What is Ultra-sonic pulse velocity meter ? Describe its use and application ?

Ultra-sonic Pulse Velocity Meter

INTRODUCTION

 

UPV (Ultra-sonic pulse velocity meter) measurement through concrete was initiated in the USA in the mid-1940s and later adopted everywhere as NDT on concrete

 

• UPV methods basically consists of transmitting the mechanically generated pulses (in the frequency ranges of 20-150/s) through concrete with the help of electro-acoustic transducers and measuring the velocity of the longitudinal waves generated by the applied pulses

 

• UPV is correlated to many desirable information pertaining to concrete, such as:

 

– Elastic modulus, strength, and uniformity of concrete

 

– Layer thickness, cracking, honeycombing, and deterioration of concrete

 

If the method is properly used by an experienced operator, a considerable amount of information about the interior of a concrete member can be obtained. However, since the range of pulse velocities relating to practical concrete qualities is relatively small (3.5–4.8 km/s), great care is necessary, especially for site usage.

 

Furthermore, since it is the elastic properties of the concrete which affect pulse velocity, it is often necessary to consider in detail the relationship between elastic modulus and strength when interpreting results.

 

Theory of pulse propagation through concrete

 

• Following three types of waves are generated by an impulse applied to a mass:

 

I. Surface waves having an elliptical particle displacement and slowest.

 

II. Shear or transverse waves with particle displacement at right angles to the direction of travel and faster than the surface waves.

 

III. Longitudinal or compressive waves with particle displacement in the direction of travel and fastest providing more useful information

 

• Electro-acoustical transducers used for UPV measurements on concrete produce longitudinal waves which, as mentioned above, are fastest and provide more useful information

 

• UPV depends primarily upon the elastic properties of the material and found to be almost independent of geometry.

 

 

 

Pulse velocity equipment and use

 

• The UPV equipment is used for the following purposes:

 

I. Generating a pulse mechanically.

 

II. Transmitting the generated pulse through concrete.

 

III. Receiving and amplifying the pulse. Measuring and displaying the transit time

UPV TEST PROCEDURE:

 

There are following test procedure should be determined are as follows:-

 

I. Coupling of transducers

 

II. Arrangement of transducers

 

III. Selection of transducers

 

IV. Equipment calibration

 

V. Velocity determination

 

I. Coupling of transducers

 

• A good acoustic coupling between the concrete surface and the face of the transducers is essential for reliable results

 

• Coupling is provided by a medium such as petroleum jelly, liquid soap or grease

 

• Air pockets must be eliminated, and it is important that only a thin separating layer exists-any surplus must be squeezed out

 

• A light medium such as petroleum jelly or liquid soap is found to be the best for smooth surfaces

 

• A thicker medium such as grease is recommended for rough surfaces which have not been cast using smooth shutters

 

• In case of very rough or uneven surfaces, grinding or preparation with plaster of Paris or quick-setting mortar may be necessary before coupling

 

 

II. Arrangement of transducers

 

Following are three basic ways in which the transducers may be arranged:

 

I. Transducers coupled on opposite faces (direct transmission).

 

II. Transducers coupled on adjacent faces (semi-direct transmission).

 

III. Transducers coupled on same faces (indirect transmission).

 

The above mentioned arrangements of transducers are shown below:

 

 

a. The direct method is the most reliable from the point of view of transit time measurement as well as path length measurement

 

b. The semi-direct method is less reliable than the direct method and should only be used if the angle between the transducers is not too great, and if the path length is not too large.

 

c. The indirect method is the least accurate because received signal is subject to errors due to scattering of pulse by discontinuities.  

III. Selection of transducers

 

Selection of the transducers for UPV test mainly depend on the following:

 

• Whether point contact is needed or not, as in case of rough or curved surface, the exponential probe transducer is suitable.

 

• The required transducer frequency, which is related to the dimensions of the member under test, for example, for 10 m path length a transducer should have a frequency of 54 kHz and the transducer should have a frequency of 82 kHz for a path length of 3 m (higher frequency required for lower energy output). IV. Equipment calibration

 

• Before use, the time delay adjustment must be made by setting the zero reading for the equipment. For this, the equipment should regularly be checked during and at the end of each period of use.

 

• The time delay adjustment is carried out with the help of a calibrated steel reference bar which has a transit time of around 25 μs.

 

• It is recommended that the accuracy of transit time measurement of the equipment should also be checked by measurement of a second reference specimen, preferably with a transit time of around 100 μs. V. Velocity determination

 

• Determination of pulse velocity requires measurement of the transit time using the UPV equipment with an accuracy of ± 0.1 μs and measurement of path length with an accuracy of ± 1%

 

• The transit time readings are repeated by complete removal and reapplication of transducers to obtain a minimum value for the transit time, which is taken as final reading

 

• Once the transit time and the path length are measured, the pulse velocity is determined by dividing the path length by the transit time, as follows: V = path length/transit time

 

• In case of direct transmission, the path length is just the thickness of the member under test. In case of semi-direct transmission, the path length is taken as distance between center to center of transducer faces.

 

• In case of indirect transmission, the pulse velocity is determined by recording the transit times by placing the receiver at different distances from the fixed position of the transmitter and then obtaining the mean pulse velocity as inverse of slope of a best fit line plotted using spacing versus transit time data.

 

APPLICATION OF UTV TEST RESULTS

 

Monitoring strength development or deterioration in laboratory specimens subjected to varying curing conditions or to aggressive environment. 

Measurement of in-situ concrete uniformity. 

Detection of cracking and honeycombing in in-situ concrete. 

Measurement of crack depth. 

Strength estimation of in-situ concrete 

Assessment of in-situ concrete deterioration 

Measurement of layer thickness in in-situ concrete 

Measurement of elastic modulus of in-situ concrete  

ADVANTAGES AND LIMITATIONS

 

ADVANTAGES

 

UPV test is truly non-destructive and can be performed both in lab as well as in-situ 

UPV measurement has been found to be a valuable and reliable method of examining interior of a body of concrete 

Modern UPV test equipment is robust, reasonably cheap and easy to operate, and reliable even under site conditions

 
LIMITATIONS

Operators must be well trained and aware of the factors affecting the readings 

It is essential that the test results are properly evaluated and interpreted by experienced engineers who are familiar with the technique. 

The UPV method only gives an estimate of the extent of cracking within concrete, however, the use for detection of flaws within the concrete is not reliable when the concrete is wet 

The UPV test is least reliable for estimation of strength of concrete because of the many factors affecting calibrations 

Application of the UPV test for determining depth of fire damage is limited to only the portions which are free from cracking due to very high temperature