Compressive Strength of Mortar -Mix Ratio and Cube Test

Compressive Strength of Mortar: Testing and Significance

The compressive strength of mortar is a critical parameter in determining the durability and load-bearing capacity of masonry structures. Mortar, a mixture of cement, sand, and water, is widely used in bricklaying, stone masonry, and plastering of walls and columns. Testing the compressive strength of mortar is essential for ensuring the structural integrity of these constructions.

Significance of Compressive Strength Test

The compressive strength test is vital for assessing the ability of masonry structures, especially load-bearing walls and columns, to withstand applied loads. Ensuring the requisite compressive strength is crucial for maintaining structural stability and safety, particularly in residential and commercial buildings where walls support vertical loads from upper floors.

Procedure for Determination of Compressive Strength

To determine the compressive strength of standard cement-sand mortar cubes, a standardized procedure is followed, as outlined in ASTM C109 / C109M.

Apparatus

• Cube molds: 7.06 cm molds with a face area of 50 cm²
• Mixing equipment: For gauging and mixing mortar
• Vibrator: For compacting the mortar
• Compression testing machine: To apply the load on the cubes

Procedure

1. Mixing the Mortar:

• Measure: 200g of cement and 600g of standard sand in a 1:3 ratio by weight.
• Sand specifications: Angular grains passing through a 2mm IS sieve.
• Mixing: Thoroughly mix the dry cement and sand for one minute. Gradually add water, maintaining a proportionate quantity based on the combined weight of cement and sand. The water content is determined by a predefined formula ensuring standard consistency.

1. Preparation:

• Uniform Mixing: Continue mixing until achieving a uniform consistency, within a duration of 3 to 4 minutes.
• Filling Molds: Transfer the mortar mixture into cube molds, compacting it by prodding 20 times within 8 seconds to remove air entrapped.

1. Compaction:

• Using a Vibrator: If a vibrator is used, subject the cube molds to vibration for 2 minutes at a specified speed of 12000±400 vibrations per minute.

1. Curing:

• Initial Curing: Place the cube molds in an environment with a temperature of 27±2°C and 90% relative humidity for 24 hours.
• Water Curing: After 24 hours, remove the cubes from the molds and submerge them in clean water until testing.

1. Testing:

• Placement: Test the cubes on their sides without additional packing.
• Loading Rate: Apply a uniform loading rate, typically 350 kg/cm²/minute.
• Testing Schedule: Test three cubes and report the average compressive strength at both 7 and 28 days.

Result of Mortar Cube Test

The results of the mortar cube test are expressed in terms of compressive strength at 7 and 28 days, measured in N/mm². The calculations involve determining:

1. Breaking Load: The maximum load applied before failure.
2. Cross-sectional Area: The area over which the load is applied.
3. Ultimate Compressive Strength: The maximum stress the mortar can withstand, calculated by dividing the breaking load by the cross-sectional area.

Summary

The compressive strength of mortar is a fundamental parameter for assessing the structural robustness of masonry constructions. Through rigorous testing procedures and precise calculations, engineers and construction professionals can ensure the durability and safety of load-bearing structures, contributing to the overall resilience of buildings and infrastructure. By adhering to standardized methods such as ASTM C109 / C109M, the reliability of these tests is maintained, providing essential data for construction projects.