--- title: "NDT importance; Schmidt hammer test" subject: "Concrete Technology and Masonry Structure" chapter: 5 key: "semester_5/concrete_technology_and_masonry_structure:ndt-importance-schmidt-hammer" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/ndt-importance-schmidt-hammer site: BCT Engineering --- # NDT importance; Schmidt hammer test Concrete Technology and Masonry Structure · Chapter 5: Testing of concrete and quality control - Asked in: 2074 Chaitra, 3+4 marks - Video: [Rebound Hammer Test on Concrete – Principle, Procedure](https://www.youtube.com/watch?v=pnaS7zKBG5I) — Civil Technology - 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-importance-schmidt-hammer - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-5 ## Question Describe the importance of non-destructive testing of concrete. Explain Schmidt hammer test. ## Short answer NDT assesses in-situ concrete without damage, allowing repeated surveys to check uniformity, detect defects, and evaluate old structures. The Schmidt hammer test measures surface hardness via rebound number; a spring-loaded mass strikes the surface, and the rebound is correlated to strength using a calibration curve. Take 12 readings, reject outliers, and average. Accuracy is ±15–25%, depth ~30 mm. ## Answer #### Importance of NDT - Tests the **actual** in-situ concrete, not only the control cubes, which may not represent the structure's compaction and curing. - It causes no damage, and repeated tests at the same place are possible, so changes with time can be tracked. - Quick and cheap; large areas can be surveyed to find weak, honeycombed or cracked zones. - Assesses old structures for repair, change of use, fire or earthquake damage, and long-term deterioration (corrosion, carbonation). - Investigates doubts when cube results fail the acceptance criteria (IS 456, Cl. 16), before costly coring or load tests (Cl. 17). - Checks uniformity, thickness, cover and the position of rebars, and gives a basis for deciding on repair. #### 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 (if it is held horizontal, or vertically up or down, an orientation correction is needed), push the plunger until the mass is released, and read the rebound number. 3. Take at least **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; check the instrument on the test anvil. **Limits:** it tests only the surface (about 30 mm deep), so it is affected by carbonation, moisture, surface finish, type of aggregate and cement, and age. The accuracy of strength is about ±15 to 25%. It is useful for comparing uniformity and for choosing places for coring or UPV. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is NDT useful compared with control cubes? A. It is more accurate than cubes B. It tests actual in-situ concrete without damage and can be repeated C. It breaks the member D. It tests only fresh concrete Answer: B. NDT assesses the real structure non-destructively. (Key point: Importance of NDT) 2. What does the rebound number of a Schmidt hammer mainly depend on? A. Density of the steel reinforcement B. Water-cement ratio measured inside the core C. Surface hardness of the concrete, which is related to strength D. Thickness of the member only Answer: C. The rebound of the spring-loaded mass depends on the surface hardness, which is correlated with strength. (Key point: Rebound hammer principle) 3. 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 3 readings at the same point, take the highest C. At least 12 readings, take the maximum value D. At least 12 readings; reject those differing from the mean by more than 6 units and average the rest Answer: D. 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) 4. Why is the rebound hammer strength estimate only approximate (about ±15 to 25%)? A. It tests only the surface (about 30 mm) and is affected by carbonation, moisture, finish and aggregate B. It tests the full depth of the member but at low load C. It needs no calibration curve D. It always reads the strength of the steel bars Answer: A. Only the surface layer is tested, so surface condition and material type change the result. 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