There are a number of polymer modified fillers that can be used. These are the same materials that are used for concrete crack injection. (See link for example.) There are also swell plugs, a though I don't have experience with these.

In the old days, the formers used cork covered with asphalt. They worked fine too.

Look for a local concrete supply company for the most appropriate products, not the big boxes.

Here is a link that might be useful: Crack Injection

A commercial roofing supply company probably would have the materials you need. NP1 by Sonolastic would be good. Best if applied on the outside.