Concrete – Building Materials for JE Exam | Ingredients, Properties, Strength, Curing & Admixtures

ᑭᑎ ᒪEᗩᖇᑎ ᗩᒪEᖇT

4 weeks ago

🧱 Building Materials – CONCRETE (JE Exam Focused)

1️⃣ INGREDIENTS OF CONCRETE (Exam-Focused Concepts)

Concrete is an artificial composite material made by mixing cement, fine aggregate, coarse aggregate, water, and sometimes admixtures.

Key Ingredients

Cement: Binding material (Ordinary Portland Cement most common)
Fine Aggregate: Sand (< 4.75 mm)
Coarse Aggregate: Gravel/stone (> 4.75 mm)
Water: Hydration + workability
Admixtures: Modify properties

🔑 JE Note:
👉 Water–cement ratio governs strength, not cement quantity.

2️⃣ PROPERTIES OF CONCRETE

(A) Fresh Concrete

Workability: Ease of mixing & placing
Measured by Slump Test
Segregation: Separation of aggregates (undesirable)
Bleeding: Water rising on surface

(B) Hardened Concrete

Strength
Durability
Impermeability

🔑 Revision Highlight:
👉 High workability ≠ High strength

3️⃣ STRENGTH OF CONCRETE (Very Important)

Types of Strength

Compressive Strength ⭐ (Most Important)
Tensile Strength ≈ 10% of compressive
Flexural Strength ≈ 15% of compressive

Standard Test

Cube Size = 150 mm × 150 mm × 150 mm
Test Age = 28 days

⭐ MOST REPEATED FORMULA

$$f_c = \frac{P}{A}$$

Where:
= compressive strength (N/mm²)
= load at failure (N)
= loaded area (mm²)

🧠 Short Trick:
👉 “28-150-Cube” → 28 days, 150 mm cube

4️⃣ CURING OF CONCRETE

Curing = maintaining moisture & temperature for hydration.

Methods

Water curing (ponding, sprinkling) ⭐
Membrane curing
Steam curing (Precast works)

⏱️ Minimum Curing Period

OPC: 7 days
Blended cement: 10 days

🔑 JE Question Trend:
👉 Inadequate curing reduces strength up to 40%.

5️⃣ ADMIXTURES (High-Scoring Area)

Admixtures are added (<5%) to modify concrete properties.

Types

Accelerators: Increase early strength
Retarders: Delay setting
Plasticizers: Improve workability
Superplasticizers: High strength concrete ⭐

🧠 Mnemonic:
👉 A-R-P-S → Accelerator, Retarder, Plasticizer, Superplasticizer

✍️ IMPORTANT ONE-LINERS (Last Day Revision)

Water cement ratio law given by Abram
Excess water ↓ strength ↑ workability
Rich mix → more cement
Lean mix → less cement
Bleeding causes weak surface layer
Steam curing gives early strength

📊 PYQ PATTERNS (SSC JE / RRB JE / OSSC JE)

✔ Repeated Topics:

Water–cement ratio
28-day strength
Slump test purpose
Types of admixtures

⚠️ Numerical Traps:

Wrong cube area
Confusing N/mm² with kg/cm²
Assuming strength ∝ cement quantity

🧮 SOLVED EXAMPLE (JE Level)

Q: A 150 mm cube fails at 450 kN. Find compressive strength.

Solution:
Area =
Load =

✅ Answer: 20 MPa

📝 MCQs – Concrete (30 Exam-Level Questions)

Q1. Strength of concrete mainly depends on?
A. Cement content
B. Aggregate size
C. Water-cement ratio ✅
D. Mixing method
📝 Lower w/c ratio → higher strength

Q2. Standard age of testing concrete cube is:
A. 7 days
B. 14 days
C. 21 days
D. 28 days ✅
📝 Characteristic strength is 28-day strength

Q3. Bleeding occurs due to:
A. Excess cement
B. Excess water ✅
C. Fine sand
D. Air entrainment
📝 Water rises to surface

Q4. The workability of concrete is directly proportional to: A. Time of transit

B. Water-cement ratio ✅

C. Grading of aggregate

D. Aggregate-cement ratio

Note: Higher water content increases fluidity, thus increasing workability.

Q5. Segregation is responsible for: A. Honeycombed concrete ✅

B. Porous layers

C. Surface scaling

D. All of the above

Note: Segregation occurs when coarse aggregates separate from the matrix, leading to voids called “Honeycombing.”

Q6. The slump test is not suitable for concrete having: A. Very high workability

B. Very low workability ✅

C. Medium workability

D. Normal workability

Note: For very dry mixes (zero slump), the Vee-Bee Consistometer test is preferred.

Q7. As per IS 456:2000, the modulus of elasticity of concrete ( $E_c$ ) can be assumed as: A. $5000 \sqrt{f_{ck}}$ ✅

B. $5700 \sqrt{f_{ck}}$

C. $0.7 \sqrt{f_{ck}}$

D. $1000 \sqrt{f_{ck}}$

Note: This is the short-term static modulus of elasticity in $N/mm^2$ .

Q8. The target mean strength ( $f_{cm}$ ) is calculated as: A. $f_{ck} + 1.65\sigma$ ✅

B. $f_{ck} - 1.65\sigma$

C. $f_{ck} + 0.825\sigma$

D. $1.5 f_{ck}$

Note: $\sigma$ is the standard deviation. This ensures 95% of test results fall above the characteristic strength.

Q9. Addition of sugar in concrete acts as a: A. Accelerator

B. Retarder ✅

C. Air entraining agent

D. Plasticizer

Note: Sugar delays the setting time of cement significantly.

Q10. For underwater concreting, which method is most commonly used? A. Pumping

B. Tremie Method ✅

C. Skip hoisting

D. Manual pouring

Note: A Tremie pipe prevents the cement from washing away during placement.

Q11. The compaction factor test is more accurate than the slump test for: A. High workability

/B. Low workability ✅

C. Concrete with large aggregates

D. Pavement concrete

Q12. Strength of concrete at 7 days is approximately ________ of its 28-day strength. A. 40%

B. 50%

C. 67% ✅

D. 90%

Q13. In the absence of a concrete mixer, “Hand Mixing” requires extra cement of: A. 5%

B. 10% ✅

C. 15%

D. 20%

Q14. Which admixture is used to improve the workability of concrete without increasing water? A. Calcium Chloride

B. Gypsum

C. Superplasticizers ✅

D. Fly Ash

Q15. The factor of safety for concrete in the Limit State Method is: A. 1.15

B. 1.50 ✅

C. 3.0

D. 1.0

Note: For steel, it is 1.15. Concrete has a higher factor due to field variability.

Q16. Air entrainment in concrete is used to increase: A. Strength

B. Density

C. Resistance to freezing and thawing ✅

D. Modulus of elasticity

Q17. The shape of the aggregate that provides the best workability is: A. Angular

B. Flaky

C. Rounded ✅

D. Irregular

Note: Rounded aggregates have minimum surface area and less internal friction.

Q18. The process of hardening of concrete in the presence of water is called: A. Carbonation

B. Hydration ✅

C. Oxidation

D. Evaporation

Q19. For a M20 grade concrete, the ratio of ingredients (Cement:Sand:Aggregate) is: A. 1:2:4

B. 1:1.5:3 ✅

C. 1:3:6

D. 1:1:2

Q20. Crazing in concrete is caused by: A. Excess vibration

B. Rapid drying of the surface ✅

C. High water-cement ratio

D. Use of admixtures

Q21. The split tensile strength of concrete ( $f_{cr}$ ) as per IS 456 is: A. $0.7 \sqrt{f_{ck}}$ ✅

B. $0.45 \sqrt{f_{ck}}$

C. $0.67 f_{ck}$

D. $0.5 \sqrt{f_{ck}}$

Q22. Non-Destructive Testing (NDT) of concrete is done using: A. Vicat Apparatus

B. Rebound Hammer ✅

C. Slump Cone

D. Le-Chatelier Apparatus

Q23. Which property of concrete is tested using the “Autoclave Test”? A. Strength

B. Soundness due to Magnesia ✅

C. Setting time

D. Durability

Q24. In cold weather concreting, which accelerator is most common? A. Gypsum

B. Calcium Chloride ( $CaCl_2$ ) ✅

C. Retarder

D. Air-entraining agent

Q25. “Creep” in concrete is a: A. Instantaneous deformation

B. Time-dependent deformation ✅

C. Thermal expansion

D. Shrinkage effect

Q26. Which grade of concrete is recommended for RCC work in moderate exposure? A. M15

B. M20

C. M25 ✅

D. M30

Q27. The ratio of tensile strength to compressive strength of concrete is approximately: A. 0.05

B. 0.10 ✅

C. 0.25

D. 0.50

Q28. “Workability” is measured using a Slump Cone of height: A. 15 cm

B. 20 cm

C. 30 cm ✅

D. 45 cm

Q29. For mass concreting (like dams), we use: A. Rapid Hardening Cement

B. Low Heat Cement ✅

C. Quick Setting Cement

D. High Alumina Cement

Q30. Characteristic strength is defined as that strength below which not more than _____ % of results are expected to fall. A. 2%

B. 5% ✅

C. 10%

D. 15%