The compressive strength test crushes a cured cube in a calibrated compression testing machine (IS 14858) at a controlled rate of loading; compressive strength = maximum load ÷ cross-sectional area. It is the number on which IS 456 acceptance, payment and most disputes turn — valid only if the specimen/curing (Clause 4), the machine calibration and the loading rate are right.
Key Requirements
•Crush in a calibrated compression testing machine conforming to IS 14858 (load accuracy ≈ ±2 %)
•Apply load at the IS 516 controlled rate — too fast over-reads strength, too slow under-reads
•Compressive strength fck = maximum load P ÷ specimen cross-sectional area A
•Centre the specimen; load the cast faces (not the trowelled top) so faces are plane and parallel
•Report individual results and the average; assess against IS 456 acceptance criteria
Formulas
f_c = P / A
Compressive strength from maximum load and area
f_c = compressive strength (N/mm²)P = maximum applied load (N)A = loaded cross-sectional area (mm²) — e.g. 22 500 mm² for a 150 mm cube
Practical Notes
✓A 150 mm cube has A = 22 500 mm², so a failure load of 750 kN gives ≈ 33.3 N/mm². Always work in consistent units (N and mm²).
✓Acceptance is on standard-cured cubes per IS 456; in-situ assessment uses cores (IS 516 Part 6), not site cubes.
Common Mistakes
⚠Using an un-calibrated machine or the wrong loading rate, then attributing the result to the concrete.
⚠Loading the trowelled top face (non-plane) instead of the cast faces.
⚠Reporting a single low cube as a failure without applying the IS 456 mean/individual acceptance rule.