Which of the following materials is NOT commonly found in CMUs?

Concrete Masonry Units (CMUs) typically consist of a combination of cement, sand, and aggregates, which are essential components that give the blocks their structural integrity and durability. Cement acts as a binding agent, while sand and aggregates provide bulk and strength.

Clay, while it is a primary ingredient in some bricks and tiles, is not a standard ingredient in traditional CMUs. Therefore, while it may be associated with masonry in general, it does not have a place in the composition of CMUs.

Glass fibers, on the other hand, are not a traditional or standard element within the composition of CMUs. They might be used in advanced masonry products or for specific applications to enhance performance, but they are not commonly included in standard CMU formulations. This distinction makes glass fibers the correct answer to identify as the material that is NOT typically found in CMUs.

In summary, CMUs are primarily made from cement, sand, and aggregates, making glass fibers the outlier in this context.