This is definitely a labor-intensive option, but it turns out looking pretty realistic. First learned it when I was a scenic intern at Williamstown, and I've used it in a number of other theaters for my own designs.
https://goo.gl/photos/QyavhdNuigVN95U88
https://goo.gl/photos/S4qaGxvvabV5TQw47
1. Cut your "bricks" out of a sheet of Homasoate (compressed newspaper board), then carefully split the
thickness of the bricks in half so you
reveal the texture of the inside of the material (it will look a
bit flaky, this is what you want). You should end up with bricks roughly a 1/4" thick, with a
flat side and a textured side.
2. Apply the bricks to your flats or sheets of plywood textured-side out, and using a mat knife cut off corners, knock down edges, generally make the bricks look less uniform.
3.
Cover the whole thing with, "goop," a mixture of 1 part joint compount to 1 part white glue (approximately, mix to taste) and add additional texture with
Jaxsan or something similar. You'll apply your "goop" with chip brushes that you don't intend to keep, go thick over the bricks and thinner in the grout lines. Experiment a
bit and see how thin you can go to keep the texture of the homasoate coming through while sealing it well, and tint the goop whatever color you want the bricks to be (or whatever color you want the grout, depending on which one you want to paint in later).
4. Paint in your grout lines or your bricks with a contrasting color (for concrete
block, maybe light gray for the bricks and middle gray for the grout).
5. Make
stock brick-covered scenic pieces so you never have to do the whole process again!
Good luck!
