During the May 2010 floods here in TN the pressure of ground water caused water to shoot through these gaps and flood the basement.
I need to dig out this old material and reseal all the way around.
Do I even need expansion in a buried basement? Isn't temperature basically constant?
The water shot up during the flood due to intense ground water pressure (hydrostatic) under the house (due to the flood) and if it hadn't come up through the expansion joint it would probably have caused the slab to float and/or severely break apart.
At any rate, a slab does shrink or expand from drying or temperature changes or as the sub grade compresses a little.
Basements are not usually subject to extreme temperature changes, but even in TN the slab temperature can vary considerably if the basement is not heated in winter. I lived next door to TN in NC for 27 years and I know what your weather is like.
At any rate, expansion joints are required by most building codes partially because no one knows how long the basement (foundation) will be uncovered or unprotected. I have seen basement slabs poured and left for weeks before construction of the house commences.
So, if the expansion strip has disintegrated or rotted away, dig, chip or chisel out what’s left of it and remove all the loose pieces with a vacuum cleaner.
Buy a new expansion strip and cut it to size. Slide the new strip into the joint and tap it with a piece of wood scrap until the top is 1/2 inch below the concrete surface. Caulk the joint with an elastomeric sealant.
Or, the top part of the bad expansion joint can be cut off and the space filled (Caulked) with an elastomeric sealant.