Rebound hammer

Franziska Schick, winter semester 2018/19

The Rebound Hammer, also called Schmidt Hammer or Swiss Hammer, is a non-destructive testing device used to determine the surface strength of concrete point by point. 

Historical

The Rebound Hammer was invented in 1948 by the Swiss civil engineer and bridge builder Ernst O. Schmidt. In contrast to the ball penetration tester device used at that time, the handling has been improved. Instead of ball penetration, the rebound index is now measured to determine the surface strength. This characteristic value can be read on a scale on the device. The Rebound Hammer was further developed in 1952 by using only one spring instead of two. This – again – led to an easier handling. In 1954, the company Proceq Sa was founded, which has been manufacturing the original Schmidt Hammer ever since. The Silver Schmidt Hammers have been available since 2007. These can measure the coefficient of restitution in addition to the rebound index.

Principle

The Rebound Hammer is composed of a spring hammer with a defined mass and a latching mechanism. After release, the mass hits a plunger, which contacts the test surface. The rebound distance of the spring hammer to the plunger or other rebound values are measured either digitally or on a graduated scale. For precise physical principles, see Leeb hardness test.

Procedure

Selection of measuring point

The concrete parts to be tested must be securely fixed within a structure and have a minimum thickness of 100 mm. Smaller components or test bodies may be tested, provided that they are firmly embedded. Surfaces with shrinkage, flaking, rough texture or high porosity should be avoided. A measuring point ought to be approximately 300 mm × 300 mm. The following factors should be considered when selecting the test surface:

• Concrete strength
• Type of surface
• Concrete type
• Moisture state of the surface
• Carbonation
• Test direction
• Other factors if any

Preparation of the measuring point

Very rough or soft surfaces and surfaces with loose mortar must be ground with a grinding stone of medium-grain silicon carbide or a comparable material until they are smooth and free of loose building material. Water on concrete surfaces requires removal.

Initial preparation

The impact surfaces of the test anvil and plunger are to be cleaned before a series of tests is carried out. At least five impact tests are mandatory on the steel test anvil. The results mustn’t deviate by more than ±3 from the guide value specified by the manufacturer. Otherwise, the Rebound Hammer has to be cleaned and/or adjusted according to the instructions of the manufacturer. The procedure described above should then be repeated. The Rebound Hammer must be operated in accordance with the manufacturer's instructions and may only be used at a temperature range from 0 °C to 50 °C.

Conducting examinations

To produce reliable results, the Rebound Hammer must be held in a position where the plunger can strike the test surface orthogonally. Furthermore, the distance between two impact points shouldn’t be less than 25 mm and each impact point must be at least 25 mm from any edge. To trigger the Rebound Hammer, the pressure on the plunger must be gradually increased. After impact, the rebound value should be recorded based on the rebound distance and/or energy or velocity measurements. Any imprint on the surface after impact is to be investigated. If the impact has crushed or penetrated a near-surface pore, the result should be disregarded. For a reliable estimate of the rebound value of the measuring point, at least nine valid measurements have to be used. The position and orientation of the Rebound Hammer must be recorded for each series of measurements.

Reference verification

After the test has been conducted, five additional measurements must be performed using the test anvil. If the results are not within ± 3 of the manufacturer's reading, the Rebound Hammer should be cleaned and/or adjusted as instructed by the manufacturer and the test should be repeated.

Examination results

The rebound value of the measuring point, or rather the test range, is the median value of all measurements. In consideration of the position of the Rebound Hammer, as specified by the manufacturer, the median has to be adjusted if necessary. The rebound value should be expressed as an integer. The whole test series is to be discarded, if more than 20 % of all results differ by more than 30 % from the median. If more than one Rebound Hammer has been used in the tests, a sufficient number of measurements should be performed with all Rebound Hammers on comparable concrete surfaces. Thus, the expected fluctuation of the measured values can be determined.

Field of application

The Rebound Hammer is used to determine the rebound value of a surface made of hardened concrete. This characteristic value can be used for the verification of the uniformity of in-situ concrete. It also serves to show areas or surfaces of low quality as well as damaged concrete in structures.

This test method is not suitable for determining the compressive strength of concrete. However, with the help of the rebound number and suitable correlations, an estimated value for the compressive strength of in-situ concrete can be inferred. For such correlations, Rebound Hammer test graphs are accessible. These are either provided by the manufacturer, can be taken from online databases or are produced directly, using a compression testing machine and a Rebound Hammer. By using the rebound value and the plunger angle, the compressive strength can be deduced from the graph.

Another field of application is comparative testing. For the comparison, a concrete of known strength or a concrete, that is proven to originate from a defined concrete volume and has a certain proven strength class, is used.

Advantages and disadvantages

Advantages

• Fast non-destructive testing device
• Inexpensive device with simple handling
• Compact, light and portable device
• Can test in-situ concrete as well as fresh concrete after final set
• Wide variety of concrete test hammers available

Disadvantages

• Continuous maintenance and cleaning of the device necessary
• Results only for a local point
• Strength and deformation of the concrete surface not directly related to test results, thus rebound value can not be directly converted into compressive strength

• Tested concrete has to be part of a larger mass or be immovably supported or secured
• Many factors influencing the results: Type of aggregate, type of cement, surface and moisture condition of the concrete, curing and age of concrete, carbonation of concrete surface
• Smooth and formed surface of the test subject necessary, thus not applicable for open textured surfaces
• Due to gravity the position of the hammer relative to the vertical will affect results. Results have to be adjusted
• Calibration curves may be necessary

Literature

• Malhotra V.M., Carino N.J.: Handbook on nondestructive testing of concrete. Second Edition. Taylor & Francis e-Library. (2006)

• Rebound Hammer Test, unter: Rebound Hammer Test (abgerufen am 02.01.2019)

• Nayan Parmar, Ajay Vatukiya, Mayanksinh Zala, Gaurav Gohil: Non-destructive testing by rebound hammer method in IJRTI. (2017) 2:4, p. 120-123.

• Proceq History, unter Proceq History (abgerufen am 02.01.2019)

• Szilágyi Katalin, Borosnyói Adorján: 50 years of experience with the Schmidt Rebound Hammer in Concrete Structures. (2009) 10, p. 46-56

• DIN EN 12504-2:2012-12, Prüfung von Beton in Bauwerken – Teil 2: Zerstörungsfreie Prüfung – Bestimmung der Rückprallzahl