I see cement trucks constantly turning the cement, I get that it may just be keeping it thoroughly mixed so no one part becomes more dried than the rest. Is that all that’s happening here?
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Concrete settles to harden. This is why heat is generated. The churning stops the settling. Concrete will harden “dry” even when completely submerged in water. The particles fall slowly into locked positions through being wedged among all the other particles.
just think of the concrete as water, and imagine the temperatur is 0 degrees, swishing around it will not harden/freeze
Concrete doesn’t “dry”. It “cures” when the individual components of the ingredients begin crystalizing and forming bonds between the minerals. Churning the mix stops these bonds from forming until it is poured into place.
For the same reason Elmer’s glue has a tough time drying when you’re playing with it on your fingers
> Is that all that’s happening here?
Yep. The rotation does not stop the cement from curing. It starts to cure inside the truck, but the cure time is calculated to ensure that there is plenty of time to unload it at the job site and get it into place before it’s too hard to handle.
In some cases, the ingredients are not mixed before they go into the truck. They are mixed *in the truck* as the truck drives so that it’s ready to go (and already curing) when it gets to the job site. The truck has a tank of water onboard and the dry ingredients are mixed during the drive, and then the water added to begin the curing process as the truck arrives (or at an appropriate time before arrives so that it’s 100% ready to go as the truck pulls up).
There is no way to stop the curing process once the chemicals responsible are combined. Keeping it churned up may slow the process down, but only because you don’t get a settling of *all* the cement in one place with the aggregate settled on top or underneath (cement is the “glue”, aggregate is the “filler” that you’re gluing together, and “concrete” is the mixture of cement and aggregate). The curing process *can* be controlled by varying how much of which chemicals get added, so that you can have a shorter or longer cure time. Still, everything I can find online suggests that if you have concrete delivered it should be no more than 90 minutes from the time it’s mixed to when it’s poured, and under an hour is preferred.
Additional note. Throw in 5lbs if sugar and cement will never set. This is kept on job sites if the site isn’t ready and the truck is stuck waiting. So instead of ruining the truck they throw the sugar in and lose that load
timing is everything
the churning does help to slow the process to give everyone a larger time window tho
but once the chemical process is started , ie by adding water to the dry concrete powder , the curing is starting and is only stopped by chemically destabilizing the cure [ie via sugar/glucose]
[https://www.quora.com/What-happens-if-sugar-is-put-in-concrete](https://www.quora.com/What-happens-if-sugar-is-put-in-concrete)
obvs ratios and concentrations/percentages matter
There are two ways to take concrete to the site in mixer trucks. Either they are prepared at plants (mist probably and better controlled mix) or the water is added during the haul time(usually not preferred as the water mix might not be as per design). While calculating the concrete mix, they factor the cure time ( what you are calling drying/hardening), and add retarders to it that slow the curing time so that there is ample time for the concrete to reach the site as well as have enough time for any on site delay that might make the concrete batch be left for extended period in the mixer. If the plant operator or the mixer operator / driver are oblivious to this , they sometimes suffer the cimcrete drying inside and they have to then break and clean the whole thing. Sometimes there are on site delays that can be a day or two so they have to dump the concrete . While on site, i have witnessed two three times where the contractor made some safety errors but called on the first batch, which then were dumped, and funnily the local rural community gets benefit from it as they usually ask them to pour it in their lawns or sitting areas. Also the concept of retarder and accelerator can be extended that if you are living in a cold area where curing is slow, you add accelerators to concrete for fast curing, and if living in hot regions, you add retarders so that mix doesnt cure faster
How do they get all the concrete out of the back of the truck? After they unload most of it there must be some residual left in the tank that hardens
The mixing in the truck does have an initial slowing down effect on the start of the curing but the breaking up of the crystalline structure caused can make the final set proceed faster and can also affect the nature and properties of the end product.
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