As I was driving to lunch today I saw a cement truck and it got me to thinking…
How long does it take before the fastest-drying type of cement hardens up? How do they keep the cement from hardening in the truck? Does spinning the drum keep it from hardening or is that just to mix it up well? Doesn’t the residual cement left in the truck harden up in there? How do they clean it out? Do they have to power wash it right after pouring the cement or something? Has a whole truckfull of cement ever hardened solid? How would they get it out? What is the drum made out of? It seems like they wouldn’t last long with all of that scouring rock and sand tumbling around in there.
Don’t know. Usually the time has to be under 1.5 hours. Engineering techs check the concrete and take break samples. If it took too long to deliver they will reject it (The delivery ticket has the time). During hot weather they add ice to keep it cool and lengthen the set up time.
How do they keep the cement from hardening in the truck? Does spinning the drum keep it from hardening or is that just to mix it up well?
The spinning keeps it mixed up and also from setting up but I believe you can only keep it mixing for so long.
Doesn’t the residual cement left in the truck harden up in there? How do they clean it out? Do they have to power wash it right after pouring the cement or something?
No. The truck has a water tank. They can add water to the concrete if required (not the right consistency). They also use the water to clean the truck after wards. Diluting the concrete with the remaining water cleans the drum and prevents the remaining concrete from setting up.
Has a whole truckfull of cement ever hardened solid? How would they get it out?
Yup. There is a manway on the drum that allows access. They give the driver (if it was his fault) a jackhammer and a pat on the back and say “Go to it.”
What is the drum made out of? It seems like they wouldn’t last long with all of that scouring rock and sand tumbling around in there.
It is made out of steel. Why it doesn’t wear out I don’t know. I suspect that it does but very slowly. The drums don’t turn all that fast until they are ready to pour. And half the time they are running empty. Also the rotation wears the whole drum evenly instead of one spot.
Even if they are careful, wash out right away, etc. some residual cement builds up over time. They then send someone with a hammer and chissel or a hand held jack into the drum to enjoy a process known in the biz as “De-burring”. Hot. Dirty. Stagnant air. Loud as all get out. Painful. Dangerous.
Thanks for this thread, these are questions I’ve been wondering about since my sandbox days…
So does the drum have blades in there or an archimedes screw? how does it pump the concrete out? How does it mix it?
Also, how are these trucks loaded? Are the individual ingredients (water, cement, gravel, sand…) added individually then mixed inside? Are the “dry” ingredients pre-mixed and dumped in with water afterwards? Is the wet mix dumped in already mixed & wet?
I have heard on a documentary on the Hibernia Oil Field platform that there were using some form of “super strong” concrete, “13” times stronger than regular highway overpass stuff… I thought concrete was concrete… What are the different types, and how do the recipes differ?
Finally, if I’ve ordered some concrete for a project: building a pad, or a foundation, or a pool, etc. How do I specify concrete so I get the “good/right” stuff, and how can I check it when it arrives? How do they do this on construction sites? What if there’s “too much sand” or not enough cement, etc. etc.
If the driver needs to take a long lunch, he can throw a bag of sugar in the drum, this will retard the cure and it may still pass a “slump” test at the jobsite, if they are doing slump tests.
Not only is this a violation of any set of specifications I’ve ever seen, it’s just a bad idea in general. While small amounts of sugar can retard the setting of Portland cement, it can also cause substantial reduction in the long-term strength of the concrete. Also, the effect of added sugar can influence setting time and strength in an unpredictable fashion, depending on the type of cement and the presence of other admixtures.
I hope your driver isn’t delivering his load to a bridge, dam, highway overpass, building, etc. :rolleyes:
You can find anything on the web. It’s a pretty extreme example, I don’t remember seeing any with spades that big, but it’s been nearly twenty years. Some just had big swirling bumps or indents. They all kind of twirled, so when you were unloading the last yard the twirl kind of pushed it out towards the chute.
They extra hard cement was made from everything from vinyl, epoxies, and polymers. Ask the Germans. Those folks are mighty fond of their cement.
You need to specify the PSI rating you want; for instance, 3000 PSI concrete for a slab. Concrete strength is derived by a variety of methods, which you can research on the web.
When it arrives, a ‘slump test’ should be performed to make sure you aren’t getting soup or mix that is too dry (workability). A slump test consists of using a 12" cone that is filled with mix from the chute. It’s worked a bit with a piece of rebar, turned upside down on a piece of plywood, the cone pulled off, and the number of inches of ‘slump’ measured. Generally, a 4" slump is a good workable mix.
Between stints as a crane operator, I would drive trucks to support my food habit!! Below is a MUCH too detailed account of what happens with cement mixers.
First the truck is backed under a large hopper, all of the components for that particular mix is added then… The actuall mixing does indeed take place within the truck itself.(and they can carry as much as 12 yards… No the whole nine yards is another story)
After the components (sand, rock,and cement) are loaded the truck is taken to a staging area called a slump stand. There we will wash off any excess cement that may have spilled onto the truck and actually set the slump of the cement(slump is the viscosity)…
Then the truck is taken to the job site and placed in the correct position… The driver can operate the truck from inside or out! Then the drum is reversed from mixing to pour by turning the drum the other direction( The fins act as an agitator during mixing and transport, But when reversed act as an auger to lift and push out the concrete.)
After the pour is made then we do a washout, Mixers carry about 110 gallons of water and use it to thin out the concrete and for washout. Gotta wash off EVERYTHING! Even gotta reach in as far as possible to try to rinse the fins…If there is any left. the unused portion is taken back to the plant and dumped…
At the end of the day we then do a drum washout Where the Batch man will load our trucks with water and we then agitate as fast as the drum will turn to try to get any residue left over from the day’s work. Then we dump the water, park the truck and go get a beer with the boys!!
Regarding how long it takes for concrete to set up, among the things that can be added to the mix is calcium carbonate, which produces what my road-building friend calls “cartoon concrete” as it sets up *very * quickly.
He tells the tale of one job where the cement truck was stuck in a traffic jam caused by road construction. Yup. The same job that the truck was trying to get to was preventing the truck from getting there. 10 or so yards of concrete that’s not only a few minutes away from being unusable, it’s about to harden inside the mixer. The hasty decision was made to pull off the road into the grassy median and dump the load. When they dumped it, it was already partially set and fell out in clumps, rather than pouring out. :eek:
I’ve wondered this for years. How heavily would you have to modify a cement truck before you could use it as a giantmilkshake machine?
You already have a place to pour in the ingredients, a big rotating drum to keep them mixed, and a pouring chute. Is there a feasable way to keep the whole drum refrigerated?
robbie I didn’t say that it was a good idea, condoned, or even legal. I merely suggested that this is a trick, sometimes used to “ride the clock”. I spent many years as a heavy equipment operator and I’ve seen many things done that were not a "good idea’. People cut corners, sometimes it’s not safe and someone just trying to get by that knows that the results may not show up for a couple years might act indiscreetly.
Regarding concrete and sugar, my father, who spent most of his career as an analytical chemist for the Portland Cement Association, led me to believe that it was common practice to keep a large bag of sugar on the delivery trucks. He said it was only used in case of emergency to extend the cure time of the concrete and keep it from setting up in the truck. Of course once the sugar was added, the load had to be dumped, but at least you wouldn’t have to jackhammer out the cured concrete.
Drivers here in Calgary have told me that they get a a single portion mixture of retardant that they can add to the mixture in heavy traffic situations. We have two seasons in Calgary - Winter and Construction, and I think it can make it challenging for concrete delivery.
In winter the calcium is added to help the mix cure in cold conditions.
Ive been told about the coke bottle trick as well but I think that is probably used as a last resort to keep a ruined load dump-able as Moe indicated.
This is a copy of my post from the other cement thread:
From the previous portion of the thread, it sounded like the water was added at the plant when the truck is filled. Why don’t the trucks get filled with dry ingredients and have the water added and mixed when the truck arrives at the destination? Or is that what is done?
