For major road projects and such they often set up a batch plant like this one.
See them all the time for highway and other construction projects. Mix the concrete right on site.
(bolding mine)
This is not correct, for the simple fact that *existing ready-mix concrete delivery trucks only have a six cubic yard capacity. And as a further example, a concrete slab 20’ x 25’, with a nominal thickness of 4" would require slightly over six cubic yards. (6.17 cu. yds.)
(And that doesn’t include the extra amount that would be required for any footings, piers or beams.)
*This being the norm in Houston, Texas.
I am trying to remember how I see them transferring concrete to the pavement laying machines. I know asphalt they use dump trucks to hall it from a batch plant.
On a construction site even if you had a batch plant, you would still need to move the concrete from it to where it was being used. Scale may be a factor too. One big plant and a stream of trucks from it to sites all over town. Construction sites are often crowded, stuff piled everywhere. Do you need piles of aggregate and a dry place to store the cement? Another complication is some projects must reuse a certain amount of old concrete.
it’s hard to store large amounts of dry mix on a site because it sits there and sucks moisture right out of the air and starts to clump up. also concrete for more advanced structural applications is highly engineered and that is best done at the plant. it can be mixed with a retardant so it doesn’t set for hours, but typical time to what we call cold joint is 45 minutes. it’s not unusual for small customers to split a load between jobs, and there are trucks that mix small batches out front of your house. never heard of a mixer truck drying out, they’ll sit in line for hours at a big pour, their only concern being they get paid by the load and ding dong dang form setters are costing them $$$$! i’ll have to ask a driver why the load doesn’t set in the truck, never thought about that…
For big contracts, the construction contractor does whatever is cheaper. Period. Contractors talk to each other and they fiddle with schedules and methods while the same sort of projects keep coming up for bid. So even if no particular Contractor figures out the optimal process, with a little time and a lot of contractors and projects, the general optimal process gets identified by a kind of evolution.
If the local specialty concrete supplier delivering the slurry by truck couldn’t beat the price of mixing concrete up on site, then that supplier would lose business. But they are specialists. They have the large scale storage areas and equipment. They have the experience to meet the specifications for the concrete.
Their labor costs are lower, too, unless the construction contractor mixes enough concrete in a year to have their own team of mixers. That’s because specialist labor is quicker at the job than general labor is. And fully automated is cheaper still.
Then there’s dust control. Construction sites have to follow air quality best practices. Moving gravel, sand, and powdered concrete without kicking up dust may be costly.
Materials testing for the project may be cheaper, too. If the supplier mixes bigger batches than can be mixed on site, say 26 batches vs 55, then there’s that many fewer compressive strength tests that have to be paid for.
It’s the money. Completely and totally.
That looks like the hopper is only a small thing at the very back of the truck. It says it can produce 70 cubic yards an hour, if other vehicles keep it continuously supplied with materials. They say that mixing onsite saves you from losing batches because of delays. For small or medium projects, that may be a significant advantage. I’m guessing it’s also cheaper per CY than the Onsite truck.
A typical in-transit mixing truck holds 8-9 CY and if your job is big, more than one can discharge at the same time. If you’ve got a big job, the schedule will be managed and you’ll notify the supplier exactly when to be there.
It may keep it from solidifying, but it won’t stop it from curing. Past a certain point, enough of the mix will have cured into granules to prevent it from ever properly curing into a strong enough solid. At that point, it will have to be dumped somewhere as waste. It will still be a slurry, but it will be useless as concrete when it dries. Wiki says it usually needs to be delivered within 90 minutes of loading and I’m not going to argue with that. It depends on the mix. Some need to be delivered a lot sooner.
Inspectors read the truck tags (paperwork from the supplier) before the load is poured and reject any that have been mixing too long. Many suppliers offer both wet mix and dry mix concrete. Contractors buy the wet for projects near the batch plant and dry for those far enough away to make overmixing a risk.
It’s all about the money.
I believe in continental Europe it is more common to deliver dry cement and mix it on the jobsite. You often see the big hoppers and silos at construction sites, and see many fewer delivery trucks.
Good to know. You reminded me of something.
It’s not always all about the money. Sometimes it’s about the regulations. I don’t know if the European standard is due to the money or the regulations. But I can imagine a city refusing to permit a line of trucks with expiriing loads in a given area, for various reasons, most of them traffic related.
And I guess it could be the way things are expected to be done, too. Locally, here, it’s usually about the money. Most areas can handle the traffic, temporarily, and everyone expects builders and contractors to do what’s cheaper.
Here are some of the reasons for not waiting to add the water to the load:
By adding the water at the plant it is a more controlled environment where the correct amount of water can be added. Having the water off by much can make a big difference in the slump (consistency) and compressive strength of the concrete. The load can be rejected if the slump or other properties of the concrete are off so the supplier needs to get it right.
There can be moisture in the gravel and sand that goes into the mixture. This moisture needs to be accounted for in the mix design. They would “start the clock” on that load of concrete as soon as the portland cement was added to the moist aggregate. There is also usually a little wash water left in the truck from the previous load. By not adding the normal mix water at the beginning, clumps would tend to form in the areas near the moisture and the load would not mix as well.