Where the clock starts, and what it is actually measuring
It is a batch clock. It starts at the plant, the moment water meets cement in the drum, and the batch time is printed on the ticket the driver hands you. Everything after that spends it. The drive out, a stop on the way, time waiting at the end of a drive while somebody finishes tying steel. On a job a fair drive from the plant, a good share of the window is gone before anything has been placed.
The revolution count is the same idea measured another way. A drum that keeps turning keeps working the load, and the mixing that keeps concrete uniform on the road is slowly spending the workability in it. Counting turns catches a truck that sat and agitated in a yard for an hour, which a clock on its own would treat the same as a truck that drove straight out and back.
Neither number is measuring the concrete itself. Both are measuring what has happened to it, which is a fair stand in when nobody is testing every load at the forms.
The clock against the concrete, and which one gets the last word
That distinction is the real fork in this question, and crews land on both sides of it. One way to treat 90 minutes is as a hard stop. The ticket says the batch time, the batch time runs out, the load does not go in. The other way is to read the concrete in front of you and treat the clock as a warning rather than a verdict.
Both have a failure mode and it is worth being honest about each. Hold it as a hard stop and you will occasionally send back a load that was still perfectly good, on a cool overcast morning with a mix carrying enough retarder to sit comfortably past the mark. Hold it as a suggestion and you will eventually place a load that had already turned, because on open ground in July concrete can be stiff and unwilling at an hour, well inside a window somebody was relying on.
The limit can be extended when the supplier and the buyer agree and the load still checks out. What it cannot be extended by is a garden hose. Loosening a stiffening load with site water buys back workability and pays for it in strength, and the strength does not come back.
So the useful way to hold the number is as the outside edge of a plan rather than a target to run up against. If a pour only works when every truck discharges at minute eighty eight, that is not a schedule, it is a hope.
Stacked trucks are how a good plan loses the clock
Most loads that run out of time do not run out on the road. They run out in line, on the job, with the drum turning and nowhere to put anything.
It happens two ways. Either the order went in as one delivery and three trucks arrived inside twenty minutes of each other, or the crew was not ready when the first one landed. Forms not finished, steel not tied, a gate still locked, a grade still being cut by somebody who thought there was more morning left. The first truck to arrive is the one carrying the most time already, and it is the one that ends up waiting.
The fix is not complicated. Work out how fast concrete can honestly go in the ground on that site, then ask the plant to space the trucks to match it. A crew that places and finishes at a steady rate can say roughly how long a load takes them, and the spacing falls straight out of that. Be honest about the start as well. If the forms will not be done until nine, the first truck gets ordered for nine, not for eight thirty in the hope of gaining half an hour.
How fast the load comes off the truck
The other half of the clock sits at the placement end, and this is where access costs more time than distance ever does.
If the mixer can back up to the forms and the chute reaches, concrete comes off about as fast as the crew can handle it. If it cannot, somebody is carrying it. Hauling a load across a yard by barrow is slow work, and a truck standing at the road while that happens is spending the same minutes as a truck stuck behind a wreck, except this is happening on your job and it is happening to every load in the order.
That is a large part of why a pump gets put on a pour with poor access, and it is the least discussed part of it. We run a trailer mounted line pump on 2.5 inch and 3 inch line, and once the line is laid and primed, the machine takes concrete off the truck steadily instead of a barrow at a time. On our own pours the line is out and primed before the first truck arrives, so the driver backs to the hopper and starts discharging rather than watching a crew set up.
A pump can move concrete faster than a crew can screed and finish it. That is the point. It moves the constraint back onto your finishers, where it belongs, instead of leaving it on the delivery clock.
When a load really has run out of time
Sometimes it happens anyway. A truck breaks down, a road closes, a load shows up stiff enough that it is arguing with the chute before anybody has touched it. A load that has begun to set on the way out is a hot load, and the clock is only one of the things that makes one.
The move is a phone call to the plant about that specific truck, before any of it goes in the forms. They batched it, they know what is in it, and they can tell you whether a correction is available or whether that load should turn around. What is not on the list is opening the water valve on site and hoping nobody asks later.
Then think about the hole a rejected load leaves. Concrete already in the forms keeps setting while a replacement is on the road, and if the wait runs long the joint between the old and the new does not knit. That is a cold joint, and on a slab it is a line you keep. Worth knowing before you decide, because sometimes the right call is to stop the pour at a joint you chose rather than run it into one you did not.