At Rhino, we use volumetric wagons, which are different to the typical spinning-barrel style we are all used to seeing and associating with concrete.
Volumetric wagons are split into sections, in order of front-to-back: water tank, aggregate stone & sand hoppers, conveyor-belt underneath, cement hopper, and finally the screw (long hydraulic swinging arm at the back). Each of these separate pieces are for keeping the aggregate, water & cement separate until the very moment you’re pouring concrete, which is why volumetric wagons exist. In order to control and alter these different elements that go into mixing concrete; the water can be adjusted, the conveyor has separate gates for sand & stone coming through, and the cement hopper (what essentially dictates what grade your concrete is) can be controlled electronically.
But how does this all equate to concrete being poured? The water, aggregate & cement have to mix somewhere, of course. At the back of the wagon, at the base of the screw there is essentially a large catch-bucket where water is plumbed, aggregate travels on the conveyor-belt, and cement comes down from its tank above the catch-bucket. The water and cement combined created a chemical reaction that occurs instantly, and mixed with sand & stone aggregates it travels up the screw and out of the chute at the end.
The benefits: having a volumetric – or “Volley” – wagon means you can arrive on-site, ask the customer what grade of concrete they’d like, and how wet or dry, and make the necessary adjustments for pouring that exact grade and consistency of concrete on the spot. This is incredibly handy, as you can make these decisions then-and-there! Whereas with a barrel wagon you get a pre-agreed same-consistency concrete quality that cannot change unless you send it back and wait for another wagon.
The only real drawback from using a volley wagon that comes to mind is that barrel wagons would be slightly quicker depending on the operator and nature of the job. However, we think that having the ability to mix any grade of concrete you require right before pouring it is a far greater bonus.