Rebound hammer test is also known as Schmidt Hammer Test is used for
assessing the strength of Concrete. The test is done as per IS: 13311 (Part 2)
– 1992in order to analyze the compressive strength of concrete by using rebound
hammer. This method is used to study the elastic properties, Uniformity of any
concrete. It also helps in determination of variation in strength of similar
structures.
Since it is a Non-Destructive Testing Method, it can be applied for
finished as well as unfinished concrete structures.
Principle
The rebound of an elastic mass depends on the hardness of the surface
against which its mass strikes. When the plunger of the rebound hammer is
pressed against the surface of the concrete, the Spring-controlled mass
rebounds and the extent of such a rebound depends upon the surface hardness of
the concrete. The surface hardness and therefore the rebound is taken to be
related to the compressive strength of the concrete. The rebound value is read
from a graduated scale and is designated as the rebound number or rebound
index. The compressive strength can be read directly from the graph provided on
the body of the hammer.
Procedure
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.
v 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.
v 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.)
v Take
the average of about 5 readings
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.
Table:- Average Rebound number and quality of concrete
Average Rebound Number
|
Quality of Concrete
|
>40
|
Very good hard layer
|
30 to 40
|
Good layer
|
20 to 30
|
Fair
|
<20
|
Poor concrete
|
0
|
Delaminated
|
Table: - Average Rebound number and quality of concrete
Using the Humboldt
Concrete Rebound Hammer
Always hold the
Concrete Rebound Hammer in both hands while in use. Also, when in use, keep the
hammer perpendiculars to the surface you are testing with the scale pointing
up, making it easier to read.
Figure 1:
The Concrete Rebound Hammer
Before the Hammer can be used for testing, the piston
must be released out of the hammer into the testing position. Typically during
storage and transportation, the Hammer piston will be locked inside the Rebound
Hammer and will need to be released from the storage position. If the piston is
not extended into the test position, place the end of the piston against a
stiff surface and gently press the Rebound Hammer firmly against the surface.
You will hear a click, and the piston will extend into the test position.
Figure 2: Always hold the Rebound Hammer with both
hands perpendicular to the surface
To perform a test, make sure the piston is extended and
gently press the Rebound Hammer against
the concrete surface to be tested. When the piston is pressed all the way into
the Rebound Hammer, continue to push harder until you hear a rattling sound.
Keep the Rebound Hammer firmly pressed against the concrete surface and read the
rebound number on the scale.
Figure 3: Push the piston in as far as it will go
As long as you hold the Rebound Hammer firmly against
the concrete, the re- bound number will remain on the scale. Record your
reading. When you pull the hammer away and allow the piston to re-extend, the
scale will return to zero. The Rebound Hammer is now ready for another test. If
desired, once you get a reading, you may press the button on the handle to lock
the piston in place. This makes your test results easier to read, as it will
also lock the indicator. Do not touch the button until you get a reading. To
release the piston again for the next test, press against a hard surface. When
you are finished using the Rebound Hammer, lock the piston inside the Rebound
Hammer for storage, as described above
Taking a Measurement
Selecting a Surface for Testing Select a concrete
surface that is:
- Smooth, dry, and fixed in place
- At least 4 inches (or 102 mm) thick
- Mature— at least 14 days old
- Free of any decay or scaling
If necessary grind a section of rough concrete smooth
before taking your readings. A grind
stone in a plastic case is provided with the hammer for this purpose
Surfaces to Avoid
Avoid testing concrete that is:
- Frozen
- Honeycombed or excessively porous
- Known to have rebar 2 cm or less below the surface
- Carbonated on the surface
If you must test a mature concrete surface that is damp
from rain or flowing water, or if you must test new concrete, keep in mind that
you will probably get. Rebound numbers that are lower than they should be.
Using the Rebound Hammer on new concrete may damage it if the strength is less
than 1,000 PSI. If you are testing concrete with a lot of surface carbonation,
the rebound readings will be significantly higher than normal. The thicker the
layer of carbonated concrete, the higher the rebound numbers.
Holding the Rebound Hammer against Walls,
Ceilings, and Floors
The Rebound Hammer will work against any concrete
surface as long as you keep it perpendicular to that surface. The Rebound
Hammer can be used against a ceiling (upward), a floor (downward), or a wall
(forward). You will need to consider gravity when estimating the compressive
strength of concrete. The rebound number found when pressing up against a
ceiling will differ from the number that appears if you are pressing down
against a floor. Please review the strike angles A, B, and C and the correlation
curves in “Reading your Results” below.
Specialty Concrete
The Rebound Hammer is designed to estimate the
compressive strength of standard concrete. The correlation diagram for standard
concrete is printed on the side of your Concrete Rebound Hammer. It shows the
correlation curves deter- mined when the compressive strength of standard
concrete, as measured with your Rebound Hammer, is directly compared to the
compressive strength from laboratory tests of concrete cylinders or cubes.
This diagram is not designed for use with:
- Lightweight concrete
- Concrete using artificial aggregates
- High performance concrete
In order to gain an accurate rebound number for these
specialty materials, you will need to create your own correlation curves
diagram intended for this use. See the
standards ACI 228.1R-2010 and ASTM C-805-08.
Reading your Results
Make at least ten readings from a concrete surface and
discard the highest and lowest rebound numbers. Average the remaining eight
numbers. If desired, take a few test readings before you complete your series
of ten regular tests. Use the average rebound number to estimate the strength
of the concrete. Compare your average rebound number to the chart shown on your
Concrete Rebound Hammer. Consider the diagram shown here:
Figure 4: Concrete Rebound Hammer correlation curves
Rebound Number
Correlation curves. This diagram shows the relationship
between the compressive strength for concrete, measured directly in a laboratory
using concrete cylinders or concrete cubes, and the rebound numbers you
receive using your Rebound Hammer.
A. Refers to using the Rebound Hammer against
a concrete floor (downward).
B. Refers to using it against a concrete wall
(forward).
C. Refers to using it against a concrete
ceiling (upward).
The strength values shown in this diagram are directly
based on compressive strength testing in a laboratory, using concrete 14 to 56
days old. The tests use concrete cylinders, 6 inches wide and 12 inches high (15cm
x 20cm), or concrete cubes six inches high.
According to ASTM C-805-08, you should discard any
readings that differ from the average
value rebound number by more than six units. Then, take an average for the
remaining readings. If more than two readings differ by more than six units
from the average value, throw out all of your readings and perform the test
over again. Take ten new readings with your Rebound Hammer in a different section of the concrete but in the
same test area.
Note that some countries have set their own national
standards for calculating test results. Therefore, follow the standards of the
country where you are using your Rebound Hammer.
Rebound Hammer Test Equipment
A simple equipment
known as Rebound Hammer or Schmidt Hammer is used for this purpose. Surface
hardness measured during the test give an idea about the soundness and quality
of cover concrete. Locations having very low rebound numbers indicate weak
surface concrete and may be affected by corrosion. The quality of concrete may
be interpreted as shown in the above table
Advantage
- Simple to use. No special experience is needed to conduct the test.
- Establishes uniformity of properties.
- Equipment is inexpensive and is readily available.
- A wide variety of concrete test hammers is available with an operational range of M10 to M70.
- For rehabilitation of old Monuments
Disadvantage
- Evaluates only the local point and layer of masonry to which it is applied.
- No direct relationship to strength or deformation properties.
- Unreliable for the detection of flaws
- Cleaning maintenance of probe and spring mechanism
Maintenance
We recommend that you calibrate your Rebound
Hammer regularly using the Humboldt
Calibration Anvil, H-2972, ordered separately. For a Rebound Hammer, you should
start verifying the readings after approximately 2,000 uses (or completed tests). You should also
verify the readings if you have reason
to suspect that your Rebound Hammer is giving you false
data
Calibrating your Concrete Rebound Hammer
When tested against the Anvil, the impact energy level
of the Rebound Hammer must be 1.64 foot-pounds or .227 kilograms-meter and the
rebound number must be a value of 80 ±2. As the Rebound Hammer approaches 2,000
uses, it may start to read outside of the range of 80 ±2 on the test Anvil. At
this point you should calibrate it.
1.
With the piston fully extended, twist the cap on the top of the Rebound Hammer to open it. Keep your hand positioned
on the top of the cap, as you will feel some slight spring compression pushing
against the cap. Set the Re- bound Hammer aside and examine the cap. The cap
has an adjustment screw and nut, which are used to calibrate the Rebound
Hammer.
Figure 5: Rebound Hammer cap
2. using a 10 mm wrench, loosen the adjustment nut away
from the cap. If the Rebound Hammer is reading low, tighten the adjustment
screw (clockwise) into the cap. If the Rebound Hammer is reading high, loosen
the adjustment screw (counterclockwise) away from the cap. When you are
finished, tighten the adjustment nut to lock the screw in this new position.
3. Put the Rebound Hammer back together, and test it on
the Anvil again to get a calibrated reading. If it reads 80 ±2, the Rebound
Hammer is calibrated. If the readings fall outside of this range, take the
Rebound Hammer apart and try to adjust it again. Keep adjusting and testing the
Rebound Hammer until it reads 80 ±2.
Clean the Rebound Hammer with a dry, clean cloth. Do
not use solvents, abrasives, or water for cleaning. The springs in the Rebound
Hammer may become worn and ineffective after more than 8,000 tests. Therefore,
if you have an older Rebound Hammer that is no longer providing accurate
measurements, and you tried to recalibrate it but it still doesn’t work
properly, send it back to Humboldt for an evaluation and/or repair.
Safety:
- When using this device wear safety glasses
- Do not fire the Rebound Hammer against any body part
- Impact Plunger
- Housing, complete
- Rider with Guide Rod
- Push-button, complete
- Hammer Guide Bar
- Guide Disk
- Cap
- Two-part Ring
- Rear Cover
- Compression Spring
- Pawl
- Hammer Mass
- Retaining Spring
- Impact Spring
- Guide Sleeve
- Felt Washer
- Plexiglass Window
- Screw
- Lock Nut
- Pin
- Pawl Spring
excellent info about rebound hammer test
ReplyDeletethis is really nice post rebound hammer test is code
ReplyDelete