IS 516:2021 Part 5/Sec 2 is the Indian Standard (BIS) for methods of tests for strength of concrete - part 5: non-destructive testing of concrete - section 2: rebound hammer. This standard specifies the method for assessing the in-situ compressive strength, uniformity, and surface hardness of hardened concrete using a Rebound Hammer. It is a non-destructive testing (NDT) code frequently used by structural and quality control engineers to quickly evaluate existing concrete structures.
Specifies the method for determining the rebound number of concrete using a rebound hammer.
Velocity grading bands, transducer frequency, path lengths and direct/indirect test modes.
| Reference | Value | Clause |
|---|---|---|
| Method | Ultrasonic Pulse Velocity (UPV) | Cl. 4 / IS 13311 Pt 1 ref |
| Transducer frequency — typical concrete | 20 kHz – 150 kHz (50 kHz common) | Cl. 5.2 |
| Path length — direct transmission (min) | 100 mm | Cl. 6.3 |
| Couplant — typical | Petroleum jelly / grease (for smooth contact) | Cl. 6.4 |
| Quality grade — V > 4.5 km/s | Excellent | Cl. 8 (Table 1) |
| Quality grade — V 3.5–4.5 km/s | Good | Cl. 8 (Table 1) |
| Quality grade — V 3.0–3.5 km/s | Doubtful / Medium | Cl. 8 (Table 1) |
| Quality grade — V < 3.0 km/s | Poor | Cl. 8 (Table 1) |
| Calibration — reference bar | Steel bar of known velocity (~5.9 km/s) | Cl. 5.3 |
| Test mode — direct (preferred) | Transducers on opposite faces | Cl. 6.2 |
| Test mode — semi-direct | Adjacent faces (90°) | Cl. 6.2 |
| Test mode — indirect (surface) | Same face — least accurate | Cl. 6.2 |
| Reinforcement effect — correction | Avoid bars in path; use shielded chart if unavoidable | Cl. 7.2 |
| Min concrete age | 14 days | Cl. 6.1 |
| Resolution — time measurement | 0.1 µs (or finer) | Cl. 5.2.1 |
| Number of readings per location | Min 3 (mean reported) | Cl. 6.5 |
IS 516 Part 5 Section 2:2021 is the rebound-hammer (Schmidt-hammer) method for non-destructive testing of concrete — measuring the rebound number of a hardened concrete surface to assess its surface hardness, uniformity, and (via correlation) likely strength. It is the fast, cheap NDT companion to UPV within the IS 516:2021 Part 5 framework, replacing the rebound provisions of the older IS 13311 Part 2.
It sits in the assessment stack:
A spring-loaded mass strikes the concrete and rebounds; the rebound number reflects the *surface* hardness/stiffness, which loosely correlates with strength only for the surface zone and only with calibration. The governing realities:
The engineering point: the rebound hammer is a comparative surface-quality and screening tool, not a strength meter — its strength estimate is only as good as a site-specific, carbonation-aware correlation.
Scenario: in-situ assessment of an existing structure of uncertain quality.
Step 1 — prepare test areas: grind off any plaster/loose surface; mark a grid of test areas (avoid edges/honeycomb).
Step 2 — take the standard set: the prescribed number of impacts per test area, discard outliers, record the median, and apply the orientation correction.
Step 3 — map uniformity: rebound-number variation across the grid flags anomalous (weak/poor) zones — this is what the hammer is genuinely good at.
Step 4 — calibrate, don't guess strength: take cores from representative zones, test per IS 516 Part 1, and build a site-specific correlation (account for carbonation on old surfaces); combine with UPV for a more reliable estimate.
Step 5 — decide per IS 456: use the mapped uniformity + correlated/core strength for the acceptance/repair decision.
The hammer told you *where* to look and *how uniform*; cores told you *how strong* — used together they answer the question, the hammer alone does not.
1. Reading strength off a generic chart. Strength needs a site-specific correlation to cores; the factory chart is not acceptance.
2. Ignoring carbonation on old concrete. A carbonated surface is hard and over-reads strength badly — the classic rebound error on existing structures.
3. No orientation correction. Up/down/horizontal readings differ; failing to correct skews results.
4. Single readings / poor test-area prep. It is a statistical surface test — needs a proper set of impacts on a prepared area, not one tap on plaster.
5. Using it alone for a verdict. Best as screening/uniformity and combined with UPV + cores — not a stand-alone strength judgement.
IS 516 Part 5 Section 2 is current (2021) and the rebound hammer is the most-used — and most-abused — NDT tool in concrete assessment: fast and cheap, but routinely over-read as a strength meter. The disciplined view: it measures surface hardness, is excellent for uniformity screening and locating where to core, and yields strength only through a site-specific correlation that accounts for carbonation (the dominant error on old structures) and orientation. Its real power is in combination — rebound for surface, UPV for the interior, cores (IS 516 Part 1) for the truth — the standard combined-NDT approach. Treat a bare rebound-to-strength number as indicative at best; the IS 456 decision rests on cores and the combined assessment, with the hammer pointing you to where they matter.
| 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 |