Non-destructive testing (NDT) is performed on underwater concrete structures as a part of structural investigation, routine preventive maintenance programs, or examinations prompted by an accident or catastrophic event. Some of the underwater concrete NDT techniques include sounding, echo sounders, side-scan sonar, etc.
This article discusses the NDT techniques and instruments used for underwater concrete investigation.
1. Sounding Technique
Sounding is one of the elementary NDT methods used in construction. In this method, soundings (data collected from sounding) are taken by striking the concrete surface. The technique locates the internal voids or delamination of the concrete cover.
An overall condition of the concrete surface is derived from the soundings obtained. The results are qualitative, but it is rapid and economical. As the test is conducted underwater, the ability to detect sounds by inspection is reduced.
2. Magnetic Reinforcing Bar Locator
A magnetic reinforcing bar locator or pachometer determines the location of reinforcing bars in concrete, the depth of concrete cover, and the size of reinforcing bars. The pachometers can be are modified for underwater use.
3. Impact Hammer
A standard impact hammer (ASTM C 805) modified for underwater use is employed for rapid surveys to determine the surface hardness of concrete. The readings obtained are generally higher compared to dry conditions. Hence, it is required to redesign the hammer for underwater uses. The readings obtained are normalized to eliminate the effect of higher underwater readings.
4. Ultrasonic Pulse Velocity
In this method, the time of transmission of a pulse of energy through a known distance of concrete is determined. The results obtained depend on the aggregate content and rebar location. The use of ultrasonics help assess the condition of the concrete structures.
Image Credits: Seaward Marine
5. Echo Sounders
Echo sounders or fathometers are used for underwater rehabilitation work using tremie concrete. It is used to fill the voids in the concrete and confirm the level of the tremie pipe in the tremie concrete.
An echo sounder consists of a transducer that sends high-frequency sound waves through the water until it strikes the surface and reflects. Then, the echo sounder device measures the transit time of these waves and calculates the water depth.
7. Radar
Radars are used to study the condition of the concrete upto 30 inches in thickness. It determines delaminations, deteriorations, cracks, and voids. The radar can also detect any inherent changes within the materials.
8. Underwater Acoustic Profilers
Standard acoustic techniques find it difficult to accurately map and check for erosion and faulting of submerged structures. Hence, acoustic profiling is a technique used to map underwater structures.
The acoustic profiling technique is similar to commercial depth sounders or echo sounders but provides greater accuracy.
NDT technologies, along with experienced professionals and specialized tools, are reliable methods for detecting and defining the extent of erosion-corrosion in difficult and inaccessible parts of underwater structures.
FAQs
The common NDT methods used for the inspection of underwater concrete structures are:
1. Sounding technique
2. Magnetic reinforcing bar Locator
3. Impact hammer test
4. Ultrasonic pulse velocity
5. Echo sounders
6. Radar
7. Underwater acoustic profilers
An echo sounder consists of a transducer that sends high-frequency sound waves through the water until it strikes the surface and reflects. Then, the echo sounder device measures the transit time of these waves and calculates the water depth.
A magnetic reinforcing bar locator or pachometer determines the location of reinforcing bars in concrete, the depth of concrete cover, and the size of reinforcing bars.
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