Reddit reviews Tremco Vulkem V116GRY 116 Polyurethane Sealant, 10.1 oz Caulk Tube, Gray

Try some vulkem 116 - it's flexible and comes in a bunch of different colors.

https://www.amazon.com/Tremco-Vulkem-Polyurethane-Sealant-Caulk/dp/B000WTXMR2/ref=sr_1_1?ie=UTF8&qid=1487266912&sr=8-1&keywords=vulkem+116

Depending on what part of the country you're in you could probably find it at lowes/home depot/menards too

Anything you put in that's hard is going to crack again. I've used vulkem for years and it's pretty good stuff.

Looks like you need to re-do the expansion joint.

I'd start by getting a carpet knife. The hook on the knife makes it easier to dig all that stuff out of there. Get rid of the old caulk, foam backer, etc.

Take a shop vac around and clean everything out of that crack. If you leave stuff behind, it'll just be a place where things can start to go wrong again.

Now, you're going to need some new backer. You can get it at most pool equipment supply places. Here's an example: http://www.poolcenter.com/p/foam-backer-rod . You'll want to get it a tiny bit wider than the hole you're filling in. You just cut it to size using a razor knife.

Then, you're going to need caulk. I used Vulkem when I had to do this. http://www.amazon.com/Tremco-Vulkem-Polyurethane-Sealant-Caulk/dp/B000WTXMR2

The guide I used is located here: http://blog.poolcenter.com/article.aspx?articleid=6289

(Note: I'm not a pool pro. I just bought a house with a fixer-upper-pool, so have had to learn how to fix things on my own.)