How long should I leave the internal bracing on a custom drainage inlet?

You need to give that concrete at least 24 to 48 hours before you even think about knocking those internal 2x4 braces out. Since you mentioned the humidity is high and the mix feels sluggish, you're looking at a slower hydration process. If you pull those supports too early, the green concrete won't have the compressive strength to fight the dead load of that top arch, and you’ll end up with structural cracks or a total slump that ruins the flow line. If you're worried about the finish, you can ussually strip the outer forms after about 4 to 6 hours to rub the exterior walls, but keep those internal structural braces pinned. For the inside edges, wait untill the concrete is 'thumbprint hard'—meaning you can press your thumb into it and it leaves an indentation but doesn't stick to your skin. At that point, you can reach in and do some minor **edge tooling** without pulling the 2x4s. If you absolutely have to smooth the throat of the inlet, just work around the wood.

Another thing to watch is that dirt embankment. Since it's unstable, any vibration from nearby equipment or even heavy foot traffic can shift the soil and put lateral pressure on your fresh walls. If the soil moves before the concrete hits that 2500 psi mark, it'll blow out your forms or bow the walls. I'd honestly throw a tarp over the whole thing to keep the moisture in and let it cook for two full days.

And watch that purple PVC riser. If you didn't anchor it well, the buoyancy of the wet pour can sometimes cause those pipes to float or shift an inch or two out of plumb. Check it one last time before it fully sets. Once you finally pull the wood, keep an eye on the **arch transition** for any hairline fractures. If you see them, you pulled the supports too early.

I'd definatly wait. I've seen arches sag just a hair—not enought to collapse, but enough that the cast-iron grate didn't sit flush later. That's a massive pain to grind down. Since you're in loose dirt, maybe throw some plastic over the top if there's any rain in the forecast. You don't want runoff washing out the bottom of your forms while it's still curing. If you gotta smooth it, just reach around the bracing with a margin trowel. It won't be perfect, but nobody's gonna be looking down the throat of a drain anyway once the lid's on.

Listen, if you're using a standard 4000 psi mix and it’s sluggish, don't rush it. I’ve seen guys pull forms too fast and the arch just deforms enugh to ruin the flow. If you want a better finish without compromising the structure, I usally spray some Sika LateX R or a similar bonding agent if I have to do any touch-up sack-rubbing later. Also, did you use any Fritz-Pak or similar mid-range water reducer? If not, that humidity is your enemy. Keep those 2x4s pinned untill tomorrow afternoon. Better to spend an hour grinding a high spot later than to watch your invert collapse becasue you wanted to play with a margin trowel too early.

Quick update—I took the advice to wait a full 48 hours beacuse of that humidity. The arch held perfectly and didn't sag, which is a huge relief. However, I'm still seeing some minor honeycombing near the base where the vibrator couldn't reach. Should I patch that now while the concrete is still green, or wait untill it's fully cured?