Fire Protection in BuildingsFollowing facts in connection with the fire protection in buildings should be remembered:

(1) It is too expensive to make all the buildings fully protected against fire. As a matter of

fact, the amount spent on fire-resisting construction should be in relation to the reduction

in loss in case fire breaks out. The preventive measures include suitable planning, proper

method of construction and satisfactory means of escape.

(2) It does not necessarily follow that a non-combustible material possesses more power

to resist fire. For instance, a timber post would resist fire in a better way than an

unprotected R.S. joist.

(3) The main purpose of making a building fire-resistant is the protection of life, goods

and activities within the building.

(4) The degree of fire resistance required will largely depend on the use of building. For

instance, a theatre or a town hall will demand greater degree of fire resistant construction

than a warehouse or a go down.

(5) In case of fire-hazard, the danger is from fire, smoke and panic. The fire is capable of

consuming and destroying and the tragic part of it is that nothing is left to rise from the

ashes except the fumes of smoke. The survivors of fire are left to carry terribly the impact

way that they remain unobstructed by smoke or fumes.

(6) It is estimated that nearly 15000 people are killed by fire every year in our country.

The direct and indirect losses are estimated as more than 1000 crores and yet, there is no

comprehensive regulation to insure fire prevention.

(7) The experience the world over has shown that if escape from a tall building is not

effected within 10 minutes or 15 minutes of the outbreak of fire, the temperatures as high

as 800°C and dense poisonous smoke can fatally trap the people within the building.

(8) It has also been seen that the fire-fighting equipment, normally available, like snorkel,

cannot go beyond 10 floors — the highest of the turntable ladder reaches only upto 1 3th

floor.

Causes and effects of fire:Following are the chief causes of fire in order of the toll or victims taken:

(1) smoking in unauthorised places and disregarding carelessly the lighted ends of

cigarettes and matches;

(2) faulty workmanship with respect to the electrical wiring;

(3) heating and cooking equipment;

(4) children playing with matches;

(5) open flames and sparks;

(6) flammable liquids;

(7) suspected arson;

(8) chimneys and flues;

(9) lighting; and

(10) spontaneous combustion.

The home contains various different materials and they produce different gases

when ignited by fire. The effects of these gases are as follows:

(1) Carbon monoxide: This gas hampers oxygen from reaching the brain. It is the most

abundant of fire gases. It is invisible and odorless.

(2) Carbon dioxide: This gas over stimulates the rate of breathing and it is thus

responsible for increasing the intake of other toxic gases.

(3) Hydrogen suiphide: This gas affects the nervous system and it causes dizziness and

pain in the respiratory system.

(4) Nitrogen dioxide: This gas is extremely toxic and it numbs or deadens the throat.

Following are some of the precautionary measures which can be taken to avoid or to

minimize the dangers of a fire:

(1) Be sure that the cigarettes and matches are thoroughly extinguished before throwing

them in rubbish.

(2) Entrust the work of wiring and electrical installations to an expert.

(3) Evacuate your home as soon as possible. Do not allow toxic fumes to take control of

your mind.

(4) Feel each door before you open it. If the handle of door

hot or if smoke is seen coming from its bottom or sides, do not open the door. There are

all the chances of meeting with a blast of fire in such cases.

(5) If there is smoke, crawl on your hands and knees to safety. Heat rises to the ceiling

and there is roughly a safety zone between 300 mm off the floor and approximately

height of the door knob.

(6) If your clothes catch on fire, drop to the floor and roll to extinguish the flames.

(7) Never smoke in bed. A cigarette will continue to burn for

about 24 minutes, if it is ignited and left alone. It takes only 10

minutes or 12 minutes, however, for sofa or beds to ignite, if a

cigarette is left carelessly on them to burn.

(8) Prepare the line of moving through the fire affected area and as you proceed with your

evacuation plan, close every door behind you.

(9) Train your family members or staff members for fighting a fire and acquaint them

with the fire preventive measures.

Fire hazards:

The fire hazards are of the following three types:

(1) Exposure hazard

(2) Internal hazard

(3) Personal hazard.

Characteristics of fire-resisting material:

Following are the characteristics of an ideal fire-resisting material:

(1) The composition of the material should be such that it does not become disintegrated

under the effect of great heat.

(2) The expansion of the material due to heat should not be such that it leads to

unstabflity of the structure of which it forms the part.

(3) The contraction of the material due to sudden cooling with water after it has been

heated to a high temperature should not be rapid.

Fire-resisting

Fire-resisting properties of common building materials:

The fire-resisting properties of common building materials such stone, brick, timber, cast-

iron, glass, steel and concrete are

Fir Protection Build,

(1) Stone:

(2) Brick.

(3) Timber.

(4) Cast-Iron

(5) Concrete.

(6) Glass- (7) Wrought-iron: (8) Aluminum]

Damp-proofing, Water Leakage and Termite—proofing

This chapter, the following three important treatments to be to the buildings to control

damp, water leakage and termites be discussed:

Damp-proofing

Water leakage

Termite-proofing.

Damp-proofing:

Earning of the term damp-proofing: One of the basic requirements in case of all the

buildings is that structure should remain dry as far as possible. If this condition satisfied,

it is likely that the building may become unhabitable unsafe from structural point of view.

Hence, in order to prevent entry of damp into a building, the courses, known as the amp-

proofing courses, are provided at various levels of entry of into a building. At present,

practically all the buildings are given the treatment damp-proofing. Thus the provision of

damp-proofing courses .ivents the entry of moisture from walls, floors and basement of

building. The treatment given to the roofs of a building for the purpose is known as the

water-proofing and it will be discussed

The dampness in a building is a universal problem and the causes which are

responsible for the entry of dampness

structure are as follows:

(1) Rising of moisture from the ground: The ground on ich the building is constructed

may be made of soils which allow the water to pass. Usually the building materials used

the foundations, absorb moisture by capillary action. Thus the pness finds its way to the

floors through the -substructure.

(2) Action of rain: If the faces of wall, exposed to heavy J of rain, are not suitably

protected, they become the urces of entry of dampness in a structure. Similarly the

leaking also permit the rain water to enter a structure.:

(3) Exposed tops of walls: The parapet walls and compound walls should be provided

with a damp-proof course on their exposed tops. Otherwise the dampness entering

through these exposed tops of such walls may lead to serious results.

(4) Condensation: The process of condensation takes Place when warm humid air is

cooled. This is due to the fact that cqol air can contain less invisible water vapour than

warm air. T moisture is deposited on the walls, floors and ceilings. This is he main source

causing dampness in badly designed kitchens.

(5) Miscellaneous: There are various miscellaneous causes of dampness as mentioned

below:

(i) If the structure is located on a site which cannot be easily drained oft, the

dampness will enter the structure.

(ii) The orientation of a building is also an important factor. The walls obtaining

less sunshine and heavy showers of rain are liable to become damp.

(iii) The newly constructed walls remain damp for a short duration.

(iv) Very flat slope of a roof may also lead to the penetration of rain water which

is temporarily stored on the roof.

(v) The dampness is also caused due to bad workmanship in construction such as

defective rain water pipe connections, defective joints in the roofs, improper

connections of walls, etc.

Thus the dampness can be summarised as follows:

(1) detective junctions between root ‘lab and parapet wall;

(2) defective roof covering of the pitched roofs;

(3) faulty eaves and valley gutters;

(4) improper rain water pipe connections;

(5) inadequate roof slope;

(6) moisture from wet ground below foundation;

(7) splashing rain water;

(8) unprotected tops of walls, parapets and compound walls; etc.

Effects of dampness:

The building materials such as bricks, timber, concrete, etc., have a moisture content

which is not harmful under normal circumstances The rise in moisture content of these

materials beyond a certain level from where it becomes visible or when it causes

deterioration leads to the real dampness. In absolute terms, the moisture Conte of

different materials may be the same. But the acceptable.

Requirements of an ideal material for damp-proofing:

Following are the requirements of an ideal material for the amp-proofing:

(1) The material should be durable. As a matter of fact, the damp-proof course

should remain effective during the useful life of building.

(2) The material should be such that it remains steady and does rot allow any

movement in itself.

(3) The material should be perfectly impervious.

(4) The material should be capable of resisting safely the loads min on it.

Materials used for damp-proofing:

Following are the materials which are commonly used for the damp-proofing:

(1) Hot bitumen: This is a flexible material and is placed on the bedding of concrete or

mortar. This material should be applied with a minimum thickness of 3 mm.

(2) Mastic asphalt: This is a semi-rigid material and it forms an excellent impervious

layer for damp-proofing. The good asphalt is a very durable and completely impervious

material. It can withstand only very slight distortion. It is liable to squeeze out in very hot

climates or under very heavy pressure. It should be laid by experienced men of the

specialist firms.

(3) Bituminous felts: This is a flexible material. It is easy to lay and is available in rolls of

normal wall width. It is laid on a layer of cement mortar. An overlap of 100 mm is

provided at the joints and full overlap is provided at all corners. The laps may be seated

with bitumen, if necessary. The bitumen felt can accommodate slight movements. But it

is liable to squeeze out under heavy pressure and it offers little resistance to sliding. The

material is available in rolls and it should be carefully unrolled, especially in cold

weather.

(4) Metal sheets: The sheets of lead, copper and aluminium can be used as the

membranes of damp-proofing.

(5) Combination of sheets and felts: A lead foil is sandwiched between asphalt or

bituminous felt. This is known as the lead core and it is found to be economical, durable

and efficient.

(6) Stones: The two courses of sound and dense stones such granites, slates, etc. laid in

cement mortar with vertical breaking joints can work as an effective damp-proofing

course. The stones $hould extend for full width of the wall. Sometimes the stones can be

fixed, as in case of roof surfaces, on the exposed faces of the all, etc.

(7) Bricks: The dense bricks, absorbing water less than 4.50% their weight, can be used

for damp-proofing at places where the damp is not excessive. The joints are kept open.

Such bricks are Widely used when a damp-proofing course is to be inserted in an ixisting

wall.

(8) Mortar: The mortar to be used for bedding layers can be prepared by mixing 1 part of

cement and 3 parts of sand by volume. small quantity of lime is added to increase the

workability. For plastering work, the water-proof mortar can be prepared. It is prepared

by mixing 1 part of cement, 2 parts of sand and pulverised alum at the rate of 1 20 N per

m of sand. In the water to be Used, 0.75 N of soft soap is dissolved per litreot water and

this Soap water is then added to the dry mix. The mortar thus preparedused to plaster the

surfaces. Alternatively some patented ater-proofng material such as Pudlo, Cido,

Dempro, etc. may be added to the cement mortar.

(9) Cement concrete: A cement concrete layer in the proportion 0:2:4 is generally

provided at the plinth level to work as a damp proofing course. The depth of cement

concrete layer varies from 40 mm to 150 mm. It stops the rise of water by capillary action

nd it is found to be effective at places where the damp is not excessive.

(10) Plastic sheets: The material is made of black polythene ving thickness of about 0.50

mm to 1 mm with usual width of all and it is available in roll lengths of 30 m. This

treatment is latively cheap but it is not permanent. general principles of damp-proofing:

It should be remembered that when a damp-proof course is Und to be ineffective, the

fault usually lies in its position or location

SCAFFOLDING

When the height above floor level becomes more than 1.50 m, temporary erection is

needed to support a number of platforms at different levels for the convenience of

workers. Scaffold enables the mason to work at various stages of a building and to lift the

materials for their immediate use at different heights.

Terms used in Scaffolding

1. Standard. Vertical member is known as standard.

2. Ledgers. The horizontal member parallel to the wall is called

3. Putlog. It is the transverse piece which is placed on the ledgers and which supported on

the wall at one end. It is at right angles to the wall.

4. Thansom. It is putlog supported on ledgers at their both ends.

5. Brace. It is diagonal or cross piece fixed on the standards.

6. Guard Rail. It is a rail fixed like a ledger at the working level.

7. Bridle. It is a piece which is used to cover an opening in a wall and it provides support

to one end of the putlog at the opening.

8. Toe Board. It is a board fixed parallel to ledgers and supported between the putlog It is

given as a protective measure on the working platform.

9. Raker. An oblique support is known as raker.

Types of Scaffolds. The following are various types of scaffolds commonly used:

1. Single scaffold or bricklayers scaffold. 4. Suspended scaffold.

2. Double scaffold or mason’s scaffold. 5. Trestle scaffold.

3. Cantilever scaffold.6. Steel scaffold.

Single Scaffold.

It is very common type of scaffolding used widely in the construction of brick work. It

consists of vertical members firmly in grounds at 2.5 to 3.0 m apart. These standards are

connected to each other by ledgers at every rise at 1.3 to 1.6 m. They are provided on the

building side of the standards and are fixed in position by rope lashings. Putlogs are

lashed on the ledgers at one end and into the holes into the wall at the other end. Putlogs

are 1 m in length and are placed 1.2 m apart. They are provided to support the working

platform. In very high scaffolding cross braces are employed to stiffen the structure

Double Scaffold.

This type of scaffold is more stronger than the single scaffold. Since it is difficult to leave

holes in the stone masonry to provide bearing for the putlogs, in double scaffold two

frames of standards, ledgers and braces are used. One is placed close to the wall and the

other at a distance of about 1.5 m from the first. Hence the double scaffold is completely

independent of the wall.

Reinforced BrickworkBrick masonry is weak in tension and cannot carry appreciable amount of tensile stresses.

Though reinforced cement concrete is a versatile material for use in all types of

constructions, but it is very costly material, hence it cannot be recommended for smaller

works such as lintels, slabs, beams, etc. Steel reinforcement is provided in between the

mortar joints of the brick masonry. Reinforced brick work can take up tensile and shear

stresses upto reasonable amount. Following are the advantages of reinforced brickwork

construction:

(a) It is cheap, strong and durable.

(b) It is fire-proof construction.

(c) It is easy to construct.

(d) It can resist appreciable amount of tensile and shear stresses.

Typical constructions in reinforced brickwork

(1) Walls. Reinforcement in wall may be provided either in the form of expanded metal

mesh or sIeei bars. Many types of patented expanded metal meshes are available in

different widths and different gauges in the market. Mortar is evenly placed on the

required course of the brickwork and the steel fabric is stretched flat on it and pressed

uniformly. The next course of bricks is further built over it.

The flat steel bars of section 25mm. x 2 mm. known as hoop iron are also used to

reinforce the walls. They are hooked at corners and junctions as shown in Fig. 1.79. The

hoop iron is dipped in hot tar and immediately sanded to increase the resistance against

rusting. Special bricks are used to provide vertical reinforcement in walls (as illustrated

(2) Columns. Special t of bricks is used for the construction of reinforced columns.

Vertical reinforcing bars are placed between these bricks. The steel plates of about 6 mm.

thickness are provided at every fourth course and the steel bars are fixed in the

foundation concrete block.

(3) Lintels. Steel bars of 6 to 12 mm. diameters are provided longitudinally in between

the vertical, joints of brick lintels. lb take up the vertical shear, 6 mm. diameter steel

stirrups are used at suitable intervals.

(4) Slabs. For laying reinforced brickwork slabs, the centering is erected at the required

level. Generally, a platform of wooden planks supported on beams and posts is used for

this purpose. It is covered with well beaten earth and fine sand is sprinkled over it. Slabs

of definite depths such as 10, 20, 30, 40 cm. etc. can only be constructed. The

reinforcements are placed as per specifications and bricks are laid to fill up the spaces.

Joints are filled up with mortar in such a way that reinforcement is fully embedded in

mortar. The slab is properly cured for 2 to 4 weeks. The centering is removed only after

28 days and the surfaces of the slabs are smoothened.

(5) Beams. Reinforced beams are constructed inlhe same way as RB. slabs. In case of

RB. beams steel bars upto 25 mm. diameter are used as reinforcement.

Maintenance of Brickwork

(1) Cleaning. Brick masonry may be cleaned with steam or hot water jets. This is useful

for fihe textured and hard burnt bricks. Sand blasting is also used for cleaning brickwork

but it spoils the texture of the bricks as well as damages the pointing done in the

brickwork.

(2) Removal of Efflorescence. Magnesium, sulphate, calcium sulphate, sodium and

potassium sulphate etc., being soluble in water are deposited on the brickwork surface as

efflorescence as a result of alternate drying and wetting of the brickwork by rain and sun

by rise and fall of ground water table. In fact, salts come in the brickwork along with

water from below and are deposited on the brickwork in layer after evaporation. Since

moisture movement is one of the causes for efflo rescence, it can be reduced to a great

extent by suitable damp preven tion of the building.

Efflorescence is removed by scrubbing the wall with water and hard steel brush. A 10%

solution of muriatic acid is also used sometimes instead of ordinary water. The wall is

washed with pure water im mediately after this treatment.

(3) Repointing old brickwork. Old pointings get damaged due to wind, rain, heat,

freezing and other weathering agencies. Repointing is done to improve the appearance of

an old brickwork. It is done as follows:

(a) The old mortar is removed from joints to a depth of at least 3 mm. The old mortar is

loosened with small hammer and removed out with steel brush to make the surface clean.

(b) Water is sprinkled over the joints and the new mortar is put with the trowel. It is

finally finished with the pointing tool to give the desired shape.

(c) Proper curing is done for 2 to 3 weeks.

(4) Repainting. Repainting is required for the surfaces which were earlier painted and

have been damaged by weathering agencies. The old paint is removed by hard steel brush

and water or steam. Then the new paint is applied in the desired colour and tint.

Defects in Brickwork

Following are the common defects that occur in brickwork:

(a) Use of weaker materials. Use of weaker materials forn small depressions with nodules

of friable materials at the joints. creates expansion and cracking in brickwork.

(b) Sulphate in mortars. Sulphate attacks the mortar causii expansion of mortar joints and

finally resulting in cracking of bric work, spalling of brick edges, and damage of mortar.

The failure is di to chemical reaction between sulphates present in bricks and ti

aluminium ingredients of portland cement and it is accelerated in ti presence of moisture.

Bricks free from suiphates should be used ai proper damp-proofing should be done to

check this type of defect.

(c) Corrosion of iron and steel. Exposed iron and steel are corroded when they come in

contact with water or moisture. The volume of corroded iron and steel is increased and

hence cracks in the brick masonry are formed. Therefore, it is advisable to give a

protective cover of 1 to 2 cm. cement mortar over reinforcing steels. For partially

embedded steel, paint of bitumen should be applied in the unbonded portion.

(d) Crystallisation of salts. It forms white deposit on the surface of the brickwork and

may cause disintegration of brickwork. The salts may come from the bricks used, with

the soil contact, with sea-water, or other weathering agencies.

(e) Linear changes due to variation in moisture contact. Shrinkage cracks develop in the

brickwork due to the first long spell of dry weather after the construction. The stepped

cracks appear on the surface but they never-continue below damp-proof course. Good

quality bricks should be used in the construction and the work should be protected from

rain during the construction, to avoid this type of defect.

t) Frost action. The volume of water is increased when it is frozen. Due to this

phenomena, the cracks are caused in the brickwork. This defect is considerably reduced if

the water accumulation is prevented.

PLASTERING

The plastering is a method of covering uneven surfaces with a plastic material to get an

even, regular, smooth, clean and durable surface. Sometimes it is used to develop

decorative finish. The main object of external plastering is to cover the surface to protect

it from rain and other weathering agencies besides concealing defective work manship

and inferior materials.

Terms related to plastering

(1) Background. The prepared surface to which first coat of plaster is applied.

(2) Cracking. It is development of one or more fissures in the plaster.

(3) Crazing. This is the appearance of a series of haphazard hair cracks on the finished

plastered surface.

(4) Blistering. It is appearance of one or more small local swell ings on the finished

plastered surface.

(5) Dado. The bottom most part of the plastered wall where special treatment is given to

plaster to give a smooth and resistant finish.

(6) Dots. They are small patches of plaster laid on the background for fixing screeds, etc.

(7) Dubbing out. It is a method of filling in hollow spaces in a solid background before

applying plaster.

(8) Flaking. In absence of proper adhesion with the previous coat, some patches of plaster

fall down and this is known as flaking.

(9) Gauging. The process of mixing the various ingredients of plaster is known as

gauging.

(10) Hacking. The process of making the background rough to have suitable key for

plastering is known as hacking.

(11) Keys. These are indentations on the surface of under coat or background to form a

mechanical bond between the plaster applied and the prepared surface.

Peeling. It is removal of plaster from the background.

Preparation of surface for plastering

Besides the quality of the mortar, its adhesion with the back ground also affects the

durability of plaster. This makes the preparation of surface of prime importance. All the

projections more than 13 mm. from the general face of the wall are removed to get a

uniform surface and to reduce the amount of plaster. To get good key of the plaster with

the wall surface, the mortar joints in the brick masonry are raked out to a depth of 13 mm.

The joints and surfaces are cleaned with hard wire brush to remove loose mortar and dust.

Efflorescence, greasy spots and other patches are also scrapped and cleaned. If the old

wall is smooth, the surface is roughened by hacking it with some tool. The surface is

washed with water and it is kept wet till applying plaster.

To maintain uniform thickness of the plaster and a true surface, patches of plaster (known

as screeds) 15 cm x 15 cm are applied horizontally and vertically at about 1.8 m. apart

over the whole surface. Mortar is applied on the surface of the wall between screeds with

trowel. Types of Plaster

Following are the various types of plaster commonly used:

(a) Lime plaster (b) Cement plaster (e) Plaster on lath.

(c) Stucco plaster (d) Water-proof plaster

(a) Lime Plaster. Fat lime or hydraulic lime may be used for plastering. Fat lime

provides best plaster. Hydraulic lime produces. harder and strong plaster, but it may

consist of unslacked particles, which will cause blistering in the plaster after some time.

Hence as a precaution, the hydraulic lime is ground dry with sand and left exposed to the

atmosphere for about 2 to 3 weeks and then reground before use.

Lime mortar contains equal volume of lime and sand, and the mixture is nicely ground in

a mortar mill. Sometimes small quantity of cement is also added to the lime mortar to

increase its strength. ‘lb prevent the appearance of cracks on the plastered surface, Gugal

(a kind of fragrant gum) and chopped hemp are added at the rates of 15 kg. and 9 kg. to

every 10 cubic metre of mortar respectively. The prepared mortar is kept for 2 days

before use and it is turned once in day to make it more uniform and homogeneous.

(b) Cement Plaster. Cement mortar comprises of one part of cement to 3-4 parts of

clean, coarse and angular river sand, The materials are thoroughly mixed in dry condition

on a water-tight platform before water is poured to it. The mortar of one full cement bag

is prepared at one time and it is consumed within 30 minutes after adding water.

Generally, cement plaster is applied in one coat but when the thickness of plaster is more

than 16mm. plaster is applied in two coats.

When plastering is to be done in one coat, cement plaster is applied on the background

surface between the screeds with trowel. The surface is smoothened by wooden float and

straight edges and finally it is polished with trowel.

When plastering is to be done in two coats, the first coat is applied as discussed above but

the final polishing is not done. The surface of the first coat is roughened with scratching

tool, to make a key for the second or finishing coat. The finishing coat is applied over the

first coat within 48 hours. It is finished smoothly. The proper curing is done for 2-3

weeks after 24 hours of applying the cement plaster.

(c) Stucco Plaster. Stucco plaster is a decorative type of plaster with elegant finish like

marble finish. It is usually applied in three coats and total thickness of the plaster is about

25 mm. The first, second and third coats are known as the scratch coat, the finer coat or

the brown coat, and white coat or finishing coat respectively. Stucco plaster can be

applied for exterior and interior surfaces. Each coat is allowed to dry before next coat is

applied.

Stucco for exterior walls. The mortar for the scratch coat consists of cement in 1: 3 ratio

and the hydrated lime is added 10% by weight to this mixture. The thickness of this coat

is about 12 mm. The brown coat has the same composition as the scratch coat but its

thickness is 10 mm. This thickness of the finishing coat may be 3 to 6 mm. and the

mortar is made up of 1 : 2 cement and sand. White or coloured cement is used in

finishing coat to obtain the desired tint.

Stucco for interior walls. The scratch coat is 12 mm. thick and it consists of lime plaster.

The brown coat is richer in lime and it is lime plaster of 10 mm thickness. The finishing

coat is 3 mm. thick and contains a mixture of finest lime and well ground marble of

quartz stone. This coat is carefully polished with lime, moist chalk and oil. The surface

obtained is very smooth and bright.

(d) Water-proof plaster. The mortar of this type of plaster

consists one part of cement, two parts of sand and pulverized alum at the rate of 12 r

cubic metri of sand. Iii one lifi water

a ou gin of soft soap is dissolved and this soap water is used to

prepare the mortar for plaster.

(e) Plaster on lath. Lath is used as base to the plaster work. It is mainly used for the

construction of ceiling or partition work, by fixing it to timber supports or steel works.

Laths may be broadly classified into two classes, i.e. (i) metal laths and (ii) wooden laths.

(i) Metal laths. Various types of metal laths are available in the market such as (I) Plain

expanded, (2) Ribbed expanded.(3) Perfora ted,(4) Dove-tailed, etc. The plain expanded

metal lath is widely used (Fig. 1.112). The metal laths are fixed and stretched tightly with

9 to 12 mm. diameter steel rods or steel channels. Plaster is then applied on both sides. It

provides a strong and solid base for plaster work and it is fire-resistant construction. All

sheets should have an overlap of not less than 25 mm, at the sides and ends. Overlaps

should always rest on solid supports. When metal lath is fixed to timber supports, only

galvanized Special materials used in plastering

(1) Plaster of Paris. It is obtained by heating Gypsum. Plaster of Paris forms paste with

water and it sets quickly. It has better adhesion with wood, metal lath, masonry and other

plastering surfaces. It is generally used with ordinary lime to fill up the small holes and

other defects in the plastered surfaces. It is not used in external works as it is soluble in

water.

(2) Keen’s Cement. It is the hardest and strongest form of gypsum plaster. It has pure

white colour and can take good polish. It is employed for ornamental work and

decorative plastering.

(3) Martin’s Cement. It is obtained by calcinations of pearl ash plaster of Paris. It sets

quickly and produces a hard and white

(4) Barium Plaster. It is produced from barium sulphate and applied as a final coat of

plaster. This is specially used in the plastering of X-ray rooms.

(5) Acoustical Plaster. It is used to prepare the surface of the auditorium or hail to

provide acoustical treatment. This plaster is a gypsum mixture and used as a final coat of

plaster. It is applied in two coats of 6 mm. thickness each.

(6) Asbestos-marble plaster. It is a mixture of asbestos, cement and finely crushed

marble. This type of plaster provides elegant mar ble-like finish to the surface.

(7) Snow-crete and colourcrete cements. These are patented cements and are employed

on external walls to improve the appearance.

(8) Granite Silicon plaster. It is a quick setting material and very elastic. Hence, there

are lesser possibilities of developing cracks over the plastered surface.

Defects in plastering

(1) Cracks. They appear on the plastered surface in the form of hair cracks or wider

cracks. Following are the reasons for the develop ment of cracks:

(a) When thick coat of plaster is applied, too much of shrinkage takes place; thus causing

cracks on the plastered surface.

(b) Due to movement of backing on account of shrinkage of the backing materials.

(c) Due to movement in the plaster itself because of expansion or shrinkage of the plaster

coat during drying.

(d) When old surface is not properly dressed.

(e) Due to bad workmanship and defective method of application of the plaster.

Efflorescence. Sometimes soluble salts are present in plaster-making materials or bricks.

They on the plastered surface in whitish patches and produce ugly appearance.

Efflorescence is re moved by brushing and washing the surface several times. In brick

work, a solution of zinc sulphate and water is applied to the surface containing

efflorescence and then the surface is brushed and washed.

(3) Blistering of plastered surface. Small patches swell out beyond the plastered surface

like boils. When partially slaked lime is used for plastering, the unslaked particles of lime

slake causing blistering in the plastered surface, Thus, properly slaked lime should be

employed in the plaster work to avoid blistering.

(4) Falling out of plaster. The following reasons n be listed for falling Qut of plaster:

(i) Due to excessive thermal variations in the backing or plaster.

(ii) Due to inadequate bond between the successive coat of the plaster.

(iii) Due to imperfect adhesion of the plaster to the background.

(iv) If the backing material absorbs too much of water, the strength of the plaster and

bond between plaster and backing gets reduced.

POINTING

Pointing is the process of finishing the mortar joints with a separate material in brick

masonry or stone masonry. The mortarjoints of the masonry are raked out to a depth of

13 to 20 mm. and the fresh mortar is inserted in the raked spaces to form the desired

shape.

Pointing is generally recommended for the finishing of exposed external walls. Its initial

cost is less but it requires replacement after few years. The use of pointing is avoided as

far as possible.

Following are the advantages of pointing:

(a) The pointing protects the joints from the adverse effect of atmospheric actions.

(b) It hides defective workmanship.

(c) It imparts better appearance to the exposed surface of the structure.

Mortar for pointing

Pointing can be done in cement or lime mortar. The lime mortar is prepared with equal

parts of fat lime and fine sand which is clean and free from any organic impurities. The

mixture is ground thoroughly in a mortar mill. The cement mortar is prepared by mixing

cement and sand in proportion 1: 1 or: 1:2. The cement should be standard portland

cement and sand should be clean, fine and free from any organic impurities. The

materials are first mixed in dry state on a water-tight

DAMP-PROOF COURSE

Dampness in building leads to unhealthy conditions and unsafe from structural point of

view. Therefore, one of the essential require ments is that the structure should be dry as

far as practicable. lb check the entry of water or moisture into a building, damp-proof

courses are placed at various levels. Now-a-days, all the buildings are provided with

damp-proof course to prevent dampness from affecting a building or the persons living in

the building.

Effects of dampness

The main effects of dampness are as follows

(1) It c; es unhealthy conditions for the occupants of the building.

(2) It causes efflorescence which may finally result in the dis integration of bricks, tiles

etc.

(3) Plasters become soft and crumbled.

(4) It causes warping and decay of timber.

(5) The metals used in the construction of building are subjected to corrosion.