I’m beginning to favour the technique described below as it gives me more modelling options. At any rate, I don’t end up with overly antiseptic look working with plastic.
The cardboard I use is from an old Priority Mailbox because it’s the thickness that works best. Using a carpenters square, I cut my slabs in rectangles 2′ X 2 1/2″. Sidewalks made first, and you can make them with thicker cardboard, Masonite or even soft pine paint stir sticks. The above picture has thicker cardboard. The gas station below was built on a thin Masonite base that includes the sidewalks, and the buildings to the right have paint stir sticks. Working with six slabs at a time, coat the layout base with Carpenter wood glue and press them in place. Because the cardboard will curl, now cover the top of the carton. Use a cardboard scrap to trawl the glue flat for both applications. Now sprinkle the #1290 Concrete powder on the wet glue. Use a pallet knife to trawl the powder flat. When the dust gets to gluey from the glue below, hit the area with a light water spray to make it workable. An occasional slab can be shimmed up slightly on one edge to model a frost heave, but don’t overdo it.
Before the glue sets hard, clean out the joints with a sharp tool. I use an angle piece of brass as a corner tool for curbs and gutters after everything is dry, sand away any lumps and rough edges.
To model large cracks, I’ll cut some of that unfinished cardboard in random jagged pieces and then install in the method described above.
I photoed the scene before everything had a chance to dry and couldn’t sweep up the loose powder on the street. It does show how deep cracks in the concrete looks.