The Schmidt Hammer Test, also known as the Rebound Hammer Test, is a widely used non-destructive testing (NDT) method to assess the compressive strength of concrete. Developed by Ernst Schmidt in 1948, this test helps evaluate the quality, uniformity, and hardness of concrete without causing damage to the structure. It’s a simple, quick, and reliable field test for determining whether a concrete surface meets required strength standards. Understanding the working principle, procedure, applications, and limitations of the Schmidt Hammer Test is essential for civil engineers and construction professionals.
What Is the Schmidt Hammer Test?
The Schmidt Hammer Test is a non-destructive test that measures the surface hardness of concrete based on the rebound principle. When the plunger of the hammer impacts a concrete surface, it releases a spring-driven mass that strikes with a specific force. The amount of rebound of this mass depends on the hardness and strength of the concrete. The rebound value, read on a calibrated scale, is then correlated with the compressive strength of the concrete using standard charts or graphs. This allows engineers to estimate in-situ concrete strength without damaging the structure.
The test is standardized by several international codes, including IS 13311 (Part 2): 1992, ASTM C805, and EN 12504-2.
Principle of the Schmidt Hammer Test
The Schmidt Hammer works on the rebound principle — the greater the surface hardness of concrete, the greater the rebound of the hammer mass. When the spring-loaded plunger impacts the concrete surface, kinetic energy is transferred to the concrete. A portion of this energy is absorbed by the surface, while the rest is returned to the hammer body as rebound. The rebound distance is measured and displayed as a Rebound Number (R-value).
The correlation between rebound number and compressive strength is determined through laboratory calibration. Typically, a higher rebound number indicates stronger and denser concrete.
Equipment Used in Schmidt Hammer Test
The key instrument in this test is the Schmidt Rebound Hammer, consisting of the following main parts:
- Plunger: A steel rod that comes in contact with the concrete surface.
- Spring-controlled mass: Strikes the plunger with a set force.
- Locking system: Holds the mass before release.
- Scale: Displays the rebound number.
- Housing: Contains all components in a compact cylindrical body.
Schmidt hammers are available in different impact energy ratings depending on the application:
| Type | Impact Energy | Suitable Application |
|——-|—————-|———————-|
| Type N | 2.207 Nm | General concrete structures (100 mm–300 mm thick) |
| Type L | 0.735 Nm | Light and thin concrete elements or small specimens |
| Type M | 29.43 Nm | Mass concrete such as dams or foundations |
| Type P | 2.207 Nm | For rock and pavement surfaces |
Procedure of Schmidt Hammer Test
Performing the Schmidt Hammer Test requires careful surface preparation and consistent application. Below is the step-by-step procedure:
1. Surface Preparation
Select a clean, smooth, and dry concrete surface. Remove loose particles, dust, and coatings such as plaster or paint. The surface should be flat and firm for accurate results.
2. Hammer Calibration
Before testing, ensure the Schmidt Hammer is properly calibrated using the manufacturer’s reference anvil. The standard rebound value should match the calibration range; otherwise, the device needs adjustment.
3. Testing Process
- Hold the hammer perpendicular to the concrete surface (horizontal, vertical, or inclined).
- Press the plunger firmly against the surface until the hammer automatically releases the spring-driven mass.
- Note the rebound number indicated on the scale.
4. Repetition and Averaging
Take at least 10 to 12 readings in a single test area (not less than 300 mm apart) to ensure accuracy. Discard outlier readings beyond ±20% of the average.
5. Conversion to Compressive Strength
Use the manufacturer’s chart or graph to convert the average rebound number into compressive strength (N/mm²). The calibration curve relates rebound numbers to cube compressive strength obtained from laboratory tests.
6. Correction Factors
Apply correction factors for parameters like surface smoothness, carbonation, direction of impact, and concrete age. For example, a vertical upward test yields a lower rebound value than a horizontal test due to gravity effects.
Factors Affecting the Schmidt Hammer Test Results
Several factors can influence the rebound number and cause variations in strength estimation:
- Surface condition: Rough, wet, or dusty surfaces reduce rebound values.
- Moisture content: Wet concrete gives lower rebound numbers than dry concrete.
- Type of aggregate: Hard aggregates like granite yield higher rebound values compared to softer aggregates.
- Age of concrete: Older concrete tends to have higher rebound values due to surface hardening.
- Direction of testing: Upward impacts result in lower rebound readings than horizontal ones.
- Carbonation: Carbonation increases surface hardness and gives misleadingly higher results.
Advantages of the Schmidt Hammer Test
The Schmidt Hammer Test offers multiple benefits in construction quality control:
- It is non-destructive, preserving the integrity of the structure.
- Provides quick and cost-effective results.
- Simple to perform and requires minimal training.
- Can be used directly on-site without laboratory facilities.
- Useful for comparing concrete uniformity and identifying weak zones.
- Portable and suitable for vertical, horizontal, and overhead surfaces.
Limitations of the Schmidt Hammer Test
Despite its advantages, the test has certain limitations:
- It measures surface hardness, not internal strength.
- Not suitable for very rough, porous, or coarse aggregate concrete.
- Requires calibration for each type of concrete mix.
- Results may vary due to operator technique and surface conditions.
- Accuracy decreases with highly carbonated concrete surfaces.
For precise strength estimation, the Schmidt Hammer Test should be combined with other NDT methods such as the Ultrasonic Pulse Velocity (UPV) Test.
Applications of Schmidt Hammer Test
The test is widely used in civil engineering for:
- Quality control during construction.
- Assessing uniformity and consistency of in-situ concrete.
- Estimating residual strength of old or deteriorated structures.
- Evaluating concrete before structural retrofitting or repair.
- Checking the effectiveness of curing and workmanship.
Interpretation of Results
The rebound number obtained is compared to standard correlations to estimate the compressive strength. Typical indicative values are:
| Rebound Number | 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 |
| These values provide a general understanding but must be verified with calibration data specific to the hammer and concrete type. |
Conclusion
The Schmidt Hammer Test is a vital non-destructive testing technique for evaluating the surface hardness and approximate compressive strength of concrete structures. Although not a substitute for standard compressive strength testing, it provides valuable insights into concrete quality, uniformity, and potential weak zones. When used with other NDT methods and proper calibration, it ensures accurate, reliable, and quick assessment of in-situ concrete strength — making it an indispensable tool in modern construction quality control.
