Water and Damp proof

 

One of the essential requirements of a building is that it should be dry. Dampness in building may occur due to bad design, faulty construction and use of poor quality of materials. Dampness not only affects the life of the building adversely, but also creates unhygienic conditions for the occupants. Hence, treatment to check dampness is considered to be one of the important items of work in the construction of a building. The treatment given to prevent leakage of water from roof is generally termed as water proofing, whereas the treatment given to keep the walls, floors and basement dry is termed as damp proofing.

DEFECTS CAUSED BY DAMPNESS

The various defects caused by dampness in buildings may be summarised as under:

·      It causes efflorescence which may ultimately result in disintegration of bricks, stones, tiles etc.

·      It may result in softening and crumbling of plaster.

·      It may result in warping, buckling and rotting of timber.

·      It may lead to the corrosion of metals.

·      It may deteriorate electrical fittings.

·      It promotes growth of termites.

CAUSES OF DAMPNESS

Absorption of moisture by the building materials is one of the chief cause of dampness. On account of granular nature of materials, moisture finds an easy access through the voids and this aided by capillary action assists the moisture to travel in different directions. Thus, either on account of faulty design of structure or bad workmanship or by use of defective materials, moisture may find its way to interior of the building either through the foundations and plinth, walls, chhajjas, floors or roof.

SOURCE OF DAMPNESS

The important sources of dampness may be summarised below:

·      Dampness rising through the foundation walling. Moisture from wet ground may rise well above ground level on account of capillary action.

·      Splashing rain water which rebounds after hitting the wall surface may also cause dampness.

·      Penetration of rain water through unprotected tops of walls, parapets, compound walls etc. may cause dampness.

·      On condensation, the moisture present in the atmosphere gets deposited on the components of the building which may cause dampness.

·      In case of flat roofs, inadequate roof slopes, improper rain water pipe connections and defective junction between roof slab and parapet wall may prove to be source of dampness.

      METHOD OF DAMP PROOFING

1.Membrane Damp proofing

This consists in providing layer or membrane of water repellent material between the source of dampness and the part of the structure adjacent to it. This type of layer is commonly known as damp proof course and it may comprise of materials like bituminous felts, mastic asphalt, silicon, epoxy, polymers, plastic or polythene sheets, cement concrete etc. depending upon the source of dampness, D.P.C. may be provided horizontally or vertically in floors.

General principles observed while laying D.P.C. are as under:

1.The D.P.C. should cover full thickness of walls excluding rendering.

2. The mortar bed upon which the D.P.C. is to be laid should be made levelled, even and free from projections. Uneven base is likely to cause damage to D.P.C.

3. When a horizontal D.P.C. is to be continued to a vertical face, a cement concrete fillet 75 mm in radius should be provided at the junction, prior to the treatment.

2.Integral Damp proofing

This consists in adding certain water proofing compounds with the concrete mix to increase its impermeability. Such compounds are available in market in powdered as well as liquid form. The compounds made from clay, sand or lime help to fill the voids in concrete and make it water proof.

Another form of compound like alkaline silicates, aluminium sulphates, calcium chlorides etc. react chemically when mixed in concrete to produce water proof concrete.

Pudlo, permo, impermo, etc. are some of many commercially made preparations of water proofing compounds commonly used. The quantity of water proofing compounds to be added to cement depends upon the manufacturers recommendations.

3.Surface treatment

As described earlier the moisture finds its way through the pores of materials used in finishing. In order to check the entry of the moisture into pores, they must be filled up. Surface treatment consists in filling up the pores of the surfaces subjected to dampness. The use of water repellent metallic soaps such as calcium and aluminium oleates and stearates is much effective in protecting the building against ravages of heavy rain. Bituminous solution, cement coating, transparent coatings, paints and varnishes fall under this category.

4. Guniting

This consists in depositing an impervious layer of rich cement mortar over the surface to be water proofed. The operation is carried out by use of a machine known as cement gun. The assembly broadly consists of a machine having arrangements for mixing materials and a compressor for forcing the mixture under pressure through a 50 mm dia flexible hose pipe. The hose pipe has nozzle at its free end to which water is supplied under pressure through a separate connection.

The surface to be treated is first thoroughly cleaned of dirt, dust, grease or loose particles and wetted properly. Cement and sand usually taken in proportion of 1:3 to 1:4 are then fed into the machine. This mixture is finally shot on the prepared surface under a pressure of 2 to 3 kg/cm^2 by holding the nozzle of the cement gun at a distance of 75 to 90 cm from the working face. The quantity of water in the mix can be controlled by means of regulating value provided in the water supply hose attachment.

5. Cavity wall construction

This consists in shielding the main wall of the building by an outer skin wall leaving a cavity in between the two. The cavity prevents the moisture from travelling from the outer to the inner wall.

MATERIALS USED FOR DAMP PROOFING

As effective damp proofing material should have the following properties:

1.It should be impervious.

2.It should be strong and durable and should be capable of withstanding both dead as well as live loads without damage.

3. It should be dimensionally stable.

4. It should be free from deliquescent salts like sulphates, chlorides and nitrates.

 DAMP PROOFING TREATMENT IN BUILDING

1.Treatment to foundations

2. Treatment to floors

3. Treatment to walls

4. Treatment to flat roofs

5. Treatment to parapet roofs

6. Treatment to pitched roof

 1.TREATMENT TO FOUNDATIONS

Before dealing with the problem of damp proofing treatment to be given to foundations, it is necessary to have an idea about the ground water level.

When it rains, the rain water seeps through the ground until it is stopped by an impervious layer in the sub soil strata.

The water in the pores of soil is known as ground water. The top level of the ground water is known as sub soil water level or ground water level. The ground water level does not remain constant. It varies with season, magnitude of rainfall and the nature of sub soil strata. The ground water generally follows the contours of the ground and flows from higher level to lower level underneath the ground. Therefore, it appears at greater depth in high ground and shallower depths in low ground. In addition, irregular distribution of soft and hard strata, in the composition of the ground may also cause the ground water level to rise or drop appreciably within small areas.

It is observed that the soil above the water table contains certain amount of moisture in their voids. Near the ground level the soil voids are only partly filled with water and they gradually get fully saturated with water by the time the depth of ground water level is reached. The strata of ground above the water table attracts water by capillary action. The height of capillary rise depends on the size of voids in the soil. In case of granular soils like gravel, coarse sand etc. the capillary rise water is almost negligible.

2. TREATMENT TO FLOORS

In places where the soil water table is low and rainfall is not much, a 75 to 100 mm thick layer of coarse sand is first spread over the entire area of flooring on the prepared bed of rammed earth. Alternatively, this layer can comprise of stone soling with voids filled with smaller stones. This layer is known as base course and its material is well rammed. A 75 to 100 mm thick layer of lean cement concrete mix or lime concrete is thereafter laid over the base course. This forms the base for the floor topping, which may comprise of tiles, stone or cement concrete etc.

In places where the sub soil water table is high, or in damp or humid areas, where there is possibility of moisture rising up in the floors, it is necessary to provide membrane D.P.C. of flexible material like bituminous felt etc. over the entire area of flooring.

3. TREATMENT TO WALLS

As explained earlier wall can get damp due to penetration of moisture from its external face to internal one, due to porosity of bricks and mortar joints. Various treatments given to exposed surface of walls to prevent dampness include pointing, plastering, painting etc. It is observed that plaster made out of cement, lime and sand mixed in proportion of 1:1:6 serves as very effective rendering to protect the wall against dampness in normal weather conditions. In areas of heavy rainfall, cement plaster 1:4 mixed with water proofing compounds like pudlo, permo etc. serves serves the purpose effectively. In exposed brick work, dampness can be prevented by painting the surface with water proof cement paint or with colourless liquid waterproofing compound.

4. TREATMENT TO FLAT ROOFS

Flat roofs require relatively heavier and costlier water proofing treatment as compared with pitched or sloped roofs. The specification of material used for the purpose should be such that it performs the function of water proofing as well as provide adequate thermal insulation. Stagnation of water on the roof is considered to be the root cause of leakage and dampness in flat roofs. This can be avoided by providing adequate roof slope and rain water pipes. In case of R.C.C. or R.B.C. slab roofing with grading above, a slope of 1 in 40 to 1 in 60 is considered desirable. This may be achieved either by varying the thickness of the terracing material or by constructing the roof slab with a slope, or by providing part slope in the roof slab and part in the terracing material.

In addition to the slope, the size and the spacing of the rain water pipes or the outlets require due consideration for the proper drainage of the roof. In general practice, one 10 cm diameter pipe is considered suitable for every 30 sq.m. of the roof area to be drained.

5. TREATMENT TO PARAPET WALL

If the flat roof has a parapet and there are cracks in it or its plaster is very porous or defective, rain water may find and easy access to the wall below and make the wall and some portion of the ceiling damp. Rain water may also leak through cracks at the junction of the parapet and roof slab. In cases, where asphalt layer is provided over the grading material for the water proofing treatment to roof slab, the asphalt layer covering the roof is turned up against the parapet for a height at least 15 cm.

In case, the water proofing of roof is carried out by providing lime terracing, the layer of terracing is taken right upto the edge of the roof and a 15 cm. high triangulated filet is built up there. This is necessarily done to prevent the soakage of water through the parapet in case the rain water pipes get chocked up and the water stands on the roof. In addition, a drip couse is also sometimes provided over the lime gola to prevent water from parapet to fall directly over its junction.

6. TREATMENT TO PITCHED ROOFS

In general, rainwater is liable to leak through a pitched or sloped roof on account of the following reasons:

1.Due to insufficient roof slope

2. Due to insufficient lap in roof covering material i.e, A.C. or C.G.I. sheets, tiles, slates etc.

3. Due to inadequate treatment to rainwater gutter.

All the above referred causes can be taken care of by proper design and construction. However, design and installation of rainwater gutter or valley gutter needs extra special attention with respect to its capacity, position, fixing, water tightness and freedom to accommodate expansion or contraction in any direction. The slope of the gutter should not be flatter than 1:100 in straight lengths and it should be made steeper in portions where the gutter is not straight. The gutter should be leak proof and all the joints in the gutter should be made tight. The work of laying should be carefully supervised and it should be ascertained that the sheets or tiles project beyond the edge of the gutter. Lead flashing should be continued upto the vertical face of the parapet walls and should stop inside the body of the wall.

If the Architect wants to provide a projection in the face wall to decorate the structure, a sloped junction should be stressed.

SPECIFICATIONS FOR LAYER 38 MM. THICK DAMP PROOF COURSE WITH CEMENT CONCRETE 1:2:4 AT PLINTH LEVEL

1.The damp proof course shall cover the full thickness of the wall.

2. The base of the damp proof course shall be clear, even and free from projections liable to cause damage to the D.P.C.

3. The side shuttering shall be strong and so fixed that it does not get disturbed during compaction and the concrete slurry does not leak out.

4. The concrete prepared by mixing ingredients in the proportion of 1:2:4 shall be of workable consistency.

5. The concrete shall be laid and tempered roughly to make a dense mass.

6. After 24 hours of its laying, the concrete layer shall be cured for at least 7 days.