Inspection of underwater concrete structures is required for maintenance and repair works. Methods, types, and objectives of underwater concrete structure inspection are discussed.
The service life of any underwater concrete structure, for example, bridges and wharves, and other marine structures, is based on maintaining the physical condition of both superstructure and substructure.
That is why it is significant to establish and carry out an adequate inspection, maintenance, and repair of the whole underwater concrete structure.
Inspection of underwater concrete is usually not easy and economical; that is why it is conducted less frequently compared with part of the structure above the water surface. However, underwater inspection is a major part of underwater structure assessment.
For instance, various organizations, such as transportation agencies and port authorities in the United States and Canada, provide time intervals for conducting underwater inspections as part of a preventive maintenance program.
In this article, the reason for performing underwater inspection, factors needed to be considered during the inspection, the objective of inspection, and the various levels of inspection will be discussed.
Figure-1: Inspection of Underwater Concrete Structure by Diver
- Why is Inspection of Underwater Concrete Structure needed?
- Factors to be considered for Underwater Inspection of Concrete Structures
- Objectives of Underwater Inspection of Concrete Structures
- Level of Underwater Inspection
Why is Inspection of Underwater Concrete Structure needed?Apart from inspection for maintenance purposes, it may be conducted for certain situations, such as new loading conditions, modification or expansion of the structure, and the newly constructed structure to ensure that the structure is built based on applicable specification and contract documents. When the new owner buys the structure, the underwater inspection might be carried out as a requirement. The underwater inspection is required when catastrophic occurs, for example, earthquakes, ship collisions, hurricanes, and floods.
Factors to be considered for Underwater Inspection of Concrete StructuresFollowing are the factors which need to be considered during inspection of underwater concrete structures
- Design considerations
- Existing operating, inspection, and maintenance record
- Condition survey
- In-situ testing
- Specifying factors led to structural deterioration.
Objectives of Underwater Inspection of Concrete StructuresGenerally, inspection is carried out to achieve data needed to evaluate structural conditions and specify whether the structure meets design requirements and future performance. The safety of the structure is a major reason for performing underwater inspections. Nature of inspection specifies the amount of data that should be provided, and the general objective of a condition of a survey required to include the following:
- Specify and describe all main damage and deterioration.
- Determine the phenomenon that caused deterioration.
- Specify the rate and extent of deterioration.
- Determine the performance characteristics of the structure under future service conditions.
- Record the types and extent of water depth, marine growth, water visibility, tidal range, and water currents that will be beneficial to plan future inspections.
- Specify any potential issues that possibly occur with the movement of equipment, personnel, and materials required for the repair process.
- Determining conformance with contract documents and verifying as-built conditions.
- Provide recommendations for appropriate techniques of repair and maintenance.
- Achieving and developing information required to produce cost estimates of the repair and maintenance.
- Recommending frequencies and types of future inspection.
Level of Underwater InspectionThere are three types or levels of underwater inspection used to evaluate the marine structure. They are different in the extent of the work needed and the techniques or means by which the task is to be conducted. In the planning stage, the level of inspection employed for a specific inspection is specified. The three types of underwater inspection are discussed in the following sections:
- Level I – General visual underwater inspection
- Level II – Close level underwater inspection
- Level III – Highly detailed underwater inspection
Level I – General Visual Underwater InspectionIt is not required to clean the structure in this method while this level of inspection is carried out, which makes this level of inspection the most rapid method among all other inspection levels. There are different purposes of general visual underwater inspection, for example, to confirm as-built conditions, produce initial data by which an inspection program is established, and find obvious deterioration or damage that might be occurred as a result of the impact overstress corrosion or biological attack.
Fig.2: Underwater Inspection of Delap and Uliga Dock
Level II – Close Level Underwater InspectionCleaning concrete surface before or during the close level underwater inspection is a need. This method is employed to detect and specify problems that are covered or hidden by marine growth. In addition, cleaning is usually applied for critical locations of the structure because cleaning the structure takes lots of time. In this method, the quantity of information used to assess the initial load-carrying capacity of concrete structures or members is not adequate and is limited. The amount and quality of cleaning are controlled by the quantity of information required to produce a general evaluation of the structure.
Level III – Highly Detailed Underwater InspectionThe level III underwater inspection is employed to observe hidden damages or damages that are about to happen and specify the homogeneity of the material. Therefore, cleaning the surface of the concrete structure or element before the beginning of the inspection is a must. Techniques that may be used in the highly detailed underwater inspection are non-destructive tests and occasionally destructive tests. The former is utilized for the critical structural region that is considered representative of the part of the structure below water level. In contrast, destructive or partially destructive tests are used to take samples for laboratory testing. Finally, if destructive or partially destructive tests are advised, experienced personnel must conduct the process because equipment and test procedures will be more sophisticated. The purpose and the sort of damage recognized by each inspection method are provided in Table-1 which the United States Navy developed: Table-1: capability of each level of underwater inspection for detecting damage to marine structures
|Method of the underwater inspection||Purpose of the underwater inspection||Detectable defects on the surface of the structure|
|Level I||General visual to confirm as-built condition and detect severe damage||Major spalling and cracking|
|Level II||Detect surface defects normally obscured by marine growth||Surface cracking and crumbling, rust straining, exposed rebar|
|Level III||Detect hidden and damage which is about to begin||Location of rebar, beginning corrosion of rebar, change in material strength|