Underwater inspections are usually conducted by scuba divers. When a very deep or long dive is required, a diver with surface-supplied air is a better option. Flexibility and speed is an advantage for the scuba diver. In clear water, a visual inspection can be done. Many types of structures are located in water that is not clear enough to perform a visual examination.
Fortunately, many types of test devices that are used above water have been adopted for use below water. Rebound hammers work underwater. Both direct and indirect ultrasonic pulse-velocity systems can be used below the water surface. These tools can give a diver a good reading on the general condition of concrete that is surrounded by water.
Underwater vehicles are commonly used to inspect submerged concrete structures. These vehicles come in five different categories of manned units: untethered, tethered, diver lockout, observation or work bells, and atmospheric diving suits. All are operated by a person inside, who has viewing ports, dry conditions, and some degree of mobility.
Unmanned vehicles are another option for underwater inspections. These include tethered, free swimming, towed, towed midwater, bottom-reliant, bottomcrawling, structurally reliant, and untethered. Unmanned vehicles are known as remotely operated vehicles (ROVs). Television cameras are mounted on the vehicles.
Control of the vehicle is done from the water surface with a navigation system, such as a joystick. These vehicles can be fitted up to perform inspections and maintenance.
ROVs offer the advantage of being operated at extreme depths. They can remain underwater for long periods of time. Repeated tasks can be completed accurately with ROVs. Another advantage is that ROVs can be operated in harsh conditions that would hamper general diving operations.
When ROVs are compared to manned vehicles, there are pros and cons. For example, manned vehicles are big, bulky, and expensive to operate. ROVs are small,flexible, and relatively inexpensive. An ROV provides a two-dimensional view, while a manned unit can provide three-dimensional assessments. Both types of vehicles have their place in underwater inspections.
Photographic tools have come a long way over the years. An underwater inspection can involve the use of either still cameras or video cameras, or both. Video systems can see through turbid water conditions. This is a big plus over the eyes of a diver.
HIGH-RESOLUTION ACOUSTIC MAPPING SYSTEM
High -resolution acoustic mapping systems can be used to check for erosion and faulting. These systems consist of three basic components: the positioning subsystem, the acoustic subsystem, and the compute-and-record subsystem. An acoustic subsystem is made up of a boat-mounted transducer array and signal-processing electronics. This type of system sends output back to a computer. The computer calculates the elevation of the bottom surface from the information supplied.
A lateral positioning subsystem has a sonic transmitter on a boat and two or more transponders in the water at a known or surveyed location. The transponders receive a sonic pulse from the transmitter. This information is radioed to the survey vessel. A time and location is determined by the survey vessel. Compute -and-record subsystems provide computer-controlled operation of the system and for processing, display, and storage of data. Real-time mapping is done in a computerized manner.
While high-resolution systems are extremely accurate, they do have limitations. These systems typically work in depths ranging from 5 to 40 feet. Another drawback is that a high-resolution system works best in calm water. If there is wave activity that exceeds 5 degrees, a hi-res system will shut down. When removing concrete forms, workers must be sure to maintain all safety and serviceability of the concrete structure.
A side scanner sonar requires two transducers mounted in a waterproof housing. When a signal is sent from the scanner, it is called a sonograph. Darkened areas and shadows are used for evaluation. The width of shadows and the position of objects can be used to calculate height. Newer versions of scanners have far fewer limitations than the earlier models did. Side scanners have been proven useful in breakwaters, jetties, groins, port structures, and inland waterway facilities, such as locks and dams.
OTHER MEANS OF UNDERWATER TESTING
Other means of underwater testing include radar, ultrasonic pulse velocity, ultrasonic pulse-echo systems, and sonic pulse-echo techniques for piles. All of these methods have their advantages. Let’s look at some of them.
Advantages of radar systems
■ The electromagnetic signal emitted from radar travels very quickly.
■ Conductivity controls the loss of energy and, therefore, the penetration depth.
■ Dielectric constant determines the propagation velocity.
Advantages of ultrasonic pulse velocity
■ Provides a nondestructive method for evaluating structures.
■ Measures the time of travel of acoustic pulses of energy through a material of known thickness.
■ Piezoelectric transducers are housed in metal casings and are excited by high impulse voltages as they transmit and receive acoustic pulses.
■ An oscilloscope in the system measures time and displays acoustic waves.
■Reliable in situ delineations of the extent and severity of cracks, areas of deterioration, and general assessments.
■ Capable of penetrating up to 300 feet of continuous concrete with the aid of amplifiers.
■ Can be transported easily.
■ Has a high data acquisition to cost ratio.
■ Can be converted for underwater use.
Advantages of ultrasonic pulse-echo systems
■Uses piezoelectric crystals to generate and detect signals and the accurate time base of an oscilloscope to measure the time of arrival of a longitudinal ultrasonic pulse in concrete.
■These systems can delineate sound concrete, concrete of questionable quality, deteriorated concrete, delaminations, voids, reinforcing steel, and other objects within concrete.
■ Can determine the thickness of concrete up to about 1.5 feet.
■ Can be adapted to water environments.
Advantages of pulse-echo techniques for piles
■ Can determine the length of concrete piles, in tens of feet, in dry soil or underwater.
■ Uses a round-trip echo time in the pile to measure an accurate time base of an oscilloscope.
■ Can be used to calculate the reference between length and diameter ratios.