I see the referenced article recommends 6" thickness and fiberglass reinforced concrete. I would agree with the 6" thinkness - this would ensure the slab would handle the weight of the BBQ and last well. I have no experience with the fiberglass reinforced concrete - I would put in wire mesh myself. I imagine someone will come along and be able to speak to the fiberglass reinforced concrete.
You might want to consider readymix concrete rather than bags. Several sources now for self-tow setups suiteable for smaller projects. You would come out with a better and stronger concrete mix and less work.
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