Concrete Masonry Units
USACE Afghanistan Engineer District-South, CC BY-SA 2.0 , via Wikimedia Commons

CMU Block (Concrete Masonry Unit) 101

Concrete masonry units are a great alternative to traditional brick or stone. They are also known as CMUs, and they are manufactured from cement and other ingredients to form a solid, durable block.

CMUs are used to create walls and floors in both residential and commercial structures. They are designed to be used in a variety of situations, from low-rise residential buildings to high-rise commercial and industrial structures.

But what exactly is a CMU block? What types are available and are they any different from cinder blocks?

What is a CMU Block?

CMU Block

A CMU block is an environmentally friendly building material made from:

  • Portland cement
  • Aggregates, such as sand, grave, fly ash or bottom ash
  • Water

Blocks may also incorporate recycled materials into the mix, such as slag cement or recycled glass. Incorporating recycled materials into the mix can change the overall look of the block and may be used to help the building get LEED certification.

CMU blocks have been an integral part of masonry structures for decades. They offer a number of advantages over other materials, including:

  • High durability
  • Easy maintenance
  • Fire resistance
  • Easy installation

Concrete blocks come in a variety of styles and sizes to suit any project. Their high durability makes them a great fit for residential, commercial and industrial projects.

Concrete Block Types

There are several types of CMU blocks, including:

  • Quion Corner Unit: A decorative block used at the ends of concrete block walls. These blocks are hollow and may be larger in size to create a stronger appearance.
  • Stretcher with Score Unit: Designed to be viewed end on and to look like a regular brick.
  • Split Wide Stretcher Unit: Has four sections with a decorative pattern.
  • Closed Bottom Bond Beam Unit: A hollow block that has parts of the crosswebs and ends formed. This creates a channel for reinforcing steel.
  • Open Bottom Bond Beam Unit: These blocks are similar to the closed bottom units, but the bottoms are open. These blocks can also accommodate steel.
  •  “L” Corner Unit: Finishes an outside corner and is designed to be viewed from three sides.
  • Bullnose Stretcher Unit: A block that has its edges rounded off.

These are some of the most common styles of CMU blocks used in construction. Some manufacturers may offer different styles.

Common CMU Block Sizes

CMU blocks come in a wide range of sizes, but the standard depth sizes are 4”, 6”, 8”, 10” and 12”. Each of these standard sizes come in full block and half-block sizes. Here’s a rundown of the nominal dimensions for each standard size (listed as D x H x L):

  • 4” full block: 4” x 8” x 16”
  • 4” half-block: 4” x 8” x 8”
  • 6” full block: 6” x 8” x 16”
  • 6” half-block: 6” x 8” x 8”
  • 8” full block: 8” x 8” x 16”
  • 8” half-block: 8” x 8” x 8”
  • 10” full block: 10” x 8” x 16”
  • 10” half-block: 10” x 8” x 8”
  • 12” full block: 12” x 8” x 16”
  • 12” half-block: 12” x 8” x 8”

As you can see, the only measurements that change are the depths and lengths. Full blocks are 16” long, and half-blocks are just 8” long.

Some manufacturers may have their own unique size options that are not listed above.

It’s important to keep in mind that the sizes listed above are the nominal sizes – not the actual sizes. The actual sizes are 3/8” less than the nominal dimensions to allow for mortar joints between the blocks.

Cinder Block vs CMU: What’s the Difference?

Are cinder blocks and CMUs the same thing? Yes, and no. When a CMU uses cinders as an aggregate material, the blocks are called cinder blocks. So, all cinder blocks are CMUs, but not all CMUs are cinder blocks.

Uses for CMU Blocks

CMU construction has its perks and drawbacks depending on the type of walls that need to be erected.

Foundation Walls

Building weight load will dictate the size of the CMU blocks used in a foundation, but they’re generally 16” long with a width of 8” to 10”. If higher weight loads are needed, the blocks may be thicker to support these walls.

Running bond patterns may be used, and to increase the strength and durability of the block, steel bars may be placed inside of the blocks.

Blocks can be secured using mortar.

A few of the benefits to creating a CMU foundation wall are:

  • Mortar can be poured inside of the blocks to add to their strength and durability.
  • Steel rebar can also be inserted to provide additional strength.
  • When heavy vertical loads are needed, the compression strength of CMUs offer optimal support.

But there are some downsides, including the blocks being heavy. Air and moisture barriers may need to be added, and if not reinforced properly, the wall can be weak. Mortar is also susceptible to water and weather wear which really demands additional support to be constructed properly.

Multiple other uses for CMUs exist, including:

  • Exterior Walls
  • Basement Walls
  • Partition Walls

When opting for a CMU wall versus other methods, keep in mind that CMUs are much larger in size than traditional bricks.

What does this mean?

Larger blocks allow for the bricks to be laid faster and at a lesser expense.

While water makes the mortar susceptible to wearing down and becoming unstable, there are corrective measures that can be taken to reduce these risks. A lot of foundation CMUs will have a glaze or some form of weather proofing on the exterior.

The added protection will extend the lifetime of the bricks so that little maintenance is required.

And thermal resistance is low, so there will need to be more emphasis on insulating these bricks.

Many of the same techniques used to create foundation walls will be utilized in all three of these wall types. Thanks to the addition of surface finishes, it’s possible to create an aesthetically pleasing exterior wall that matches high-end buildings.

Residential and commercial buildings are starting to leverage CMUs as a viable building material.

CMU Surface Finishes

CMUs are large, concrete blocks, so they’re not the most aesthetically pleasing building material. You can, at a premium cost, find suppliers that will offer an array of finishes for concrete masonry units. The most common finishes are:

  • Colored. Multiple colored pigments can be added to the mortar that allow it to have a nice color, but it’s important to look through multiple block samples to see variations in color. Even the same batch of blocks can have drastically different color variations which need to be considered.
  • Glazed. A glazed finishing can be placed on the CMU and is made with resin. The coating allows for a variety of patterns and resistance, including resistance to certain chemicals.
  • Split-faced. If you’re looking for the look of natural stone, a split-faced option may be the best choice. CMUs are molded side-by-side and then split apart to create this look.
  • Soft-split. If a regular split-face isn’t ideal, a soft-split face may suffice. This type of texture is similar to its split-faced counterpart, but molds are used to create the texture rather than mechanically splitting the blocks.
  • Polished or burnished. The concrete’s natural aggregates may be exposed using a polishing or burnishing process. These blocks have a nice, exterior texture that is rough and looks natural.
  • Sandblasted. A sandblasting process can be performed on the CMUs to create a weathered look that is smoother than the polished or burnished look. This is a very natural look which does make the blocks look like they’ve weathered naturally.
  • Raked. During the molding process, the material can be raked to create an appealing outside texture for the blocks.

Concrete masonry units have come a long way in recent years allowing the blocks to offer affordability and durability while also providing exceptional exteriors.

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