Pond Boss Forums INTRO Questions & Observations Best caulk for cracks in concrete spillway

I have used Sika brand products. I believe this is what we used. https://usa.sika.com/en/construction/repair-protection/mortars/vertical-overhead/sikaset-plug.html

It's not a caulk but a hydraulic cement that will set up under water. I worked with a guy that repaired concrete and we injected epoxy into a floating 4 lane highway that was towed down to San Diego bay from Washington state. It developed cracks on the journey and was taking on water. We used epoxy on the inside to inject into the cracks, and we had a diver under the water on the outside using this product to patch the cracks when he saw epoxy leaking out. We'd mix up a double handful in the inside rubber half of a basketball, he'd go underwater and apply it to the crack so we could continue to inject the epoxy from the inside. You can't mix up a lot at a time, as it has a 5 minute working time before it's set up.

Your issue is the water. Caulk will not stick to the concrete if it is wet, and everything else besides (I believe) butyl caulk will shrink as it cures. This Sika product actually expands a bit as it cures. They also make a caulk that I have used on a driveway to patch a wide crack. I used a foam backer rod so I wasn't using too much caulk. It's held up for 10+ years, but now it's starting to pull away from the sides so it will have to be replaced.