Inspection of underwater concrete structures is required for maintenance and repair works. Methods, types and objectives of underwater concrete structure inspection is discussed.
The service life of any underwater concrete structure for example bridges and wharves and other marine structures are based on maintaining 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 an easy task and economical, that is why it is conducted less frequently compared with part of structure above water surface. However, underwater inspection is a major part of underwater structure assessment.
There are various organizations for instance transportation agencies and port authorities in the United States and Canada that provide time intervals for conducting underwater inspection as part of preventive maintenance program.
In this article, reason for performing underwater inspection, factors needed to be considered during inspection, 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?
Apart from inspection for maintenance purposes, it may be conducted for certain situations, for example, in the case of new loading conditions, modification or expansion of the structure, and for new constructed structure to ensure that the structure built based on applicable specification and contract documents.
When the structure is bought by new owner, 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 Structures
Following 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 Structures
Generally, inspection is carried out to achieve data needed to evaluate structural conditions and specify whether the structure meet design requirements and future performance. The safety of the structure is a major reason for performing underwater inspection.
Nature of inspection specifies the amount of data that should be provided, and general objective of a condition of a survey required to include the following:
- Specify and describe all main damage and deterioration.
- Determine the phenomenon 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 possible occur with the movement of equipments, 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 Inspection
There are three types or levels of underwater inspection used to evaluate the marine structure. Not only do they are different in the extent of the work needed but also in techniques or means by which the task is to be conducted.
In 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 Inspection
It is not required to clean the structure in this method while this level of inspection is carried out and this 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 an initial data by which an inspection program is established, and find obvious and clear deterioration or damage which might be occurred as a result of impact, overstress, corrosion, or biological attack.
Fig.2: Underwater Inspection of Delap and Uliga Dock
Level II – Close Level Underwater Inspection
Cleaning concrete surface prior to or during close level underwater inspection is need and this method is employed to detect and specify problems which are covered or hidden by marine growth. Cleaning is usually applied for critical locations of the structure because cleaning of the structure takes lots of time.
In this method, the quantity of information which can be used to assess initial load carrying capacity of concrete structure or member is not adequate and is limited.
The amount and quality of cleaning is controlled by the quantity of information required to produce a general evaluation of the structure.
Level III – Highly Detailed Underwater Inspection
Not only is the level III underwater inspection employed to observe hidden damages or damages that is about to happen but also to specify the homogeneity of the material. Cleaning the surface of 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 to be representative of the part of the structure below water level, whereas the destructive or partially destructive tests are used to take samples for laboratory testing.
Finally, if destructive or partially destructive tests are advised, it is necessary that experienced personnel conduct the process because equipment and test procedure will be more sophisticated.
The purpose and the sort of damage recognized by each inspection method are provided in Table-1 which was developed by the United States Navy:
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|