Wood separators in concrete rotting problem

I have a large concrete driveway which is about 20 years old, and the wooden separators (for concrete expansion) in between the slabs is severely rotting.
What are my best options? I don't really want to replace the wood with wood, because it will just rot again eventually and attract even more bugs.
What else can I use to replace the wood? Does it even have to be replaced?
Thanks in advance.
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