BE Civil Engineering (IOE, TU) Civil Engineering Materials (IOE, CE 501) Question Paper 2078 Nepal

(a) Primary vs. secondary bonds

Primary (chemical) bonds are strong inter-atomic bonds (≈ 100–1000 kJ/mol) formed by transfer or sharing of valence electrons:

Secondary (physical) bonds are weak (≈ 1–50 kJ/mol), arising from dipole attractions:

Metallic bonding explains why steel is ductile and a good conductor; covalent/ionic bonding explains why stone and glass are hard but brittle.

(b) Crystalline vs. amorphous structure

• Crystalline: atoms arranged in a regular, repeating 3-D lattice (long-range order). Metals (steel, aluminium) and most minerals are crystalline. They have a definite melting point and often exhibit anisotropy and slip along crystal planes, giving rise to plastic deformation.
• Amorphous (non-crystalline): atoms have no long-range order (short-range order only), e.g. glass, bitumen, many plastics. They soften gradually over a temperature range rather than melting sharply and tend to be brittle (no slip planes).

Influence on mechanical behaviour: in crystalline metals, dislocations move along close-packed planes, allowing plastic flow (ductility). Defects, grain size and impurities (alloying) impede dislocation motion and raise strength/hardness (grain refinement → higher strength, Hall–Petch). Amorphous solids lack slip systems, so they fracture in a brittle manner with little plastic deformation.

(c) Mechanical properties

• Elasticity: ability to recover original shape fully on removal of load (deformation ∝ stress, Hooke’s law).
• Plasticity: ability to retain permanent deformation after the load is removed without rupture.
• Ductility: ability to be drawn into thin wires / undergo large permanent tensile strain before fracture (measured by % elongation, % reduction of area).
• Brittleness: tendency to fracture suddenly with negligible plastic deformation.
• Toughness: ability to absorb energy up to fracture (area under the stress–strain curve); resistance to fracture under impact.
• Hardness: resistance to localised surface indentation, scratching or abrasion.

(i) Mild steel: dominated by ductility and toughness (large plastic plateau, high energy absorption, ~20–25 % elongation).

(ii) Cast iron: dominated by brittleness and hardness (high compressive strength, fails suddenly in tension with very little elongation).

(a) Manufacture of OPC — Wet process

Limestone (CaCO3) + Clay/Shale  -->  Crushing & grinding with water
        |
        v
   SLURRY (35-45% water)  -->  Slurry tanks (correction)
        |
        v
   ROTARY KILN (~1400-1450 C, slight slope, rotating)
   Zones: Drying -> Calcination (CaCO3 -> CaO + CO2) -> Clinkering
        |
        v
   CLINKER (cooled rapidly)  +  ~3-5% GYPSUM
        |
        v
   BALL MILL grinding  -->  PACKING & storage (OPC)

Four major Bogue compounds:

(b) Hydration of cement and role of gypsum

When water is added, the anhydrous compounds react exothermically to form hydration products, chiefly calcium silicate hydrate (C-S-H gel) — the main binding/strength-giving phase — and calcium hydroxide, Ca(OH)₂ (portlandite).

Representative reactions:

$$2C_3S + 6H \rightarrow C_3S_2H_3 \;(\text{C-S-H gel}) + 3Ca(OH)_2$$ $$2C_2S + 4H \rightarrow C_3S_2H_3 + Ca(OH)_2$$

Role of gypsum (CaSO₄·2H₂O): C₃A reacts almost instantly with water causing flash (instantaneous) set, which is undesirable. Gypsum (3–5 %) reacts with C₃A to form ettringite, a protective coating that retards the C₃A reaction, thereby controlling the setting time and giving workable cement.

(c) Fineness by dry sieving

Given: total sample $W = 100\,\text{g}$, residue retained $R = 7.5\,\text{g}$ on the 90 µm sieve.

$$\text{Fineness (\% residue)} = \frac{R}{W}\times 100 = \frac{7.5}{100}\times100 = 7.5\%$$

Result: residue = 7.5 %. Since this is less than the IS limit of 10 %, the cement satisfies the fineness requirement.

(a) Classification of aggregates

By size:

• Fine aggregate — passes 4.75 mm IS sieve (sand).
• Coarse aggregate — retained on 4.75 mm IS sieve (gravel, crushed stone).
• (All-in / combined aggregate contains both.)

By source/origin:

• Natural — river sand, pit sand, gravel, crushed rock.
• Artificial — broken brick, blast-furnace slag, sintered fly-ash, expanded clay.

Importance of grading: A well-graded aggregate (good distribution of particle sizes) gives minimum voids, so less cement paste is needed to fill voids and coat particles. This improves workability, density, strength and economy and reduces segregation and bleeding. Poor (gap) grading increases voids, demands more water/cement, and lowers durability.

(b) Specific gravity and absorption

Given $W_d = 2480\,\text{g}$, $W_{ssd} = 2510\,\text{g}$, $W_w = 1560\,\text{g}$.

Volume of aggregate (incl. permeable pores) $\propto (W_{ssd}-W_w) = 2510-1560 = 950\,\text{g (of water displaced)}$.

(i) Bulk specific gravity (oven-dry):

$$G_{bulk} = \frac{W_d}{W_{ssd}-W_w} = \frac{2480}{950} = \mathbf{2.611}$$

(ii) Apparent specific gravity:

$$G_{app} = \frac{W_d}{W_d-W_w} = \frac{2480}{2480-1560} = \frac{2480}{920} = \mathbf{2.696}$$

(iii) Water absorption:

$$\text{Absorption} = \frac{W_{ssd}-W_d}{W_d}\times100 = \frac{2510-2480}{2480}\times100 = \mathbf{1.21\%}$$

(c) Fineness modulus of fine aggregate

Fineness modulus (FM) = (sum of cumulative % retained on standard sieves) / 100.

$$FM = \frac{2+12+28+52+78+92}{100} = \frac{264}{100} = \mathbf{2.64}$$

Comment: An FM of 2.64 lies in the typical range for fine aggregate (2.2–3.2) and corresponds to medium sand. The grading is consistent with Zone II / Zone III fine aggregate, which is suitable for general structural concrete.

(a) Workability

Workability is the ease with which concrete can be mixed, transported, placed, compacted and finished without segregation or bleeding.

Factors affecting workability:

• Water content / water–cement ratio (most significant)
• Aggregate properties: size, shape (rounded > angular), texture, grading
• Aggregate–cement ratio (richness of mix)
• Use of admixtures (plasticizers, air-entraining agents)
• Time and ambient temperature

Slump test: fresh concrete is filled in a slump cone (300 mm high) in 3 layers, each tamped 25 times; the cone is lifted and the subsidence (slump) in mm is measured — a measure of consistency. Suitable for medium/high workability.

Compaction-factor test: concrete is dropped through two hoppers into a cylinder; the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete is the compaction factor (0–1). More sensitive for low-workability concrete.

(b) Absolute volume mix design (per 1 m³)

Step 1 — Water content: $w/c = 0.50$, cement $= 400\,\text{kg}$

$$\text{Water} = 0.50 \times 400 = \mathbf{200\,kg} \;(=200\,\text{litres})$$

Step 2 — Absolute volumes (m³): (mass / (SG × 1000))

$$V_{cement} = \frac{400}{3.15\times1000} = 0.1270\,\text{m}^3$$ $$V_{water} = \frac{200}{1\times1000} = 0.2000\,\text{m}^3$$ $$V_{air} = 0.02 \times 1 = 0.0200\,\text{m}^3$$

Step 3 — Volume available for aggregate:

$$V_{agg} = 1 - (0.1270 + 0.2000 + 0.0200) = 1 - 0.3470 = 0.6530\,\text{m}^3$$

Step 4 — Split aggregate (FA = 35 %, CA = 65 % by absolute volume):

$$V_{FA} = 0.35\times0.6530 = 0.22855\,\text{m}^3,\quad V_{CA} = 0.65\times0.6530 = 0.42445\,\text{m}^3$$

Step 5 — Masses:

$$M_{FA} = V_{FA}\times G_{FA}\times1000 = 0.22855\times2.65\times1000 = \mathbf{605.7\,kg}$$ $$M_{CA} = V_{CA}\times G_{CA}\times1000 = 0.42445\times2.70\times1000 = \mathbf{1146.0\,kg}$$

Step 6 — Mix proportion by mass (cement : FA : CA):

$$1 : \frac{605.7}{400} : \frac{1146.0}{400} = \mathbf{1 : 1.51 : 2.87}\;(w/c = 0.50)$$

Summary per m³: Cement 400 kg, Water 200 kg, Fine aggregate ≈ 605.7 kg, Coarse aggregate ≈ 1146.0 kg.

(a) Stress–strain curve of mild steel (tension)

Stress
  ^
  |                         U (ultimate stress)
  |                  ____----•----____
  |              __--               --__ • B (breaking/fracture)
  |   P,E  Y_u
  |    •----• • (upper yield)
  |   /      \_• Y_l (lower yield, plateau)
  |  /
  | /  (linear, elastic)
  |/
  +------------------------------------------> Strain
  O

• O–P (Limit of proportionality): stress ∝ strain (Hooke’s law); slope = Young’s modulus E.
• E (Elastic limit): maximum stress up to which the material returns to original shape on unloading (just above P).
• Y_u (Upper yield point): stress at which yielding begins suddenly.
• Y_l (Lower yield point): stress sustained during the yield plateau (plastic flow at nearly constant load).
• U (Ultimate stress): maximum nominal stress carried by the specimen.
• B (Breaking/fracture point): specimen necks down and fractures; nominal stress at fracture is below U.

Nominal (engineering) stress = load / original cross-sectional area; true stress = load / instantaneous (actual) area. After necking, true stress keeps rising while nominal stress falls, so the true stress–strain curve continues upward to fracture.

(b) Tension test calculations

Cross-sectional area of 16 mm bar:

$$A = \frac{\pi}{4}d^2 = \frac{\pi}{4}(16)^2 = 201.06\,\text{mm}^2$$

Yield stress:

$$f_y = \frac{P_y}{A} = \frac{62\times10^3}{201.06} = \mathbf{308.4\,N/mm^2\;(MPa)}$$

Ultimate tensile strength:

$$f_u = \frac{P_u}{A} = \frac{95\times10^3}{201.06} = \mathbf{472.5\,N/mm^2\;(MPa)}$$

Percentage elongation:

$$\%\,\text{elongation} = \frac{L_f - L_0}{L_0}\times100 = \frac{96-80}{80}\times100 = \mathbf{20\%}$$

These values (f_y ≈ 308 MPa, ~20 % elongation) are consistent with ductile mild steel (Fe 250-grade).

Qualities of a good brick: uniform deep-red colour and regular rectangular shape with sharp edges; uniform size; hard and well-burnt (rings with a clear metallic sound on striking); compressive strength ≥ about 3.5 N/mm² (higher for first class); low water absorption; free from cracks, flaws and lime nodules; should not show efflorescence; should resist abrasion (no fingernail scratch).

Water absorption:

$$\text{Absorption} = \frac{W_{wet}-W_{dry}}{W_{dry}}\times100 = \frac{3680-3200}{3200}\times100 = \frac{480}{3200}\times100 = \mathbf{15\%}$$

Comment: For a first-class brick water absorption should not exceed 20 % (by IS practice). Since 15 % < 20 %, the brick is acceptable as first class.

Seasoning of timber is the process of reducing the moisture content of freshly felled (green) timber to a level in equilibrium with the surrounding atmosphere (usually 10–15 %), so the timber is fit for use.

Objectives:

• Reduce/remove sap moisture and shrinkage in service
• Increase strength, stiffness and durability
• Reduce weight (easier handling, transport)
• Make timber less liable to decay, fungal attack and warping
• Make it suitable for painting, polishing and gluing

Comparison:

Common defects: any two of — knots, shakes (cracks), warping/twisting, wane, checks, decay/rot.

Classification of lime:

• Fat lime (quick/high-calcium lime): obtained from pure limestone; slakes vigorously, sets only by carbonation in air (non-hydraulic). High fattiness/plasticity.
• Hydraulic lime: obtained from limestone containing clay; can set under water due to silica/alumina content. Sub-classes: feebly, moderately and eminently hydraulic.
• Poor (lean) lime: from impure limestone (>30 % impurities); slakes slowly, weak.

Fat lime vs. hydraulic lime:

Mortar is a workable paste made by mixing a binding material (cement or lime) with fine aggregate (sand) and water, used to bind masonry units together.

Functions of mortar (any two): binds bricks/stones into a monolithic mass; fills and seals joints to make the wall weather-tight; distributes loads uniformly over the bed; provides a level bedding surface; can give a finished/decorative appearance.

Qualities of a good building stone:

• High compressive strength and hardness
• Durability and weather resistance
• Low water absorption / low porosity
• Good appearance, colour and ability to take a fine polish (for facing)
• Toughness and resistance to abrasion/impact
• Free from cracks, cavities and soft mineral patches; fire resistance; ease of dressing

Compressive strength of cube:

Cross-sectional area $A = 150 \times 150 = 22500\,\text{mm}^2$.

$$f_c = \frac{P}{A} = \frac{320\times10^3\,\text{N}}{22500\,\text{mm}^2} = \mathbf{14.22\,N/mm^2\;(MPa)}$$

(Note: had the specimen been a 200 mm cube failing at 320 kN, the strength would be $320{,}000/40000 = 8.0\,\text{N/mm}^2$. For the 150 mm cube specified, the answer is 14.22 N/mm².)

Bitumen vs. Tar:

Two laboratory tests on bitumen (any two): Penetration test, Softening point (Ring-and-Ball) test, Ductility test, Flash and Fire point test, Viscosity test, Specific gravity test.

(i) Constituents of oil paint:

• Base (white lead, zinc oxide, titanium oxide) — main body, gives opacity and durability.
• Vehicle/binder (linseed/drying oil) — holds pigment particles, forms the film, gives adhesion.
• Pigment — imparts colour and hiding power.
• Solvent/thinner (turpentine) — reduces viscosity for easy application; evaporates.
• Drier — accelerates oxidation/hardening of the film.
• Extender/filler — reduces cost and modifies properties.

(ii) Glass — properties and uses: Amorphous, hard, brittle, transparent/translucent, chemically inert, weather- and corrosion-resistant, good thermal/sound insulation when treated, can be coloured and moulded. Uses: glazing of windows and doors, partitions, facades and curtain walls, skylights, decorative work, glass blocks, and reinforcement (glass fibre/GRC).

(iii) Thermoplastics vs. thermosetting plastics: