Ultrasonic Pulse Velocity Test
The ultrasonic pulse velocity test is a non-destructive test used to determine the quality of concrete on site. This test basically involves the assessment of the velocity of electronic pulse passing through the concrete from a transmitting transducer to a receiver transducer.
The principle of the ultrasonic pulse velocity test is that the velocity sound in a solid material is a function of the square root of the ratio of its modulus of elasticity E to its density P. The density and the elastic properties of the material are related to its quality and strength, respectively.
The velocity of electronic pulses ranges from 3 km/h to 5 km/h, on average. The frequency of the electronic pulse generated varies from 15 kHz to 175 kHz.
The apparatus used for the ultrasonic pulse velocity test consists of a transmitter and a receiver which are held against two faces of concrete. The electronic pulse of ultrasonic frequency is generated by the apparatus which is transmitted through concrete using the transmitter. The transmitted pulses are received by the receiver, which is on the other face of the concrete. When the electronic pulses are received, they are also recorded by the apparatus.
The travel time of electronic pulses is measured when they travel from the transmitter to the receiver. The length of the path traveled by pulses is divided by the travel time of pulses which gives the average velocity of wave propagation. The pulse velocity is correlated to the strength of concrete. The higher the pulse velocity, the greater is the strength of concrete.
| Ultra Sonic Pulse Velocity Km/s | Quality Of Concrete |
| Below 3.0 | Doubtful |
| 3.0 to 3.5 | Medium |
| 3.5 to 4.5 | Good |
| Above 4.5 | Excellent |
Arrangement Of Transducer
The measurement of electronic pulse velocity through concrete can be done in three different ways. They are as follows:
1. Direct Transmission – Opposite Faces
The direct method of transmission is the most reliable method of measuring pulse velocity through concrete since the maximum pulse energy is transmitted at right angles to the face of the transmitter. Also, the path of travel of the pulse can be clearly and accurately measured as it can be easily defined.
2. Semi–Direct Transmission – Adjacent Faces
The semi-direct method of transmission is used relatively less than the direct method as the results of this method are not accurate, but satisfactory. It is because the angle between the transducers and the path length is not that large.
3. Indirect Transmission – Same Face
The indirect method of transmission is the least used method as the results from it are least satisfactory and inaccurate. Due to this, the indirect method is rarely used.
Factors Affecting The Measurement Of Pulse Velocity.
The factors affecting the measurement of pulse velocity are as follows:
1. Smoothness Of Contact Surface.
The surface of contact of concrete, where the transducer is to be placed, should be even and smooth enough. If the contact surface is rough or uneven, it can be made smooth using a tool. In addition to that, the use of coupling medium such as jelly, oil, or a thin film of soap is recommended.
2. Length of path
To avoid inaccuracy or any errors due to heterogeneity of concrete, the path length should be sufficiently long. The length of path may be anywhere between 300mm in the case of concrete columns to 23m in mass concrete dams.
The length of path is directly related to the accuracy of results and errors. Longer the length of path, the more accurate results you get with minimum or no errors.
In some cases, when the concrete samples are to be evaluated in the laboratory, it usually has a smaller length of path, which affects the pulse velocity readings.
3. Moisture Content Of Concrete
The pulse velocity through concrete increases, when the moisture content of concrete increases. The pulse velocity of saturated concrete is said to be higher than that of relatively dry concrete approximately by 2% to 3%.
4. Temperature Of Concrete
The optimum temperature range for accurate readings of pulse velocity is 5 degree Celsius to 30 degree Celsius. In this range, the pulse velocity readings are not affected much.
The pulse velocity increases when the temperature of concrete goes below 5 degree Celsius. As the temperature goes below 5 degree Celsius, the water content present in concrete starts freezing, resulting in an increase in pulse velocity.
On the other hand, when the temperature of concrete rises above 30 degree Celsius, the pulse velocity decreases by up to 5%.
5. Presence Of Steel Reinforcement In Concrete
The pulse velocity of Reinforced Cement Concrete (RCC) is approximately 1.2 to 2 times the pulse velocity of plain cement concrete (PCC). When the path of pulse is parallel to the steel bars, the pulse velocity will be higher.
Applications
The applications of ultrasonic pulse velocity test are as follows:
1. To measure the uniformity of concrete.
2. To discover the development of cracks in various concrete structures and also to check deterioration due to frost action or chemical action.
3. To find out the voids or honeycomb in concrete structure.
4. To determine the strength of concrete.
5. To measure the thickness of concrete slabs and concrete roads.
6. To measure elastic modulus.
7. It is useful for studies conducted on the durability of concrete.
8. It is used for strength development monitoring.
9. It is also useful in pre-casting and also in deciding the removal of formwork.
Advantages Of UPV Test:
The advantages of the ultrasonic pulse velocity test are as follows:
1. The electronic pulses have high penetrating power.
2. It is highly sensitive.
3. It is highly accurate.
4. It has a wide variety of applications.
5.The apparatus of ultrasonic pulse velocity test is portable. Hence it is easy to carry from one site to another.
Disadvantages Of UPV Test:
The disadvantages of ultrasonic pulse velocity test are as follows:
1.This test is done manually and requires careful attention from experienced technicians.
2. It is difficult to inspect the irregular parts of the apparatus.
3. Couplants like soap, oil, and jelly are required at the contact surface.
4. The elements which are going to be tested should be water resistance.
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