Rebound Hammer new

Cedric Belau, winter semester 2024/25

The Rebound Hammer, also known as Schmidt Hammer or Swiss Hammer is a handheld, non-destructive testing device used to determine the surface hardness and penetration resistance of materials, primarily concrete or rock.

Historical Overview

The Rebound Hammer was invented in 1948, and shorty after patented, by the Swiss engineer Ernst Schmidt. The Schmidt Hammer replaced the previously employed ball penetration method. With the use of a rebound index displayed on the device, faster and easier measuring was possible. With a revision in 1952, which reduced the mechanism to a single impact spring instead of two, the handling of the device was further simplified. Following the founding of the company Proceq SA in 1954, the rebound hammer has been distributed and utilized globally. Since then, there were no significant changes until 2007, when Proceq released a new, digital version that, in addition to Schmidt's rebound index, also provides the coefficient of restitution COR. This version is referred to as Silver Schmidt Hammer [1], [2]. Over time, additional variations of the rebound hammer have been developed for materials such as stone, wood, leather, and paper [3]. To standardize the use and results of the Schmidt hammer, DIN and ASTM standards were established in 1972 and 1975, followed by a DIN EN standard in 2001 (See Standards) [4], [5], [6].

Working Principle 

The Rebound Hammer is a handheld device without the need of an external electricity source. In case of the original design, no electric components are needed. The Silver Schmidt on the other hand has a digital display, battery and a circuit board implemented. The essential components are the housing, plunger, impact spring, release mechanism, guide bar, hammer mass and scale (see figure 1).

The Schmidt Hammer tests the hardness of a surface by determining the energy absorbed by the testing material, which is correlated by the stress-strain relationship of concrete [7]. Generally, harder concrete (higher stress endurance) has a higher resistance to deformation (lower resultant strain) under a given stress and thus absorbs less energy. A defined impact energy (hammer mass) is released by the spring when the plunger is pressed against the testing material. The hammer mass then contacts the plunger and rebounds (see figure 1) [7].

Originally, the distance after impact was measured to determine a hardness value. With the introduction of the Silver Schmidt and digitalization, the method transitioned to measuring the velocities before and after impact [1].

Distance Measurement

Measuring the distance travelled by the hammer mass, allows for a purely mechanical scale. Original Schmidt Hammers [8] measure a rebound index ($//<![CDATA[ \begin{array}{l}\mathrm{R}\end{array} //]]>$). The R-value is not an SI unit and has to be converted to a surface hardness with so called correlation curves (see chapter Calibration and Correlation Curves) after the test. $//<![CDATA[ \begin{array}{l}\mathrm{R}\end{array} //]]>$ is the ratio of hammer mass displacement $//<![CDATA[ \begin{array}{l}\mathrm{x_0}\end{array} //]]>$ at the moment of triggering and $//<![CDATA[ \begin{array}{l}\mathrm{x_1}\end{array} //]]>$ as the maximum rebound distance after impact. Considering that the potential energy stored in the impact spring  is constant for every measurement and proportional to the square of $//<![CDATA[ \begin{array}{l}\mathrm{x}\end{array} //]]>$, $//<![CDATA[ \begin{array}{l}\mathrm{R}\end{array} //]]>$ can be calculated:

$//<![CDATA[ \begin{array}{l}R = 100\cdot\sqrt{\cfrac{E_1}{E_2}}=100\cdot\sqrt{\cfrac{\cfrac{1}{2}Dx_1}{\cfrac{1}{2}Dx_2}}=100\cdot\cfrac{x_1}{x_2}}\end{array} //]]>$

Where $//<![CDATA[ \begin{array}{l}\mathrm{D}\end{array} //]]>$ is the spring constant and $//<![CDATA[ \begin{array}{l}\mathrm{E_0}\end{array} //]]>$ and $//<![CDATA[ \begin{array}{l}\mathrm{E_1}\end{array} //]]>$ the potential energy before and after the impact, respectively [9].

Because of its mechanical nature, the measurements of travel distance result in different uncertainties, such as: energy loss due to friction on the guide rod and index rider, due to dissipation of mechanical waves inside the steel plunger and due to concrete crushing under the testing device. Another source of error is the missing or incorrect application of correction values for the hammer's orientation with respect to gravity [1].

Verlocity Measurement

With the release of the Silver Schmidt, the coefficient of restitution (COR / Q-value) was introduced to concrete hardness testing. The Q-value is calculated the same as the COR established by Dietmar Leeb and Marco Brandestini in the 1970s primarily for metal hardness testing (see Leeb hardness test):

$//<![CDATA[ \begin{array}{l}Q = 100\cdot\sqrt{\cfrac{E_1}{E_2}}=100\cdot\sqrt{\cfrac{\cfrac{1}{2}m v_1^2}{\cfrac{1}{2}m v_2^2}}=100\cdot\cfrac{v_1}{v_2}}\end{array} //]]>$

Instead of measuring the hammer mass distance, the Silver Schmidt measures the respective velocity $//<![CDATA[ \begin{array}{l}\mathrm{v_0}\end{array} //]]>$ directly before and $//<![CDATA[ \begin{array}{l}\mathrm{v_1}\end{array} //]]>$ directly after impact, while $//<![CDATA[ \begin{array}{l}\mathrm{m}\end{array} //]]>$ is the mass of the hammer mass.

The Q-value is therefore independent from friction and orientation of the hammer. As a result, is the Q-value the desired measurement instead of the R-value, considering the accuracy of the absorbed impact energy.

Handling and Operation 

The Rebound Hammer is operated by one person on one or more testing samples. To gather qualitative data, several measurements have to be taken with the same device. See Limitations and Standards for further information about testing conditions.

Preparation of Measuring Point

According to DIN EN 12504-2:2021-10 [10] a steel reference anvil has to be used, to verify the functional capability of the Rebound Hammer. To do so, ten impacts have to be performed on the cleaned anvil, where the last five are recorded. If one of these values is above/below +- 3, a cleaning or adjusting of the hammer is needed.

The testing sample should have a thickness of at least 100mm and should be fixed in place. Furthermore, the standard recommends:

• Avoidance of areas exhibiting honeycombing, scaling, rough texture, or high porosity.
• Grinding of heavily textured surfaces until they are smooth and free of loose material.
• Using a dry surface of 300 mm by 300 mm.
• Testing temperatures within the range of 0°C to 50°C.

Conduction of Measurement

Figure 1 depicts the process of taking one measurement with the Rebound Hammer. To begin, the plunger – test ready, unlocked and turned on (in case of digital hammer)- has to be slowly pressed against the testing sample on the measuring point (1-2). Thereby, the Rebound Hammer should be perpendicular to the sample surface. If not, the angle needs to be measured and documented. At a certain vertical pressure or distance along the guide bar, the release mechanism is triggered (2-3). The hammer mass accelerates due to the potential energy of the impact spring towards the plunger. After the impact (3), the mass rebounds along the guide rod and the hammer measures the rebound distance (4). In case of the Silver Schmidt, a digital recording of the velocities is made [8], [7].

After a measurement, the examiner must manually read and record the displayed R- or Q-value and visually check the measuring point. If the impact has crushed or penetrated a near-surface pore, the result should be disregarded. A minimum of nine measuring points - at least 25 mm distant to each other or to an edge – should be conducted [10].

Afterwards, five measurements on the reference anvil should be conducted. If one of these values is above/below +- 3, a cleaning or adjusting of the hammer is needed. This is referred to as reference checking in DIN EN 12504-2:2021-10 chapter 7.3.

Interpretation of Measurements

The standard orientation of measurement is perpendicular to vertical surfaces (e.g. a concrete wall). Any change from the surface direction results in altered R-values, since the gravity influences the friction on the guide rod and travel distance. Correction values are typically given for horizontal surfaces up or down and at an angle of 45° [9].

The final measurement is the median of all measurements. Notice the difference between median an arithmetic mean. If all measurements have been taken in the same orientation, the manufactures correction value is applied on the median. If not, all measurements have to be corrected before determining the median. To determine the compressive strength, the user has to input the rebound number in a fitting correlation curve. See chapter Calibration and Correlation Curves.

The European standard also specifies: “The rebound number shall be expressed as a whole number.” and “If more than 20 % of all the readings differ from the median by more than 25 % the entire set of readings shall be discarded.” [10].

Additionally, the standard proposes the content of a test report for R-value testing [10].

Calibration and Correlation Curves

To make a qualitative statement about the compressive strength of concrete, the reliability and calibration of the Rebound Hammer are important issues [11]. The calibration consists, on the one hand, of comparing the R-values with a reference anvil and, on the other hand, of adjusting the correlation curve to the material being tested. Detailed information on what, when, and how to calibrate the Rebound Hammer using the calibration anvil can be found in DIN EN 12504-2:2021-10 [10] as well as the manufacturers operation handbook.

The first empirical correlations between the rebound number and concrete strength emerged in the 1950s [9]. Unfortunately, as stated in Limitations, several factors influence the energy absorbed by the Rebound Hammer. As a result, there have been countless different correlation curves and various correction factors published over the years [1]. A typical correlation curve for the Original Schmidt from Proceq is seen in figure 2. It is usually printed on the device, so that the rebound value is already aligned with the scale of the hammer. Figure 2 depicts a correlation curve for a cylindrical sample with three functions for different hammer orientations.

The biggest single influence on the rebound number was found to be the water-to-cement ratio [12], followed by the carbonation [1], which is not possible to differentiate with the given graphs in the operation manual. This is just one example, why the correct determination with universal curves is prone to high uncertainty. It is recommended to use individual curves if the respective concrete quality is known.

While there are no standards for correlation curves, EN 13791 proposes calibration based on a basic curve, which is adjusted through compressive strength core testing [13]. To gain a deeper understanding of the influences by the concrete structure, theoretical approaches for the rebound number and how to interpret correlation curves, Szilágyi’s and Borosnyoi ‘s article [1] is highly advised.

With state-of-the-art digital rebound hammers and the measurement of the Q-value, the calibration curve issue remains unchanged. Although the uncertainty of the Q-value itself is smaller, empirical correlation curves are still required [9].

Limitations

Several factors influence the rebound number. While some may be the objective of the measurement (age of test sample, type of cement), others are unwanted uncertainties. Influencing factors to be considered for a measurement are:

• Regarding the rebound hammer:
  • The device has never been calibrated.
  • Debris on the guide rod or other components leads to higher friction and thus a lower rebound number. The device has to be cleaned and re-calibrated.
  • The hammer orientation is either not taken into account or is taken into account incorrectly.
• Regarding the testing conditions:
  • Local properties of the testing material differ: air voids or generally inhomogeneous cement on the measuring point and surface quality.
  • Shape and size of testing sample, e.g. cylinder, cube other geometry.
  • Carbonation and moisture of concrete have not been controlled and recorded.
  • Trivial but essential: type and strength of testing material influence the absorbed energy and thus the rebound number.
  • Temperature during the measurement is below 0°C or higher than 50°C

Testing conditions are described in DIN EN 12504-2 chapter six and seven [9], [7], [10].

Fields of Application

With the Rebound Hammer and appropriate correlation curves, the surface hardness of a material can be determined in a nondestructive manner. While this testing method has multiple limitations and uncertainties, is has become the method of choice for civil engineers to assess the compressive strength of concrete, primarily because the fast and easy handling of the Schmidt Hammer.

Furthermore, a statement about the homogeneity and quality of concrete with respect to standard properties can be made. Similarly, different samples can be set into relation to another. [7]

Standards

For the calibration, use and interpretation of results of the Rebound Hammer are three main standards should be consulted:

• ASTM C805/C805M-18, Standard Test Method for Rebound Number of Hardened Concrete, 2018.
• DIN EN 12504-2, Testing concrete in structures – Part 2: Non-destructive testing – Determination of rebound number, 2021-10.
• EN 13791, Assessment of in-situ compressive strength in structures and precast concrete, 2020-02.

The manufacturers operation manual should be consulted for the proper cleaning and execution of a Rebound Hammer.

Literature

[1] Szilágyi, Katalin & Borosnyoi-Crawley, Dorian. (2009). 50 years of experience with the Schmidt rebound hammer. Concrete Structures. 10. 46-56.

[2] Proceq SA (Hrsg.): silver schmidt. The Original SCHMIDT fully integrated electronic Concrete Test Hammer. Switzerland. Technical Specifications. URL: proceq-silverschmidt-datasheet.pdf

[3] Proceq SA: Website. URL: https://www.screeningeagle.com/en/product/concrete/rebound-hammer

[4] ASTM C805/C805M-18, Standard Test Method for Rebound Number of Hardened Concrete, 2018.

[5] DIN EN 12504-2: Testing concrete in structures – Part 2: Non-destructive testing – Determination of rebound number, 2001-12.

[6] DIN 1048-2: Testing concrete – Part 2: testing of hardened concrete (specimens taken in situ), 1972-01.

[7] Nayan Parmar, Ajay Vatukiya, Mayanksinh Zala, Gaurav Gohil: Non-destructive testing by rebound hammer method in IJRTI. (2017), p. 120-123.

[8] Proceq SA (Hrsg.): Concrete Test Hammer. Original Schmidt. Operating Instructions. URL: OrginalSchmidt_Operating Instructions_Multilingual_high.pdf

[9] Breysse, Denys, ed.: Non-Destructive Assessment of Concrete Structures. Reliability and Limits of Single and Combined Techniques. State-Of-the-Art Report of the RILEM Technical Committee 207-INR. Dordrecht: Springer Netherlands, 2012.

[10] DIN EN 12504-2: Testing concrete in structures – Part 2: Non-destructive testing – Determination of rebound number, 2021-10.

[11] Carino N.J.: Nondestructive testing of concrete: history and challenges, in ACI SP-144, Concrete Technology – Past, Present and Future, P.K. Mehta Ed., Detroit, (1994) pp. 632–678.

[12] Kolek, J.: Discussion of the paper 3A: Non-destructive testing of concrete by hardness methods, Proceedings of the Symposium on Non-destructive testing of concrete and timber, 11-12 June 1969, Institution of Civil Engineers, London, (1970), pp. 27-29

[13] EN 13791: Assessment of in-situ compressive strength in structures and precast concrete, 2020-02.