That is a complex question to address. In some cases they are simple to spot, and in other cases they are very near impossible to detect. The GPR does not directly distinguish between post-tension cables and rebar, so the skilled GPR technician is left with a host of clues to distinguish between the two. These clues can sometimes be easier or harder to decipher. They include questions such as (but not limited to):
- Are post tension grout pockets visible?
- Is the scan location in a column line?
- How bright is the GPR reaction?
- Is the reaction mid-slab?
- Is the GPR reaction changing in depth over time?
- Is the reaction at the expected depth for PT (shallow nears columns, deep between columns)?
- Does the reaction stop mid-slab?
- Is the reaction visible when cross-polarized?
- Are there other bands at the same depth at the expected intervals?
- Is the reaction running at an angle or on an XY grid?
A skilled technician can fly though these mental questions to quickly determine whether PT cables are a serious risk. It is important to understand, though, that no one of these questions is definitive on its own. PT cables generally run in a patterned grid, but not always. They normally rise and fall in the slab, but there are exceptions. They normally do not end mid-slab, but it is occasionally possible. They often create a distinct GPR reaction, but not always. They can run in groupings of anywhere from 1-5 cables.
It is important to have a full understanding of how these cables run, including all the exceptions noted above, in order to be able to accurately assess the risk of drilling. It is important, in other words, to have expertise. Most (not all) GPR companies provide a technician who can adequately read the data on the screen, but that is still only half the battle. It is equally important to understand the macro view of how PT cables are run, one’s current place within the slab layout, and what risks are associated with each area of the slab.
Quintin Short (pictured above) scanned 12 locations at a resort for a major Orlando attraction, and had the expertise to discern a number of irregularities in the Post Tension layout. The cables seen here were turning due to an architectural flair in the buildings design, and Quintin accurately followed 5 PT bands through the turn. The customer drilled safely, and everyone went home to their families, proud of what we built together.