Buildings / Bridges / Concrete Structures

Applications

The owners or professionals responsible for a building will need information about its arrangement or integrity at some point in its lifetime. Frequent reasons for investigation include problems associated with workmanship, deleterious materials, or durability or to the need to alter the fabric due to change of use.

Non-destructive methods offer a 'big picture' of hidden construction detail or problems within a structure without significant damage to the building or disturbance to its users. Infrared thermography, for example, can characterise thermal performance, find defects and map hidden fixings from a remote position, allowing a large building to be surveyed in two hours without expensive access arrangements or even entry to the building. For more detailed resolution a combination of methods should be chosen.

Methods for the determination of structural arrangement of detail include:

Application	Recommended Techniques
Location of fixings, wall ties	Metal Detectors, Radar, Thermography
Concrete/masonry thickness	Radar
Thickness of steel	Ultrasonic thickness gauge
Foundations	Radar
Buried Services	'Cat & Jenny' type service locators, Radar
Mapping flues	Radio tracking, Smoke tracking, Ball tracking, Radar

Methods for the determination of condition include:

Application	Recommended Techniques
Moisture	Thermography, Nuclear density/Moisture gauge, Capacitance or Conductivity/Resistivity base moisture meters
Cracks in concrete/masonry	Ultrasonics, Crack width gauge
Delamination & debonding	Impact echo, Radar
Concrete/masonry strength	Schmidt hammer, Surface Hardness, Ultrasonics
Voids in concrete	Radar
Durability of concrete	Resistivity
Corrosion	Half cell potential, Radar
Thermal performance	Thermography

Benefits

The pros and cons of each method are complex, but a summary relevant to NDI in general would include:

Rapid imaging methods such as radar and thermography offer very rapid sampling enabling statistically significant samples to be collected in a fraction of the time required for conventional testing.
Many methods are effective working from only one accessible surface.
Highly cost effective on large buildings or where similar conditions are repeated eg. System built housing.

Limitations

Point measurement methods such as ultrasonics, radiography and Schmidt Hammer are relative slow
Verification or calibration usually needed
Relatively little documentation to guide specification and use
Surveys can be uneconomic for isolated problems (eg typical domestic properties)

Approach

Due to the cost of instrumentation and the complexity of analysis most NDI work is conducted by specialists. Conclusions often rely on qualitative interpretation of indirect findings so the competence and experience of the specialist in dealing with comparable materials is vital. To get the best results there must be an open dialogue between specialist and client, with a free exchange of historic and even anecdotal evidence. There must also be an acceptance that the non destructive survey may require limited exposure of the structure to clarify or calibrate interpretation.

Reporting

Reporting formats are highly flexible. Where requirements are sufficiently simple results may be marked up on site (eg. Locating a buried pipe or mapping ferrous wall ties), normally however, data must be removed from site for analysis and reporting. Findings from building surveys are typically presented on CAD generated engineering drawings as plans, sections or elevations and the format is tailored from project to project to suit client needs.

Further Reading

Bungey,J.H. 1993. Radar inspection of structures: in proceedings of I.C.E. Structures and Buildings Journal.
Halabe,U.B., Maser,K.R., Kausel,E.A. (1995) "Condition of Reinforced Concrete Structures Using Electromagnetic Waves," ACI Materials Journal, 92(5) September/October.
Concrete Society (UK) Technical Report No.48 "Guidance on Radar Testing of Concrete Structures".

Typical Projects

Roof slab investigations to determine structural detail at various locations for mobile phone antenna systems.
Investigation to determine construction detail and condition of reservoir linings in Brisbane, Sydney and Geelong.
Investigation of reinforced concrete chimney to determine condition of reinforcement and depth of cracking.
Investigation of subsurface support for concrete slab bus depot and heavy vehicle weighing station.
Determination of post tension tendon duct location, for specification compliance, for major bridge crossing in Melbourne.
Determination of post tension tendon ducts in building slabs prior to coring.

GBG Australia have undertaken investigations of: