Rebound hammer
The rebound hammer (Schmidt hammer) is an instrument for non-destructive testing of materials , with which the compressive strength z. B. concrete can be measured point by point ( Ref . : DIN EN 12504-2: 2012-12).
The rebound hardness test enables a quick overview of the condition z. B. a concrete structure, also in places where no core samples for a laboratory analysis can be taken for static or structural reasons (e.g. lack of space, difficult to access places).
scope of application
DIN EN 12504-2: 2012-12 (D) describes the determination of the rebound number of a surface made of hardened concrete using a rebound hammer (spring hammer made of steel). According to NOTE 1 to the standard, the method is used to verify the uniformity of in-situ concrete and to represent areas or surfaces of poor quality or for damaged concrete in structures. NOTE 2 allows the strength of in-situ concrete to be estimated using a suitable correlation, but expressly denies the use of the estimate as a criterion for strength ( Ref .: DIN EN 12504-2: 2012-12 (D)).
DIN EN 13791: 2008-05 specifies the missing correlation in DIN EN 12504-2 in the National Annex NA, Table NA.2
principle
When kinetic energy acts on a material, part of this energy is always converted, for example into heat in the case of deformation of the material (cf. energy conservation law ). The harder the material, the less energy it absorbs (deformation) and converted. By measuring this energy conversion, the hardness or compressive strength of the material can be determined. It should be noted that the compressive strength can vary with the same hardness depending on the elasticity module of the material .
execution
Test area
Smoothly shaped or smoothly painted surfaces may be tested unsanded, otherwise the surfaces must be removed from loose, soft mortar and rough surfaces must be sanded smooth. Remove water from the test surface. The test area must measure approximately 300 mm × 300 mm. The test area must be at least 100 mm thick. For the evenness of a concrete surface, several rebound values and thus several test surfaces are necessary. At least nine test points or impact points are necessary to determine the rebound number of a test surface. The individual test points or impact points must be at least 25 mm apart. For this purpose, it is advisable to attach a regular grid with intervals of 25 to 50 mm on the test area. The rebound number for the test surface is the median of the series. The row and thus the area is discarded if more than 20% of the readings deviate from the median value by more than 6 (reading) units.
Checkpoint
A test point or impact point describes the measurement made at a specific point. The rebound hammer is placed at right angles for each test point on the point to be tested and the pressure must be increased gradually until the rebound hammer is triggered. The measured value is read and noted. Every impression on the surface after impact must be examined, and if the impact has crushed or broken through a pore near the surface, the result is disregarded.
general description
When determining a measured value, a tensioned spring in the rebound hammer accelerates a weight, the momentum of which is transmitted to a firing pin, which hits the test object with its round end and rebounds. The harder the material, the further the bolt will rebound. The rebound distance is displayed on a scale and is a measure of the rebound energy. The strength is then read from a table assigned to the rebound hammer.
However, the nature of a material can vary greatly due to its internal structure. So there is z. B. Concrete made from materials of different compressive strength and grain size (sand, gravel, cement, air pores). A single test process is therefore only a non-representative sample, which is why a series of measurements is necessary, the results of which are then averaged in order to compensate for fluctuations in strength.
There are also rebound hammers that use electronic data acquisition to calculate the mean during the series of measurements.
comment
The no longer valid DIN 1048 Part 2 as of June 1991 still provided a table with comparative values for the rebound distance (median value) and concrete strength class.
credentials
- DIN 1048 Part 2: Test methods for concrete, hardened concrete in structures and components.
- DIN EN 12504-2: 2012-12 Testing of concrete in buildings, part 2: Non-destructive testing Determination of the rebound number
- ÖNORM EN 12504-2: 2001 Testing of concrete in buildings, part 2: Non-destructive testing Determination of the rebound number
- DIN EN 13791: 2008-05 "Assessment of the strength of concrete in structures or in parts of structures"