Codes & standards referenced
Schmidt hammer calibration, test grid (≥16 readings), angle and atmospheric corrections. Limits applicability to surface layer (~30 mm depth) and relational comparisons only.
Method for measuring UPV through concrete. Velocity grading: > 4.5 km/s good, 3.5–4.5 doubtful, < 3.5 poor. Confirm the latest published section number via the BIS catalogue.
Procedure for extracting, preparing, and testing cores for compressive strength. Critical when cubes and structure disagree. Pair with ASTM C42 / BS 1881:207 cross-references for international project reports.
Useful cross-reference for combined NDT correlation curves. Cite IS standards in Indian project reports.
1 — Rebound hammer (IS 516 Pt 5/Sec 4)
The Schmidt hammer measures surface hardness by firing a spring-loaded mass against concrete and reading the rebound distance on a 0–100 scale. It tests only the outer ~30 mm of concrete — carbonated surface gives misleadingly high readings.
The values below are taken from the Original Schmidt Type N manufacturer's published correlation curve, assuming horizontal impact on standard 150 mm cubes at 28 days, on an uncarbonated, well-prepared surface. Real-world readings vary ±20–30% from this curve. Corrections must be applied for:
- Hammer orientation — vertical-down reads ~5–7 points higher than horizontal; vertical-up (soffit) reads ~5–7 points lower. Use the angle-correction chart from the hammer manual.
- Concrete age & curing — 7-day readings run ~15% lower than 28-day; early-age tests overstate later strength gain.
- Carbonation depth — on a 5+ year-old structure, uncorrected rebound numbers overstate strength by 30–50% (see warning below).
- Surface preparation — grind off laitance; rough formwork marks and honeycombing give false-low readings.
- Aggregate hardness — basalt and quartzite read higher than limestone for the same paste strength.
IS 516 Pt 5/Sec 4 forbids using rebound hammer alone for acceptance. Build a project-specific curve using trial mixes + at least 3 paired (cube + rebound) data points before relying on this table for any decision beyond uniformity assessment.
| Average rebound number (R) | Indicative compressive strength (MPa, horizontal impact, 28-day) | Quality interpretation |
|---|---|---|
| > 50 | ≥ 50 (typical 45–55) | Very high strength — typical M50 and above; verify with cores before any acceptance decision |
| 40 – 50 | 32 – 50 (typical M40 grade) | High-strength structural concrete (typical M40) |
| 30 – 40 | 22 – 40 (typical M25–M35) | Normal structural concrete (typical M25–M30) |
| 20 – 30 | 14 – 28 (typical M15–M20) | Low strength or suspect quality — verify with cores before any decision |
| < 20 | < 18 (typically < M15) | Poor — immediate investigation required (cores + UPV + structural review) |
Source: Original Schmidt Type N manufacturer's correlation curve, horizontal impact on 150 mm cubes at 28 days, uncarbonated surface. References: Proceq Operating Instructions — Original Schmidt Concrete Test Hammer; Bungey, The Testing of Concrete in Structures, 4th ed.; IS 516 Pt 5/Sec 4 calibration clause. Apply angle, age, and carbonation corrections before use. Build a project-specific correlation per IS 516 Pt 5/Sec 4 for any acceptance decision.
Concrete surface carbonates over time, increasing hardness. On a 5+ year-old structure, uncorrected rebound readings overstate strength by 30–50%. Either grind 5 mm off the test point, or apply a correction factor based on phenolphthalein-spray depth-of-carbonation measurement.
Field protocol (per IS 516 Pt 5/Sec 4)
- · Test grid: minimum 16 readings in a 300 × 300 mm square, discard outliers (±7 from median).
- · Surface must be smooth — grind if necessary. Avoid honeycomb, laitance, and rough formwork marks.
- · Hold hammer perpendicular to surface (90° ± 5°) — angle corrections are mandatory.
- · Calibrate daily on the test anvil supplied by manufacturer.
- · Atmospheric correction: -3 to +5% per 10°C deviation from 27°C reference.
2 — Ultrasonic pulse velocity (IS 516 series)
UPV sends a 20–150 kHz pulse from a transmitter and times the arrival at the receiver. Velocity in well-compacted, dense concrete is 4.0–5.5 km/s; lower velocity indicates internal cracks, voids, or honeycombing.
UPV grading (IS 516 series)
Dense, well-compacted, homogeneous. No defects expected.
Some voids / minor cracking. Further investigation warranted.
Significant defects. Core sampling and structural review required.
Velocity in km/s. Values indicative — use combined-method assessment per IS 516.
Probe configuration
Both probes on opposite faces. Most accurate. Required for beams, columns. Path length must be measured accurately.
Probes on adjacent faces (90° apart). Used for slabs on inaccessible soffit. Slightly less accurate.
Both probes on same face. Detects only outer ~50 mm. Used for one-sided access only (retaining walls). Less reliable.
3 — Core sampling (IS 516 Pt 5)
The core test is the only direct measurement of in-place strength in the disputed region. IS 516 Pt 5 / IS 1199 Annex specify 100 mm nominal diameter cores (75 mm acceptable where reinforcement congestion prevents 100 mm), tested as-moulded (L/D = 2.0 ± 0.05) or capped.
Core test decision flow
- Cubes fail (< fck + 1.65σ) at 28 days.
- Verify cubes: re-test using capped cubes, retained sample, or witness cubes.
- If cubes still fail — independently sample fresh concrete for trial mixes.
- Run combined NDT (rebound + UPV) on suspected structural region.
- If NDT suggests in-place strength is low, schedule core extraction.
- Extract ≥ 3 cores per zone. Core size 100 mm (avoid bars; GPR-scan first).
- Test cores at 28-day equivalent (if tested earlier, apply 0.85 correction).
- Apply strength reduction factor for L/D ratio (0.87 for L/D = 1.0).
- Compare core strength to fck + 3 MPa for acceptance (Indian Engineering Practice).
Every core leaves a hole. The structure must be restored with epoxy-bonded patch using parent-equivalent or higher-grade concrete. Failure to restore properly turns the inspection into a defect.
Other methods — when they fit
Measures force to extract an embedded insert. Strong correlation with compressive strength (~±10%). Excellent for in-place assessment. Less common in India.
Steel pin driven into concrete; depth of penetration correlates to strength. Quick but destructive and inconsistent. Rare in Indian practice.
Estimates rebar corrosion probability (not strength). Used in durability audits of bridges, coastal structures.
Locates rebars, voids, delamination before coring. Reduces "oops, hit a bar" damage to structure.
Common pitfalls on Indian sites
One impact at one point is meaningless. Read 16+ on a 300 mm grid, discard outliers, report the median. IS 516 Pt 5/Sec 4 forbids single-impact acceptance.
Carbonation overstates strength; saturated concrete understates UPV velocity. Always note the surface condition and time of test in the report.
The correlation curve supplied with a Schmidt hammer is generic. For acceptance work, build a project-specific curve using trial mixes + tested cores (3+ pairs).
Hitting a rebar invalidates the core. Always scan with cover meter + GPR before coring. Reposition 80 mm if bar detected.
Frequently Asked Questions
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Amit Haridas
Founder & Proprietor, ConcreteInfo. Has performed combined NDT assessments (rebound + UPV + core) on RMC plant acceptance failures, bridge durability audits, and post-fire damage assessments across India. IS-codes-aligned reporting; results survive cross-examination in arbitration.