CMU Block Dimensions and Sizes Explained

CMU blocks, also known as concrete masonry units, are an essential building material for construction projects. Understanding the standard dimensions and sizes of CMU blocks is key for engineers, architects, and builders to optimize layouts and coordinate with other systems.

Standard CMU Block Sizes

CMU blocks come in standard nominal dimensions based on their width, height, and length. However, the actual dimensions will be slightly smaller due to manufacturing tolerances and mortar joints.

Here are some of the most common standard CMU sizes:

• 8 x 8 x 16 inches (nominal)
• 12 x 8 x 16 inches (nominal)
• 16 x 8 x 16 inches (nominal)

There can be regional variations in the popularity of certain standard CMU sizes. For example, the 8 x 8 x 16 CMU is very common in the Western U.S., while the 12 x 8 x 16 size is more popular in the Midwest and Eastern regions.

Nominal vs. Actual Dimensions

The nominal CMU dimensions refer to the manufacturer's specified size. However, the actual dimensions will be slightly smaller due to:

• Manufacturing tolerances - ASTM C90 allows up to 1/8 inch variance.
• Mortar joints - Mortar joints add between 1/4 and 3/8 inches.
• Shrinkage during curing - Concrete shrinks as it cures.

It's important for builders to account for these factors and not assume the nominal size is the exact installed size.

CMU Block Shapes

Beyond the basic rectangular blocks, CMUs come in a variety of shapes including:

• L-shaped corner units
• U-shaped units with two open ends
• Curved units for rounded walls

These special blocks reduce cutting requirements and speed installation around corners or curves. Manufacturers can also create custom shapes by request.

Solid vs. Hollow CMUs

CMU blocks are categorized as either solid or hollow:

• Solid blocks are completely filled with concrete.
• Hollow blocks have voids or cavities inside accounting for a certain percentage of the total area.

Hollow CMUs are more commonly used as they offer insulation benefits and reduce the weight. Most hollow blocks have two cores, but four- and six-core units are also produced.

Factors Affecting CMU Dimensions

Several factors impact the final installed size of CMU blocks beyond just the nominal dimensions:

• Manufacturing Tolerances: As per ASTM C90, units can vary up to 1/8 inch from nominal dimensions.
• Mortar Joints: Mortar joints between CMU blocks add between 1/4 and 3/8 inches.
• Shrinkage: As concrete cures, minor shrinkage can occur affecting dimensions.
• Grout Space: Some CMU walls are grouted, which changes spacing.

Engineers and architects should consult with manufacturers and account for these factors in layouts to avoid issues down the road.

Hollow CMU Core Size

For hollow CMU blocks, the size of the internal cavities or cores is another key dimension. Common core sizes include:

• 2 inches x 16 inches
• 4 inches x 16 inches
• 6 inches x 16 inches

The core size impacts structural strength, so engineers must select an appropriate thickness. Cores can also be filled with concrete, rebar, or insulation depending on project needs.

Structural Considerations

Using smaller 2-inch cores may provide enough strength for non-load bearing walls. For structural elements like lintels, larger 4- or 6-inch cores are recommended.

Fill Options

If additional strength is needed, cores can be filled with materials like grout or concrete. Insulation or sound-proofing can also be added to the cores before installation.

Custom CMU Block Options

Beyond the standard sizes, custom CMU dimensions are an option for unique project needs. Some key considerations for custom blocks include:

• Higher unit costs due to special production requirements.
• May require a minimum order quantity from manufacturer.
• Lead times may be longer than standard blocks.

When planning a project, consult with manufacturers early to determine feasibility and costs for custom blocks.

Reasons to Use Custom Dimensions

Some instances where custom CMU sizes may be useful include:

• Matching dimensions of existing historical buildings.
• Coordinating block coursing with special window or door sizes.
• Creating unique architectural patterns or textures.
• Accommodating metric-sized projects.

Specifying Custom Dimensions

Manufacturers will require detailed drawings showing the exact custom size needed. Be sure to indicate actual dimensions required, not just nominal sizes.

Checking CMU Dimensions On-Site

When the CMU blocks arrive to the jobsite, best practices include:

• Visually inspecting units for damage.
• Taking measurements of a sample to validate size within tolerance.
• Accounting for mortar joint thickness in coursing plans.

Catching any discrepancies early allows time for replacements long before installation.

Importance of Verifying Size

It's crucial to verify CMU dimensions on-site for a few reasons:

• Avoid problems if manufacturer dimensions are off.
• Confirm blocks will align with project drawings and plans.
• Identify needed adjustments to coursing plans.

Measuring Tools

Use an appropriate tape measure, calipers, or ruler to check sample CMU dimensions. Measure to the nearest 1/8 inch for acceptable construction tolerances.

Accounting for Mortar

Remember that the installed size will be larger than just the bare CMU once mortar is applied. Adjust coursing plans accordingly.

Optimizing Layouts with CMU Blocks

Proper planning of CMU positioning and placement in the layout optimizes the installation process. Here are some tips:

• Use full units as much as possible to reduce cutting time.
• Strategically use special shapes for corners and openings.
• Plan exact coursing in advance, accounting for mortar.
• Minimize partial block widths to speed installation time.

Planning Courses

Map out exact vertical coursing in your drawings, coordinated with doors, windows, and other openings. Account for mortar joints in this layout planning.

Reducing Cuts

Avoid partial blocks sizes, as cutting CMUs is time-consuming. Use standard sizes as much as possible for faster installation.

Using Special Shapes

Take advantage of U, L and curved CMU blocks around openings or corners. This reduces cutting requirements compared to rectangles.