BUILDING MATERIALS : lime, mortar, aggregates, admixtures and Structural steel Notes

By Deepanshu Rastogi|Updated : March 30th, 2021

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           

Lime

Some Basic Definitions

  1. Calcination: The heating of limestone to redness in contact with air is known as the calcinations.
  2. Hydraulicity: It is the property of lime by which it sets or hardens in damp places, water or thick masonry walls where there is no free circulation of air.
  3. Quick Lime: The lime which is obtained by the calcination of comparatively pure limestone is known as the quick lime or caustic lime. It is capable of slaking with water and has no affinity for carbonic acid.
    • Its chemical composition is (CaO) oxide of calcium and it has great affinity for moisture.
    • The quick lime as it comes out from kilns is known as the lump lime.
  4. Setting: The process of hardening of lime after it has been converted into paste form is known as the setting. It is quite different from mere drying.
  5. Slaked Lime The product obtained by slaking of quick lime is known as the slaked lime or hydrate of lime. It is in the form of white powder and its chemical composition is Ca(OH)2 or hydrated oxide of calcium.

byjusexamprep

  1. Slaking: When water is added to the quick lime in sufficient quantity a chemical reaction takes place.
    • Due to this chemical reaction the quick lime cracks, swell and falls into a powder form which is the calcium hydrate Ca (OH)2 and it is known as the hydrated lime.
    • This process is known as the slaking.

Classification of Limes

  1. Fat Lime: This lime is also known as the high calcium lime. Pure Lime, rich lime or white lime. It is popularly known as the fat lime as it slakes vigorously and its volume is increased to about 2-2-5 times the volume that of quick lime. The percentage of impurities in such limestone is less than 5%.
  2. Hydraulic Lime: This lime is also known as the water lime as it sets under water. It contains clay and some amount of ferrous oxide. Depending upon the percentage of clay present the hydraulic lime is divided into following three types.
    1. Feebly hydraulic lime
    2. Moderately hydraulic lime
    3. Eminently hydraulic lime

The hydraulic lime can set under water and in thick walls where there is no free circulation of air.

  1. Poor Lime: This lime is also known as the impure or lean lime. It contains more than 30% of clay. It slakes very slowly.

Impurities in Limestones

  1. Magnesium carbonate
    • The magnesium limestones are hard, heavy and compact in texture.
    • The magnesium limestones display irregular properties of calcination, slaking and hardening.
    • Upto 5% of magnesium oxide imparts excellent hydraulic properties to the lime.
  2. Clay
    • It is mainly responsible for the hydraulic properties of lime.
    • The percentage of clay to produce hydraulicity in time stone usually varies from 10 to 30.
    • Limes containing 3-5 per cent of clay do not display any hydraulic property and do not set and harden under water.
  3. Silica: In its free form it has a detrimental effect of the properties of lime.
  4. Iron Compounds
    • Iron occurs in small proportions as oxides, carbonates and sulphides.
    • Pyrite or iron sulphide is regarded to be highly undesirable.
    • For hydraulic limes 2-5 per cent of iron oxide is necessary.
  5. Sulphates: Sulphates if present slow down the slaking action and increase the setting rate of limes.
  6. Alkalis: When pure lime is required the alkalis are undesirable. However, up to 5 per cent of alkalis in hydraulic lime do not have any ill effect.

 

 

Mortar

Some Basic Definition

  • Building mortar is defined as a mixture of cement, sand and water.
  • Mortar is similar to concrete but it does not contain coarse aggregate.
  • Mortar are used for filling joints as a binder in stone and brick masonry.

Bulking of Sand

  • In the case of aggregates there is another effect of the presence of moisture viz. bulking which is an increase in the volume of a given mass of sand (fine aggregate) caused by the films of water pushing the sand particle apart. For a moisture content of about 5-8% this increase of volume may be as much as 20-40% depending upon grading of sand.
  • Finer the materials more will be the increase in volume for a given moisture content.

Classification of Mortars

  • Mortars are classified on the basis of the following:
  • Bulk density
  • Kind of binding materials
  • Nature of application
  • Special mortars

Properties of Good Mortar Mix and Mortar

The important properties of a good mortar mix are mobility, place ability and water retention.

  • Mobility
  • It is used to indicate the consistency of mortar mix which may range from stiff to fluid.
  • The mobility of mortar mix depends on the compositions of mortar and the mortar mixes to be used for masonry work are made sufficiently mobile.
  • Placeability
  • The placeability of mortar mix should be such that a strong bond is developed with the surface of the bed.

Properties of a Good Mortar

  • It should be capable of developing good adhesion with the building units such as bricks, stones etc.
  • It should be capable of developing the designed stresses.
  • It should be cheap
  • It should be durable.
  • It should be easily workable.
  • It should set quickly so that speed in construction may be achieved.

Uses of Mortar

  • To bind the building units such as bricks, stones.
  • To carry out pointing and plaster work on exposed surfaces of masonry.
  • To form an even and soft bedding layer for building units.
  • To form joints of pipes.
  • To hide the open joints of brickwork and stonework.
  • To improve the general appearance of structure.

Functions of Sand in Mortar

  1. Bulk
  2. Setting
  3. Shrinkage
  4. Strength

Tests for Mortars

  1. Adhesiveness to Building Units: Mortar is placed to join them so as to from a horizontal joint. If size of bricks is 19 cm x 9 cm x 9, a horizontal join of 9 cm x 9 cm = 81 cm2 will be formed. Ultimate adhesive strength of mortar per cm2 area is obtained by dividing maximum load with = 81 cm2 area.
  2. Crushing Strength: Brick masonry or stone masonry laid in mortar to be tested are crushed in compression machine. The load at which the masonry crushes gives the crushing strength
  3. Tensile Strength: The briquettes are tested in a tension testing machine. Cross-sectional area of central portion is 38 mm x 38 mm or 1444 mm2 or 14.44 cm2.

GUNITING

  • The guniting is the most effective process of repairing concrete work which has been damaged due to inferior work or other reasons. It is also used for providing an impervious layer.
  • Gunite is a mixture of cement and sand, the usual proportion being 1:3. A cement gun is used to deposit this mixture on the concrete surface under a pressure of about 2 to 3 kg/cm2.
  • The surface to be treated is cleaned and washed. The nozzle of gun is generally kept at a distance of about 75 cm to 85 cm from the surface to be treated and velocity of nozzle varies from 120 to 160 m/sec.

 

Aggregate

 

Aggregates are the materials basically used as filler with binding material in the production of mortar and concrete. They are derived from igneous, sedimentary and metamorphic rocks or manufactured from blast furnace slag, etc. Aggregates form the body of the concrete, reduce the shrinkage and effect economy. They occupy 70-80 per cent of the volume and have considerable influence on the properties of the concrete. It is therefore significantly important to obtain right type and quality of aggregates at site. They should be clean, hard, strong, durable and graded in size to achieve utmost economy from the paste. Earlier aggregates were considered to be chemically inert but the latest research has revealed that some of them are chemically active and also that certain types exhibit chemical bond at the interface of aggregates and cement paste. To increase the bulk density of concrete aggregates are used in two markedly different sizes—the bigger ones known to be coarse aggregate (grit) and the smaller ones fine aggregate (sand). The coarse aggregate form the main matrix of concrete and the fine aggregate from the filler matrix between the coarse aggregate

 

Classification of aggregates:

a.On the basis of origin: Natural and artificial

b.On the basis of size: According to size aggregates are classified as coarse aggregate, fine aggregate and all-in- aggregate

Aggregate retained on 4.75 mm sieve are identified as coarse.

Naturally available aggregates of different fractions of fine and coarse sizes are known as                                      all-in-aggregate.

Aggregate most of which passes through a particular size of sieve are known as graded aggregate. For example, a graded aggregate of nominal size 20 mm means an aggregate most of which passes IS sieve 20 mm.

Aggregate passing through 4.75 mm sieve are defined as fine.

c.On the basis of shape: Aggregates are classified as rounded, irregular, angular, and flaky.

Rounded aggregate: These are generally obtained from river or sea shore and produce minimum voids (about 32 per cent) in the concrete.

Irregular aggregate: They have voids about 36 per cent and require more cement paste as compared to rounded aggregate. Because of irregularity in shape they develop good bond and are suitable for making ordinary concrete.

Angular aggregate:They have sharp, angular and rough particles having maximum voids (about 40 per cent). Angular aggregate provide very good bond than the earlier two, are most suitable for high strength concrete and pavements; the requirement of cement paste is relatively more.

Flaky aggregate:These are sometimes wrongly called as elongated aggregate. However, both of these influence the concrete properties adversely. The least lateral dimension of flaky aggregate (thickness) should be less than 0.6 times the mean dimension.

Notes:

1 Very sharp and rough aggregate particles or flaky and elongated particles require more fine material to produce a workable concrete. Accordingly, the water requirement and, therefore, particles should be roughly cubical in shape.

  1. The flakiness and elongation tests are described is IS : 2386 (Part I).

 

Characteristics: 

The properties to be considered while selecting aggregate for concrete are strength, particle shape, specific gravity, bulk density, voids, porosity, moisture content and bulking.

A.Strength:

The strength should be at least equal to that of the concrete.

The test conducted for strength evaluation are crushing test, impact-test and ten per cent fines test. Of these the first one is the most reliable.

B.Stiffness:

The modulus of elasticity of concrete is approximately equal to the weighted average of the moduli of the cement paste and the aggregate, as such the modulus of the coarse aggregate has an important influence on the stiffness of concrete.

  1. Bond strength: The strength of the bond between aggregate and cement paste thus has an important influence on the strength of concrete. There is no standard test for bond but it is known that the rougher the surface texture of the particles, the better the bond.
  2. Shape and texture: The shape influences the properties of fresh concrete more than when it has hardened. Rounded aggregate are highly workable but yield low strength concrete. Same is the case with irregular shaped aggregate. Flaky aggregate require more cement paste, produce maximum voids and are not desirable. Angular shape is the best. Crushed and uncrushed aggregates generally give essentially the same strength for the same cement content.
  3. Specific gravity: The specific gravity of most of the natural aggregates lies between 2.6-2.7. The specific gravity and porosity of aggregates greatly influence the strength and absorption of concrete. Specific gravity of aggregates generally is indicative of its quality. A low specific gravity may indicate high porosity and therefore poor durability and low strength.
  4. Bulk density: The bulk density of aggregate depends upon their packing, the particles shape and size, the grading and the moisture content. For coarse aggregate a higher bulk density is an indication of fewer voids to be filled by sand and cement.
  5. Void ratio:

The void ratio is calculated as Void ratio = 1 – Bulk density/ Apparent specific gravity .

If the voids in the concrete are more the strength will be low.

  1. Porosity: The porous aggregate absorb more moisture, resulting in loss of workability of concrete at a much faster rate.
  2. Moisture content: High moisture content increases the effective water/cement ratio to an appreciable extent and may render the concrete weak.
  3. Bulking: The increase in the volume of a given mass of fine aggregate caused by the presence of water is known as bulking. There will be chances of segregation, honeycombing and reduced yield of concrete.

 

Fineness modulus:

It is a numerical index of fineness, giving some idea about the mean size of the particles in the aggregates. The fineness modulus (F.M.) varies between 2.0 and 3.5 for fine aggregate, between 5.5 and 8.0 for coarse aggregate, and from 3.5 to 6.5 for all-in aggregate. Aggregate, whose F.M. is required, is placed on a standard set of sieves (80, 63, 40, 20, 12.5, 10, 4.75, 2.36, 1.18 mm and 600, 300, 150 m) and the set vibrated. The material retained on each sieve after sieving represent the fraction of aggregate coarser than the sieve in question but finer than the sieve above. The sum of the cumulative percentages retained on the sieves divided by 100 gives the F.M. A fineness modulus of 3.0 can be interpreted to mean that the third sieve i.e., 600 m is the average size. The test procedure is given IS: 2386 (Part I). The object of finding F.M. is to grade the given aggregate for the required strength and workability of concrete mix with minimum cement. Higher F.M. aggregate result in harsh concrete mixes and lower F.M. result in uneconomical concrete mixes

 

FINE AGGREGATES: Sand (> 0.07 mm) is used as a fine aggregate in mortar and concrete. It is a granular form of silica. Sand used for mix design is known as standard sand (IS: 650). In India Ennore Sand is standard sand.

Sand used in mortars for construction purposes should posses at least 85 per cent of the strength of standard sand mortars of like proportions and consistency.

Depending upon its size sand is classified as coarse sand—fineness modulus (F.M.) 2.90-3.20; medium sand—F.M.: 2.60-2.90 and; fine sand—F.M. : 2.20-2.60.

Based on particle size distribution fine aggregate have been divided in four grades from grading zone I to grading zone IV as given in Table.

Percentage Passing for

Sieve  Designation

Grading Zone I

Grading Zone II

Grading Zone III

Grading Zone IV

10 mm

100

100

100

100

4.75 mm

90-100

90-100

90-100

95-100

2.36 mm

60-65

75-100

85-100

95-100

1-18 mm

30-70

55-90

75-100

90-100

600 micron

15-34

35-59

60-79

80-100

300 micron

5-20

8-30

12-40

15-50

150 micron

0-10

0-10

0-10

0-15

 

COARSE AGGREGATES: 

These may be uncrushed, crushed or partially crushed gravel or stone most of which is retained on 4.75 mm IS sieve. They should be hard, strong, dense, durable, clear and free from veins and adherent coatings; and free from injurious amounts of disintegrated pieces, alkali, organic matter and other deleterious substances.

The functions of coarse aggregate are almost same as that of fine aggregate.

 

CINDER AGGREGATES:

They are well-burnt furnace residue obtained from furnaces using coal as fuel and are used for making lime concrete. They should be clean and free from clay, dirt, wood ash or other deleterious matter.

 

BROKEN BRICK AGGREGATES:

They are prepared from well-burnt or over-burnt broken bricks free from under-burnt particles, soil and salt and are used in lime concrete.

 

TESTING OF AGGREGATES:

various tests on aggregates are:

1.PARICLE SIZE DISTRIBUTION TEST

2.FLAKINESS AND ELONGATION INDEX TEST

3.DETRIOUS MATERIAL AND ORGANIC IMPURITY TEST

4.SPECIFIC GRAVITY AND WATER ABSORPTION TEST

5.CRUSHING VALUE TEST

6.ABRASION VALUE TEST

7.SOUNDNESS TEST.

 

 

 

 

 STEEL

Metals:

Metals are aiming the most useful building materials. They exist in nature as compounds like oxides, carbonates, sulphides and phosphates and are known as ores. Metals are derived from ores by removing the impurities. Those used for engineering purposes are classified as ferrous metals, with iron as the main constituent, e.g. cast iron, wrought iron and steel and others like aluminium, copper, zinc, lead and tin in which the main constituent is not iron as non ferrous metals.

Iron:

Iron is by for the most important of the metals used in engineering construction. It is available in abundance, but does not occur freely in nature. It is extracted from ores by Heating the ores in the presence of a reducing agent will result in the formation of CO or CO2, librated as a gas, and metallic iron

Types of iron after extraction:

A.Pig iron :

Composition: 

Pig iron contains 3–4% carbon, 0.5–3.5% silicon, 0.5–2% manganese, 0.02–0.1% sulphur and 0.03–1% Phosphorus.

Properties:

Pig iron is hard and brittle with fusion temperature of 1200°C and melts easily. It can be hardened but cannot be tempered and magnetised. Its compressive strength is high but is weak in tension and shear. Pig iron does not rust and cannot be riveted or welded.

Uses:

Pig iron is most suitable for making columns, base plates, door brackets, etc

B.Cast iron:

Composition:  cast iron contains about 2–4% of carbon in two forms, i.e., as the compound cementite—in a state of chemical combination; and as free carbon—in a state of mechanical mixture.

Properties:

Cast iron is hard and brittle. It can neither be riveted nor welded. It is strong in compression (600 N/mm2 ) but weak in tension (150 N/mm2 ) and shear. Its specific gravity is 7.50. It has low melting point (1200°C) and is affected by sea water. It cannot be magnetized and is not suitable for forging.

Uses:

cast iron is the most used of the cast metals employed in engineering constructions and machines. Some of the more common uses of cost iron are making ornamental castings such as wall brackets, lamp posts; bathroom fittings such as cisterns, water pipes, sewers, manhole covers, sanitary fittings.

It is used as basic material for manufacturing wrought iron and mild steel.

C.Wrought iron:

Composition:

Wrought iron considered to be pure iron, is produced by removing the impurities of cast iron. The total impurities are limited to 0.5 per cent with a maximum percentage of carbon as 0. 15, silicon 0.15–0.2%, Phosphorus 0.12–0.16%, sulphur 0.02–0.03% and manganese 0.03–0.1%.

Properties:

Wrought iron is ductile, malleable, tough and moderately elastic. Its ultimate crushing strength is 200 N/mm2 and ultimate tensile strength is 40 N/mm2.

The modulus of elasticity of wrought iron is 1.86 × 105 N/mm2 — The melting point of wrought iron is 1500°C and sp. gr. about 7.80.

Uses:

Roof coverings, rivets, chains, ornamental iron works such as gates, etc. are made of wrought iron.

STEEL: Steel is the most suitable building material among metallic materials. This is due to a wide range and combination of physical and mechanical properties that steels can have. By suitably controlling the carbon content, alloying elements and heat treatment, a desired combination of hardness, ductility and strength can be obtained in steel.

As per as carbon content is concerned, steel forms anIntermediate stage between cast iron and wrought iron. Cast ironContains carbon from 2 to 4 percent and wrought iron contains 0.15Percent. In steel the carbon content varies from 0.25 to 1.5 percent.

 

Type of steel

Carbon content (%)

Dead mild steel

< 0.15

Mild steel

0.15–0.3

Medium carbon steel

0.3–0.8

High carbon steel

0.8–1.5

or hard steel

(> 1 is also called cast steel or tool steel

 

The prominent steel-making processes are:

  1. Bessemer process
  2. Cementation process
  3. Crucible process
  4. Open Hearth process
  5. Electric Smelting process
  6. Duplex process
  7. Lintz and Donawitz (L.D.) process

Properties and uses:

  1. Mild steel: Also known as low carbon or soft steel. It is ductile, malleable; tougher and more elastic than wrought iron. Mild steel can be forged and welded, difficult to temper and harden. It rusts quickly and can be permanently magnetised. The properties are: Sp. gr. = 7.30, ultimate compressive and tensile strengths 800–1200N/mm2 and 600–800N/mm2 . Mild steel is used in the form of rolled sections, reinforcing bars, roof coverings and sheet piles and in railway track.
  2. High carbon steel: The carbon content in high carbon steel varies from 0.55 to 1.50%. It is also known as hard steel. It is tougher and more elastic than mild steel. It can be forged and welded with difficulty. Its ultimate compressive and tensile strengths are 1350 N/mm2 and 1400–2000 N/mm2 , respectively. Its Sp. gr. is 7.90. High carbon steel is used for reinforcing cement concrete and prestressed concrete members. It can take shocks and vibrations and is used for making tools and machine parts.
  3. High Tensile Steel: The carbon content in high tensile steel is 0.6–0.8%, manganese 0.6%, silicon 0.2%, sulphur 0.05% and phosphorus 0.05%. It is also known as high strength steel and is essentially a medium carbon steel. The ultimate tensile strength is of the order of 2000 N/mm2 and a minimum elongation of 10 per cent. High Tensile steel is used in prestressed concrete construction

Properties of Steel:

The factors influencing the properties of steel are chemical composition, heat treatment, and mechanical work.

A.Chemical Composition:

The presence of carbon in steel gives high degree of hardness and strength. The addition of carbon to iron decreases the malleability and ductility of the metal, and reduces its permeability to magnetic forces.

The tensile strength of hot rolled steel bars is maximum between 1.0 and 1.2 per cent carbon.

The ductility of steel decreases as the carbon content increases.

The resistance of steel to heavy shocks or blows decreases with increase of carbon content.

B.Effect of principal impurities on steel:

It is not feasible to entirely remove impurities in making either iron or steel. The final product always contains small percentages of the metallic impurities like silicon, manganese, sulphur, and phosphorus besides iron and carbon.

In well made steel these impurities generally range between 0.2 and 1.0 per cent.

Of the common impurities, Phosphorus cannot be eliminated in the process of manufacture, whereas most of the silicon and manganese are introduced to improve the metal.

  • Copper: increases resistance to corrosion when present in small percentage.
  • Arsenic: has a tendency to raise the strength and brittleness.
  • Non-Metallic Impurities: are mechanically suspended in the metal and are often called slag inclusions causing brittleness.

C.Heat Treatment: The object of heat treatment is to develop desired properties in steel.

A steel of given composition may be made soft, ductile and tough by one heat treatment, and the same steel may be made relatively hard and strong by another. Heat treatment affects the nature, amount, and character of the metallographic properties.

Some of the principle purposes of heat treatment are as follows:

  1. To enhance properties such as strength, ductility, hardness and toughness.
  2. To relieve internal stresses and strains.
  3. To refine the grain.
  4. To remove gases.
  5. To normalize steel after heat treatment.

D.Hardening:

The objective of this treatment may be to secure a given hardness to a desired depth in steel.

Fully hardened steel are not suitable for most commercial uses because they are hard and brittle and have poor toughness.

E.Tempering:

Since hardened steels do not usually have the combination of properties desired for specific uses, modification is affected by tempering.

When a thick piece of steel is cooled rapidly it develops additional strains as the surface cools quicker than the interior. To relieve this strain, steel is subjected to the process tempering which consists in slowly heating the steel to a predetermined subcritical temperature and then cooling it slowly.

F.Annealing:

It is a general term used for heating and slow cooling of metal, glass or any other material, which has developed strain due to rapid cooling.

Rolled Steel Sections:

Structural steel can be rolled into various shapes and sizes in rolling mills. Usually sections having larger moduli of section in proportion to their cross-sectional areas are preferred. Steel sections are usually designated by their cross-sectional shapes. The shapes of the rolled steel sections available today have been developed to meet structural needs.

 Bureau of Indian Standards provides the dimensions, weights and geometrical properties of various sections. Structural shapes are abbreviated by a certain system described in the hand book for use in drawings, specifications and designs.

The types of rolled structural steel sections are as follows:

  1. Rolled steel I-sections
  2. Rolled steel channel sections
  3. Rolled steel T-sections
  4. Rolled steel angle-sections
  5. Rolled steel tube-sections
  6. Rolled steel bars
  7. Rolled steel flats
  8. Rolled steel plates
  9. Rolled steel sheets
  10. Rolled steel strip

Reinforcing steel bars:

Cement concrete is one of the most versatile and established construction material throughout the world. Concrete being extremely weak in tension requires reinforcement, which is in variably steel. Steel reinforcement is available in the form of bars of specific diameters with different chemical composition, e.g., mild steel and high tensile steel, and surface characteristics— plain or deformed.

Good steel should not have carbon content of more than 0.25%, sulphur content of more than 0.05% and phosphorus content of more than 0.05%. Effectiveness of concrete reinforcement may be enhanced by the use of low-alloy steel, or by mechanical strengthening, or by heat treatment. Mechanical strengthening of steel is done by drawing, stretching, twisting; the yield point of steel rises by about 30%. Heat treatment increases strength and improves mechanical properties of steel and effects 30 to 40% savings on reinforcement. Steel bars can also be strengthened by cold drawing after rolling. Mild steel has a definite yield point. Cold working increases the yield stress of mild steel. Higher yield strength of reinforcing steel bars lowers the steel requirement and thus the cost of reinforcement and its fixing is reduced.

Classification:

Steel for reinforcing bars can be classified according to its use. The earliest steel used for construction purposes was plain mild steel bars, designated as Fe- 250 and so on.

High yield strength deformed bars (HYSD):

bars have lugs, ribs, or deformations on the surface , which inhibit longitudinal movement of the bar relative to the surrounding concrete. Thus, the deformed surface ensures better bond between reinforcement and concrete.

These bars do not have a definite yield point. HYSD bars result in a considerable increase in yield, tensile and bond strength when twisted hot or cold. Cold twisted deformed (CTD) bars are most suitable for building purposes and are widely used in India. CTD bars with trademark TOR are called TOR-steel. Tor-steel is high strength deformed bars with high yield and bond strength.

Thermo Mechanically Treated (TMT):  bars are extra high strength reinforcing bars, which eliminate any form of cold twisting. In this process, the steel bars receive a short intensive cooling as they pass through a water cooling system after the last rolling mill stand.

TMT steel exhibits a definite yield point. It can resist high temperature up to 500°C with no loss of strength. These are more ductile compared to CTD bars. TMT bars possess excellent bendability due to the unique feature of uniform elongation.

TMT bars have high percentage of uniform elongation—thus high formability.

These bars can be used for general concrete reinforcement in buildings, bridges and various other concrete structures. They are highly recommended for use in high-rise buildings because of the saving in steel due to the higher strength.

Rusting andCorrosion:

When steel is exposed to atmosphere, it is subjected to action of atmospheric agencies. The humid air causes the rusting of steel (the formation of oxides on the surface of steel), also the atmospheric conditions along with rain produces oxidation and corrosion. Consequently, the physical and mechanical properties are affected.

To safeguard iron and steel from rusting and corrosion some of the prevalent methods are enamelling; applying metal coatings – galvanizing, tin plating, electroplating; and applying organic coatings – painting and coal tarring. Of these methods painting is the most common. Enamelling consists in melting a flux on the surface of iron in muffle furnace and then coating it with a second layer of more fusible glaze. Galvanising is the process of coating iron with a thin film of zinc, whereas in tin plating a film of tin is coated.

Alloy Steel:

In general, the properties desired in a metal to be used as building material are not present to the best advantage in any single metal. To develop specific properties a combination of metals or metallic substances is done and are classed as alloys. Some of the most common alloys, their properties and uses are given in Table

S. NO.

Alloy steel

Composition

Properties

Uses

1.

Stainless steel

Chromium 16%

Very hard and tough High elastic and ultimate strength  Acid and rust proof

Ball bearings, dies, crushing machines, razors.

2.

Nickle steel

Nickel 3.5%

More elastic Higher tensile strength Lesser brittle than mild steel Improved hardness and ductility

Automobile and airplane parts

3.

Invar steel

Nickel 30 – 40%

Low coefficient of thermal expansion

Delicate instruments

4.

Vanadium steel

Vanadium 0.1 – 2%

High tensile and yield strength Resistance to softening at high temperatures

High speed tools, Locomotive castings, auto parts, chassis

5.

Tungsten steel

Tungsten 14 – 20%

High cutting hardness Resistant to abrasion

Drilling machines, high speed tools

6.

Manganese steel

Manganese 12 – 15%

Hard, tough and strong Difficult to machine high electrical resistance

Points and crossing in railways, rollers, jaws of crushers, Heavy earth and mining equipments

7.

Molybdenum steel

Molybdenum 0.2 – 0.3%

Maintains tensile strength at high temperatures

Gears, axles, shafts.

The important reasons for alloy additions are:

  1. To increase the hardenability of steel. The steel in this group are usually heat treated by quenching and tempering, for it is only this way that the added expanse of the alloys can be justified through the better combination of properties that is obtained.
  2. To strengthen the steel when it is to be used without special heat treatment. The steels that fall in this category are designed specifically for constructional purposes.

 3. To confer some special property such as machinability, corrosion resistance wear resistance, etc.

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