Concrete structures deteriorate with time despite the fact that concrete is inherently a durable material. This is because the concrete's durability, under any given set of exposure conditions, varies with the concrete mixture proportions; the presence and positioning of reinforcement; and the detailing, placing, finishing, curing, and protection it received.
Additionally, it may be exposed to the conditions of abrasion, moisture cycles, cycles of freezing and thawing, temperature cycles, corrosion of reinforcement, and chemical attack which can lead to deterioration of the concrete and the potential reduction of its life-service.
The anatomy of the lifecycle of a typical concrete structure is shown in Fig.1. Following a trouble-free period, the deterioration continues to build up and the cumulative damage is required to be repaired at some stage to restore its original strength. That is why all structures need to be inspected periodically.
In an inspection, a process or object is viewed closely with critical appraisal. Inspections are usually carried out by an inspector or a professional engineer. The inspector must identify the type of deterioration, its causes and recommend ways to repair or correct the deficiency.
Reasons for Conducting Structural Inspections
- To determine the current condition of the structure.
- To specify type; cause; and extent of deterioration, the rate of deterioration, and whether deterioration is active or not.
- To estimate the remaining time before repair or replacement.
- To evaluate the influence of deterioration on serviceability if repair or replacement is deferred, and identify whether proposed changes to the structure would affect safety and service-life of the structure or not.
- To estimate the most cost-effective means to prevent further deterioration to the structure.
- To study the performance of a material under a specific exposure condition
Inspection Phases of Concrete Structure
The preliminary inspection is the first and most important single effort in the evaluation of a structure before rehabilitation. The preliminary inspection provides the initial analytical data that are used to assess the structural adequacy of an existing structure. Typical preliminary inspection involves the following phases:
1. Gather Relevant Information
Before conducting a preliminary inspection, the inspector normally reviews the available plans, specifications, and construction records. Not only do these documents expedite the site inspection but also facilitate an evaluation that is more accurate and easier to perform.
Some of the data items that are useful in most investigations are- Name, location, type and size of the structure, Date constructed, Project engineer, contractor who built it, Concrete supplier, Materials used and sources; and test reports.
In addition to code requirements that were applicable at the time of design, previous inspection reports, sketch map, photographs, exposure conditions, and changes to the structure since it was constructed are assessed.
2. Identify Problems
- Specify whether the structure is safe to use or not.
- If the structure appears to pose a safety hazard, defer the inspection. Continue the inspection only after correction of or hazardous conditions.
- Record physical damage or deterioration, displacement or misalignment of elements, foundation settlement, joint movement, staining, and dampness.
- Additionally, the effects of humidity: condensation, steel corrosion, sulfate attack, and popouts should also be recorded.
3. Determine Possible Causes
It is extremely important that the causes of a problem are properly identified in order to achieve a successful repair.
The major causes of concrete problems are the environment, construction practices, materials, structural movement and previous repairs.
4. Classification of Identified Problems
Problems found in the are classified on the basis of the area affected, the depth affected, the volume of concrete affected, the width of cracks, the total length of cracks in a given area, or other relevant factors. Identified problems are classified into:
- Very Severe.
5. Record Problems and Related Data
- During the preliminary inspection, the inspector records the nature and extent of the observed problems and identifies the "'affected members.
- Record the frequency and severity of the problems throughout the structure.
- If the location is accessible, inspect both the external and internal surfaces of a structure.
- If possible, photograph the areas of significant distress for later reference.
- Place a familiar object or scale in the photograph to show the relative size of the included area.
- Describe each observed problem in clear, concise detail.
- Use three-dimensional isometric drawings to show offsets or distortions of structural features, depth of delamination, location of reinforcement steel, and extent of steel corrosion.
6. Preliminary Inspection Report
Preliminary inspection report shall contain the following objectives:
- Record the nature and extent of the observed problems.
- Identify the affected members or areas.
- Estimate or define the causes of the problems.
- State the requirement for a detailed investigation.
7. Recommended Rehabilitation Method
If the preliminary inspection has identified the concrete problems, and a detailed investigation is not required, the report may also state the conclusions and make recommendations for the rehabilitation of the structure. Rehabilitation options can be divided into the following four categories:
- No repair
- Cosmetic Repair
- Structural Repair
Reasons for Detailed Inspection
- When the information gathered during a preliminary inspection indicates a need for closer examination.
- When either the inspectors or plant operators note potentially serious deterioration.
- When there is a plan to increase the loads on a structure.
- When the condition of the structure has changed significantly since the last inspection
Detailed Inspection Process
A detailed inspection generally involves several tasks which are shown schematically in a flow chart in Fig. 5.
Field measurements and tests include Delamination; voids; and crack mapping, Corrosion activity/potential mapping, Carbonation, Concrete cover, of rebars, Concrete strength and variability, Concrete quality (other than strength), and various nondestructive test methods are used in the field tests.
Samples are collected from the site to determine various properties of in-situ concrete. Samples are generally in the form of cores, broken concrete pieces, powdered samples extracted by drilling, and sawed beams.
Samples collected from the fields are tested in the laboratory to obtain necessary information which may include the strength of concrete, cement content, chloride and sulfate content, permeability, aggregate type and gradation, alkali-aggregate reactivity, density, and air voids.
Inspectors should exercise caution in the interpretation of test results. The test results are easily misrepresented.