I've just taken a carpet up prior to putting a new one down. The floor is concrete but a channel has been chipped out at some point to lay a gas pipe. The problem now is that where the channel has been concreted after laying the pipe some of the concrete is now crumbly and has come out, leaving the channel 1 to 2cm. deep in places. Has anyone any suggestions as to how to remedy this? It's not deep enough for a bit of concrete mix, and plaster would just come out when walked on. Thanks.