The compressive strength of mortar is an important test to check the quality and performance of cement mortar used in masonry, plastering, and repair works.
This test is commonly asked in:
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🧱 Site inspections
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🧪 Laboratory tests
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🎓 Civil engineering exams
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💼 QA/QC interviews
Why This Test Is Important
Determining mortar strength helps to:
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✅ Ensure proper bonding in masonry
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✅ Prevent cracks and failures
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✅ Match mortar strength with masonry units
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✅ Maintain construction quality
Applicable Standard
📘 IS Code: IS 2250 / IS 4031 (Part 6)
📘 Cube Size: 70.6 mm × 70.6 mm × 70.6 mm
Materials Required
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Cement
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Standard sand
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Clean potable water
Equipment Required
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Cube moulds (70.6 mm)
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Weighing balance
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Mixing tray
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Trowel
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Vibrating table / tamping rod
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Compression Testing Machine (CTM)
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Curing tank
Mortar Mix Proportion
The standard mortar mix used for testing is:
1 : 3 (Cement : Sand) by weight
Step-by-Step Procedure
1️⃣ Preparation of Mortar
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Weigh cement and sand accurately.
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Mix dry cement and sand until uniform.
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Add required quantity of water.
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Mix thoroughly to obtain a uniform mortar.
2️⃣ Preparation of Cube Moulds
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Clean the cube moulds.
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Apply a thin layer of oil on inner surfaces.
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Place moulds on a level, rigid base.
3️⃣ Filling of Moulds
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Fill the mould in two equal layers.
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Compact each layer:
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By tamping rod (25 strokes per layer), or
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Using a vibrating table.
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Level the surface smoothly.
4️⃣ Initial Setting
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Keep filled moulds in a moist atmosphere for 24 hours.
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Cover with wet cloth if required.
5️⃣ Curing of Specimens
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After 24 hours, remove cubes from moulds.
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Submerge cubes in clean water.
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Cure for:
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7 days, or
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28 days (standard)
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6️⃣ Testing of Cubes
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Remove cube from curing tank.
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Wipe off surface moisture.
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Place cube centrally in the Compression Testing Machine.
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Apply load uniformly until failure.
Calculation of Compressive Strength
Formula:
Cube Area:
Unit:
📏 N/mm² or MPa
Reporting of Results
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Take average strength of three cubes.
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Report strength at:
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7 days
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28 days
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Precautions
⚠️ Ensure accurate weighing
⚠️ Proper compaction
⚠️ Clean water for curing
⚠️ Uniform load application
Site Engineer Tip 💡
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Mortar strength should not exceed masonry unit strength.
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Over-strong mortar can cause cracks in bricks/blocks.
Interview Questions (Quick Revision)
Q: What is the size of mortar cube?
👉 70.6 mm
Q: What is the standard mix for testing?
👉 1 : 3 (cement : sand)
Conclusion
The compressive strength test of mortar ensures:
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Structural durability
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Quality workmanship
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Compliance with standards
Every civil engineer and QA/QC professional should understand this test thoroughly.
Call to Action
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👉 Comment if you want a comparison with concrete compressive strength test
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