Schmidt hammer

The rebound hammer ( Schmidt Hammer ) is an instrument for non-destructive material testing, the compressive strength can be measured point by point, for example, of concrete ( Ref: DIN EN 12504-2:2012-12 ).

The rebound hardness testing provides a quick overview of the condition, for example, a concrete building, even in places where, because of static or structural reasons (eg lack of space, hard to reach places ) can be taken from any core samples for laboratory analysis.

Scope

The DIN EN 12504-2:2012-12 (D ) describes the Determination of rebound number of a surface of hardened concrete by means of a rebound hammer ( hammer spring made ​​of steel). After NOTE 1 norm, the method of proof of the uniformity of in situ concrete, as well as the representation of regions or areas poor quality or damaged concrete in structures used. Although it can NOTE 2 the estimate of the strength of in situ concrete using a suitable correlation, however, expressly denied the use of the estimate as a criterion for the strength ( Ref: DIN EN 12504-2:2012-12 ( D)).

DIN EN 13791:2008-05 indicates the in DIN EN 12504-2 lack of correlation and indeed in the National Annex NA, Table NA.2

Principle

If kinetic energy is applied to a material, a portion of this energy is always converted, for example, in heat, in the case of deformation of the material (see conservation of energy ). The harder the material, the less energy is converted received by it ( deformation ), and. By measuring this energy conversion, the hardness and the compressive strength of the material can be determined. It should be noted that depending on the modulus of elasticity of the material, the compressive strength may vary for the same hardness.

Implementation

Test surface

Straight shaped or smooth painted surfaces may be tested unpolished, otherwise the surfaces of loose, soft mortar shall be removed and glattzuschleifen rough surfaces. Water on the test surface must be removed. The test surface shall have approximately the dimensions of 300 mm × 300 mm. The test area must have at least a thickness of 100 mm. For the uniformity of a concrete surface that is more rebound values ​​and thus several test areas are needed. There are at least nine probes or impact points required to determine the rebound speed of the test surface. The individual test points and impact points must be at least 25 mm apart. For this purpose, it is advantageous to apply a regular grid with a spacing of 25 to 50 mm on the test surface. The rebound speed of the test area is the median value of the range. The series, and thus the surface is rejected when more than 20% of the readings differ by more than 6 ( readout ) units from the median.

Checkpoint

A test point or point of impact refers to the respective measurement taken at a particular location. The rebound hammer is placed at right angles to each checkpoint on the area to be tested and the pressure is while gradually increase until the rebound hammer triggers. The measured value is read and recorded. Every impression on the surface after the impact is to investigate and if the surcharge has crushed or broken through a near-surface pore, the result is ignored.

General Description

In determining a measured value, a weight is through a tensioned spring in the rebound hammer accelerates whose pulse is transferred to a firing pin, which strikes with its round end on the test specimen and rebounds. The harder the material, the more bounces back the bolt. The rebound distance is indicated on a scale, and is a measure of the rebound energy. The strength is then read from a rebounding hammer assigned table.

However, the nature of a material can vary greatly due to its internal structure. Thus, for example, concrete made ​​of materials of different compressive strength and grain size (sand, gravel, cement, air entrainment ). A single test process thus illustrates not only a representative sample, which is why a series of measurements is necessary from the results then an average is calculated to compensate for fluctuations in strength.

There are also rebound hammer that calculate by electronic measurement recording the averaging during the measurement series.

Remark

The no longer valid DIN 1048 Part 2 status in June 1991 yielded even a table of comparative values ​​of rebound distance ( median) 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 structures, Part 2: Non-destructive testing Determination of rebound number
  • ÖNORM EN 12504-2:2001 Testing concrete in structures Part 2: Non-destructive testing Determination of rebound number
  • DIN EN 13791:2008-05 " assessment of the strength of concrete in structures or structural parts "
  • Material testing
  • Hardness Measurement
Structural analysis
695724
de