Underwater reinforced concrete structures are commonly inspected using visual inspection, which is considered to be a cost-effective method that requires neither extensive equipment nor a significant amount of time. This technique is useful not only for identifying surface damages but also for detecting areas of the structure that have deteriorated severely. To carry out visual inspection, inspectors may use a combination of tools such as photography, video cameras, and their own eyes while diving underwater. The limitations of the visual inspection method and the necessary tools required for conducting this type of assessment are discussed in detail in the following sections.
Visual Inspection of Underwater Reinforced Concrete Structures
Visual inspection is a crucial part of assessing the condition of underwater reinforced concrete (RCC) structures. Various visual inspection tools are used to examine the structures, such as underwater cameras, remotely operated vehicles (ROVs), and divers. These tools provide a visual overview of the RCC structures and allow inspectors to identify any signs of deterioration or damage.
In addition to visual inspection tools, measuring equipment is also used to obtain quantitative data about the structures. For example, ultrasonic thickness gauges are used to measure the thickness of the RCC structures, while corrosion potential meters can be used to assess the potential for corrosion.
However, there are limitations to the visual inspection method. For instance, the visibility in underwater environments can be poor, which can make it difficult to identify certain types of damage. Additionally, the RCC structures may have complex geometries, making it challenging to assess all areas of the structure visually.
To overcome these limitations, tactile assessment can be performed. This involves physically touching and feeling the RCC structures to identify any defects or inconsistencies. Tactile assessment can be performed by divers or by using ROVs equipped with manipulator arms. Tactile assessment can provide more detailed information about the condition of the RCC structures and can help to identify defects that may not be visible during visual inspection alone.
Visual Inspection Tools for Underwater Reinforced Concrete Structures
Underwater reinforced concrete structures are subject to visual inspection using various tools and equipment. These tools include pry bars, hammers, probing bars, and picks that aid in conducting concrete soundings. During the inspection, delamination and voids are typically observed, and chipping equipment is necessary to chip off parts of the concrete and determine the extent of the damage. However, it should be noted that the results obtained from these tools should only be used as guidelines for further evaluation and analysis.
To enhance visibility during underwater inspections, flashlights and clear water masks are crucial. The use of such devices improves visibility underwater and leads to better inspection results. Successful underwater inspections have been reported using both thallium iodide and quartz iodide lamps.
Measuring Equipments for Visual Inspection of Underwater RCC Structures
Divers use various straightforward tools to gather limited data while inspecting underwater reinforced concrete structures. These tools are not costly and can be easily used without much time consumption. However, their capacity to determine the level of deterioration is significantly restricted. The measuring tools used by divers can be categorized into three groups: linear measurement, circular measurement, and spall or deformation depth.
Linear Measurement for Visual Inspection
Devices employed for linear measurements while underwater reinforced concrete structure is inspected are provided in Table-1.
Table-1: Linear Measurement Tools, their Applications and Accuracies
| Device types | Application of the device | Accuracy of measurements |
| Ruler, Figure-2 | Measure crack length and spall width and length | +/- 0.5 mm |
| Tap measure, Figure-3 | Used for measurements of up to 100m | +/- 5 mm |
| Magnetic tape | Used for circular measurements and employed for maximum of 3m | +/- 1 mm |
| Scales | Used in combination with photography | +/- 5 mm |
| Comparator | Measure crack width | +/- 0.025 mm for dry condition but the accuracy is decreased underwater |
Fig.2: Ruler
Fig.3: Tape Measure
Circular measurements
The devices used to obtain circular measurement during the process of visual inspection underwater reinforced concrete structure are provided in Table-2
. Table-2: Circular Measurement Tools, their Applications and Accuracies
| Device types | Application of the device | Accuracy of measurements |
| Calipers, Figure-4 | Take measurements of up to 2 m in diameter | +/- 0.5 mm |
| Special jigs | Measure member ovality | +/- 5 mm |
Fig.4: Caliper
Deformation or Spall Depth Measurements
There are number of devices used in the underwater inspection to measure deformation or spall depth and these tools are provided in Table 3.
Table-3: Deformation of Spall Depth Measurement Devices and their Applications and Accuracies
| Device types | Application of the device | Accuracy of measurements |
| Profile gauge | Used to record mirror image of defects on concrete elements | +/- 0.5 mm |
| Taut wire | Employed for great deformation or spalling regions, in addition to measure out of plumbness of members | +/- 5 mm |
| Casts | Applied to achieve mirror image of defects and employed to take samples to the surface for visual inspection |
Limitation of Visual Inspection Method for Underwater RCC Structures
Underwater visual inspection is a cost-effective and relatively speedy method of conducting inspections. However, there are certain conditions that can impede this technique. For instance, strong waves can limit the diver’s capacity, while turbid water can significantly reduce visibility. Additionally, marine growth can obscure structural defects, making them difficult to detect if they are not cleaned.
Furthermore, the experience and skills of the diver can also play a significant role in the restrictions related to this inspection technique. Inexperienced divers may struggle to perform a thorough visual inspection in challenging environmental conditions. Therefore, it is crucial to consider these factors and ensure that inspections are carried out by appropriately skilled divers in optimal environmental conditions to minimize the limitations of this technique.
Tactile Assessment of Underwater Reinforced Concrete Structures
Underwater concrete structures may require inspection when visibility is significantly limited by harsh environmental conditions or marine growth covering the structure. In such cases, divers can rely on their sense of touch and feel to inspect the structure, a technique known as tactile inspection. However, this method requires extensive preparation compared to structural assessment in clean water. The diver must be familiar with the structure to be inspected through drawings and must communicate effectively with individuals on the surface.
To repair underwater concrete structures, specific methods and procedures must be followed. Various types of underwater concrete repair methods are available, each with its own purpose. Anti-washout admixture is used to prevent the segregation of concrete components when placing underwater concrete. This admixture is available in different types and offers several advantages in underwater concreting. Tremie method is one of the underwater concreting techniques used to place concrete underwater.
Overall, underwater inspection and repair of concrete structures require specialized knowledge and preparation to ensure safe and effective execution.