First the trailer gets parked and the hose route gets walked
The machine tows in on a trailer and works from the ground, so the first thing settled on pour morning is where it stands. It wants footing that is firm and near enough to level, with space beside it for a mixer to pull in and empty into the hopper. A drive, a road shoulder, the edge of a field: any of those hold it fine.
The line goes down next, and it goes down early rather than while loaded trucks sit waiting. Hose arrives in sections with clamps at the joints, so the run gets assembled to fit what is actually in front of us. Sections go on to reach around a building and come back off when the forms turn out nearer than expected. Every bend in it stays wide and gradual.
Resistance is the reason for all that care. Length works against you, bends work against you, and so does every foot of climb between the machine and the forms. Lay the route badly and the operator spends his morning fighting his own hose, which is why the access questions get asked on the phone days before anybody loads a truck.
Nothing goes down a dry line
Hose gets primed before concrete ever enters it. A thin slurry runs through first and leaves a wet coating on the inside wall, so the load behind it slides along instead of dragging. Whatever comes out the far end during priming is not part of your pour and does not go in your forms. It has a spot picked out for it ahead of time.
Priming is not a step worth trading for ten saved minutes. Dry hose is one of the more reliable ways to start a blockage, and a blockage partway through a pour turns the whole morning into pulling line apart while trucks keep arriving.
What happens inside the machine once concrete arrives
The mixer backs in and empties into the hopper on the trailer rather than into your forms. From there a piston pulls concrete down out of the hopper and drives it into the line under pressure. Trailer machines generally use a pair of pistons working opposite one another, one filling while the other delivers, with a valve handing off between the two. That is why the stream at the placing end arrives even rather than in surges.
The hopper has a paddle turning through it, keeping the load moving so nothing sits still long enough to stiffen or come apart before its turn. Over all of it sits the throttle, and on flatwork the throttle is the control that matters, because the correct speed for the machine is whatever speed the crew can screed and float at. That number moves around several times over a morning.
Line size is the other variable. Ours is 2.5 inch and 3 inch. The narrow one weighs less and is far easier to steer by hand down a footing trench or in through a doorway. The wider one gives the concrete more room, so it rubs less on the way past and delivers more volume without the machine working as hard for it. Which one belongs on a given pour is a subject with its own page here.
Two people, one at each end, talking the whole pour
The job takes two and neither one can cover for the other. One stays with the machine, feeding the hopper off the truck and watching pressure. Somebody who has run one a while picks up a change in the sound of it before a gauge shows anything, and that is usually the first warning that a line is beginning to pack. The second man is out at the discharge, putting concrete exactly where the finishers ask for it.
The two of them keep talking the entire time, because only the man holding the end of the hose can see whether the pour is getting away from the crew. Where concrete starts and which way it travels is decided before the first yard moves. A slab gets filled from the far side back toward the exit, so nobody has to cross fresh concrete to leave. A trench gets fed from one end and run through in a single pass. An interior floor gets placed from inside, with the line brought in through whatever door or window opening the framing allows.
Cleanout is the last thing that happens and the first thing agreed
With the final yard in the forms, the line gets cleared and the hopper washed down. Water and slurry from that go to the place chosen back when the route was planned, which is neither a lawn nor anywhere it can find a storm drain. Hose gets coiled, the trailer gets hitched, and the machine is out of the finishers' way while the slab is still setting up.
That is the whole cycle, and it is meant to be dull. On our own pours the two men who ran the machine walk back onto the slab and finish it, because the pump belongs to us rather than to a yard we booked it from. Most concrete crews rent one when a job calls for it. Where the pour belongs to another contractor, we do the pumping for them and their people stay on the finish.