CONCRETE CURING & POUR PLANNING · CONCRETE COMPRESSIVE STRENGTH CALCULATOR
Compressive Strength of Concrete Cube
Cube strength = failure load ÷ 22,500 mm² for a 150 mm cube. A cube that fails at 675 kN has a strength of 30.0 MPa. M25 concrete should average about 33 MPa at 28 days, a load of about 743 kN, with no cube far below 25 MPa.
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Compressive strength
30.0MPa
In psi
4,351 psi
Expected mean at 28 days
33.0 MPa
Result ÷ expected
91 %
Result ÷ specified strength
120 %
Cube strength is higher than cylinder strength: compare with a cube grade (IS 456 M-grades, or the second number of EN C25/30).
Acceptance uses groups of results, not one specimen: see IS 456 Table 11 or ACI 318 §26.12.3.
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Loaded area of a 150 mm cube = 22,500 mm² (34.88 in²)
Strength = 675.0 kN ÷ area = 30.0 MPa
Expected mean = (specified + 8 MPa) × exp{0.25 × [1 − √(28 ÷ 28)]} = 33.0 × 1.000 = 33.0 MPa (EN 1992-1-1 Eq. 3.2, standard 20 °C curing)
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Fresh concrete is filled into 150 mm steel moulds in layers and compacted, left for 24 hours, then cured in water at about 27 °C (IS 516) until the test day. Each cube is crushed in a testing machine at a steady rate, and the maximum load is recorded.
How many cubes are taken?
Usually three cubes per sample for each test age, for example three for 7 days and three for 28 days. The average of the three is the test result. IS 456 cl. 15.2 sets how many samples to take for the volume poured.
What load should an M20 cube fail at?
At 28 days the mean target is about 28 MPa, which is 630 kN on a 150 mm cube. Single cubes may be lower; acceptance follows IS 456 Table 11.
References
EN 1992-1-1 Eq. 3.2 (strength development) and Table 3.1 (fcm = fck + 8 MPa)
IS 516 (cube tests), ASTM C39 (cylinder tests); acceptance: IS 456:2000 Table 11, ACI 318-19 §26.12.3
