The Reason Cement Trucks Are Always Spinning (And What Happens If They Stop)

Cement trucks are as familiar a sight at a construction site or roadwork job alongside workers in yellow hats, their large drums spinning the day away. Even when the truck itself is stationary and even when no active work is going on, that drum never stops spinning. To a casual observer, it might seem like an odd little waste of energy to have the drum spinning all the time like that. However, despite what the jovial patterns on some cement truck drums might imply, the workers don't leave that thing spinning all day just for fun.

The drum of a cement truck is designed to spin constantly not just to put on a little show, but to ensure that the cement mix within remains in a fluid, pliable state that workers can pour and apply as necessary. If that drum stopped spinning for too long, you'd have an entire truck's worth of cement you wouldn't be able to use anymore. That's not even mentioning how difficult it'd be to get out of there.

The drum of a cement truck doesn't just contain cement — it also contains a large quantity of water. You know how cement dispenses from the back of the drum as kind of a runny, sludge-like substance? That's because of all the water in the mix. When the cement is in that sludgy state, it can be easily dispensed from the truck where it needs to go, whether you're just making tiles on the ground or large, elaborate constructs. There's a bit of science behind it, but the short version is that water activates cement's inherent bonding properties, transforming it from nonspecific dust into a solid mass of concrete. It's kind of like how a sponge gets bigger when it's full of water.

However, as anyone who's worked with cement will tell you, that stuff dries quite fast. That's part of the appeal of cement, after all — you don't need to stand around all day waiting for it to solidify. Even with all the water in the drum alongside the cement, the drum needs to keep spinning to keep everything in a mixed, fluid state so it can still be dispensed and poured. This is why even small construction jobs need a dedicated cement mixer, even if it's a small one from a Harbor Freight.

The drum of a cement mixer needs to keep spinning to ensure the cement mixture remains liquid, so it can be poured wherever it needs to go. What if, however, the truck suffered some manner of failure and the drum stopped spinning? You'd have a brief window of time before all of the cement and water sets within the drum and then completely solidifies into concrete. Congratulations, you no longer have a cement truck, you have a truck with a gigantic solid rock stuck to the back.

At this point, the cement is completely unsalvageable. You can't return solidified concrete into a liquid state, after all. The drum itself won't fare much better, as you obviously can't pull several tons of solid stone out through a little spigot in the back of the drum. In this circumstance, the construction company would need to send the truck back to the yard, giant rock and all, then open up the drum's access panel to get at the insides. The concrete would need to be carefully broken down with a chemical agent or a powerful tool like a hydraulic splitter. If they can get enough of the concrete out, the drum can be salvaged.

There have been some experiments to remedy this situation with other methods. For instance, in an episode of "Mythbusters," they attempted to dislodge a mass of solidified concrete from the inside of a drum with dynamite. Unfortunately, all this did was completely blow up the truck, so perhaps it's best to just make sure the drum never stops spinning.