BE Civil Engineering (IOE, TU) Civil Engineering Materials (IOE, CE 501) Question Paper 2076 Nepal

(a) Manufacture of OPC -- Wet Process

Raw materials: calcareous material (limestone, chalk -- source of CaO) and argillaceous material (clay, shale -- source of $SiO_2$, $Al_2O_3$, $Fe_2O_3$).

Sequence of operations:

1. Crushing & grinding: Limestone is crushed; clay is washed in wash mills. The two are combined with water to form a slurry (35-50% water).
2. Slurry correction: Chemical composition is adjusted in correcting/blending tanks (lime saturation factor controlled).
3. Burning in rotary kiln: The slurry is fed at the upper end of a slightly inclined, rotating kiln (length up to ~150 m). Hot gases flow counter-current.

  Slurry feed end (top)                         Firing end (bottom)
  +-----------------------------------------------------------+
  | Drying  | Pre-heating | Calcination | Clinkering | Cooling|
  | <100 C  |  100-500 C  |  ~900 C     | 1400-1500 C|        |
  +-----------------------------------------------------------+
        Gas flow  <----------------------------- Flame/fuel

• Drying zone (up to ~100 C): free water evaporates.
• Pre-heating zone (100-500 C): combined water driven off, organic matter burns.
• Calcination zone (~900 C): $CaCO_3 \rightarrow CaO + CO_2\uparrow$.
• Clinkering / burning zone (1400-1500 C): lime, silica, alumina, iron oxide combine; partial fusion forms hard greyish clinker nodules (5-25 mm).
• Cooling zone: clinker cooled rapidly.

4. Grinding with gypsum: Cooled clinker is ground with 3-5% gypsum ($CaSO_4\cdot 2H_2O$) to retard flash setting. The fine powder is packed.

(b) Bogue Compounds

(c) Effect of $C_3S$ and $C_2S$

• $C_3S$: hydrates rapidly, generates high heat of hydration, and is mainly responsible for early strength (up to ~28 days). High-early-strength (rapid hardening) cements are richer in $C_3S$.
• $C_2S$: hydrates slowly, gives low heat of hydration, and contributes to later (long-term) strength beyond 28 days, improving ultimate strength and durability.

Therefore, increasing the $C_3S/C_2S$ ratio speeds strength gain but raises heat output (undesirable for mass concrete); a higher $C_2S$ proportion suits low-heat cement for dams and massive pours.

(a) Quantities per m3 of compacted concrete (absolute volume method)

Take 1 kg of cement as basis. For the mix $1:1.8:3.4$ with w/c $=0.48$:

Sum of solid + water absolute volumes:

$$V_s = 0.0003175 + 0.0006792 + 0.0012593 + 0.0004800 = 0.0027360\ m^3$$

With 2% entrapped air, this material fills 98% of the concrete volume. Volume of concrete produced per 1 kg cement:

$$V_c = \frac{V_s}{0.98} = \frac{0.0027360}{0.98} = 0.0027918\ m^3$$

Cement content per m3:

$$\frac{1.00}{0.0027918} = \mathbf{358.2\ kg/m^3}$$

Multiply the basis quantities by 358.2:

• Cement $= 1.00 \times 358.2 = \mathbf{358.2\ kg/m^3}$
• Fine aggregate $= 1.80 \times 358.2 = \mathbf{644.8\ kg/m^3}$
• Coarse aggregate $= 3.40 \times 358.2 = \mathbf{1217.9\ kg/m^3}$
• Water $= 0.48 \times 358.2 = \mathbf{171.9\ kg/m^3}\ (\approx 172\ litre/m^3)$

Check (absolute volumes): $0.1138 + 0.2434 + 0.4511 + 0.1719 = 0.9802\ m^3$ solids+water, $+0.02$ air $= 1.00\ m^3$. OK.

(b) Factors governing water-cement ratio

1. Required strength/durability -- lower w/c gives higher strength and lower permeability (Abrams' law).
2. Required workability for placing and compaction -- higher w/c improves workability but reduces strength, so a balance set by exposure conditions and available compaction is chosen.

(a) Seasoning of Timber

Seasoning is the process of reducing the moisture content of freshly felled (green) timber to a level in equilibrium with the atmosphere in which it will be used, in a controlled manner so as to avoid defects.

Objectives:

• Reduce moisture content and weight.
• Increase strength, hardness and durability.
• Reduce shrinkage and warping after fixing in place.
• Make timber resistant to decay/fungal attack and suitable for painting/polishing and gluing.
• Improve workability and dimensional stability.

Natural (Air) seasoning vs Kiln seasoning

(b) Four Common Defects

1. Shakes -- separations or cracks along the grain.
   • Cup shake: a curved crack separating one annual ring partly from another.
   • Heart/Star shake: radial cracks running from the pith outwards (wider near centre).

   Cross-section:   (((O)))   <- rings;  star shake = radial cracks from pith O
   

2. Warping -- distortion of a board out of plane due to uneven drying/shrinkage. Forms: bow (curving along length), cup (curving across width), twist (spiral distortion).
3. Knots -- bases of branches embedded in the trunk; they disrupt grain continuity and reduce strength. Dead/loose knots are more harmful than live/tight knots.
4. Checks / Splits -- cracks separating wood fibres, not extending through the whole cross-section (checks) or extending fully (splits), caused by rapid surface drying. End checks appear at the ends of boards.

(Other acceptable defects: rind galls, upsets/ruptures, foxiness/decay, wane.)

Cross-sectional area

Original area:

$$A_0 = \frac{\pi}{4}d^2 = \frac{\pi}{4}(14)^2 = \frac{\pi}{4}(196) = 153.94\ mm^2$$

(a) Limit-of-proportionality stress

$$\sigma_{LP} = \frac{56\times10^3\ N}{153.94\ mm^2} = \mathbf{363.8\ N/mm^2}\ (\approx 363.8\ MPa)$$

(b) Yield stress

$$\sigma_y = \frac{60\times10^3}{153.94} = \mathbf{389.8\ N/mm^2}$$

(c) Ultimate tensile stress

$$\sigma_u = \frac{92\times10^3}{153.94} = \mathbf{597.6\ N/mm^2}$$

(d) Percentage elongation

$$\%\,elongation = \frac{L_f - L_0}{L_0}\times100 = \frac{252-200}{200}\times100 = \frac{52}{200}\times100 = \mathbf{26\%}$$

(e) Percentage reduction in area

Final (neck) area:

$$A_f = \frac{\pi}{4}(9.6)^2 = \frac{\pi}{4}(92.16) = 72.38\ mm^2$$ $$\%\,reduction = \frac{A_0 - A_f}{A_0}\times100 = \frac{153.94-72.38}{153.94}\times100 = \frac{81.56}{153.94}\times100 = \mathbf{52.98\%}\ (\approx 53\%)$$

(f) Stress-strain curve for mild steel

  Stress
  ^
  |                         U (ultimate)
  |                      ___/\___
  |                  ___/        \___ F (fracture)
  |   B (upper yield)/                \
  | A_____.--C (lower yield, plateau)
  | /  P (limit of prop.)
  |/
  +-------------------------------------> Strain
   O

Salient points: O-A linear elastic (Hooke's law obeyed); P limit of proportionality; A elastic limit; B upper yield point; C lower yield point (yield plateau where strain increases at nearly constant stress); rising strain-hardening region to U ultimate stress; then necking with falling load to F fracture (cup-and-cone failure).

(a) Characteristics of a Good Brick

• Uniform in shape (truly rectangular with sharp straight edges and even surfaces) and standard size.
• Uniform deep red / copper colour when burnt.
• Should give a clear metallic ringing sound when struck against another.
• Hard enough that no impression is left when scratched by a fingernail.
• Compressive strength not less than $\approx 3.5\ N/mm^2$ (higher for first class).
• Water absorption not more than ~20% of dry weight after 24 h immersion (<= ~15% for first class).
• Free from cracks, flaws, stones and lumps of lime; low efflorescence.
• Sufficiently tough -- should not break when dropped flat from about 1 m height.
• Low thermal conductivity, sound-proof and fire-resistant; should not show appreciable wear.

(b) Classification by Field Quality

(c) Field Tests

1. Visual inspection & dimension test: Check colour uniformity, shape, sharp edges, freedom from cracks; stack 20 bricks and compare cumulative length/width/height with standard to assess size tolerance.
2. Sound test: Strike two bricks together -- a good (well-burnt) brick gives a clear metallic ringing sound; a dull sound indicates an under-burnt or cracked brick.

(Other field tests: hardness test by fingernail scratch; structure test by breaking and examining the cross-section for uniform compact texture; soundness by dropping from ~1 m.)

Cumulative analysis

Total mass $= 20+80+150+280+290+150+30 = 1000\ g$ OK

Fineness Modulus

FM $= \dfrac{\sum (\text{cumulative \% retained on standard sieves})}{100}$ (pan excluded):

$$FM = \frac{2.0 + 10.0 + 25.0 + 53.0 + 82.0 + 97.0}{100} = \frac{269.0}{100} = \mathbf{2.69}$$

Comment

For fine aggregate (sand) the usual FM range is about 2.2 - 3.2. A value of 2.69 lies comfortably in this band, indicating a medium sand that is well graded and suitable for general concrete work. (Higher FM => coarser sand; lower FM => finer sand.)

(a) Classification of Lime & Fat vs Hydraulic

Based on composition / setting behaviour lime is broadly classified as:

• Fat lime (pure / high-calcium / quick lime): obtained from nearly pure limestone (<= ~5% impurities); rich in CaO.
• Hydraulic lime: contains clay (silica + alumina) which gives it the property of setting under water; sub-grades -- feebly, moderately and eminently hydraulic.
• Poor (lean) lime: more than ~30% impurities; slakes slowly, gives weak mortar.

(b) Slaking of Lime

Slaking is the chemical reaction of quicklime (calcium oxide) with water to form slaked (hydrated) lime, calcium hydroxide, accompanied by liberation of heat and expansion:

$$CaO + H_2O \rightarrow Ca(OH)_2 + \text{heat}$$

The slaked lime is then used in mortar/plaster. (Fat lime slakes rapidly with much heat and volume increase; hydraulic lime slakes slowly.)

(c) Mortar

Mortar is a workable paste of a binder (cement or lime) + fine aggregate (sand) + water, used to bind masonry units, fill joints, plaster surfaces and bed/level.

(a) Workability

Workability is the property of freshly mixed concrete that determines the ease and homogeneity with which it can be mixed, placed, compacted and finished without segregation or bleeding.

Factors affecting workability:

• Water content / water-cement ratio (most significant).
• Aggregate properties: size, shape (rounded vs angular), texture, grading.
• Aggregate-cement ratio (richness of mix).
• Use of admixtures (plasticizers, superplasticizers, air-entraining agents).
• Cement properties & fineness.
• Ambient temperature and time since mixing.

(b) Slump Test

Apparatus: a slump cone (frustum: bottom dia 200 mm, top dia 100 mm, height 300 mm), tamping rod (16 mm dia, 600 mm long).

Procedure: place cone on a smooth surface, fill in 4 layers, each tamped 25 times; strike off top; lift cone vertically; measure the vertical drop (slump) of the concrete from the top of the cone.

   |   100   |        Lift cone ->     ____
   | (top)   |                        /    \  } slump
   |\        /|                      /      \__
   | \______/ |   300 mm           /  concrete \
   |          |                   /  subsides    \
   |__200(bot)|                  /________________\

Types of slump:

• True slump -- concrete subsides evenly (valid result).
• Shear slump -- top half shears off and slides (lean/harsh mix; retest).
• Collapse slump -- concrete collapses completely (very wet/high w/c mix).

Typical slump values: very low 0-25 mm (road/pavement, vibrated); low 25-50 mm (mass concrete, foundations); medium 50-100 mm (normal RCC slabs, beams, columns); high 100-175 mm (heavily reinforced/congested sections, pumped concrete).

(c) Interpretation of 75 mm slump

A true slump of 75 mm lies in the medium-workability range. It indicates concrete that is readily placeable and compactable with ordinary reinforcement. It is suitable for normal reinforced concrete work such as slabs, beams and columns of moderate reinforcement, placed with hand or limited vibration.

(a) Bitumen vs Tar

(b) Three Laboratory Tests on Bitumen

1. Penetration test -- measures the consistency/hardness. A standard needle under a 100 g load penetrates the sample for 5 s at 25 C; depth of penetration in tenths of a mm = penetration value. Softer bitumen => higher penetration; used to grade bitumen (e.g. 60/70, 80/100).
2. Softening point test (Ring & Ball) -- measures the temperature at which bitumen attains a particular softness; a steel ball sinks through a bitumen disc in a ring as it is heated in a water/glycerine bath, dropping 25 mm. Higher softening point => less susceptible to high temperature.
3. Ductility test -- measures adhesion/elongation (flexibility); a briquette of bitumen is pulled apart at 25 C at 50 mm/min, and the elongation in cm at break is the ductility. Adequate ductility (often >= 50-75 cm) ensures the binder will not crack under traffic/temperature movement.

(Other acceptable tests: flash & fire point, viscosity, specific gravity, loss on heating, solubility.)

(a) Qualities of a Good Building Stone

• High crushing strength (>= ~100 N/mm2 for good stones).
• Hardness and toughness to resist wear and impact (especially for road/floor work).
• Low water absorption / low porosity (< ~5%) to resist frost and weathering.
• Durability -- resistant to atmospheric and chemical action.
• Good appearance and uniform colour, capable of taking a polish where required.
• Adequate specific gravity (~2.4-2.8) for stability of heavy structures.
• Fire resistance, soundness (free from cavities/cracks) and easy workability/dressing.
• Good resistance to weathering and seasoning without disintegration.

(b) Rocks Used in Construction

(c) Dressing and Seasoning of Stones

• Dressing of stone: the process of giving a quarried stone a proper shape, size and finish/surface suitable for its position in the work (e.g. hammer-dressed, chisel-dressed, polished). It reduces weight for transport and produces good bedding and appearance.
• Seasoning of stone: allowing a freshly quarried stone to dry out its natural quarry sap (moisture) for some time (a few weeks to months) before use, so it becomes harder and more durable; stones are easier to dress when freshly quarried (still containing sap) and gain strength after seasoning.

(i) Good Paint -- Characteristics & Constituents

Characteristics of a good paint: spreads easily and freely over the surface; forms a thin, uniform, durable and hard film on drying; adheres well; gives a pleasing, stable colour; is impervious/water-resistant and protects the surface from corrosion, decay and weather; dries in reasonable time without cracking, flaking or blistering.

Constituents:

• Base -- principal solid pigment giving body & opacity (e.g. white lead, zinc oxide, titanium dioxide).
• Vehicle / binder -- drying oil (linseed oil) or resin that holds pigment and forms the film.
• Pigment -- gives colour & hiding power.
• Solvent / thinner -- (turpentine, spirit) lowers viscosity for application.
• Drier -- accelerates drying/oxidation.
• Extender / filler -- cheapens and adds bulk.

(ii) Thermoplastic vs Thermosetting Plastics

(iii) Glass -- Properties & Uses

Properties: transparent/translucent; hard and brittle; chemically inert (resists acids except HF); good electrical and thermal insulator; can be coloured, toughened, laminated; weather-resistant and easily cleaned; can be moulded/blown to shapes.

Uses: glazing of windows, doors and curtain walls; partitions and facades; mirrors; glass blocks and skylights; insulation (glass wool); decorative and safety glazing (toughened/laminated).

(Answer any two of the three.)