Step-by-Step Concrete Compressive Strength Testing Procedure

🧪 Step-by-Step Concrete Compressive Strength Testing Procedure

(As per IS 516 & IS 456 – commonly followed in Indian projects)

Since you're working on electrical & site execution assignments, this format will also help you for site documentation and QA/QC records.

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1️⃣ Casting of Test Specimens (Cube Preparation)

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Step 1: Preparation of Mould

• Use 150mm × 150mm × 150mm cube mould.

• Clean and apply light oil inside the mould.

• Assemble and tighten properly to avoid leakage.

Step 2: Filling the Mould

• Fill concrete in 3 equal layers.

• Compact each layer:

  • 35 strokes per layer using tamping rod
    OR

  • Use vibrating table (5–10 seconds).

Step 3: Leveling

• Finish top surface using trowel.

• Mark cube ID (Date, Grade, Location, Cube No.).

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2️⃣ Initial Storage (First 24 Hours)

• Keep moulds undisturbed for 24 ± ½ hours.

• Store in shade, free from vibration.

• Maintain temperature approx. 27 ± 2°C.

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3️⃣ Curing of Cubes

Step 4: Demoulding

• After 24 hours, remove cube carefully.

• Check for surface defects.

Step 5: Water Curing

• Immerse cubes in curing tank.

• Maintain temperature at 27 ± 2°C.

• Cure for:

  • 7 Days (Early strength)

  • 28 Days (Final strength)

  • 14 Days (if required)

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4️⃣ Compression Testing Procedure

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Step 6: Preparation Before Testing

• Remove cube from water.

• Wipe surface dry.

• Measure dimensions (if required).

• Ensure no loose particles.

Step 7: Placing in CTM

• Place cube centrally on machine base plate.

• Align properly to avoid eccentric loading.

Step 8: Load Application

• Apply load gradually without shock.

• Loading rate: approx. 140 kg/cm² per minute
  (As per IS guidelines)

• Continue until cube fails.

Step 9: Record Failure Load

• Note maximum load (kN).

• Observe failure pattern.

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5️⃣ Calculation of Compressive Strength

Formula:

$$\text{Compressive Strength (MPa)}=\frac{\text{Failure Load (N)}}{\text{Loaded Area (mm²)}}$$

For 150 mm cube:

Area = 150 × 150 = 22500 mm²

Example:

If failure load = 600 kN

$$\text{Strength}=\frac{600000}{22500}=26.67\text{ MPa}$$

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6️⃣ Acceptance Criteria (As per IS 456)

As per Bureau of Indian Standards – IS 456:2000

For example:

• M25 Grade → Characteristic Strength = 25 MPa

• Average of 3 cubes ≥ Required strength

• Individual cube should not fall below permissible limit

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7️⃣ Documentation to Maintain at Site

✔ Cube Register
✔ Test Report Format
✔ Calibration Certificate of CTM
✔ Cube Identification Tagging
✔ Lab Temperature Record

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📌 Important Site Tips (Very Practical)

• Always cast minimum 3 cubes per test.

• Do not test partially cured cubes.

• Ensure CTM calibration every 6 months.

• Avoid shock loading.

• Keep curing tank clean.

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