Visual testing is probably the most important of all non-destructive tests. It can often provide valuable information to the well trained eye. Visual features may be related to workmanship, structural serviceability, and material deterioration and it is particularly important that the engineer is able to differentiate between the various signs of distress which may be encountered. These include for instance, cracks, pop-outs, spalling, disintegration, colour change, weathering, staining, surface blemishes and lack of uniformity. Extensive information can be gathered from visual inspection to give a preliminary indication of the condition of the structure and allow formulation of a subsequent testing programme. The visual inspection however should not be confined only to the structure being investigated. It should also include neighbouring structures, the surrounding environment and the climatic condition. This is probably the most difficult aspect of the whole structural investigation or any diagnostic works since what appears obvious to one may not be so to another. The importance and benefits of a visual survey should not be underrated. Often the omission of what appears to be insignificant evidence can lead to a wrong conclusion being made. The advantage of a trained eye is best described by Sherlock Holmes when he wrote: “I see no more than you but I have trained myself to notice what I see.”
TOOLS AND EQUIPMENT FOR VISUAL INSPECTION
An engineer carrying out a visual survey should be well equipped with tools to facilitate the inspection. These involve a host of common accessories such as measuring tapes or rulers, markers, thermometers, anemometers and others. Binoculars, telescopes, borescopes and endoscopes or the more expensive fibre scopes may be useful where access is difficult. A crack width microscope or a crack width gauge is useful, while a magnifying glass or portable microscope is handy for close up examination. A good camera with the necessary zoom and micro lenses and other accessories, such as polarized filters, facilitates pictorial documentation of defects, and a portable colour chart is helpful in identifying variation in the colour of the concrete. A complete set of relevant drawings showing plan views, elevations and typical structural details allows recording of observations to be made.
GENERAL PROCEDURE OF VISUAL INSPECTION
Before any visual test can be made, the engineer must peruse all relevant structural drawings, plans and elevations to become familiar with the structure. Available documents must also be examined and these include technical specification, past reports of tests or inspection made, construction records, details of materials used, methods and dates of construction, etc.
The survey should be carried out systematically and cover the defects present, the current and past use of the structure, the condition of adjacent structures and environmental condition. All defects must be identified, the degree classified, similar to those used for fire damaged concrete and, where possible, the causes identified. The distribution and extent of defects need to be clearly recognized. For example whether the defects are random or appear in a specific pattern and whether the defect is confined to certain locations of members or is present all over the structure. Visual comparison of similar members is particularly valuable as a preliminary to testing to determine the extent of the problems in such cases. A study of similar structures or other structures in the local area constructed with similar materials can also be helpful in providing ‘case study’ evidence, particularly if those other structures vary in age from the one under investigation. There is a need to identify associated or accompanying defects, especially which particular defect predominates.
Segregation or excessive bleeding at shutter joints may reflect problems with the concrete mix, as might plastic shrinkage cracking, whereas honeycombing may be an indication of a low standard of construction workmanship. Lack of structural adequacy may show itself by excessive deflection or flexural cracking and this may frequently be the reason for an in situ assessment of a structure. Long term creep defections, thermal movements or structural movements may cause distortion of doorframes, cracking of windows, or cracking of a structure or its finishes.
Material deterioration is often indicated by surface cracking and spalling of the concrete and examination of crack patterns may provide a preliminary indication of the cause. Systematic crack mapping is a valuable diagnostic exercise when determining the causes and progression of deterioration. Observation of concrete surface texture and colour variations may be a useful guide to uniformity. Colour change is a widely recognized indicator of the extent of fire damage.
Visual inspection is not confined to the surface but may also include examination of bearings, expansion joints, drainage channels and similar features of a structure. Any misuse of the structure can be identified when compared to the original designed purpose of the structure.
An assessment may also need to be made of the particular environmental conditions to which each part of the structure has been exposed. In particular the wetting and drying frequency and temperature variation that an element is subjected to should be recorded because these factors influence various mechanisms of deterioration in concrete. For example, in marine structures it is important to identify the splash zone. Settlement of surrounding soil or geotechnical failures need to be recorded. Account must also be taken of climatic and other external environmental factors at the location, since factors such as freeze thaw conditions may be of considerable importance when assessing the causes of deterioration. A careful and detailed record of all observations should be made as the inspection proceeds. Drawings can be marked, coloured or shaded to indicate the local severity of each feature. Defects that commonly need recording include:
- cracking which can vary widely in nature and style depending on the causative mechanism
- surface pitting and spalling
- surface staining
- differential movements or displacements
- variation in algal or vegetative growths
- surface voids
- bleed marks
- constructional and lift joints
- exudation of efflorescence.
Classification of the degree of damage or condition requires experience and engineering judgement but guides are available. Where a large structure is to be examined it may beappropriate to produce a plan or a series of plans indicating ‘climate exposure severity’ to overlay the engineer’s plans of the structure.
APPLICATIONS OF VISUAL INSPECTION
For existing structures, presence of some feature requiring further investigation is generally indicated by visual inspection, and it must be considered the single most important component of routine maintenance. It will also provide the basis for judgements relating to access and safety requirements when selecting test methods and test locations. As mentioned earlier, a visual inspection provides an initial indication of the condition of the concrete to allow the formulation of a subsequent testing programme. It is also through such inspections that proper documentation of defects and features in the concrete structure can be effected. With a trained eye, visual inspection can reveal substantial information regarding the structure such as the construction methods, weathering, chemical attack, mechanical damage, physical deterioration, abuse, construction deficiencies or faults and many others.