Concrete Cube Test Procedure as per IS 516 — Step by Step Guide
The concrete cube compression test is the single most important quality test in Indian construction. Every concrete pour, every structural element, every project — the cube test is what proves your concrete meets the design strength. A failed cube test can halt construction, trigger demolition orders, and end careers. Getting it right is non-negotiable.
This article walks through the complete procedure from cube casting to result interpretation, with IS code references at every step. Whether you're a fresh site engineer or a seasoned QC professional, this is your definitive reference.
Key Standards: IS 516:2018 (test method), IS 456:2000 Cl. 15-16 (sampling frequency and acceptance), IS 1199:2018 (workability test before cube casting), IS 10262:2019 (mix design target strength).
Step 1: When and How Many Cubes to Cast
IS 456 Clause 15.2.2 specifies the minimum sampling frequency. This is not optional — it's a contractual and regulatory requirement.
| Volume of Concrete per Day | Minimum Samples (1 sample = 3 cubes) | Practical Recommendation |
|---|---|---|
| 1 – 5 m³ | 1 sample | 1 sample per pour |
| 6 – 15 m³ | 2 samples | 1 per 5-8 m³ |
| 16 – 30 m³ | 3 samples | 1 per 8-10 m³ |
| 31 – 50 m³ | 4 samples | 1 per 10-12 m³ |
| >50 m³ | 4 + 1 per additional 50 m³ | 1 per truck for critical pours |
Each "sample" consists of 3 cubes: typically 1 cube tested at 7 days (early strength indicator) and 2 cubes tested at 28 days (acceptance strength). Some projects add a 3-day cube for early stripping decisions.
Step 2: Equipment and Mould Preparation
| Item | Specification | IS Reference |
|---|---|---|
| Cube mould | 150mm × 150mm × 150mm (standard), rigid, leak-proof | IS 516 Cl. 3.1 |
| Tamping rod | 16mm dia, 600mm long, rounded ends | IS 516 Cl. 3.2 |
| Trowel | For filling and finishing | — |
| Mould oil | Light mineral oil or approved release agent | IS 516 Cl. 4.1 |
| Vibrating table | Optional — for high-workability mixes | IS 516 Cl. 4.2 |
Preparation
- Clean the mould thoroughly — remove any hardened concrete from previous use
- Apply a thin film of mould oil on all internal surfaces — prevents concrete from sticking
- Assemble the mould and base plate — ensure joints are tight (no grout leakage)
- Keep moulds on a level, vibration-free surface
Step 3: Sampling the Concrete
The sample must be representative of the batch being poured. IS 1199 and IS 516 specify:
| Rule | Requirement |
|---|---|
| Where to sample (RMC truck) | After discharging first 0.25 m³ — not from the beginning or end of the load |
| Where to sample (site mix) | From the middle of the batch, not from edges of the mixer |
| Workability test first | Perform slump test (IS 1199) BEFORE casting cubes — record slump value on cube label |
| Sample quantity | Minimum 30 litres (enough for 3 cubes + slump test + spare) |
| Time limit | Cast cubes within 15 minutes of sampling — concrete properties change rapidly |
Critical: Always perform the slump test BEFORE casting cubes. If slump is out of range, the concrete should be rejected — there's no point casting cubes from rejected concrete. Record the slump value on the cube identification label.
Step 4: Filling and Compacting the Cubes
Method A: Hand Compaction (Standard Method)
| Step | Action | Detail |
|---|---|---|
| 1 | Fill first layer | Fill mould to approximately 1/3 depth (~50mm) |
| 2 | Tamp 35 times | 35 strokes with tamping rod, distributed uniformly over cross-section |
| 3 | Fill second layer | Fill to 2/3 depth (~100mm) |
| 4 | Tamp 35 times | Rod should penetrate into the first layer by ~25mm |
| 5 | Fill third layer | Overfill slightly above mould top |
| 6 | Tamp 35 times | Rod penetrates into second layer by ~25mm |
| 7 | Level off | Strike off excess with trowel, smooth the top surface |
| 8 | Label | Mark cube ID, date, grade, element, slump, truck number |
Method B: Vibration Compaction (for stiff mixes, slump <25mm)
Fill in 2 layers, vibrate each layer on a vibrating table for minimum 15 seconds or until air bubbles stop rising. Do not over-vibrate — it causes segregation.
Common Mistake: Under-compaction is the #1 cause of low cube strength that does NOT reflect actual concrete quality. If cubes are poorly tamped, they'll have voids and test low even though the placed concrete is fine. Always complete the full 35 strokes per layer.
Step 5: Initial Curing (First 24 Hours)
| Parameter | Requirement | IS Reference |
|---|---|---|
| Location | Keep in shade, away from vibration and foot traffic | IS 516 Cl. 5.2 |
| Temperature | 27 ± 2°C (cover with wet hessian/plastic in hot weather) | IS 516 Cl. 5.2 |
| Duration | Keep in mould for 16-24 hours (demould at 24 ± ½ hour) | IS 516 Cl. 5.3 |
| Moisture | Cover top surface with wet cloth or plastic sheet to prevent drying | IS 516 Cl. 5.2 |
Site Reality: Cubes left in direct sun on an Indian summer day (45°C+) will crack and give false low results. ALWAYS shade cubes and keep them moist during the first 24 hours. This is where most cube test failures originate — not from bad concrete.
Step 6: Demoulding and Water Curing
- Demould at 24 hours — carefully remove mould without damaging cube edges
- Mark the cube — rewrite identification (may have been lost during demoulding)
- Submerge immediately in clean water at 27 ± 2°C
- Cure continuously until test date — do not remove from water until testing
- Water quality — change water if it becomes turbid or contaminated
- Cube register — log cube ID, date cast, date to test, element, grade in the register
Download Cube Test Tracker Register → — a ready-to-use log for tracking all cube samples from casting through testing and results.
Step 7: Testing — Compression Test
| Parameter | Requirement | IS Reference |
|---|---|---|
| Testing age | 7 days ± 2 hours, 28 days ± 8 hours | IS 516 Cl. 6.2 |
| Remove from water | Remove cube, wipe surface dry, test within 30 minutes | IS 516 Cl. 6.3 |
| Loading surface | Test perpendicular to casting direction (side faces, NOT the finished top) | IS 516 Cl. 6.4 |
| Loading rate | 14 N/mm²/min (= 315 kN/min for 150mm cube) | IS 516 Cl. 6.5 |
| Record | Maximum load at failure, calculate compressive strength | IS 516 Cl. 6.6 |
Compressive Strength Calculation
| Formula | Expression |
|---|---|
| Compressive strength | f = P / A |
| Where: | P = maximum load at failure (N), A = cross-section area = 150 × 150 = 22,500 mm² |
| Example: | Load = 675 kN = 675,000 N → Strength = 675,000 / 22,500 = 30.0 MPa |
Important: The test result of a "sample" is the average of 3 cubes (or 2 at 28 days if 1 was tested at 7 days). Individual cube results are NOT used for acceptance — only the sample average.
Step 8: Acceptance Criteria (IS 456 Clause 16)
This is where cube results become pass or fail decisions. IS 456 Table 11 gives the acceptance criteria:
| Criterion | Requirement | What It Means |
|---|---|---|
| Mean of any 3 consecutive samples | ≥ fck + 0.825 × σ | Average must exceed characteristic strength + margin |
| Any individual sample | ≥ fck - 3 MPa | No single result can be more than 3 MPa below specified grade |
Practical Acceptance Values (assuming σ = 4 MPa)
| Grade | fck | Mean of 3 must be ≥ | Any individual must be ≥ | Mix design target |
|---|---|---|---|---|
| M20 | 20 MPa | 23.3 MPa | 17 MPa | 26.6 MPa |
| M25 | 25 MPa | 28.3 MPa | 22 MPa | 31.6 MPa |
| M30 | 30 MPa | 33.3 MPa | 27 MPa | 36.6 MPa |
| M35 | 35 MPa | 38.3 MPa | 32 MPa | 41.6 MPa |
| M40 | 40 MPa | 43.3 MPa | 37 MPa | 46.6 MPa |
If cubes fail: Raise an NCR immediately. Options include: core testing (IS 516 Part 4), load test, UPV assessment (IS 13311), or in worst case — demolition and reconstruction. Download NCR Format →
7-Day vs 28-Day Strength Relationship
| Age | % of 28-Day Strength (OPC) | % of 28-Day Strength (PPC) |
|---|---|---|
| 3 days | 40-50% | 25-35% |
| 7 days | 65-75% | 45-55% |
| 14 days | 85-90% | 70-80% |
| 28 days | 100% | 100% |
| 56 days | 105-110% | 110-120% |
Rule of Thumb: If 7-day OPC cube gives ≥67% of target 28-day strength, the concrete is likely on track. If <60%, investigate immediately — don't wait for 28 days.
Cube vs Cylinder — Why India Uses Cubes
| Parameter | Cube (150mm) — IS 516 | Cylinder (150×300mm) — ASTM C39 |
|---|---|---|
| Used in | India, UK, EU, most of Asia | USA, Canada, Middle East |
| Strength value | Higher (~20% more than cylinder) | Lower (reference value) |
| Conversion | f'c (cylinder) ≈ 0.8 × fck (cube) | |
| Size effect | Friction at platens increases apparent strength | H/D ratio of 2:1 gives true uniaxial strength |
For international projects: M30 cube strength (IS) ≈ C24 cylinder strength (Eurocode) ≈ 3500 psi (ACI). Read our detailed cube vs cylinder comparison →
Frequently Asked Questions
What is the standard cube size for concrete testing?
150mm × 150mm × 150mm as per IS 516. For aggregate size >38mm, use 200mm cubes. The 150mm cube is standard for all normal construction concrete in India.
How many cubes per truck of RMC?
IS 456 requires minimum 1 sample (3 cubes) per 50 m³. A standard RMC truck carries 6-7 m³. For critical pours, many projects cast 1 sample per truck. For routine work, 1 sample per 2-3 trucks is common practice.
What if only 1 cube out of 3 fails?
The sample result is the average of the cubes. If 1 cube is significantly lower, check for casting/curing defects in that specific cube. If the average still meets acceptance criteria, the sample passes. Report the individual values along with the average.
Can I test cubes at 56 days instead of 28 days?
PPC concrete can be specified for 56-day strength per IS 456 Amendment. This must be stated in the specification. Default acceptance age remains 28 days. If 28-day cubes fail for PPC, testing at 56 days may be considered with consultant approval.
What causes low cube results but good in-situ concrete?
Poor cube making practices: insufficient tamping (most common), cubes left in sun during first 24 hours, inadequate water curing, testing cubes wet (should wipe dry), wrong loading rate, and damaged cube edges. Fix the cube-making process before questioning the concrete quality.
Download QA/QC Templates
| Template | Use | Download |
|---|---|---|
| Cube Test Report | Record 7-day and 28-day results | Excel / PDF ↓ |
| Cube Test Tracker | Log all samples from casting to results | Excel / PDF ↓ |
| Slump Test Report | Record workability before cube casting | Excel / PDF ↓ |
| Pre-Pour Checklist | Complete QC gate before concrete placement | Excel / PDF ↓ |
| NCR Format | Document non-conformance if cubes fail | Excel / PDF ↓ |
Related Resources
- IS 516:2021 — Concrete Testing Methods
- IS 456:2000 — Plain and Reinforced Concrete
- Concrete Mix Design Calculator (IS 10262)
- Cube vs Cylinder Strength Comparison
- Concrete Grade Guide — M20, M25, M30
- Water Cement Ratio Guide — IS 456 Table 5
- All Concrete QA/QC Templates (14 formats)
References
- IS 516:2018 — Method of Tests for Strength of Concrete — Part 1: Compressive, Flexural and Split Tensile Strength
- IS 456:2000 — Plain and Reinforced Concrete — Code of Practice — Clauses 15, 16
- IS 1199:2018 — Methods of Sampling and Analysis of Concrete
- IS 10262:2019 — Concrete Mix Proportioning — Guidelines
- IS 13311:2022 — Non-Destructive Testing of Concrete
- SP 23:1982 — Handbook on Concrete Mixes