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6 Dampness a~d Damp-Proofing General The moisture present in the ground or rain water may penetrate into the building from below, from sides and ,also sometimes from the top of its roof, causing the structure to remain in damp condition and thus deteriorating the structure as a whole. It is, therefore, very essential to prevent any moisture penetrating into a building. Now-a-days, care is taken while designing and constructing any building so thaf dampness can be prevented from affecting it and its inhabitants. In this chapter, all the important topics concerning Dampness and Damp-proofing have been discussed in order to provide adequate knowledge to the students / readers about this important aspect in building construction. 6.1. DAMPNESS The access or penetration of moisture contents inside a building through its walls,floors or roof is known as dampness. Dampness is not only injurious to buildings but it affects adversely the health of their inhabitants. It becomes, therefore, essential to know the causes of dampness and methods of its prevention so that the buildings can be constructed damp-proof. 6.1.1. III effects of dampness. Following are the common ill or harmful effects of dampness in a building: (i) It causes dry rot to the wooden members provided in the building. (ii) It causes corrosion of metals used in the construction of a building. (iii) It causes peeling otl and removal of the plaster. (iv) It causes the paints to get blistered and bleached, and the surface I thu.sgets disfigured. (v) It causes floors of the building to remain ugly since they cannot be cleaned well. (vi) Carpet, if used on floor of a damped building, gets destroye~ I earlier. J (vii) All electric installations get deteriorated. (viii) It causes efflorescence which affects the exposed surface of the brickwork to disintegrate and fall to powder. J (ix) It reduces the life of the structure as a whole. . - ( (x) It causes unhygienic conditions for the occupants of.the building and affects adversely their health. 6.2. CAUSES OF DAMPNESS Following are the causes of dampness in a building: V Rain penetration; 21Level of the site; 3yDrainability of the soil; <kClimatic conditions; ~ Defective orientation; . 6./ Moisture entrapped during construction; ¥I. Defective materials; ~ Defective construction; 9:' Moisture which originates in the building itself. 6.2.1. Rain penetration. It is considered as the chief source of damp- ness. Some buildings can withstand heavy rains for a short duration whereas they fail to withstand continuous mild showers for a number of days. Pro- perly constructed walls offer considerable resistance to penetration of the moisture but its rapid penetration takes place through the joints and porous brick or stones. Rain penetration is also possible through the roof compo- nents,cracks and the join~s between walls and the roof. 6.2.2. Level of the site. The elevation of the building site has also effect on dampness. Structures, built on a higher ground, can be drained off easily and hence they are less liable to dampness. But low laying area cannot be easily drained off and causes dampness in the structures. 6.2.3. Drainability of the soil. The drainability of the soil on which the building is constructed is an important factor for causing the dampness or its preventi~n. Gravel and sandy soil allow water to pass through ve~y easily, whereas clayey soil retains moisture and also causes dampness due to capil- lary rise. , 6.2.4. Climatic condition. Dampness is also caused due to condensation of the moisture present in the atmosphere under very cold climate. Conden- sation of atmospheric rnoisture can often be identified by the drops of mois- ture present on the ceilings walls, floors, etc. ,~2.5. Defective orientation. The building having its walls subjected to constant splashes of rains or getting less direct sun rays due to defective orientationis liable to dampness. '6.2.6. Moisture entrapped during construction. Walls, while being constructed are in wet conditions. These may persist moisture for a long period after the construction is over due to the use of salty or alkaline water whichcauses dampness in the building,. 6.2.7. Defective materials. Dampness is also caused due to soakage of moistureby the defective materials like porous bricks, soft stones, etc. espe- "dallywhen they are used in external walls. 6.2.8. Defective construction. Defective construction of various partsI such as joints between parapet and the roof copings, iron fixtures, etc. in aI building causes dampness by transmitting rain water. t 6.2.9. Moisture which originates in the building itself. In case there iso any leakage in sewers, down water pipes, etc., in kitchens or bath rooms, it~ will be causing dampness in the building. 6.3. METHODS OF PREVENTING DAMPNESS a Thefollowing methods are generally employed for preventing dampness~ in a building: 1 1. By providing a damp-proof course; tl C 2. By surface treatment; S 3. By integral water-proof construction; 4. .By special devices. IY 6.3.1. By providing a damp-proof course. The continuous layer of an . impervious material, which is provided in between the sDurce of dampness and part of the structure is called a damp-proof course. The function of providing a damp-proof course is to prevent dampness and to maintain dry and sanitary conditions in the interior of a building. Damp-proof courSe (D.P.C.) is laid either horizontally or vertically. In practice, horizontal D.P.C. is generally laid 150 to 200 mm above the ground level on the external walls and at plinth level in case of internal walls of a. building (see Figs. 6.1 and 6.2). '. Vertical D.P.C. is provided to connect the horizontal D.P.C. in the exter-,t nal walls with the floor level to check the flow of moisture from the under- side of a floor into the walls. Vertical D.P.C. is also provided to prevent the dampness into the walls of the basement from the adjacent soil as shown inn Fig. 6.5. Method of laying damp-proof course. Horizontal D.P.C. is generallyla laid by spreading a layer of cement concrete (1 : 2 : 4) of specified thickness (generally 40 mm) on the top of the wall either at 150 to 200mm above the' ground level in case of external walls or at plinth level on internal walls. On the top of concrete layer, two coats of hot bitumen, at the rate of 1.75 kg/m2, are provided. Horizontal D.P.C. can also be laid by spreading bitumen felt over the top of concrete layer. Sometimes, sheets of lead, copper or aluminium are.S also laid on the top of the wall at specified level for this purpose. 'Iertical D.P.C. can be laid by applying 20 mm thick cement plasteree (1 : 3) on the surface. Sometimes, either two coats of hot bitumen are provided on the plastered surface or any other damp-proofing material is added in the cement plaster during its preparation. ... 6.3.2. By surface treatment. This method consists in painting the ex- :s posed surface of the wall with a water repellant paint. Material to be used for a painting the wall surface must be durable and effective to check the absorp- tion of moisture by the wall. Moreover, it must not disfigure the appearance s of the structure. Surface treatment can also be done by erecting 'Thatties' [t (consisting of grass fixed to split up bamboo mat and painted with tar) over the ext~rnal walls during rainy season. 6.3.3. By integral water-proof constructiou. The method consists of adding certain compound in the mortar or concrete to make it more dense by Slfilling the pores through chemical action or mechanical effect. "Ironfillings, if added, react chemically and fill the pores. If 5% sunlight soap is added in the water to be used for preparing the mortar, the pores get clogged and a coating of water repellant substance sticks to the wall surface which makes it sufficiently damp-proof. 6.3.4. By special devices. Dampness can be prevented to a large extent by using special devices which are given below: ~ (i) By constructing the external walls of sufficient thickness. " (ii) By using the bricks of good quality for constructing the external walls. (iii) By building the walls in rich cement mortar. (iv) By providing string courses and cornices. (v) By fixing down-water pipes in sufficient number so that water may not leak through the junction of wall and the roof. 6.4. DAMP-PROOFING MATERIALS The materials used for damp-proofing purposes in buildings and in other engineering structures are called damp-proofing materials. There are many damp proofing materials used for preventing dampness in different parts of a building or any otl1erengineering structure. 6.4.1. Requirements of a good damp-proofing material. A good damp-proofingmaterial should fulfil the following requirements: (i) It should be impervious to mC)isture. (ii) It should be durable. (iii) It should not disintegrate unoer loading. (iv) It should not move sideways under loading. (v) It should provide unifonn and homogeneous layer. (vi) It should cover full thickness of the wall. (vii) It should be cheaply and easily available. 6.5. CLASSIFICA nON OF DAMP-PROOFING MATERIALS The various damp-proofing materials used in buildings and other engi. neering structures are broadly classified into thefollowing two categories .- I. Flexible materials; 2. Rigid materials.
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