Brick Classes and Field Tests: How to Check Brick Quality

A truckload of bricks arrives on site, and within minutes you have to decide: accept it or send it back. The lab report, if one exists at all, may take days. That is why every site engineer needs to know two things cold — how bricks are classified, and how to test them on the spot with nothing more than a bucket, a tape measure and a bit of judgement. Get this wrong and you end up with walls that crack, plaster that peels and damp that never goes away.

This guide covers the brick classes used in practice, the field tests you can run in ten minutes, and the laboratory numbers behind them, so you can judge a delivery the way an experienced engineer does.

Why brick quality matters more than brick price

Bricks carry load, resist weather and form the skin of the building. A weak or porous brick does not just look bad — it absorbs water, swells and shrinks with the seasons, and slowly weakens the mortar bond around it. In load-bearing walls, under-strength bricks reduce the wall's overall capacity. In exposed brickwork, poor durability shows up as spalling, efflorescence stains and frost damage within a couple of winters.

The cheapest brick on the quotation is rarely the cheapest brick in the finished building. Rework, replastering and damp repairs cost many times what a better class of brick would have cost at the start.

Brick classes: what the grades actually mean

Under IS 1077, burnt clay bricks are classified mainly by their average compressive strength, tested on a bed-face area of 190 mm × 90 mm after 24 hours of immersion in water. The classes run from 3.5 up to 35 N/mm²:

• Class 3.5 and 5: low-strength bricks for non-load-bearing partitions and temporary work.
• Class 7.5 and 10: the workhorses of ordinary residential construction, suitable for most load-bearing walls up to two storeys.
• Class 12.5 and 15: good-quality bricks for heavier loads and multi-storey load-bearing walls.
• Class 20, 25, 30 and 35: high-strength engineering bricks for heavy loads, foundations and damp-prone situations.

Alongside strength, water absorption matters just as much. First-class bricks should absorb no more than 20% of their dry weight after 24 hours of immersion (IS 3495 Part 2). Second-class bricks may go up to 22%, and third-class bricks up to 25% — but third-class bricks are soft, under-burnt and really only fit for temporary work.

In the United States, ASTM C62 uses a different system — grades SW (severe weathering), MW (moderate weathering) and NW (negligible weathering) — based on strength, absorption and saturation coefficient rather than a single strength number. The idea is the same: match the grade to the exposure. A brick that is fine for a sheltered internal wall can fail quickly on an exposed parapet.

Field tests you can run on site in ten minutes

These tests do not replace laboratory testing, but they catch most bad deliveries before a single brick is laid.

1. Visual inspection and dimensional check

Good bricks are uniform in colour — a deep, even red or copper — with sharp, square edges and smooth, flat faces. Reject bricks that are pale and soft (under-burnt), blackened and distorted (over-burnt), or cracked and chipped. Measure a sample: the standard modular size is 190 mm × 90 mm × 90 mm, and IS 1077 allows a tolerance of ±3 mm on length and ±1.5 mm on width and height for first-class bricks. Lay twenty bricks end to end — if the total length drifts more than a few centimetres from the expected 3.8 metres, joint thicknesses will wander and the mason will struggle to keep courses level.

2. The sound test (ringing test)

Hold a brick in one hand and strike it with another brick, or tap it with a small hammer. A good, well-burnt brick gives a clear metallic ringing sound. A dull, thudding sound means the brick is under-burnt, cracked inside, or full of soft material. It takes five seconds and it is surprisingly reliable — experienced engineers trust their ear more than they trust a supplier's brochure.

3. The drop test

Drop a brick flat from about 1 metre onto hard ground. A good brick should not break. If it shatters or crumbles, the batch is weak regardless of what the delivery note claims. Test two or three bricks from different parts of the load, not just the ones on top.

4. The scratch and hardness test

Try to scratch the brick surface with your fingernail. A properly burnt brick should not leave a mark. You can also break a brick and look at the fractured face: it should be compact, homogeneous and free of lumps of lime, stones or hollow pockets. White or grey lumps in the fracture are a warning sign — lime nodules can hydrate later and blow small craters out of the finished wall.

5. The water absorption test

Weigh a dry brick, soak it in water for 24 hours, wipe off the surface water and weigh it again. The percentage gain in weight is the water absorption:

Water absorption (%) = (Wet weight − Dry weight) ÷ Dry weight × 100

A first-class brick should stay under 20%. In practice, bricks absorbing more than 22% will drink water from the mortar, weaken the bond, and invite damp and efflorescence.

6. The efflorescence test

Soak a brick in a shallow tray of water for 24 hours, then let it dry in the shade. If white, powdery salt deposits appear on the surface, the brick contains soluble salts. A light deposit covering under 10% of the surface is classed as slight and usually acceptable; deposits over 50% of the surface are serious and the batch should be rejected for exposed work, because the salts will keep migrating out and staining the wall.

Worked example: checking a delivery with numbers

Say a supplier delivers bricks claimed to be Class 10, and you pull five bricks for testing. In the lab (or at a site testing rig), each brick is soaked for 24 hours and then crushed flat on its bed face. The average failure load comes out at 187.5 kN.

The bed-face area of a standard brick is:

Area = 190 mm × 90 mm = 17,100 mm²

Compressive strength is load divided by area:

Strength = 187,500 N ÷ 17,100 mm² = 10.96 N/mm²

That clears the 10 N/mm² minimum for Class 10, so the strength claim holds. Now check absorption. One brick weighs 2.85 kg dry and 3.30 kg after 24 hours of soaking:

Absorption = (3.30 − 2.85) ÷ 2.85 × 100 = 15.8%

At 15.8%, the brick is comfortably under the 20% limit for first-class bricks. Combined with a clear ring on the sound test and no breakage on the drop test, this is a delivery you can accept with confidence.

Now imagine the same test on another load: average failure load 120 kN, giving 7.0 N/mm², and absorption of 24%. That batch is not Class 10 at all — it is closer to Class 7.5 in strength and third-class in absorption. Reject it, or at most accept it for non-load-bearing partitions at a reduced price, and record the decision in the site diary.

Acceptance checklist for a brick delivery

• Check the delivery note against the order: class, size and quantity.
• Inspect at least 20 bricks from different parts of the load for colour, shape and damage.
• Run the sound test and the drop test on three or four bricks.
• Measure a sample for size tolerance, and lay bricks end to end to check consistency.
• Start a 24-hour water absorption test on five bricks the day the load arrives.
• Look for efflorescence after soaking, especially for exposed brickwork.
• Keep one sample brick from each delivery, labelled with the date and supplier, in case of later disputes.
• Record the result — accepted, accepted with conditions, or rejected — in the site measurement book.

Common site mistakes with bricks

Soaking, or not soaking, before laying. Dry bricks suck water out of the mortar before it can cure, weakening the bond. Bricks should be soaked until saturated but surface-dry before laying — wet on the inside, not dripping on the outside.

Mixing classes in one wall. Different classes shrink and absorb water differently, which shows up as cracking along the boundary between batches. Keep one class per wall panel where possible.

Storing bricks on bare ground. Bricks stacked directly on soil pick up moisture and salts from below. Stack them on a raised, level platform, covered against rain, and never higher than about 1.2 metres so the bottom courses are not crushed.

When to send bricks to the laboratory

Field tests catch most problems, but laboratory testing under IS 3495 is still the right call when the structure is load-bearing and the brick class is critical, when a delivery fails a field test and the supplier disputes it, or when the project specification explicitly demands test certificates. The standard lab suite is compressive strength, water absorption, efflorescence and dimensional tolerance, with warpage checked for high-quality facing bricks.

The cost of a lab test is small compared with the cost of a wall built from the wrong bricks. As a rule of thumb, test one sample of five bricks per 50,000 bricks delivered, and always test the first delivery from a new supplier.

Final word

Good brickwork starts before the first course is laid. Know the class you ordered, run the five-minute tests the moment the truck arrives, and back them up with a 24-hour absorption check and proper records. A site engineer who checks bricks properly saves the project from damp walls, cracked plaster and disputes that drag on long after handover — and that is exactly the kind of quiet competence that builds a reputation on site.