What makes a load turn hot on the way to the site
Concrete starts its setting reaction the moment cement and water meet at the plant, and the clock does not pause for the drive. Heat speeds that reaction along, so a summer afternoon shortens the usable window on its own before anything else goes wrong. Time in the drum does the same thing more slowly, whether that is a long haul, a wait at another job first, or trucks stacked up behind each other on a site that is not ready for them. A mix that is not carrying enough retarder for the conditions will set fast on a normal day, let alone a hot one.
Forcing a hot load into the forms
A load that has already begun to stiffen does not pour the way a fresh one does, and it does not pump the way a fresh one does either. Pressure has to climb to move it, and material that does not want to slide packs against the inside of the hose rather than traveling through it, which is one of the more common ways a line plugs. On an open pour it fights the chute and the shovel the same way, showing up lumpy and unwilling to settle into the forms or around the steel.
Why adding water on site is the wrong fix
A stiff load standing in the truck tempts a crew to open the water valve and loosen it back up. That buys workability by spending strength, and the strength does not come back once it is gone. It is the single most common way a slab quietly ends up weaker than what was ordered, and none of it shows until much later, when it shows for good. A hot load is not a reason to reach for water. It is a reason to call the plant and talk about that specific truck before it goes anywhere near the forms.