Standard Method for Testing Concrete Cube Compressive Strength

Know how to test concrete quality using cube specimens as per IS 456:2000. Learn step-by-step procedure, equipment, and acceptance criteria.

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Concrete strength testing helps ensure the material meets structural load-bearing capacity and safety standards. It's crucial for quality control on site.

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The test is performed using a CTM (Compressive Testing Machine), which applies gradual load until the concrete cube fails, helping measure its strength.

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As per IS 456:2000, the acceptance criteria for concrete strength and sampling are defined. IS 516 is referred for the testing method and procedure.

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You need cube moulds (150×150×150 mm), tamping rod (16mm × 600mm), scoop, finishing plate, weighing scale, CTM machine, oil, and water tank for curing.

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1 sample = 3 cubes. For 1–5 m³ = 1 sample, 6–15 m³ = 2, 16–30 m³ = 3, 31–50 m³ = 4, and for every extra 50 m³, add 1 sample. Follow IS 456:2000.

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Cube molds are cleaned, oiled, and placed on a flat surface. Concrete is filled in 3 layers and each layer is compacted 35 times with a tamping rod.

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After the final layer is compacted, the surface is leveled with a steel trowel. Once slightly hardened, date and grade are marked on each cube.

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After 24 hours, the cube is demolded and kept submerged in water for curing at 27 ± 2°C. Proper curing ensures strength development of concrete.

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Cube testing is done after 7 and 28 days. 6 cubes are usually cast: 3 for 7-day testing and 3 for 28-day testing to evaluate early and final strength.

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Remove cube from water, dry surface, and weigh it. Ideal cube weight is around 8.1 kg. Underweight cubes may indicate compaction or material issues.

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Place the cube in CTM machine, apply gradual load till failure. Note the max load. Use it to calculate compressive strength using the cube's area.

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Strength = Load (N) ÷ Area (mm²). For a 150 mm cube, area = 22500 mm². Average of 3 cube values gives the final result for one concrete sample.

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Target strength: 70% in 7 days, 87.5% in 14 days, and 100% in 28 days. Concrete can gain strength up to 365 days depending on curing and quality.

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If cube fails to meet required strength, conduct further tests like core cutting or NDT on actual structure. Take corrective steps as per IS 456:2000.

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