Saturday, 16 April 2016

Miscellaneous Building Materials

Miscellaneous Building Materials

Hello there, 
Glass, plastics, bitumen, asbestos, paints, distemper and varnishes are some of the miscellaneous materials used in building constructions. Their properties and uses are briefly explained in this post below.


Silica is the main constituent of glass. But it is to be added with sodium potassium carbonate to bring
down melting point. To make it durable lime or lead oxide is also added. Manganese oxide is added to nullify the adverse effects of unwanted iron present in the impure silica. The raw materials are ground and sieved. They are mixed in specific proportion and melted in furnace. Then glass items are
manufactured by blowing, flat drawing, rolling and pressing.
Important Properties of Glass
1. It absorbs, refracts or transmits light. It can be made transparent or translucent.
2. It can take excellent polish.
3. It is an excellent electrical insulator.
4. It is strong and brittle.
5. It can be blown, drawn or pressed.
6. It is not affected by atmosphere.
7. It has excellent resistance to chemicals.
8. It is available in various beautiful colours.
9. With the advancement in technology, it is possible to make glass lighter than cork or stronger than        steel.
10. Glass panes can be cleaned easily.

Types of Glass
The glass may be broadly classified as:
1. Soda-lime glass 
2. Potash lime glass
3. Potash lead glass 
4. Common glass and
5. Special glasses.

1. Soda Lime Glass: 
It is mainly a mixture of sodium silicate and calcium silicate. It is fusible at low temperature. In the fusion condition it can be blown or welded easily. It is colourless. It is used as window panes and for the laboratory tubes and apparatus.

2. Potash Lime Glass:
 It is mainly a mixture of potassium silicate and calcium silicate. It is also known as hard glass. It fuses at high temperature. It is used in the manufacture of glass articles which have to with stand high temperatures.

3. Potash Lead Glass:
 It is mainly a mixture of potassium silicate and lead silicate. It possesses bright lustre and great refractive power. It is used in the manufacture of artificial gems, electric bulbs, lenses, prisms etc.

4. Common Glass: 
    It is mainly a mixture of sodium silicate, calcium silicate and iron silicate. It is brown, green or yellow in colour. It is mainly used in the manufacture of medicine bottles.

5. Special Glasses:
    Properties of glasses can be suitably altered by changing basic ingradients and adding few more ingradients. It has now emerged as versatile material to meet many special requirement in engineering.

 The following is the list of some of the special glasses:
(a) Fibre glass 
(b) Foam glass
(c) Bullet proof glass 
(d) Structural glass
(e) Glass black
 (f) Wired glass
(g) Ultraviolet ray glass 
(h) Perforated glass


Plastic is an organic material prepared out of resin. It may or may not contain fillers, plasticisers and
solvents. Plastic may be defined as a natural or synthetic organic material which are having the property of being plastic at some stage of their manufacture when they can be moulded to required size and shape.
     Shellac and bitumen are the natural resins used as plastic for a long time. In 1907, Blackland
produced synthetic resin from the reaction of phenol and formaldehyde. The resin was hardened under pressure and heat to produce useful plastic articles.

Types of Plastics
Primarily there are two types of plastics:
1. Thermosetting and
2. Thermoplastic.

1. Thermosetting Plastics:
 It needs momentary heated condition and great pressure during shaping. When heated cross linkage is established between the molecules and chemical reaction takes place. During this stage shape can be changed with pressure. This change is not reversible. The scrap of such plastic is not reusable. Bakelite is an example of such plastic.

2. Thermoplastic: 
  In this variety, the linkage between the molecules is very loose. They can be softened by heating repeatedly. This property helps for reuse of waste plastic. These plastic need time to cool down and harden. These plastics are to be kept in moulds till cooling takes place completely. Bitumen, cellulose and shellac are the examples of this variety of plastics.

Properties of Plastics:-
1. Colour: Some plastics are completely transparent. Using pigments plastics of any attractive
                 colour can be produced.
2. Dimensional Stability: It is dimensionally stable to a great extent.
3. Durability: Plastic offers great resistance to moisture and chemicals and hence more durable.
4. Electrical Insulation: The plastics possess excellent electrical insulating property.
5. Fire Resistance: The phenol-formaldehyde and urea-formaldehyde plastics resist fire to a
                         great extent and hence they are used as fire proofing materials.
6. Strength: The plastics are reasonably strong. Their strength may be increased by reinforcing
with various fibrous materials. Attempts are being made to produce structurally sound plastics.
7. Specific Gravity: The specific gravity of plastics is very low and hence convenient to handle.
8. Ductility: The plastics are not ductile and hence they fail without giving warning.
9. Fixing: Plastics can be bolted, drilled, glued, clamped or simply push fitted in position.
10. Maintenance: There is no maintenance cost for plastic articles i.e., they do not need painting
    and polishing.

Uses of Plastics:-
There are variety of plastics made to suit different uses. The typical uses of plastics in buildings is listed below:
1. Corrugated and plain sheets for roofing.
2. For making jointless flooring.
3. Flooring tiles.
4. Overhead water tanks.
5. Bath and sink units.
6. Cistern hall floats.
7. Decorative laminates and mouldings.
8. Window and door frames and shutters for bathroom doors.
9. Lighting fixtures.
10. Electrical conduits.
11. Electrical insulators.
12. Pipes to carry cold waters.


Ashalt, bitumen and tar are referred as bituminous materials, which are essentially hydrocarbon materials. The asphalt is a mixture of inert mineral matter lime alumina, lime, silica etc. and a hydrocarbon known as asphaltic bitumen. In some places like Trinidad and Bermudez, asphalt is available in nature at a depth of 3 to 60 metres. It is known as natural asphalt. Common variety used all over the world is residual asphalt, which is obtained by fractional distillation of crude petroleum oil. Bitumen is the binding material which is present in asphalt. It is a hydrocarbon. It is obtained by partial distillation of crude oil. It contains 87 per cent carbon, 11 per cent hydrogen and 2 per cent oxygen. Tar is obtained in the distructive distillation of coal, wood or other organic materials. When coal or wood is heated to redness in an closed chamber, it yields volatile product and residue coke. After separating and cooling volatile product gives tar.
Comparison between asphalt, bitumen and tar is guven below figure.


Asbestos is a general name for several varieties of fibrous minerals which are available in nature. But
presently, most of the commercial asbestos produced is ‘chriotile’ [Mg6SiO11(OH)6.H2O].

Properties of Asbestos:-
1. It is flexible, soft and non-porous.
2. It is fire proof and acid proof material.
3. It is a good insulator of heat and electricity.
4. When it is mixed with cement and water, it retains shape firmly.
5. Its colour is brown or grey.
6. It can be cut into pieces or can be drilled.
7. It possesses high tensile strength in the direction of its fibres.
8. Its specific gravity is 3.10.

Uses of Asbestos:-
1. Asbestos cement sheets are the cheapest roofing materials.
2. Asbestos cement pipes are used as down take pipes of rain water from the roof.
3. With bitumen it forms good damp proof layer.
4. It is used for preparing fire proof ropes and clothes.
5. It is used as covering material for fuse and electric switch boxes.
6. It is useful for insulating boilers, furnaces etc.


Paints are applied on the surfaces of timber, metals and plastered surfaces as a protective layer and at
the same time to get pleasant appearance. Paints are applied in liquid form and after sometime the
volatile constituent evaporates and hardened coating acts as a protective layer.
Constituents of Paint
The essential constituents of paints are:
1. Base 
2. A vehicle 
3. A pigment
4. A drier and 
5. A thinner.

1. Bases:
   It is a principal constituent of paint. It also possesses the binding properties. It forms an opaque coating. Commonly used bases for paints are white lead, red lead, zinc oxide, iron oxide, titanium white, aluminium powder and lithophone. A lead paint is suitable for painting iron and steel works, as it sticks to them well. However it is affected by atmosphere action and hence should not be used as final coat. While zinc forms good base but is costly. Lithophone, which is a mixture of zinc sulphate and barytes, is cheap. It gives good appearance but is affected by day light. Hence it is used for interior works only.

2. Vehicles:
    The vehicles are the liquid substances which hold the ingredients of a paint in liquid suspension and allow them to be applied on the surface to be painted. Linseed oil, Tung oil and Nut oil are used as vehicles in paints. Of the above four oils, linseed oil is very commonly used vehicles. Boiling makes the oil thicker and darker. Linseed oil reacts with oxygen and hardens by forming a thin film.

3. Pigment:
     Pigments give required colour for paints. They are fine particles and have a reinforcing effect on thin film of the paint. The common pigments for different colours are: 
Black—Lamp black, suit and charcoal black.
Red—venedion red, red lead and Indian red.
Brown—burned timber, raw and burned sienna
Green—chrome green, copper sulphate.
Blue—prussian blue and ultra marine
Yellow—ochre and chrome yellow.

4. The Drier: 
   These are the compounds of metal like lead, manganese, cobalt. The function of a drier is to absorb oxygen from the air and supply it to the vehicle for hardening. The drier should not be added until the paint is about to be used. The excess drier is harmful because it destroys elasticity and causes flaking.

5. The Thinner:
     It is known as solvent also. It makes paint thinner and hence increases the coverage. It helps in spreading paint uniformly over the surface Terpentine and neptha are commonly used thinners. After paint applied, thinner evaporates and paint dries.

Properties of an Ideal Paint:-
1. It should be possible to apply easily and freely.
2. It should dry in reasonable time.
3. It should form hard and durable surface.
4. It should not be harmful to the health of workers.
5. It should not be easily affected by atmosphere.
6. It should possess attractive and pleasing appearance.
7. It should form a thin film of uniform nature i.e., it should not crack.
8. It should possess good spreading power.
9. It should be cheap.

Types of Paints:-
Depending upon their constituents there are various types of paints. A brief description of some of them which are commonly used are given below:

1. Oil Paint:-These paints are applied in three coats-primer, undercoat and finishing coat. The
presence of dampness while applying the primer adversely affect the life of oil paint. This paint is cheap and easy to apply.

2. Enamel Paint:- It contains white lead, oil, petroleum spirit and resinous material. The surface
provided by it resists acids, alkalies and water very well. It is desirable to apply a coat of titanium white before the coat of enamel is applied. It can be used both for external and internal walls.

3. Emulsion Paint:- It contains binding materials such as polyvinyl acetate, synthetic resins etc.
It dries in 1 1 2 to 2 hours and it is easy to apply. It is more durable and can be cleaned with
water. For plastered surfaces, first a coat of cement paint should be applied and then the
emulsion point. Emulsion paint needs sound surfaces.

4. Cement Paint:- It is available in powder form. It consists of white cement, pigment and other
additives. It is durable and exhibits excellent decorative appearance. It should be applied onrough surfaces rather than on smooth surfaces. It is applied in two coats. First coat is applied on wet surface but free from excess water and allowed to dry for 24 hours. The second coat is then applied which gives good appearance.

5. Bituminous Paints:- This type of paint is manufactured by dissolving asphalt or vegetable
bitumen in oil or petroleum. It is black in colour. It is used for painting iron works under water.

6. Synthetic Rubber Paint: -This paint is prepared from resins. It dries quickly and is little affected by weather and sunlight. It resists chemical attack well. This paint may be applied even on fresh concrete. Its cost is moderate and it can be applied easily.

7. Aluminium Paint:- It contains finely ground aluminium in spirit or oil varnish. It is visible in
darkness also. The surfaces of iron and steel are protected well with this paint. It is widely used for painting gas tanks, water pipes and oil tanks.

8. Anti-corrossive Paint:- It consists essentially of oil, a strong dier, lead or zinc chrome and finely ground sand. It is cheap and resists corrossion well. It is black in colour.

Application of Paint:-
                  Preparation of surface for application of paint is the most important part in painting. The surface to be painted should not be oily and it should be from flakes of the old paint. Cracks in the surface should be filled with putty and then with sand paper. Then primer is applied. Painting work should be carried out in dry weather. The under coats and first coats must be allowed to dry before final coat is applied.


           Distempers are the cheaper variety of paints in which chalk is used as base and water is used as a carrier. The emulsifying agent which is commonly used is glue or casein. Distempers are available in powder form or in the form of paste. They are to be mixed with hot water before use.
The surface to be distempered should be thoroughly rubbed and cleaned. The cracks, if any should be filled by lime putty. The surface should be kept dry for about two months before applying distemper. Thus a primary coat is applied and is allowed to dry. Distemper is usually applied in two coats.

Properties of Distemper:-
1. They are generally light in colour.
2. The coatings are generally thick.
3. They give reflective coating.
4. They are less durable than oil paints but are cheaper.


Varnish is the solution of resins or resinous substances like amber, copal, shellac, gum resin etc. in
solvents like oil, turpentile, alcohol etc. Depending upon the solvents used varnishes are classified as,
oil varnishes, turpentile varnishes, spirit varnishes and water varnishes. The desirable characteristics of an ideal varnish are as follows;-
1. It should give glossy surface.
2. Should be durable.
3. It should dry rapidly after application.
4. It should not develop cracks after drying.
It is commonly used on wooden surfaces.


     Solid and hollow concrete blocks are manufactured in factories to meet the requirements of building blocks in cities and towns. These blocks may be called as artificial stones, since they replace the stones in the masonry construction. They are manufactured with lean mixes of cement, sand and aggregates of sizes less than 12 mm. Instead of sharp edged aggregates, round aggregates are professed in the manufacture of these blocks. The properties and uses of these blocks is given in this article.
(i) Solid Concrete Blocks:-       Solid concrete blocks of size 400 mm × 200 mm × 150 mm are
commonly manufactured. To reduce the weight of the block no fine concretes are preferred. No fine
concrete is the concrete in which fine aggregate is not used, but round aggregates of size less than
12 mm are used. IS:2185 (part I) 1983 covers the requirement, for such blocks. The blocks should satisfy the strength requirement of 4 N/mm2. Their density should be as low as possible, so that handling is not difficult. They should have sharp edges which are at right angles to each other.
These blocks are used for load nearing wall construction also.

(ii) Hollow Concrete Blocks:-       To reduce the weight of concrete blocks, they may be made hollow
as shown in Fig. 5.1. Hollow blocks of sizes 400 mm × 200 mm × 190 mm (nominal size 400 × 200 ×
200 mm) and also of sizes 400 mm × 300 mm × 190 mm (nominal size 400 × 300 × 200 mm) are
manufactured. IS:2185 (part I) 1983 covers the specifications for these blocks. These block need richer mixes. Fine aggregates upto 60% and coarse aggregates upto 40% are used.
These blocks also should satisfy the strength requirement of 4 N/mm2. They should have truely
right angled corners. Advantage of using concrete blocks is that the construction activity is fast. Mortar requirement for finishing the surface is less. Pointing alone is sufficient, in other words plastering is not necessary. below Table gives the differences between solid and hollow concrete blocks.

Both solid and hollow blocks can be used for the construction of load bearing as well as partition
walls. They are ideally suited for the construction of compound walls.


These are also clay products like brick but are thin. Depending upon their use, building tiles may be
further classified as,
1. Roofing tiles
2. Flooring tiles and wall tiles.

1. Roofing Tiles:-     Roofing tiles are used to cover sloping roofs. They are supported on wooden
reapers. Sometimes light gauge steel or steed rods are also used as reapers. After supporting on reapers these tiles should be strong enough to take load of a man safely. The tiles should he leak proof. Normally these tiles are having curved surface having ribbed sections, so that with thin section they are sufficiently strong to resist the load. However many times flat tiles are used under curved/ribbed tiles. These tiles are not subjected to load directly. They serve in reducing adverse thermal effects. Mangalore, Allahabad tiles, and corrugated tiles are popularly used roofing tiles.

Allahabad tiles are generally laid side by side and the joints are covered with half round tiles.
Mangalore tiles are red in colour and they are of interlocking type. These tiles are manufactured in
Mangalore, Calicut, Cochin and Gujarat.
Corrugated tiles satisfy the requirements of appearance and leak proof but they can be easily
blown away by wind.

The desirable properties of the roofing tiles are:-
1. they should not absorb moisture more than 20 per cent by weight.
2. they should give pleasing look.
3. they should be capable of taking load of a man safely, after they are supported on reapers.
4. they should be durable.
5. they should be uniform in shape and size.
6. warpage should not exceed 2% along the edges and 1.5% along the diagonal.

2. Flooring Tiles and Wall Tiles:-     These tiles are manufactured by burning pressed green tiles
twice. First they are burnt at 700°C. Then they are dipped in the glaze solution and again burnt at
1250°C to fuse them with glaze. The thickness of these tiles vary from 15 to 20 mm. These tiles are flat and they have pleasing appearance.
 There are two types of flooring tiles:
(a) Glazed Tiles: These tiles are used as finish surfaces for floors and walls in kitchen and bathrooms.
These tiles are glazed and are provided with attractive colours and designs.
(b) Mosaic Tiles: These are precast concrete tiles with marble chips on the top surface. After
fixing these tiles polishing is done.

The desirable properties of flooring and roofing tiles are:-
1. Tolerance for length = ± 5 mm.
2. Tolerance for thickness = ± 2 mm.
3. Should be uniform in shape and colour.
4. They should be sound, hard and durable.
5. They should have very low percentage of water absorption.
6. They should give a clear ringing sound when struck with each other.
7. They should show good resistance to abrassion.

Friday, 8 April 2016

Standard Consistency Test and Setting time test for Concrete

Standard Consistency Test

                      For finding out initial setting time, final setting time and soundness of cement, and strength a parameter known as standard consistency has to be used. It is pertinent at this stage
to describe the procedure of conducting standard consistency test. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould shown in below fig. The appartus is called Vicat Appartus. This appartus is used to find out the percentage of water required to produce a cement paste of standard consistency.

The standard consistency of the cement paste is some time called normal consistency (CPNC). The following procedures is adopted to find out standard consistency. Take about 500 gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger. Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top. That particular
percentage of water which allows the plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency. This percentage is usually denoted as ‘P’. The test is required to be conducted in a constant temperature (27° + 2°C) and constant humidity (90%).

Setting time of Concrete

Setting Time:

 Initial setting time and final setting time are the two important physical properties of cement. Initial setting time is the time taken by the cement from adding of water to the starting of losing its plasticity. Final setting time is the time lapsed from adding of the water to complete loss of plasticity. Vicat apparatus is used for finding the setting times. Vicat apparatus consists of a movable rod to which any one of the three needles shown in figure can be attached. An indicator is attached to the movable rod. A vicat mould is associated with this apparatus which is in the form of split cylinder.
Before finding initial and final setting time it is necessary to determine water to be added to get
standard consistency. For this 300 gms of cement is mixed with about 30% water and cement paste
prepared is filled in the mould which rests on non porous plate. The plunger is attached to the movable rod of vicat apparatus and gently lowered to touch the paste in the mould. Then the plunger is allowed to move freely. If the penetration is 5 mm to 7 mm from the bottom of the mould, then cement is having standard consistency. If not, experiment is repeated with different proportion of water fill water required for standard consistency is found. Then the tests for initial and final setting times can be carried out as explained below:

Initial Setting Time: 

300 gms of cement is thoroughly mixed with 0.85 times the water for standard consistency and vicat mould is completely filled and top surface is levelled. 1 mm square needle is fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its plasticity and offers resistance to penetration. When needle can penetrate up to 5 to 7 mm above bottom of the paste experiment is stopped and time lapsed between the addition of water and end if the experiment is noted as initial setting time.

Final Setting Time.

 The square needle is replaced with annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste. This time is noted as final setting time.

Friday, 1 April 2016

Types of Foundations and their Functions


Hello friends, in this post The definition and functions of foundation and broad guidelines for fixing the dimensions of foundation are given and different types of foundations are explained.

Guidelines for minimum dimensions are given below:
(a) Depth of Foundation: For all types of foundations minimum depth required is calculated
using Rankine’s Formula:

H= (p/w)*(1-sinϕ/1+sinϕ)^2

where p = safe bearing capacity of soil
w = unit weight of soil
ϕ = angle of repose of soil.

However in any case it is not less than 0.9 m. Finding safe bearing of the soil is an expert’s job, and it is found after conducting tests in field or in Laboratories. However general values for common soils are listed below,

Type of the soil and SBC in Kn/m2

1.Igneous rocks (granite, troy etc.) -3300
2. Sedimentary rocks (sand, stone etc.)- 1650
3. Residual deposits, hard shale, cemented materials -900
4. Soft rock, coarse sand- 450
5. Medium sand- 250
6. Fine sand- 150
7. Loose gravel or sand gravel- 250
8. Soft shale, hard clay- 450
9. Medium clay, readily indented with thumb nail -250
10. Moist clay, clay and sand mixture -150
11. Soft clay -100
12. Black cotton soil, peat and made up of ground- nill

(b) Width of Foundation: Width of wall foundations or size of column footing is determined by
first calculating the expected load and then dividing that with SBC. Thus,

Width of wall foundation =(Load per unit length of wall/S.B.C. of soil)

Area of column footing =(Load carried by column/S.B.C. of soil)


This type of foundations are commonly used for walls and masonry columns. These foundations are
built after opening the trenches to required depth. Such footings are economical up to a maximum depth of 3 m. As these foundations are suitable depth, they are grouped under shallow foundations.

Figure 1 shows a conventional spread footing for a wall and Fig. 2 shows it for a masonry

Before building these footing trenches are opened to required depth and the soil is rammed well. Then a plain concrete of mix 1 : 4 : 8 is provided. Its thickness varies from 150 to 200 mm. Over this
bed, stone masonry footing is built. It is built in courses each course projecting 50 to 75 mm from the
top course and height of each course being 150 to 200 mm. In case of wall footing the projections are
only one direction while in case of columns, they are in both directions. The projection of bed concrete from the lowest course of foundation masonry is usually 150 mm.


There are mainly two types of R.C.C. footings:
1. One way reinforced footings.
2. Two way reinforced footings.

1. One Way Reinforced Footing:
These footings are for the walls. In these footings main reinforcements are in the transverse direction of wall. In longitudinal directions there will be only nominal reinforcement.

2. Two Way Reinforced Footings:
 For columns two way reinforced footings are provided. The following types of the footings are  common:
(i) Isolated Column Footings:
 If separate footings are provided for each column, it is called isolated column footing.below  Figure shows a typical isolated column footing. The size of footing is based on the area required to distribute the load of the columns safely over the soil . These footings are provided over a 100 to 150 mm bed concrete. Required reinforcements and thickness of footing are found by the design engineers. Thickness may be uniform or varying.

(ii) Combined Footings:
 Common footings may be provided for two columns. This type of footing is necessary when a column is very close to the boundary of the property and hence there is no scope to project footing much beyond the column face.below Figure shows a typical combined footing. The footing is to be designed for transferring loads from both columns safely to the soil. The two columns may or may not be connected by a strap beam.

(iii) Continuous Footings:
 If a footing is common to more than two columns in a row, it is called continuous footing. This type of footing is necessary, if the columns in a row are closer or if SBC of soil is low.below Figure shows this type of footing.

(iv) Mat Footing/Raft Footing:
 If the load on the column is quite high (Multistorey columns) or when the SBC of soil is low, the sizes of isolated columns may work out to be to such an extent that they overlap each other. In such situation a common footing may be provided to several columns as shown in below Fig.Such footings are known as raft footings. If the beams are provided in both directions over the footing slab for connecting columns, the raft foundations may be called as grid foundation also. The added advantage of such footing is, settlement is uniform and hence unnecessary stresses are not produced.

High rise buildings are built with steel columns encased in concrete. Such columns carry very heavy
load and hence they need special foundations to spread the load to a larger area of soil. Grillage foundation is one such special foundation. It consists of one tier or more tiers of I-sections steel beams.below Figure shows a typical two tier grillage foundation. Top tier consists of less number but large size steel section while lower tier consists of larger number but smaller size steel sections. Column load is transferred to the top tier through a base plate. The grillage beams are unpainted and are encased in concrete with minimum cover of 100 mm beyond the edges of steel sections. A minimum clear space of 75 mm should be maintained between the flanges of adjacent grillage beams so that concreting can be made properly. To maintain spacing, pipe separators are used.

Inverted arch foundations are provided in the places where the SBC of the soil is very poor and the load of the structure is through walls. In such cases inverted arches are constructed between the walls. End walls should be sufficiently thick and strong to withstand the outward horizontal thrust due to arch action. The outer walls may be provided with buttress walls to strengthen them.below Figure shows a typical inverted arch footing.

These foundations are known as deep foundations. A pile is a slender column made of wood, concrete or steel. A pile is either driven into the soil or formed in situ by excavating a hole and then filling it with concrete. A group of piles are driven to the required depth and are capped with R.C.C. slab, over which super structure is built. The pile transfer the load to soil by friction or by direct bearing, in the latter case, piles being taken up to hard strata. This type of foundations is used when top soil is not capable of taking the load of the structure even at 3–4 m depth.

Pile foundations are classified according to the materials used and also on the nature of load transfer.
Classification According to Materials Used:Piles may be classified as:
(a) Timber piles
(b) Concrete piles
(c) Steel piles and
(d) Composite piles.

(a) Timber piles:
Circular seasoned wood can be used as piles. Their diameter may vary from 200 mm to 400 mm. Similarly square piles of sizes 200 mm to 400 mm are also used. The length of timber pile should not be more than 20 times its lateral dimension. The bottom of the pile is sharpened and is provided with iron shoe, so that it can be driven in the ground easily by hammering. These piles should be always kept below water table; otherwise alternating wet and dry condition cause the decay. These piles are cheap and can be easily driven rapidly. The main disadvantage is their load carrying capacity is low and are likely to be damaged during driving in the soil.

(b) Concrete piles:
These piles may be further classified as precast piles and cast in situ piles. Precast piles are reinforced with steel and are manufactured in factories. The cross-section diameter/dimension varies from 200 mm to 500 mm. Square, circular and octagonal sections are commonly used. The length of piles may be up to 20 m. They are provided with steel shoe at the lowest end. These piles can carry fairly large loads. These piles are highly resistant to biological and chemical actions of the soil. The disadvantage of these piles is they need more time to manufacture and are heavy to handle.
Cast in situ concrete piles are formed first by boring the holes in the soil and then concreting them. Concreting is usually made using casing tubes. If the hole is filled with only plain concrete it is pressure pile. The load carrying capacity of the piles may be increased by providing enlarged base.
The reinforcement caging may be inserted in the bored holes and to increase load carrying capacity
one or two under reams may be formed. After that concreting may be carried out. Such piles are known as under reamed piles. These piles are provided at regular interval of 2 to 4 m and capping beam is provided over them.

(c) Steel Piles:
 A steel pile may be a rolled steel I sections, tubes or fabricated in the form of box. These piles are mostly used as bearing piles since surface available for friction is less and also the coefficient of friction is less. If tubes are used the soil inside the tube is driven out by compressed air and concrete is filled. These piles are very useful for driving close to existing structures since they disturb the soil least.

(d) Composite Piles:
 Composite piles may be of concrete and timber or of concrete and steel. Wooden piles should not be subjected to alternating wet and dry conditions. Hence they are preferred for the portion below water table. The portion above water table are built with cast in situ concrete piles. If the required length of steel piles is less than the depth of pile, many times upper portions are built with concrete. Thus steel and concrete composite piles are sometimes used.

Classification of Piles According to Load Transfer:
According to the load transfer to the soil piles may be classified as
(a) Bearing piles and
(b) Friction piles.

Bearing piles rest on hard strata and transfer the load by bearing. Such piles are preferred. These piles are used if the hard strata is available at reasonable depth.
Friction piles transfer the load to the soil by the friction between soil and the pile. Such piles are used if hard strata is not available to a considerable depth. The friction developed is to be properly assessed before deciding the length of the pile. The surface of such piles is made rough to increase the skin friction so that required length of pile is reduced.


Black cotton soil swells during rainy season and cracks in summer due to shrinkage. These shrinkage
cracks are 100 mm to 150 mm wide and 0.5 m to 2 m deep. Swelling creates upwards pressure on the
structure and shrinkage creates downward pull. It results into cracks in foundations wall and roof.
Hence foundation in black cotton soil need special care. In case black cotton soil is only to a depth of 1.0 m and 2.0 m it is economical to remove entire black cotton soil from the site and build the foundation on red soil. Apart from this black cotton soil should be removed from the sides of the foundation and filled with sand and gravel.

In case the depth of black cotton soil is more, the following type of foundation may be provided
1. Strip or pad foundation
2. Pier foundation with arches and
3. Under reamed pile foundation.

1. Strip or Pad Foundation: 

   Strip foundation are for walls while pad foundations are for columns. In these foundation the attempt is to keep black cotton soil from foundation by interposing layers of sand and gravel. These foundations should be constructed during dry season. Suitable plinth protection should be made around external walls with its slops away from the wall, so that moisture do not penetrate the foundation during rainy season.below Figure shows such foundations.

2. Pier Foundation with Arches:

 A pier is a vertical columns of relatively larger cross-section than piles. For walls carrying heavy loads, piers are dug at regular intervals and filled with plain concrete. These piers are taken up to good bearing strata. Then the piers are connected by concrete or masonry arch. Over these arches regular masonry is built.below  Figure shows a typical pier foundation with arches.

3.Under Reamed File Foundations: 

Under reamed piles are bored and then concreted at the sites. Their length may vary from 3 to 6 m. They are provided with reams and reinforcement. The pile spacing varies from 2 to 4 m. The top of piles are provided with capping beams over which walls are built.

Thank you,:)