RCD tripping with fault on live wire only
AC power does not have positive and negative,  it has active (or live) and neutral 

The voltage between active and ground is much greater than the voltage between neutral and ground, so connecting a weak fault between active and ground will trip the breaker while connecting it between neutral and ground does not.

The only temporary fix is to stop using the faulty wire. You could run a temporary wire on the surface of the wall.

Next time use electrical junction boxes and conduit inside the wall so that the cables do not bind to the concrete and can be serviced or replaced should they develop a fault.