What material can you insert on expansion joint?

First we removing old material from a concrete expansion joint then foam backer rods act to fill in space between concrete joints so you don't need to use a ton of self-leveling sealant and one thing you should be remember that the diameter of the backer rod should be1/8 inch larger than the width of your concrete expansion joint and after that squeez the sikaflex the sikaflex sealant into concrete expansion joint and allow it to flow and self level itself. And we can also use any other material to filling this but the procedure are same.