Why rubber? I remember way back in the day they changed the surfaces a couple times, but we always preferred gravel. Sand we never had, but wood chips had splinters and rubber got really hot in the summer and gave us more burns or scrapes when we fell. Gravel was dusty but cushioned our falls better.

Man, I remember when it was just grass. It was removed, IIRC, because parents got annoyed their kids came home with grass stains on their pants.