BE Civil Engineering (IOE, TU) Concrete Technology and Masonry Structures (IOE, CE 605) Question Paper 2078 Nepal

Chemical composition of OPC

OPC is produced by burning a calcareous material (limestone) and an argillaceous material (clay) at about $1450^{\circ}C$. The principal oxides are lime ($CaO$), silica ($SiO_2$), alumina ($Al_2O_3$) and iron oxide ($Fe_2O_3$), with minor amounts of $MgO$, $SO_3$ and alkalis. During clinkering these oxides combine into four major compounds (Bogue compounds).

Role of the four Bogue compounds

Bogue equations

$$C_3S = 4.071\,CaO - 7.600\,SiO_2 - 6.718\,Al_2O_3 - 1.430\,Fe_2O_3$$ $$C_2S = 2.867\,SiO_2 - 0.7544\,C_3S$$ $$C_3A = 2.650\,Al_2O_3 - 1.692\,Fe_2O_3$$ $$C_4AF = 3.043\,Fe_2O_3$$

Step 1 — $C_3S$

$C_3S = 4.071(64.0) - 7.600(21.0) - 6.718(5.5) - 1.430(3.0)$ $= 260.544 - 159.600 - 36.949 - 4.290 = \mathbf{59.71\%}$

Step 2 — $C_2S$

$C_2S = 2.867(21.0) - 0.7544(59.705) = 60.207 - 45.041 = \mathbf{15.17\%}$

Step 3 — $C_3A$

$C_3A = 2.650(5.5) - 1.692(3.0) = 14.575 - 5.076 = \mathbf{9.50\%}$

Step 4 — $C_4AF$

$C_4AF = 3.043(3.0) = \mathbf{9.13\%}$

Result

The high $C_3S$ content indicates good early-strength characteristics typical of OPC.

Method

In the absolute-volume method the volumes of all solids, water and air sum to $1\ \text{m}^3$.

Step 1 — Cement content

$$C = \frac{\text{water}}{w/c} = \frac{186}{0.50} = 372\ \text{kg/m}^3$$

Step 2 — Absolute volumes of water, cement, air

• Water volume $= \dfrac{186}{1000} = 0.1860\ \text{m}^3$
• Cement volume $= \dfrac{372}{3.15\times 1000} = 0.11810\ \text{m}^3$
• Air volume $= 0.02\times 1 = 0.0200\ \text{m}^3$

Step 3 — Volume available for total aggregate

$$V_{agg} = 1 - (0.1860 + 0.11810 + 0.0200) = 1 - 0.32410 = 0.67590\ \text{m}^3$$

Step 4 — Split into fine and coarse (by absolute volume)

• Fine aggregate volume $= 0.38\times 0.67590 = 0.25684\ \text{m}^3$
• Coarse aggregate volume $= 0.62\times 0.67590 = 0.41906\ \text{m}^3$

Step 5 — Convert aggregate volumes to mass

• Fine aggregate $= 0.25684\times 2.65\times 1000 = 680.6\ \text{kg}$
• Coarse aggregate $= 0.41906\times 2.70\times 1000 = 1131.5\ \text{kg}$

Mix per $\text{m}^3$

Proportions by mass (cement : FA : CA)

$$372 : 680.6 : 1131.5 = 1 : 1.83 : 3.04 \quad (w/c = 0.50)$$

Check: total absolute solid + water + air volume $= 0.11810+0.25684+0.41906+0.1860+0.0200 = 1.000\ \text{m}^3$. OK.

Workability

Workability is the ease with which freshly mixed concrete can be mixed, transported, placed, compacted and finished without segregation. It is governed mainly by the water content and the internal friction of the mix.

Factors affecting workability

1. Water content (most influential).
2. Water-cement ratio and aggregate-cement ratio.
3. Size, shape, texture and grading of aggregate (rounded, well-graded aggregate improves workability).
4. Use of admixtures (plasticizers/superplasticizers increase it).
5. Use of supplementary cementitious materials (fly ash improves it; silica fume reduces it).
6. Ambient temperature and time after mixing.

Slump test

A frustum mould (top $100\ \text{mm}$, bottom $200\ \text{mm}$, height $300\ \text{mm}$) is filled in 3 layers, each tamped 25 times. On lifting the cone, the slump (vertical subsidence in mm) is measured.

   |---100---|
   _________            true slump
  /         \           |
 /           \          v  measured drop
/_____________\  -----  ===
 |---200---|

Suitable for medium to high workability (slump 25-150 mm); poor for very dry or very wet mixes.

Compaction-factor test

Concrete is dropped through two hoppers into a cylinder; the compaction factor = (weight of partially compacted concrete) / (weight of fully compacted concrete). More sensitive at the low-workability (dry-mix) end, where the slump test is insensitive.

Effect of segregation and bleeding on durability

• Segregation: separation of coarse aggregate from mortar gives a non-uniform, honeycombed structure with voids, lowering strength and increasing permeability, hence reduced durability.
• Bleeding: upward migration of water creates a weak, porous, laitance-rich top surface and water channels under aggregates/reinforcement, increasing permeability, promoting carbonation, corrosion of steel and scaling. Both reduce the impermeability that protects concrete from aggressive agents.

Step 1 — Slenderness ratio (SR)

Least lateral dimension $t = 400\ \text{mm}$.

$$SR = \frac{\text{effective height}}{\text{least lateral dimension}} = \frac{3000}{400} = 7.5$$

Step 2 — Eccentricity ratio

Eccentricity is along the $500\ \text{mm}$ direction, so the relevant thickness is $t = 500\ \text{mm}$.

$$\frac{e}{t} = \frac{40}{500} = 0.08$$

Step 3 — Stress-reduction factor $k_s$ (interpolation)

For $e/t = 0.08$, interpolate between $SR = 6$ ($k_s = 0.90$) and $SR = 8$ ($k_s = 0.84$) at $SR = 7.5$:

$$k_s = 0.90 - \frac{(7.5-6)}{(8-6)}(0.90-0.84) = 0.90 - 0.75(0.06) = 0.90 - 0.045 = 0.855$$

Step 4 — Permissible compressive stress

$$f_{perm} = k_s \times f_b = 0.855 \times 1.5 = 1.2825\ \text{N/mm}^2$$

Step 5 — Cross-sectional area

$$A = 400 \times 500 = 200000\ \text{mm}^2$$

Step 6 — Safe axial load

$$P = f_{perm}\times A = 1.2825 \times 200000 = 256500\ \text{N}$$ $$\boxed{P = 256.5\ \text{kN}}$$

The column can safely carry an axial load of about 256.5 kN.

Classification of chemical admixtures

1. Water-reducing admixtures (plasticizers) — reduce water demand for a given workability.
2. High-range water reducers (superplasticizers) — give very high water reduction / very high workability.
3. Retarders — delay setting (useful in hot weather, long hauls).
4. Accelerators — speed up setting/hardening (e.g. cold weather, calcium chloride for plain concrete).
5. Air-entraining agents — introduce fine air bubbles for freeze-thaw resistance.
6. Others — waterproofing, grouting, gas-forming, corrosion-inhibiting, colouring admixtures.

Mechanism of plasticizers / superplasticizers

Cement particles tend to flocculate because of surface charges, trapping mixing water.

• Plasticizers (lignosulphonates) adsorb on cement grains and impart a negative charge, causing electrostatic repulsion that disperses the flocs and releases trapped water, increasing workability (water reduction 5-12%).
• Superplasticizers (sulphonated melamine/naphthalene formaldehyde, or polycarboxylate ethers) act by stronger electrostatic and, for PCE, steric hindrance from side chains, giving water reduction of 20-40% and enabling high-strength / flowing concrete.

Special concretes

Self-Compacting Concrete (SCC)

Highly flowable concrete that spreads and fills formwork under its own weight without vibration, passing through congested reinforcement without segregation (achieved with superplasticizers + viscosity modifiers + high fines). Advantages: no compaction labour, excellent finish, works in congested sections, less noise.

Fibre-Reinforced Concrete (FRC)

Concrete with dispersed steel, glass, polypropylene or natural fibres. Fibres bridge cracks and resist crack propagation. Advantages: improved tensile/flexural strength, ductility, toughness, impact and fatigue resistance, and control of shrinkage cracking.

Rebound (Schmidt) hammer

A spring-loaded mass strikes a plunger held against the concrete surface; the rebound number depends on the surface hardness, which correlates with compressive strength. It is a quick, non-destructive surface-hardness test, calibrated against a graph.

Ultrasonic pulse velocity (UPV)

An ultrasonic pulse is sent through the concrete between a transmitter and a receiver. The velocity of the pulse is related to the elastic modulus and density, hence to homogeneity, voids/cracks and quality.

Calculation

$$V = \frac{\text{path length}}{\text{transit time}} = \frac{0.300\ \text{m}}{66\times 10^{-6}\ \text{s}}$$ $$V = 4545.5\ \text{m/s} = \mathbf{4.55\ \text{km/s}}$$

Comment

Since $V = 4.55\ \text{km/s} > 4.5\ \text{km/s}$, the concrete quality is rated excellent.

Cross-sectional area

$$A = 150 \times 150 = 22500\ \text{mm}^2$$

Individual strengths $\left(f = \dfrac{P}{A}\right)$

• Cube 1: $\dfrac{720\times 10^3}{22500} = 32.00\ \text{N/mm}^2$
• Cube 2: $\dfrac{765\times 10^3}{22500} = 34.00\ \text{N/mm}^2$
• Cube 3: $\dfrac{750\times 10^3}{22500} = 33.33\ \text{N/mm}^2$

Mean strength

$$f_{mean} = \frac{32.00 + 34.00 + 33.33}{3} = \frac{99.33}{3} = 33.11\ \text{N/mm}^2$$

Acceptance check (no individual result below $0.85\,f_{mean}$)

$$0.85 \times 33.11 = 28.15\ \text{N/mm}^2$$

Lowest individual result $= 32.00\ \text{N/mm}^2 > 28.15\ \text{N/mm}^2$.

All three results satisfy the rule; the test results are acceptable. Mean strength = 33.11 N/mm².

Step 1 — Cumulative % retained

Total sample $= 1000\ \text{g}$.

Step 2 — Fineness modulus

FM = (sum of cumulative % retained on the standard sieves 4.75 to 0.15) / 100. Pan is excluded.

$$\sum = 2.0 + 13.0 + 31.0 + 56.0 + 79.0 + 95.0 = 276.0$$ $$FM = \frac{276.0}{100} = 2.76$$

Result

$$\boxed{FM = 2.76}$$

Since $2.6 < 2.76 < 2.9$, the sand is classified as medium sand.

Desirable properties of good bricks

1. Regular shape, sharp edges, uniform size.
2. Uniform deep-red colour, well-burnt.
3. Compressive strength not less than about $3.5\ \text{N/mm}^2$ (common bricks).
4. Water absorption not more than $20\%$ (by mass) after 24 h immersion.
5. Low efflorescence (salt deposits).
6. Should give a clear ringing sound when struck, and not break when dropped from about 1 m.
7. Good resistance to weathering and fire.

Tests for brick quality

• Compressive strength test (crushing in a compression machine).
• Water absorption test (24 h immersion; % increase in mass).
• Efflorescence test (immerse and observe salt deposits on drying).
• Soundness test (strike two bricks; clear ring = sound).
• Hardness test (scratch with a fingernail; no impression for good brick).
• Dimension/shape test (stacking 20 bricks and measuring).
• Structure test (broken brick should be compact, homogeneous, no voids).

English bond vs Flemish bond

Causes of deterioration

Sulphate attack

Sulphates (from soil, groundwater, seawater) react with hydrated $C_3A$ and calcium hydroxide to form ettringite and gypsum, which are expansive, causing cracking, spalling and softening. Controlled by using sulphate-resisting cement (low $C_3A$) and low $w/c$.

Alkali-aggregate reaction (AAR)

Reactive silica in certain aggregates reacts with alkalis ($Na_2O$, $K_2O$) in cement to form an expansive gel that absorbs water and swells, producing map cracking. Controlled by using low-alkali cement, non-reactive aggregate or pozzolans.

Carbonation

Atmospheric $CO_2$ reacts with $Ca(OH)_2$ to form $CaCO_3$, lowering pore-water pH from ~13 to below ~9. This destroys the passive layer on steel, making it vulnerable to corrosion.

Corrosion of reinforcement

Loss of passivity (from carbonation or chloride ingress) lets steel corrode; rust occupies more volume, generating internal pressure that cracks and spalls the cover concrete.

Measures for an aggressive coastal environment

1. Use low water-cement ratio ($\le 0.45$) and adequate cement content for a dense, impermeable concrete, with sufficient cover to reinforcement.
2. Use sulphate-resisting / blended cement (with fly ash, slag or silica fume) to resist chloride and sulphate ingress, plus good compaction and curing.

Step 1 — Load per metre run on a 1 m strip

Consider a $1\ \text{m}$ (1000 mm) length of wall. Axial load on this strip $P = 180\ \text{kN} = 180\times 10^3\ \text{N}$.

Step 2 — Bearing area of the strip

$$A = \text{thickness} \times \text{length} = 230 \times 1000 = 230000\ \text{mm}^2$$

Step 3 — Actual compressive stress

$$f = \frac{P}{A} = \frac{180\times 10^3}{230000} = 0.7826\ \text{N/mm}^2$$

Step 4 — Adequacy check

$$f = 0.78\ \text{N/mm}^2 < f_{perm} = 0.9\ \text{N/mm}^2$$

The actual stress (0.78 N/mm²) is less than the permissible stress (0.9 N/mm²); the wall is SAFE in axial compression.