Compressive Strength Test of Concrete Cubes

The compressive strength test is a vital quality control method in concrete construction. This test ensures that concrete meets its strength requirements, providing a reliable indicator of durability and structural performance. Here’s a detailed guide on conducting this test, complete with updated example calculations.

Aim

To determine the compressive strength of concrete cubes and verify if the concrete meets its design specifications.

Apparatus & Equipment

The required tools for the compressive strength test include:

1. Compression testing machine (CTM).
2. Cube moulds: 15 cm .
3. Curing tank for water immersion.
4. Tamping bar for compaction.
5. Weighing balance for material measurement.

Preparation of Concrete Cube Specimens

1. Proportioning

The concrete mix should follow design specifications, such as M20 or M25 grades.

2. Mixing Concrete

Concrete can be mixed by machine or hand:

a) Machine Mixing

1. Add half of the coarse aggregates into the mixer drum.
2. Add cement and fine aggregates, followed by the remaining coarse aggregates.
3. Gradually pour in water and mix until a uniform texture is achieved.

b) Hand Mixing

1. Dry mix cement and fine aggregates on a watertight platform.
2. Add coarse aggregates and mix thoroughly.
3. Gradually add water while mixing until the desired consistency is achieved.

3. Casting Concrete Cubes

1. Fill the moulds in layers, each approximately 5 cm deep.
2. Compact each layer with a tamping bar (35 strokes for 15 cm cubes, 25 for 10 cm cubes).
3. Smooth the top surface with a trowel and cover to prevent evaporation.

4. Curing of Concrete Cubes

1. Keep the specimens in a moist, vibration-free environment at 27°C ± 2°C for 24 hours.
2. Remove the moulds and immerse the cubes in clean water for curing. Replace the water every 7 days to maintain freshness.

Testing Procedure

1. Remove the cubes from the curing tank 30 minutes before testing.
2. Dry the cubes and clean off loose materials.
3. Place the cube in the compression testing machine, ensuring the load is applied on opposite faces.
4. Align the cube properly, ensuring uniform load application.
5. Apply load at a steady rate of 140 kg/cm² per minute until failure occurs.
6. Record the maximum load applied before failure.

Calculations for Compressive Strength

The compressive strength is calculated using the formula:

Compressive Strength (N/mm²) =

Where:

• P = Maximum load applied (in Newtons)
• A= Cross-sectional area of the cube (in mm²)

Example Calculation:

Results and Reporting

The test results for multiple specimens are as follows:

Key Considerations

1. Testing Age:
   • Conduct the test at 7 days and 28 days for accurate strength assessment.
   • The 28-day strength is used as the standard.
2. Number of Specimens:
   • Test at least 3 cubes for each age.
3. Temperature:
   • Maintain the environment at 27°C ± 2°C during testing and curing.
4. Load Application:
   • Ensure uniform and steady application of load.

Compressive Strength Over Time

• 7 Days: Achieves approximately 65-70% of the 28-day strength.
• 28 Days: Reaches full design strength.

Conclusion🎯

The compressive strength test of concrete cubes is a fundamental procedure to ensure concrete quality and structural safety. Following the prescribed procedures, precautions, and calculations ensures reliable results. This test, conducted as per IS 516: 1959, is essential for quality control in the construction industry.