Field Tests of Bricks: Site Checks and IS 1077 / IS 3495 Tests
A good burnt clay brick has a uniform size with sharp edges and an even copper-red colour. It gives a clear ringing sound when two bricks are struck together, takes no mark from a fingernail, does not break when dropped from about 1 m onto hard ground and shows no white salt patches after soaking.
For acceptance, IS 1077 requires lab tests done as per IS 3495:
- Compressive strength: this sets the class, from 3.5 to 35 N/mm²
- Water absorption: at most 20% by weight, or at most 15% for classes above 12.5
- Efflorescence: not more than "moderate", or not more than "slight" for classes above 12.5
- Dimensional tolerance: checked on 20 bricks laid in a row
Field tests of bricks
| Test | How to do it | A good brick |
|---|---|---|
| Size and shape | Lay 20 bricks end to end, then side by side, then stacked, and measure each total | Totals within tolerance (see below); straight edges, sharp corners, no warping |
| Colour | Look at the whole stack | Uniform copper-red. Pale yellow means under-burnt; dark, glassy or distorted means over-burnt |
| Sound | Strike two bricks together | A clear metallic ring, not a dull thud |
| Hardness (scratch) | Scratch the surface with a fingernail | No impression left |
| Drop test | Drop a brick flat from about 1 m onto hard ground | Does not break |
| Fracture | Break one brick and look at the broken face | Uniform, compact and fine-grained; no voids, lumps or white lime nodules |
| Soaking | Weigh a dry brick, soak it in water for 24 hours, weigh again | Weight gain well below 20%; no change in shape |
| Efflorescence (field) | Stand a brick in a little water and let it dry out | No noticeable white deposit on the surface |
Classes of bricks as per IS 1077
IS 1077 classifies common burnt clay bricks by their average compressive strength. The class number is the minimum average strength in N/mm², and the classes are:
35, 30, 25, 20, 17.5, 15, 12.5, 10, 7.5, 5 and 3.5
The standard also limits individual results: no single brick may fall more than 20% below the class value.
Dimensional tolerance (20 bricks measured together)
| Brick size (mm) | Length of 20 bricks (mm) | Width of 20 bricks (mm) | Height of 20 bricks (mm) |
|---|---|---|---|
| Modular, 190 × 90 × 90 | 3800 ± 80 | 1800 ± 40 | 1800 ± 40 |
| Non-modular, 230 × 110 × 70 | 4600 ± 80 | 2200 ± 40 | 1400 ± 40 |
Lab tests as per IS 3495
Compressive strength (IS 3495 Part 1)
Remove any unevenness from the bed faces and soak the bricks in water for 24 hours. Fill the frog with 1:1 cement-sand mortar, keep the bricks under damp sacking for 24 hours, then store them in water for 3 days. Test each brick flat, frog upward, in a compression machine. Strength = maximum load ÷ area of the bed face. A set of five bricks is normally tested.
Water absorption (IS 3495 Part 2)
Dry the brick in an oven at 105–115 °C until its mass stops changing, then cool it and weigh it (M1). Immerse it in clean water at room temperature for 24 hours, wipe off the surface water and weigh it again (M2).
Water absorption = (M2 − M1) ÷ M1 × 100.
Efflorescence (IS 3495 Part 3)
Stand the brick on end in a shallow dish of distilled water and let the water be absorbed and evaporate in a warm, ventilated room. Repeat the process, then examine the deposit of salts on the surface.
| Rating | Deposit on the surface |
|---|---|
| Nil | No perceptible deposit |
| Slight | Thin deposit covering up to about 10% of the exposed area |
| Moderate | Heavier deposit covering about 10% to 50% of the area, without powdering or flaking |
| Heavy | Deposit covering more than 50% of the area, without powdering or flaking |
| Serious | Heavy deposit with powdering and/or flaking of the surface |
Warpage (IS 3495 Part 4)
Measure the concave or convex distortion of the faces and edges with a straight edge or flat surface and a wedge-shaped measuring gauge.
Worked example: classifying a lot of bricks
Five non-modular bricks (230 × 110 × 70 mm) were tested for compressive strength. The frogs were filled, so the bed area = 230 × 110 = 25,300 mm².
| Brick | Failure load (kN) | Strength = load ÷ 25,300 mm² (N/mm²) |
|---|---|---|
| 1 | 263 | 10.40 |
| 2 | 283 | 11.19 |
| 3 | 248 | 9.80 |
| 4 | 306 | 12.09 |
| 5 | 276 | 10.91 |
- Average = (10.40 + 11.19 + 9.80 + 12.09 + 10.91) ÷ 5 = 10.88 N/mm²
- The highest class not exceeding 10.88 is class 10.
- Check the individual results: 80% of 10 = 8.0 N/mm². The lowest result is 9.80, which is above 8.0, so the lot is OK.
- Water absorption. The dry mass M1 is 3.10 kg and the soaked mass M2 is 3.55 kg. Absorption = (3.55 − 3.10) ÷ 3.10 × 100 = 14.5%. That is at most 20%, as required for class 10, so the result is OK.
- Dimensions. Twenty bricks laid end to end measured 4630 mm. This is within 4600 ± 80 mm, so the result is OK.
Result: the lot is accepted as class 10 bricks, subject to a satisfactory efflorescence rating, which must be no worse than "moderate" for this class.
To understand how burnt clay, fly ash and other bricks compare, see types of bricks. To estimate brick quantities for a compound wall, use the boundary wall estimator.
Site tips and common mistakes
- Testing only the best bricks. Pick samples at random from different parts of the stack and from different truckloads.
- Ignoring efflorescence. Salts in bricks cause white patches and can make plaster and paint peel. Bricks rated "heavy" or "serious" should not go into exposed or damp-prone walls. To keep ground moisture and its salts out of the walls, also provide a proper damp proof course at plinth level.
- Using under-burnt bricks in load-bearing walls. Yellowish bricks that give a dull sound absorb a lot of water and are weak.
- Laying dry bricks. Soak bricks before laying so they do not draw water out of the mortar, but let the surface dry before use.
- Accepting on the drop test alone. It shows toughness, not the strength class. When a class is specified, do the lab test.
Frequently asked questions
What are the field tests for bricks?
Check size and shape, uniform colour, a clear ringing sound when two bricks are struck, no fingernail scratch mark, no breakage when dropped from about 1 m, and low water absorption and no white salt after soaking.
What is the maximum water absorption of bricks as per IS 1077?
Water absorption after 24 hours of immersion must not exceed 20% by weight for bricks up to class 12.5 and 15% for higher classes.
What are the classes of bricks as per IS 1077?
Common burnt clay bricks are classified by average compressive strength into classes 35, 30, 25, 20, 17.5, 15, 12.5, 10, 7.5, 5 and 3.5 N/mm².
Which IS code covers testing of bricks?
IS 3495 covers the methods. Part 1 is compressive strength, Part 2 is water absorption, Part 3 is efflorescence and Part 4 is warpage. IS 1077 gives the requirements.
How much efflorescence is allowed in bricks?
IS 1077 allows not more than moderate efflorescence for bricks up to class 12.5 and not more than slight for higher classes.
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