Creating More Complex Man-made Materials - Blender 2.6 Cycles: Materials and Textures - page 213

After this we built the Concrete material: from step 20 to step 21 again the basic
shader combination, from step 22 to step 32 we built the bump for the concrete, and
from step 33 to step 35 the color for the concrete.
Then it was the time of the brick masking, from step 37 to step 41; the second Brick
Texture is combined with the first one so as to leave a distorted border only on the
mortar space, to give an irregular shape to the bricks. At step 43 we added the black
and white values of the brick masking to the overall bump effect.
As the last thing we added a slight dirty effect on the bricks, from step 44 to step 49.