Why is concrete delivered as a slurry?

It sure seems like it would eliminate a lot of potential problems if the mix was delivered as dry ingredients, and then mixed with water at the site. But, there must be a really good reason why it’s delivered as a slurry, with the clock ticking…

I believe part of it is that it’s just easier to mix it that way (it’s being mixed as it’s being delivered, so it saves a lot of time). Also, they’re starting the drying process. I think if they just mixed it up on site and drop it into the forms it would just pour right through them. They also wouldn’t be able to finish the top or sides. This way it was some time to start stiffening up before it’s poured.

You can get it delivered dry, or buy it that way at all sorts of stores. Mixing it is big work, especially enough to pour a foundation or something similar. You need big powerful equipment. Therefore, for anything big (which is typically a pretty scheduled event anyway), it makes more sense to mix it in the truck on the way over.

I would consider the relative consistency between large controlled truck batches to be much better than a series of manually churned smaller batches. More consistency in creation means better control over the end result.

There isn’t always a ready supply of where the concrete is to be poured. If a mixer showed up at my house , to pour a driveway for example, how long would it take to fill the thing using my little garden hose (no penis jokes, please.)

Plus, it is my understanding that a mixer might have concrete for more than one customer at a time. I need 5 yards, the guy done the street needs 10 and another guy needs 10. One mixer comes out and services us all rather than going back and forth.

That would only work if everyone ordered the same mix at could take delivery literally within minutes of each other. Last time I had concrete delivered it was fiberglass reinforced and dyed red…I don’t think they were going anywhere else with that other then back to the factory to clean out the mixer.
OTOH, it wouldn’t surprised me if they had one ‘standard’ mix that was much cheaper just to try to get more customers to order it in an attempt to get as many deliveries onto one truck as they can.

I realize that for medium-sized jobs (like, pouring a home driveway), a mixer full of pre-mix has significant benefits. But, this is also done for really big jobs, like pouring the floors in huge office buildings. I guess the answer is probably what **Joey P ** pointed out - that the time benefits outweigh the possibly of having to dump a load because the truck got stuck in traffic.

If they do get stuck in traffic they can add something to keep it wet longer, but they risk having rejected because of the additive.

One of my survivial jobs included hydroseeding. Coming by a quick 500 gallons of water isn’t easy.

Slurries are easier to handle. Very fine particle materials such as portland cement and lime can be a pain. The bulk plant has scales and conveying equipment plus a big water meter.

The factory I once managed made the lead diphthalate into a slurry to control the exposure to lead. It worked quite well. That kept OSHA and the EPA off my back.

Can we make a penis joke about this line?

I had the floor and walls of an addition done that way. Worked out pretty good.

And outfit like this - http://onsiteconcrete.net/

Concrete doesn’t dry, it cures. I do not know what the tolerance is for mixing to when it sets, but adding water to concrete to delay curing will weaken it. Some water will dry up, but water chemically binds with the ingredients.

I thought the constant motion of the mixer truck slowed or somehow delayed that curing reaction, thus eliminating the “stuck in traffic” problem mentioned above. Can anyone clarify if I somehow made that up or something?

mixing delays curing but not for huge amounts of time.

On site mixing is indeed available as pointed out by Enipla, but for reasons that I do not know, it is not favored for big construction (as in high-rise building and such).

I would guess that on-site mixed concrete is more expensive than the other kind. For a small home improvement project, a slight difference in cost isn’t a big deal, and you can also get a less-than truckload quantity, but when your need is multiple truckloads, it’s worth it to use the premixed stuff.

Vast amounts of cement are used in oilfield drilling (to isolate the different formations drilled through and to support steel casing that lines the hole) and pretty much all of it is delivered as dry materials and batch-mixed on site.

Er, just to expand on that a bit, oilfield cementing jobs generally require customized additives that vary depending on downhole temperature and pressure conditions, and are often in locations where it would be impractical to deliver pre-mixed product.

I think the bottom line answer to the OP is simply that it works well delivering it mixed. Any additional time spent messing about adding water and mixing at the delivery site is time lost for the truck that could have been used delivering more concrete. Issues with trucks becoming snarled in traffic and having to jack hammer out their load are rare enough that any potential gain from avoiding this is swamped by the simple economics of keeping the trucks moving and delivering more loads per day. I would imagine that if you have a load to be delivered to somewhere where there was a significant risk that the load would cure in transit that a post mix delivery would be an option.

Further, I suspect that the majority of concrete orders are not simple single loads. Where I am building foundations probably account for a huge fraction of the business, and even for a small house, this requires a number of loads, and a coordinated effort with a concrete pump, quite a number of workers and trucks arriving and delivering their load in a continual stream. A major building foundation pour is a sight to behold, and is always done at night to allow the entire effort to run with minimum disruption.

My understanding is that the elements and aggregate of dry concrete can settle out during transport to a work site. A slurry will better hold everything in suspension for a more even mix when ready to use, and quantities of water can be better managed from a central mixing site.

My 2¢.