Two destinations, and somebody picks one before a truck rolls
The honest fork is whether the rinse stays on your property or leaves with the truck. Both are normal and neither is a trick question.
Leaving with the truck is the tidier of the two on paper. A ready mix driver rinses the tail of the chute into a bucket or a catch pan, carries it back and washes out at the plant, where they have a proper setup for it. It costs you nothing and leaves nothing behind. It works when the volume is small and when the driver has the room and the time to do it that way.
Staying on site means a contained low spot, lined so the solids stay put, in a place you agreed to before anybody started. It is the workhorse option, because on most jobs there is something that has to be washed out where it stands. It costs you a patch of ground for a day and a lump of hardened concrete that has to be dealt with at the end.
The thing that usually settles it is the pump. A trailer sits where the mixer can reach it, not where the truck is going next, so the hopper wash and whatever comes out of the line have to be caught at the machine. On a pumped pour, a spot on site is not really optional. It is just a question of which spot.
A chute rinse and a line cleanout are different amounts of material
People picture washout as a bucket of grey water and on a chuted pour that is close to right. The chute holds a little, the driver rinses it, and that is the end of it.
A pumped pour produces more, and it produces it at both ends of the day. Before any concrete goes down the hose, the line gets primed, and that priming slurry is there to coat the inside of the hose so concrete slides instead of grabbing. It comes out ahead of the concrete and it does not go in your forms. Somebody has to catch it, and that is the first thing the washout spot does.
At the other end the line gets blown clear, the hopper gets washed and the pump hose gets rinsed before it is coiled. That is the bigger share of the day's washout and all of it happens at the trailer. None of it is a surprise, which is exactly why the spot gets picked in advance rather than while a hose is dripping.
Picking the spot on your property
A good washout spot has four things going for it. It is somewhere water cannot leave from, so not a swale, not the head of a ditch, not the high side of a bank running down toward a neighbour or a creek. It is away from a well head and away from anything that drains to the road. It is reachable, meaning the trailer or the truck can get to it without a second trip around the house. And it is ground you were not precious about anyway.
On a lot of jobs the obvious answer is already on the site. If old concrete is coming out, the excavation is right there. If a shed pad or a footing is getting dug the same week, that hole works. On a sloped Blue Ridge lot the choice matters more than it does on flat ground, because everything on those lots eventually goes downhill. Where a site already carries a silt fence, the washout spot belongs inside it rather than below it.
You get the final say on this and you should use it. If there is a corner you would rather keep clean, or a spot where the ground stays wet, say so when the route gets planned. Moving the spot is a two minute conversation on the phone and an argument on pour morning.
Why it is contained rather than just poured out somewhere
Wash water off fresh concrete is not plain dirty water. It is alkaline and it carries fine cement in suspension, so run across open ground it will burn grass and leave a pale dead patch that takes a while to come back. Sent into a ditch it clouds the water and carries sediment on downstream.
Contained, the solids drop out and the water has somewhere to sit instead of somewhere to go. That is the whole reason for a washout pit, and there is more on what goes into one in our glossary.
What happens to it once it hardens
It sets up. Whatever went in the pit becomes a lump of concrete in the shape of the hole, usually within a day.
From there it either gets broken out and hauled off with the rest of the site cleanup, or it stays where it is if the spot was going under fill anyway and that was the agreement from the start. What it does not do is sit open on your property after the crew has gone. If the pit was dug for the day, the day includes filling it back in.
Leftover concrete in the drum is a separate question from washout and it gets handled differently, usually by putting it to work somewhere on the job rather than dumping it. There is an answer on this site that walks through that.