--- title: "Non-destructive testing methods for concrete" subject: "Concrete Technology and Masonry Structure" chapter: 5 key: "semester_5/concrete_technology_and_masonry_structure:ndt-methods-concrete" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/ndt-methods-concrete site: BCT Engineering --- # Non-destructive testing methods for concrete Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control - Asked in: 2079 Baisakh, 6 marks; 2076 Chaitra, 6 marks; 2075 Chaitra, 6 marks; 2068 Chaitra, 6 marks (asked 4 times) - Video: [Introduction to Non Destructive Testing of Concrete - Advanced Concrete Technology](https://www.youtube.com/watch?v=T-853btlzCA) — Ekeeda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/ndt-methods-concrete - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5 ## Question Explain the methods (techniques) of non-destructive testing of concrete. ## Short answer Key NDT methods: rebound hammer (surface hardness, IS 13311-2), ultrasonic pulse velocity (uniformity, cracks, IS 13311-1), combined SONREB, penetration resistance, pull-out, cover meter, half-cell potential, radiography, and impact echo. They provide indirect strength estimates and locate internal flaws. ## Answer NDT methods give indirect estimates of strength, quality and condition without damaging the structure. | Method | What it measures | Standard / note | |---|---|---| | Rebound hammer | Surface hardness, related to strength | IS 13311 Part 2 | | Ultrasonic pulse velocity | Pulse velocity, uniformity, voids, cracks | IS 13311 Part 1 | | Combined (SONREB) | Rebound number plus UPV for better strength estimate | | | Penetration resistance (Windsor probe) | Depth of a probe fired into the surface | Surface strength | | Pull-out / pull-off test | Force to pull out an embedded insert | Local strength | | Cover meter / ferroscan | Cover and bar position by magnetic field | Cover check | | Half-cell potential | Probability of rebar corrosion | ASTM C876 | | Radiography, gamma ray | Internal voids and bars | Special safety needs | | Infrared thermography, impact echo, GPR | Delamination, voids | Special tools | | Carbonation depth (phenolphthalein) | Depth of carbonated layer | Semi-destructive | #### Rebound hammer (Schmidt hammer) test (IS 13311 Part 2) **Principle:** a spring-loaded mass strikes the concrete surface through a plunger. The rebound of the mass, read as the **rebound number** $R$ on a scale of 10 to 100, depends on the surface hardness, which is related to strength. **Procedure** 1. Select a smooth, clean, dry surface of a structural member at least 100 mm thick; grind off plaster, loose scale and rough texture. 2. Hold the hammer at right angles to the surface (a horizontal shot is the reference; shots vertically up or down need an orientation correction), push the plunger until the mass is released, and read the rebound number. 3. Take about **9 to 12 readings** at spots 20 to 50 mm apart; reject readings that differ from the mean by more than 6 units, and average the rest. 4. Read the compressive strength from the manufacturer's calibration curve (for the position of the hammer), which should preferably be made from cubes of the same concrete. **Precautions:** avoid edges, honeycombed and wet surfaces, and spots over reinforcement; the instrument should be checked on the test anvil. **Limits:** it tests only the surface (about 30 mm), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. Accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV. #### Ultrasonic pulse velocity (UPV) test (IS 13311 Part 1) **Principle:** the velocity of an ultrasonic pulse through concrete depends on its density and elastic properties. Sound, dense concrete transmits the pulse faster than porous or cracked concrete. **Apparatus:** pulse generator and receiver with two transducers (usually 54 kHz), a timer reading to 0.1 microsecond, a coupling gel, and a reference calibration bar. **Procedure** 1. Calibrate the instrument on the reference bar. 2. Clean the surface and apply grease or gel to ensure good acoustic contact. 3. Put the transducers on the concrete, either **directly** (opposite faces, most reliable), **semi-directly** (adjacent faces) or **indirectly** (same face, least reliable). 4. Measure the path length $L$ and the transit time $T$. 5. Calculate the velocity: $$ V = \frac{L}{T} $$ Take several readings and average them. Avoid paths parallel to reinforcement, as steel carries the pulse faster. **Quality of concrete (IS 13311 Part 1)** | Pulse velocity (km/s) | Quality | |---|---| | Above 4.5 | Excellent | | 3.5 to 4.5 | Good | | 3.0 to 3.5 | Medium | | Below 3.0 | Doubtful (poor) | A combination of two or more methods gives a better estimate, and the results are best correlated with some cores. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Rebound number mainly reflects: A. Surface hardness related to strength B. Internal voids C. Rebar corrosion D. Cover depth Answer: A. The rebound of a spring-loaded mass depends on surface hardness. (Key point: Rebound principle) 2. Procedure for rebound hammer readings: A. 1 reading is enough B. At least 12 readings, rejecting those over 6 units from mean C. 5 readings, no rejection D. 12 readings, reject those over 20 units Answer: B. Take 12 readings 20-50 mm apart, reject deviations > 6, average rest. (Key point: Rebound procedure) 3. UPV velocity is calculated as: A. V = T/L B. V = L x T C. V = L/T D. V = L/(2T) Answer: C. Path length over transit time. (Key point: UPV velocity) 4. Pulse velocity 3.8 km/s indicates: A. Good quality B. Excellent C. Medium (doubtful) D. Poor Answer: A. 3.5 to 4.5 km/s is good; above 4.5 excellent. (Key point: Quality) 5. The half-cell potential test measures: A. Cover thickness B. Surface hardness C. Internal voids by gamma rays D. Probability of rebar corrosion Answer: D. Half-cell (ASTM C876) indicates corrosion probability. 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