In the quality assurance (QA) and quality control (QC) of concrete works, the Rebound Hammer Test plays an important role as a non-destructive testing (NDT) method. It provides a quick, simple, and economical means of obtaining an indirect indication of concrete compressive strength. This article focuses on the test procedure, interpretation of results, and limitations of the rebound hammer test.
What is a Rebound Hammer?
A Rebound Hammer is a hand-held non-destructive testing (NDT) instrument, commonly known as a Schmidt Hammer or Swiss Hammer, used to evaluate the surface hardness of concrete and the quality of hardened concrete. It was invented by Ernst Heinrich Wilhelm Schmidt, a Swiss engineer, to provide a simple and quick method for assessing concrete quality without damaging the structure.
Types of Rebound Hammers (N-Type and L-Type)
Rebound hammers are classified based on their impact energy and intended application. The most commonly used types in concrete testing are N-type and L-type rebound hammers.
1. N-Type Rebound Hammer
The N-type rebound hammer is the standard and most widely used instrument for testing normal structural concrete. It has an impact energy of 2.207 Nm, making it suitable for ordinary concrete structures such as beams, slabs, columns, and walls.
2. L-Type Rebound Hammer
The L-type rebound hammer has a lower impact energy and is designed for testing weaker or more delicate concrete surfaces. It has an impact energy of 0.735 Nm, making it suitable for low-strength concrete (generally below about 10 MPa) and thin concrete sections.
What is a Rebound Hammer Test?
The rebound hammer test is a non-destructive method used on hardened concrete surfaces to evaluate surface hardness and indirectly assess concrete quality without damaging the concrete member.
Based on this surface hardness, it provides an indirect indication of the concrete’s compressive strength and uniformity, without damaging the structure.
The test is widely used on construction sites for quick quality checks, condition assessment of existing concrete, and comparative testing of different concrete areas.
Also, read: Tests Conducted on Concrete: A Complete Guide for Civil Engineers
Objective of the Rebound Hammer Test
The rebound hammer test could be used for :
- To estimate the probable compressive strength of concrete using established correlations between the rebound number and compressive strength.
- To assess the uniformity and consistency of concrete within a structural element or across different locations.
- To evaluate the surface quality of concrete in relation to specified standards and acceptance requirements.
- To compare the quality of concrete between different structural elements or between different zones of the same element.
Principle of the Rebound Hammer Test
The principle of the Rebound Hammer Test is based on the measurement of the elastic rebound of a spring-controlled mass after it impacts the surface of hardened concrete.
When the rebound hammer is pressed against the concrete surface, a spring-loaded plunger strikes the surface with a fixed amount of energy. The mass then rebounds, and the rebound distance is measured and indicated as a rebound number. This rebound number is directly related to the surface hardness of the concrete.
Since the surface hardness of concrete is influenced by its compressive strength, density, and quality, the rebound number provides an indirect indication of concrete strength and uniformity. Higher rebound numbers generally indicate harder and stronger concrete, while lower values suggest weaker or deteriorated concrete.
The rebound numbers obtained are compared with standard calibration or correlation charts (as specified in relevant codes) to assess the quality of concrete or to estimate its probable compressive strength.
Factor Affects Rebound Hammer Readings
The rebound hammer reading is influenced by several factors related to the concrete properties, surface condition, and testing procedure. For reliable interpretation, these factors must be properly understood and controlled during testing.
1. Surface Condition
Rough, uneven, or laitance-covered surfaces can give lower rebound values. Smooth, clean, and dry surfaces produce more consistent readings.
Also, read: Concrete Laitance: Definition, Cause & Remedy
2. Moisture Condition of Concrete
Wet or saturated concrete surfaces generally result in lower rebound numbers compared to dry surfaces due to reduced surface hardness.
3. Age of Concrete
As concrete gains strength with time, rebound numbers increase. Readings taken at early ages may not represent the long-term strength of concrete. As per the IS code, the readings taken 3day and 3month are not considered.
4. Type of Cement
Different cement types (OPC, PPC, PSC, etc.) affect the rate of strength gain and surface hardness, influencing rebound values.
Also, read: Types of Cement: 10 Types and Their Usage in Construction
5. Direction of Testing
Readings vary depending on whether the hammer is used horizontally, vertically upward, or vertically downward. Directional correction factors must be applied.
6. Number and Distribution of Readings
Insufficient readings or poor spacing may lead to misleading averages. Multiple impacts at well-distributed locations are essential.
★ Important Note: Because there are many variables influence rebound hammer readings, results should be used mainly for comparative assessment and uniformity checks, and not as the sole basis for determining compressive strength.
Procedure For Rebound Hammer Test
1. Surface Preparation
- Clean the concrete surface from dust, dirt, laitance, oil, or loose particles.
- Smooth the surface if it’s too rough (grinding may be necessary).
- The surface should be dry or at normal moisture conditions, as wet surfaces can reduce rebound values.
★ Proper surface preparation is crucial for reliable readings.
2. Equipment Calibration
- Check the Rebound Hammer calibration using the test anvil provided by the manufacturer.
- Adjust or note any corrections if the hammer is out of calibration.
- Ensure the plunger moves freely and the scale reads zero when at rest.
★ Calibration ensures consistency and reliability of results.
3. Test Positioning
- Hold the hammer perpendicular (90°) to the concrete surface.
- Rest the hammer firmly on the surface without tilting.
- If testing a vertical surface, support the hammer properly to avoid slippage.
★ Improper positioning can result in inaccurate readings.
4. Conducting the Test
- Press the hammer plunger against the concrete surface until it releases and impacts the surface.
- Record the rebound number shown on the scale.
- Move the hammer to a different nearby spot (at least 25 mm apart) for the next reading.
★ Avoid repeated impacts on the exact same spot as it may cause local crushing.
5. Taking the Readings
- For reliable results, take 10–15 readings per surface or area.
- If testing a large structural element, divide the surface into grids and take readings at each grid point.
6. Calculating Average and Strength
- Discard outliers if any reading is obviously erroneous.
- Calculate the average rebound number.
- Use the manufacturer’s correlation chart or site-specific calibration chart to estimate the compressive strength of concrete.
Concrete Quality vs Rebound Numbers
| Average Rebound Number | Concrete Surface Quality |
|---|---|
| >40 | Very good hard layer |
| 30-40 | Good layer |
| 20-30 | Fair |
| <20 | Poor |
| 0 | Delaminated |
Limitations of a Rebound Hammer Test
Very important from a professional liability perspective
- Calibration Requirement: The rebound hammer must be calibrated regularly against concrete of known strength. Different hammers can produce slightly different values. Thus, one should not compare results from different hammers without calibration.
When NOT to Rely on It
The rebound hammer test is not recommended or reliable for conducting the test on:
- Surface is rough, contaminated, or heavily carbonated.
- The concrete member is very thick or heavily reinforced.
- You need precise compressive strength values for structural design or safety assessment.
- Testing high-strength or lightweight concrete without proper calibration.
- Critical decisions like evaluating structural safety or failure investigation.
Also, read: Types of Cement: 10 Types and Their Usage in Construction
Frequently Asked Questions
Below are some FAQs on the Rebound Hammer Test conducted on concrete, helping site engineers, students, and QA/QC engineers/ professionals quickly understand the procedure, interpretation & limitations.
Q: Is the Rebound Hammer Test destructive?
Answer: No. It is a non-destructive test (NDT), meaning it does not damage the concrete member during testing.
Q: Can Rebound Hammer readings give exact, concrete strength?
Answer: No. Rebound Hammer results provide an indirect estimate of compressive strength. Other factors, such as surface condition, moisture, curing, and aggregate type, can affect readings. For precise strength, core testing or other NDT methods may be required.
Q: What unit is used in a Rebound Hammer Test?
Answer: The Rebound Hammer test measures the rebound number, which is a dimensionless value. It does not have a physical unit like MPa or N/mm². The rebound number is used to estimate concrete surface hardness and indirectly assess compressive strength using standard correlation charts.
Q: At what age of concrete can the test be performed?
Answer: The test is generally reliable after the concrete has hardened, typically beyond 3 days. According to IS 13311 (Part 2), readings between 3 days and 3 months can generally be used, but early-age results should be interpreted cautiously.
Q: How many rebound hammer readings should be taken on a concrete specimen?
Answer: As per IS 13311 (Part 2) and standard site practice, a minimum of 10 rebound hammer readings should be taken on one test area of a concrete specimen or structural element.
Q: Are all rebound hammer readings considered for averaging?
Answer: No. Readings that differ significantly from the average (commonly more than ±20%) should be discarded. The average of the remaining readings is taken as the representative rebound number.
Q: Is a single rebound hammer impact sufficient for testing concrete?
Answer: No. A single impact is not reliable. Multiple readings are required to account for surface variability and to obtain meaningful results.
References:
- Bureau of Indian Standards. (1992). IS 13311 (Part 2): Non-Destructive Testing of Concrete—Methods of Test. Part- 2. Rebound Hammer. BIS.
- British Standards Institution. (2001). Testing concrete—Part 2: Non-destructive testing—Determination of rebound number (BS EN 12504-2:2001). BSI. https://knowledge.bsigroup.com/products/testing-concrete-in-structures-non-destructive-testing-determination-of-rebound-number-2
- Wikipedia contributors. (2025, October 2). Schmidt hammer. Wikipedia. https://en.wikipedia.org/wiki/Schmidt_hammer
- Table 3 .1: Rebound Number v/s Quality of concrete surface. (n.d.). ResearchGate. https://www.researchgate.net/figure/Rebound-Number-v-s-Quality-of-concrete-surface_tbl1_285674540
