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Ultrasonic Pulse Velocity Test - Method where ultrasonic pulses are transmitted through concrete
Principle
This is one of the most commonly used methods, in which the ultrasonic pulses are transmitted through the concrete. In solids, the particles can oscillate along the direction of sound propagation as longitudinal waves, or the oscillations can be perpendicular to the direction of sound waves as transverse waves. At surfaces and interfaces, various types of elliptical or complex vibrations of the particles occur.
 
A complex system of stress waves is developed which includes longitudinal (Compressional ) , shear ( transverse ) and surface ( releigh ) waves. Piezoelectric transducers are designed to generate longitudinal and transverse (shear) waves. The active element of most acoustic transducers is piezoelectric ceramic These transducers converts electrical signals into mechanical vibrations (transmit mode) and mechanical vibrations into electrical signals (receive mode). The travel time is measured with an accuracy of +/- 0.1 microseconds. Transducers with natural frequencies between 20 kHz and 200 kHz are available, but 50 kHz to 150 kHz transducers are common.
 
Ultrasonic Pulse Velocity testing Instruments
 
This instrument basically is dependent on the Dynamic Young’s Modulus , density , Poisson’s ratio of the material. Under certain specified conditions the velocity and strength of concrete are directly related.
 
 
Thus it is clear from the above equations that pulse velocity is directly proportional to the compressive strength of concrete.
 
Procedure
There are three possible ways of measuring pulse velocity :
 
 
The transducers are placed on the smooth concrete surface to measure the time required for travel. A coupling media such as petroleum jelly, grease are applied to the surface to have good acoustical coupling. The velocity is calculated as
V = L / T
Where, L is the distance between two probes and
T is the time required to travel the distance between two transducers.
 
If the quality of concrete in terms of density, homogeneity, strength and uniformity is good then the velocity obtained is higher
 
In practice it is convenient to establish the relation between strength of cube and pulse velocity. Calibration charts are prepared based on the above relationship by testing sufficient numbers of cubes under various conditions and for various mix designs. This calibration charts also has to applied proper modifications / correction factor for various influencing conditions such as Grade of cement , Moisture content, Type of aggregate, water cement ratio, % reinforcement etc.
 
At CDC we have carried out extensive research on quality assessment and it has been observed that, the quality gradation as per IS – 13311 (part-1)- 1992 is valid ONLY for M – 15 grade concrete. For concrete with more than M – 20, we recommend to grade the quality of concrete as per below given table –
 
 

 

Gradation of Quality of concrete ( as per CDC ) - 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 ( as per CDC ) - 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

 

Influence of Test Conditions
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.
 
Type of Aggregate : -
For aggregate having higher specific gravity pulse transmission is faster.
 
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.
 
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.
 
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.
 
Stress :-
At higher stress level significant reduction in pulse velocity is observed.
 
Path length :-
The minimum path length is governed by the frequency of the transducers.
 

 

Natural frequency transducers kHz
 Minimum path length ( mm )
For concrete with
V = 3.5 Km/sec. V = 4.5 Km/sec.
 V = 3.5 Km/sec.
V = 4.5 Km/sec.
150
 23 30
100
 35 45
 50 70 90
 20 175225

 

There is no significant drop in velocity up to 3.0 m path length. For 3m to 6 m path length, reduction of 5 % in pulse velocity has been reported.
 
Application
The ultrasonic pulse velocity method is used to assess
  • The homogeneity of the concrete
  • The presence of cracks, voids and other imperfections, depth of crack
  • Changes in the structure of the concrete which may occur with time
  • The quality / compressive strength of concrete of one element in relation to another element / standard requirement.
  • The values of dynamic elastic modulus of concrete
 
 Ultrasonic Pulse Velocity testing of Bridge Girder – Direct Method
 
Ultrasonic Pulse Velocity testing of Column– Direct Method
 
 
 
 
 Ultrasonic Pulse Velocity testing of Slab – Direct Method – Probe from Top
 
Ultrasonic Pulse Velocity testing of Concrete Road – Indirect Method for crack depth detection
 
Ultrasonic Pulse Velocity testing of Silo
 
Reliability & Limitations
Ultrasonic pulse velocity has been found to be a valuable and reliable method of detecting interior of concrete member in truly a non destructive way.

If the data about factors influencing pulse velocity as stated above is not available , then, the assessment of compressive strength will have limitations. The accuracy of the strength prediction by this method at about 95 % confidence level is about ± 15 - 20 % depending upon the concrete mix design data available and the correction factors applied. It is recommended to use this test in combination with rebound hammer & core test for new concrete and with core test for old concrete.