Cracks on the service of a hearth slab are an indication of something not right. There are number of reasons why the slab might crack. The first is if the slab is not reinforced with strengthening materials. The most common materials are polypropylene fibers which are added to the hearth mix and wire mesh which is placed in the slab frame before the mix is poured.

The second reason why your slab might crack is due to unfavorable drying. If the slab is exposed to direct heat, mostly sunlight, then it will crack before it dries. Consequently, if the oven is built and the first fire is lit before the slab fully dries, then cracking will happen.

Regardless of why tour slab is cracking, you can repair it. All you need is the self leveling concrete mix. It is available from hardware shop. Mix it and poor it in the inside of the oven, or on the slab if the oven is not yet build. The self leveling concrete will fill the cracks and forms a top layer.

Further information: Proper Use of Self-Leveling Compound and How to build a pizza oven