Compressive Strength of First Class Bricks

🧱 Compressive strength of first class bricks:
the complete engineering guide (standards, testing & safety)

🔍 1. What is compressive strength of first class bricks? (definition)

Compressive strength is the capacity of a first class brick to withstand axially directed pushing forces. It is determined by crushing bricks in a compression testing machine (CTM) and expressed in N/mm² or MPa. For first class bricks, the average compressive strength must not be less than 10 N/mm² (IS 1077) with no individual brick below 8 N/mm². This property guarantees load-bearing capacity, durability, and uniform quality.

⚡ Core definition: Compressive strength = Maximum load at failure (N) / gross cross-sectional area (mm²). First class designation also requires water absorption ≤20%, efflorescence nil, and dimensional tolerance ±3%.

📌 2. Why is compressive strength critical for classification?

First class bricks are engineered for structural masonry. Their high compressive strength ensures safety in multi-storey buildings, piers, and retaining walls. It also correlates with low porosity, frost resistance, and long-term durability. Without adequate strength, bricks may crack under design loads, leading to catastrophic failures.

🏭 3. How manufacturing affects compressive strength of first class bricks

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Raw clay

High silica + alumina content with low lime/iron; well-proportioned particles increase strength.

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Firing (1000–1200°C)

Complete vitrification creates a dense, strong matrix. Under-burnt bricks show low compressive strength.

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Moulding

Machine-pressed (vs hand-moulded) gives uniform shape, higher green density → higher final strength.

📊 4. Compressive strength classes (first, second, third & special)

Brick classMin compressive strength (N/mm²)Typical use
First class≥ 10 (avg), ≥ 8 (individual)Load-bearing walls, exposed masonry, columns
Second class≈ 7–10Partition walls, plastered surfaces
Third class≈ 3.5–7Temporary structures, non-load bearing
High strength / Grade A≥ 15 (sometimes 20, 25)Bridges, high-rise, industrial floors

🧪 5. How to test compressive strength: complete step‑by‑step

Standard: IS 3495 (Part 1) or ASTM C67. Below is the detailed procedure for first class bricks.

  • Sampling: Select 10 bricks randomly; 5 for dry test, 5 for wet (optional).
  • Preparation: Immerse in water at 27±2°C for 24h. Remove, wipe, and fill frog (if any) with cement mortar 1:3. Store under damp cloth for 24h, then immerse again for 48h.
  • Condition: Test in saturated surface-dry condition (wet strength) or dry (as required).
  • Setup: Place brick flat between two 3-ply plywood sheets, centered in compression testing machine (capacity 500–2000 kN).
  • Loading: Apply load uniformly at rate 14 N/mm² per minute until no further increase.
  • Calculation: Compressive strength = failure load / average area of bearing faces (mm²). Report average of 5 bricks.

📋 6. Acceptance norms for first class bricks (as per IS 1077)

PropertyRequirement for first class
Avg compressive strength≥ 10 N/mm²
Lowest individual≥ 8 N/mm²
Water absorption (24h)≤ 20% by weight
EfflorescenceNil to slight
Dimensional tolerance±3% for length, ±5% for width/height

⚙️ 7. Factors affecting compressive strength of first class bricks

Raw material: Clay composition (excess lime reduces strength, causes lime popping).
Firing temp & duration: Underfired = weak; overfired = brittle, deformed.
Shape & texture: Warped or cracked bricks fail early.
Water absorption: Inverse relation – higher absorption usually lowers compressive strength.
Testing conditions: Wet strength ~80% of dry. Rate of loading influences results.
Age: Bricks gain some strength with time due to continued pozzolanic action? (minor).

✅ 8. Advantages of high compressive strength in first class bricks

✔️ Higher load capacity → thinner walls, more floor space.
✔️ Superior durability & weather resistance.
✔️ Low maintenance, good fire rating.
✔️ Uniform size saves mortar.
✔️ Suitable for earthquake resistant masonry (with reinforcement).

⚠️ 9. Disadvantages / limitations

❌ Cost 15–25% more than second class.
❌ Heavier → higher transport cost.
❌ Requires skilled labour for testing.
❌ Not all local kilns produce consistent first class quality.

🛡️ 10. Is it safe? – structural safety & factors

Yes, when compressive strength is verified. Design codes (IS 1905) apply partial safety factors: for first class bricks with M1 mortar (1:3 cement-sand), allowable compressive stress can reach 1.2–1.5 N/mm² in masonry, ensuring ample safety. Always combine with proper workmanship.

🏗️ 11. Where are first class bricks used (based on strength)?

Load-bearing walls >3 storeys, reinforced brick slabs, lintels, columns, arches, sewerage structures (with low absorption), exposed façades, and boundary walls requiring strength.

🔄 12. Field estimation vs laboratory test

Lab: definitive compressive strength via CTM. Field: rebound hammer (screwdriver test) gives approximate correlation; drop test (breaking sound) is crude. Only lab results certify first class.

📎 13. Correlation with water absorption & efflorescence

Typically, first class bricks with strength >10 N/mm² have water absorption below 15% (IS requirement 20%). Lower absorption means less efflorescence and higher durability.

❓ 14. Frequently asked questions (20+ FAQ)

1. What is the compressive strength of first class brick in N/mm²?
Minimum 10 N/mm² average; usually 10–15 N/mm².
2. In kg/cm², what is the crushing strength?
10 N/mm² ≈ 102 kg/cm². So first class bricks range 102–153 kg/cm².
3. What is the difference between first class and second class brick strength?
First class ≥10 MPa; second class ~7 MPa. Also, first class has better shape, less absorption.
4. How many bricks are tested for compressive strength?
Usually 5 bricks (or 10 as per some specifications).
5. Can first class bricks be used in foundations?
Yes, if damp-proof course is provided; they resist moisture better.
6. What is the compressive strength of first class brick as per ASTM?
ASTM C62 requires first grade (similar) to have min 10.3 MPa (1500 psi).
7. What happens if compressive strength is below 10 N/mm²?
Brick cannot be classified as first class; may be downgraded to second class or rejected.
8. Does soaking bricks reduce strength?
Yes, wet strength is about 15–20% lower. First class wet strength must be >8 MPa.
9. What is the size of test specimen?
Full brick (190×90×90 mm) tested flat.
10. What is the rate of loading for brick test?
14 N/mm² per minute (or 2–5 kN/s).
11. What is the minimum compressive strength for first class brick as per British Standard?
BS 3921: engineering brick A ≥70 N/mm²? That’s different; for first class facing bricks typically ≥10 N/mm².
12. Can we use first class bricks in sewerage?
If strength >10 MPa and absorption <10%, salt resistant, yes.
13. How to visually identify first class bricks?
Uniform deep red, smooth surfaces, no cracks, ringing sound when struck.
14. What is the relation between strength and density?
Higher density usually means higher compressive strength. First class bricks density ~1800–2000 kg/m³.
15. What is the characteristic compressive strength of masonry?
Depends on brick strength and mortar; for first class bricks with M1 mortar, characteristic strength ~3–5 MPa.
16. Why do we soak bricks before testing?
To simulate worst-case (saturated) condition and because bricks absorb moisture from mortar.
17. Is 10 N/mm² safe for 4-storey building?
Yes, with proper wall thickness and mortar design.
18. What does “first class” mean in brick classification?
Highest quality among common burnt clay bricks: best strength, shape, low absorption.
19. How often should bricks be tested at site?
For every 50,000 to 1 lakh bricks, one set of 5 bricks tested.
20. Which IS code governs first class brick strength?
IS 1077 (specification) and IS 3495 (test methods).
21. What is the tolerance for compressive strength variation?
Individual bricks should not fall below 8 MPa if average is ≥10 MPa.
22. Can first class bricks be used in fireplaces?
Yes, they resist high temperatures better than lower classes.

🌍 15. International standards for first class brick strength

StandardMinimum compressive strength (MPa)
IS 1077 (India)10
ASTM C62 (USA) – Grade NW10.3 (1500 psi)
BS 3921 (UK) – Class B engineering50 (but facing bricks lower ~10)
AS/NZS 4455 (Australia)≥10 for structural bricks