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
Having driven a concrete truck I’d be happy to give some insight.
So you can be left with 2 different types of leftovers. One is a big pour where pretty well all your concrete is used up. In this case you use your hose, plus you also have a quicker non-hose filler which can move water from the holding tank to the drum. And you just spray everything down. Spray the chutes, drum, and the fins in the drum. You’re turning the barrel while you’re doing this. You water the tiny leftover down so much. Go back to the yard, dump it, and if you’re done for the day you do the water thing all over again.
The second scenario is you have a fair bit of concrete leftover, like half a metre or more. In this case you add a little water to save it, go back to the yard and pour it into a block mold, then if you’re done for the day you hose everything out.
End of day wash out probably takes 25 mins. You spray water everywhere. And the hoses at the plant are high powered. And you get the drum going full speed with a fair bit of water in it and it really cleans the inside well.
Eventually…a year 2 years later, whatever, the build up will happen on hard to reach areas, like the back side of fins. I always found the worst build up will happen on long hot days. You may have 6 or 7 pours, but you never have a chance to really clean out the drum. You dump your semi dirty water and you get a fresh load. Do that over 10 hours and you start to get a bit of build up. So after a few years of this you have to go inside with a small jackhammer and break it off. Some drums are better designed (in my opinion) to have less build up, with better shaped fins. It never builds up so bad that you’ve lost space in the drum, it just doesn’t mix quite as well…and just a maintenance thing. Plus when build up gets a little too thick, again sime is on the back side of a fin which is tricky to see, you can have a chunk break off while you’re pouring and it can clog a pump truck. It honestly felt like half the time we spent just cleaning. You’re always spraying everything down.
We had a couple mixer trucks 25+ years old, drums still looked great. It’s very easy to see on the back down in the drum and you’d see right away how clean/dirty it is. Guys would be all over you if you didn’t hose down properly.
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