Compressive Strength Test of Concrete Cubes: Procedure and Results
The cube test is the standard way of proving that concrete placed on site has reached its specified grade. Every slab pour, every RMC delivery and every lab session on concrete ends with cubes being cast, cured and crushed. In this article you will learn the full procedure from casting to crushing, how to calculate strength from the failure load, how to judge whether a set of three cubes is valid, and how many samples are required for a given volume of concrete.
Equipment Required
The test method is given in IS 516, which has been revised in recent years into a multi-part standard; the basic cube procedure described here is unchanged in principle.
| Item | Details |
|---|---|
| Cube moulds | 150 x 150 x 150 mm steel or cast iron, with base plate |
| Tamping bar | 16 mm diameter, about 600 mm long, rounded end |
| Curing tank | Clean water maintained at 27 ± 2 °C |
| Compression testing machine (CTM) | Capacity typically 2000 kN, with load gauge or digital readout |
| Others | Mould oil, trowel, scoop, marker, weighing balance |
Casting and Curing Procedure
- Clean the moulds, tighten the bolts and apply a thin coat of mould oil on inner faces.
- Take a representative sample of concrete, from the middle portion of the discharge if it comes from a transit mixer.
- Fill each mould in three layers of about 50 mm each.
- Compact each layer with not less than 35 strokes of the tamping bar, distributed evenly, or use a vibrating table until no more air bubbles rise.
- Strike off the top surface level and smooth it with a trowel. Mark each cube with the date, grade and location of pour.
- Keep the filled moulds undisturbed in a place free from vibration, in moist conditions, at 27 ± 2 °C for 24 hours.
- After 24 hours, demould the cubes and immerse them in the curing tank until the day of testing.
- Test cubes in a surface-dry condition at 7 days and 28 days, or as specified.
Crushing the Cubes
Remove the cube from water, wipe off surface water and grit, and note its mass and dimensions. Place the cube in the CTM so that the load is applied on two opposite side faces as cast, not on the top trowelled face and bottom. Centre the cube on the lower platen. Apply the load gradually without shock at a rate of about 140 kg/cm² per minute, which is roughly 14 N/mm² per minute, until the cube fails. Record the maximum load.
Compressive strength (N/mm²) = Failure load (N) / Loaded area (mm²) For a 150 mm cube: area = 150 x 150 = 22,500 mm² 1 kN = 1000 N
A good failure is an hourglass or double-pyramid shape, where all four exposed faces crack roughly equally and the cube is left with two cones. Failure with cracks on one side only, or a tension crack running down one face, usually points to an off-centre cube, an uneven face or a faulty platen.
Worked Example: M25 Slab Cubes
Three cubes from an M25 slab pour were tested at 28 days. The failure loads were 702 kN, 655 kN and 690 kN.
| Cube | Failure load (kN) | Strength (N/mm²) |
|---|---|---|
| 1 | 702 | 702000 / 22500 = 31.20 |
| 2 | 655 | 655000 / 22500 = 29.11 |
| 3 | 690 | 690000 / 22500 = 30.67 |
| Average | - | 30.33 |
IS 456 treats the average of three cubes as the test result of one sample, provided the individual results do not vary by more than ±15 percent of the average. If they do, the sample is not valid as a test result.
Average = 30.33 N/mm²
Allowed range = 30.33 x 0.85 to 30.33 x 1.15
= 25.78 to 34.88 N/mm²
All three values (29.11, 30.67, 31.20) lie inside the range -> sample validThe sample strength of 30.33 N/mm² is above fck = 25 N/mm². Final acceptance of the concrete, however, is judged on a group of consecutive samples using the acceptance criteria in IS 456, which check both the mean of the group and each individual result against limits based on fck.
7-day strength as an early indicator
Seven-day results are used as an early warning. For OPC concrete, a 7-day strength of roughly 65 to 70 percent of the 28-day strength is common. For the M25 example, a 7-day average around 19 to 21 N/mm² would be normal. Concrete with PPC or slag cement gains strength more slowly, so its 7-day percentage is lower; this alone does not mean failure.
How Many Samples to Take
IS 456 sets a minimum sampling frequency for each grade of concrete in a pour. Each sample is three cubes for 28-day testing, and extra cubes are cast for 7-day or other tests.
| Quantity of concrete in the work (m³) | Number of samples |
|---|---|
| 1 to 5 | 1 |
| 6 to 15 | 2 |
| 16 to 30 | 3 |
| 31 to 50 | 4 |
| 51 and above | 4 + 1 for each additional 50 m³ or part |
Example: slab pour of 72 m³ Samples = 4 + 1 (for the 21 m³ beyond 50) = 5 samples Cubes for 28 days = 5 x 3 = 15 cubes
Common Mistakes
- Leaving cubes on site in the sun for days before sending them to the lab, which reduces strength.
- Loading on the rough trowelled face, which causes early local failure.
- Applying load too fast, which gives falsely high strength.
- Not recording the cube mass; an unusually light cube often indicates poor compaction.
- Discarding a low result without investigation. Low cubes call for core tests or rebound hammer checks on the structure.
Summary
Cast 150 mm cubes in three layers with at least 35 strokes each, cure in water at 27 ± 2 °C, and crush them on their side faces at a steady loading rate. Strength is failure load divided by 22,500 mm². Three cubes make one sample, valid when each is within ±15 percent of the average, and the number of samples depends on the volume poured.