CMU Units: Your Complete Guide to Sizes, Shapes & Uses

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What are Concrete Masonry Units (CMUs)?

Concrete Masonry Units, or CMUs, are those gray, rectangular blocks you often see used in construction. They’re made from a mixture of Portland cement and aggregates like sand and crushed stone. They’re precast, which means they’re formed and hardened before being brought to the construction site.

Why are CMUs so popular? Well, they’re incredibly durable and can last for up to 100 years. They’re also cost-effective and relatively easy to install. Plus, CMUs offer a lot of design flexibility. They come in various sizes, shapes, and finishes, allowing for a wide range of architectural styles and applications. Because they can be recycled and often use recycled materials, CMUs are considered a sustainable building material.

This article will dive into the many aspects of CMUs, including their different dimensions, specifications, shapes, finishes, and applications. We’ll also explore their sustainability features, highlighting why CMUs are a great choice for environmentally conscious construction projects.

CMU Dimensions: Nominal vs. Actual

When you’re working with CMUs, it’s important to understand the difference between “nominal” and “actual” dimensions. It’s a distinction that can save you a lot of headaches on the construction site.

Nominal dimensions refer to the size of the CMU including the mortar joint. Think of it as the “name” size. Mortar joints are typically 3/8 of an inch.

Actual dimensions are simply the physical dimensions of the block itself. It’s what you’d measure with a tape measure.

Why does this matter? Because knowing the difference is crucial for accurate calculations in construction and design. Building designs typically use nominal dimensions. This helps to standardize construction and minimize wasted materials. If you design using actual dimensions, you’ll likely end up with odd gaps and needing to cut blocks unnecessarily.

For example, an “8 x 8 x 16″ CMU (that’s the nominal dimension) actually measures 7 ⅝” x 7 ⅝” x 15 ⅝”. See the difference? Keeping that in mind will help ensure your projects go smoothly.

Standard CMU Sizes and Shapes

Concrete masonry units come in a range of sizes and shapes to suit a variety of construction needs. Here’s a look at some common options.

Common CMU Block Sizes

CMUs are referred to by their nominal size, which includes the mortar joint. So, while the actual dimensions are slightly smaller, these are the sizes you’ll typically see listed:

  • 4″ x 8″ x 16″
  • 6″ x 8″ x 16″
  • 8″ x 8″ x 16″
  • 10″ x 8″ x 16″
  • 12″ x 8″ x 16″

You’ll also find half-block versions of these, which are half as long:

  • 4″ x 8″ x 8″
  • 6″ x 8″ x 8″
  • 8″ x 8″ x 8″
  • 10″ x 8″ x 8″
  • 12″ x 8″ x 8″

The size of the CMU you choose depends on the structural requirements of your project. Smaller CMUs might be suitable for non-load-bearing walls, while larger CMUs are often used for load-bearing walls that need to support more weight.

CMU Shapes and Configurations

While the standard rectangular block is most common, CMUs also come in a variety of specialty shapes. These shapes are designed for specific purposes and can simplify construction in certain situations. Some examples include:

  • “U” blocks
  • “L” blocks
  • Rounded blocks
  • Interlocking blocks
  • Stretcher blocks
  • Kerf blocks
  • Double Corner blocks
  • Single Corner blocks
  • Open End blocks
  • Double Open End blocks
  • Bond Beam blocks
  • Knockout Bond Beam blocks
  • U Lintel blocks
  • Sash blocks
  • Bullnose blocks
  • Column blocks
  • Column blocks with Pilaster

Bond beam units, for example, are designed for horizontal reinforcement within a wall. Lintel units are used to span openings, like doorways or windows. These specialty shapes help create stronger, more efficient structures.

CMU Specifications and ASTM Standards

CMU production and quality in the United States are heavily regulated by ASTM International standards. These standards set minimum quality requirements for manufacturers.

Some of the most important ASTM standards for CMUs include:

  • ASTM C90: Standard Specification for Loadbearing Concrete Masonry Units
  • ASTM C55: Standard Specification for Concrete Building Brick
  • ASTM C1634: Standard Specification for Concrete Facing Brick and Other Concrete Masonry Facing Units
  • ASTM C744: Standard Specification for Prefaced Concrete Masonry Units
  • ASTM C1877: Standard Specification for Adhered Manufactured Stone Masonry Veneer Units and Adhered Concrete Masonry Veneer Units

These standards govern key CMU properties like:

  • Dimensional tolerances
  • Minimum compressive strength
  • Maximum water absorption
  • Drying shrinkage

CMUs are also classified by density:

  • Lightweight: Less than 105 lb/ft³
  • Medium weight: Between 105 lb/ft³ and 125 lb/ft³
  • Normal weight: More than 125 lb/ft³

The density of a CMU impacts its fire resistance, sound control, and water penetration.

CMU Surface Finishes and Architectural Applications

CMUs aren’t just for plain, utilitarian walls anymore. They can be manufactured with a variety of surface finishes and textures, making them a versatile option for many architectural designs.

Common CMU Surface Finishes

Here are some of the most common CMU surface finishes:

  • Burnished/Polished CMU: These units have a smooth, polished surface, often achieved by grinding and polishing the concrete.
  • Colored CMU: Pigments are added to the concrete mix to create CMUs in a wide range of colors.
  • Glazed CMU: A ceramic glaze is applied to the surface of the CMU, providing a durable, colorful, and graffiti-resistant finish.
  • Raked CMU: The mortar is recessed to add visual interest.
  • Sandblasted CMU: Sandblasting removes the outer layer of concrete, exposing the aggregate and creating a textured surface.
  • Soft-Split CMU: These units are split to create a rough, textured surface, but the edges are softened for a more refined look.
  • Split-Faced CMU: These units are split to create a rough, textured surface that resembles natural stone.

Each finish offers different aesthetic and functional benefits. For example, glazed CMU is known for its color consistency and resistance to graffiti, making it a popular choice for schools and public buildings. Split-face CMU provides a textured, rugged appearance that’s often used in landscaping and exterior walls. Burnished CMU offers a smooth, polished look that’s suitable for interior applications.

Architectural Concrete Masonry Units

Architectural CMUs are designed to be visually appealing and can be used in a variety of ways to create unique architectural features. Some common types of architectural CMUs include:

  • Split-face units
  • Scored units
  • Ribbed units
  • Ground face units (burnished, honed)
  • Slump block units

When working with architectural CMUs, it’s important to create sample panels to evaluate the finished work against the specifications. Sample panels allow you to see how the CMUs will look in place and ensure that the color, texture, and overall appearance meet your expectations.

Real-World Applications of CMUs

You’ll see CMUs used in all kinds of construction projects, from the house next door to the tallest skyscrapers.

Residential Construction

In residential construction, CMUs are often used to build:

  • Foundational walls: CMUs offer a strong and stable base for a home, especially when properly reinforced.
  • External walls: CMUs can provide excellent insulation and fire resistance for exterior walls. They stand up well to all kinds of weather.
  • Basements: CMUs are a popular choice for basement walls because they can withstand soil pressure and moisture.

Commercial Buildings

Commercial construction relies on CMUs for their durability and cost-effectiveness. You’ll see them used in:

  • Factories, warehouses, and high-rise buildings: CMUs provide the strength and stability needed for large-scale structures.
  • Interior partitions and veneer walls: CMUs can create durable and fire-resistant interior walls and can also be used as a base for attractive veneer finishes.

Landscaping and Exterior Projects

CMUs aren’t just for buildings. They’re also great for landscaping and exterior projects, such as:

  • Garden walls: CMUs can be used to create attractive and functional garden walls. They can also be painted or stained to match any landscaping style.
  • Retaining walls: CMUs can provide the strength and stability needed to build retaining walls that hold back soil and prevent erosion.
  • Outdoor fireplaces: CMUs are fire-resistant and can be used to build safe and durable outdoor fireplaces.

Other Applications

CMUs are also used in schools, hospitals, government buildings, and many other types of construction projects. Their versatility, durability, and cost-effectiveness make them a popular choice for a wide range of applications.

Are CMUs sustainable?

CMUs offer several environmental advantages, making them a good choice for sustainable construction.

For starters, CMUs themselves can be made sustainably. Many manufacturers incorporate recycled materials into their blocks. Recycled CMUs typically contain around 30% recycled content, diverting waste from landfills and reducing the demand for virgin resources.

Using CMUs can also contribute to earning LEED (Leadership in Energy and Environmental Design) points in green building projects. LEED awards points for various sustainable practices, and CMUs can help in several areas:

  • Recycled Content: Using CMUs made with recycled materials earns points.
  • Regional Materials: Sourcing CMUs from local manufacturers reduces transportation emissions.
  • Construction Waste Management: CMUs are durable and generate minimal on-site waste.
  • Energy Efficiency: CMU walls can contribute to a building’s thermal mass, reducing energy consumption for heating and cooling.

CMUs can also contribute to LEED points due to their mold resistance and the fact that they don’t necessarily need painting.

Finally, the use of supplementary cementitious materials like fly ash and blast furnace slag in CMU production further reduces their environmental impact. These materials are byproducts of other industrial processes, and their use in concrete can reduce CO2 emissions by up to 70 percent, although typical reductions range from 15-40 percent.

To Conclude

Concrete masonry units, or CMUs, offer a compelling combination of durability, design flexibility, and sustainability. They stand up to weather and time, and they can be used in a wide range of construction projects.

CMUs are available in a variety of sizes, shapes, colors, and finishes. The range of options means you can get just what you need for your specific project. But with so many choices, it’s important to make sure you select the right CMU for the job. Think about the structural demands of your project, the aesthetic you’re trying to achieve, and the environmental impact of your choice.

If you aren’t sure which CMU is the best for your needs, consult with a qualified professional. An experienced architect, engineer, or contractor can help you assess the project and make sure you get the right materials for the job.

So, as you plan your next construction project, don’t overlook CMUs. They may be the perfect solution for your needs.