Finally figured out the sound check for calling my slump before hitting the brake

I learned to listen for that 'flop' back when I was driving an old Mack DM with a 10-yard Rex mixer. You're right about that suction sound; if it sounds like a wet boot pulling out of deep mud, you’re usually in that 4 to 5 range. I also started keeping a close eye on the pressure gauge when I'm spinning it up to charge speed. On my current Granite, if the needle sits right around 1,200 psi when I’m at 1500 RPMs, I know that 3500-psi pump mix is going to be a perfect 5 without me even looking at the chute. The tricky part is when they start throwing additives in at the plant. Last month on a bridge deck pour, we were running a high-early mix with alot of plasticizer. The drum sounded way wetter than it actually was becuase the rock was sliding so fast. I told the foreman it was a 6, but it came out looking like a stiff 3. Now I always give the truck a quick lurch forward and back while I'm still in the seat. If the load surges and moves the whole cab, it's usually looser than it sounds. It definitely beats climbing up and down that ladder twenty times a shift just to eyeball the fins.

I’ve found that listening to the drum can be tricky depending on the mix design, especially if you’re hauling a 4000 psi pea gravel mix versus a heavy 1-inch stone exterior wall mix. The sound changes too much between loads for me to rely on it 100%. Instead, I’ve gotten used to watching the "climb" in my driver-side mirror while I’m backing down toward the forms.

On my Oshkosh front-discharge, I watch exactly where the concrete sits in the drum while it’s spinning at agitation. If the mud is riding high up the ribs, I know she’s stiff, probly a 3 or 3.5. If it’s sliding back down and won't catch the blades, I’m looking at a 6 or 7. I also pay attention to the "rebound" of the chassis when I hit the brakes to stop. A stiff load has a very sharp, quick jerk to it, while a wet load feels like there is a heavy pendulum swinging inside that takes an extra second to settle. It’s a physical sensation in the seat of your pants that tells you more than a gauge ever will.

That sounds like a solid system until the sun hits the drum for an hour. Have you ruled out the effect of ambient temperture on your drum shells? I've seen guys get confident with the 'slap' sound in the morning, but once that steel gets hot in the afternoon, the mix starts setting up faster on the fins. It changes the acoustics and makes it sound looser than it relly is. I’m always worried that trusting the sway of the truck might lead to a stiff load if the slump loss is hitting harder than you realize between the gate and the forms.

This is all so cool to read beacuse I honestly thougt slump was just somethig you had to measure with a cone and a ruler every single time. I didn't realize you guys could acutally hear how wet the concrete is just from the cab. Since the sound changes based on the rock size or the heat, how long does it take for a new driver to actually get good at guessing? Is there a way to practice that without looking like a total amateur if you get it wrong on a big job?