Compressive Strength Test of Concrete as per Indian Standard (IS 516:1959)

Compressive Strength Test of Concrete as per Indian Standard (IS 516:1959)

Aim
To determine the compressive strength of concrete by testing concrete cubes of standard dimensions after curing at specified intervals (7 days, 28 days) according to IS 516:1959.

Apparatus

1. Compression Testing Machine (CTM): Capacity of at least 2000 kN.
2. Cubes: Standard dimensions of 150 mm x 150 mm x 150 mm (or smaller depending on the size of the compression machine).
3. Mold for Concrete: Metal molds with tight-fitting baseplates.
4. Tamping Rod: A steel rod, 16 mm diameter, 600 mm long with a rounded end.
5. Weighing Balance: For measuring the weight of materials.
6. Curing Tank: A water tank to cure the concrete cubes.
7. Measuring Scale: For measuring cube dimensions.

Theory
The compressive strength test is conducted to measure the ability of concrete to resist loads that tend to reduce size. Compressive strength is one of the most important parameters for determining the quality and durability of concrete.

The compressive strength of concrete is calculated by the following formula:

$$\text{Compressive Strength} = \frac{\text{Load at Failure}}{\text{Area of Cube Face}}$$

The test is conducted on hardened concrete, usually at the ages of 7 days and 28 days. A typical strength value for M25 concrete is 25 MPa (N/mm²) at 28 days, though the actual value depends on the mix design, type of cement, and curing process.

Procedure

1. Preparation of Concrete Mix:
   Prepare the concrete mix as per the mix design (e.g., M25, M30). Ensure the concrete is homogeneous, and maintain the correct water-cement ratio.

2. Filling the Cube Mold:

   • Pour the fresh concrete into the cube mold in three layers, ensuring each layer is compacted by tamping 25 times with the tamping rod.
   • After filling and compacting the top layer, smooth the surface to level it with the mold's top.

3. Curing of Concrete Cubes:

   • Leave the filled molds for 24 hours under moist conditions (covered with a damp cloth).
   • After 24 hours, demold the cubes and immerse them in the curing tank filled with clean water at a temperature of 27°C ± 2°C for the specified curing period (usually 7 or 28 days).

4. Testing Procedure:

   • Remove the cubes from the curing tank 30 minutes before testing.
   • Check the dimensions of the cube and ensure there are no significant deformations.
   • Place the cube centrally on the base plate of the compression testing machine (CTM).
   • Apply a uniform load at a rate of 140 kg/cm²/min until the cube fails.
   • Record the maximum load at failure.

5. Calculation of Compressive Strength:
   Calculate the compressive strength using the formula:

$$\text{Compressive Strength} = \frac{\text{Failure Load}}{\text{Area of Cube Face}}$$

Observation Table

Sample No.	Curing Period (Days)	Failure Load (kN)	Area of Cube Face (mm²)	Compressive Strength (N/mm²)
1	7	440	22500	19.56
2	28	850	22500	37.78
3	28	820	22500	36.44

Sample Readings

• Cube Size: 150 mm x 150 mm = 22500 mm²
• Load at Failure: 440 kN (for 7 days)
• Compressive Strength = $\frac{440 \, \text{kN}}{22500 \, \text{mm}^2}$ $$\text{Compressive Strength} = 19.56 \, \text{N/mm}^2$$

Reporting the Results The compressive strength should be reported for both 7 days and 28 days. It is recommended to take the average value of three specimens and report the mean compressive strength in N/mm² (MPa). For instance:

• 7-day compressive strength: 19.56 N/mm²
• 28-day compressive strength: 37.78 N/mm²

Results should also indicate whether the concrete meets the target design strength. For example, if the mix design is M25, the concrete should achieve a compressive strength of at least 25 N/mm² at 28 days.

Conclusion The compressive strength test is critical in determining the quality of concrete. In this test, the concrete achieved a 28-day compressive strength of 37.78 N/mm², which exceeds the typical target strength of 25 N/mm², indicating that the concrete mix is suitable for its intended structural application. This suggests the concrete possesses good durability and strength characteristics, meeting the design specifications.