NON-DESTRUCTIVE TEST

TESTS ON HARDENED CONCRETE

1. NON-DESTRUCTIVE TESTS

1.1 REBOUND HAMMER

AIM

To assess the likely compresive strength of concrete by using

rebound hammer as per IS: 13311 (Part 2) - 1992.

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.

APPARATUS

Rebound hammer

PROCEDURE
i) 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.
ii) 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.
iii) 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 15 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.

1.2 ULTRASONIC PULSE VELOCITY

AIM
To assess the quality of concrete by ultrasonic pulse velocity
method as per IS: 13311 (Part 1) - 1992.

PRINCIPLE
The method consists of measuring the time of travel of an
ultrasonic pulse passing through the concrete being tested.
Comparatively higher velocity is obtained when concrete quality
is good in terms of density, uniformity, homogeneity etc.

APPARATUS

 Ultrasonic pulse velocity meter

PROCEDURE
i) Preparing for use: Before switching on the 'V' meter, the
transducers should be connected to the sockets marked
"TRAN" and " REC".
The 'V' meter may be operated with either:
a) the internal battery,
b) an external battery or
c) the A.C line.
ii) Set reference: A reference bar is provided to check the
instrument zero. The pulse time for the bar is engraved on it.
Apply a smear of grease to the transducer faces before
placing it on the opposite ends of the bar. Adjust the 'SET
REF' control until the reference bar transit time is obtained
on the instrument read-out.
iii) Range selection: For maximum accuracy, it is
recommended that the 0.1 microsecond range be selected
for path length upto 400mm.
iv) Pulse velocity: Having determined the most suitable test
points on the material to be tested, make careful
measurement of the path length ‘L’. Apply couplant to the
surfaces of the transducers and press it hard onto the
surface of the material. Do not move the transducers while a
reading is being taken, as this can generate noise signals
and errors in measurements. Continue holding the
transducers onto the surface of the material until a
consistent reading appears on the display, which is the time
in microsecond for the ultrasonic pulse to travel the distance
'L'.

The mean value of the display readings should be taken
when the units digit hunts between two values.

Pulse velocity = Path length / Travel time

v) Separation of transducer leads: It is advisable to prevent the
two transducer leads from coming into close contact with
each other when the transit time measurements are being
taken. If this is not done, the receiver lead might pick-up
unwanted signals from the transmitter lead and this would
result in an incorrect display of the transit time.

INTERPRETATION OF RESULTS
The quality of concrete in terms of uniformity, incidence or
absence of internal flaws, cracks and segregation, etc.,
indicative of the level of workmanship employed, can thus be
assessed using the guidelines given below, which have been
evolved for characterising the quality of concrete in structures in
terms of the ultrasonic pulse velocity.

Pulse Velocity Concrete Quality
(km/second) (Grading)
Above 4.5 Excellent
3.5 to 4.5 Good
3.0 to 3.5 Medium
Below 3.0 Doubtful