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

1. Objective

The Concrete Cube Compressive Strength Test, conducted in accordance with IS 516 (Part 1/Sec 1), is the most widely accepted method for assessing the compressive strength of hardened concrete. It is one of the most critical quality control tests used to verify that concrete has achieved the specified design strength and complies with the requirements of IS 456, MoRTH Specifications, project specifications, and contractual acceptance criteria. The test is routinely performed on highways, bridges, buildings, industrial facilities, airports, and other infrastructure projects to ensure the structural safety, durability, and long-term performance of concrete.

For EPC contractors, consultants, and QA/QC teams, concrete cube testing forms an integral part of the Quality Assurance and Quality Control (QA/QC) system. The reliability of the test depends not only on the Compression Testing Machine (CTM) but also on the accuracy of every preceding activity, including representative sampling, cube casting, compaction, curing, specimen handling, identification, and laboratory testing. Failure to follow standard procedures at any stage can significantly influence the measured compressive strength and may result in incorrect acceptance or rejection of concrete.

Concrete test specimens are generally prepared using standard cube moulds of 150 × 150 × 150 mm, which are widely used for routine structural concrete testing. Where the nominal maximum size of coarse aggregate does not exceed 20 mm, 100 × 100 × 100 mm cube moulds may also be used in accordance with the applicable provisions of IS 516. Specimens are normally tested at 7 days to evaluate early strength development and at 28 days to determine the characteristic compressive strength. Additional testing ages may be specified for high-performance concrete, mass concrete, or project-specific requirements.

Scope of the Concrete Cube Test Procedure

A standard Concrete Cube Compressive Strength Test consists of the following sequential activities:

• Inspection, cleaning, assembly, and lubrication of cube moulds before casting.
• Collection of representative fresh concrete samples from the point of discharge or placement.
• Recording of specimen identification, concrete grade, batch details, casting location, date, and time for complete traceability.
• Layer-wise placement of concrete into cube moulds followed by proper compaction using a tamping rod or vibration table, depending on the workability of the concrete.
• Finishing the top surface flush with the mould and marking each specimen for identification.
• Protecting freshly cast specimens from vibration, direct sunlight, excessive moisture loss, and mechanical disturbance during the initial setting period.
• Demoulding the specimens after 24 ± ½ hours and transferring them immediately to a curing tank maintained under standard curing conditions.
• Continuous water curing until the specified testing age.
• Testing the specimens using a calibrated Compression Testing Machine (CTM) in accordance with IS 516.
• Calculating the compressive strength by dividing the maximum failure load by the loaded cross-sectional area of the specimen.
• Comparing the test results with the acceptance criteria specified in IS 456, MoRTH Specifications, project Quality Assurance Plans (QAP), and other applicable contract documents.

Importance of Proper Testing

The accuracy of the Concrete Cube Compressive Strength Test depends on strict adherence to standard laboratory procedures. Common errors such as improper sampling, inadequate compaction, poor curing, damaged specimens, incorrect loading rates, or inaccurate specimen identification can produce misleading test results. Therefore, every stage of the testing process shall be carried out in accordance with IS 516 and the approved Quality Assurance procedures to ensure reliable, repeatable, and technically valid results.

Benefits of Routine Concrete Cube Testing

• Verifies that the concrete has achieved the specified grade (M20, M25, M30, M40, and higher).
• Detects deficiencies in batching, mixing, transportation, placement, compaction, and curing practices at an early stage.
• Evaluates the consistency and quality of concrete production throughout the project.
• Confirms the performance of the approved concrete mix design under actual site conditions.
• Demonstrates compliance with IS 456, IS 516, MoRTH Specifications, consultant requirements, and contractual QA/QC procedures.
• Provides documented laboratory records for inspections, audits, quality reviews, and client approvals.
• Enhances confidence in the structural safety, durability, and long-term service performance of concrete structures.

This guide presents the complete IS 516 Concrete Cube Compressive Strength Test Procedure in a practical, step-by-step sequence followed on major NHAI, MoRTH, EPC, HAM, BOT, and O&M projects. It is intended as a comprehensive reference for site engineers, material engineers, QA/QC engineers, laboratory technicians, consultants, contractors, engineering students, and project managers responsible for concrete quality control and acceptance testing.

2. Apparatus Required

The following apparatus and equipment are required for casting, curing, and testing concrete cube specimens in accordance with IS 516 (Part 1/Sec 1). All equipment shall be clean, properly maintained, and calibrated (where applicable) to ensure accurate and reliable test results.

3. Preparation of Cube Moulds

Proper preparation of cube moulds is essential to obtain concrete specimens with accurate dimensions, smooth surfaces, and reliable compressive strength results. Before casting, all moulds shall be inspected, cleaned, assembled, and prepared in accordance with IS 516 (Part 1/Sec 1). Any defect in the mould or improper preparation may affect the shape of the specimen and lead to inaccurate test results.

1. Clean the cube moulds thoroughly to remove hardened concrete, cement paste, dust, rust, oil residues, and any foreign material from previous tests. All internal surfaces shall be clean and free from imperfections.
2. Inspect each mould for wear, distortion, cracks, damaged corners, or loose bolts. Replace or repair any defective mould before use to ensure dimensional accuracy.
3. Assemble the mould correctly and tighten all bolts uniformly to prevent movement or leakage of cement slurry during casting and compaction.
4. Apply a thin and uniform coat of approved mould release oil to the internal faces of the mould using a clean brush or cloth. Avoid excessive oil application, as it may affect the surface finish of the concrete specimen.
5. Ensure that all excess oil is removed and that no oil collects at the corners or bottom of the mould.
6. Check that the assembled mould is clean, dry, square, properly aligned, and conforms to the specified dimensions before concrete placement.
7. Place the mould on a rigid, level, vibration-free, and non-absorbent surface capable of supporting the mould during filling and compaction without movement.
8. Assign a unique identification number or tag to each mould and verify that the identification corresponds with the Cube Casting Register to ensure complete specimen traceability.

4. Sampling and Mixing of Concrete

The accuracy of concrete cube test results depends significantly on obtaining a proper and representative concrete sample. Sampling shall be carried out in accordance with IS 1199 and the approved project Quality Assurance Plan (QAP). The collected sample shall represent the actual concrete being placed at site and shall be handled carefully to avoid segregation, contamination, or loss of workability.

1. Collect a representative sample of freshly mixed concrete from the point of discharge or placement as specified in the project requirements. Avoid collecting samples from the first discharge or final portion of the batch, as these may not represent the actual concrete quality.
2. Ensure that the sampling equipment, such as trays, shovels, and containers, is clean and free from hardened concrete, dust, oil, or other contaminants that may affect the concrete properties.
3. Place the collected sample on a clean, rigid, and non-absorbent mixing tray. Remix the concrete thoroughly using a shovel until a uniform colour, consistency, and distribution of aggregates are achieved.
4. During remixing, check the concrete visually for segregation, excessive bleeding, abnormal consistency, or loss of workability. Any unusual observation shall be recorded and reported to the concerned engineer.
5. Do not add water or any other material to the collected sample after sampling. Any adjustment to concrete properties shall only be carried out through approved mix procedures.
6. Complete the cube casting operation as soon as possible after sampling to minimize changes in concrete properties due to evaporation, temperature variation, hydration, or initial setting.
7. Ensure that the sample remains protected from direct sunlight, rain, wind, and contamination during handling and before placing into cube moulds.

5. Casting of Concrete Cubes

The accuracy of concrete cube compressive strength results largely depends on proper specimen preparation. Cube casting shall be carried out immediately after sampling to ensure that the test specimens represent the actual condition of fresh concrete at the time of placement. Proper filling, compaction, and finishing are essential to avoid segregation, honeycombing, entrapped air, and non-uniform density within the specimen.

1. Place the freshly mixed concrete into the prepared cube mould in three approximately equal layers. The concrete shall be placed carefully and uniformly to ensure complete filling of all corners and edges of the mould.
2. Avoid free fall of concrete from excessive height during placement, as it may cause segregation of coarse aggregates and variation in concrete composition. Use a scoop or suitable tool for controlled placement.
3. Spread and distribute each layer evenly throughout the mould before compaction. Ensure that concrete reaches all corners without leaving voids or pockets of entrapped air.
4. Compact each layer immediately after placing using the specified method. For manual compaction, each layer shall be compacted uniformly using a 16 mm diameter tamping rod as per the applicable requirements of IS 516. Where vibration is used, avoid excessive vibration that may cause segregation or bleeding.
5. During compaction, ensure that the tamping rod penetrates the layer being compacted and slightly enters the underlying layer to achieve proper bonding between layers.
6. Continue filling and compacting the remaining layers until the mould is completely filled. After placing the final layer, slightly overfill the mould to compensate for settlement during compaction.
7. Strike off the excess concrete carefully using a steel trowel or straight edge to obtain a smooth, level surface flush with the top edge of the mould. Avoid excessive finishing that may bring mortar to the surface or disturb the concrete structure.
8. Remove any spilled concrete from the external surfaces of the mould and ensure that the specimen remains undisturbed after finishing.
9. Immediately mark or tag the specimen with complete identification details, including:
   • Concrete grade
   • Cube identification number
   • Casting date and time
   • Location / structural element
   • Batch or pour reference
   This ensures complete traceability from casting to final test reporting.

6. Compaction of Concrete

• Compact each layer using the appropriate method specified in the relevant IS Code or project specification.
• For manual compaction, rod each layer uniformly with 35 strokes using the standard tamping rod.
• Ensure the tamping rod penetrates approximately 25 mm into the previous layer to achieve proper bonding.
• When using a vibrating table or needle vibrator, avoid excessive vibration that may cause segregation or bleeding.
• Lightly tap the external sides of the mould with a rubber mallet to remove entrapped air and improve surface finish.
• After compaction of the final layer, strike off the excess concrete and finish the surface with a steel trowel without overworking the concrete.

7. Identification and Initial Storage

• Assign a unique identification number to each cube specimen.
• Clearly record the date and time of casting.
• Record the concrete grade (e.g., M20, M30, M40).
• Record the mix identification, batch number, chainage/location, structural member, and sampling point where applicable.
• Enter all specimen details in the Cube Casting Register or Laboratory Test Register for complete traceability.

Store the moulded specimens on a rigid, level, vibration-free surface under suitable conditions for 24 ± ½ hours at a temperature of 27 ± 2°C. Protect the specimens from vibration, direct sunlight, rain, and accidental disturbance during the initial setting period.

8. Curing of Concrete Cubes

1. Carefully remove the specimens from the moulds after 24 ± ½ hours without damaging the edges or corners.
2. Immediately immerse the cubes in a curing tank containing clean potable water or lime-saturated water.
3. Maintain the curing water temperature at 27 ± 2°C throughout the curing period.
4. Ensure the specimens remain completely submerged with adequate spacing to allow free circulation of water.
5. Replace the curing water periodically to keep it clean and free from contaminants.
6. Continue curing until the specified testing age, typically 7 days, 14 days, or 28 days, unless otherwise specified by the project.

9. Compression Testing of Concrete Cubes (IS 516: Part 1/Sec 1)

The compressive strength test is the most widely used method for evaluating the quality and strength of hardened concrete. The test shall be carried out in accordance with IS 516 (Part 1/Sec 1) using a calibrated Compression Testing Machine (CTM). The primary objective is to determine whether the concrete has achieved the specified characteristic strength and complies with the requirements of IS 456, MoRTH Specifications, and the project quality control plan.

Testing Procedure

1. Remove the cube specimens from the curing tank at the specified testing age (typically 7, 14, or 28 days). Wipe off excess surface water and ensure the specimen is in a Saturated Surface Dry (SSD) condition before testing.
2. Verify the specimen identification against the Cube Casting Register. Record the concrete grade, casting date, testing date, curing age, sample location, and specimen identification number.
3. Inspect each cube for visible defects such as cracks, honeycombing, broken edges, chipped corners, segregation, or surface damage. Any abnormality shall be recorded before testing.
4. Clean the bearing faces of both the concrete specimen and the CTM platens to remove dust, loose particles, laitance, or moisture. Clean and level bearing surfaces ensure uniform load transfer during testing.
5. Measure the dimensions of the specimen using a calibrated vernier caliper or steel scale. If the cube dimensions differ from the standard size, calculate the actual loaded area before determining the compressive strength.
6. Position the specimen centrally on the lower platen of the Compression Testing Machine with proper alignment. Incorrect positioning may produce eccentric loading and inaccurate test results.
7. Bring the upper platen into contact with the specimen carefully without impact. Ensure full and uniform contact between the bearing surfaces before applying the load.
8. Apply the compressive load continuously, uniformly, and without shock at the loading rate specified in IS 516 (approximately 140 kg/cm²/min) until the specimen fails. Avoid sudden changes in loading rate throughout the test.
9. Record the maximum load at failure displayed by the CTM. Observe and document the failure pattern, including diagonal shear, cone failure, columnar cracking, splitting, or any abnormal mode of failure.
10. Calculate the compressive strength using the following equation and compare the result with the specified concrete grade and applicable acceptance criteria.

Compressive Strength (MPa) = Maximum Load (N) ÷ Loaded Area (mm²)

10. Calculation of Compressive Strength

The compressive strength of a concrete cube is determined by dividing the maximum load carried by the specimen at failure by the loaded area of the cube. The calculated value represents the compressive strength of the concrete and is expressed in MPa (N/mm²).

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

Calculation Procedure

1. Record the maximum load at failure from the Compression Testing Machine (CTM) in kN.
2. Convert the load into Newtons (N) if required.
   1 kN = 1000 N
3. Determine the actual loaded area of the specimen. If the cube dimensions differ from the nominal size, calculate the loaded area using the measured dimensions.
4. Calculate the compressive strength using the standard formula.
5. When three specimens are tested, calculate the average compressive strength and compare it with the applicable acceptance criteria specified in IS 456, MoRTH Specifications, or the project Quality Assurance Plan (QAP).

Worked Example

A 150 mm concrete cube failed under a load of 900 kN.

• Maximum Load = 900 kN = 900,000 N
• Loaded Area = 150 × 150 = 22,500 mm²
• Compressive Strength = 900,000 ÷ 22,500 = 40.0 MPa (N/mm²)

Result: The compressive strength of the concrete cube is 40 MPa.

11. Standard Cube Size and Loaded Area Reference

Concrete cube specimens are manufactured in standard sizes specified by IS 516. The compressive strength is calculated using the loaded area of the specimen. Therefore, it is essential to use the correct cube dimensions during strength calculations. If the actual specimen dimensions differ from the nominal size due to manufacturing tolerances or damage, the measured dimensions shall be used for calculating the loaded area.

📌 Selection of Cube Size

• 150 mm cubes are the standard specimens for routine quality control of concrete in most construction projects.
• 100 mm cubes are generally used when the aggregate size is small or when specified by the project specifications or laboratory testing procedures.
• Always use the cube size specified in the approved Quality Assurance Plan (QAP), contract documents, or project specifications.

⚡ Quick Field Calculation (For Reference Only)

For rapid verification at site, the following simplified formulas may be used. These calculations are intended only for preliminary checking and shall not replace the official laboratory calculations.

12. Sampling Frequency (As per IS Practice / QA Plan)

Sampling shall be carried out as per approved Quality Assurance Plan (QAP) and IS provisions. Minimum frequency is as follows:

Control Requirement: Sampling shall be random, representative, and witnessed by authorized QA/QC personnel.

13. Reporting and Acceptance Criteria

• Individual cube strengths shall be calculated separately.
• Average strength of three specimens shall be considered for acceptance.
• Results shall be reported in standard format with traceability (date, batch, location, mix ID).
• Any variation beyond permissible limits shall trigger investigation and corrective action as per QA/QC procedure.

Engineering Control Note: Consistency in curing, handling, and testing is critical to ensure reliable compressive strength results representing in-situ concrete quality.

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

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

Reasons for Rejection

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

Final Decision: ❌ CONCRETE REJECTED

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

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