Table of Contents
6.1 Introduction
- Metals are used extensively in structures
- Metals used for engineering purposes are called ferrous metals with iron as the main constituent — e.g. Wrought iron, Cast iron, Stainless steel etc.
- Other type of metals include non-ferrous metals like copper, zinc, tin, lead etc. wherein iron is not the main constituent
6.2 Ferrous Metal
- Iron is an element with atomic number 26; occurs in four allotropic forms viz. α, β, γ and δ
- α iron — weak, ductile, magnetic, bcc structure; β iron — hard, brittle, non-magnetic; γ iron — fcc structure; δ iron — non-magnetic, absorbs little carbon
- γ iron containing carbon = austenitic; α iron containing carbon = ferritic
6.2.1 Iron — Major Ores
| Iron Ore | Formula | % Iron |
|---|---|---|
| Haematite | Fe₂O₃ | 70% |
| Magnetite | Fe₃O₄ | 75% |
| Limonite | 2Fe₂O₃·3H₂O | 60% |
| Iron Pyrite | FeS₂ | 47% |
| Siderite | FeCO₃ | 40% |
6.2.2 Pig Iron
- Iron obtained from the blast furnace — ore crushed, calcined to remove moisture, smelted in blast furnace where iron gets reduced. Limestone added as flux to remove sulphur, ash etc. — separates as slag.
- Composition: 3–4% carbon, 0.5–3.5% silicon, 0.5–2% manganese, 0.02–0.1% sulphur, 0.03–1% phosphorous
- Hard and brittle; melts easily at 1200°C; cannot be magnetized but can be hardened; high compressive strength but weak in tension and shear; does not rust; riveting/welding cannot be done
- Uses: base plates, columns, door brackets etc.
6.2.3 Cast Iron
- Pig iron re-melted with limestone and coke in Cupola furnace; poured into moulds — product called cast iron
- Contains 2–4% carbon as: (i) combined carbon (cementite) and (ii) free carbon (graphite)
- Difference: Steel is plastic and forgeable; cast iron is neither plastic nor forgeable
Figure 6.1 — Types of Cast Iron: Properties, Composition and Uses (from source table)
Fig. 6.1 — Types of Cast Iron (from source). Grey CI = most common; Malleable CI = automobile parts; Chilled CI = hardened outer surface for mill applications.
6.2.3.2 Properties of Cast Iron
- Hard and brittle; specific gravity = 7.5
- Cannot be riveted/bolted nor welded; cannot be magnetized; not suitable for forging
- Strong in compression (600 N/mm²) but weak in tension (150 N/mm²) and shear
- Low melting point (1200°C); gets affected by sea water
6.2.3.3 Impurities in Cast Iron
| Impurity | Effect on Cast Iron |
|---|---|
| Carbon | Most important. As % carbon increases → melting temperature reduces; shrinkage reduces |
| Silicon (0.5–3%) | Increases fluidity of molten CI; decreases blow holes; increases density. Si >6% → iron hard with mirror-like fracture. |
| Sulphur (≤0.1%) | Highly undesirable. Combines with Mn → MnS. If Mn very low → iron sulphide (FeS) forms → makes CI brittle and weak at higher temperatures. Neutralized by addition of silicon. |
| Phosphorous (0.1–1.5%) | No considerable effect <0.5%. >2% → makes CI brittle and reduces strength. High phosphorous → more fluid, suitable for ornamental castings. |
| Manganese (0.4–1.2%) | Combines with sulphur + carbon → manganese carbide. Increases hardness and tensile strength. Excess of manganese → increases shrinkage of iron. |
6.3 Wrought Iron
- Obtained by removing impurities from cast iron — considered the pure iron
- Total impurities limited to 0.5%; max carbon 0.15%, Si 0.15–0.2%, P 0.12–0.16%, S 0.02–0.03%, Mn 0.03–0.1%
- Produced in a reverberatory (pudding) furnace; molten iron refined by blasting air → puddle balls → groove rollers → flat bars (repeated to remove impurities)
6.3.1 Properties of Wrought Iron
- Ductile, malleable, tough, moderately elastic (modulus of elasticity = 1.86 × 10⁶ N/mm²)
- Ultimate compressive strength ≈ 200 N/mm²; tensile strength ≈ 40 N/mm²
- Melting point = 1500°C; specific gravity = 7.8
- Unlike cast iron, it can be forged and welded; resists corrosion effectively; tough and can withstand shocks
- At ~900°C becomes soft enough that two pieces can be joined by hammering
- Alloying elements: Nickel (Ni) 1.5–3.5% → increases elastic limit and tensile strength; Copper (Cu) → increases corrosion resistance
- Uses: Roof coverings, rivets, chimneys, gates etc.
6.4 Steel — Types and Properties
Most suitable building material among all metallic materials. By suitably controlling carbon content and other alloying elements and heat treatment, a desired combination of strength and ductility can be obtained.
Figure 6.2 — Types of Steel by Carbon Content with Key Properties
Fig. 6.2 — Steel classification by carbon content. MS (0.15–0.3%): sp. gr. 7.3, UCS 800–1200 N/mm², UTS 600–800 N/mm². High Carbon (0.8–1.5%): UCS 1350, UTS 1400–2000 N/mm².
6.5 Heat Treatment of Steel
- Given to develop desired properties; properties can be controlled and changed by various heat treatment processes
- Purpose: remove gases; refine grain size; relieve internal stresses and strains; enhance strength, ductility etc.
Figure 6.3 — Four Types of Heat Treatment of Steel
Fig. 6.3 — Four types of heat treatment: (1) Hardening: heat above critical temp + quench → martensite (very brittle); (2) Tempering: re-heat 100–700°C → relieves strains; (3) Annealing: heat 500–600°C + slow cool → removes strain, imparts softness; (4) Normalizing: heat above critical + cool in air → refines grain structure.
Note: Quenching medium is generally salt water, water or oil depending on desired cooling rate. This heat treatment is given in order to have a desired hardness up to a certain depth in steel. Higher tempering temperature (100–700°C) → softer resulting steel.
6.6 Rolled Steel Sections
- Steel sections can be rolled into various shapes and sizes in rolling mills
- Sections rolled to give large section modulus with minimum cross-sectional area
- IS Handbook No. 1 gives various rolled steel sections — I section, Tee section, channel section etc.
6.7 Reinforcing Steel Bars
- Concrete being weak in tension requires steel as reinforcement to take up tensile stresses
- Effectiveness can be increased by use of low alloy steel or by mechanical strengthening or by heat treatment
| Bar Type | Standard / Description | Key Properties |
|---|---|---|
| Mild Steel (MS) Bars | Fe 250; IS 432 (Part–I):1982; plain round bars; dia 6–50 mm | More ductile than HYSD; absorbs shocks better; has definite yield point |
| HYSD Bars (TOR steel) | High Yield Strength Deformed; ribs on surface; cold twisted deformed = TOR steel bars | Better bond with concrete; no definite yield point; higher tensile strength, bond strength; cold twisted bars more suitable for building purposes |
| TMT Bars | Thermo-Mechanically Treated; extra strength bars; short intensive cooling through water system after last rolling mill stand | Better than MS bars; resist temp up to 500°C; more ductile than cold twisted; excellent bending; very good weldability; no pre/post heating required for welding; resist fatigue and dynamic loading; TMT-HCR: superior resistance to aggressive weather; high thermal resistance up to 600°C |
6.8 Alloy Steel
An alloy is a mixture of two or more metals. Desired properties not available in any single type of steel — combination is done by alloying.
| Type | Composition | Properties | Uses |
|---|---|---|---|
| Stainless Steel | Chromium 16% | Hard, tough, elastic; acid resistant and rust proof | Utensils, ball bearings, dies |
| Nickel Steel | Nickel 3.5% | More elastic; high tensile strength; more hardness; more ductility | Automobile and airplane parts |
| Invar Steel | Nickel 30–40% | Low coefficient of thermal expansion | Precision instruments, measuring tapes |
| Vanadium Steel | Vanadium 0.1–2% | High tensile and yield strength; resistance to softening at high temperature | High speed tools, locomotive castings, chassis |
| Tungsten Steel | Tungsten 14–20% | High cutting hardness; abrasion resistant | Drilling machines, high speed tools |
| Manganese Steel | Manganese 12–15% | Hard, tough, strong; difficult to machine; high electrical resistance | Railway points and crossings, milling equipment, crusher jaws |
| Molybdenum Steel | Molybdenum 0.2–0.3% | High tensile strength at high temperature | Gears, axles, shafts |
6.9 Other Construction Materials
6.9.1 Stone
Stone is a natural, hard substance formed from minerals and earth material present in rocks. Classified as: Igneous rock (crystallization of molten magma e.g. granite), Metamorphic rock (changed by heat and pressure e.g. marble, slate), Sedimentary rock (deposition by glacial action e.g. limestone, sandstone, shale).
- Forms used in construction: (a) Rubble (b) Dimension stone (c) Flagstone (d) Crushed stone
| Stone Type | Rock Type | Properties & Uses |
|---|---|---|
| Granite | Igneous | Hard, strong, durable, capable of high pressure polish. Red, pink, yellow, green, blue, white, brown. Used for flooring, wall paneling, column, mullion facing, stair threads and flagstone. |
| Limestone | Sedimentary | Like dolomite; no cleavage lines; low absorption; smooth, uniform structure; high compressive and tensile strength. Used for wall and floor surfaces. |
| Marble | Metamorphic | Re-crystallized limestone — carrara, parian, onyx and vermont types. Used for flooring, wall and column facing. |
| Sandstone | Sedimentary | Cemented silica grains; texture very fine to coarse. Porous — up to 30% of volume may be pores. Colors from buff, red and light brown. |
| Slate Rock | Metamorphic | Metamorphosis of clay and shale; separated into thin tough sheets (slates). Colors: black, green, red, grey, purple. Used for flooring, window sills, stair threads and facing. |
| Travertine | Sedimentary | Pleasing texture with small natural pockets on cut surface. Used for interior decorative stone. |
6.9.2 Glass
- Transparent or translucent non-crystalline substance made from silica (sand), soda (Na₂CO₃) and lime (CaO or CaCO₃)
- When heated, does not melt at specific temp but becomes plastic — can be moulded by blowing, casting, rolling or extrusion
- Glass is amorphous; super cooled mould with very high viscosity
- Pure silica melts at 1700°C; Na₂CO₃ and CaCO₃ added to bring down melting temperature
- Fe, Mn, Cu, Cr, Co, Ni oxides added for colour control
- Thermal expansion: 3.6 × 10⁻⁶ per °C
- Wired glass: Wire sandwiched between molten glass; Toughened glass: Air glass interface placed into pre-compression; produced by thermal (600–670°C) or chemical process
6.9.3 Plastics
| Type | Description |
|---|---|
| Thermo-plastics | Softens by heat and hardens when cooled; can be used by remoulding as many times as required. When heated → melt to liquid; freeze to glassy state when cooled enough. Can be moulded into any shape. |
| Thermo-setting plastics | Becomes rigid when moulded at suitable pressure and temperature. Cannot be reused. Passes through thermo-plastic stage at 127°–177°C → sets permanently. At ~343°C charring occurs (peculiar characteristic of organic substances). |
| Advantages | Corrosion resistance; low electrical and thermal conductivity; easily formed into complex shapes; wide choice of appearance, colors and transparencies |
| Disadvantages | Low strength; low useful temperature range (upto 600°F); less dimensional stability; ageing effect; sensitive to environment, moisture and chemicals |
6.9.4 FRP (Fibre Reinforced Polymer)
- Fibres: Provide strength and stiffness (e.g. Carbon, Glass, Aramid)
- Matrix: Protects and transfers load between fibres (e.g. Polyester, Epoxy, Vinyl Ester, Urethane)
- FRP is Anisotropic: High strength in direction of fibres; anisotropic behavior affects shear strength, dowel action and bond performance; linear elastic until failure
- Composites features: Impervious to chloride ion and chemical attack; tensile strength greater than steel; 1/4th weight of steel; transparent to magnetic fields and radar frequencies; electrically and thermally non-conductive
- FRP Rebar uses: Concrete members susceptible to chloride corrosion; members requiring non-ferrous reinforcement; alternative to epoxy/galvanized/stainless steel rebars; mining and tunnelling; thermal non-conductivity applications
6.9.5 Ceramics
- Inorganic, non-metallic solid prepared by action of heat and subsequent cooling
- Properties: Hard and brittle; strong in compression; weak in shearing and tension; withstands chemical erosion and high temperatures
- Structural ceramics: Enhanced mechanical properties — bricks, pipes, floor and roof tiles
- Refractories: Retain strength at high temperatures; linings of furnaces, kilns, incinerators. Main oxides: aluminium, silicon and magnesium.
- White wares: Porcelain, china, pottery, stoneware, vitreous tile — impervious to fluids; low conductivity; chemically inert
- Building ceramics: Terracotta (clay-based earthenware), Stoneware (vitreous/semi-vitreous), Tiles (roof, floor, pebble, ceiling, wall), Earthenware, Porcelain (1200–1400°C firing)
6.9.6 Aluminium
- 3rd most plentiful element in earth’s crust; present in 8% of planet’s soil and rocks; found only in chemical compounds in nature
- Raw material: BAUXITE (Al₂O₃·2H₂O — hydrated oxide of aluminium); consists of 45–60% aluminium oxide + sand, iron, other metals
- Major producers: Australia (>1/3 of world supply), Brazil and Jamaica
Key Properties of Aluminium
Melting point660°C | Boiling point: 2470°C
Specific gravity2.70 (density = 1/3 that of steel)
Tensile strengthPure Al: 7–11 MPa | Al alloys: 200–260 MPa
Above 100°CStrength decreases continuously
MagnetismNon-magnetic (paramagnetic) — used in magnet X-ray devices
ConductivityGood conductor of heat and electricity
AppearanceSilvery white metal with bluish tinge; smooth bright finish
6.9.7 Fly Ash
Figure 6.4 — Fly Ash: Sources, Types, Uses and Advantages