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Concrete Cube Test – IS 516 Procedure, Compressive Strength & Formula

1. Objective

The Concrete Cube Test as per IS 516 is the standard method used to determine the compressive strength of concrete for pavements, buildings, bridges, and structural elements. It verifies whether the concrete supplied at site meets the specified grade requirements and ensures structural safety and durability.This test involves preparing, casting, curing, and testing concrete cube specimens of size 150 × 150 × 150 mm or 100 × 100 × 100 mm to determine compressive strength at specified ages — typically 7 days (early strength assessment) and 28 days (characteristic strength verification).The procedure includes:

• Proper mould preparation and oiling
• Sampling of fresh concrete as per standard practice
• Layer-wise filling and compaction (tamping/vibration)
• Initial setting period and demoulding after 24 ± ½ hours
• Water curing under controlled temperature conditions
• Compression testing using a calibrated Compression Testing Machine (CTM)
• Calculation of compressive strength (Load / Cross-sectional Area)

Accurate execution of cube testing ensures:

• Verification of concrete grade (M20, M25, M30, etc.)
• Early detection of batching or mix design issues
• Compliance with project specifications and quality control norms
• Long-term durability and structural performance

Designed for site engineers, QA/QC teams, laboratory technicians, and project managers, this guide provides practical, step-by-step instructions to ensure reliable results and compliant construction practices.

2. Apparatus Required

• Cube moulds – 150 mm or 100 mm
• Mixing tray and scoop
• Tamping rod (16 mm diameter)
• Trowel
• Concrete mixer (if required)
• Curing tank (27 ± 2°C)
• Compression Testing Machine (CTM)

3. Preparation of Cube Moulds

1. Clean moulds to remove dust and hardened mortar.
2. Assemble moulds properly and tighten bolts.
3. Apply thin uniform oil layer on internal faces.
4. Check alignment and squareness of mould.

4. Sampling and Mixing of Concrete

1. Sample concrete from freshly mixed batch.
2. Mix thoroughly until uniform colour and consistency are achieved.
3. Start casting immediately to avoid loss of workability.

5. Casting of Concrete Cubes

1. Fill mould in three equal layers.
2. Distribute concrete evenly around mould.

6. Compaction of Concrete

• Compact each layer by rodding or vibration.
• Manual compaction: 35 strokes per layer.
• Rods to penetrate into the previous layer.
• Tap mould sides gently to remove air voids.
• Finish top surface smoothly using trowel.

7. Identification and Initial Storage

• Date of casting
• Grade of concrete
• Cube number / location

Store cubes undisturbed for 24 hours at 27 ± 2°C.

8. Curing of Concrete Cubes

1. Demould cubes after 24 ± ½ hours.
2. Immediately immerse in clean water.
3. Maintain curing temperature at 27 ± 2°C.
4. Continue curing till testing age.

9. Compression Testing of Cubes (IS:516)

1. Remove cube from curing tank (SSD condition).
2. Clean cube and CTM platens.
3. Measure dimensions (nearest 0.2 mm).
4. Place cube centrally on CTM platen.
5. Apply load gradually at ≈140 kg/cm²/min.
6. Record maximum load at failure.

Note: Improper centring causes eccentric loading and wrong test results.

10. Calculation of Compressive Strength

Compressive Strength (N/mm²) = Maximum Load at Failure ÷ Loaded Area

11. Cube Size – Area – Thumb Rules

Cube Size	Loaded Area
150 mm Cube	225 cm²
100 mm Cube	100 cm²

Fast Site Calculation Rules

• 150 mm cube → Load (kg) ÷ 225 | Load (kN) ÷ 22.5
• 100 mm cube → Load (kg) ÷ 100 | Load (kN) × 10

12. Sampling Frequency (IS Practice – Simplified)

Concrete Quantity	No. of Samples	Total Cubes
1 – 5 m³	1	3
6 – 15 m³	2	6
16 – 30 m³	3	9
31 – 50 m³	4	12
Each additional 50 m³	+1	+3

13. Reporting of Results

• Calculate strength of each cube.
• Round off to nearest whole number.
• Average of 3 cubes = representative strength.
• Variation limits must be satisfied.

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Concrete Strength Acceptance Criteria (±15% Rule Explained)

Basic Rule

For any set of 3 cubes (one sample):

• Calculate average strength
• Each cube must lie within:
  • 0.85 × Average (−15%)
  • 1.15 × Average (+15%)

If even one cube is outside this range, the sample is REJECTED, irrespective of average strength.

Key Strength Values – M25 Concrete

• Characteristic strength (fck) = 25 N/mm²
• Standard deviation (assumed) = 4 N/mm²
• Target mean strength = fck + 1.65 × S = 25 + (1.65 × 4) = 31.6 N/mm²

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CASE–1: Single Sample (Small Quantity Concrete)

Concrete Quantity = 5 m³ As per IS practice → 1 sample (3 cubes)

Acceptance Criterion (Special Case)

When only one sample is available:
Average strength ≥ fck + 4 = 29 N/mm²

Cube Strengths (N/mm²)	Average	0.85 × Avg	1.15 × Avg
19, 26, 16	20.3	17.3	23.3

Reasons for Rejection

• ❌ Average strength less than 29 N/mm²
• ❌ Cubes 26 and 16 N/mm² outside ±15% range

Final Decision: ❌ CONCRETE REJECTED

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CASE–2: Multiple Samples (Normal Quantity Concrete)

Concrete Quantity = 28 m³ Samples required = 3 samples (9 cubes)

Acceptance Criteria

• Each cube ≥ fck − 2 = 23 N/mm²
• Overall average ≥ fck + 4 = 29 N/mm²
• ±15% variation satisfied for each sample

Sample-wise Results

Sample	Cube Strengths (N/mm²)	Average	0.85 × Avg	1.15 × Avg
1	33, 29, 32	31.3	26.6	36.0
2	24, 32, 28	28.0	23.8	32.2
3	25, 29, 32	28.7	24.4	33.0

Overall Average Strength

(31.3 + 28.0 + 28.7) ÷ 3 = 29.3 N/mm²

Acceptance Check (As per IS Acceptance Criteria)

• ✅ ±15% Variation Check:
  All individual cube strengths fall within the permissible range of 0.85 × Average to 1.15 × Average for their respective samples. This confirms uniformity in batching, mixing, compaction, and curing of concrete.
• ✅ Minimum Individual Strength Check:
  Each tested cube has achieved a compressive strength greater than or equal to fck − 2, i.e. 23 N/mm² for M25 concrete. No cube strength is below the minimum permissible limit.
• ✅ Average Strength Check:
  The overall average compressive strength of all samples is 29 N/mm² or higher, which satisfies the requirement of fck + 4 for acceptance of concrete under normal sampling conditions.
• ✅ Quality and Compliance Confirmation:
  Since variation, individual strength, and average strength criteria are all satisfied, the concrete meets the strength acceptance requirements prescribed under IS practice for M25 grade.

Final Decision: ✅ CONCRETE ACCEPTED

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One-Line Site Memory Rules

• Single sample → Average ≥ fck + 4
• Multiple samples → Each cube ≥ fck − 2
• ±15% variation is compulsory