Ultrasonic Pulse Velocity Test
The Ultrasonic pulse velocity test is performed on concrete to assess the quality of concrete by passing ultrasonic pulse velocity through it as per IS: 13311 (Part 1) – 1992.
The method consists of measuring the time of travel of an ultrasonic pulse passing through the concrete. The high velocity of ultrasonic pulse velocity and lower time of travel indicates a good quality of concrete in terms of density, uniformity, homogeneity, etc.
Objectives of Test
The ultrasonic pulse velocity test used to measure
- The concrete homogeneity,
- The availability of cracks, voids, and other imperfections,
- Structural changes of concrete which may occur with time,
- To assess concrete quality,
- To compare the quality of one member of concrete with another,
- To measure the value of the dynamic modulus of elasticity.
Read More: Setting Time Of Concrete & Method Of Testing
Principle of UPV Test
The ultrasonic pulse is generated by an electro-acoustical transducer. When the pulse is induced into the concrete from a transducer, it undergoes multiple reflections at the boundaries of the different material phases within the concrete.
A series of ways are induced in concrete which includes longitudinal (compression), shear (transverse) and surface (Rayleigh) waves. The transducer placed on another side detects fasted longitudinal waves.
As the velocity of ultrasonic pulse primly depends on the shape, size, and geometry of concrete material through which they pass and depends only on its elastic properties. This method is suitable for assessing the structural properties of concrete.
As per the ultrasonic pulse velocity principle when concrete quality is good in terms of density, homogeneity, and uniformity, it comparatively obtains higher velocity in concrete. On another hand, if the concrete quality is poor, the velocity obtained is lower.
In case, if the concrete has cracked, voids inside, the strength of pulse velocity is reduced and it passes around discontinuity, thereby follow the longer path for travel and Consequently, lower velocities are obtained.
The velocity of the pulse depends on concrete material or mix proportion. Aggregate density and modulus of elasticity are also had a significant effect on the velocity of the pulse.
The Instruments and device used for ultrasonic pulse velocity test:
a) Electrical pulse generator,
b) Transducer – one pair,
c) Amplifier, and
d) Electronic timing device.
Any suitable type of transducer operating within the frequency Lange of 20 kHz to 150 kHz ( see Table 1 ) may be used. Piezoelectric and magnetostrictive types of transducers may be used, the latter being more suitable for the lower part of the frequency range.
Table – 1 Natural Frequency of the transducer for different path lengths
|Path Length (mm)||Natural Frequency of Transducer (kHz)||Minimum Transverse Dimension of Members (mm)|
|Up to 500||150||25|
|500 – 700||> 60||70|
|700 – 1500||> 40||150|
|Above 1500||> 20||300|
1. Electronic Timing Device
The electronic timing device is capable of measuring the time travel of pulse at the transmitting transducer and the onset of its arrival at the receiving transducer.
2. Performance of the Assembly of Apparatus
The Ultrasonic pulse velocity apparatus used test is capable of measuring pulse velocity to the accuracy of +1 % over a range of 20 microseconds to 10 milliseconds.
For this, it is essential to check the accuracy of the apparatus by making measurements on two standard reference specimens in which the pulse transit times are known accurately.
The check should be done on two steel specimens and should have minimum transit times of about 25 microseconds to 100 microseconds respectively. Generally, this transit time is specified by the supplier of apparatus to an accuracy of 0.2 microseconds.
It is recommended that shorter steel specimens should be used to set zero for equipment and longer steel should be used to check the accuracy of transit time measurement of the apparatus.
Ultrasonic Pulse Velocity Test Procedure
During the test, the transducer held in contact with one surface of concrete and it traverse a known path length Q in the concrete and then an electrical signal passed the second transducer held in contact with the other surface of the concrete member and the transit time (T) of the pulse to be measured.
The pulse velocity (V) is given by:
V = L/T
Once the path is discovered by the transducer the pulse velocity is transmitted at a right angle to the surface of the concrete to get the best result.
It is essential that pulse velocity propagated or transmission by the transducer is detected by receiving transducer. To ensure that they keep sufficient coupling between the concrete and the face of each transducer. Generally, copulates are petroleum jelly, grease, liquid soap, and kaolin glycerol paste.
In case, if there is a very rough concrete surface it is essential to smoothen it for placing transducer.
Influence of Test Condition on Ultrasonic Pulse Velocity Test
1) Surface condition and Moisture Content
Concrete surface must smooth as it affects the test result.
In most cases, the concrete surface is finished smoothly against which transducer can be easily placed by the use of a coupling medium and by pressing the transducer against the concrete surface. In case the concrete is not finished well it necessary to make it smooth before the test to make the pulse velocity measurement possible.
It is a fact that the velocity of the pulse increases with the increase in moisture condition of the concrete. This influence is more for low-grade concrete than high-grade concrete. it is estimated that the velocity of pulse in saturated concrete is 2 to 3 % more than of dry concrete.
2) Influence of Path length
As we know that concrete is heterogeneous material, so it essential to select as long as a path for pulse velocity test to avoid error due to concrete heterogeneous property.
On-site it is carried out on thick structural members as the pulse velocity measurements are not much affected in such a situation. However, while performing the test in laboratory path length is shorter due o small specimens and results are affected.
3) Influence of Temperature of Concrete
Pulse velocity tests generally not affected by temperature ranges from 5°C and 30°C in concrete. The test result may affect when the temperature is between 30 to 60°C, there is a reduction in pulse velocity up to 5 percent. At freezing, temperature test results affected up 7.5 % as water within concrete also gets a freeze in it.
4) Influence of Stress:
When concrete is subjected to high stress which higher than normal stress design for a concrete member can create small crack within the concrete and which affects the result of the pulse velocity test. The influence of stress comes into existence when stress in concrete exceeds more than 60 % of the ultimate strength of the concrete.
5) Effect of Reinforcing Bars
The presence of reinforcement in concrete affect pulse velocity test result. It is because of the fact that the pulse velocity in steel is 1.2 to 1.9 times the velocity in plain concrete.
Interpretation of Test Result
The ultrasonic pulse velocity of concrete primly depends on concrete density and modulus of elasticity property. Also, depends on concrete ingredients quality such as cement, sand, and aggregate, there mix proportion as well as the method of placing, compaction, and curing of concrete.
For example, if the compaction of concrete is not done properly, or if concrete segregation is done during placing or there are internal cracks or flaws. For such condition pulse velocity in concrete will be lower although the same mix proportion and materials are used.
However, if the comparison is made between different components of the building which are made of the same material and proportioning, the evaluation of quality check becomes easy and more accurate. Considering the velocity parameters of the concrete following are concrete quality grading classification.
Table – 2 Velocity Criterions for Concrete Quality Grading
|Sr. 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|