The Swiss Hammer Test, also known as the Rebound Hammer Test or Schmidt Hammer Test, is a non-destructive testing (NDT) method used to determine the surface hardness and compressive strength of concrete. This test was invented by Ernst Schmidt in Switzerland in 1948, which is why it is often referred to as the Swiss Hammer Test. It remains one of the most widely used tools for assessing in-situ concrete quality due to its simplicity, portability, and quick results.
What Is the Swiss Hammer Test?
The Swiss Hammer Test measures the rebound number of concrete when a spring-loaded mass strikes its surface. The rebound distance or rebound value indicates the hardness of the surface, which can be correlated to the compressive strength of the concrete. The test does not damage the concrete surface, making it ideal for quality control, inspection, and comparative assessment of different concrete areas.
The test is standardized by major international codes such as:
- IS 13311 (Part 2): 1992 – Method of Non-destructive Testing of Concrete.
- ASTM C805 – Standard Test Method for Rebound Number of Hardened Concrete.
- EN 12504-2 – Testing Concrete in Structures – Part 2: Non-destructive Testing – Determination of Rebound Number.
Objective of the Swiss Hammer Test
The main objectives of performing the Swiss Hammer Test are:
- To determine the surface hardness of concrete.
- To estimate the compressive strength of concrete using rebound values.
- To check the uniformity of concrete in different parts of a structure.
- To assess the quality of in-situ concrete without damaging the structure.
- To identify areas of poor or deteriorated concrete.
Principle of the Swiss Hammer Test
The working principle of the Swiss Hammer Test is based on the rebound mechanism. When the spring-controlled plunger of the hammer impacts a concrete surface, part of the kinetic energy is absorbed by the concrete and part is returned to the hammer as rebound energy. The rebound distance is displayed as a rebound number (R-value) on a calibrated scale.
The rebound number is directly proportional to the surface hardness of the concrete, which in turn is related to its compressive strength. A higher rebound number indicates harder and stronger concrete, while a lower rebound number suggests weaker or deteriorated concrete.
Apparatus Used in the Swiss Hammer Test
The main instrument used is the Swiss Rebound Hammer (Schmidt Hammer), which consists of:
- Plunger: A steel rod that comes in contact with the concrete surface.
- Spring-controlled mass: Strikes the plunger at a constant energy level.
- Rebound scale: Displays the rebound number after impact.
- Locking button: Controls the release of the internal spring.
- Body housing: Encloses all mechanical components.
Types of Swiss Hammers
Depending on the application and concrete strength range, there are different types of hammers:
| Type | Impact Energy | Use |
|---|---|---|
| Type N | 2.207 Nm | Standard hammer for normal concrete (100–300 mm thick). |
| Type L | 0.735 Nm | For small or thin concrete sections (40–100 mm). |
| Type M | 29.43 Nm | For mass concrete like dams or heavy foundations. |
| Type P | 2.207 Nm | Used for testing rock or pavement surfaces. |
Test Procedure for Swiss Hammer Test
The Swiss Hammer Test follows a systematic procedure to ensure accuracy and consistency.
1. Surface Preparation
Select a clean, smooth, and dry concrete surface. Remove dust, plaster, paint, or loose particles using sandpaper or a wire brush. Avoid testing near edges, joints, or reinforcement.
2. Hammer Calibration
Before the test, verify the hammer calibration using the reference anvil provided by the manufacturer. The rebound number should fall within the specified calibration range (usually around 80 ± 2).
3. Positioning the Hammer
Hold the hammer firmly perpendicular to the surface. The direction of impact may be vertical, horizontal, or inclined, but it should remain consistent for each set of readings.
4. Performing the Test
- Press the plunger against the concrete surface until it clicks (impact occurs).
- Note the rebound number shown on the scale.
- Take 10 to 12 readings at least 25 mm apart within a test area.
- Discard outliers (values differing more than ±20% from the mean).
- Calculate the average rebound number for that area.
5. Determination of Compressive Strength
The average rebound number is correlated to the compressive strength using standard calibration charts provided with the hammer or established through laboratory calibration.
6. Correction Factors
Correction factors are applied for parameters like:
- Impact direction (vertical up or down).
- Surface moisture (wet or dry).
- Carbonation of concrete surface.
- Age of concrete.
- Type of aggregate.
Formula and Calculation Example
Although the test relies on correlation charts, a general empirical relationship between compressive strength (fck) and rebound number (R) can be expressed as:
fck = aR – b
where:
- fck = compressive strength (N/mm²)
- R = rebound number
- a and b = constants determined experimentally (depend on concrete type and calibration).
For example, if the average rebound number = 35, and for a specific calibration fck = 0.97R – 1.5, then
fck = (0.97 × 35) – 1.5 = 32.45 N/mm².
Interpretation of Results
Typical indicative values of rebound number and corresponding concrete quality:
| Rebound Number (R) | Concrete Quality | Approx. Compressive Strength (N/mm²) |
|---|---|---|
| < 20 | Poor | < 15 |
| 20–30 | Fair | 15–25 |
| 30–40 | Good | 25–35 |
| 40–50 | Very Good | 35–45 |
| > 50 | Excellent | > 45 |
Advantages of Swiss Hammer Test
- Non-destructive: Does not damage the concrete structure.
- Quick and portable: Ideal for on-site testing.
- Cost-effective: No need for specialized laboratory setup.
- Easy to use: Requires minimal training.
- Useful for comparison: Helps check concrete uniformity across a structure.
Limitations of Swiss Hammer Test
- Measures surface hardness, not internal strength.
- Results affected by surface roughness, moisture, carbonation, and aggregate type.
- Not suitable for lightweight or very coarse concrete.
- Requires correlation with laboratory data for accurate strength estimation.
- Inconsistent results if performed carelessly or on uneven surfaces.
Applications of Swiss Hammer Test
- Quality control during construction.
- Evaluation of in-situ concrete strength in existing structures.
- Assessment of uniformity and homogeneity of concrete.
- Inspection of old or deteriorated structures.
- Verification of curing effectiveness.
- Cross-checking concrete strength before load testing.
Difference Between Swiss Hammer Test and Other NDT Methods
| Parameter | Swiss Hammer Test | Ultrasonic Pulse Velocity (UPV) Test |
|---|---|---|
| Principle | Rebound hardness | Sound wave velocity |
| Measurement | Surface hardness | Internal quality and density |
| Nature of Test | Surface-based | Volume-based |
| Accuracy | Moderate | High when correlated |
| Damage to Structure | None | None |
For more accurate concrete assessment, engineers often combine both methods (Swiss Hammer + UPV Test) to get a composite strength value.
Precautions
- Always perform calibration before use.
- Avoid testing on rough, cracked, or wet surfaces.
- Take readings only after removing surface laitance.
- Maintain consistent hammer orientation during readings.
- Do not reuse a single spot for multiple impacts.
Conclusion
The Swiss Hammer Test is a simple yet effective non-destructive method to assess the surface hardness and estimated compressive strength of concrete. Although it provides only an indirect measure, it is invaluable for field inspections, maintenance assessments, and quality control. When properly executed and correlated with other testing methods, it gives reliable insights into the structural condition and performance of concrete — making it an essential tool in modern construction quality assurance.
