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
It does! There was an episode of ‘Dirty Jobs’ back in the day about chipping the cement off the inside of a mixer ….
https://generalchipping.com/dirty-jobs-chipping-concrete/
Typically the concrete yard is close enough to the job site that the concrete won’t set between the job and back at the yard. The typical cure time on concrete is a few hiurs. That’s plenty of time for a two way trip to still have the concrete be liquid.
So the driver just has to get a hose out and rinse off the residue leftover. Occasionally the weather and timing does help it dry somewhat, in which case the yards have acid washes and other chemicals which will allow the driver to take care of any thin buildup. If a larger amount is left inside due to a bad driver or unforseen circumstances then the driver has to hop inside with power tools and chisel it out.
If there’s a large amount of concrete leftover in the drum from a job they actually will dump it into a mould to create those moveable highway concrete dividers people called “k rails”
Concrete, which is a combination of cement, water, and aggregate (gravel and sand), doesn’t set or cure well unless it’s still. Therefore, after the load is delivered, the barrel of the mixer is still moving, and agitating the mix.
It’s easy enough to pour water in the mixer at the end of the day, reverse the barrel to combine the leftovers with the water, then pour the resulting slurry out into a waste pile.
In the event concrete is left to cure inside the barrel, it’s a painstaking process involving something akin to jackhammers to chip out the hardened mix. It’s grueling, hot, and loud.
Take a look at any construction site, and concrete mixers both coming and going have their barrels rotating.
At the end of the day, the mixer returns to its terminal, where company staff spend a considerable amount of time spraying plain water into the mixer vessel, causing a thin slurry of the mix to come out of it, eventually leaving no accumulation of concrete in the truck.
the inside of the drum is washed in some fashion (pressure, acid bath, chemical, etc) to prevent a big amount of cement hardening. there will always be a little bit of cement left over that will harden and accumulate over time. when it gets to the point where there’s too much cement, someone goes into the mixer with a jackhammer or chisel and breaks up the cement.
When we call a concrete truck on a construction site we always need to keep a space for the truck to dump any residue of concrete left and to wash. Each truck have a tank of water to wash on the site. You can see an example of this [here](https://www.youtube.com/shorts/dDZrp11yopw).
Obviously this isn’t perfect, but it get the job done in-between site. At the end of the day, the truck will do a more thorough clean like seen [here](https://www.youtube.com/watch?v=_nzsTKQBUAc).
If your mixer gets stuck in a traffic or breaks down, you can slow curing of the concrete with sugar. Coca-Cola works perfectly.
Really great questions and answers. I’ve taken many loads of this and often wondered how they kept the barrels clean. I knew the guy with the hose wasn’t the final clean.
The trucks are washed out with water before the concrete residue hardens.
The have a water tank on board to do this, with a hose attached.
They do have to occasionally have someone climb in and jackhammer it out. I saw it on dirty jobs.
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