BE Civil Engineering (IOE, TU) Concrete Technology and Masonry Structures (IOE, CE 605) Question Paper 2080 Nepal

Step 1 — Target mean strength

$$f_{target} = f_{ck} + 1.65\,s = 30 + 1.65\times 5.0 = 30 + 8.25 = 38.25\ \text{N/mm}^2$$

Step 2 — Selection of w/c ratio

From the strength requirement a w/c of about 0.48 corresponds to ~38 N/mm². The exposure (moderate) caps the free w/c at 0.50. The lower value governs, so adopt w/c = 0.48.

Step 3 — Water content

Base water = 186 kg/m³ at 50 mm slump. Required slump = 75 mm, i.e. 25 mm extra → add 3%:

$$W = 186\times(1+0.03) = 186\times 1.03 = 191.6\ \text{kg/m}^3$$

Step 4 — Cement content

$$C = \frac{W}{(w/c)} = \frac{191.6}{0.48} = 399.2\ \text{kg/m}^3$$

Check minimum cement (300 kg/m³): 399.2 > 300 → OK.

Step 5 — Aggregate volumes (absolute volume method)

Total volume = 1 m³. Entrapped air = 2% = 0.02 m³.

Volume of cement: $\dfrac{399.2}{3.15\times 1000} = 0.1267\ \text{m}^3$

Volume of water: $\dfrac{191.6}{1\times 1000} = 0.1916\ \text{m}^3$

Volume of all-in aggregate:

$$V_{agg} = 1 - (0.1267 + 0.1916 + 0.02) = 1 - 0.3383 = 0.6617\ \text{m}^3$$

Step 6 — Split into coarse and fine

Volume of CA = 0.62 of total aggregate; volume of FA = 0.38.

Coarse aggregate volume = $0.62\times 0.6617 = 0.4103\ \text{m}^3$

Fine aggregate volume = $0.38\times 0.6617 = 0.2514\ \text{m}^3$

Mass of CA = $0.4103\times 2.70\times 1000 = 1107.8\ \text{kg/m}^3$

Mass of FA = $0.2514\times 2.65\times 1000 = 666.3\ \text{kg/m}^3$

Step 7 — Final quantities per m³

Mix proportion by mass (Cement : FA : CA)

$$\frac{399.2}{399.2} : \frac{666.3}{399.2} : \frac{1107.8}{399.2} = \mathbf{1 : 1.67 : 2.78},\quad \text{w/c} = \mathbf{0.48}$$

(a) Slenderness ratio (SR)

Effective height: $h_{eff} = 0.85\times H = 0.85\times 3000 = 2550\ \text{mm}$

Effective thickness (solid wall): $t_{eff} = 230\ \text{mm}$

$$SR = \frac{h_{eff}}{t_{eff}} = \frac{2550}{230} = \mathbf{11.09}$$

(b) Permissible stress and check

Stress reduction factor $k_s$ — interpolate between SR = 10 (0.89) and SR = 12 (0.84) for eccentricity 0:

$$k_s = 0.89 - (0.89-0.84)\times\frac{11.09-10}{12-10} = 0.89 - 0.05\times 0.545 = 0.89 - 0.0273 = 0.863$$

Area reduction factor — loaded area per metre run = $1.0\ \text{m}\times 0.230\ \text{m} = 0.230\ \text{m}^2 \ge 0.2\ \text{m}^2$, so $k_a = 1.0$.

Shape factor $k_p = 1.0$.

Permissible compressive stress:

$$f_{perm} = k_s\,k_a\,k_p\times f_b = 0.863\times 1.0\times 1.0\times 0.96 = 0.828\ \text{N/mm}^2$$

Actual stress (load per metre run over the wall cross-section per metre):

$$f_{actual} = \frac{180\times 10^3\ \text{N}}{1000\ \text{mm}\times 230\ \text{mm}} = \frac{180000}{230000} = 0.783\ \text{N/mm}^2$$

Check: $f_{actual} = 0.783\ \text{N/mm}^2 < f_{perm} = 0.828\ \text{N/mm}^2$.

The wall is SAFE (the applied stress is about 95% of the permissible stress, leaving a small margin).

(a) Factors affecting compressive strength of hardened concrete

1. Water-cement ratio (Abrams' law): For fully compacted concrete, strength is inversely related to the w/c ratio: $f_c = \dfrac{A}{B^{w/c}}$, where $A$ and $B$ are empirical constants. A lower w/c gives a denser hydrated paste with fewer capillary pores, hence higher strength. Excess water beyond that needed for hydration leaves capillary voids on evaporation.
2. Gel/space ratio: Strength depends on the ratio of the volume of hydrated cement gel to the total space available (gel + capillary pores): $f_c = a\,x^n$ where $x$ is the gel/space ratio (typically $n\approx 3$). A higher gel/space ratio (more hydration, less capillary space) yields higher strength independent of age and w/c.
3. Degree of hydration / age: Strength increases with curing age as hydration proceeds.
4. Cement type and content; aggregate quality, grading, and bond (transition zone).
5. Degree of compaction: Voids drastically reduce strength (≈5–6% strength loss per 1% voids).
6. Curing temperature and moisture, and testing conditions (rate of loading, specimen shape/size, moisture state).

(b) Cube strength and cylinder strength

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

$$f_{cube} = \frac{P}{A} = \frac{742\times 10^3\ \text{N}}{22500\ \text{mm}^2} = 32.98\ \text{N/mm}^2 \approx \mathbf{33.0\ \text{N/mm}^2}$$

Cylinder strength = $0.80\times 32.98 = \mathbf{26.4\ \text{N/mm}^2}$.

(c) Rebound hammer limitations and complementary method

Limitations (any two): (i) it assesses only the surface zone (top ~30 mm), so it does not reflect the interior or core strength; (ii) results are strongly affected by surface moisture, carbonation, smoothness, aggregate type near the surface, and hammer orientation, so site calibration is essential. Complementary NDT method: Ultrasonic Pulse Velocity (UPV) test (the combined SonReb method improves reliability); coring for a direct strength check is also acceptable.

(a) Durability

Sulphate attack: Sulphates (from soil, groundwater, or seawater) react with hydrated cement compounds. SO₄²⁻ reacts with calcium hydroxide to form gypsum, and with the calcium aluminate hydrate / monosulphate to form ettringite (calcium sulphoaluminate). Both products are expansive, causing internal pressure, cracking, spalling, softening, and loss of strength. Preventive measures: use Sulphate-Resisting Portland Cement (SRPC, low C₃A) or blended cements with PPC/PSC (fly ash/slag); low w/c ratio (≤0.45) for low permeability; adequate cement content and dense, well-compacted concrete; protective coatings in severe exposure.

Chloride-induced corrosion: Chloride ions (from de-icing salts, marine environment, or contaminated aggregates/water) penetrate the cover, depassivate the protective oxide film on the steel once a threshold concentration is reached, and initiate electrochemical corrosion. The rust occupies a larger volume than steel, generating expansive forces that crack and spall the cover. Preventive measures: adequate, dense cover; low w/c and use of mineral admixtures (silica fume, fly ash, slag) to reduce permeability and chloride diffusion; limit chloride content in mix ingredients; corrosion-inhibiting admixtures, coated/galvanised or stainless reinforcement; surface coatings/cathodic protection in aggressive zones.

(b) Chemical admixtures

Classification: plasticizers (water reducers), superplasticizers (high-range water reducers), retarders, accelerators, air-entraining agents, and combinations (e.g., retarding/accelerating water reducers), plus special types (waterproofing, corrosion inhibitors, etc.).

(i) Plasticizer/superplasticizer: Surface-active (anionic) molecules adsorb onto cement particles and impart a negative charge, causing mutual electrostatic repulsion (and, for polycarboxylate ethers, steric hindrance). This deflocculates the cement, releases trapped water, and disperses particles — giving either higher workability at the same w/c or the same workability at a reduced w/c (hence higher strength).

(ii) Air-entraining agent: Surfactants that stabilise billions of tiny (10–300 µm), well-distributed, discrete air bubbles in the paste. These voids relieve hydraulic pressure during freeze–thaw cycles, improve workability and cohesion, and reduce bleeding/segregation (with a small strength penalty).

(c) Special concretes (any two)

• Self-compacting concrete (SCC): Highly flowable, non-segregating concrete that spreads and fills formwork and encapsulates reinforcement under its own weight without vibration; achieved with superplasticizers, viscosity-modifying agents, high fines, and limited coarse aggregate.
• Fibre-reinforced concrete (FRC): Concrete with dispersed fibres (steel, glass, polypropylene, etc.) that bridge cracks, improving tensile/flexural strength, toughness, ductility, impact and fatigue resistance, and crack control.
• High-performance concrete (HPC): Concrete engineered for superior properties (high strength and/or high durability, low permeability) using low w/c, silica fume and other mineral admixtures, and high-range water reducers.

(a) Slenderness ratio

For a column, the least lateral dimension governs $t_{eff} = 460\ \text{mm}$.

$$SR = \frac{h_{eff}}{t_{eff}} = \frac{3450}{460} = \mathbf{7.5}$$

(b) Permissible compressive stress

Stress reduction factor — interpolate between SR = 6 (1.00) and SR = 8 (0.95):

$$k_s = 1.00 - (1.00-0.95)\times\frac{7.5-6}{8-6} = 1.00 - 0.05\times 0.75 = 1.00 - 0.0375 = 0.9625$$

Area reduction factor — column cross-sectional area:

$$A = 0.460\times 0.460 = 0.2116\ \text{m}^2 \ge 0.2\ \text{m}^2 \Rightarrow k_a = 1.0$$

(The section is larger than 0.2 m², so no area reduction applies.)

Shape factor taken as $k_p = 1.0$.

$$f_{perm} = k_s\,k_a\,k_p\times f_b = 0.9625\times 1.0\times 1.0\times 1.10 = 1.059\ \text{N/mm}^2$$

(c) Safe axial load capacity

Gross cross-sectional area = $460\times 460 = 211600\ \text{mm}^2$.

$$P_{safe} = f_{perm}\times A = 1.059\ \text{N/mm}^2\times 211600\ \text{mm}^2 = 224{,}083\ \text{N}$$ $$\boxed{P_{safe} \approx \mathbf{224\ \text{kN}}}$$

The four major Bogue compounds of OPC:

Roles:

• C₃S (alite): Hydrates rapidly; chiefly responsible for early strength (up to ~7–28 days); liberates a high heat of hydration. Major strength contributor.
• C₂S (belite): Hydrates slowly; responsible for later-age (long-term) strength beyond 28 days; low heat of hydration; contributes to durability.
• C₃A: Reacts very fast (flash set) and gives high early heat; contributes little to ultimate strength; gypsum is added to retard its flash set. High C₃A reduces sulphate resistance.
• C₄AF: Hydrates moderately fast but contributes little to strength; gives cement its grey colour; moderate heat; improves resistance to sulphate attack relative to C₃A.

(a) Workability

Workability is the property of fresh concrete that determines the ease and homogeneity with which it can be mixed, placed, compacted, and finished without segregation or bleeding. Factors affecting it: water content/water–cement ratio, aggregate grading, shape and texture, maximum aggregate size, aggregate–cement ratio, use of admixtures (plasticizers, air-entrainers), cement properties and fineness, mix proportions, and ambient temperature/time since mixing.

(b) Slump and workability class

$$\text{Slump} = \text{cone height} - \text{subsided height} = 300 - 235 = \mathbf{65\ \text{mm}}$$

A slump of 65 mm falls in the medium workability range (typically 50–100 mm).

Suitable application: normal reinforced concrete work — beams, slabs, columns, and footings placed with conventional vibration. (Very low/low slump suits pavements and mass concrete; high slump suits heavily reinforced/congested sections.)

(a) Bulking of sand

Bulking is the increase in the apparent (loose) volume of fine aggregate when it contains moisture. A thin film of water around each sand particle pushes the particles apart through surface tension, increasing the bulk volume. Bulking increases with moisture content up to about 4–6% (where it can reach 20–40%) and then decreases, becoming zero when the sand is fully saturated/inundated. Significance: In volume batching, moist sand occupies more volume for the same mass, so if not corrected the mix gets less actual sand than intended, making it richer in cement and prone to incorrect proportions. The sand volume must be increased to compensate, or batching by mass should be preferred.

(b) Volume to be batched

Bulking increases the loose volume by 28%. To still place the mass equivalent of 1.0 m³ of dry sand, the moist sand must be batched at the bulked volume:

$$V_{bulked} = V_{dry}\times(1+\text{bulking fraction}) = 1.0\times(1+0.28) = \mathbf{1.28\ \text{m}^3}$$

Check via mass: mass of dry sand required = $1.0\times 1600 = 1600\ \text{kg}$. Batching 1.28 m³ of the bulked sand (whose loose unit weight has fallen to $1600/1.28 = 1250\ \text{kg/m}^3$) delivers $1.28\times 1250 = 1600\ \text{kg}$ of dry sand. Confirmed.

(a) Properties of good building bricks

• Uniform size, shape, and sharp, square edges; uniform deep red/copper colour.
• Adequate compressive strength (commonly ≥ 3.5 N/mm² for common burnt-clay bricks; higher for structural grades).
• Low water absorption (preferably ≤ 20% of dry weight by 24-hour immersion).
• Hard, well-burnt, sound (gives a clear metallic ringing sound when struck); free of cracks, flaws, lumps, and nodules of lime.
• Low efflorescence (soluble salts), good durability, weather and fire resistance, and good thermal insulation.

(b) English bond vs. Flemish bond

English bond: Consists of alternate courses of headers and stretchers. One course shows only stretchers (long faces), the next shows only headers (short faces). Continuity of vertical joints is broken using queen closers placed next to the quoin header. It provides excellent overlap and breaking of vertical joints through the wall thickness.

Course 1 (stretchers): [____][____][____][____]
Course 2 (headers):    [_][_][_][_][_][_][_][_]
Course 3 (stretchers): [____][____][____][____]

Flemish bond: Each course shows headers and stretchers laid alternately in the same course, with each header centred over the stretcher below. It gives a more pleasing, symmetrical appearance and uses fewer facing bricks, but has more continuous (less broken) internal joints.

Every course: [____][_][____][_][____][_]
              (stretcher-header-stretcher-header...)

Stronger bond: English bond is generally stronger and is preferred for load-bearing/structural walls, because it provides better breaking of vertical joints and superior transverse strength; Flemish bond is chosen mainly for its better appearance and economy.

(a) Curing

Curing is the process of maintaining adequate moisture and a favourable temperature in concrete for a sufficient period after placing, so that hydration of cement continues and the concrete develops its design strength, durability, and impermeability while controlling shrinkage cracking.

Common methods (any four):

1. Water/ponding (immersion) for slabs and pavements.
2. Wet covering — gunny bags, hessian, sand, or straw kept continuously moist.
3. Sprinkling / fog spraying.
4. Membrane curing using curing compounds or polythene sheeting (sealing moisture in).
5. Steam curing / accelerated curing (for precast elements).

(b) Drying shrinkage vs. creep

Two factors that increase creep: (i) higher sustained stress level and higher w/c ratio / lower-strength concrete; (ii) loading the concrete at an early age (less mature), low ambient humidity, and higher temperature. (Higher cement paste content and finer/poorer aggregate stiffness also increase creep.)

Pulse velocity

$$V = \frac{\text{path length}}{\text{transit time}} = \frac{300\ \text{mm}}{68\ \mu s} = \frac{0.300\ \text{m}}{68\times 10^{-6}\ \text{s}}$$ $$V = 4411.8\ \text{m/s} = \mathbf{4.41\ \text{km/s}}$$

Comment: 4.41 km/s lies in the 3.5–4.5 km/s band, so the concrete quality is rated GOOD (sound, well-compacted concrete, just below the 'excellent' threshold).