Keeping NDT credible: CSI tackles the misconception about GPR technology

Background Non-destructive testing (NDT) involves inspection, testing, evaluation of materials, components or assemblies for defects or differences in characteristics without […]

Background

Non-destructive testing (NDT) involves inspection, testing, evaluation of materials, components or assemblies for defects or differences in characteristics without disturbing, generally on a macroscopic level, the serviceability of the part or system.  NDT has many applications in the fields of manufacturing, medicine, and construction. In the construction industry, NDT is synonymous with the inspection of welds using ultrasonic testing, however, it encompasses a much broader range of technologies. For example, NDT has played an important role in collecting information for assessment of heritage buildings, where destructive testing methods must be minimised or completely avoided.

NDT application in RC structure

Reinforced concrete (RC) structures are prevalent in New Zealand, be they concrete block masonry, in-situ beam and columns, or precast walls and floor slabs. One major application of NDT in the construction industry in New Zealand is scanning of existing RC structures, typically for collecting information of the reinforcing bar arrangement (cover, spacing, diameter, etc.). Theoretically this information should be available from construction drawings, however often these drawings are missing, especially for older/heritage constructions. Another, more ominous reason to scan is that the built structures may not reflect the specification on the drawings at all.

With seismic rating assessment becoming a pivotal aspect of the ever-growing New Zealand property market, structural scanning has become increasingly in-demand. This has raised the concern that some claims had been made on what the scanning technologies can or cannot provide, without adequate knowledge of the basic theory. This can lead to structural engineers, who make the call on the adequacy of structures to sustain loads, having fundamentally incorrect information when doing their job.

Scanning on RC structures

There are two main technologies developed to detect reinforcing bars (rebar) inside concrete structures: Ground Penetrating Radar (GPR) and magnetic techniques such as cover meters and the Hilti Ferroscan. GPR utilises electromagnetic waves while the magnetic techniques detect changes in an applied magnetic field caused by the ferromagnetic rebar.  Each of these technologies has its own advantages. GPR can be used with more complicated arrangements where there may be multiple layers of bars or non-ferrous objects to be detected, whereas in some situations the magnetic techniques can provide a direct estimate of the bar diameter. The discussion in this paper will be largely limited to GPR technology.

Basic theory (and limitation) of GPR

A GPR scanner detects hidden objects by transmitting an electromagnetic wave, which the objects reflect back.

Scan images can be digitally processed to aid analysis. Filtering and gaining can be used to highlight certain reflections and reduce noise. Focussing algorithms can be used to collapse the hyperbola down to single points at their peaks. And edge detection can then be used to highlight these peaks.

Processing is done by default on the Hilti PS 1000, a GPR scanner that CSI and other companies use to scan reinforced concrete in New Zealand. The result is the “Standard” view, a sample of which is shown below.

A sample of Hilti Profis PS 1000 Standard view

In this view, the size of the dots representing reinforcing bars is purely a product of the wavelength of the radar pulse. Unfortunately, unsuspecting contractors or engineers may blindly rely on this visualisation and think they can provide bar diameters to the client, simply by taking the difference in depth at the top and bottom edge of each dot. This is a fundamental mistake; the Hilti software analysis manual states that “The center of a reflection described above generally defines the object depth”. Moreover, in the same manual, it is stated that “A metal object (steel reinforcing bar) is not transparent to radar waves…The underside of an object of this kind cannot be detected” as well as “If the diameter of a plastic pipe or conduit is <50mm, the reflections from the top and bottom surfaces are superimposed. The reflections then cannot be isolated”. Here transparent means allowing the pulse to continue through the bar, which does not occur as it is a conductive medium. Even for large dielectric objects such as plastic and air, the change in wave speed and complicated reflection/refraction effects make directly measuring thickness impractical.

Magnetic devices such as the Hilti Ferroscan, on the other hand, can provide an estimation of the diameter of single reinforcing bars by comparing the deflection of the magnetic field to known rebar magnetic behaviour.

Ground penetrating radar can be used in some special circumstances to give a very rough estimate of the thickness of some bars. If two orthogonal bars are known to be touching, or if both sides of a bar can be seen by scanning opposing sides of a concrete element, the difference in cover values can be established. However, due to the inhomogeneity of concrete resulting in a variable wave speed, and the long wavelength of the pulse resulting in a low precision, such values can be more than ±10mm which is of limited value in structural calculations.

Why it matters

It is now clear that providing estimation, let alone exact, rebar diameter using the GPR technology is fundamentally not possible. Unfortunately, structural engineering consultancies who collect this kind of rebar arrangement information, either through separate contractors or by themselves, may not possess adequate knowledge on the basic theory and limitation of the technology. This is understandable since structural engineers most likely did not learn about the concept while pursuing their qualifications. This is the case of the first author (Ronald), who is a structural engineer by qualification. Nevertheless, it is structural engineers who make the call of whether a concrete floor slab has adequate moment capacity to support additional dead load on it; whether a concrete shear wall has sufficient moment capacity to form well-distributed cracks in the plastic hinge regions so that it fails in a ductile manner during earthquake, and so on. These scenarios, commonly encountered during design calculations for the structural upgrade or assigning seismic ratings, require the information of the rebar arrangement (spacing, cover) as well as the diameter. However, if they are given a claim that the bar diameter can be provided using the GPR technology, it is the hope of the authors that after reading this article, they will question the claim. Bar diameter can at best be estimated if two straight, orthogonal bars are known to be touching, and the diameter is taken as the difference in covers. However, the error margin with this method is very high. The consequence of potential under-design if the incorrect bar diameter is used in the calculation is not something to be taken lightly.

Conclusion

This paper presents a brief overview of the GPR scanning technology, in particular, its application to detect rebar configurations in RC structures. The motivation to author the paper is the concern that misconception of what the GPR technology can and cannot do may damage not only the reputation of the industry but moreover the danger to health and safety in the presence of incorrect information. The paper presents the basic theory of the GPR, leading to its limitation which demonstrates why some claims of its capability, in New Zealand particularly, are fundamentally at fault. It is hoped that this paper gives a better basic understanding of the scanning technologies, pushes engineers to be inquisitive toward questionable claims. This, in turn, will maintain the reputation of NDT services as the forefront for diagnostic testing to in-service structure, with the ultimate goal of preserving the health and safety of our infrastructure.

Please see full paper in our Premium Login Section for complete detail – note: you will need to register and login to access this premium content section

Register for Premium content here

If you have already registered then Login for Premium content here

If you have any questions about this blog or the full paper or would just like to discuss any aspect of NDT with the authors please contact CSI on phone – 0800 33 77 67 or email: info@csiscan.nz

Our thanks to authors Ronald Gultom, Daniel Traeger, Jerome O’Connor and Sam O’Connor from CSI for their work on this paper,

Leave a Reply

Your email address will not be published. Required fields are marked *