Title says it all; I’m sure a thin layer of cement will be left over after they poor it all, and I would imagine that thin layer would harden and then the next time they use the mixer another thin layer would be leftover and so on and so forth. After a while I would imagine it would accumulate to the point where it renders the mixer unusable.
Why is this not the case?
In: Engineering
The short answer is that most companies require the customer to provide a dumping spot for leftover concrete, because it would cause this issue if left in while they drive back. And some even require water so they can hose down the inside before it hardens.
Depending on how thick the layer becomes or how much is left, sometimes people will literally have go inside with a jackhammer and spend hours removing the hardened concrete.
Which is why these days you can order on-site concrete for smaller projects, where special trucks will mix it on demand and pump it into place.
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