Many times at construction sites, due to various factors such as ineffective batching, poor workability, disproportionate admixture content, inadequate water-cement ratio, poor monitoring etc, the concrete cubes prepared may not produce the desired strength at 7 days, 14 days and 28 days. The project team is left with no other option but to dismantle the structure if the strength is not achieved.
The demolition leads to wastage of money, time and labour as a lot of progress must have been achieved in 28 days (time for concrete cube testing). This leads to time and cost overruns.
In this blog, I am going to tell you all there is to know about Ultrasonic Pulse velocity Test conducted at the construction site for concrete structures.
1. What is ultrasonic pulse velocity test?
An Ultrasonic Pulse velocity test is a non-destructive test where the strength and quality of concrete in the structure is tested in-situ without causing any damage of any structure.
An ultrasonic pulse (also known as Ultrasound, is a sound wave that has a higher frequency than a human ear can hear) is passed through a concrete structure to be tested. The time taken by the pulse to pass through the structure is measured and its velocity is calculated. The quality of concrete can be judged by comparing it with acceptable limits.
What is Ultrasonic Pulse?Also known as Ultrasound, Ultrasonic Pulse is sound waves that have a higher frequency than the human ear can hear. Know more
2. Why is the Ultrasonic Pulse velocity test performed on concrete?
The UPV method has a number of advantages, such as low influence on the structure tested, a simple evaluation procedure, and the ability to evaluate the changes in the internal structure of concrete.
This test is performed to ascertain any of the following:-
- Analyse the Homogeneity of Concrete Material
- Detect Internal flaws such as cracks, Air voids etc
- Quality of concrete and material characterisation
- Estimation of Compressive strength of Concrete Structures. (Enter 5-6 here)
- Determine the thickness of a material
- Estimate the depth of cracks in Concrete.
- Evaluation of concrete degradation depending on heating conditions by ultrasonic pulse velocity
3. Is ultrasonic pulse velocity a laboratory test?
Yes, Ultrasonic pulse velocity can be conducted in the laboratory for experimental purposes. Generally experimental analysis is done to evaluate corelations between fck (Compressive strength of concrete) in Normal Weight (NWC) and Light Weight (LWC) and UPV values. Many studies have been carried out by various scholars over the world by performing Ultrasonic Pulse Velocity Test and Compressive Strength Test on CTM on concrete cubes.
But since Ultrasonic Pulse Velocity is a non-destructive test (Structure is not demolished), it finds its 100% commercial application to find out the quality and strength of old and new made concrete structures. This test is used mostly for those concrete structures which have obtained cracks at site.
Hence, this test is conducted 100% in-situ.
4.What is the meaning of non destructive test?
Non-destructive testing(NDT) is a descriptive term used for the examination of materials and components to enable materials to be examined without altering or destroying the future usefulness of the material and structure. NDT can be used to find, size and locate surface and subsurface defects and defects.
With Non destructive testing, one is able to locate the discontinuities in the material (Concrete in this case) that cannot be identified by the naked eye. For example – internal cracks, air voids, honeycombing etc
NDT is performed while components or materials or structures are in use. It can detect service-related conditions caused by wear, fatigue, corrosion, stress, or other factors.
NDT allows for careful and thorough materials evaluation without the need for deconstruction or damage.
- Fast and highly accurate results.
- It has no harm to environment or humans.
- Practical and easy to carry equipments.
- Requires minimum preparation.
- Thickness can be measured
- Internal discontinuities can be detected.
- Surface must be accessible.
- Linear defects oriented parallel to the sound wave might not be found.
- Equipments are expensive.
- Materials in different shape, small or thin are difficult to inspect.
- Requires a coupling to transfer the sound wave to test material and an effective bond between concrete and transducers.
5. How is the Ultrasonic Pulse Velocity Test performed?
If you prefer to watch a video I have a video with a very good explanation about Ultrasonic Pulse Velocity Test on youtube.
In ultrasonic NDT, transducers convert a pulse of electrical energy from the test instrument into mechanical energy in the form of sound waves that travel through the test piece. The transducer converts the sound waves reflecting from the test piece into a pulse of electrical energy. The test instrument then processes and displays the electrical energy. In effect, the transducer acts as an ultrasonic speaker and microphone, generating and receiving pulses of sound waves at frequencies much higher than the range of human hearing.
- Amplifier may be used to amplify the signal
In this test, electronic transducer(transmitter) produces the ultrasonic pulse at one surface. A similar electronic transducer (receiver) is placed at another surface. It receives the signal sent by the transmitter. The travel time of ultrasonic pulse is calculated.
Then, Ultrasonic pulse velocity is calculated by the following formula:-
There are three different arrangements to find out ultrasonic pulse velocity:-
01. Opposite Faces (Direct Transmission or Cross Probing)
In the Direct method, one transducer is placed at one surface and the other transducer is placed exactly at the opposite surface. This is the most preferable method. This gives the most accurate results because maximum energy is propagated at right angles. Ultrasonic Pulse Velocity Calculation is also easier as the values of L(length of path travelled and time(s) is easily available.
02. Adjacent Faces (Semi-Direct Transmission)
In this method, both transducers are placed on the adjacent surface of the concrete. The receiver receives the ultrasonic pulse coming after striking the molecules of the concrete
03. Same Face (Indirect Transmission or Surface Probing):
It is mostly used when opposite faces are not available for concrete members. Surface probing is not so efficient as cross probing, because the signal produced at the receiving transducer has an amplitude of only 2 to 3 percent of that produced by cross probing and the test results are greatly influenced by the surface layers of concrete which may have different properties from that of concrete inside the structural member. Surface probing in general gives lower pulse velocity than in case of cross probing and depending on number of parameters, the difference could be of the order of about 1 km/sec.
6. Acceptable Criteria for Ultrasonic Pulse Velocity Values for Concrete Quality
As mentioned before the Ultrasonic pulse velocity is calculated by the formula:-
We shall explain this with a small example of a concrete cube:-
Question:- Concrete quality is to be determined in a laboratory setting and correlation is to be established between Compressive strength of Concrete and Ultrasonic pulse velocity test. Concrete cubes of dimensions 150mm x 150mm x 150mm shall be tested for quality using Ultrasonic Pulse Velocity Test. If the time taken on the V-meter is .000036 seconds, calculate the Ultrasonic Pulse Velocity.
Answer: As we know that the concrete cube size is 150mm. Transducers are places on the opposite faces of the cube.
Hence, L = 150mm = .00015km
We will perform the test as mentioned in the infographic and the time period is found to be .000036 seconds
Hence by the below mentioned formula
Ultrasonic Pulse Velocity, v = L/T = .00015/ .000036 = 4.1 km/ sec.
Now, we will tally the above values for ultrasonic pulse velocity in the table below
|S.no||Pulse Velocity by Cross Probing(km/sec)||Concrete Quality Grading|
|2.||3.5 to 4.5||Good|
|3.||3.0 to 3.5||Medium|
Hence, our quality of the concrete is good.
Note – If we had the ultrasonic velocity less than 3, it would have been a case of ‘doubtful’ quality, it would be necessary to carry out further tests.
7. Precautions To Be Taken While Performing UPV Test
Following precautions must be taken while performing the Ultrasonic Pulse Velocity Test at Site.
- The transducers should not be moved while taking the reading, as this can generate noise signals and errors in measurements.
- It is essential that there be adequate acoustical coupling between the concrete and the face of each transducer for effective transmission of the signals.
- A minimum path length of 150 mm is recommended for the direct transmission method involving one unmoulded surface and a minimum of 400 mm for the surface probing method along an unmoulded surface
- Since size of aggregates influences the pulse velocity measurement, it is recommended that the minimum path length should be 100 mm for concrete in which the nominal maximum size of aggregate is 20 mm or less and 150 mm for concrete in which the nominal maximum size of aggregate is between 20 to 40 mm.
- Sufficient number of readings must be taken by dividing the entire structure in suitable grid markings of 30 x 30 cm or even smaller and average should be taken for evaluation of time(s).
- Surface probing in general gives lower pulse velocity than in case of cross probing and depending on number of parameters, the difference could be of the order of about 1 km/sec.
- It is advisable to prevent the two transducer leads from coming into close contact with each other when the transit time measurements are being taken. If this is not done, the receiver lead might pick-up unwanted signals from the transmitter lead and this would result in an incorrect display of the transit time
8.What Affects the accuracy of Ultra Sonic Pulse Velocity Test
(a) Moisture Content and Surface Condition of Concrete:
- The concrete structure surface should be smooth and clean, only then pulse velocity test is possible. The test is not carried out if the concrete surface is a rough and uneven.
- If the moisture content is increased the pulse velocity may be increased. The pulse velocity of saturated concrete may be 2% greater than of similar dry concrete.
(b) The Temperature of Concrete:
- Variations of the concrete temperature between 5 and 30°C do not affect the pulse velocity measurements in concrete.
- At temperatures between 30 to 60°C there can be a reduction in pulse velocity up to 5 per cent.
- Below freezing temperature, the free water freezes within the concrete, resulting in an increase in pulse velocity up to 7.5 per cent.
(c) Path Length, Shape and Size of the Concrete Member
(d) Effect of Reinforcing Bars:
- The pulse velocity measured in reinforced concrete in the vicinity of reinforcing bars is usually higher than in plain concrete of the same composition.
- This is because, the pulse velocity in steel is 1.2 to 1.9 times the velocity in plain concrete and, under certain conditions, the first pulse to arrive at the receiving transducer travels partly concrete and partly in steel.
- The apparent increase in pulse velocity depends upon the proximity of the measurements to the reinforcing bar, the diameter and number of the bars and their orientation with respect to the path of propagation.
9. Why this test is used to find out cracks?
Ultrasonic pulse velocity method is a relatively easy method to determine the presence of cracks in a concrete medium.
Time period in homogenous concrete medium is much less than the time period in a medium where cracks are available. This is because the ultrasonic pulse is not able to pass through the cracks and have to take a different route to reach the receiver.
10. How are crack depths determined by Ultrasonic Pulse Velocity test?
Ultrasonic Pulse Velocity (UPV) is an effective non-destructive testing (NDT) method for quality control of concrete materials and detecting damages in structural components. The UPV methods have traditionally been used for the quality control of materials, mostly homogeneous materials such as metals and welded connections. With the recent advancement in transducer technology, the test has been widely accepted in testing concrete materials. Ultrasonic testing of concrete is an effective way for quality assessment and uniformity, and crack depth estimation.
UPV method can be used for estimating the depth of surface cracks. To do so, each transducers should be place on each side of the crack, for a given distance. Then, the distance between transducers is changed in the same trajectory. The UPV measurement will be repeated for different spacing of transducers.
An estimate of the depth of a visible crack at the surface can be obtained by the transit times through the crack for two different arrangements of the transducers placed on the surface.
An appropriate arrangement is one in which the transmitting and receiving transducers are placed on opposite sides of the crack and away from it. Two values of X are chosen, one being twice that of the other, and the transmission times corresponding thereto are measured.
An equation can be derived by assuming that the plane of the crack is perpendicular to the concrete surface and that the concrete near the crack is of reasonably uniform quality. It is important that the X distance is accurately measured and that the very good coupling is developed between the transducers and the concrete surface.
12. What is the prevailing rate of Ultrasonic Pulse Velocity Test?
Ultrasonic Pulse Velocity Test could be conducted by an NABL Accredited Laboratory at around Rs 4000 per face of the structure. Test are also conducted by the host of this article. You may contact them for more details.
13. Where Can I download the IS code for Ultrasonic Pulse Velocity Test?
You can download the IS code -13311 (Part 1):1992 (Reaffirmed- May 2013) “Non-Destructive Testing of Concrete- Methods of Test (Ultrasonic Pulse Velocity)”
14. Curious FAQs
1. Can UPV be used for alphalt mixtures?
The initiation of adopting ultrasonic pulse velocity (UPV) method for determining dynamic modulus (E) of asphalt mix by utilising ultrasonic pulse propagation time has been recently accepted globally as the unique as well as less time consuming approach. But there are still studies going on as to the use of geometric densities or bulk densities to be used for calculation as propagation of waves largely depends upon the density of the medium and presence of air voids.
Contreras J. Norambuena, Fresno D. Castro, Zamanillo A. Vega, M. Celaya, Vozmediano I. Lombillo, “Dynamic modulus of
asphalt concrete by ultrasonic direct test”, “Journal of NDT and E International”, (2010), 43(7), 629-634.
Reliability of Ultrasonic Pulse Velocity Method for Determining
Dynamic Modulus of Asphalt Mixtures
Debanjan Majhi*, Sandip Karmakar, Tapas Kumar Roy
Indian Institute of Engineering Science and Technology, Shibpur, Howrah-711103, India
2. What is the variation of ultrasonic Pulse velocities with age of Concrete?
There is a very small variation in the difference of values of Ultrasonic Pulse Velocities with time.
For an structure with M25 grade concrete, concrete values can be as follows.
|Age of concrete||UPV Value*|
|3 Days||4.1 km/s|
|7 Days||4.2 km/s|
|14 Days||4.3 km/s|
|28 Days||4.3 km/s|
|90 Days||4.6 km/s|
Note – * These values are observational only and author’s opinion. No supporting documents could be produced for the same. Reader is advised to use these values at his own discretion.
3. What is the difference in values of ultrasonic pulse velocities for concrete and mortar?
ANSWER:- Ultrasonic pulse velocities is higher in concrete than in mortar. At early ages, the ultrasonic pulse velocity of concrete is approximately 16% higher on average than that of mortar. This appears to be due to the influence of coarse aggregate with higher specific gravity than mortar. While examining the relationship between the ultrasonic pulse velocity and compressive strength of concrete over time, the curing conditions of constant
temperature and constant humidity exhibited a smaller error range.
4. Can a relation be determined between Compressive Strength of Concrete (fck) and Ultrasonic Pulse Velocities(v)?
Various studies have been conducted on determination of Compressive strength of Concrete. The relation between UPV and fck is not unique and can be affected by factors such as the type and size of aggregate, physical properties of the cement paste, curing conditions, mixture composition, concrete age and moisture content.
15. Further Numericals For Practice
I have put some numericals for your further understanding and practice. You may write your answers in the comments section below.
Structure 1 – We have a structure whose UPV for 28 day strength was found to be 3.9 km/s.
Structure 2 – We have another structure whose UPV for 14 day strength is 2.9 km/s.
Which structure is expected to perform better over its period?
The length(L) of a structure during Ultrasonic Pulse Velocity test is found to be 30m What is the value of reading in V-meter (time(s)), if the concrete quality was found to be excellent?
Ultrasonic test was carried out on a structure but unfortunately due to site conditions, the test could not be conducted by cross-probing arrangement? If the test was then conducted by surface probing and UPV was found to be 2.5 km/s, what could be the possible value of UPV if it was conducted by cross probing method?