If you are going to do a pad, then you may not need to attach anything to the tub, just build around it.
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I use an old washing machine tub in my yard for an outdoor fire and the above suggestion is probably the best. The metal will not allow the concrete to adhere well and once it gets hot the metal will expand at a different rate than the concrete. It will end up cracking. Leave a small gap and build around it for best results. Mine is an enamelled tub and i just cut some holes in the front and that's all. It sits on old bricks and i hide it in a garden come
spring..
It's great for watching the stars and losing myself in thought on a cold night.