A friend of mine who is a Civil Engineer is investigating the condition of the steel reinforcement in a concrete deck approx 1000mm thick. It is not possible to expose the rebar in this instance so he is looking for non destructive possibilities.
X-rays have been suggested but for various reasons have been rejected, would gamma rays have greater penetration and clarity of image?
If so what are the drawbacks and how can a beam of gamma rays be produced?
I am a complete layman, but there must be some sort of sonic imaging that could handle something like this?
There are different definitions of X-rays and Gamma rays. By one older and more classic definition the same photon would be defined as an X-ray if it came from an electron tube and as a Gamma ray if it came from a disintegrating nucleus. By another definition Gamma rays are of shorter wavelength and higher photon energy than are X-rays. Yet other definitions show overlapping ranges for the two.
Practically speaking, you get Gamma rays from radioactive sources, and they are more penetrating than X-rays, given the available sources for both. Yes, you can use Gamma rays for imaging pretty solid pieces of metal. Before they moved the Liberty Bell a few years ago, for example, they used a Gamma emitting source (maybe cobalt 60?) and a huge piece of photographic film to study it for weaknesses. The setup was highly impressive - the source per se, which couldn’t have been very big, was carried in a Gamma-blocking container that required an enormous truck. God knows what the paperwork’s like to do something like that in downtown Philadelphia…
How about half-cell potential monitoring? Would that work for this situation? Otherwise, I’m thinking radiography of some sort (x-ray or gamma) would be your best bet. Clarity of image is not a significant factor in the choice of radiation source.
Here is a link to a half-cell potential meter.
I understand that all of the standard NDT concrete tests such as half cells, chloride tests etc have been carried out. The bridge deck in question has a hinge joint (will try to post details if I can work out how), which has steel in an x shape in section crossing thorugh the joint between two decks. It is the condition of this crossover steel that is required in order to properly assess the carrying capacity of the structure.
As I understand it, it is not possible to break out too much concrete to physically inspect the steel for several reasons.
Firstly it is very expensive and will involve road closures etc.
Secondly a probably most important, once the concrete has been removed the steel looses some of it’s dead load capacity.
I agree that there are serious H&S implications to using x rays/gamma rays to image the steel, I don’t know if they have tried accoustics, I will ask.
Radiographic imaging is typically limited to 45cm of concrete (cite). I believe larger thicknesses could be imaged, but exposure times may be extremely long. This may be this impracticality mentioned in the OP. A gamma source can be used for long exposure times, since it is always ‘on’.
Other than driving consecutively bigger trucks across the bridge until it breaks, then rebuilding it, I don’t know any better way to determine it’s strength.