Before any strength figure is meaningful the specimen, sampling, compaction and curing must follow IS 516. Cube specimens are 150 mm (or 100 mm where the nominal max aggregate ≤ 20 mm), sampled from fresh concrete per IS 1199, compacted in the standardised way, demoulded at 24 h, then cured in water at 27 ± 2 °C until test (commonly 7 and 28 days). A strength result is only as valid as the specimen and curing behind it.
Key Requirements
•Cube size 150 mm standard; 100 mm permitted when nominal maximum aggregate size ≤ 20 mm — be consistent within a batch
•Sample fresh concrete per IS 1199; compact specimens by the standardised method (rodding/vibration) — under-compacted cubes read low
•Demould at 24 ± 0.5 h; cure in clean water at 27 ± 2 °C until testing (poor/late curing depresses strength independent of the mix)
•Test at the specified age (commonly 7 and 28 days); record the actual age
•Faces in contact with platens must be plane and at right angles — cap/grind if not
Reference Tables
Standard specimens (IS 516 Part 1)
Test
Specimen
Typical size
Compressive strength
Cube
150 mm (or 100 mm if max agg ≤ 20 mm)
Flexural strength
Beam (prism)
150 × 150 × 700 mm (or 100 × 100 × 500 mm)
Split tensile strength
Cylinder
150 mm dia × 300 mm
Confirm sizes/tolerances against the current BIS edition and the project specification.
Practical Notes
✓Most disputed low cube results trace to the specimen/curing or the testing machine — audit this clause and IS 14858 before blaming the concrete.
✓Site-cured cubes read lower than standard-cured cubes; standard curing (27 ± 2 °C water) is the acceptance basis — don't confuse the two.
Common Mistakes
⚠Mixing 150 mm and 100 mm cubes in one batch without applying the size relationship.
⚠Poor or interrupted curing of acceptance cubes — a first-order, invisible cause of low strength.
⚠Testing non-plane / off-centre specimens and blaming the concrete.