Our concrete patio step has been cracking and crumbling a bit, mostly just some cracks but there's this one spot where a hole has formed. I would have expected this was a solid concrete step but there's a hollow space inside the step and wood? Is it a normal thing to leave concrete forms in or is this something else?

Anyways what is the best way to remedy this? I assume I should seal it up with concrete, but should i try to fill the hollowed out part with anything?

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