Ultrasonic testing of concrete, also known as ultrasonic pulse velocity test, is a non-destructive method used to evaluate the uniformity and soundness of concrete structures. This test allows for a qualitative assessment of the strength of the concrete and its gradation in various locations within the structural members.
Moreover, the ultrasonic test can also detect any discontinuities in the cross-section of the concrete, such as cracks or cover concrete delamination. By using this method, it is possible to assess the depth of surface cracks as well.
Overall, the ultrasonic test on concrete is a valuable tool in the assessment of concrete structures, as it provides information about the quality and integrity of the concrete without causing any damage to the structure. This information can be used to identify potential areas of weakness, allowing for proactive maintenance and repair.
Ultrasonic Testing of Concrete
The ultrasonic pulse velocity test is a non-destructive testing method used to measure the travel time (T) of an ultrasonic pulse with a frequency of 50 to 54 kHz. The test involves using an electro-acoustical transducer that is held in contact with one surface of the concrete member being tested, while another transducer of similar characteristics receives the pulse at the opposite surface. By measuring the time taken for the pulse to travel through the material (T) and knowing the distance between the two probes (L), the pulse velocity (V=L/T) can be calculated.
The pulse velocity is an indicator of the elastic modulus, density, and integrity of the concrete being tested. Higher values of pulse velocity typically indicate higher values of these properties. The test relies on the fact that the speed of sound in a material is influenced by its density and elastic properties. Therefore, by measuring the pulse velocity, information about the density and elastic properties of the concrete can be obtained without damaging the material.
In summary, the ultrasonic pulse velocity test is a non-destructive method for measuring the travel time of an ultrasonic pulse through concrete. It provides valuable information about the density and elastic properties of the material, with higher pulse velocities indicating higher values of these properties.
Fig.1: Ultrasonic Pulse Velocity Testing Instrument
Pulse velocity is often used as an indicator of the crushing strength of concrete, although statistical correlation may not always be applicable. Several factors can influence pulse velocity in concrete.
One such factor is the path length, which can have a negligible influence as long as it is not less than 100mm for 20mm size aggregate or less than 150mm for 40mm size aggregate. The shape of the specimen being tested, specifically its lateral dimensions, can also affect pulse velocity. As long as the least lateral dimension of the specimen (measured at right angles to the pulse path) is not less than the wavelength of the pulse vibrations (which is about 80mm for a pulse frequency of 50Hz), the shape of the specimen should not significantly impact pulse velocity results.
The presence of reinforcement steel in the concrete can also affect pulse velocity measurements. Steel bars generally have a higher velocity of pulses compared to concrete, which means that pulse velocity measurements taken near reinforcing steel may be higher and not representative of the concrete itself. However, the influence of reinforcement steel is usually minimal if the bars are running perpendicular to the pulse path and the quantity of steel is small compared to the path length.
Moisture content of the concrete can also have an impact on pulse velocity, albeit small but significant. In general, higher moisture content tends to result in increased pulse velocity, with the influence being more noticeable in lower quality concrete.
In summary, pulse velocity in concrete may be influenced by factors such as path length, lateral dimensions of the specimen, presence of reinforcement steel, and moisture content of the concrete. It is important to consider these factors when interpreting pulse velocity measurements and assessing the crushing strength of concrete.
Fig.2: Method of propagating and receiving pulses
Assessing the homogeneity of concrete in a structure can be reliably done by measuring pulse velocities at points on a regular grid on its surface. The size of the grid to be chosen depends on factors such as the size of the structure and the amount of variability encountered. This method allows for a systematic evaluation of the concrete quality across the structure, helping to identify any potential variations or inconsistencies in its composition. By measuring pulse velocities at multiple points on the surface of the structure and analyzing the results, an assessment of the overall homogeneity of the concrete can be made. The grid size for measuring pulse velocities should be determined based on the specific characteristics of the structure being evaluated, such as its size and the expected variability in the concrete quality. This approach provides a quantitative and reliable means of assessing the uniformity of concrete in a structure, aiding in quality control and ensuring the integrity of the concrete construction.
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 1 presents the guidelines that can be used for qualitative assessment of concrete based on the results obtained from the ultrasonic pulse velocity (UPV) test. This test is commonly used to evaluate the quality and integrity of concrete, and the guidelines provided in Table 1 can help to determine the condition of a structural member based on the UPV test results.
However, to obtain a more accurate assessment of the surface condition of a structural member, the pulse velocity test can be combined with the rebound number test. This approach involves using both tests together to obtain more comprehensive information about the concrete’s condition.
Table 2 provides guidelines for identifying locations where corrosion may occur by combining the results of the pulse velocity and rebound number tests. By using both tests together, it is possible to identify areas of concrete that are more susceptible to corrosion, which can help to inform maintenance and repair decisions. Overall, using a combination of tests can provide more detailed information about the condition of concrete and can help to ensure the safety and longevity of structures.
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 Concrete
When an ultrasonic pulse moves through concrete and encounters a concrete-air boundary, there is an insignificant amount of energy transmitted across this boundary. This means that any crack or empty space filled with air that lies directly between the transducers will obstruct the direct beam of the ultrasonic pulse, but only if the void has a projected area larger than the area of the faces of the transducers.
In the case where the void does obstruct the direct beam of the ultrasonic pulse, the first pulse that reaches the receiving transducer will have been redirected around the periphery of the defect. This redirection causes the time it takes for the pulse to arrive at the receiving transducer to be longer than it would be in concrete without a defect.
Therefore, in concrete with air-filled cracks or voids larger than the area of the transducer faces, the ultrasonic pulse will be obstructed, causing a delay in the time it takes for the pulse to reach the receiving transducer. This delay can be detected and used to identify the presence and location of the defect in the concrete.
Estimating the depth of cracks
A technique to estimate the depth of a visible crack on a surface involves using transducers and measuring the transit times across the crack in two different arrangements. One suitable arrangement involves placing the transmitting and receiving transducers on opposite sides of the crack, away from it. Two different values of X are chosen, with one being twice the size of the other, and the corresponding transmit times are measured. Assuming that the crack runs perpendicular to the concrete surface and that the concrete near the crack is of consistent quality, an equation can be derived. It’s crucial to measure the distance X with accuracy and to ensure a solid connection between the transducers and the concrete surface. This method is reliable only if the crack isn’t filled with water. This ultrasonic test follows the guidelines of IS: 13311 (Part 1) – 1992.
Procedure for Ultrasonic Pulse Velocity
To prepare the ‘V’ meter for use, the transducers should be connected to the sockets labeled “TRAN” and “REC” before switching it on. The ‘V’ meter can be operated using either the internal battery, an external battery, or the AC line.
A reference bar is provided with the instrument to check its zero. The pulse time for the bar is engraved on it. Before placing the transducers on the opposite ends of the bar, a smear of grease should be applied to their faces. Adjust the ‘SET REF’ control until the instrument read-out shows the reference bar transit time.
For path lengths up to 400mm, it is recommended to select the 0.1 microsecond range for maximum accuracy. To determine the pulse velocity, carefully measure the path length ‘L’ and apply couplant to the transducer surfaces. Press the transducers firmly onto the material surface and avoid moving them while taking a reading to prevent generating noise signals and errors in measurements. Continue holding the transducers in place until a consistent reading appears on the display, which indicates the time in microsecond for the ultrasonic pulse to travel the distance ‘L’. The mean value of the display readings should be taken when the units digit oscillates between two values.
It is advisable to separate the two transducer leads to prevent them from coming into close contact with each other during the transit time measurements. Failure to do so may result in the receiver lead picking up unwanted signals from the transmitter lead, leading to an incorrect display of the transit time.