SOUNDNESS TEST FOR CEMENT

 The Soundness Test determines the ability of cement to retain its volume after setting. If cement is unsound, it can lead to expansion and cracking in hardened structures. This test is crucial for ensuring durability and preventing failure due to delayed expansion caused by excess lime or magnesia in cement.

ASTM Standard Code

The ASTM C151/C151M standard outlines the Autoclave Expansion Test, which is used to determine the soundness of hydraulic cement. This test measures the expansion of a cement specimen subjected to high pressure and temperature in an autoclave.

Apparatus Required

• Autoclave – A pressure-controlled vessel capable of maintaining 2.07 MPa (21 kg/cm²) pressure and a temperature of 216°C.
• Le Chatelier Moulds – Alternative apparatus used in non-ASTM methods for expansion testing.
• Cement Sample – Ordinary Portland Cement (OPC) or other hydraulic cement.
• Weighing Balance – Accuracy 0.01g.
• Mixing Tools – Trowel, spatula, and a mixing pan.
• Measuring Cylinder – For accurate water measurement.
• Glass Plate – For specimen preparation.
• Curing Tank – Filled with water at 23±2°C.
• Compressive Testing Machine – For strength determination (optional).

Test Procedure

1. Preparation of Cement Paste

• Prepare a cement paste with standard consistency (as per ASTM C187).
• Fill the moulds (25 mm × 25 mm × 285 mm prisms) with cement paste.
• Smooth the surface and cover the specimens to prevent moisture loss.

2. Curing of Specimens

• Place the specimens in a humidity-controlled chamber (23±2°C, 90% RH) for 24 hours.
• After 24 hours, remove the specimens from the moulds and submerge them in water at 23±2°C for further curing.

3. Autoclave Expansion Test

• Measure the initial length of the specimens using a length comparator.
• Place the specimens in the autoclave, ensuring they are positioned correctly.
• Increase the temperature gradually to 216°C over 1 hour while maintaining 2.07 MPa (21 kg/cm²) pressure.
• Hold these conditions for 3 hours.
• Allow the autoclave to cool down slowly before removing the specimens.

4. Measurement and Expansion Calculation

• Measure the final length of the specimen using a length comparator.
• Calculate the expansion (%) using:

Expansion (%) = (Final Length - Initial Length/Initial Length) ×100

According to ASTM C151, the maximum expansion limit for Ordinary Portland Cement (OPC) should not exceed 0.80%.

 

Limitations of the Test

• High-Pressure Equipment Sensitivity – Autoclave calibration is crucial, as incorrect pressure can distort results.
• Gradual Cooling Requirement – Rapid cooling can crack the specimen, leading to false expansion values.
• Material Sensitivity – Different cement types (e.g., high-alumina cement) require specific procedures.
• Specimen Preparation Errors – Improper mould filling or air voids can affect expansion measurements.

Conclusion

The Soundness Test ensures that cement does not undergo delayed expansion, which could lead to cracks and structural failure. By following ASTM C151, engineers can ensure cement quality and durability, preventing long-term damage in construction projects.