--- title: "UPV and rebound hammer tests" subject: "Concrete Technology and Masonry Structure" chapter: 5 key: "semester_5/concrete_technology_and_masonry_structure:upv-and-rebound-hammer" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/upv-and-rebound-hammer site: BCT Engineering --- # UPV and rebound hammer tests Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control - Asked in: 2081 Baisakh, 6 marks; 2076 Ashwin, 6 marks (asked 2 times) - Video: [Rebound hammer complete test | start to end | How to take reading | practical on site | graph](https://www.youtube.com/watch?v=8gmI0CHAJck) — Civil Site visit - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/upv-and-rebound-hammer - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5 ## Question Explain the Rebound Hammer test and the Ultrasonic Pulse Velocity test (procedure for both tests in concrete). ## Short answer The rebound hammer test strikes the surface with a spring-loaded mass; rebound number R correlates with hardness and strength. Take 9-12 readings, discard those beyond +/-6 from mean, and average; read strength from calibration curve. The UPV test measures pulse velocity V = L/T; direct transmission is best. Quality per IS 13311: >4.5 km/s excellent, 3.5-4.5 good, 3.0-3.5 medium, <3.0 doubtful. Strength estimated with +/-20% accuracy. ## Answer #### 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) | **Strength estimation:** read from a calibration curve of $V$ against cube strength for the same mix; accuracy about ±20%. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the rebound number of a Schmidt hammer mainly depend on? A. Surface hardness of the concrete, which is related to strength B. Density of the steel reinforcement C. Water-cement ratio measured inside the core D. Thickness of the member only Answer: A. The rebound of the spring-loaded mass depends on the surface hardness, which is correlated with strength. (Key point: Rebound hammer principle) 2. How many readings are taken and how are they treated in the rebound hammer test? A. One reading at the centre of the member B. At least 12 readings; reject those differing from the mean by more than 6 units and average the rest C. At least 3 readings at the same point, take the highest D. At least 12 readings, take the maximum value Answer: B. At least 12 readings 20 to 50 mm apart are taken, outliers beyond 6 units from the mean are rejected and the rest averaged. (Key point: Rebound hammer readings) 3. Which transducer arrangement is the most reliable in the UPV test? A. Indirect, on the same face B. Semi-direct, on adjacent faces C. Direct, on opposite faces D. All three are equally reliable Answer: C. Direct transmission gives the strongest and most reliable pulse; indirect is least reliable. (Key point: UPV transducer arrangement) 4. According to IS 13311 Part 1, a pulse velocity of 4.0 km/s indicates concrete of what quality? A. Excellent B. Medium (doubtful) C. Poor D. Good Answer: D. 3.5 to 4.5 km/s is Good; above 4.5 is Excellent. 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