Engineering materials

Every civil structure is only as good as the materials it is built from. A civil engineer must know the properties of materials (physical, chemical, mechanical, thermal), their types and composition, and how to select the right one for a job. In Nepal, bricks from Kathmandu Valley and Terai kilns, local stone, cement from domestic plants, sal timber, TMT rebar and bitumen for roads are the everyday materials.

Key concepts

Properties of materials

Group	Properties
Physical	Density, bulk density, specific gravity, porosity, water absorption, permeability, durability, frost and fire resistance
Chemical	Resistance to acids, alkalis, sulphates, corrosion, efflorescence
Mechanical	Strength (compressive, tensile, shear, flexural), elasticity, plasticity, ductility, brittleness, hardness, toughness, creep, fatigue
Thermal	Thermal conductivity, specific heat, thermal expansion, fire resistance

Stones

• Classified geologically (igneous – granite, basalt; sedimentary – sandstone, limestone; metamorphic – marble, slate, quartzite), physically (stratified, unstratified, foliated) and chemically (siliceous, argillaceous, calcareous).
• Good building stone: crushing strength > 100 N/mm² for heavy work, water absorption < 5%, specific gravity > 2.7, durable, free from cracks. Granite and basalt are strong and durable; slate for roofing; marble for flooring.

Bricks

• Composition of good brick earth: silica 50–60%, alumina 20–30%, lime < 5%, iron oxide 5–6%, magnesia < 1%. Excess lime → melting/splitting; iron oxide gives red colour; alkalis cause efflorescence.
• Manufacturing: preparation of clay → moulding → drying → burning (clamp, Bull's trench, Hoffman, zig-zag kilns — many Nepali kilns were rebuilt as cleaner zig-zag kilns after 2015).
• Traditional classification: first-class bricks have compressive strength of about 10.5 N/mm² or more and water absorption ≤ 20%. Standard modular size 190 × 90 × 90 mm (200 × 100 × 100 with mortar).

Tiles

Thin clay or cement products: roofing (Mangalore, country/pantile), flooring (ceramic, vitrified, terrazzo, mosaic), wall glazed tiles. Vitrified tiles have very low water absorption (<0.5%).

Cement

• Ordinary Portland Cement (OPC) composition: lime (CaO) 60–67%, silica 17–25%, alumina 3–8%, iron oxide 0.5–6%, gypsum added (~2–3%) to control setting.
• Bogue compounds: C₃S (early strength, high heat), C₂S (later strength), C₃A (flash set, heat, sulphate attack), C₄AF.
• Types: OPC 33/43/53 grade, PPC (Portland pozzolana – fly ash), PSC (slag), rapid-hardening, low-heat, sulphate-resisting, white cement, quick-setting.
• Nepal Standards (NS) issued by NBSM specify requirements for OPC and PPC sold in Nepal.

Lime

• Quick lime (CaO) from burning limestone; slaked (hydrated) lime Ca(OH)₂ from adding water; sets by carbonation (fat lime) or partly hydraulic action (hydraulic lime, with clay impurities).
• Fat lime: high CaO, slow setting, used in plaster/whitewash. Hydraulic lime: sets under water, used in masonry mortar.

Timber

• Hardwood (sal, sisau, teak — dense, strong; sal is widely used in Nepal for doors/frames) vs softwood (pine, deodar, chir — light, easy to work).
• Seasoning reduces moisture to 10–12% (natural/air, kiln, chemical, water seasoning) to prevent warping and decay.
• Defects: knots, shakes, rot, warp, twist. Preservation by creosote, ASCU (copper-chrome-arsenic) treatment.
• Strength is highest parallel to the grain.

Metals and alloys

• Cast iron (2–4.5% C): strong in compression, brittle. Wrought iron (<0.15% C): ductile. Mild steel (~0.15–0.25% C): ductile, weldable. High-carbon steel: harder, less ductile.
• Rebars: Fe 415, Fe 500, Fe 500D/550 TMT (thermo-mechanically treated) — the number is characteristic yield strength in N/mm²; "D" means higher ductility.
• Alloys: stainless steel (Cr, Ni), aluminium alloys, brass (Cu+Zn), bronze (Cu+Sn).

Paints and varnishes

• Paint constituents: base (white lead, zinc oxide, titanium dioxide — gives opacity), vehicle/binder (linseed oil, acrylic), pigment (colour), thinner/solvent (turpentine), drier, extender/filler.
• Types: oil, enamel, emulsion (water-based), cement paint, bituminous, anti-corrosive (red oxide, zinc chromate primer).
• Varnish = resin dissolved in oil/spirit; transparent protective coat on timber; French polish is a spirit varnish.

Asphalt, bitumen and tar

• Bitumen: black hydrocarbon binder from petroleum distillation; soluble in carbon disulphide.
• Tar: obtained by destructive distillation of coal or wood; more temperature-susceptible and toxic, now rarely used.
• Asphalt: mixture of bitumen with inert mineral matter (natural or mixed asphalt concrete).
• Forms: penetration grades (e.g. 60/70, 80/100 — common in Nepal roads), cutback (bitumen + volatile solvent), emulsion (bitumen in water, used for prime/tack coats in wet weather).
• Uses: road pavements, damp-proofing, waterproofing roofs.

Selection of materials

Criteria: strength, durability, availability, cost, workability, appearance, maintenance, fire and seismic behaviour.

High-yield points

• OPC: CaO 60–67%, SiO₂ 17–25%; gypsum is added to retard setting.
• First-class brick: ≥ 10.5 N/mm², water absorption ≤ 20%; modular size 190 × 90 × 90 mm.
• Seasoned timber moisture ≈ 10–12%; timber is strongest parallel to the grain.
• Bitumen comes from petroleum; tar from destructive distillation of coal or wood.
• Fe 500: characteristic yield strength 500 N/mm²; "D" grades have higher ductility.

Formulas

$\text{Water absorption}(\%) = \frac{W_2 - W_1}{W_1}\times100,\qquad \text{Porosity}(\%) = \frac{V_v}{V}\times100$ Bulk density $= M/V$ (kg/m³); specific gravity $G = \rho_s/\rho_w$; compressive strength $= P/A$.

Worked example

A dry brick weighs 3.00 kg and 3.48 kg after 24 h immersion; its 190 × 90 mm bed face fails at 200 kN.

• Water absorption $= (3.48 - 3.00)/3.00\times100 =$ 16% (≤ 20% ✓)
• Strength $= 200\,000/(190\times90) = 200\,000/17\,100 =$ 11.7 N/mm² (≥ 10.5 ✓, first class)

Exam tips

• C₃S → early strength; C₂S → long-term strength; C₃A → initial setting and heat; gypsum retards setting.
• Excess lime in brick earth causes splitting/melting; alkalis cause efflorescence.
• Tar is from coal; bitumen from petroleum; asphalt = bitumen + mineral aggregate.

Exam-style questions

From Calmon's question bank, which has 25 questions on this topic.

1. Brass is an alloy of

• A. Iron and nickel
• B. Copper and zinc
• C. Copper and tin
• D. Lead and tin

Answer: B. Brass is copper with zinc; bronze is copper with tin; lead and tin form solder.

2. In an oil paint, the vehicle (binder) most commonly used is

• A. Turpentine
• B. Linseed oil
• C. White lead
• D. Litharge

Answer: B. Linseed oil is the vehicle that binds the pigment and forms the film; turpentine is a thinner, white lead a base and litharge a drier.

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