IS 3495 Part 1 : 1992Methods of Tests of Burnt Clay Building Bricks: Part 1 Determination of compressive strength

IS 3495 Part 1:1992 specifies the method of test for compressive strength of burnt clay building bricks (the IS 3495 series also covers Part 2 water absorption, Part 3 efflorescence, Part 4 warpage). It is the acceptance test that lets you assign a brick to a class designation and decide whether a delivered lot is fit for masonry.

It is read with the brick product and masonry stack:

• IS 1077 — common burnt clay building bricks (the *classes* and acceptance criteria)
• IS 5454 — methods of sampling of clay building bricks (how many to test)
• IS 2212 — code of practice for brickwork · IS 1905 — structural use of unreinforced masonry

The brick is conditioned, the frog (and any voids) filled flush with 1:1 cement–sand mortar, the bed faces capped/levelled, and the unit cured, then crushed in a compression testing machine at the specified loading rate. The compressive strength is the failure load divided by the gross bed area.

The result is then read against IS 1077:

• A brick is given a class designation equal to its average compressive strength (e.g. class 3.5, 7.5, 10, 12.5 N/mm² …)
• Acceptance is on the average of the sample meeting the class, with no individual brick more than 20% below the class strength
• Common burnt clay bricks have a minimum class of 3.5 N/mm²; structural masonry typically needs higher classes per IS 1905

Proper frog-filling and capping is essential — an unfilled frog or unlevelled bed gives a falsely low result.

Sample: 5 bricks drawn per IS 5454 from the lot; bed area 190 mm × 90 mm = 17,100 mm².

Failure loads → strengths: 8.4, 7.6, 9.1, 6.9, 8.0 N/mm² (= load / 17,100).

Step 1 — mean: (8.4+7.6+9.1+6.9+8.0)/5 = 8.0 N/mm².

Step 2 — class check against IS 1077: the mean (8.0) comfortably meets class 7.5; does any unit fall more than 20% below 7.5 (i.e. below 6.0)? Lowest is 6.9 > 6.0 ✓.

Step 3 — verdict: the lot qualifies as class 7.5 burnt clay brick. (It does *not* make class 10 — the mean is below 10.) Acceptance for use also requires the Part 2 water absorption and Part 3 efflorescence results to pass — strength alone does not clear the lot.

1. Not filling the frog / not capping the beds. An open frog or rocking bed face concentrates load and gives a falsely low strength — and an unjustified rejection or dispute.

2. Testing fewer than the IS 5454 sample size. Brick strength is highly variable kiln-to-kiln; a 2–3 brick 'test' is not an acceptance.

3. Judging the lot on individual values. Class is on the average, with only the 20%-below cap on individuals — neither 'all must exceed' nor 'one passed' is the rule.

4. Strength only, ignoring absorption and efflorescence. A strong brick with high water absorption or heavy efflorescence still fails IS 1077 — run Parts 2 and 3.

5. Wrong area basis. Strength is on the gross bed area of the standard unit; using net or nominal dimensions skews the class.

• IS 1077 — common burnt clay building bricks: classification and acceptance
• IS 5454 — methods of sampling of clay building bricks
• IS 3495 Part 2/3/4 — water absorption / efflorescence / warpage
• IS 2212 — code of practice for brickwork
• IS 1905 — structural use of unreinforced masonry (design strength from class)
• IS 2180 — heavy-duty burnt clay building bricks · IS 13757 — fly-ash bricks
• IS 12894 — fly-ash–lime bricks · IS 2185 — concrete masonry units

IS 3495 Part 1 is reaffirmed and remains the contractual acceptance test for clay bricks, but the field reality is high variability — strength swings widely with clay source, moulding and kiln firing, so test *per lot/per source*, never once for the project. The most common avoidable dispute is a falsely low result from poor specimen preparation (frog not filled, beds not capped); enforce the prep procedure and witness it.

The larger trend is substitution: fly-ash bricks (IS 13757/12894) and concrete/AAC blocks (IS 2185/IS 2185 Part 3) are displacing burnt clay bricks for environmental and dimensional-consistency reasons, each with its own strength test and class system. Whichever unit is used, the engineering discipline is the same — sample to the relevant sampling code, test strength *with* absorption and efflorescence, and map the result to the masonry design class in IS 1905.