IS 516:1959, Methods of Tests for Strength of Concrete, is the Indian Standard that defines how concrete cubes are made, cured, and crushed in the laboratory. The code was first published in 1959 and remains the reference for cube preparation across India, so its procedures appear in almost every concrete acceptance report. The clauses covered here walk through the sequence in order: sampling the materials, mixing the batch, compacting the specimens, and curing them under controlled conditions.
Sampling Materials for Cube Tests
The strength of a cube means nothing if the sample does not represent the concrete actually used on site. Clause 2.6.1 of IS 516 opens the testing sequence with careful sampling of the constituent materials.
The guiding principle is representativeness. A sample drawn from one corner of a stockpile or from a single cement bag describes that corner or that bag, not the material as a whole, so the code builds the sampling rules around spreading the draw across the lot.
Cement Samples
Test samples of cement are made up of a small portion taken from each of a number of bags on site. Drawing from several bags spreads the effect of bag-to-bag variation, so the laboratory receives a composite picture of the cement rather than the extremes of a single sack. The portions are combined and mixed before any test runs.
Aggregate Samples
Aggregates are taken from a larger lot by quartering. The lot is shovelled into a conical heap, flattened, and divided into four quarters; two opposite quarters are combined, re-mixed, and quartered again until the sample is reduced to laboratory size. Quartering keeps the fine and coarse fractions in the same proportion as the original lot, so the grading of the sample matches the grading of the stockpile.
Sampled materials should be protected from moisture and contamination between sampling and testing. Cement in particular absorbs moisture from the air quickly, and a damp sample tests softer than the cement actually delivered to the site.
Mixing Concrete in the Laboratory
Once the materials are sampled, the batch must be mixed so that every cube comes from uniform concrete. IS 516 allows machine mixing or hand mixing, and each method carries its own rules. The mixing water is weighed or measured before the batch starts, because the water-cement ratio of the laboratory mix must match the ratio intended for the site concrete.
Machine Mixing
For machine mixing, the code requires a mixing period of not less than 2 minutes after all the materials are in the drum, continuing until the concrete is uniform in appearance. The 2-minute floor applies to the whole batch, not to a single drum revolution, and it exists because short mixing leaves streaks of paste and pockets of dry aggregate.
When pan mixers are used, the concrete is heaped together before sampling, so the sample spoon draws from the full batch rather than from a segregated corner. A few seconds of re-mixing before sampling removes the coarser material that settles at the edges of the pan.
Hand Mixing Procedure
Hand mixing follows a fixed sequence:
- Mix the cement and fine aggregate dry until the mixture is thoroughly blended and uniform in colour.
- Add the coarse aggregate and mix until it is uniformly distributed through the batch.
- Add the water and mix the entire batch until the concrete appears homogeneous and reaches the desired consistency.
The dry blending in the first two steps is what makes hand mixing reliable. Coarse aggregate added before the cement and sand are blended leaves dry patches that no amount of final mixing fully removes.
The clause adds a strict rule about trial adjustments. If repeated mixing becomes necessary because water is added in increments while adjusting the consistency, the batch must be discarded and a fresh batch made. Trial consistency tests cannot interrupt the mixing of a batch that will become test specimens, because interrupting the process changes the mixing history of the concrete and produces cubes that do not match the intended mix.
Compacting Cube Specimens
Compaction drives out entrapped air and settles the concrete around the mould. IS 516 specifies the tools and the effort in clause 2.10.1, with hand compaction as the standard laboratory method.
The Tamping Bar
Clause 2.9.3 defines the tamping bar used for cubes: a steel bar 16 mm in diameter and 0.6 m long, with a bullet-pointed lower end. The rounded point spreads the tamping force and stops the bar from punching a channel through the layer. A flat-ended bar would compact a narrow column of concrete and leave the rest of the layer loose.
Strokes Per Layer
For cubical specimens, the concrete must not receive less than 35 strokes per layer for 15 cm cubes, or 25 strokes per layer for 10 cm cubes. The strokes are distributed evenly over the surface of each layer, and each stroke should penetrate the layer below so the two layers knit together.
The difference between 35 and 25 strokes follows the area of the layer surface. A 150 mm cube has a larger surface than a 100 mm cube, so it needs more strokes to cover the surface at the same spacing.
| Specimen size | Minimum strokes per layer | Tamping bar |
|---|---|---|
| 150 mm cube | 35 | 16 mm diameter, 0.6 m long, bullet-pointed |
| 100 mm cube | 25 | 16 mm diameter, 0.6 m long, bullet-pointed |
Filling the Mould in Layers
The cube mould is filled in layers of roughly 50 mm depth, and each layer is compacted before the next is placed. After the final layer is compacted, the surface is struck off level with the top of the mould, and the mould is kept on a rigid base so the tamping energy goes into the concrete rather than into the bench.
Curing Cube Specimens
Curing conditions control the hydration of the cement and therefore the strength the cube develops. IS 516 sets precise temperature, humidity, and timing requirements for laboratory curing, and the margins are tight because small deviations change the measured strength by a measurable amount.
Storage Before Demoulding
Freshly made specimens are stored in a place free from vibration, in moist air of at least 90 percent relative humidity, at a temperature of 27 plus or minus 2 degrees Celsius, for 24 hours plus or minus 0.5 hours from the time water was added to the dry ingredients. The tolerance on the 24-hour window matters: early demoulding damages the green cube, and late demoulding changes the curing history.
Submersion After Demoulding
After this period the specimens are marked and removed from the moulds. Unless a test is required within 24 hours, they are immediately submerged in clean fresh water or a saturated lime solution and kept there until taken out just before testing. The water or solution is renewed every seven days and maintained at 27 plus or minus 2 degrees Celsius.
Saturated lime solution is used because fresh water can slowly dissolve lime out of the surface of the specimen, softening the paste near the face. The saturated solution stops that exchange, so the cube keeps the composition it was cast with.
Specimens must not be allowed to become dry at any time until they have been tested. Drying, even once, arrests hydration in the surface zone and produces a cube that understates the strength of the concrete.
The curing requirements can be summarised in a checklist:
- Store specimens free from vibration in moist air of at least 90 percent relative humidity.
- Hold the temperature at 27 plus or minus 2 degrees Celsius throughout.
- Renew the immersion water or lime solution every seven days.
- Never allow the specimens to dry out before testing.
Field Curing
Clause 3.3 extends the same discipline to cubes stored on site. Field specimens are kept under conditions that match the laboratory requirements as closely as the site allows, protected from vibration, sun, and wind, so the acceptance record reflects the concrete rather than the weather.
Testing and Reporting Cube Strength
At the test age the cubes are crushed and the results reported. The most common ages in Indian practice are 7 days and 28 days, with the 28-day strength used as the design strength and the 7-day result treated as an early indicator. Each sample produces at least three specimens for each test age, and the reported value is based on the group rather than on a single cube.
The Compression Test
Each cube is placed in the compression testing machine so that the load is applied to the faces opposite the trowelled face. The load increases steadily, without shock, at a rate around 14 N per square millimetre per minute, and the crushing load divided by the cross-sectional area gives the compressive strength of the concrete.
Reporting the Results
Results are reported to the nearest 0.5 N per square millimetre for the average of the specimens in the sample. When individual results differ by more than the permitted margin, the test is repeated, because a wide spread usually points to a fault in specimen making rather than a property of the concrete.
The recorded strength feeds the acceptance criteria in IS 456. The acceptance rules look at two things at once: the average of consecutive test results must clear the characteristic strength plus a margin built from the standard deviation, and no individual result may fall below the characteristic strength by more than the code tolerance.
Common Sources of Variation
Bad sampling, uneven compaction, drying during curing, and cubes that are not exactly square all push results around. A laboratory that follows IS 516 clause by clause produces numbers that can be compared across projects; one that skips steps produces numbers that only describe its own mistakes.