Compressive strength of clay bricks is carried out as per

IS 3495 Part 1 2019.pdf 456.53 KB

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1. Immersion of Bricks

• Immerse the brick specimens in water at a temperature of 27 ± 2°C for 24 h.
• Remove the specimens and drain out any surplus moisture.

2. Grinding the surface

• Remove unevenness observed in the bed faces to provide two smooth and parallel faces by grinding.
• However, If the grinding process is expected to significantly alter the contact area of the tested faces then the capping shall be done using cement mortar to achieve two smooth and parallel faces.

3. Filling of Frog with Mortar

• Use a cement/sand mortar for capping and filling the frog (where provided) and all voids in the bed faces
• Store under the damp jute bags for 24 h followed by immersion in clean water for 3 days at a temperature of 27 ± 2°C. Remove, and wipe out any traces of moisture.

4. Procedure for Testing

• Place the specimens with flat faces horizontal, and mortar filled face facing upwards between two 3-ply plywood sheets each of 3 mm thickness and carefully centered between plates of the testing machine.
• Apply load axially at a uniform rate of 7 ± 1 N/mm2/min till failure occurs and note the maximum load at failure.

5. Compressive Strength Calculation

Failure/ Pass Criteria for Compressive Strength

1. Average compressive strength of samples as Sampling Criteria should not be less than class strength of the bricks. 
2. Any individual brick strength comes out to be less than 15% of Class Strength

1. For Bricks of Class-10, the average compressive strength of say a sample size of 10 bricks for a lot size of 30000 bricks shall not be below 10 N/mm² 
2. Of the 10 bricks tested above, the compressive strength of none of those bricks should fall below 15% of class strength of 10 N/mm² i.e. 8.5 N/mm²

Originally the criteria of failure was 20% as per IS 5454 Clause 5.2.1.1, however, as per the amendment issued in 2008 for IS 1077 the criteria for failure has been made more stringent by reducing it from 20% to 15%.