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.
I used to work for an oilfield company that did Plug and Abandon work. Basically we took old wells that went below a certain production threshold and cut out the old pipes and pumped cement down to plug them back to certain EPA standards. The offshore mixers would routinely need to have someone with a pneumatic needle scaler go inside them and knock it clean. As the guy that did the job for awhile, they very much do get clogged up and someone is paid by the hour to knock it off.
It can. There are known cases of it not getting used fast enough and curing in the barrel. This can cause the entire barrel to become a loss and need replacement.
But concrete companies and drivers know about this and are usually smart enough to know when the load needs dumping. It’s the dummy drivers who end up ruining their trucks by allowing the curing to occur.
I’m a former concrete truck driver. At the end of a job, the construction site usually has an area for me to rinse off my truck.
My truck has a water tank and a hose on it to rinse. I rinse off the chutes and rinse down the walls inside the barrel. The site usually doesn’t have a spot for me to dump excess concrete, so it gets taken back to my company’s property, where they have a designated concrete dumping site. I rinse the inside and empty it a few times.
Concrete still does accumulate inside. The truck needs constant chiseling during our downtime. We usually take a jackhammer to the inside of the drum during the winter when work is slow.
You notice how that big barrel spins? It’s not only to mix it … but it keeps anything that cures from sticking to the sides.
Concrete driver here. There will always be a little bit of a buildup, you can only slow down the process. We rinse after every load and if possible(only on big construction sites) we add about half our water tank into the drum, let it spin back and forth and let the slurry out. Over here on the other side of the pond we dont go in to the drums to clear the concrete but we take the truck to a specialized company instead, where they will close off the drum, add gas and oxygen and let that explode 2-3 times. That will clear most of the concrete inside the drum, and keep it from looking like someone shot a shotgun at it. Last time we did this with one of our trucks about 1 m3 came out.
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.
It does, what normally happens is the truck driver will dump the concrete before it sets otherwise you’re inside the drum with a Jack hammer breaking it out
one I can actually help with
concrete trucks will wash out after pouring ( God help you if you use all of a trucks water, drivers are miserable) This will help wet down remaining concrete in the truck. Here’s the thing, once concrete hits the air the process of hydration occurs rapidly ( less rapidly in the drum of the truck) unless the concrete has been in there for a long time ( hours) then the stones that rub together as the drum turns create friction and create a ” hot load”
Whatever yardage they have left over is poured into 1 yard blocks that are floated off and sold as retaining walls ( you often see them separating materials at gravel pits) under that amount is poured off into a pile at the back of the plant and sometimes they will get it trucked out ( I’ve seen concrete mountains)
That being said there always remains some residue and many drivers will spend days in the winter jackhammering drums ( and more importantly, the fins of the drum )clean
You will also see 1 foot by 4 inch chunks sometimes in a mix as they break off the fins during the mixing process
I had a mate that used to own his concrete truck, he would hose the drum between jobs, after discharging at a job site he used the onboard water to spray the inside of the drum and keep the drum rotating until he was back at the plant to reload, there he washed out the drum and then reloaded.
His truck broke down one time while he had half a load in the drum, it set hard before he could get it working and cleaned, he spent almost two days in the drum with a jack hammer cleaning it out.
The alternative was a new drum which was an exorbitant price so he elected to clean it.
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