Ultrasonic Pulse Velocity Test for concrete

Ultrasonic test on concrete is a recognized non-destructive test to asses the homogeneity and integrity of concrete. With this ultrasonic test on concrete, following can be assessed:

1. The homogeneity of the concrete.
2. The presence of cracks, voids and other imperfections.
3. Changes in the structure of the concrete which may occur with time.
4. The quality / compressive strength of concrete of one element in relation to another element / standard requirement.
5. The values of dynamic elastic modulus of concrete.
6. Qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same.
7. Any discontinuity in cross section like cracks, cover concrete delamination etc.
8. Depth of surface cracks.

This test essentially consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz, produced by an electro-acoustical transducer, held in contact with one surface of the concrete member under test and receiving the same by a similar transducer in contact with the surface at the other end. With the path length L, (i.e. the distance between the two probes) and time of travel T, the pulse velocity (V=L/T) is calculated. Higher the elastic modulus, density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material being tested.

Though pulse velocity is related with crushing strength of concrete, yet no statistical correlation can be applied.

The pulse velocity in concrete may be influenced by:

         a.   Moisture content : -

The pulse velocity through saturated concrete is higher by about 2 to 10 % than dry concrete. For high strength and well-compacted concrete the influence is less as against low strength and less compacted concrete.

b.   Type of Aggregate : -

For aggregate having higher specific gravity pulse transmission is faster.

c.   Type of Cement : -

It has observed by the author that, the pulse velocity does not have any significant effect with the different grades of cement.

d.   Type of Mix & water / cement ratio : -

With the change in mix proportion the % change in velocity is less as compared with the % change in compressive strength. For change in w/c ratio, % change in velocity is comparable with % change in the strength.

e.   Reinforcement : -

The pulse velocity through steel is about 5.9 Km/sec. as against 3 to 4.5 Km/sec. of concrete. Thus it can be seen that, for weaker concrete the effect of reinforcement is more than for higher strength concrete. As far possible reinforcement shall be avoided while selecting the test point.

If the axis bar is parallel to the pulse path ( which is a rare case ), the increase in velocity may be to the extent of 5 to 20 % depending on the bar size and the location of bar from the path of the pulse.

If the axis of bars is perpendicular to the pulse path, the increase in velocity may be to the extent of 1 to 5 % depending on the bar size.

f.    Stress :-

At higher stress level significant reduction in pulse velocity is observed.

g.   Path length :-

The minimum path length is governed by the frequency of the transducers.

Gradation of Quality of concrete - Direct & Semi Direct velocity Km/Sec.
Quality of Concrete	M - 20 to M - 25	M - 30 to M - 35	> M - 40
Excellent	More than 4.400	More than 4.600	More than 4.900
Good	3.750 to 4.400	3.900 to 4.600	4.150 to 4.900
Medium	3.400 to 3.750	3.600 to 3.900	3.800 to 4.150
Doubtful	Less than 3.400	Less than 3.600	Less than 3.800
Gradation of Quality of concrete - Indirect velocity Km/Sec.
Quality of Concrete	M - 20 to M - 25	M - 30 to M - 35	> M - 40
Excellent	More than 3.900	More than 4.100	More than 4.400
Good	3.250 to 3.900	3.400 to 4.100	3.650 to 4.400
Medium	2.900 to 3.250	3.100 to 3.400	3.300 to 3.650
Doubtful	Less than 2.900	Less than 3.100	Less than 3.300

Procedure for Ultrasonic Pulse Velocity:

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.