Assessing steel culverts for corrosion using conventional wet-coupled ultrasonic instruments presents significant challenges. The location and size of these structures often necessitate scaffolding or complete road closures to achieve proper sensor contact with the substrate using couplant. Additionally, due to the presence of sprayed concrete on some areas of the galvanized steel walls and heavily corroded sections, it is not possible to fully conduct the inspection with conventional wet-coupled ultrasonic equipment.
RoadTek and Revo Group sought to test a faster, more accurate method for assessing steel culverts for corrosion across Queensland, Australia. The objective of the trial was to quickly and accurately measure the remaining wall thickness of the steel culvert walls using a predetermined grid pattern, record the results for future comparison, and investigate any areas of wall loss to map the extent of detected corrosion.
The first culvert measured, at Dalveen, was in very good condition. The culvert was 5.3m high, with a dual-lane road passing underneath a main road overpass.
The culvert was marked out along the concrete footpath in 1m intervals to assist in taking measurements in the horizontal plane.
The horizontal rows of bolts along the culvert walls assisted in vertical measurement points. Each row of measurements was estimated at the halfway point between each horizontal row of bolts.
The wall thickness was verified with conventional Vernier calipers on a number of accessible overlap sections.
The measurement grid is outlined in the following diagrams.
Further trials were conducted on a smaller culvert 12km from Dalveen, underneath the road. This culvert had some sprayed concrete on some areas of the galvanized steel walls and heavily corroded sections in the base of the culvert toward the western end.
Innerspec recommends two different techniques suitable for faster and more accurate testing under these conditions: Electromagnetic Acoustic Transducer (EMAT) and Dry-Coupled Ultrasonic Testing (DCUT). Both EMAT and DCUT techniques, along with 20+ sensors for different materials and conditions, are available with Innerspec’s latest instrument, CODA (EMAT, DCUT, UT).
The Thickness Grid application available with CODA permits visualizing and storing multiple thickness readings in a grid-type view. The pre-configured grid from the component is replicated in the software for easier data entry and retrieval. It supports up to 250 rows and columns and allows the export of data in .csv (Excel) format.
EMAT or Electro Magnetic Acoustic Transducer is an Ultrasonic Testing (UT) technique that generates the sound in the part inspected instead of the transducer. The main EMAT advantages for this application include:
· Dry, non-contact inspections (no need for couplant gels)
· Transducers not affected by surface coatings and condition. Best for Coated and Rough Surfaces, Curved parts.
· Able to inspect extreme temperatures
For the test, the High-Temperature EMAT Probe (P/N 274AO272) was used. It is a non-contact normal beam probe with a permanent magnet. The probe is held in a protective collar and attached to a telescoping handle, which is adjustable from 0.9 to 3.3 meters in length.
DCUT piezoelectric transducers are designed to withstand high voltages and can be efficiently coupled through rubber without the need for couplant gels. DCUT transducers are also impervious to surface conditions and coatings.
For the test, a Flexible Dry-Coupled Ultrasonic Transducer (DCUT) Probe (Part No. 281AO251) was used. This probe is mounted on an extendable handle, adjustable from 1.3 to 2.5 meters in length. It features a cushioned, flexible point of contact and is ideal for measuring the thickness of conductive materials with irregular geometries.
The EMAT Thickness Probe with the 0.9m to 3.3m handle was trialed first. These measurements are recorded in Table 1. This probe needed to be aligned at exactly 90 degrees to the peak or side of each corrugation to obtain a reliable reading. It easily gave a reading in millimeters, to 2 decimal places, within a second or two of contact.
The fully extended handle (3.3m) with an average height worker (in this case 1.73m) can reliably reach the culvert to approximately 4.8m. This was not quite high enough to measure the top of this culvert. A small work stand of 0.5m would be sufficient to reach all areas of the culvert.
The Flexible DCUT probe on the shorter 1.3 to 2.5m handle was then trialed. The shorter handle could only reach approximately 4m high on the culvert walls, however, it would be relatively simple to fabricate another extension handle and lead to attach to this pole and reach the 5.3m ceiling.
The Flexible DCUT sensor had three advantages over the EMAT probe:
1. It obtained a reading slightly faster than the EMAT probe.
2. It did not need to be aligned at 90 degrees to the surface.
3. It could measure any point on the sinusoidal culvert corrugation, including the concave troughs.
The Flexible DCUT Probe was the most successful of the equipment at obtaining accurate wall thickness readings on the concrete sprayed areas and the heavily corroded sections. It was noted that when the corrosion is extensive, both of the Innerspec probes struggle to obtain and retain a steady thickness reading due to the sound waves echoing in various directions from the irregular corrosion pits. While it is more difficult in these areas, it is still possible to detect the remaining wall thickness.
This measurement would not be possible with conventional wet-coupled ultrasonic equipment.
The Dalveen Culvert wall thickness measurements obtained using the EMAT Thickness Probe are listed below.
While collecting the results a small patch of chalky chemical residue (from the galvanizing process) and oxidization(red corrosion product) was noticed at the top of Segment 11, close to position 5. This patch of approximately 100mm square was measured using the EMAT probe and found to have a wall thickness loss of up to 0.5mm.
Both the EMAT and DCUT probes proved successful in rapidly and accurately mapping the wall thickness of the culvert to a height of 4.8m.
The Flexible DCUT Probe proved more effective and versatile, eliminating the requirement to be at exactly 90 degrees to the wall surface and also measuring in the trough of the corrugation.
With a small 0.5m work stand, or a modified extension handle, this method of ultrasonic measurement could quickly and easily assess all culverts from the ground without the need for ultrasonic couplant, scaffolding, or complete road closures.
These methods can measure curved, rough, painted, dirty, or corroded surfaces with speed and accuracy.
EMAT and DCUT techniques are available with CODA. Learn more about the Instrument.