The order is where a hot load gets headed off
Concrete begins its chemistry the moment cement and water meet in the drum at the plant, and everything that happens afterward is happening on that clock. So the first place a hot load is prevented is on the phone before anybody orders, not on the driveway.
That conversation covers the mix, the weather you are pouring into and the time the load will spend riding. Warm air shortens the working window on its own, and a mix dosed for a mild morning will behave differently on the first hot afternoon of the season. Your supplier adjusts for that once they know what the load is for and where it is going, including whether it is coming through a pump. Loads also have limits on how long they can ride before the plant will not stand behind them, which is a clock with its own answer on this site.
The site has to be ready before the truck is, not after
The second place hot loads get made is a site that is not ready when the concrete arrives. Forms still being staked, steel not tied, a gate nobody unlocked, no water for priming, no agreement yet on where washout goes. Every one of those turns into minutes with the drum turning and the clock running.
So the morning of a pour is deliberate. We show up ahead of the concrete. The pump gets set on solid ground where the mixer can back up to the hopper, the line gets run and primed, and the finishers are standing there with tools in hand when the first truck rolls in. Nobody is waiting on anybody. On a multiple load pour that discipline carries all the way through, because the second truck sitting at the end of the drive is a load quietly getting older.
How you recognize one at the chute
A hot load announces itself in the first thirty seconds. It comes off the chute in lumps instead of a smooth flow, or it stalls on the chute entirely and needs help. Slump is visibly down from what was ordered. In the forms it does not level out and settle around the steel, it sits where it lands and has to be worked, and the surface starts to close up sooner than the crew is ready for.
On a pumped pour the tell is at the machine. Pressure climbs on a line that ran fine ten minutes earlier, because material that has stiffened does not slide through hose, it packs. A line that plugs halfway through a slab is the worst possible time to stop, because the concrete already in the forms does not wait while anybody breaks hose apart.
What happens next, and what should not
What should happen is a phone call about that specific truck, from the people on site to the plant that batched it. They know what went in it, when, and what can honestly be done about it. Sometimes the load can be corrected properly at the truck. Sometimes the answer is that it should not go in your slab and another one is coming.
What should not happen is somebody opening the water valve to loosen it up. Water thrown at a stiff load makes it move again, and it does that by trading away the strength you ordered. That trade is permanent. The slab ends up weaker than what is written on the ticket, nobody can see the difference on the day, and the place it eventually turns up is in the surface a few winters later. If you are standing on a pour and see water going into a stiff load, that is a fair moment to ask who authorized it.
Deciding whether the pour keeps going
The last decision is the hardest one and it belongs to whoever is running the pour. Placing a stiff load next to concrete that is already setting up gives you a cold joint through the middle of a slab, which is a weak line in a place nobody chose. On a driveway or a garage floor, that is the thing you are trying hardest to avoid.
Sometimes the honest call is to stop, place what is good to a joint you pick on purpose, and come back. That is a worse morning and a better slab. Losing an hour is a cheap price next to putting concrete in the ground that we already know is not what was ordered, and if that is where a day is heading we will say so while there is still time to do something about it.