Inspection of Concrete Structures: Motivations, Types, and Procedures

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

1. To determine the current condition of the structure.
2. To specify type; cause; and extent of deterioration, the rate of deterioration, and whether deterioration is active or not.
3. To estimate the remaining time before repair or replacement.
4. 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. 
5. To estimate the most cost-effective means to prevent further deterioration to the structure.
6. To study the performance of a material under a specific exposure condition

Inspection
Phases of Concrete Structure

Preliminary
Inspection

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

1. Specify whether the structure is safe to use or not.
2. If the structure appears to pose a safety hazard, defer the inspection. Continue the inspection only after correction of the unsafe or hazardous conditions.
3. Record physical damage or deterioration, displacement or misalignment of elements, foundation settlement, joint movement, staining, and dampness.
4. 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 struture 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:

1. Light
2. Medium
3. Severe
4. Very Severe.

5. Record Problems and Related Data

1. During the preliminary inspection, the
   
   inspector records the nature and extent of the observed problems and identifies
   
   the “‘affected members.
2. Record the frequency and severity of the
   
   problems throughout the structure.
3. If the location is accessible, inspect both the
   
   external and internal surfaces of a structure.
4. If possible, photograph the areas of
   
   significant distress for later reference.
5. Place a familiar object or scale in the
   
   photograph to show the relative size of the included area.
6. Describe each observed problem in clear,
   
   concise detail.
7. 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:

1. Record the
   
   nature and extent of the observed problems.
2. Identify the
   
   affected members or areas.
3. Estimate or
   
   define the causes of the problems.
4. 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:

1. No repair
2. Cosmetic Repair
   
3. Structural
   
   Repair
4. Replacement

Detailed
Inspection

Reasons for
Detailed Inspection

1. When the information gathered during a preliminary inspection indicates a need for closer examination.
2. When either the inspectors or plant operators note potentially serious deterioration.
3. When there is a plan to significantly increase the loads on a structure.
4. 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, spacing 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 compressive 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.

Specific Types
of Detailed Inspections

1. Delamination Survey
2. Crack Survey
3. Half-Cell Potentials
4. Pachometer (Cover) Survey
5. Chloride Content
6. Moisture Content
7. Ultrasonic Testing
8. Rebound Hammer Test
9. Impact-Echo Test
10. Concrete Core Extraction and Test