Ultrasonic pulse velocity test for concrete : objective, principle, equipment, procedure, application

Ultrasonic pulse velocity

Ultrasonic pulse velocity test is done to check the quality of the targeted concrete or rocks. This test is in-situ and non-destructive test (after the test, concrete specimen will not be broken). 
Sometimes it is necessary to check some properties of the concrete without damaging the constructed structure. Ultrasonic pulse velocity test is suitable to check those parameters in concrete after the construction is done. 
This test can assess the parameters like, strength, density, elastic modulus, surface absorption and hardness, reinforcement detail, presence of cracks or void, etc.

Why do we need this test?

Quality of fresh concrete is depends upon cement types, aggregate, water-cement ratio, compaction, environmental condition, curing, etc. 
We can check the workability by slump test, compressive strength of that concrete instantly, but we all know after that making concrete in actual site may be different as compare to concrete made in laboratory due to some factors like curing, compaction, etc.
We cannot achieve desired strength in concrete due to that factor. On the other hand, we cannot achieve desired quality in structure after 3 or more years due to factors like rain, wind, disasters, acid attack, etc. 
We need to do quality assessment in structure after certain period of time without damaging the structures, Thus, this test can be very useful in this condition. 

Objective

Upv test can be done for following objective,
  • The homogeneity of the concrete,
  • To check the detail of presence of cracks or void and other imperfection,
  • Changes in the properties of the concrete structures which may be occurs with time,
  • To check the quality of specific element or member compare to other members.
  • To check the value of dynamic elastic modulus in structure.

Principle

The ultrasonic pulse is generated by electro-acoustical transducer which are transmitted through the concrete. While pulses are transmitted through the concrete, it undergoes multiple reflections at the limits of the different phases of the material inside the concrete.
A complex system of stress waves namely, longitudinal (congressional), shear (transverse) and surface (Rayleigh) are generated.
The velocity of this waves does not depend on the geometry of the element, it depends upon density, Poisson's ration and modulus of elasticity.
Higher the obtained velocity higher the quality of concrete in terms of uniformity, homogeneity and density. Lower velocity indicates the presence of cracks or honey-combing and non-uniformity in concrete.
While there is presence of cracks in the concrete, generated velocity has to travel longer time to reach transducer and thus, indicates the lower velocity.
The transducer first converts electrical signals in to the mechanical signal and then mechanical signals in to electrical signal after reaching the receiver.
The time taken by the pulse is measured within the accuracy of ± 0.1 microsecond. Transducer of frequencies between 50 kHz to 100 kHz are generally used.

Ultrasonic pulse velocity test equipment

Ultrasonic pulse velocity testing equipment are as follow,
  • Electrical pulse generator,
  • Transducer (one pair),
  • Electronic timing device and
  • Amplifier.
Transducer


Ultrasonic pulse velocity equipment
UPV testing equipment


Transducer arrangement

There are three different ways to measure the pulse velocity as shown in figure.
  • Direct transmission
  • Semi-direct transmission
  • Indirect transmission

upv test equipment
Arrangement of transducer

Due to some undesired site condition we may not be able to arrange the transducer in the opposite face (direct transmission), For example, slab. In that case, we can use alternative method of measuring pulse velocity.
Among all these method, direct transmission method gives more reliable result and indirect transmission gives least satisfactory results.

Ultrasonic pulse velocity test procedure

The transducers are held in position according to method (direct, semi-direct or indirect). 
One transducer is transmitter and another one is receiver. The length between transducer (L) is known. 
The electrical signal coming from transmitter and passes through concrete and reach to the receiver. 
The time total time of signal from transmitter to receiver is measured and is known as transit time (T). Finally pulse velocity(V) is measured from the following equation.

                                                               V = L/T        ; V = Pulse velocity in km/sec,

                                                                                       L = Path length in cm and

                                                                                       T = Transit time.

The dynamic young's modulus of elasticity can be measure with the help of pulse velocity and Poisson's ratio from the follow equation, given in figure.

Equation for pulse velocity and young's modulus of elasticity
Equation for dynamic young's modulus of elasticity.

Acoustic coupling is provided between the surface of concrete and the transducers, to ensure that the generated signal are coming from the transducers are passes through the concrete. 
Petroleum jelly, grease, kaolin glycerol paste and liquid soap are typically used as couplants. 
                 

     Results based on upv test

The quality of the tested concrete specimen can be checked by the following table.

Grading of concrete quality
Quality of concrete
[ IS: 13311, PART-1]

     Testing condition 

          Surface condition and moisture content

Before test is started make sure the surface of the concrete is smooth and even (where the transducers are held) for achieving results and if not then we can use carborundury stone to smoothen the surface of the concrete. also, couplants should be used.
As the moisture content increase in concrete the pulse velocity will also increase.
In saturated concrete, the pulse velocity is 2-3% higher as compare to dry concrete.

          Path length

Due to heterogeneity of concrete, the path length should be enough long to avoid the errors. The path length in field are enough long which will not cause any difficulty as compare to testing cubes in laboratory.

          Temperature

The temperature between 5 to 30℃ does not affect the pulse velocity, temperature around 30 to 60℃ can reduce the pulse velocity about 5%.
Below the freezing temperature, water in concrete will freeze and it will increase the pulse velocity.

          Reinforcing bars

Pulse velocity in reinforcing concrete is 1.2 to 2 times higher in the comparison of that plain concrete.
If the reinforcing bars are parallel to the direction of pulse path it will increase the pulse velocity and if the reinforcing bars of low quantity and it is the perpendicular to the propagation of the pulse path then the pulse velocity will be small.  

Application

  • To measure the uniformity of the concrete.
  • To check the quality of the concrete.
  • To detect the honeycombing, cracks or void in the existing structures as well as new concrete.
  • To estimate the strength of the concrete.
  • To measure the thickness of members or slab.
  • To measure the modulus of elasticity.
  • To study durability aspect of the concrete.
  • In case of structure audit.
  • To check the development of the cracks or void in dam structure.

Here are some standard codes for the practice of ultrasonic pulse velocity test,
  • IS: 13311 PART-1 (1992)
  • ASTM C597
  • BS 1881 PART-203.


          










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