Water – How to mitigate and/or repair concrete frets (slab edge wetting)

Your best bet for a good finish relies on doing three things:

• Making sure that the foundation for the extension is well compacted and onto good material
• Cut back and scable the joint with the existing 10x10 slab so that there is a good bond when the new concrete is cast.
• Drill and fix dowel bars into the existing slab across the joint with the extension.

The number and type of bars will vary depending on the load from the new shed but you should be fine with something like 8mm (3/8") bars at 200mm (8") centres.

You should probably note that this will give you a full structural connection and so may be too much for your particular problem unless you want to be absolutely sure that the slab and the extension don't separate.

You're on the right track. I've repaired chips and missing chunks in 80-100 year old public stairways in similar ways. Start with a hammer and chisel, and be really aggressive like a dentist to find and remove all the soft spots.

I've used the tapcon screws, but generally find it easier to chisel an irregular shape that does the same thing.

For your patch I might chisel a channel all the way to the top, so it is easier to get the patching mortar past temporary formwork. Remove your formwork as the stuff starts to get hard, and brush or trowel to match the old surface. Or ignore that as your "concrete restore" will hide minor surface differences.

You can build up the repair in layers, but that requires mixing a fresh batch of rapid set cement every 15 minutes or so.