I'm on the breach first too.
When roots grow, it's by the very tip which is very weak but is able to sneak in every little crevice. After all, it's their job to explore the ground. Then the diameter growth cames in play and this time, it's like a jack. Tiny at first, but more and more strong as the contact area increases. The cells can develop an inside pressure up to 200psi by osmotic pumps and that's what they can oppose to any obstacle. The sidewalk taken by the underside stands no chance at all. But a continuous surface (like metal, plastic or good concrete) forces the root's tip to turn away. After that, the root's side pushes against the surfaces, slowly but powerfully. Inverted tug of war. Unlike the concrete, the plastic/rubber membrane is souple enough to give room to the root's diameter growth without breaking. If there's a defect though, like a bad seam, a puncture from a stone, a wood chunk or a rodent's shew, you can bet that the roots will find it /them and go through.