CMU (Concrete Masonry Unit)
CSI Division 04, Masonry
Standardized, precast concrete blocks, commonly called "concrete block" or "cinder block", laid in a mortar-bonded pattern to build walls, both structural (load-bearing) and non-structural (partition or veneer backup).
Why it matters
CMU walls can be built either as the actual structural system of a building or purely as a backup wall behind a different exterior finish, and knowing which role a given CMU wall plays changes everything about how it's reinforced, inspected, and detailed.
On a real project
A single-story retail building uses reinforced CMU walls as its actual structural system, with vertical steel reinforcing bars grouted solid inside certain block cells according to the structural engineer's spacing requirements.
Who this matters most to
A Mason lays CMU to precise coursing and plumb tolerances, since a wall that's out of plumb becomes increasingly difficult to correct as it rises. A Quality Control Manager verifies reinforcing steel placement and grout consolidation in structural CMU walls, since the reinforcing, like rebar in concrete, is hidden once the wall is finished.
Where this goes wrong
A mason crew builds a load-bearing CMU wall without grouting the reinforced cells as frequently as the structural drawings specify, trying to save material cost. The wall passes a casual visual inspection since grouted and ungrouted cells look identical from the outside, but the wall doesn't have the load capacity the engineer designed for until the error is caught by a more careful special inspection.