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I have one in my yard very similar. The blocks I use crack as does the foundation of 16x16 concrete pavers. To prevent any cracking of the concrete base it might be a good idea to put a layer of the 16x16 pavers. they are $4.00 to replace. Using charcoal directly or having a fire instead of embers may result in cracking.
If you line it with firebricks and lay them up with whats called heatstop50, leave a pinch of room behind them, it will last for years and years and years.