Cube Test (Compressive Strength)

The cube test is the standard method to measure the characteristic compressive strength of hardened concrete in India, governed by IS 516:1959 (now IS 516 Part 1:2021). Three 150 mm cubes are cast from each batch, demoulded after 24 hours, water-cured, then crushed in a calibrated compression testing machine — typically one cube at 7 days (early-age check) and two cubes at 28 days (acceptance). Strength is reported as load at failure divided by the loaded face area, in N/mm² (MPa).

For 150 mm cubes the failure load typically ranges from 450 kN (M20) to 1100 kN (M50). The compression machine must apply load at 14 N/mm² per minute (Cl. 5 of IS 516) — too fast gives an inflated reading. The cube must fail in a satisfactory mode (truncated pyramid, type T-shape); shear failure or column failure indicates poor capping or off-axis loading and the result must be discarded. The 7-day strength is typically 65-70% of 28-day for OPC and 50-55% for PPC, useful as an early indicator but not for acceptance.

Acceptance per IS 456 Cl. 16 is statistical, not pass-fail per cube. Two criteria must BOTH be satisfied: mean of any 4 consecutive non-overlapping samples ≥ fck + 4 MPa, AND no individual cube < fck − 4 MPa. A single sample is the average of two cubes from the same batch tested at 28 days. If either criterion fails, the structural engineer initiates the IS 456 Cl. 17 review (core test, ultrasonic pulse velocity, rebound hammer, structural assessment) — the affected work is NOT automatically demolished.