DPC. The thickness may vary from 4 to 15 cm. Such a layer can effectively check the water rise due to capillary action. • Plastic sheets: – This is relatively new type of DPC material, made up from black polythene, 0.5 to 1 mm thick in usual walling width and roll length of 30 m. The treatment is cheaper but not permanent.
- 25. DPC Treatment in Buildings: • Treatment to foundations against gravitational water: – Foundation may receive water percolating from adjacent ground, and this moisture may rise in the wall. – This can be checked by providing air drain parallel to the external wall. – The width of air drain may be about 20 to 30 cm. The outer wall of the drain is kept above the ground to check the entry of surface water. – A RCC roof slab is provided. Openings with gratings are provided at regular interval, for passage of air.
- 27. Treatment to basements: • Provision of Foundation drains and DPC • Provision of RCC raft and wall slab • Asphalt tanking
- 28. • Treatment to basements: – Provision of foundation drain and DPC: • When basement rests on soil which are not properly drained, great hydrostatic pressure is exerted and the floor as well as wall receive water continuously. • In such a case it becomes necessary to make a trench all round, up to foundation level and fill it with gravel, coke and other pervious materials. • Open jointed drains may be provided to collect the under ground water or drainage pipes embedded in gravel bed may also be provided before foundation concrete.
- 30. – Provision of RCC floor and wall slab: • Where underground water pressure is severe, the drainage system may not solve the problem effectively. Also, constant pumping out water may be costly. • In such case, floor slab as well as walls may be constructed in rigid RCC structure. Horizontal and vertical DPC treatment is also provided. Atleast, 3 layers of bituminous felts are used as DPC • Half brick thick wall protective wall provided at outer face of RCC wall slab.
- 32. Asphalt tanking o Subsoil water table is not very high. o Horizontal DPC in the form of asphalt layer of 30 mm thick in three coats over the entire area of basement floor and extending it in the form of 20 mm thick vertical DPC on the external face of basement walls o A 1 ½ brick thick outer wall is constructed o Vertical DPC 15 cm from ground o Layer of flat bricks on foundation concrete (1:3:6) is provided to protect DPC from damage during floor construction slab
- 33. Asphalt Tanking for Basement
- 34. • Treatments to floors: – for location where ground moisture is not present, subsoil is rammed well and a 7.5 to 10 cm thick layer of coarse sand is spread over the entire area under flooring. – Alternatively stone soling may first be provided and then 7.5 to 10 cm thick layer of lean cement concrete (1:3:6 or 1:4:8) may be provided over it and over this base, flooring may be laid. – In a damp soil, where water table is near GL, it is essential to provide membrane DPC over entire area. – A layer of flat bricks is laid on a cushion of fine sand over DPC to protect it from damage during the construction of floor slab.
- 35. DPC for Flooring
- 36. • Treatments to walls: – For basement wall, a vertical DPC is laid over the external face of wall. – This vertical layer of DPC is laid over the base of water- cement plaster grouted on the external face of the wall. – This vertical DPC is further protected by external protective walls of half brick thickness. – The vertical DPC should be carried atleast up to a level of 15 cm above GL and similarly, horizontal DPC in external wall, extending from the floor is provided atleast 15 cm above GL. – In the internal walls DPC is provided in level with the lower surface of concrete floor.