the differences between rusticated concrete blocks and concrete masonry units (cmu) are quite minimal. in fact, rusticated concrete blocks could be considered a type of concrete masonry unit. the main difference here is the treatment of the face of the concrete. as its name suggests, rusticated concrete block involves rustication, leaving the face that will be on the outside rough and often raised towards the middle. while architects may specify rcb as a cladding element, cmu is more often used in behind-the-scenes structural work.