Concrete Cube Test IS Code – IS 516 & IS 456 Complete Guide (2025)

Concrete Cube Test – IS Code Quick Answer

• IS code for concrete cube test: IS 516:1959
• Cube casting, frequency & acceptance criteria: IS 456:2000
• Standard cube size: 150 × 150 × 150 mm
• Testing age: 7 days & 28 days
• Rate of loading: 140 kg/cm²/min

The concrete cube test is a fundamental quality control test used to verify whether the concrete supplied and placed at site meets the design compressive strength and complies with Indian Standards. It is one of the most reliable methods to assess the load-bearing capacity, durability, and uniformity of concrete used in structural elements such as slabs, beams, columns, footings, and foundations.

Carried out in accordance with IS 516:1959 and IS 456:2000, this test involves casting concrete into standard cube moulds, curing them under controlled conditions, and testing them under compression at specified ages—commonly 7 days and 28 days.

For site engineers, QA/QC teams, consultants, and contractors, the cube test is not just a routine activity—it is a critical checkpoint that confirms whether the structure is safe to proceed further.

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Purpose of Concrete Cube Test in RCC Construction

The primary purpose of the concrete cube test is to confirm that concrete used in a structure can safely carry the loads for which it is designed. Since concrete primarily resists compressive forces, its compressive strength directly governs the safety and performance of RCC members.

The cube test helps to:

• Verify compliance with design grade of concrete
• Detect mixing, batching, or curing problems
• Ensure uniform quality of concrete
• Provide documented proof of structural safety

Concrete Cube Test IS Code (IS 516 & IS 456 Explained)

The concrete cube test is a standardized laboratory procedure used to determine the compressive strength of hardened concrete. In RCC construction, compressive strength is the most important mechanical property because concrete is primarily designed to resist compressive stresses.

• IS 516:1959 specifies the method of testing, including cube size, curing, loading rate, and testing procedure.
• IS 456:2000 defines sampling frequency, acceptance criteria, and quality control requirements for concrete used in structures.

Together, these two codes ensure that concrete used on site is safe, durable, and structurally reliable.

Standard Concrete Cube Test Size and Specifications

As per IS standards, the standard size of concrete cube used for compressive strength testing in India is:

• 150 mm × 150 mm × 150 mm

This size provides a representative volume of concrete and allows uniform stress distribution during testing.

Key Specifications

• Dimensions: 150 × 150 × 150 mm
• Dimensional tolerance: ±0.2 mm
• Mould material: Cast iron, steel, or rigid non-absorbent material
• Surface finish: Smooth interior faces
• Joint condition: Watertight to prevent leakage of cement slurry

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:

1. Cube Moulds (150mm × 150mm × 150mm)
   • Material: Cast iron or steel
   • Surface: Machine-finished smooth interior
   • Joints: Watertight assembly
   • Quantity: Sufficient for planned testing frequency
2. Tamping Rod
   • Diameter: 16mm
   • Length: 600mm
   • Material: Steel with rounded ends
   • Weight: Approximately 2kg
3. 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:

1. Clean all mould surfaces thoroughly with wire brush
2. Remove any concrete residue or rust particles
3. Apply thin layer of mould release agent
4. Ensure all joints are tight and leak-proof
5. Check dimensional accuracy with measuring tape

Equipment Calibration:

• Verify compression testing machine (CTM) calibration certificates
• Check tamping rod weight and dimensions
• Ensure weighing balance accuracy
• Test vibrating equipment functionality

Cube Test Procedure as per IS Code (Step-by-Step)

Phase 1: Concrete Sampling and Preparation

Concrete sampling is a critical step in cube testing. The objective is to ensure that test specimens accurately represent the concrete actually placed in the structure.

Sampling Guidelines (As per IS 456 & IS 516)

• Concrete samples shall be taken from fresh concrete at the point of placing, such as:
  • Transit mixer discharge, or
  • Pump delivery line during steady flow
• Sampling shall not be done from:
  • The first discharge of the batch
  • The last portion of the batch where segregation may occur
• The sample taken shall be representative of the entire batch
• Sampling and cube casting shall be carried out as soon as practicable after discharge
• During sampling and handling:
  • Protect concrete from direct sunlight
  • Avoid exposure to wind, rain, or drying
  • Prevent loss of moisture before casting

Improper sampling is one of the most common reasons for misleading cube test results.

Cube Filling Procedure as per IS Code

The cube filling procedure is one of the most critical steps in the entire concrete cube test process. Even if the mix design and batching are correct, improper filling and compaction can lead to weak, unrepresentative cube specimens — causing failures that do not reflect the actual concrete quality placed in the structure.

The procedure is governed by IS 516:1959, which specifies the exact method of filling, compacting, and finishing concrete cube moulds.

Equipment Required for Cube Filling

• 150 mm × 150 mm × 150 mm cube mould (cast iron or steel)
• Tamping rod — 16 mm diameter, 600 mm length, steel with rounded ends
• Vibrating table or needle vibrator (for mechanical compaction)
• Steel float for surface finishing
• Mould release agent (light mineral oil)
• Permanent marker for cube identification

Step-by-Step Cube Filling Procedure as per IS 516

Step 1 — Mould Preparation and Oiling

Before filling, the mould must be clean and properly prepared. This prevents concrete from sticking to the mould walls and ensures clean demoulding after 24 hours.

• Clean the mould interior thoroughly using a wire brush to remove any old concrete residue or rust
• Apply a thin, even coat of mould release agent (light mineral oil) to all interior surfaces
• Ensure all joints are tight and watertight — any leakage of cement slurry will cause honeycombing and weak corners
• Place the assembled mould on a firm, level, vibration-free surface
• Mark the mould with cube identification number, date of casting, grade of concrete, and location of pour

Important: Do not apply excess oil. A heavy oil coat traps air at the surface and affects the surface finish of the cube.

Step 2 — Concrete Sampling at Site

As per IS 456:2000, the concrete sample for cube filling shall be taken from the point of placing — either from the transit mixer discharge or from the pump delivery line during steady flow. Do not collect the sample from the first or last discharge of the batch.

• Collect the concrete sample in a clean, non-absorbent container
• Proceed to filling immediately — do not allow the sample to sit in direct sunlight or dry out
• Remix the sample gently with a trowel before filling if any initial segregation is visible

Step 3 — Filling the Mould in Three Equal Layers

As per IS 516:1959, the cube mould must be filled in three approximately equal layers. Each layer must be compacted before the next layer is added.

Do not fill the mould in one single go. Filling in one pour prevents uniform compaction of the bottom layers, causing lower density and false strength results.

Step 4 — Compaction of Each Layer

Each layer must be fully compacted before adding the next. IS 516:1959 permits two methods of compaction.

Manual Compaction — Tamping Rod Method

• Use the standard tamping rod (16 mm dia, 600 mm length)
• Apply 35 strokes per layer uniformly distributed across the full cross-section of the cube
• For the second and third layers, the rod shall penetrate into the layer below by approximately 25 mm
• Strokes must be applied in a systematic pattern — do not concentrate strokes in one area
• Avoid hitting the mould base or walls with the tamping rod

Mechanical Compaction — Vibrator Method

• Use a vibrating table or needle vibrator
• Apply vibration for 30 to 60 seconds per layer
• Stop vibration as soon as the concrete surface becomes smooth and large air bubbles stop appearing
• Over-vibration causes segregation — the coarse aggregate sinks and cement paste rises, giving a false high strength reading at the surface
• Needle vibrator tip should not touch the mould walls

IS 516 Note: For concrete with slump less than 25 mm (stiff mix), mechanical compaction using a vibrating table is preferred. For normal workability concrete, manual tamping is acceptable.

Step 5 — Surface Finishing

After the final layer is compacted, the cube top surface must be finished smoothly and level with the top of the mould.

• Remove excess concrete using a straightedge or trowel, striking off the top surface flush with the mould rim
• Finish the surface with a steel float to produce a smooth, level top face
• Do not add water, cement paste, or neat cement to the surface for finishing — this weakens the surface zone and gives an artificially smooth appearance
• The finished surface should be plane within ±1 mm across the full width of the mould

Step 6 — Initial Curing (First 24 Hours)

After surface finishing, the filled moulds must be protected immediately.

• Cover the filled moulds with wet burlap, damp cloth, or polythene sheeting
• Store in a sheltered location away from direct sunlight, rain, and vibration
• Maintain ambient temperature as close to 27 ± 2°C as possible
• Do not move or disturb the moulds for the first 24 hours — movement at this stage can crack the freshly set concrete internally

Common Mistakes in Cube Filling Procedure

Summary — Cube Filling Procedure

1. Prepare and oil the mould — clean surface, tight joints, thin oil coat
2. Collect representative sample — from steady flow, not first or last discharge
3. Fill in three equal layers — approximately 50 mm each for a 150 mm cube
4. Compact each layer — 35 tamping strokes (manual) or 30–60 sec vibration (mechanical)
5. Finish the top surface — level with mould rim, smooth with steel float, no added water
6. Cover and protect — wet burlap, 27 ± 2°C, no movement for 24 hours

Cube Test Formula and Calculation as per IS Code

Once the cube specimen fails under compression in the testing machine, the compressive strength is calculated using a simple formula. Understanding this calculation helps site engineers and QA teams verify results on the spot and confirm whether the concrete meets the acceptance criteria as per IS 456:2000.

IS 516:1959 — Compressive Strength Formula

f_c = P ÷ A

For a standard 150 mm cube: A = 150 × 150 = 22,500 mm²
CTM load is usually displayed in kN — multiply by 1000 to convert to N before dividing.

Unit Conversion — kN to N/mm²

The compression testing machine (CTM) records the failure load in kilonewtons (kN). The compressive strength result must be in N/mm² (which is the same as MPa). Always convert before calculating:

• 1 kN = 1,000 N
• Load in kN × 1000 = Load in N
• Then divide by 22,500 mm² (for a 150 mm cube)

Worked Example 1 — M20 Concrete (28-Day Test)

Given data

• Concrete grade: M20
• Characteristic strength: 20 N/mm²
• Cube size: 150 mm × 150 mm × 150 mm
• Failure load recorded by CTM: 450 kN

Calculation

Cross-sectional area = 150 × 150 = 22,500 mm²

Load in Newtons = 450 × 1,000 = 450,000 N

Compressive strength = 450,000 ÷ 22,500 = 20 N/mm²

Acceptance check as per IS 456:2000

• Required characteristic strength = 20 N/mm²
• Minimum individual cube strength = 0.85 × 20 = 17 N/mm²
• Result = 20 N/mm² ✓
• PASS — cube meets acceptance criteria

Worked Example 2 — M25 Concrete (28-Day Test)

Given data

• Concrete grade: M25
• Characteristic strength: 25 N/mm²
• Cube size: 150 mm × 150 mm × 150 mm
• Failure load recorded by CTM: 675 kN

Calculation

Cross-sectional area = 150 × 150 = 22,500 mm²

Load in Newtons = 675 × 1,000 = 675,000 N

Compressive strength = 675,000 ÷ 22,500 = 30 N/mm²

Acceptance check as per IS 456:2000

• Required characteristic strength = 25 N/mm²
• Minimum individual cube strength = 0.85 × 25 = 21.25 N/mm²
• Result = 30 N/mm² ✓
• PASS — cube exceeds acceptance criteria

Worked Example 3 — M30 Concrete (7-Day Test)

Given data

• Concrete grade: M30
• Target 28-day strength: 30 N/mm²
• Expected 7-day strength: 65–70% of 28-day = 19.5–21 N/mm²
• Cube size: 150 mm × 150 mm × 150 mm
• Failure load at 7 days recorded by CTM: 450 kN

Calculation

Cross-sectional area = 150 × 150 = 22,500 mm²

Load in Newtons = 450 × 1,000 = 450,000 N

7-day compressive strength = 450,000 ÷ 22,500 = 20 N/mm²

Interpretation

• 7-day result = 20 N/mm² — falls within the expected 65–70% range (19.5–21 N/mm²)
• This indicates normal strength development
• Proceed with 28-day cubes to confirm final acceptance
• 7-day result is satisfactory — monitor 28-day cubes

Note: 7-day strength is used only as an early indicator. Formal acceptance is based on 28-day strength as per IS 456:2000. If 7-day strength falls significantly below the expected range, investigate curing conditions and batching immediately — do not wait for 28-day results.

Quick Reference — Cube Test Calculation Table

The table below gives the expected failure loads and minimum acceptable strengths for common concrete grades, based on a 150 mm cube and standard CTM:

Minimum failure load = (Characteristic strength × 0.85 × 22,500) ÷ 1,000. These are minimum acceptable values; actual results should ideally exceed the characteristic strength.

Curing Method and Duration as per IS Code

Initial Curing (First 24 Hours)

• Store cubes in a moist environment
• Maintain temperature close to 27 ± 2°C

Water Curing (After 24 Hours)

• Demould cubes after 24 hours
• Immerse completely in clean water
• Maintain curing water temperature at 27 ± 2°C

Curing Time Standards

• 7 days: Early strength assessment
• 28 days: Characteristic strength
• 56 / 90 days: Special or mass concrete

Testing Procedure and Machine Setup as per IS 516

• Remove cubes from curing tank and wipe surface moisture
• Measure cube dimensions
• Place cube centrally on CTM platen
• Apply load continuously and uniformly until failure
• Record maximum load and observe failure pattern

Rate of Loading for Concrete Cube Test as per IS 516

The standard rate of loading for concrete cube testing is:

140 kg/cm² per minute

Loading must be continuous and uniform. Incorrect loading rates can lead to false strength values and unreliable results.

Cube Test Frequency as per IS 456 (How Many Cubes?)

Cube test frequency ensures continuous monitoring of concrete quality.

General Practice (IS 456 Based)

• One sample = three cubes
• Minimum one sample per day or as directed by the Engineer-in-Charge
• Additional samples for:
  • Important structural elements
  • Large pours
  • Suspected quality variation

Typical Frequency Table

How Many Cubes Are Taken in One Sample as per IS Code?

As per IS 456:2000:

• One concrete sample consists of three cubes
• All cubes are cast from the same batch
• Common testing practice:
  • One cube tested at 7 days (optional)
  • Two cubes tested at 28 days (average strength considered)

Acceptance Criteria for Cube Test as per IS 456

Individual Cube Strength

• No individual cube strength shall be less than 0.85 × characteristic strength

Average Strength

• Average strength of the sample shall be equal to or greater than characteristic strength

Example (M25 Concrete):

• Characteristic strength = 25 N/mm²
• Minimum individual cube strength = 21.25 N/mm²

7 Days and 28 Days Cube Strength as per IS Code

• 7-day strength generally achieves 65–70% of 28-day strength
• 28-day strength is used for acceptance and structural verification

Understanding Test Results and Calculations

Compressive Strength Formula

Compressive Strength = Maximum Load / Cross-sectional Area

Example

• Load = 675 kN
• Area = 22,500 mm²
• Strength = 30 N/mm² (PASS)

Common Reasons for Concrete Cube Test Failure

• Non-representative sampling
• Inadequate compaction
• Improper curing
• Incorrect loading rate
• Misaligned cube placement

The concrete cube test is the most important quality control test in RCC construction. When carried out correctly as per IS 516 and IS 456, it ensures that concrete placed at site is structurally safe, durable, and compliant with design requirements.

Accurate sampling, proper compaction, controlled curing, and standardized testing are essential. Ultimately, the safety of a structure depends on concrete quality—and the cube test provides the final confirmation of that quality.