Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is this test? It finds the compressive strength of cement by making small mortar cubes (cement + standard sand + water), curing them in water and crushing them at 3, 7 and 28 days.
Why it matters: strength is the most important property of cement. The test confirms the grade of cement (33, 43 or 53) and catches weak, old or adulterated cement before it goes into a structure.
This guide follows IS 4031 (Part 6). (Concrete cubes are a different test, IS 516.)
What Is “Strength”?
Apparatus Required
Cube mould (70.6 mm)
Steel mould with a base plate; face area about 5000 mm².
Vibration machine
12 000 ± 400 vibrations per minute, with a hopper (IS 10080).
Poking rod & trowel
Prod the mortar; mix and finish with a 210 g trowel.
Balance & standard sand
Weigh accurately; use only IS 650 standard sand.
Moist cabinet & curing tank
90%+ humidity for 24 h, then water at 27 ± 2 °C.
Compression testing machine
Calibrated; loads the cube steadily until it fails.
Materials for One Cube
🧱 Cement
200 g
🏖️ Standard sand (IS 650)
600 g: 200 g each of 2–1 mm, 1 mm–500 µm and 500–90 µm (1 : 3 by weight)
💧 Water
(P/4 + 3)% of cement + sand weight. P = standard consistency. For P = 30%: 10.5% × 800 g = 84 ml
Procedure
- Work in a room at 27 ± 2 °C and 65 ± 5% humidity. Weigh cement, standard sand and water for one cube.
- Dry-mix cement and sand with the trowel for 1 minute. Add the water and mix until the colour is uniform. Total mixing time: 3 to 4 minutes. (If it takes longer, discard and start again.)
- Place the oiled mould on the vibration machine with its hopper. Put in the mortar and prod it 20 times in about 8 seconds with the rod.
- Vibrate for 2 minutes at 12 000 ± 400 vibrations per minute. Remove the mould and smooth the top with the trowel.
- Keep the moulds in a moist room or cabinet (90% humidity or more) at 27 ± 2 °C for 24 hours.
- Remove the cubes from the moulds, mark them, and put them in clean water at 27 ± 2 °C. Change the water every 7 days.
- Test 3 cubes at each age: 3 days (72 ± 1 h), 7 days (168 ± 2 h) and 28 days (672 ± 4 h).
- Wipe the cube surface dry, place it in the compression machine on its smooth side faces (not the trowelled top), and load it steadily at about 35 N/mm² per minute until it fails. Note the maximum load.
Calculation
For a 70.6 mm cube, area = 70.6 × 70.6 ≈ 4984 mm². So strength (N/mm²) ≈ load (kN) × 1000 ÷ 4984. Report the mean of 3 cubes to the nearest 0.5 N/mm² (or two decimals, as your format requires).
📝 Worked example (28 days)
Loads: 245, 252 and 238 kN
Strengths: 49.2, 50.6 and 47.8 N/mm²
Mean = 49.2 N/mm² → meets 43 grade ✓
Strength Requirements
| Cement | 3 days | 7 days | 28 days |
|---|---|---|---|
| OPC 33 / PPC / PSC | 16 N/mm² | 22 N/mm² | 33 N/mm² |
| OPC 43 | 23 N/mm² | 33 N/mm² | 43 N/mm² |
| OPC 53 | 27 N/mm² | 37 N/mm² | 53 N/mm² |
Minimum values as per IS 269, IS 1489 and IS 455. These codes also set a maximum 28-day strength for OPC 33 and 43. Follow the current code and your contract specification.
Try It: Cube Strength Calculator
Enter the failure load of each cube in kN. The values filled in are the worked example.
Water for each cube
Precautions
⏱️ Mixing 3–4 minutes
Use a stopwatch. Too short gives poor mixing; too long lets the mortar start setting.
📳 Full vibration
Vibrate for the full 2 minutes so no air pockets remain. Honeycombed cubes break early.
🔲 Test on side faces
Place the cube so the load acts on two smooth moulded faces, never on the rough top face.
⚙️ Steady loading
Load smoothly at about 35 N/mm² per minute, without shock. Fast loading gives falsely high results.
Practical Tips for Site Engineers
- Don’t rely only on the test certificate: the manufacturer’s certificate is useful, but test cement from site deliveries yourself, especially for important works.
- Use the 3 and 7 day results for early warning: they give an early warning about cement quality; don’t wait 28 days to find a problem. 7-day strength is usually about 65–75% of the 28-day value.
- Sample correctly: take samples from several bags in the delivery, mix them, and keep the sample in an airtight container until testing.
- Keep a cube register: record the cement brand, grade, week of manufacture, delivery date, casting date, test dates and results. Mark each cube clearly.
- Cure cubes properly: keep them fully under water at the right temperature until the test. Cubes left in the sun or air give low results.
- Check the machine: make sure the compression testing machine is calibrated and the platens are clean and flat.
- Low result? Check the procedure first (water, mixing time, vibration, curing). If the procedure was right, test another sample and inform the supplier before using more of that lot.
Key Takeaways
- Mortar cube: 200 g cement + 600 g standard sand + (P/4 + 3)% water; 70.6 mm cube.
- Mix 3–4 min, prod 20 times, vibrate 2 min; 24 h moist, then cure in water at 27 °C.
- Test 3 cubes at 3, 7 and 28 days; load at about 35 N/mm² per minute.
- Strength = load ÷ area (about 4984 mm²); minimum 28-day strength is 33, 43 or 53 N/mm² for each grade.
- On site: test deliveries, use 3- and 7-day results as early warnings, keep a cube register, and cure cubes properly.
References: IS 4031 (Part 6): Determination of Compressive Strength of Hydraulic Cement; IS 650: Standard Sand for Testing Cement; IS 10080: Vibration Machine; IS 269: Ordinary Portland Cement; IS 1489: Portland Pozzolana Cement; IS 455: Portland Slag Cement.