The concrete cube test is a fundamental part of quality control in concreting work which is used to assess whether the compressive strength of concrete matches the structural requirements specified in the design. Carried out according to IS 516:1959 and IS 456:2000, the test involves casting standard-size cubes—typically 150 mm × 150 mm × 150 mm—curing them under controlled conditions, and testing them under compression at specific intervals, commonly on the 7th and 28th day. The concrete cube test results help us to determine if the concrete used on site requirement/meets the expected load bearing capacity and structural performance parameters.
For site engineers, quality control teams, and construction supervisors, understanding this test is very important to ensure that the concrete mix used on construction sites complies with national codes and can withstand real-world loads over time. The cube test is not just a routine task—it’s a critical checkpoint for evaluating the durability, uniformity & reliability of the material being used in structural components such as foundations, beams, slabs, and columns.
What Is the Concrete Cube Test?
The concrete cube test is a standardized laboratory procedure designed to determine the compressive strength of hardened concrete specimens. This cube test in civil engineering (RCC_Reinforced Concrete) involves casting concrete into standardized cubic moulds, curing them under controlled conditions, and subjecting them to compressive loading until failure occurs.
The compressive strength test measures the maximum load that a concrete cube can withstand per unit area before crushing, expressed in (N/mm²) or (MPa). This value represents the concrete’s ability to resist compression forces, which is the primary loading condition in most structural applications.
Standard Concrete Cube Test Size and Specifications
According to IS 516:1959 and IS 456:2000, the standard cube test size in India is 150mm × 150mm × 150mm. This cube mould standard size provides a representative sample volume while maintaining practical handling capabilities for laboratory testing.
Key Specifications:
- Cube dimensions: 150mm × 150mm × 150mm
- Tolerance: ±0.2mm on each dimension
- Surface finish: Smooth, non-absorbent material
- Mould material: Cast iron, steel, or high-grade plastic
- Joint sealing: Watertight to prevent mortar leakage
Materials and Tools Required for Concrete Cube Test
Accurate results in a concrete cube compressive strength test depend on the use of standardized equipment and proper material handling. Below is the complete list of essential tools and materials required for carrying out the test as per IS 516:1959 and IS 456:2000 guidelines.
Primary Equipment:
- Cube Moulds (150mm × 150mm × 150mm)
- Material: Cast iron or steel
- Surface: Machine-finished smooth interior
- Joints: Watertight assembly
- Quantity: Sufficient for planned testing frequency
- Tamping Rod
- Diameter: 16mm
- Length: 600mm
- Material: Steel with rounded ends
- Weight: Approximately 2kg
- Compression Testing Machine
- Capacity: Minimum 2000 kN
- Accuracy: ±1% of applied load
- Loading rate: 140 kg/cm²/minute
- Calibration: Annual certification required
Supporting Materials:
- Clean potable water for curing
- Mould release agent (light mineral oil)
- Vibrating table or needle vibrator
- Weighing balance (accuracy ±1g)
- Measuring cylinder for water
- Clean cloth for mould cleaning
- Permanent markers for identification
- Curing tank or wet burlap
- Steel float for surface finishing
Preparation and Cleaning Procedures
Mould Preparation:
- Clean all mould surfaces thoroughly with wire brush
- Remove any concrete residue or rust particles
- Apply thin layer of mould release agent
- Ensure all joints are tight and leak-proof
- Check dimensional accuracy with measuring tape
Equipment Calibration:
- Verify compression testing machine calibration certificates
- Check tamping rod weight and dimensions
- Ensure weighing balance accuracy
- Test vibrating equipment functionality
Step-by-Step Concrete Cube Test Procedure (IS 516:1959)
Phase 1: Concrete Sampling and Preparation
Sampling Guidelines:
The concrete cube test procedure begins with proper sampling techniques that ensure representative specimens:
Sampling Frequency:
- One sample per 50m³ of concrete for building works
- One sample per 25m³ for critical structural elements
- Minimum three samples per day of concrete work
- Additional samples for quality assurance purposes
Sampling Method:
- Collect concrete samples from the discharge point
- Avoid sampling from the beginning or end of the batch
- Ensure samples represent the entire concrete mix
- Complete sampling within 15 minutes of batching
- Protect samples from direct sunlight and wind
Phase 2: Cube Casting and Compacting
Detailed Casting Procedure:
Step 1: Mould Preparation
- Place clean, oiled moulds on a firm, level surface
- Ensure moulds are properly assembled and secure
- Check for any damage or dimensional variations
Step 2: Concrete Placement
- Fill moulds in three equal layers (approximately 50mm each)
- Place concrete gently to minimize segregation
- Avoid dropping concrete from excessive height
- Distribute concrete evenly across mould surface
Step 3: Compaction Process
- Manual Compaction: Use tamping rod with 35 strokes per layer
- Mechanical Compaction: Vibrate for 30-60 seconds per layer
- Ensure complete compaction without over-vibration
- Remove entrapped air bubbles completely
- Level surface with steel float
Step 4: Surface Finishing
- Strike off excess concrete level with mould top
- Smooth surface with float in circular motion
- Avoid adding water or cement paste to surface
- Create uniform, flat finished surface
Phase 3: Curing Method and Duration
Standard Curing Procedure:
Initial Curing (0-24 hours):
- Store cubes in moist conditions at 27±2°C
- Cover with wet burlap or place in humid chamber
- Prevent water loss through evaporation
- Maintain ambient temperature control
Water Curing (24 hours to test age):
- Remove cubes from moulds after 24 hours
- Immerse completely in clean water tank
- Maintain water temperature at 27±2°C
- Ensure continuous water circulation
- Prevent direct sunlight exposure
Cube Curing Time Standards:
- 7-day strength: 65-70% of 28-day strength
- 28-day strength: Standard design strength
- 56-day strength: For mass concrete structures
- 90-day strength: For long-term strength assessment
Phase 4: Testing Procedure and Machine Setup
Pre-Testing Preparation:
Cube Conditioning:
- Remove cubes from curing tank 30 minutes before testing
- Wipe surface moisture with damp cloth
- Check for visible defects or damage
- Measure and record cube dimensions
- Calculate cross-sectional area
Machine Setup:
- Clean compression testing machine platens
- Check machine calibration and loading rate
- Set loading rate to 140 kg/cm²/minute
- Zero the load indicator
- Verify safety mechanisms
Testing Execution:
Loading Procedure:
- Place cube centrally on lower platen
- Align cube axis with loading direction
- Apply initial seating load (0.5-1.0 N/mm²)
- Begin continuous loading at specified rate
- Record maximum load at failure
- Note failure pattern and characteristics
Data Recording:
- Cube identification and casting date
- Test date and age at testing
- Cube dimensions and weight
- Maximum failure load
- Compressive strength calculation
- Failure mode description
Concrete Cube Test Frequency (As per IS 456:2000)
Mandatory Testing Requirements
The cube test frequency as per IS code ensures adequate quality control throughout construction projects. IS 456:2000 specifies minimum testing frequencies based on concrete volume and structural importance.
Standard Frequency Table:
| Concrete Quantity (m³) | Minimum Number of Samples | Number of Cubes per Sample |
|---|---|---|
| 1-5 | 1 | 3 |
| 6-15 | 2 | 3 |
| 16-30 | 3 | 3 |
| 31-50 | 4 | 3 |
| >50 | 4 + 1 per additional 50m³ | 3 |
How Often Cube Testing is Required:
Daily Requirements:
- Minimum one sample per day of concrete work
- Additional samples for different concrete grades
- Separate samples for different structural elements
- Extra samples for quality assurance verification
Special Circumstances:
- Critical structures: Double the standard frequency
- Mass concrete: Extended testing at 56 and 90 days
- Prestressed concrete: Enhanced frequency requirements
- Ready-mix concrete: Additional samples at delivery point
How Many Cubes to Cast for Each Pour
Standard Practice:
- Three cubes per sample: Standard requirement
- Additional cubes: Quality control backup
- Witness cubes: For third-party verification
- Contingency cubes: Replace damaged specimens
Cube Distribution:
- Test 1 cube at 7 days (optional)
- Test 2 cubes at 28 days (average result)
- Reserve additional cubes for extended testing
Acceptance Criteria for Cube Test
IS 456:2000 Compliance Requirements
The cube test acceptance criteria establish minimum performance standards for concrete strength. Understanding these requirements is crucial for quality control and structural safety.
Primary Acceptance Criteria:
Individual Cube Strength:
- No individual cube strength shall fall below 0.85 times the characteristic strength
- For M25 concrete: Minimum individual strength = 0.85 × 25 = 21.25 N/mm²
Average Strength Requirements:
- Average strength of any sample (2 cubes) ≥ Characteristic strength + 0.825 × standard deviation
- For established production: fck + 0.825 × 4 = fck + 3.3 N/mm²
- For limited data: Use higher factors of safety
Detailed Acceptance Table:
| Concrete Grade | Characteristic Strength (N/mm²) | Minimum Individual Strength (N/mm²) | Minimum Average Strength (N/mm²) |
|---|---|---|---|
| M15 | 15 | 12.75 | 18.3 |
| M20 | 20 | 17.00 | 23.3 |
| M25 | 25 | 21.25 | 28.3 |
| M30 | 30 | 25.50 | 33.3 |
| M35 | 35 | 29.75 | 38.3 |
What to Do if Concrete Cube Failure Occurs
Immediate Actions:
Investigation Procedure:
- Verify test procedures: Check for testing errors
- Review concrete production: Examine batching records
- Assess site conditions: Evaluate curing and handling
- Additional testing: Cast more cubes for verification
- Core testing: Extract cores from actual structure
Remedial Measures:
- Mix design revision: Adjust concrete proportions
- Enhanced quality control: Increase testing frequency
- Structural assessment: Evaluate load-carrying capacity
- Repair/strengthening: If structural capacity is compromised
Documentation Requirements:
- Detailed failure investigation report
- Corrective action implementation plan
- Enhanced quality control procedures
- Approval from structural engineer/consultant
Understanding Test Results and Calculations
How to Calculate Cube Strength
The compressive strength calculation involves straightforward mathematical formulas, but proper understanding ensures accurate interpretation.
Basic Formula:
Compressive Strength (N/mm²) = Maximum Load (N) ÷ Cross-sectional Area (mm²)Example Calculation for M25 Cube Test Result:
Given Data:
- Cube dimensions: 150mm × 150mm × 150mm
- Maximum failure load: 675 kN = 675,000 N
- Cross-sectional area: 150 × 150 = 22,500 mm²
Calculation:
Compressive Strength = 675,000 N ÷ 22,500 mm²
= 30.0 N/mm²Result Interpretation:
- Target strength (M25): 25 N/mm²
- Achieved strength: 30.0 N/mm²
- Result: PASS (exceeds minimum requirement)
Statistical Analysis of Test Results
Standard Deviation Calculation:
For a series of test results, calculate:
Standard Deviation = √[Σ(xi - x̄)² ÷ (n-1)]Where:
- xi = Individual test result
- x̄ = Average of all results
- n = Number of test results
Coefficient of Variation:
Coefficient of Variation = (Standard Deviation ÷ Mean) × 100%Quality Assessment:
- CV < 10%: Excellent control
- CV 10-15%: Good control
- CV 15-20%: Fair control
- CV > 20%: Poor control (investigation required)
Common Mistakes to Avoid During Cube Testing
Critical Cube Test Errors and Their Impact
Understanding why cube test fails helps prevent costly mistakes and ensures reliable results. These testing mistakes in construction can significantly affect result accuracy.
Sampling and Preparation Errors:
1. Poor Sampling Techniques:
- Error: Sampling from truck discharge or pump line end
- Impact: Non-representative samples with segregated material
- Solution: Sample from middle portion of concrete discharge
2. Delayed Testing:
- Error: Testing cubes beyond specified age tolerance
- Impact: Inaccurate strength assessment and invalid results
- Solution: Maintain strict testing schedule with ±2 hour tolerance
3. Inadequate Compaction:
- Error: Insufficient tamping or vibration during casting
- Impact: Reduced strength due to entrapped air voids
- Solution: Follow standard compaction procedures precisely
Curing and Storage Mistakes:
4. Improper Curing Conditions:
- Error: Inconsistent temperature and humidity control
- Impact: Altered hydration process affecting strength development
- Solution: Maintain 27±2°C and >95% relative humidity
5. Premature Demoulding:
- Error: Removing cubes from moulds before 24 hours
- Impact: Surface damage and distorted cube geometry
- Solution: Wait minimum 24 hours before demoulding
6. Contaminated Curing Water:
- Error: Using chemically treated or dirty water for curing
- Impact: Affected surface integrity and strength development
- Solution: Use clean, potable water meeting IS 456 requirements
Testing Procedure Errors:
7. Incorrect Loading Rate:
- Error: Loading too fast or too slow during testing
- Impact: False strength readings and invalid test results
- Solution: Maintain 140 kg/cm²/minute loading rate consistently
8. Misaligned Cube Placement:
- Error: Off-center positioning on testing machine platens
- Impact: Eccentric loading causing premature failure
- Solution: Center cubes carefully and check alignment
9. Wet Cube Testing:
- Error: Testing cubes with surface moisture
- Impact: Reduced friction and altered failure mechanism
- Solution: Surface dry cubes before testing
Quality Assurance Strategies
Prevention Measures:
- Training: Regular training for testing personnel
- Calibration: Periodic equipment calibration verification
- Procedures: Documented standard operating procedures
- Supervision: Quality control engineer oversight
- Documentation: Comprehensive test record maintenance
Frequently Asked Questions (FAQs)
1. What is the standard size of a concrete cube used for compressive strength testing?
The standard size of a concrete cube used for strength testing, as per IS 516:1959 and IS 456:2000, is 150 mm × 150 mm × 150 mm. This size ensures uniformity in test conditions and provides reliable compressive strength results for concrete grades up to M50.
2. Why is the concrete cube test done on the 7th and 28th days?
The cube test is conducted on the 7th day to assess the early strength development of concrete, and on the 28th day to measure its characteristic compressive strength. These two points provide insight into curing efficiency and long-term performance.
3. What is the minimum frequency of concrete cube testing as per IS standards?
As per IS 456:2000, at least one sample (set of 3 cubes) should be taken for every 5 cubic meters of concrete or 150 square meters of surface area placed. For large-scale pours, testing frequency increases to ensure quality control.
4. What is the acceptance criteria for concrete cube strength?
Concrete is considered acceptable if:
Average of three cubes ≥ specified characteristic strength
No individual cube strength is less than specified strength minus 4 MPa
This is detailed under IS 456:2000, Clause 15 and ensures uniform structural performance.
5. What are the causes of concrete cube test failure?
Common reasons include:
Improper mixing or batching
Inadequate compaction
Poor curing practices
Incorrect mould handling or loading rate
Each of these can significantly impact compressive strength and structural safety.
6. Can I use a different cube size for higher-grade concrete?
Yes. For high-strength concrete (above M50), smaller cube sizes (100mm) or cylinder specimens (150mm dia × 300mm height) may be preferred to reduce internal thermal gradients. However, the testing method should still comply with relevant standards.
