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Technical Note – Mechanical Rebound Hammer

DRC Srl – [email protected] R_102013

DRC Srl

Michele Massaccesi

Investigation Procedure

Non-Destructive Investigations: Estimate of concrete resistance with Rebound Hammer

This document provides information for performing proper in situ using a rebound hammer,

describes the potential and limitations of the method, procedures for performing investigations,

for collecting and analysing data, and introduces the characteristics of the ECTHA 1000 mechanical

rebound hammer instrumentation manufactured by DRC.

This document is divided into 3 sections + 1

T – General Theory Test information

P – Performing the test

E – Instrumentation

Finally, the Report R - Report document

Investigation Procedure

Investigation with Rebound hammer.

This test procedure is in accordance with the following regulations:

• EN 12504-2

• ASTM C805

We have analysed rebound hammer operation in the Theory document.

We have defined that the rebound hammer, as a non-destructive investigation instrument, absolutely

cannot be used as an alternative method to destructive tests to determine the mechanical characteristics

of material.

The rebound hammer test is an "indirect" test and therefore can be used both to estimate the compressive

strength of a material and as a comparative method.

The significance of the results and the precision are completely independent from the type of instrument,

the difference between the mechanical and electronic rebound hammer.

The reliability and repeatability of results is closely linked to the quality of the instrument, its mechanical

parts, springs, sliding components, materials and of course the experience of the operator.



A1 Instrument: Rebound hammer

Rebound hammers are mechanical instruments which base their operation on the movement of metal

components along a pre-set direction thanks to the force exerted by the compression springs - traction.

The result is expressed as a "rebound index". The initial kinetic energy is partly dissipated by friction, partly

absorbed by the material under investigation, transforming it into plastic deformation, and partly

transformed in sound energy, and the residue is measured in terms of the rebound height.

It is therefore necessary to verify correct operation of the "couplings" of the various internal components

before performing tests, through the calibration verification procedure:

Operating principle

Rebound hammer operation A1) Vertically press the rebound hammer against the impact surface. A2) The hammer and impact

spring are loaded (spring extension). At its maximum impact mass, it is "freed" by the opening of the ratchet. A3) The impact mass

is pulled downward by the spring, which hits the strike piston, transferring energy A4) The mass "rebounds" upward, dragging the

reading index. We thus obtain the IRb rebound index.

Equipment necessary for performing testing with the rebound hammer:

a. Mechanical and/or electronic rebound hammer

b. Abrasive disk

c. Calibration anvil

d. Pencil

e. Ruler and/or measuring grid



B1 Performing the test (EN 12504-2 – ASTM C805)

Testing with the rebound hammer to determine the rebound index in accordance with standard EN 12504-

2 – ASTM C805 is as follows. This procedure describes how to use the rebound hammer to acquire rebound

index values and their representation.

Selecting the test area

• The thickness of the component to be investigated must be greater than or equal to 100 mm. The

area must be inserted in the structure; that it, it should not be able to move.

• Components with smaller thickness can be tested only if rigidly secured.

• The test investigation area must be dry. Avoid areas which high levels of cavities, gravel, flaking, or

high porosity.

• Avoid zones with a suspected presence of carbonation. If carbonation is detected after testing,

repeat the measurement session.

• Select a test area that is the most representative of the component and of the structure.

Test Procedure

Preliminary operations

• Read instrument manufacturer instructions.

• Perform instrument operating verification, activating it 2-3.

• Perform calibration verification (see calibration procedure), recording the values through the

calibration anvil.

• Verify that the mean of values obtained is in compliance with the information contained in the

manufacturer's certificate.

Perform instrument operation verification operations.

• Remove any roughness from the test area using an abrasive disk.

• Perform careful analysis with a covermeter (BS 1881-204). Identify the position of framework in the

structure.

• Draw a measuring grid in the test area, keeping a minimum distance of 25 from each point, from

edges and from the reinforcing bars.

Measurement operations

• Position the rebound hammer horizontally (preferable), making sure that the strike piston (impact

pin) assumes an orthogonal position to the test surface.

• Put pressure on the external tube, pushing it against the surfaces until an impact of the internal

mass on the strike piston.

• Press the external lock button.

• Read the rebound index value on the graduated scale and record the value. Note the position of

the test station and the positioning of the instrument.

• Perform a minimum of 9 strikes with the rebound hammer (16-20 recommended).


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Verification operations

• Examine the impact surfaces, making sure that no crushed areas due to the presence of sub

voids. If there are, discard the acquired value.

• Perform rebound hammer verification on the calibration anvil, recording values.

Results

The final rebound index value is expressed as the arithmetic mean of the values

whole number. The rebound index value must be expressed with horizontal orientation (A= 0°). If

necessary, adjust the values with the correction coefficients.

If more than 20% of all values acquired differ by more than 6 units from the mean, the entire test should

be discarded.

Calibration procedure

A Verification Activate the instrument, positioning it against a rigid

wall and perform 2

operation.

B Calibration Insert the rebound hammer inside the anvil guide.

Keeping it vertical, perform a minimum of 10

rebound hammer strikes, recording the values. The

mean must fall within the tolerance specified by the

anvil manufacturer and, however, with a rebound

value between 77

C Calibration Repeat the calibration "verification" operation at the

end of each investigated component.

D Adjustment Record values read in a notebook to be included with

the report.

Mechanical Rebound Hammer

Examine the impact surfaces, making sure that no crushed areas due to the presence of sub

. If there are, discard the acquired value.

Perform rebound hammer verification on the calibration anvil, recording values.

The final rebound index value is expressed as the arithmetic mean of the values read. The value must be a

The rebound index value must be expressed with horizontal orientation (A= 0°). If

with the correction coefficients.

acquired differ by more than 6 units from the mean, the entire test should

Activate the instrument, positioning it against a rigid

wall and perform 2-3 "strikes" to verify correct

Insert the rebound hammer inside the anvil guide.

Keeping it vertical, perform a minimum of 10

rebound hammer strikes, recording the values. The

mean must fall within the tolerance specified by the

anvil manufacturer and, however, with a rebound

value between 77-83 (Irbm mean rebound index).

Repeat the calibration "verification" operation at the

end of each investigated component.

Record values read in a notebook to be included with

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Examine the impact surfaces, making sure that no crushed areas due to the presence of sub-surface

Perform rebound hammer verification on the calibration anvil, recording values.

read. The value must be a

The rebound index value must be expressed with horizontal orientation (A= 0°). If

acquired differ by more than 6 units from the mean, the entire test should



C1 In situ operating procedures

The rebound hammer can be used for different applications to verify the homogeneity of concrete in situ.

Use of the rebound hammer must always be made within its own physical - mechanical limits to avoid

providing wrong information.

The main applications are, however, divided into:

a. Newly constructed structures

i. Inspections

ii. Checks during construction

b. Existing structures

i. Verifications for seismic vulnerability

ii. Verifications of buildings subjected to thermal shock

C1.1 New Structures

In inspection operations, rebound hammers cannot be used as an alternative method to destructive tests

but, when properly calibrated, can be excellent instruments in situ for investigations and checks.

Proceed as indicated below to use the rebound hammer in inspection and check operations during

construction of new structures.

1. Extract the identifying concrete cube samples put in place, uniquely identifying them (if this has not

already been done). This operation should have already been carried out by the Site Engineer during the

extraction phase.



2. After having been ensured that the samples have carried out proper maturation as indicated in the

reference standard (EN 12390-2:2012), perform rebound hammer testing.

a. Verify that the sides of the samples are free of bumps. If any are present,

using an abrasive disk. Also check that there is no water or the sample is too wet.

Create a grid on the sample sides to be subject to the test. The vertices of the grid repr

where strikes will be carried out with the rebound hammer. The distance of the points must be in

accordance with information contained in the standard (minimum distance 25 mm between two points and

the edge).

Fig. 03 – Create a grid on the sample sides to be subject to the test. The distance of the measurement points should not be less

than 25 mm. Keep a distance of between 25 and 50 mm between two measurement points and from the edge of the sample.



2:2012), perform rebound hammer testing.

a. Verify that the sides of the samples are free of bumps. If any are present, proceed with grinding of sides

using an abrasive disk. Also check that there is no water or the sample is too wet.

Fig. 02 – Side grinding using an abrasive disk.

Create a grid on the sample sides to be subject to the test. The vertices of the grid repr



the sample sides to be subject to the test. The distance of the measurement points should not be less


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proceed with grinding of sides

Create a grid on the sample sides to be subject to the test. The vertices of the grid represent the points



the sample sides to be subject to the test. The distance of the measurement points should not be less




b. Position the sample between the press plates, activating a pre

that the sample be rigidly secured. No movements should take place after rebound hammer impact.

Fig. 04 – Strikes with the rebound hammer on the rigidly secured sample between

c. If it is not possible to insert the sample between the press plates, position it on a surface, resting the side

opposite to the one selected for the investigation against a solid wall.

Fig. 05 – Strikes with the rebound hammer on the


the press plates, activating a pre-load of about 1 N/mm


Strikes with the rebound hammer on the rigidly secured sample between press plates


opposite to the one selected for the investigation against a solid wall.

Strikes with the rebound hammer on the sample resting against a wall to prevent movement

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load of about 1 N/mm2

. It is important


press plates


sample resting against a wall to prevent movement



d. Perform strikes with the rebound hammer on the sample. Tests should be carried out on the 4 sides of

the sample, discarding those free of concrete and its opposite side.

Perform a minimum of 9 strikes on each side, keeping the rebound angle of the instrument horizontal.

Record all acquired values, uniquely associating them with the sample.

e. Perform a breaking test on the sample in accordance with standard EN 12390

breakage is compliant. Note the obtained strength value.

f. Enter the compressive strength values obtained from the direct investigation in a table, associating them

with the mean values of the rebound index obtained by tests wit

View the results obtained on a scatter plot, entering the rebound index value in the axis of abscissas (X

and the compressive strength values in the axis of ordinates (Y

Fig. 07 – Summary and correlation of values



the sample, discarding those free of concrete and its opposite side.

es on each side, keeping the rebound angle of the instrument horizontal.

Record all acquired values, uniquely associating them with the sample.

e. Perform a breaking test on the sample in accordance with standard EN 12390-3:2012, making sure that

is compliant. Note the obtained strength value.

Fig. 06 – Sample breaking test


with the mean values of the rebound index obtained by tests with the rebound hammer.

View the results obtained on a scatter plot, entering the rebound index value in the axis of abscissas (X

and the compressive strength values in the axis of ordinates (Y-axis).

Summary and correlation of values obtained by non-destructive and destructive tests

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es on each side, keeping the rebound angle of the instrument horizontal.

3:2012, making sure that


h the rebound hammer.

View the results obtained on a scatter plot, entering the rebound index value in the axis of abscissas (X-axis)

destructive and destructive tests



Assuming you have used N samples, we obtain a graph as follows:

Fig. 08 – Representation of a curve – linear regression obtained by correlating mean rebound index values (IRbm) with those of

relative cubic strength values (RcK). The values shown are purely random and for demonstration purposes only.

The curve is described by a linear regression. The axis of abscissas (X

while the axis of ordinates (Y-axis) shows

The curve created shows the direct ratio between the rebound hammer value with the cubic strength of

the concrete used in construction.

Structural verification

a. Analyse and select the components to be investigated. Perform

order to detect the presence of the reinforcing bars. Remove any bumps on the component using an

abrasive disk.

b. The technician performing testing with the rebound hammer can perform tests on all structural

components of the building, noting and recording all rebound index values.

The acquired values can be compared to the curve obtained experimentally in situ.

Fig. 09 – Analysis of structural components with the correlation curve obtained experimentally


Assuming you have used N samples, we obtain a graph as follows:

linear regression obtained by correlating mean rebound index values (IRbm) with those of

cubic strength values (RcK). The values shown are purely random and for demonstration purposes only.

The curve is described by a linear regression. The axis of abscissas (X-axis) shows the rebound index values,

axis) shows the Cubic Strength values.


a. Analyse and select the components to be investigated. Perform a careful covermeter investigation in



s of the building, noting and recording all rebound index values.

The acquired values can be compared to the curve obtained experimentally in situ.

Analysis of structural components with the correlation curve obtained experimentally

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linear regression obtained by correlating mean rebound index values (IRbm) with those of

cubic strength values (RcK). The values shown are purely random and for demonstration purposes only.

axis) shows the rebound index values,


a careful covermeter investigation in



Analysis of structural components with the correlation curve obtained experimentally



C 1.2 Existing Structures

Rebound hammers in investigation campaigns for evaluation of the mechanical characteristics of materials

making up the structure cannot be used as an alternative method to destructive tests.

Proceed as indicated below to use the rebound hammer in verification and check operations on existing

structures.

a. After having removed the plaster and performed a correct and careful covermeter investigation, proceed

with smoothing the investigation surfaces using an abrasive disk.

b. Draw a measuring grid with distances of 25-30 mm minimum between two vertices and from the edge of

the component.

Fig. 10 – Removing plaster and creating a measurement grid

c. Perform strikes with the rebound hammer in the area defined by the grid. Perform a minimum number of

12-16 strikes per each area, noting the values individually. Record the position of the measurement

station.



Fig. 11 – Performing strikes with the rebound hammer on the selected surface

d. Perform coring on selected structural components. The number of cores is defined by standards and

regulations and is based on the type of investigation being performed, the size of the building and designer

assessment. The choice of components where cores will be created is at the di

Director.

Coring must be carried out in accordance with EN 12504

Identify the extracted core uniquely, noting in the report if any strikes were performed by the rebound

hammer in the same measurement station. This link a

to the material found in situ.


Performing strikes with the rebound hammer on the selected surface

tructural components. The number of cores is defined by standards and


assessment. The choice of components where cores will be created is at the discretion of the Technical

Coring must be carried out in accordance with EN 12504-1:2009.


hammer in the same measurement station. This link allows for the creation of a correlation curve dedicated

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Performing strikes with the rebound hammer on the selected surface

tructural components. The number of cores is defined by standards and


scretion of the Technical


llows for the creation of a correlation curve dedicated



Fig. 12 – Performing coring in the test area where investigations have been performed with the rebound hammer

f. Perform non-destructive testing on all accessible structural components in order to obtain very large

number of values that therefore properly identify the entire structure.

Fig. 13 – Performing investigations with the rebound hammer on remaining structural components

e. Perform crushing tests on the cores to obtain cylindrical resistance values converted in cubic strength.

Tests must be performed in accordance with EN 12390-3:2009.



Fig. 14

f. In a table, record the rebound index values (mean rebound index) obtained for each measurement

station, also indicating, where present, the cubic strength value obtained through destructive testing on

cores.

Fig. 15 – Recording index and correlation values with core breaking val

g. In a graph, depict the values obtained, correlating the mean rebound index values with those of cubic

strength obtained by core breaking.


Fig. 14 – Performing breaking tests on extracted cores

ound index values (mean rebound index) obtained for each measurement


Recording index and correlation values with core breaking values


strength obtained by core breaking.

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ound index values (mean rebound index) obtained for each measurement


ues




Fig. 16 – Linear regression curves between rebound index and cylindrical resistance values

h. It is possible to assess the mechanical strength values of the stations where cores were not performed

through the trend line obtained by linear regression.

This system allows us to obtain an indication of the mechanical strength values of the concrete, reducing

the number of direct tests that, given their high level of invasiveness, inevitably contribute to reducing the

resistance of the structure.

C 1.2 Conclusions

Non-destructive testing, if performed within its limits, allows us to obtain satisfactory and useful results for

understanding the conditions of a structure.

Non-destructive testing cannot however be a substitute for destructive tests ment

decrees.

Assessment of the mechanical strength of concrete through non

regulated by EN 13791:2008.

Investigation with a rebound hammer: Brief instructions Creative Commons Attribuzione 4.0

Stazione RcK (carota) Irbm

ST1 210 32

ST2 230 34

ST3 36

ST4 32

ST5 36

ST6 28

ST7 32

ST8 30

ST9 191 28


Linear regression curves between rebound index and cylindrical resistance values

possible to assess the mechanical strength values of the stations where cores were not performed

through the trend line obtained by linear regression.



destructive testing, if performed within its limits, allows us to obtain satisfactory and useful results for

understanding the conditions of a structure.

destructive testing cannot however be a substitute for destructive tests ment

Assessment of the mechanical strength of concrete through non-destructive and destructive testing is

Investigation with a rebound hammer: Brief instructions by Michele Massaccesi4.0 Internazionale License.

Stazione RcK (carota) Irbm

ST1 210 32

ST2 230 34

ST3 239,5 36

ST4 214,5 32

ST5 239,5 36

ST6 189,5 28

ST7 214,5 32

ST8 202 30

ST9 191 28

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Linear regression curves between rebound index and cylindrical resistance values

possible to assess the mechanical strength values of the stations where cores were not performed



destructive testing, if performed within its limits, allows us to obtain satisfactory and useful results for

destructive testing cannot however be a substitute for destructive tests mentioned in laws and

destructive and destructive testing is

Michele Massaccesi is distributed with a

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