Condemned fire but pipes are under a concrete floor

Check the meter if there is a different gas route to the fire. If so it could be cut of there. Or you might have to the room it is in floor and cut of from there

Hi, the gas pipe must not to buried in the concrete and have to be ventilated.

Get professional in and they will acess and advise

First of all the fire has an isolation tap, as should all gas appliances which could have been turned off and then the house retested. Having done that if there is still a pressure droip detected then the only possible cause is then the installation pipework.
If this pipework is in the floor, then ideally you would need to cap the pipework off before it enters the floor and rerun new pipework to feed you gas appliances

If you re-pipe the Gas pipes from meter to new Gas Appliances then they can be left in floor as they will be redundant and also there are open ends to allow any remaining gas to go away

Can you trace near the gas meter and cut the pipe there.