What is an Ultrasonic Pulse Velocity Test?
Ultrasonic pulse velocity test is a method of assessing the quality of concrete by measuring the time of travel of an ultrasonic pulse of longitudinal vibrations, passing through the concrete to be tested.
The first report of the measurement of velocity appeared in the USA in the mid-1940s. It is based on the concept that the velocity of the pulse depends upon the elastic properties of the material.
It is one of the Non-Destructive tests of concrete and is widely used and accepted all around the world. If properly used, a considerable amount of information about the interior of the concrete member can be obtained.
Principle of Ultrasonic pulse velocity test
The ultrasonic pulse that is generated by an electroacoustical transducer is induced into the concrete. When the pulse is induced into the concrete from a transducer, a complex system of stress waves is developed which includes longitudinal (compressional), shear (transverse), and surface (rayleigh) waves.
Among these, Longitudinal waves are the fastest and provide more useful information about the interior of the concrete.
When the quality of concrete in terms of density, homogeneity, and uniformity is good, then the longitudinal waves can travel at a higher velocity and reach the receiving transducer quickly.
When the quality of concrete is poor and if there is any crack, void, or flaw inside the concrete which comes in the way of transmission of the pulses, then the pulse strength is weakened and it passes around the discontinuity, thereby making the path length longer. Consequently, the velocity of the pulse is also reduced and the time to reach the receiving transducer is also increased.
The ultrasonic pulse velocity test consists of the following apparatus
- Electrical pulse generator,
- Transducer – one pair,
- Amplifier, and
- Electronic timing device.
Ultrasonic pulse velocity test procedure
1) The surface of the specimen or concrete structure should be cleaned for the test.
2) If there is a very rough concrete surface, then it is required to be smoothened and leveled to place the transducer.
3) To ensure that the ultrasonic pulses generated at the transmitting transducer pass into the concrete and are then detected by the receiving transducer, there must be an adequate acoustical coupling between the surface of the concrete and the face of each transducer.
Typical couplants used are petroleum jelly, grease, liquid soap, and kaolin glycerol paste.
4) The ultrasonic pulse that is produced is projected into the concrete specimen with the help of a transmitting transducer. The pulse travels through the concrete and reaches the receiving transducer which is held in contact with the other surface of the specimen.
5) An electronic circuit measures the time taken (T) by the pulse to reach the receiving transducer.
Since we already know the path length (L) of the pulse and the time taken (T) by the pulse, the velocity of the pulse can be calculated by the given formula.
Velocity, V = L/T
By using the velocity of the pulse, we can assess the quality of the concrete.
The velocity of the pulse measurement can be used to establish,
- Homogeneity of concrete structure,
- Presence of cracks, voids, and other imperfections
- The quality of one concrete element to the another
- Establish the value of the elastic dynamic modulus of concrete.
Types of transmission
The ultrasonic pulse can be projected into the concrete specimen by three ways of transmission. They are,
- Direct transmission
- Semi-direct transmission
- Indirect or surface transmission
1) Direct transmission
Once the transducer touches the surface of the concrete, it propagates its maximum energy at the right angle to the face of the transmitting transducer. Therefore, the best results are obtained only when the receiving transducer is placed on the opposite face of the concrete member.
A minimum path length of 150 mm is recommended for the direct transmission method.
2) Semi-direct transmission
If the opposite face of the concrete is not accessible, then we can use the semi-direct transmission method. In this method, the transducers are almost placed at 90 degrees to each other on the adjacent face of the concrete.
This arrangement has a sensitivity lesser than the direct transmission method.
3) Indirect or Surface transmission
However, in many situations, two opposite faces of the structural member may not be accessible. In such cases, the receiving transducer is also placed on the same face of the concrete members.
It is also called Surface probing.
Surface probing is not so efficient as direct (or) semi-direct transmission because the signal produced at the receiving transducer has an amplitude of only 2 to 3 percent of that produced by the above two methods.
behaviour of ultrasonic pulse wave
At this point, you may wonder how the velocity of the ultrasonic pulse changes according to the interior of the concrete. Well, Let’s see how,
a) Normal Plain Reinforced Concrete (PCC)
Sound waves travel by the shortest distance between the two transducers.
b) Reinforced Cement Concrete
The sound waves travel through both concrete and steel. However, the sound waves travel through the steel arrive before those through the concrete.
The pulse velocity in steel is 1.2 to 1.9 times the velocity in plain concrete.
c) Shallow crack
If there is any shallow crack present in between the concrete, then the sound waves travel around the crack to reach the other end.
d) Narrow thin crack
If there is any narrow thin crack present in the concrete, then the sound waves are partially reflected and partially transmitted with a large loss of amplitude but only a slight increase in time.
e) Wide crack
If there is any wide crack present in the concrete, then the sound waves are wholly reflected and no signals are received.
If there are any voids present in the concrete, then the sound waves may travel across or around them with an increase in time.
Factors affecting test conditions
Various factors that affect the ultrasonic pulse velocity test results are discussed below.
a) Surface conditions
The concrete surface must be sufficiently smooth to ensure good acoustical contact between the transducer and the concrete surface.
When the concrete surface is rough and uneven, it is necessary to smoothen and level the surface to make the experiment possible.
b) Moisture conditions
The velocity of the pulse through concrete increases with an increase in the moisture content of concrete.
The pulse velocity of the saturated concrete may be up to 2 percent higher than that of a similar dry concrete specimen.
c) Path length
The amount of path length influences the velocity of the ultrasonic pulse.
In site conditions, it may not play a major role but in a laboratory test, the lab specimen should be selected in such a way that it has the minimum path length.
d) Temperature of concrete
Variations in the concrete temperature between 5 and 30°C do not significantly affect the pulse velocity measurements in concrete.
At temperatures between 30 to 60°C, there can be a reduction in the pulse velocity up to 5 percent.
Below freezing temperature, the free water freezes within the concrete, resulting in an increase in the pulse velocity up to 7.5 percent.
When concrete is subjected to stress, it develops small micro-cracks inside the structure which in turn reduces the pulse velocity.
This influence of stress is generally insignificant unless the stress is greater than about 60 percent of the ultimate strength of the concrete.
f) Reinforcing steel
The waves that travel through the steel are of higher velocity than the sound waves that travel in concrete. This led to an increase in pulse velocity.
The increase in velocity depends upon the diameter of the reinforcing bar and its orientation with respect to the path length.
Interpretation of test results
The following table gives an idea about the quality of concrete from the Ultrasonic pulse velocity.
|Pulse velocity (Km/s)||Quality of concrete|
|3.5 to 4.5||Good|
|3.0 to 3.5||Medium|
Hence from the above table, we can get a fair idea of
- Concrete uniformity
- Presence of cracks or voids
- Possibility of segregation and
- The level of workmanship.
Hope you understand everything you need to know about the Ultrasonic Pulse Velocity test and its procedure. If you have any queries (or) if you find this article helpful, let us know in the comment section.