sidewalk

it’s easy to find long deposites of sidewalk next to most any street
it’s slightly harder to find chuncks
where: retaining wall construction sites, corners and edges of side walk squares 

appears in stretches of 200ft-300ft, but in other parts of the country (like utah) you can easily find ones that stretch 660ft
you can tell the age of some sidewalks by stamps

most often sidewalk is made of of concrete containing river rock that are sunken below the smoother surface, you can see these rocks if you flip over a broken piece, or on end sections that have been ground away to re-level the sidewalk. 

if you’re trying to collect, I’d suggest looking at the edges of side walk strips, there you might find pieces that have broken off and remain
don’t take so much that it becomes an inconvience to hazard to anyone, cause then it’ll have to be repaired. 

also beware of fake pieces

uses:
pathways
vegetation supression
retaining walls