Yes, you can add it anytime, just a bit trickier to place it. I contacted my local concrete guys and rolloff dumpster rental places and they were all more than eager to dump their concrete at my farm. I have had a few deliveries - it saves them a ton of money in dump fees unless they sell it to a concrete crushing company already. Sometimes it's simply easier to dump I think. You should have no problems sourcing broken concrete, just use ingenuity and be resourceful.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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