Table of Contents
Introduction to Cube Testing
Compressive strength testing of concrete cubes is the most fundamental quality control test in construction. It determines whether the concrete supplied meets the specified grade requirements and structural design assumptions.
The procedure is governed by IS 516:1959 (Methods of Tests for Strength of Concrete), which specifies standardized methods for sampling, casting, curing, and testing concrete specimens. The acceptance criteria are defined in IS 456:2000 (Plain and Reinforced Concrete - Code of Practice).
Key Standards Referenced
- IS 516:1959 - Methods of Tests for Strength of Concrete
- IS 456:2000 - Plain and Reinforced Concrete Code
- IS 1199:1959 - Methods of Sampling and Analysis of Concrete
- IS 10086:1982 - Specification for Moulds for Concrete Cubes
Why Cube Testing Matters
Concrete cube testing serves several critical purposes:
- Quality Verification: Confirms concrete meets specified grade (M25, M30, etc.)
- Process Control: Identifies issues in batching, mixing, or materials
- Documentation: Provides legal evidence of concrete quality
- Structural Safety: Ensures concrete can carry design loads
- Contract Compliance: Meets specification requirements
Sampling Procedure (IS 516)
Proper sampling is the foundation of accurate test results. A sample that doesn't represent the actual concrete will give misleading results regardless of how carefully the test is conducted.
Sampling Frequency
As per IS 456, the minimum sampling frequency is:
| Concrete Quantity | Minimum Samples Required |
|---|---|
| 1-5 m³ | 1 sample |
| 6-15 m³ | 2 samples |
| 16-30 m³ | 3 samples |
| 31-50 m³ | 4 samples |
| 51 m³ and above | 4 plus 1 additional for each 50 m³ or part thereof |
Important Note
For RMC, collect samples from the truck mixer after discharging about 0.3 m³ to ensure representative sampling. Do not collect from the very beginning or end of the discharge.
Step-by-Step Sampling Procedure
1
Prepare Equipment: Have ready - clean shovel/trowel, sampling tray, sampling scoop, and cube moulds. Ensure all equipment is clean and free from old concrete.
2
Select Sampling Point: For RMC, sample after initial 0.3 m³ discharge. For site-mixed concrete, sample from at least 3 different points in the batch.
3
Collect Composite Sample: Take minimum 0.02 m³ (20 liters) of concrete. Collect small quantities from different parts of the batch and combine.
4
Remix Sample: Thoroughly remix the composite sample on a clean, non-absorbent platform before casting cubes.
5
Record Details: Document batch time, truck number, grade, pour location, weather conditions, and any observations.
Cube Casting Procedure
Cube Mould Specifications (IS 10086)
- Standard Size: 150 mm × 150 mm × 150 mm
- Alternative: 100 mm cubes for M40 and above (max aggregate 20mm)
- Material: Cast iron or steel, rigid and watertight
- Surface: Smooth, machined faces
- Tolerance: ±0.2 mm for dimensions
Casting Steps
1
Clean & Oil Moulds: Clean moulds thoroughly and apply a thin coat of mould oil to all internal faces. Assemble moulds ensuring watertight joints.
2
Fill in Layers: Fill mould in 3 approximately equal layers. Each layer should be approximately 50mm deep for 150mm cubes.
3
Compact Each Layer:
- By Hand: 35 strokes per layer with 16mm diameter steel tamping rod, 600mm long, rounded at one end
- By Vibrator: Internal vibrator (25mm needle) - vibrate until mortar just appears on surface
4
Finish Top Surface: After compacting top layer, trowel the surface smooth and level with top of mould. Cover with metal or glass plate.
5
Initial Storage: Store cubes at 27±2°C for 24±0.5 hours, protected from sun, wind, and rain. Mark each cube with unique identification.
Pro Tips for Casting
- Compact uniformly - under-compaction reduces strength, over-compaction causes segregation
- Strokes should penetrate into the underlying layer
- Distribute strokes evenly over the entire cross-section
- Cast cubes within 15 minutes of sampling
- Always cast 3 cubes from each sample
Curing Requirements
Standard Curing Procedure
After 24 hours of initial setting:
- Demoulding: Carefully remove cubes from moulds (24±0.5 hours after casting)
- Transfer to Curing Tank: Immediately place in clean water at 27±2°C
- Maintain Until Testing: Keep cubes submerged until just before testing
- Testing Ages: 7 days and 28 days are standard
Curing Tank Requirements
- Water Temperature: 27±2°C maintained constantly
- pH: 7 to 9 (change water if pH exceeds 9)
- Coverage: Minimum 25mm water above cubes
- Location: Cubes should not rest on tank bottom (use racks)
- Identification: Maintain proper identification system
Testing Ages and Strength Expectations
| Grade | 7-Day Target (65-70% of 28-day) | 28-Day Target (100%) |
|---|---|---|
| M15 | 10-11 MPa | ≥15 MPa |
| M20 | 13-14 MPa | ≥20 MPa |
| M25 | 17-18 MPa | ≥25 MPa |
| M30 | 20-21 MPa | ≥30 MPa |
| M35 | 23-25 MPa | ≥35 MPa |
| M40 | 26-28 MPa | ≥40 MPa |
Testing Procedure
Pre-Test Preparation
- Remove from Water: Take cube out of curing tank and wipe off surface water
- Measure Dimensions: Record actual dimensions of each cube face
- Check for Defects: Inspect for honeycombing, cracks, or surface defects
- Weigh: Record weight for density calculation
- Test Within Time: Test within 30 minutes of removal from water
Compression Testing Machine Setup
- Calibration: Verify machine is calibrated (calibration valid for 1 year)
- Capacity: Machine should be capable of 2000 kN for 150mm cubes
- Platens: Clean, flat steel platens - 25mm thick minimum
- Centering: Cube must be centered on lower platen
- Orientation: Test on the face perpendicular to casting direction
Critical: Loading Rate
The loading rate must be 14 MPa/min (140 kg/cm²/min) - equivalent to approximately 315 kN/min for 150mm cubes. This is the most common source of testing error!
- Too fast → Higher apparent strength (incorrect)
- Too slow → Lower apparent strength (incorrect)
Test Execution
1
Position Cube: Place cube on lower platen with casting direction perpendicular to loading direction. Ensure cube is centered.
2
Initial Contact: Bring upper platen into contact with cube. Apply small initial load to seat the cube.
3
Apply Load: Apply load continuously at 14 MPa/min (315 kN/min for 150mm cube) until failure.
4
Record Failure Load: Note maximum load applied before failure. Observe and record failure pattern.
5
Calculate Strength: Compressive Strength = Failure Load (N) / Cross-sectional Area (mm²)
Failure Pattern Interpretation
The way a cube fails provides information about the quality of concrete and testing:
- Normal Conical Failure: Cone-shaped fragments at top and bottom - indicates good concrete and proper testing
- Columnar Failure: Vertical cracks - may indicate eccentric loading or too-rapid loading
- Shear Failure: Diagonal failure plane - may indicate weak plane or improper compaction
- Explosive Failure: Sudden, violent failure - indicates very strong, brittle concrete
IS 456 Acceptance Criteria
IS 456:2000 (Clause 16.1) specifies acceptance criteria based on both individual test results and group averages:
Criteria 1: Group Average
Average of 3 Consecutive Tests
The average of 3 consecutive test results (9 cubes total) should be ≥ fck + 0.825σ
Or ≥ fck + 4 MPa (whichever is higher)
Criteria 2: Individual Result
Single Test Result
Any individual test result (average of 3 cubes) should be ≥ fck - 3 MPa
Example: M30 Grade Concrete
For M30 concrete (fck = 30 MPa):
- Group Average Required: 30 + 0.825 × 5 = 34.125 MPa OR 30 + 4 = 34 MPa → 34.125 MPa (higher governs)
- Individual Result Minimum: 30 - 3 = 27 MPa
Assuming standard deviation σ = 5 MPa (established from site records)
Calculate Acceptance Criteria
Use our Compressive Strength Calculator to check if your results meet IS 456 criteria
Open CalculatorStandard Deviation Values
Per IS 456, when sufficient test data (≥30 samples) is not available:
| Grade | Assumed Standard Deviation (σ) |
|---|---|
| M10 - M15 | 3.5 MPa |
| M20 - M25 | 4.0 MPa |
| M30 - M55 | 5.0 MPa |
| M60 - M80 | 6.0 MPa |
Common Errors to Avoid
Sampling Errors
- ❌ Sampling only from beginning of discharge
- ❌ Not remixing composite sample before casting
- ❌ Delayed casting (>15 minutes after sampling)
- ❌ Insufficient sample quantity
Casting Errors
- ❌ Using damaged or distorted moulds
- ❌ Insufficient or excessive compaction
- ❌ Uneven distribution of tamping strokes
- ❌ Not finishing top surface level
Curing Errors
- ❌ Delayed transfer to curing tank (>30 hours)
- ❌ Inconsistent curing temperature
- ❌ Cubes resting on tank bottom (uneven curing)
- ❌ Contaminated curing water
Testing Errors
- ❌ Incorrect loading rate (most common!)
- ❌ Eccentric loading (cube not centered)
- ❌ Testing on wrong face (parallel to casting)
- ❌ Uncalibrated testing machine
- ❌ Delayed testing after removal from water
Practical Checklists
Pre-Casting Checklist
- ☐ Cube moulds clean and dimensionally correct
- ☐ Mould oil applied and excess removed
- ☐ Tamping rod or vibrator ready
- ☐ Sampling tray and scoop available
- ☐ Identification system prepared
- ☐ Data recording sheets ready
- ☐ Curing tank at correct temperature
During Casting Checklist
- ☐ Sample collected from correct point
- ☐ Composite sample properly remixed
- ☐ 3 layers of equal depth placed
- ☐ 35 strokes per layer (hand compaction)
- ☐ Strokes penetrate previous layer
- ☐ Top surface leveled and finished
- ☐ Covers placed on moulds
- ☐ Cubes labeled with unique ID
- ☐ All details recorded in log
Pre-Testing Checklist
- ☐ Testing machine calibrated
- ☐ Loading rate verified (14 MPa/min)
- ☐ Platens clean and flat
- ☐ Cube dimensions measured
- ☐ Cube weight recorded
- ☐ Any defects noted
- ☐ Correct testing orientation confirmed
Key Takeaways
- Sample correctly - composite sample from multiple points, remix before casting
- Compact properly - 35 strokes per layer, uniform distribution
- Cure consistently - 27±2°C from 24 hours until testing
- Load at correct rate - 14 MPa/min is critical for accurate results
- Meet both criteria - group average AND individual minimum
- Document everything - test records are legal evidence of quality
