Measuring concrete thickness with ultrasonic pulse echo (Pundit 250 Array)

Evaluate concrete thickness variations

Ultrasonic pulse echo technology is excellent for detecting variations in the thickness of concrete structures. In particular, the panorama B-Scan function of the Pundit 250 Array is ideally suited for this.

This example shows how a typical structural assessment can be carried out. In this case, there is a relatively thick section of concrete. The transmission range of pulse echo instruments depends on the quality of the concrete and the amount of reinforcement steel. The current practical range of commercially available instruments is typically around 1 meter. It is apparent that the structure here is close to that limit.

The first step is to do simple spot checks to establish whether or not it is even possible to see the back-wall echo in the thick section of the structure. The same procedure would be used if the task was to locate objects such as pipes, tendon ducts, delaminations or honeycombing within the structure. Begin with rapid spot checks to locate anomalies then carry out more detailed scans in the areas of interest.

B-Scan with back wall echo

In the B-Scan, (2nd image) the back wall echo can be seen at a depth somewhere between 0.8 m and 0.9 m. For ultrasonic pulse echo measurements, depth information is determined by pulse velocity. For this scan, the pulse velocity (2690 m/s) was estimated by measuring surface velocity. This is typically the starting point for any investigation and must often be used when access is only available from one side and there is no accurate documentation available on the structure. However, it is well known that the pulse velocity at the surface of a concrete structure can differ significantly from the pulse velocity when measured directly through the structure. According to literature, this difference can be as much as 20%. Better results can be obtained if the pulse velocity is calibrated at a point on the structure where the actual thickness is known.

Calibrated B-Scan

This scan (3rd image) shows a B-Scan at the same location after the pulse velocity has been calibrated by measuring at an area of the structure where the thickness is known. The A-Scan to the left of the B-Scan is used to align the cursor to the beginning of the echo. The difference in pulse velocity from the surface velocity measurement is 227 m/s which resulted in a measured thickness error of about 7 cm or 8 % in this case. This illustrates the importance of pulse velocity calibration if accurate depth information is required. The gain has also been optimized to give a clear back wall signal. These settings can then be used to generate a wide area scan using the panorama B-Scan.

23 B-Scans stitched together

This scan is approximately 3.5 m long and consists of 23 separate B-Scans stitched together. The overlapping distance is set to 6 cm, which is equivalent to two channels. It took less than 2 minutes to generate this scan. After 2.7 m from the beginning of the scan there is a step change in the wall thickness. From here on, there are multiple back wall reflections due to the short path length.

The 5th image shows how this scan relates to the construction drawings. As you can see, the wall thickness variations are mapped very accurately

Gain settings

There are three gain adjustments available to the user. Two of them, gain and time gain compensation (TGC) can be controlled directly by the buttons on the measurement probe. Both of these affect the raw signal that is saved as part of the measurement and cannot be adjusted afterwards. The color gain, on the other hand, is a digital gain which can be used as part of the post-processing to obtain the best image.

The scans in the 6th image had a relatively high gain and high TGC in order to achieve a strong back wall echo at a depth of 90 cm. For the section on the right-hand side, a much lower gain could be used. 

Once a panorama B-Scan has been started, it is not possible to change the gain. Because of this, when significant thickness changes occur, it can lead to a highly saturated display. If we zoom into the right-hand side of the scan where the wall thickness is 25 cm, we can reduce the color gain to optimize the image in this section during analysis.

The 7th image is of the same section of the previous scan, but with the gain optimized for the 25 cm section. In this case, TGC is set to zero. However, with this setting, the back-wall reflection at 90 cm is no longer visible.