This can be divided into many situations. First, is your water stable layer subsidence? Is there any settlement in the base? If not, everything is in good condition. You can directly build new concrete. If there is, it is recommended to repair it first, but excavation is not recommended. The amount of work is large and the cost is high. You can try the current geopolymer repair. The materials are repaired before new concrete is added.

The above is the repair I made, as shown in the picture.