CMU Materials: Types, Sizes, and Specifications

What Is a Concrete Masonry Unit?

A concrete masonry unit (CMU), commonly called a block, is a precast building product made from Portland cement, water, and aggregate. CMUs are manufactured in a wide variety of shapes and sizes to meet the structural and architectural demands of walls, foundations, retaining walls, and other assemblies. Understanding how to read a project specification and match it to the correct unit is a core skill for every mason.

ASTM Standards That Govern CMUs

The industry relies on published standards from ASTM International to define quality requirements. The most important standard for hollow and solid load-bearing CMUs is ASTM C90, Standard Specification for Loadbearing Concrete Masonry Units. ASTM C90 sets minimum requirements for dimensions, compressive strength, moisture absorption, and the thickness of face shells and webs. Whenever a project specification calls for load-bearing block, ASTM C90 is the governing document. Always verify that the units delivered to the job site carry documentation confirming C90 compliance before laying a single course.

Nominal Versus Actual Dimensions

CMU dimensions are described two ways, and confusing them causes costly layout errors.

• Nominal dimension — the dimension used for planning and layout. It includes the unit itself plus one mortar joint (typically 3/8 inch). Nominal dimensions are the numbers used in architectural drawings.
• Actual dimension — the physical size of the manufactured unit as you would measure it with a tape or calipers.

The most common CMU in North American construction has a nominal size of 8 × 8 × 16 inches (height × width × length). Its actual dimensions are approximately 7-5/8 × 7-5/8 × 15-5/8 inches, because the 3/8-inch mortar joint on each face makes up the difference to reach the 8-inch and 16-inch nominal modules.

Other common nominal widths include 4-inch, 6-inch, 10-inch, and 12-inch units. Each follows the same rule: actual dimension equals nominal dimension minus 3/8 inch on each mortared face.

Hollow Versus Solid Units

ASTM C90 recognizes two categories based on net cross-sectional area:

• Hollow units — the net cross-sectional area in every plane parallel to the bearing surface is less than 75 percent of the gross cross-sectional area. Most standard stretcher blocks are hollow. The open cores reduce weight and can be filled with grout and reinforcing steel where structural requirements demand it.
• Solid units — the net cross-sectional area is 75 percent or more of the gross area. Solid units are used where concentrated loads, high compressive stress, or below-grade moisture resistance is required.

Lightweight Versus Normal-Weight Aggregate

The aggregate used in manufacturing determines the unit's weight class, which affects structural loads, thermal performance, and fire ratings.

• Normal-weight CMUs use aggregates such as sand, gravel, or crushed stone. They are denser and heavier, typically above 125 pounds per cubic foot of concrete.
• Lightweight CMUs use expanded shale, clay, slate, or similar lightweight aggregates. Density is lower, reducing the dead load on a structure and making individual units easier to handle.

Project specifications will state the required weight class. Never substitute a lighter unit for a heavier one, or vice versa, without written approval from the engineer of record.

Common CMU Unit Types

| Unit Type | Key Feature | Typical Use |

|---|---|---|

| Stretcher | Two open cores, both ends plain | Running bond field units |

| Half block (closure) | One core, half the length | Course endings, openings |

| Corner | One flat end, one core | Wall corners and pilasters |

| Bond beam | Depressed web, open top channel | Horizontal reinforcement courses |

| Lintel | Deep U-shaped channel, solid bottom optional | Spanning door and window openings |

| Split-face | Fractured texture on face | Architectural/decorative walls |

Face-Shell and Web Thickness

ASTM C90 specifies minimum face-shell and web thicknesses based on the nominal width of the unit. Face shells are the flat outer walls of a hollow unit; webs are the interior partitions connecting the shells. Thicker shells and webs provide greater compressive capacity and better fire resistance. Always consult the current edition of the standard for the precise table values—specifications can change between editions.

Safety on the Block Yard and in the Lab

CMUs are heavy and have sharp edges. Observe these practices at all times:

• Personal Protective Equipment (PPE): Wear safety glasses or goggles, cut-resistant or leather work gloves, steel-toed boots, and a hard hat whenever handling block or working in a material storage area.
• Lifting: Use proper two-person lifts or material-handling equipment for units heavier than 50 pounds. Bend at the knees, keep the load close to your body, and never twist the spine under load.
• Silica dust: Cutting, grinding, or breaking CMUs generates respirable crystalline silica. When cutting is required, use wet methods or local exhaust ventilation and an appropriate NIOSH-approved respirator in accordance with OSHA's silica standard for construction.
• Stacking: Never stack CMU samples or pallets in a way that creates a tipping hazard. Follow the supplier's pallet height guidelines on the job site.