A pipe or cable being traced using HF locating which passes under or through concrete, will have the locating ability adversely affected by the HF Signal bleeding off into the ReBar, making it nearly impossible to trace a pipe or cable in the HF induction mode through or very near concrete.
Therefore, if you trace a pipe or cable under a through concrete or a building, LF is best, assuming if course that the pipe or cable is continuous metal without gaps or breaks, or, you are looking for a break in a cable. For the same reason, LF may be best in a building, provided you can connect to the target, and you can get a proper ground (remember a completed circuit by the signal from the transmitter through the target line and back to ground, is needed in the conductive mode). It is possible to ‘light-up” the entire building if everything is running to the same ground. You can also clip onto a terminal of a cable or a pipe at one end, then use a long return wire back to ground. Some people make up their own long ground wire for this purpose.