Ultrasonic pulse velocity test is an example of the non-destructive tests of concrete. Non-destructive tests (NDT) and destructive tests (DT) are the tests done on hardened concrete. Concrete is the oldest and most important construction material in the world. For confirming, the structural stability, strength, durability, and condition, testing of concrete plays an important role.
The Non-destructive testing of concrete is the method of evaluating concrete structure without damages. This helps in assuring the quality and condition of structures. The strength of the concrete depends on certain other properties such as hardness, density, curing conditions, quality of ingredients, water-cement ratio, etc.
This article is about Ultra sonic pulse velocity test or UPV test.
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Ultra sonic Pulse Velocity test (UPV Test)
UPV tests are the most convenient and fast method of concrete testing. UPV results interpret the time of travel of ultrasonic pulse waves for evaluating the concrete quality. The frequency of the Ultrasonic pulse wave is to be maintained around 50-55 kHz. The pulse generator of the UPV tester produces the pulses and is allowed to pass through the concrete. The pulse velocity is calculated by measuring the traversing distance and the time. Higher velocity means the concrete has a higher elastic modulus and density.
UPV tests detect the cracks and flaws in the structure. Major differences in pulse velocity values indicate defective and deteriorated concrete. The velocity of waves relates to the density of concrete. So this test has a great potential to check concrete quality.
Relevant IS code
- IS-13311 (Part 1):1992 (Reaffirmed- May 2013) “Non-Destructive Testing of Concrete- Methods of Test (Ultrasonic Pulse Velocity)”
Ultrasonic Pulse Velocity tester
The equipment used for measurement of Ultrasonic pulse velocity is called UPV tester.Ultrasonic pulse velocity tester constitutes the following accessories
- Electrical Pulse generator
- Pair of Transducers (probes)
- Electronic timing device
Principles of Ultrasonic Pulse Velocity test
The Electrical pulse generator generates pulse and is transmitted through the transducer of the UPV tester. The pulse undergoes multiples reflections through the concrete surfaces. The pulse velocity is measured as per the below formula.
Pulse velocity, V = L/T
where L is the traverse distance, T is the time for the receiver to receive the pulse
UPV test is independent of the geometry of the material. Higher velocity means better concrete strength and vice versa. The ultrasonic pulse velocity test is one of the dynamic tests of concrete.
The main objectives of the ultrasonic pulse velocity test are
- To learn the homogeneity of the concrete.
- Determines the presence of cracks, voids and imperfections.
- To calculate the elastic modulus of concrete.
- Finds the quality of concrete relative to the standard requirements.
- To determine the age of concrete.
Factors affecting Ultrasonic pulse velocity test
The UPV test detects cracks and helps in the development of the structures. But several factors affect the measurements of pulse velocity. Therefore, the pulse velocity is not a general indicator of compressive strength. The factors which affect the UPV test are as follows.
- Presence of reinforcement
- Water content
- Mix proportion
- Temperature of concrete
- Concrete age
- Stress level of concrete
The suitable type of transducer for the UPV test is Piezoelectric and magnetostrictive types. Also, its frequency range should be 20 kHz to 150 kHz. The electronic timing device measures the time with an accuracy of 0.1 microseconds.
The transducer transmits the waves that pass through the concrete surface. The pulse waves get converted into electric signals which is received by the receiver transducer. The traversing measurement will be shown as ( L).
The Electronic timing device measures the receiving time of the signals. The time is denoted as (T).
Pulse velocity (v) = L/T
There are three common methods for doing UPV tests. They are direct method and indirect method.
The maximum energy is transmitted at right angles to the face of the transmitter. Therefore for getting the best results the receiving transducer shall be placed on the opposite side of the transmitting transducer. This method is known as the direct method or cross probing.
The opposite side of the structure may not be accessible in some cases. In this case, the receiving transducer and transmitting transducer is placed on the same face of concrete members. This method is called the indirect method or surface probing. This method is not efficient as the direct method. The test results are mostly influenced by the surface concrete which got different properties in comparison to the core concrete of structural members.
The ultrasonic pulse velocity is related to the density and modulus of elasticity of concrete. This in turn depends on the ingredients, mixing, placing methods, compaction, and curing of concrete, casting temperature, etc.
Lack of compaction and concrete segregation are the prime factors responsible for internal cracks and pockets. These concrete faults result to lower pulse velocity values. However, the lab tests might have confirmed a well-designed concrete.
The range of pulse velocity in the direct method is as shown below.
- Above 4.5 Excellent
- 3.5 to 4.5 Good
- 3.0 to 3.5 Medium
- Below 3.0 Doubtful
The final assessment of compressive strength from UPV is not the only criteria to finalise the strength of concrete. The strength is confirmed by co-relating an estimate of compressive strength derived from the same ingredient mix and conditions. The values of the UPV test and site tests with the same ingredients can be co-related. The values may have a difference of around 20% when compared with actual UPV strengths.