IS 516:2021 Part 1/Sec 1 is the Indian Standard (BIS) for methods of tests for strength of concrete - part 1: hardened concrete - section 1: compressive, flexural and split tensile strength. This standard details the exact laboratory procedures for determining the compressive, flexural, and split tensile strengths of hardened concrete. Quality control engineers and material testing laboratories use it to verify that cast concrete meets the characteristic strength required by structural design.
Specifies procedures for determining compressive, flexural, and split tensile strength of concrete specimens.
Cube size, curing, age tolerances, loading rate and acceptance limits for hardened-concrete strength tests.
| Reference | Value | Clause |
|---|---|---|
| Standard cube size | 150 mm cube (also 100 mm for max agg. ≤ 25 mm) | Cl. 5.1 |
| Cube tolerance — dimensions | ± 0.2 mm (planeness 0.05 mm/100 mm) | Cl. 5.2 |
| Number of specimens — one strength test | 3 cubes | Cl. 9 / IS 456 Cl. 15.4 |
| Curing — temperature | 27 ± 2 °C | Cl. 7.2 |
| Curing — relative humidity | ≥ 90 % (or water immersion) | Cl. 7.2 |
| Test ages — typical | 3, 7, 28 days | Cl. 9.1 |
| Test age tolerance — 28 day | ± 1 day (for 28-d test) | Cl. 9.1 (Table 1) |
| Loading rate — compression | 140 kg/cm²/min (≈14 N/mm²/min) | Cl. 9.4 |
| Compressive strength — calculation | fc = P / A (cube), MPa | Cl. 9.5 |
| Cube/cylinder ratio | ≈ 0.80 (cylinder = 0.80 × cube) | Annex A |
| 100 mm cube — strength correction factor | Multiply by 0.97 to get 150-mm equivalent | Cl. 5.1.1 |
| Acceptance — single result variation from mean | ≤ 15 % | Cl. 9.5.1 |
| Reporting — significant figures | Nearest 0.5 N/mm² | Cl. 9.6 |
| Capping — required if planeness exceeded | Sulphur or HA capping per IS 9013 | Cl. 8.2 |
| Sample handling — demoulding age | 24 ± 8 hours | Cl. 7.1 |
| Specimen marking | Indelible, on top finished face | Cl. 7.1 |
IS 516 (Part 1, Section 1) specifies the method of testing strength of concrete — compressive strength test on hardened concrete cubes — the headline acceptance test for all RCC work in India. Every cube test certificate cited in concrete acceptance traces to IS 516 Part 1 Section 1.
Use IS 516 Part 1 Section 1 for: - Routine cube testing for concrete acceptance (IS 456:2000 Clause 16) - Source qualification of new cement / aggregate / mix design - Forensic strength assessment of in-situ structures (via cores) - Mix design verification (IS 10262:2019) - Quality control on construction projects
IS 516:2021 is the latest revision; supersedes the older 1959 / 1969 editions. The 2021 revision has clearer reporting requirements + alignment with international ISO/EN standards.
The IS 516 series is comprehensive: - Part 1 / Sec 1: Compressive strength on cubes (this code) - Part 1 / Sec 2: Compressive strength on cylinders - Part 5: Flexural strength (modulus of rupture) on prisms - Part 6 / 9: Splitting tensile strength (replaces IS 5816:1999 in some applications)
For Indian practice, 150 mm cubes are the standard specimen — vs 150 × 300 mm cylinders (US / international). Cube strength is approximately 1.25 × cylinder strength of same concrete.
Specimen: - 150 × 150 × 150 mm cube — standard - 100 × 100 × 100 mm cube — for max aggregate ≤ 20 mm (smaller, less material)
Casting: 1. Sample fresh concrete from point of placement (not from mixer outlet — reduces sampling error) 2. Per IS 1199 Part 1:2018 sampling protocol 3. Compact in 3 layers using tamping rod (35 strokes per layer for 150 mm cube) OR use vibrating table 4. Top surface trowelled smooth 5. Marked with date + identification
Curing: 1. Cover specimens immediately to prevent moisture loss 2. Demould after 24 ± 4 hours 3. Cure in water at 27 ± 2 °C until test day 4. For 7-day test: 7 days curing 5. For 28-day test: 28 days curing (standard acceptance) 6. Other ages: per project (3-day for early indication; 90-day for long-term)
Testing: 1. Remove cube from water; wipe dry; weigh 2. Place in compression testing machine 3. Apply load at constant rate of 140 ± 7 kg/cm² per minute (~14 N/mm² per min) 4. Record peak load (kN) 5. Compute compressive strength: Strength (N/mm²) = Peak load (N) / Specimen cross-section area (mm²) 6. For 150 mm cube: area = 150 × 150 = 22,500 mm²
Reporting: - Specimen ID, date of casting, date of testing, age (days) - Peak load (kN), compressive strength (N/mm²) - Failure mode (proper failure: cube cracks all 4 faces simultaneously; abnormal: top/bottom only or single-face) - Mean of 3 specimens = test result - Standard deviation across 3 specimens (consistency indicator)
Concrete grades + characteristic strength (per IS 456:2000):
| Grade | Characteristic strength f_ck (N/mm²) at 28-d | Target mean strength (mix design) | |---|---|---| | M15 | 15 | f_ck + 1.65 × σ | | M20 | 20 | f_ck + 1.65 × σ | | M25 | 25 | f_ck + 1.65 × σ | | M30 | 30 | f_ck + 1.65 × σ | | M40 | 40 | f_ck + 1.65 × σ | | M50 | 50 | f_ck + 1.65 × σ | | M60 | 60 | f_ck + 1.65 × σ | | M70+ | 70+ | f_ck + 1.65 × σ |
Where σ = assumed standard deviation per IS 456 Table 8 (typical 4-5 N/mm² for routine RCC).
Acceptance criteria (IS 456:2000 Clause 16):
7-day vs 28-day: - 7-day strength typically 65-70 % of 28-day for OPC concrete - For PPC: 60-65 % at 7-day; 100 % at 28-day; gain continues to 90-day - 7-day test = early indication; 28-day = legal acceptance
Test machine calibration: - Calibrated load cell or proving ring - Annual calibration certificate from NABL lab - Capacity 1.5 × max test load
Specimen tolerance (Clause 4.1): - Dimensions: ± 0.5 mm of nominal - Surface flatness: ≤ 0.05 mm under 150 mm straightedge - Squareness: 90° ± 0.5° - Surfaces in contact with platens machined / ground if not flat
Failure modes: - Acceptable: cube failure across all 4 sides, hourglass / cone shape on inclined plane - Suspect: top-only failure (capping issue), single-face failure (specimen fault) - Rejected: specimen damage during handling; out-of-tolerance dimensions
1. Specimens cured outside water. Air-cured cubes test 30-40 % lower than water-cured. Standard is water curing at 27 ± 2 °C. 2. Inadequate compaction during casting. Voids reduce strength; cube failure low. Use proper tamping (35 strokes per layer for 150 mm) or vibrating table. 3. Test at non-standard age. Acceptance is 28-day; 21-day or 35-day requires correlation. Stick to specified age. 4. Loading rate too fast. Strength reads higher than design (rate-dependent); not matching IS 516. Stick to 14 N/mm² per minute. 5. No sample identification. Tracking lost; can't assign result to batch / pour. Mark each specimen with project, date, location. 6. Sample from mixer outlet only. Workability + segregation may differ from placement point. Sample from point of placement. 7. Oversize aggregate in 100 mm cube. Wall effect dominates; result invalid. Use 150 mm cube for max aggregate up to 40 mm. 8. Failure mode not recorded. Suspect failure mode (single face, capping issue) invalidates result; not noted. Always record + photograph if abnormal. 9. Calibration of test machine stale. ±10-15 % uncertainty in result. Annual NABL calibration mandatory. 10. Poor specimen-to-platen contact. Uneven surface causes stress concentration; premature failure. Surface preparation per Clause 4. 11. Test on same day as mix change. Operator unfamiliar with new mix; sampling errors. Trial mix sequence + sampling validation. 12. No rejection of damaged specimen. Damaged in handling; result invalid; not rejected. Discard if visible damage.
Concrete acceptance cascade per IS 456:2000:
1. Mix design (IS 10262:2019) — target mean strength = f_ck + 1.65 × σ 2. Trial mix — 9-12 cubes (3 per age × 3 ages); verify target mean strength achievable 3. Production sampling — per IS 456 Clause 16.2 frequency 4. Cube casting (IS 1199) — fresh sample, 6 cubes per sample 5. Cube curing — water tank 27 ± 2 °C 6. Cube testing (this code, IS 516 Part 1 Sec 1): - 3 cubes at 7 days (early indication) - 3 cubes at 28 days (legal acceptance) 7. Statistical analysis (IS 456 Annex A): - 3-result rolling mean check - Individual minimum check - σ trend monitoring 8. Acceptance / rejection decision: - All meet criteria → Acceptance - Any fail → Investigation, retesting if applicable, possibly rejection of pour 9. In-situ verification (if dispute): UPV (IS 13311), core testing (IS 516)
Statistical SD value: - Routine RCC at established RMC: σ ~ 4 N/mm² - Field-batched concrete: σ ~ 5-6 N/mm² - High-strength concrete: σ ~ 5-6 N/mm²
Common acceptance issues: - Single low result → 3-result mean still above; acceptable - Multiple low results → mean drops below criteria; investigate (cement source, mix design, cube quality) - Trend of declining strength → systematic issue (workability change, cure, batching error)
IS 516 Part 1 Section 1 is the foundation of concrete quality control in India. Every RCC structure's acceptance traces back to this standard.
| Parameter | IS Value | International | Source |
|---|---|---|---|
| Standard Compressive Strength Specimen Type (Primary) | Cube | Cylinder | ASTM C39/C39M |
| Compressive Strength Loading Rate (Stress/time) | 14 N/mm²/minute | 0.25 ± 0.05 MPa/s (approx. 15 ± 3 N/mm²/min) | ASTM C39/C39M |
| Curing Temperature for Standard Specimens (Water/Moist Curing) | 27 ± 2 °C | 23 ± 2 °C | ASTM C31/C31M (referenced by C39/C39M) |
| Flexural Strength Test - Span-to-Depth Ratio (Third-Point Loading) | 4 (e.g., 600 mm span for 150 mm depth) | 3 (e.g., 450 mm span for 150 mm depth) | ASTM C78/C78M, EN 12390-5 |
| Number of Layers for Compaction of 300mm High Cylinders (by rodding) | 6 layers (approx. 5 cm deep each) | 3 layers (approx. 10 cm deep each) | ASTM C31/C31M |
| Maximum Time from Curing to Compressive Test | Within 30 minutes | Within 15 minutes | EN 12390-3 |