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
There are lots of things the driver does to slow down the build up. The biggest of which is just rinsing the drum before it sets up. When you leave a job site, you can throw in a bunch of extra water, and bring that back to the plant where you then dump out the watery leftovers, and rinse more as needed.
At the end of the day, we might “rock out” a truck that maybe got stuck out for a long time. (Maybe stuck in traffic, or sunk in soft ground and had to wait to be pulled out, or a job had a hang up and the truck was on site for hours). So I might drop a couple tons of rocks and water in the drum and let him spin that for while to break things up.
But at the end of it all, build up does happen. We regularly measure the empty weight of the truck, and the bad ones get put on a chip out list. Once a month or so, we’d have a company come to the yard overnight, and they specialized in confined space entry into the drum to chip out the build up. It was not uncommon for a truck to have 5,000 lbs of build up removed during a chip out.
We had one truck that rolled over coming off a highway exit. 35k lbs of concrete set up solid in the drum. That was a $300k replacement, because that drum was forever toast.
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