Is it normal for concrete to separate right after the pump?

That separation while it's moving through the pump is definately what's known as segregation, and it's a common problem when concrete gets under a high head of pressure. When the pump pushes the mix, the force increases the friction and velocity, which acts like a centrifuge for the mix. If the concrete wasn't fluid enough to begin with, or if the mix is approaching the upper limits of its slump, the heavier coarse aggregate will start to settle out of suspension before it hits the forms. What you're seeing is usally caused by two things: poor workability for the intended pump distance, or the mix design just isn't optimized for pumping. When you check the slump in the wheelbarrow, that's just a snapshot, but the mixing and pumping process introduce shear forces that can easily break down the structure of the mix. The liquid paste holds the rocks in suspension, but if that paste is too stiff, the rocks will drop. To handle this specific situation, you need to tell the truck that the concrete is going to be pumped, not just poured. This means adjusting the slump slightly higher than you might typically want for a floor, or perhaps ensuring the producer is adding enough liquid or a suitable plasticizer to maintain a stable consistency under strain. If the aggregate is floating, it means the flowability is too low.

If you can't adjust the mix, then the boom operation needs to be managed carefully. You want a slower, more consistent flow through the boom, and avoid any sudden stops or sharp turns in the line. Fast deceleration can cause immediate dropout of the heavy stones, which is exactly what is happening now.

I think people tend to overlook the interaction between the aggregate shape and the paste itself when this happens because simple slump tests are totally misleading during transport. Even with a higher-than-average slump, if the aggregate is poorly graded, that voids the suspension ability of the cement paste prematurely. It relly depends on how much workability index the supplier is actually aiming for under extreme shear forces, not just what the wheelbarrow looked like. Have you considered if the pump station itself, like a V-notch pump, might be contributing to the localized high shear that forces the paste to fail?

I ran into this exact issue pouring my driveway last year. We were having the same problem with heavy aggregate dropping out while the pump was running. The thing that changed it for us was using an air entrainment additive in the mix. It seemed to dramatically improve the stability of the flow and kept the stones much better suspended even under that high pressure. Are you guys finding that the pump itself is maintaining consistent pressure, or is the flow rate shifting as the line fills up?

I've run into this when the concrete just starts dropping too quickly into the forms. If the pump is moving the mix across a large distance and then dumps it all at once, you’re guaranteed to see the heavy stones drop out. What helped me was asking the pump operator to slow the feed rate significantly when they got close to the pour area.

It sounds counterintuitive, but giving the mix more time to settle into place at the forms rather than just blasting it out really kept the heavier material suspended. You've got to manage the flow, not just the pump pressure.

It sounds like the mix itself is the main issue here. I've had similar problems using specific types of crushed rock that don't have the right gap between the stones. Even if your slump is high, those bigger voids mean the cement paste can't hold everythign up under pump stress. You might have to check the aggregate grading with the producer. It's frustrating because you're just trying to get the job done without issues.

Update: slowing the pump feed rate down made a big difference, way less of the stone dropping out at the drop point. But the mix still looks a bit off coming out of the hose, more watery than it went in. Has anyone actually tried the air entrainment thing on a pumped pour, or did you just call the supplier and ask for a stiffer mix instead?