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UNIVERSITY OF GAZIANTEP FACULTY OF ENGINEERING CIVIL DEPARTMENT

CE-550

NONDESTRUCTIVE TESTING AND EVALUATION IN STRUCTURAL ANALYSIS

Report About :

(Using of Schmidt Hammer as anon destructive test method

to determine compressive strength of structures )

Submitted to:

Do.Dr.ESSRA GUNAYISI

Prepared by:

Chalak Ahmed Mohammed chalak.mohammed@gmail.com

2014 45056

Date : 10.04. 2015

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INTRODUCTION

SCHMIDT REBOUND HAMMER TEST

The Schmidt rebound hammer is principally a surface hardness tester. It

works on the principle that the rebound of an elastic mass depends on the

hardness of the surface against which the mass impinges. There is little

apparent theoretical relationship between the strength of concrete and the

rebound number of the hammer. However, within limits, empirical correlations

have been established between strength properties and the rebound number.

Further, Kolek has attempted to establish a correlation between the hammer

rebound number and the hardness as measured by the Brinell method.

EQUIPMENT FOR SCHMIDT/REBOUND HAMMER TEST The Schmidt rebound hammer is shown in (Fig. 1). The hammer weighs

about 1.8 kg and is suitable for use both in a laboratory and in the field. A

schematic cutaway view of the rebound hammer is shown in ( Fig. 2). The

main components include the outer body, the plunger, the hammer mass, and

the main spring. Other features include a latching mechanism that locks the

hammer mass to the plunger rod and a sliding rider to measure the rebound of

the hammer mass. The rebound distance is measured on an arbitrary scale

marked from 10 to100. The rebound distance is recorded as a rebound

number corresponding to the position of the rider on the scale.

fig.1

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1. Concrete surface 5. Hammer guide 9. Housing

2. Impact spring 6. Release catch 10. Hammer mass

3. Rider on guide rod 7. Compressive spring 11. Plunger

4. Window and scale 8. Locking button

Fig. 2 Components of a Rebound Hammer

ROCEDURE TO DETERMINE STRENGTH OF HARDENED CONCRETE BY REBOUND HAMMER:

Before commencement of a test, the rebound hammer should be tested against the test anvil,

to get reliable results, for which the manufacturer of the rebound hammer indicates the range

of readings on the anvil suitable for different types of rebound hammer.

Apply light pressure on the plunger - it will release it from the locked position and allow it to

extend to the ready position for the test.

Press the plunger against the surface of the concrete, keeping the instrument perpendicular to

the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not

touch the button while depressing the plunger. Press the button after impact, in case it is not

convenient to note the rebound reading in that position.)iv) Take the average of about 11

readings....

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INTERPRETATION OF RESULTS

The rebound reading on the indicator scale has been calibrated by the manufacturer of the

rebound hammer for horizontal impact, that is, on a vertical surface, to indicate the

compressive strength. When used in any other position, appropriate correction as given by

the manufacturer is to be taken into account.

Hammer test can provide fairly accurate estimate of concrete compressive

strength, it performed with precautions and co- relations :

1. Concrete Age : The concrete should be 14 to 56 days old.

2. Surface Condition : The surface of the concrete at the point tested must be smooth

dry and free of honey combing. Otherwise, rebound readings will be low indicating a

weaker concrete than is actually the case .

3. Location of Test Point : The concrete to be tested must be at least 100 mm thick.

Care must be taken that the test points are no closer than 25 mm from the edge.

4. No. of Readings : Calculate the average of 11 readings and exclude extreme

readings (if any) and recalculate average using the remaining readings.

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TEST METHODOLOGY:

For taking a measurement, the hammer should be held at right

angles to the surface of the structure. The test thus can be

conducted horizontally on vertical surfaces and vertically upwards

or downwards on horizontal surfaces, as shown below:

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RANGE AND LIMITATIONS OF SCHMIDT REBOUND

HAMMER TEST

Although the rebound hammer does provide a quick, inexpensive

method of checking the uniformity of concrete, it has some serious

limitations. The results are affected by:

1. Smoothness of the test surface

Hammer has to be used against a smooth surface, preferably a

formed one. Open textured concrete cannot therefore be tested. If

the surface is rough, e.g. a trowelled surface, it should be rubbed

smooth with a carborundum stone.

2. Size, shape and rigidity of the specimen

If the concrete does not form part of a large mass any movement

caused by the impact of the hammer will result in a reduction in the

rebound number. In such cases the member has to be rigidly held or

backed up by a heavy mass.

3. Age of the specimen

For equal strengths, higher rebound numbers are obtained with a 7

day old concrete than with a 28 day old. Therefore, when old concrete

is to be tested in a structure a direct correlation is necessary between

the rebound numbers and compressive strengths of cores taken

from the structure. Rebound testing should not be carried out on low

strength concrete at early ages or when the concrete strength is less

than 7 MPa since the concrete surface could be damaged by the

hammer.

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4. Surface and internal moisture conditions of concrete

The rebound numbers are lower for well-cured air dried

specimens than for the same specimens tested after being soaked

in water and tested in the saturated surface dried conditions.

Therefore, whenever the actual moisture condition of the field

concrete or specimen is unknown, the surface should be pre-

saturated for several hours before testing. A correlation curve for

tests performed on saturated surface dried specimens should then be

used to estimate the compressive strength.

5. Type of coarse aggregate

Even though the same aggregate type is used in the concrete mix, the

correlation curves can be different if the source of the aggregate is

different. An example is shown in Fig. 4.5 where correlation curves

for four different sources of gravel are plotted

6.Type of cement

High alumina cement can have a compressive strength 100% higher

than the strength estimated using a correlation curve based on ordinary

Portland cement. Also, super sulphated cement concrete can have strength

50% lower than ordinary Portland cement.

7.Carbonation of the concrete surface

In older concrete the carbonation depth can be several millimeters

thick and, in extreme cases, up to 20 mm thick. In such cases the

rebound numbers can be up to 50% higher than those obtained on an

uncarbonated concrete surface.

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Our laboratory test:

SAMPLE#1 (horizontal reading for specimeb concrete cube)

--As we discussed above we take 11 no of reading(position

A=horizontal) after that we neglect the highest and lowest

reading and after we take the average we compare to the

special graph to get the result of compressive strength of that

reading of the instrument 30

25

30

30

22 canceled

36 canceled

25

25

27

29

23

Average= 27.11

After putting that number in the graph(horizontal position) we get the

result =19.50 N/mm2 as shown in the graph no.1

Graph No. 1

SAMPLE#2 (Vertical reading for specimen concrete slab manhole)

--As we discussed above we take 11 no of reading(position

B=Vertical) after that we neglect the highest and lowest reading

and after we take the average we compare to the special graph to

get the result of compressive strength of that

reading of the instrument 32

27

30

32

34

26

22 canceled

24

32

33

36 canceled

Average= 30

After putting that number in the graph(vertical position) we get the result

=28 N/mm2 as shown in the graph no.2

Graph No.2

If concrete compression test fails, should Schmidt

hammer test be adopted as an alternative test to

prove the concrete strength?

The Scmidt hammer test is based on the elastic rebound of

hammer which presses on concrete surface and it measures

the surface hardness of concrete. Since the test is very

sensitive to the presence of aggregates and voids at the

concrete surface, it is necessary to take more than 10

readings over the area of test. However, it should be noted

that Schmidt hammer test measures surface hardness only

but not the strength of concrete. Therefore, it may not be

considered a good substitute

Regards...