I found some white styrene sheets at a hobby shop in Calgary that have squares indented in it. You only need to cut it to length and width and paint it concrete if you like.
I've made sidewalks with illustration board (or you could use charcoal artist paper) it's a thick cardstock that is slightly textured on the surface. I paint it neutral gray and then drybrush it with white. Gives a neat concrete-like effect. Plus the price can't be beat! a HUGE 20" x 30" sheet for about $2.50.