wrote in message
Bob, I was under the impression (apparently wrong) that if water gets
into the gap and under the sidewalk, winter freezing and thawing could
cause the gap to widen or cause the sidewalk to heave and end up
higher than the driveway instead of level with it. I thiought the
expansion joint was meant to prevent that. Is this incorrect?

I'm not Bob, but the water issue is a possibility. You'll never get a
perfect seal but ice can do strange things from the expansion. I'd fill the
gap at least as well as I could with something flexible and top it with the
caulk.

That felt material is actually made from sugar cane. It was stamped right
on the last batch that I bought.