Concrete pour timing: the 90-minute window and the cold joint it prevents

Most trades can lose an hour and make it up later. Concrete can't. The moment water meets cement the clock starts, and it doesn't care whether the pump is late, the crew is short, or the GC moved your gate time. Pour timing is unforgiving in a way that nothing else on the job is — and understanding exactly why is the difference between a clean placement and a cold joint you're explaining to the inspector.

The 90-minute window, where it comes from

The rule of thumb every concrete hand knows — “you've got about 90 minutes” — isn't folklore. It comes from ASTM C94, the standard specification most ready-mix is sold under. C94 sets a guideline that concrete be discharged within 90 minutes, or 300 revolutions of the drum, whichever comes first, measured from the time water is added to the cement. It's adjustable — a producer can extend the working time with a retarder, and a buyer and supplier can agree to different limits for a given mix — but 90 minutes is the number you plan around until someone tells you otherwise.

And that's the outside limit, not the target. Hot weather shortens the practical window; a 95°F afternoon can take usable time off the back end because the mix stiffens faster. The clock is real, and on a bad day it's shorter than the spec sheet says.

What a cold joint actually is

A cold joint is what you get when concrete that's already placed takes its initial set before the next concrete lands on top of it. The two lifts don't knit into one monolithic mass — they sit against each other as two separate placements with a seam between them. You see it as a line. What you don't see is the plane of weakness: a spot where the pour is easier to crack along, and where water and chlorides can later find a path in. On a wall or a water-retaining structure that seam is a real durability problem, not a cosmetic one.

Cold joints don't happen because someone mixed the concrete wrong. They happen because the next truck didn't get there in time — a gap in delivery long enough for the last lift to set. Which makes the cold joint a scheduling failure wearing a chemistry costume.

Where the schedule blows the window

Walk back from a cold joint and you almost always land on a timing breakdown that had nothing to do with the mix:

• Trucks stacked or starved. Order the rate too high and trucks sit on the street burning their 90 minutes in line; too low and the pump outruns delivery and the placement stalls.
• The pump or crew wasn't confirmed. A pump that shows late or a crew that's a man short slows placement to where the mix wins the race.
• A late start. The pour that kicks off 90 minutes behind schedule spends its whole day fighting the clock.
• A change nobody heard about. The gate time moved, the pour got bumped a day — and half the people on the delivery didn't get the message.

Timing is a coordination problem, not a chemistry problem

Here's the honest part: software doesn't change how fast concrete sets. Nothing on a screen buys you back a minute of working time. What coordination can control is whether the trucks, the pump, and the crew show up synchronized to the same start — and whether, the moment a pour moves, everyone who's rolling toward it hears about it before they leave the yard.

That's the whole job of a live pour board. In Place & Finish, the pour, its start time, and its full contact list — ready-mix, pump operator, crew lead, the GC's super — live on one card. When the pour is a go, one click confirms it to everyone on that list, so nobody's working off a start time that changed yesterday. When it has to move, dragging it to the new day sends the reschedule notice automatically, old date and new — so the delivery that would have blown your window simply doesn't roll. The chemistry is fixed. The coordination is the part you can actually win.

Protect the next pour

You can't negotiate with the 90-minute window, so the move is to stop losing pours to avoidable timing gaps. Put your next real pour on a live board, get its contacts and start time in one place, and let one click keep ready-mix, the pump, and the crew on the same clock. Planning Ops is built for concrete subs, veteran-owned, with a 14-day free trial and your whole crew riding free as viewers. Run Your Next Pour on a Live Board →