Foam Backer Rod
Use Foam Backer Rod to fill in the expansion joint below the caulking. Buy the size that is slightly larger than the width of your expansion joint. If the joint has uneven widths, use a razor knife to slice the backer rod into smaller widths, or buy more than one size backer rod. Measure your pool perimeter for the proper length, and buy a little extra.
Using sand in your expansion joint is a Bad Idea, as it will not allow compression as foam will, and as your deck expands it will exert forces on your pool beam, tile and coping.
SWIMMING POOL CAULKING
Insert the backer rod to a depth of 1/4" into a clean, dry expansion joint. It is important that you remove all debris or sand first. The expansion joint should be "True", that is, extend between the deck and the coping stone, all the way to the earth.
If the joint is very uneven in width, you can slice the backer rod with a razor knife to fill in the gaps, or buy more than one size of backer rod. The rod must fit tightly, as shown in the diagram. Self leveling caulk can be very runny, especially on a warm day, and you will lose a great deal of caulk that will run down into the joint.
If you have a new pool, and there is a strip of rough, white foam, about 3/8" wide, already in the joint, this can either be cut down 1/2" with a razor knife, or burned down with a small gas torch, or just pushed down a bit into the joint, before adding your backer rod.
Begin with a dry joint, and make sure there is no rain forecast for the day. Insert your tubes into the caulking gun, snip the tip, and puncture the inner seal. Self leveling caulk