IS 516 Part 1 : 2021Methods of Test for Strength of Concrete: Part 1 Destructive tests on hardened concrete

IS 516 Part 1:2021 specifies methods of test for strength of concrete — compressive strength determination using cube specimens. It is the primary test method referenced by IS 456:2000 Clause 16 for concrete acceptance.

Every structural concrete pour in India is accepted or rejected based on 28-day cube tests performed per IS 516 Part 1. You reference it whenever: - Specifying concrete test frequency and sample size in your DBR / tender documents - Conducting cube tests at site or sending to accredited labs - Evaluating cube test results against IS 456 acceptance criteria - Preparing QA/QC procedures and ITPs - Troubleshooting failed tests (curing issues, sampling errors, machine calibration)

Pair with: - IS 456:2000 Clause 16 — acceptance criteria for cube test results - IS 1199 — sampling methods for fresh concrete (slump test, density, air content) — companion to IS 516 - IS 516 Part 2 — flexural strength test on beam specimens - IS 516 Part 3 — split tensile strength test on cylinder specimens - IS 516 Part 4 — non-destructive testing (rebound hammer, UPV) - IS 516 Part 5 — accelerated curing methods (for early-age strength prediction)

The 2021 revision of Part 1 modernized the test procedure, tightened tolerances on cube mould dimensions, and updated the compression testing machine calibration requirements.

Per IS 516 Part 1:2021, the compressive strength test procedure:

Step 1 — Sampling (per IS 1199): Collect representative sample from 4+ locations in each batch. Minimum sample size = 3 × cube volume + 10% contingency.

Step 2 — Mould preparation: 150 mm × 150 mm × 150 mm cubes (standard). Moulds of cast iron or steel, machined to tolerances per IS 10086:1982 (side flatness 0.2 mm per 100 mm). Oil the interior before filling.

Step 3 — Filling in 3 layers: Fill each layer ~50 mm thick. Compact each layer by 35 strokes of tamping rod (16 mm diameter, 600 mm long) OR vibrate on vibrating table for 1-2 minutes. Avoid over-vibration (causes segregation).

Step 4 — Finishing: Strike off excess concrete with trowel; finish flat surface flush with mould rim. Label with pour ID, date, and mix grade.

Step 5 — Initial curing (24 ± 8 hours): Keep cubes covered in mould at 24-30°C ambient, high humidity (wet jute or plastic cover). Do NOT move cubes for at least 16 hours after casting.

Step 6 — Demoulding and water curing: Carefully remove from mould at 24h ± 8h. Label each cube. Submerge in clean water tank (20-30°C) for 27 days (for 28-day test) or 6 days (for 7-day early test).

Step 7 — Testing: At test age, weigh cube (for density check), dimension check (each side), surface-dry (not saturated-dry — per 2021 revision). Place on compression machine with load-bearing face perpendicular to casting direction. Load uniformly at 140 kg/cm²/min (14 N/mm²/min) until failure.

Step 8 — Strength calculation: Compressive strength (N/mm²) = Failure load (N) / Cross-sectional area (150 × 150 = 22,500 mm²). Report 3-cube average (one sample = 3 cubes). If individual cube differs from average by >15%, investigate and possibly repeat.

Project: Residential RCC slab pour, 45 m³ of M30 concrete. Exposure: Moderate. Mix design cement content 350 kg/m³.

Step 1 — Sampling frequency per IS 456 Clause 15: For 45 m³ pour: 1 sample (3 cubes) per 15 m³ or part. So 3 samples = 9 cubes total. Label: M30-A (0-15 m³), M30-B (15-30 m³), M30-C (30-45 m³). Each sample gets 3 cubes: 2 for 28-day, 1 for 7-day.

Step 2 — Cast at site per IS 516 Part 1:2021 procedure. Oil moulds, fill in 3 layers, 35 tamps each, finish flat.

Step 3 — 7-day results (early indicator, not acceptance): Sample A-7day: 25 N/mm² Sample B-7day: 24 N/mm² Sample C-7day: 23 N/mm²

Expected 7-day strength for M30 is 65-75% of 28-day target. M30 target mean = 30 + 1.65 × 5 = 37.25 MPa. 7-day target ≈ 25-28 MPa. Results at 23-25 MPa are on the low end — watch 28-day results carefully.

Step 4 — 28-day results (test per IS 516 Part 1): Sample A (avg of 2 cubes): 38.2 MPa (individual cubes 37, 39) Sample B: 32.8 MPa (individual cubes 31, 34) Sample C: 36.1 MPa (individual cubes 35, 37) Mean of 3 samples = (38.2 + 32.8 + 36.1) / 3 = 35.7 MPa

Step 5 — Acceptance check per IS 456 Clause 16:

Criterion 1 — Individual sample: Each sample (mean of 3 cubes) ≥ 0.85 × f_ck = 0.85 × 30 = 25.5 MPa. ✓ (smallest = 32.8)

Criterion 2 — Mean of any 4 consecutive samples: ≥ f_ck + 4 = 34 MPa OR ≥ f_ck + 0.825 × σ = 30 + 0.825 × 5 = 34.1 MPa. Here mean of 3 (not 4) = 35.7 MPa ≥ 34.1. ✓

Criterion 3 — Individual cube: Smallest = 31 MPa (Sample B, Cube 1). ≥ f_ck - 3 = 27 MPa. ✓

Acceptance: All three criteria met. Concrete is accepted.

Observation: Sample B at 32.8 MPa is close to the f_ck lower bound (f_ck + 4 = 34). If next pour has similarly borderline results, investigate: check mix design compliance, curing practice at site, and compression machine calibration.

If the mean had been 32 MPa (below 34.1 criterion), rejection protocol per IS 456 Clause 16.3 would kick in: core sampling, load testing, or structural adequacy evaluation by the designer.

1. Under-curing cubes before test. Cubes must be water-cured continuously for 27 days (for 28-day test). Site practice sometimes moves cubes to open air or places them under a tarp — inadequate moisture leads to 10-20% strength loss. Always use a dedicated curing tank on-site; check temperature and water replacement weekly.

2. Using uncalibrated compression testing machines. IS 516 requires calibration per IS 1828 annually. Uncalibrated machines can over- or under-read by 10-15%. Accept test results only from NABL-accredited labs with current calibration certificate. Keep calibration cert on file.

3. Testing cubes at wrong moisture condition. The 2021 revision changed from 'saturated surface dry' to 'surface dry' at test. Over-saturated cubes (dripping wet) can give lower strengths; dry cubes can give higher. Remove cube from water, wipe surface dry, weigh, test within 30 minutes.

4. Wrong loading rate. IS 516 specifies 140 kg/cm²/min (14 N/mm²/min). Too fast → higher apparent strength (overestimate). Too slow → lower apparent strength (underestimate). Modern CTMs have programmable rates; ensure this is set correctly. Manual rate control is unreliable.

5. Accepting 1 or 2 cubes as a valid sample. IS 456 Clause 15.3 defines a 'sample' as 3 cubes. If you test only 2 and one fails, you cannot statistically reject or accept. Always cast 3 per sample, test all 3, and take the mean. Some sites cast 2 and 'keep 1 in reserve' — not compliant.

• IS 456:2000 — Clause 15 (sampling) and Clause 16 (acceptance criteria)
• IS 1199 — fresh concrete sampling and tests (slump, density, bleed)
• IS 10086 — concrete cube moulds specification
• IS 14858 — compression testing machine specification
• IS 1828 — calibration of compression testing machines
• IS 2506 — general requirements for vibrating tables (cube compaction)
• IS 516 Part 2 — flexural strength test
• IS 516 Part 3 — split tensile test
• IS 516 Part 4 — NDT (rebound hammer, UPV) for in-situ strength
• IS 516 Part 5 — accelerated curing (boiling water method for 24h strength)
• IS 13311 Part 1 — NDT by UPV for concrete integrity assessment

IS 516 Part 1:2021 replaced the 1959 edition after 62 years. The revision updates are practical: - Changed from 150 mm cubes as the default; 100 mm cubes are now permitted for small-aggregate concretes (< 20 mm MSA) - Tightened mould tolerance from 1.0 mm to 0.5 mm flatness - Added 'surface dry' condition at test (replacing 'saturated surface dry') - Clarified loading rate as 14 N/mm²/min (was 140 kg/cm²/min — same value, metric units) - Added Annex A for statistical analysis of multi-cube samples

Field reality: many labs still follow the 1959 procedure out of habit. Mill certificates and compression test reports must reference IS 516 Part 1:2021 explicitly. When procuring testing services, ask which edition the lab follows; if they say 1959, they are behind.

Site cube-casting quality is the single biggest source of test result variance. Labs routinely see poorly compacted cubes (honeycombed faces), un-oiled moulds causing sticking, and inadequate curing. A good QA/QC engineer witnesses cube casting personally for every batch on structurally critical pours (columns, foundations, transfer slabs).

For high-strength concrete M60+, 150 mm cubes give slightly higher apparent strength than 100 mm cubes (confinement effect). Check which size the mix design assumed. Most Indian labs test on 150 mm; international labs test on 100 mm (cylinders). Specify cube size in your test procedure.

For core sampling from hardened concrete (investigating failure, old structure evaluation), IS 516 Part 4 governs. Cores are typically 75-100 mm diameter; strength is corrected by height-to-diameter ratio per Table 1 of IS 516 Part 4.