• As a guess the step looks about 6" deep, so concrete shuttered to that height is going precisely nowhere. Presuming the area to be filled is about 6' x 2', 8 25kg bags of ballast and 2 of cement (4:1 mix) will do the job nicely. A few non-touching slabs laid directly on the existing concrete in a single layer would do no harm. Clean the existing concrete off first. Half a day should do the whole job easily, even if you haven't got a mixer.

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