Why does the top slab use 5000PSI concrete mix but the base slab uses standard concrete mix? It has me curious.
Welcome to the BrickWood forums!
Curiosity is a good thing. There is indeed a difference in the specs for the two slabs.
The base slab is supported by a very stable substrate (base rock and optionally footings) across its entire surface. Stresses are distributed by a grid of rebar. Standard concrete is well suited for this.
The hearth slab? That’s 4 to 5 feet above the ground, and most of it is suspended in the air. In the recommended design, part of it is hollowed out and replaced with an insulative material. Even though the load of the oven itself is directly over the walls of the base, the hearth slab has to carry unequal loads AND withstand a fair amount of thermal stress. That’s the reason for the high-strength concrete in that part of the structure.
Practically speaking, you can downgrade to 4,000-psi concrete in the hearth slab if that is more readily available. You can also use high-strength concrete in the base slab if you prefer. I found it simpler to do that, and it didn’t add a lot of cost to my project.
Hope this helps!