Hitting it a sledge hammer is the general reconized way to breaking one up for removal.
BE well protected and use heavy branket or canvas over the tub except where you are hitting. The shards are like glass.
I have not had to break up CI, but recently did a concrete pad.
It took a large number of hits before the first cracks started showing. Then it started quickly crumbling.
I have also heard of a using a sawzall.
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