My commute to school often involves being stuck behind countless cement trucks. Since I see them pretty much every day, I tend to notice mundane things that are different.
This morning, I was behind a cement truck whose barrel-thingy was spinning about 3x faster than any cement truck I’ve seen before. Were they in a hurry or something?
Well I checked HowStuffWorks and couldn’t find anything. I think cement mixing trucks have at least two speeds. One to actually mix the batch and the other and slower to just keep the ingedients from separating on the trip. Oh yes and a third, which I think reverses the spin, to elevate the mix out of the truck.
I know its spinning to keep the stuff from hardening/settling, it just seemed strange this particular cement truck was in ‘turbo mode’. Today was a particularly hot day…might the temperature/humidity have anything to do with it?
Quick-set cement?
Hmm, maybe I shouldn’t be driving behind these particular cement trucks then…
:eek:
The cement mixer is not just a delivery vehicle, it’s part of the manufacturing process. Basically, the ingredients are put in the truck, and mixed on the way to the delivery site. A certain amount of mixing must be done, and too little or too much can affect the quality of the cement. Perhaps the mixer you saw was delivering to a very close job site.
Cement is supposed to be delivered within 90 minutes or 300 revolutions, whichever comes first. The type of mix and additives determines the formula for each load, and mix speed and time are part of this formula. The 300 revolution / 90 minute limit implies that the drum rpm is less than 3.33 at standard speeds.
Depends. How much oil on the road, how fast they were going beforehand, whether the driver tries opposite lock, all that kinda stuff.
We used special cement they called “hydraulic” when I worked on this one construction site. It had special chemicals added for quick setting, chloride I believe. Maybe the truck with a fast spinner was carrying hydraulic? Just a WAG.
This is correct. Too few turns will mean the concrete is not fully mixed. There may be something wrong with mixing the concrete too quickly, but too many turns is probably just a warning that the concrete may be already trying to set inside the drum-- it will be hard to work and will be weak once it sets and cures.
The drum is turned by a hydraulic motor and speed is adjusted to give the correct number of turns. There is also a counter that measures the number of times the drum has turned since the truck left the cement plant. This isn’t usually checked when pouring a home foundation, but it may be checked for a commercial building and it will be checked for road construction.