COMPRESSIVE STRENGTH TEST OF CEMENT

 The Compressive Strength Test is conducted to determine the cement's ability to withstand axial loads. This is a critical property for ensuring the durability and strength of concrete structures. The test is performed on cement mortar cubes as per ASTM standards.

ASTM Standard Code:

The ASTM C109/C109M standard outlines the procedure for testing the compressive strength of hydraulic cement mortar using cube specimens.

Apparatus Required:

• Compressive Testing Machine – Capacity of at least 1000 kN for applying load.
• Cube Moulds – Standard size 50 mm × 50 mm × 50 mm (2-inch cubes).
• Cement – Ordinary Portland Cement (OPC) or any other type being tested.
• Standard Sand – As per ASTM C778 (graded sand for mortar mix).
• Weighing Balance – Accuracy 0.01g.
• Mixing Tools – Trowel, spatula, and a mixing pan.
• Measuring Cylinder – For accurate water measurement.
• Curing Tank – Filled with water at a temperature 23±2°C for proper hydration.

Test Procedure:

1. Preparation of Cement Mortar

• Prepare a mix of 1:2.75 (cement to standard sand) by weight.
• Add water equal to 10% of the total dry mix weight (as per ASTM C109).
• Mix the components thoroughly for 3-4 minutes to form a uniform paste.

2. Moulding the Specimens

• Fill the 50 mm cube moulds with the prepared mortar in two layers.
• Compact each layer using a tamping rod with 25 strokes.
• Remove excess mortar and smooth the surface with a trowel.

3. Curing the Specimens

• Leave the cubes in the mould at a temperature of 23±2°C for 24 hours.
• After 24 hours, remove the cubes from the moulds and submerge them in a curing tank at 23±2°C until testing.

4. Testing for Compressive Strength

• Test the cubes at 3, 7, and 28 days using the compressive testing machine.
• Place the cube centrally in the machine.
• Apply load at a rate of 900 to 1800 N/sec until the cube fails.
• Record the maximum load at failure.

5. Calculation of Compressive Strength

Compressive Strength=Load at Failure (N) / Cross-Sectional Area (mm)

The results are compared with standard values from ASTM C109, where the minimum strength requirements for OPC are:

• 3 Days: ≥ 12 MPa
• 7 Days: ≥ 19 MPa
• 28 Days: ≥ 33 MPa

Limitations of the Test

• Specimen Size Effect – The cube size influences strength values, as smaller specimens give higher results.
• Curing Conditions – Variations in curing temperature and humidity can significantly impact results.
• Water-to-Cement Ratio – Inconsistent mixing can lead to inaccurate strength measurements.
• Loading Rate Sensitivity – A too-fast or too-slow loading rate may alter failure characteristics.
• Human Error – Improper mould filling or compaction can introduce voids, reducing strength.

Conclusion

The Compressive Strength Test is essential for ensuring cement meets the required strength for construction applications. Following ASTM C109 ensures standardized testing conditions, leading to reliable and comparable results for cement quality assessment.