Class 11 Basic Civil Engineering — Exercise Solutions
Key Concepts Quick Recap
| Material | Key Property | Common Use |
|---|
| OPC Cement | Hydraulic binder; reacts with water | Structural concrete, plaster |
| Modular brick | 190×90×90mm; 10mm mortar → 200mm unit | Walls |
| M20 concrete | Mix 1:1.5:3 (cement:sand:aggregate); 20 N/mm² strength | Columns, beams, slabs |
| RCC | Reinforced cement concrete; steel + concrete | Foundations, structures |
| Plinth | Raised base of building above ground level | Protects from moisture |
| DPC | Damp Proof Course; horizontal membrane | Prevents rising damp |
I. Choose the Best Answer (MCQs)
1. The standard size of a modular brick (excluding mortar) is: (a) 230×110×75mm (b) 190×90×90mm (c) 200×100×100mm (d) 250×120×80mm
Answer(b) 190×90×90mm Modular brick dimensions: 190mm (L) × 90mm (W) × 90mm (H). With 10mm mortar on all sides: 200×100×100mm → easy modular calculations. Non-modular (traditional Indian) brick: 230×115×75mm (with mortar: 240×125×85mm). BIS (Bureau of Indian Standards) recommends modular bricks.
2. Concrete gains strength through: (a) Drying (evaporation of water) (b) Hydration (chemical reaction between cement and water) (c) Freezing (d) Addition of sand
Answer(b) Hydration (chemical reaction between cement and water) Hydration: exothermic reaction between cement compounds (C₃S, C₂S, C₃A, C₄AF) and water → forms calcium silicate hydrate (C-S-H gel) → binds aggregate → concrete strength develops. Concrete reaches: 70% strength at 7 days; ~100% at 28 days (standard testing); continues gaining strength for years. Drying ≠ strength — concrete must be kept moist (curing) for 7-28 days.
3. The primary purpose of steel reinforcement in RCC is to: (a) Make concrete heavier (b) Add colour (c) Resist tensile stresses (concrete is weak in tension, strong in compression) (d) Reduce cost
Answer(c) Resist tensile stresses (concrete is weak in tension, strong in compression) Plain concrete: compressive strength ~15-40 N/mm²; tensile strength ~10-15% of compressive. Steel rebar: tensile strength ~250-550 N/mm². RCC (Reinforced Cement Concrete): steel placed in tension zones of beams, columns, slabs → composite material handles both compression (concrete) and tension (steel).
4. A Dumpy level is an instrument used for: (a) Measuring horizontal angles (b) Measuring vertical angles (c) Levelling — determining difference in elevation between points (d) Measuring slope of land
Answer(c) Levelling — determining difference in elevation between points Dumpy level: surveying instrument for levelling work. Used with a levelling staff to find differences in elevation (height). Applications: setting out floors, drainage gradients, roads. Theodolite: measures both horizontal and vertical angles. Total station: measures angles and distances electronically.
5. Damp Proof Course (DPC) is provided: (a) At roof level (b) Horizontally at plinth level (above ground) to prevent rising damp (c) Inside walls only (d) In foundations only
Answer(b) Horizontally at plinth level (above ground) to prevent rising damp DPC: horizontal layer of impermeable material (bitumen felt, rich cement mortar 1:1.5, slate) placed at plinth level (~150-450mm above ground). Prevents rising dampness (moisture from soil moving up through walls by capillary action). Also: vertical DPC in basements and wet areas.
6. The concrete mix designation M20 means: (a) Mix with 20% cement (b) Mix with characteristic compressive strength of 20 N/mm² at 28 days (c) Mix with 20 kg cement per bag (d) 20th standard mix of India
Answer(b) Mix with characteristic compressive strength of 20 N/mm² at 28 days M20: nominal mix ratio 1:1.5:3 (cement:fine aggregate:coarse aggregate by volume). Strength: 20 N/mm² (MPa) at 28 days (standard 150mm cubes tested). Common grades: M15 (1:2:4, foundations), M20 (1:1.5:3, beams/slabs), M25 (1:1:2, heavy structures), M30+ (high-rise, bridges).
7. The instrument used to measure horizontal and vertical angles is: (a) Dumpy level (b) Chain (c) Theodolite (d) Plumb bob
Answer(c) Theodolite Theodolite: precision optical instrument for measuring horizontal and vertical angles. Used for: traversing, triangulation, setting out buildings, alignment of structures. Transit (repeating) theodolite: telescope can be rotated 360° in vertical plane. Total station: electronic theodolite + EDM (Electronic Distance Measurement) + data recorder.
II. Short Answer Questions
Q1. Describe the types of foundations and when each is used.
Answer
Foundation: lowest part of a structure that transfers loads to the soil. All loads (dead, live, wind) must safely transfer to soil without excessive settlement.
Shallow Foundations (depth < width): 1. Strip foundation: Continuous strip under load-bearing walls. Simple, economical for light loads on good soil. 2. Isolated (pad) footing: Square or rectangular slab under individual columns. Most common for framed buildings. 3. Combined footing: Serves two or more closely-spaced columns. 4. Raft/mat foundation: Continuous slab under entire building. Used when soil is weak or loads are heavy; spreads load over maximum area; also for soft clay, fill, or collapsible soils.
Deep Foundations: 1. Pile foundation: Slender columns (timber, concrete, steel) driven or cast deep into ground to reach firm strata or to transfer load by skin friction. Used when shallow soil is weak or when loads are very heavy. 2. Well (caisson) foundation: Large diameter cylinder sunk into ground; used for bridges over rivers.
Selection depends on: soil bearing capacity, load magnitude, type of structure, cost, groundwater table, settlement tolerance.
Q2. What are the types of surveys used in civil engineering?
Answer
Chain surveying: Simplest form; uses chain and tape to measure linear distances on ground; plots map by triangulation of measurements. Suitable for: small flat areas with no obstructions.
Levelling: Determines elevation differences between points. Uses: dumpy level + levelling staff. Applications: drainage, roads, floor levels.
Plane table surveying: Field plotting directly on drawing board while looking at the terrain. Suitable for: small-scale, rapid surveys.
Theodolite traversing: Measures angles (horizontal and vertical) using theodolite; combined with distance measurements for precise surveys. For: roads, railways, large areas.
Total station surveying: Electronic; measures angles and distances simultaneously; data stored digitally. Fastest and most accurate for most modern surveys.
GPS/GNSS surveying: Satellite-based; provides accurate positioning for large-scale projects (highways, dams); limited accuracy in dense urban areas (signal reflection).
Exam Checklist
- Modular brick: 190×90×90mm (with 10mm mortar = 200×100×100mm).
- Hydration: cement + water → strength (NOT drying). Curing: 7-28 days moist.
- RCC: concrete (compression) + steel rebar (tension) = strong in both.
- M20 concrete: 1:1.5:3 mix; 20 N/mm² at 28 days.
- Dumpy level: levelling (elevation differences). Theodolite: angles (horizontal + vertical).
- DPC: at plinth level; prevents rising damp.
- Shallow foundations: strip, isolated, combined, raft. Deep: piles.
- w/c ratio: optimal 0.4-0.6; lower = stronger; higher = weaker.
