Ultrasonic testing of concrete or ultrasonic pulse velocity test on concrete is a non-destructive test to assess the homogeneity and integrity of concrete.
With this ultrasonic test on concrete, following can be assessed:
- Qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same.
- Any discontinuity in cross section like cracks, cover concrete delamination etc.
- Depth of surface cracks.
Ultrasonic Testing of ConcreteUltrasonic pulse velocity test consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz, produced by an electro-acoustical transducer, held in contact with one surface of the concrete member under test and receiving the same by a similar transducer in contact with the surface at the other end. With the path length L, (i.e. the distance between the two probes) and time of travel T, the pulse velocity (V=L/T) is calculated. Higher the elastic modulus, density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material being tested.
Fig.1: Ultrasonic Pulse Velocity Testing InstrumentThough pulse velocity is related with crushing strength of concrete, yet no statistical correlation can be applied. The pulse velocity in concrete may be influenced by:
- Path length
- Lateral dimension of the specimen tested
- Presence of reinforcement steel
- Moisture content of the concrete
Fig.2: Method of propagating and receiving pulsesMeasurement of pulse velocities at points on a regular grid on the surface of a concrete structure provides a reliable method of assessing the homogeneity of the concrete. The size of the grid chosen will depend on the size of the structure and the amount of variability encountered.
Table: 1 – Concrete Quality based on Ultrasonic Pulse Velocity Test
|PULSE VELOCITY||CONCRETE QUALITY|
|>4.0 km/s||Very good to excellent|
|3.5 – 4.0 km/s||Good to very good, slight porosity may exist|
|3.0 – 3.5 km/s||Satisfactory but loss of integrity is suspected|
|<3.0 km/s||Poor and los of integrity exist.|
Table:2 – Identification of Corrosion Prone Location based on Pulse Velocity and Hammer Readings
|Sl. No.||Test Results||Interpretations|
|1||High UPV values, high rebound number||Not corrosion prone|
|2||Medium range UPV values, low rebound numbers||Surface delamination, low quality of surface concrete, corrosion prone|
|3||Low UPV, high rebound numbers||Not corrosion prone, however to be confirmed by chemical tests, carbonation, pH|
|4||Low UPV, low rebound numbers||Corrosion prone, requires chemical and electrochemical tests.|
Detection of Defects with Ultrasonic Test on ConcreteWhen ultrasonic pulse travelling through concrete meets a concrete-air interface, there is a negligible transmission of energy across this interface so that any air filled crack or void lying directly between the transducers will obstruct the direct beam of ultrasonic when the void has a projected area larger than the area of transducer faces. The first pulse to arrive at the receiving transducer will have been directed around the periphery of the defect and the time will be longer than in similar concrete with no defect.
Estimating the depth of cracksAn estimate of the depth of a crack visible at the surface can be obtained by the transit times across the crack for two different arrangements of the transducers placed on the surface. One suitable arrangement is one in which the transmitting and receiving transducers are placed on opposite sides of the crack and distant from it. Two values of X are chosen, one being twice that of the other, and the transmit times corresponding to these are measured. An equation may be derived by assuming that the plane of the crack is perpendicular to the concrete surface and that the concrete in the vicinity of the crack is of reasonably uniform quality. It is important that the distance X be measured accurately and that very good coupling is developed between the transducers and the concrete surface. The method is valid provided the crack is not filled with water. This ultrasonic test is done as per IS: 13311 (Part 1) – 1992.
Procedure for Ultrasonic Pulse Velocityi) Preparing for use: Before switching on the ‘V’ meter, the transducers should be connected to the sockets marked “TRAN” and ” REC”. The ‘V’ meter may be operated with either:
- The internal battery,
- An external battery or
- The A.C line.
Pulse velocity=(Path length/Travel time)v) Separation of transducer leads: 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.