IS 13311 Part 2 : 1992Non-destructive testing of concrete - Methods of test - Part 2: Rebound hammer

IS 13311 (Part 2):1992 specifies Non-Destructive Testing of Concrete by Rebound Hammer (also called Schmidt Hammer) — the fastest and most common in-situ concrete strength estimation method on Indian sites.

Use it when: - Estimating in-place concrete strength without destructive coring - Quality control during construction — check concrete surfaces shortly after striking the formwork - Forensic / retrofit assessment of existing structures - Comparing concrete uniformity across columns / beams / slabs in a single project - Field investigation of suspect zones — rebound hammer maps zones of unusual surface hardness for further investigation

IS 13311 Part 2 is paired with Part 1 (UPV — Ultrasonic Pulse Velocity). Combined use is recommended (IS 14591:1999) — rebound hammer measures surface hardness, UPV measures bulk quality; together they give a much better picture than either alone.

Principle: a spring-loaded plunger is pressed against the concrete surface; the plunger releases a hammer mass against the concrete; the rebound distance of the hammer is measured by a slider on a calibrated scale. The rebound number (R) depends on: - Hardness of the concrete at and just below the surface - Density of the concrete - Energy of the rebound (related to elastic modulus + density of the concrete)

Empirical correlation: rebound number R correlates loosely with compressive strength f_c (MPa). The relationship is NOT universal — depends on: - Hammer type and calibration - Concrete mix (cement type, aggregate, age, curing) - Surface conditions (moisture, smoothness, carbonation) - Direction of impact (vertical down, horizontal, vertical up — each gives different reading)

Standard equipment (Clause 4): - N-type Schmidt hammer: 2.207 N·m impact energy; for normal concrete testing - L-type Schmidt hammer: 0.735 N·m; for thin or low-strength concrete (rare in India) - NR-type, M-type, P-type: specialty variants — rarely used

Surface preparation (Clause 5): - Grinding stone supplied with the hammer used to smooth the test area - 100 × 100 mm test area required - Surface must be free of plaster, loose aggregate, paint, or oil films - Test on firm support — slender / unsupported elements give artificially low readings due to flex

1. Calibration check: at start of each day, test the hammer against a standard test anvil supplied with the equipment. Reading should be within manufacturer's specified range (typically 75-85 ± 2 for N-type). Otherwise, send for re-calibration.

2. Mark grid on the structural element being tested. Typically 10 × 10 readings within a 200 × 200 mm area, or distributed across larger features.

3. Position the hammer: - Horizontal direction: standard — directly perpendicular to a vertical surface (typical column / wall test) - Downward: hammer pointing down, hitting top surface (slab testing). Correct R reading downward by adding ~3-5 to compare with horizontal calibration. - Upward: hammer pointing up, hitting underside (slab soffit testing). Subtract ~3-5 from R. - All readings should be corrected to the horizontal reference for comparison.

4. Take readings: - Press the hammer plunger against the prepared surface (avoid aggregate pieces and large air voids) - Apply pressure smoothly until trigger releases - Record the rebound number R from the scale - Take 10-15 readings per test area; discard the highest and lowest 2; average the rest - If readings vary by > 6 within a test area, the concrete is heterogeneous — investigate the cause

5. Correlate with strength: - Without site-specific calibration: use generic correlation curves from IS 13311 Part 2 Annex A or hammer manufacturer's chart. Indicative strength only — accuracy ± 30%. - With site-specific calibration: cast cube samples from the same concrete mix; test on cubes at various ages (3, 7, 28 days); develop a project-specific R vs f_c curve. Accuracy improves to ± 15-20%. - With paired UPV testing: pair R + V (pulse velocity from IS 13311 Part 1) with cube strength for a 'combined' correlation per IS 14591. Accuracy ± 10-15%.

Typical rebound number ranges for Indian concrete: - M15: R = 20-30 - M20: R = 25-35 - M25: R = 28-40 - M30: R = 32-44 - M35: R = 35-46 - M40: R = 38-48 - M50: R = 42-52

These are rough indicative ranges; actual project-calibrated correlations vary.

1. Using strength correlation without site calibration — generic R-vs-f_c charts from hammer manuals or IS Annex A are indicative only. Project-specific calibration with same mix design, same age, same curing is essential for any meaningful strength estimation.

2. Wrong direction correction — horizontal, downward, upward give different readings. Always note the direction; convert to horizontal-equivalent before comparison or correlation.

3. Test on cracked or honeycombed surface — rebound reads low at defects, giving false-low strength indication. Inspect surface for visible defects before testing. Move to adjacent intact area.

4. Carbonated surface — old concrete (> 5 years) develops carbonation in the surface 5-15 mm. Carbonated surface is HARDER than non-carbonated (CaCO₃ harder than Ca(OH)₂), giving artificially HIGH R. For old structures, sandblast or grind off the carbonated layer before testing. Note: this contradicts the surface-grind requirement — IS 13311 Part 2 surface prep is for cleanliness; carbonation removal requires deeper grinding.

5. Wet surface — water films damp the rebound; readings drop ~5-10%. Test on dry surface; ideally same moisture state as the calibration test on cubes.

6. Rebound on or near reinforcement — rebar very close to the surface gives high R reading (steel is harder than concrete). Use a covermeter to locate rebar; offset test points to avoid steel within 50 mm of the impact point.

7. Skipping cube calibration — many sites use generic correlation curves and present 'strength' values that are 20-40% wrong. Combined with the IS 13311 advisory that strength is 'indicative only', this creates false confidence. For strength acceptance, always use direct cube testing per IS 516 or core testing per IS 1199. Use rebound hammer for relative comparison and screening.

8. Calibration verification skipped — hammer calibration drifts. Daily check on test anvil is mandatory. Sites that skip this collect data of unknown validity.

• IS 13311 Part 1:1992 — Non-destructive testing — Ultrasonic Pulse Velocity (UPV) (paired with rebound hammer per IS 14591)
• IS 516 Part 1 Sec 1:2021 — Compressive strength of cubes (the destructive reference)
• IS 516 Part 4:1980 — Non-destructive testing methods (parent of IS 13311)
• IS 14591:1999 — Code for evaluation of strength of concrete in existing structures (uses combined UPV + rebound + cores)
• IS 1199 Parts 1-7 — Sampling concrete (including core extraction per Part 5)
• IS 456:2000 — General RCC code
• IS 1786:2008 — TMT bar specification (relevant when rebound encounters rebar in path)
• IS 13935:1993 — Repair and seismic strengthening of buildings (uses NDT for damage assessment)
• IS 4326:2013 — Earthquake resistant design (uses NDT for assessment of existing structures)
• EN 12504-2:2012 — European standard for rebound hammer (similar methodology)
• ASTM C805 — US standard for rebound number
• ISO 8048 — International standard

IS 13311 Part 2:1992 is 33 years old but remains the working code. The rebound hammer methodology is mature; no significant revision is in sight.

Practical role: - First-line surface QC: every site QA team has a rebound hammer. Cost ₹10,000-25,000 for a basic N-type, ₹40,000-80,000 for digital recording models. - Quick acceptance screening: in 30 minutes, an inspector can map rebound numbers across 10-20 columns. Strength deviations of > 20% from cube average flag suspect zones. - Comparative analysis: relative differences between adjacent elements (column A vs column B in the same building) are more reliable than absolute strength estimates. The hammer is best used as a comparator, not a strength meter.

Where it falls short: - Absolute strength — without site calibration, error is too large to use for compliance decisions. - Sub-surface assessment — measures only top 25-30 mm of concrete. Bulk-quality assessment needs UPV (Part 1) or coring. - Old concrete — carbonation, moisture variations, and surface deterioration mask the underlying strength. Treat readings on > 10-year-old concrete with high skepticism.

Modern alternatives: - Digital rebound hammer (Proceq Silver Schmidt): automatic recording, eliminates operator-error in scale-reading; ~₹50,000-1.5 lakh. - Ultrasonic + rebound combined instrument (Proceq Hammer N + Pundit PL-200): use both methods on same point, automatic combined-correlation analysis; ~₹4-8 lakh combined. - Probe penetration (Windsor probe) per ASTM C803: gun-fired hardened-steel probe into concrete; measure penetration depth; correlates with strength. More invasive than rebound hammer but better correlation. Some Indian forensic labs use it.

For Indian project QA: use rebound hammer for uniformity check + screening, not as primary strength acceptance. For strength acceptance, cube testing per IS 516 remains mandatory. The IS 456 acceptance criteria (Clause 16) are based on cube strength, not rebound.

For forensic investigation of suspect structures: combine rebound hammer + UPV + targeted coring. The rebound hammer maps the spatial distribution; cores quantify the worst zones. This is the standard methodology in IS 14591:1999 for evaluation of existing structures, and remains the gold standard for retrofit planning.