Compressive strength test; acceptance criteria

Compressive strength test (IS 516)

1. Sampling and casting: take a representative sample from the mix (IS 456, Cl. 15). Fill 150 mm cube moulds in 3 layers, each layer compacted by a table vibrator or tamped with a 16 mm bar (35 strokes per layer for 150 mm cubes). Level and finish the top.
2. Curing: keep the mould covered for 24 ± 0.5 h at 27 ± 2 °C, demould, and cure in water at 27 ± 2 °C until testing (normally 28 days; also 3 and 7 days).
3. Testing: take the cube out of water, wipe the surface, and place it in the compression testing machine so that the load acts on two opposite faces of the cube as cast (not on its top and bottom as cast). Apply the load without shock at 14 N/mm$^2$ per minute (about 140 kg/cm$^2$/min), until the specimen fails. Record the maximum load $P$.
4. Result: $f_{c} = P/A$, where $A = 150\times150 = 22500$ mm$^2$. The test strength is the average of three specimens; if any one differs by more than ±15% from the average, that result is discarded (IS 456, Cl. 15.4).
5. For cylinders (150 mm dia × 300 mm high) the same procedure is followed, with capped ends; $f_{cyl} \approx 0.8\,f_{cube}$.

Acceptance criteria (IS 456, Cl. 16.1, Table 11)

• Strength of a sample = mean of three specimens; discard if one differs by more than 15% from the mean (Cl. 15.4).
• The mean of every group of 4 non-overlapping consecutive results must be at least $f_{ck}+0.825\,s$ (rounded to the nearest 0.5 N/mm$^2$), and not less than $f_{ck}+3$ (M15) or $f_{ck}+4$ (M20 and above), whichever is larger.
• Each individual result $\geq f_{ck}-3$ N/mm$^2$.
• If these limits are not met, the concrete may be checked by cores, NDT and load tests (Cl. 16.3, Cl. 17).