The residential and commercial construction industries make wide use of a form of concrete building material known officially as a concrete masonry unit (CMU). These hollow-core blocks can be made of standard concrete with traditional sand and gravel aggregate held together with Portland cement. Or, they can substitute lighter-weight industrial waste materials, such as fly ash or coal cinders, for the sand and gravel aggregate, in which case they are usually known as cinder block.
CMUs have many uses in the construction trades, from use in foundation walls to support framed construction, to exposed exterior walls for buildings, to freestanding landscape walls and retaining walls.
Types of CMUs
Concrete block and cinder block come in several different sizes and shapes to fit different applications. Common sizes and shapes of concrete block include:
- 4 x 8 x 8
- 4 x 8 x 16
- 6 x 8 x 8
- 6 x 8 x 16
- 8 x x 9
- 8 x 8 x 16
- 10 x 8 x 8
- 10 x 8 x 16
- 12 x 8 x 8
- 12 x 8 x 16
Actual sizes of concrete blocks are slightly less than the nominal sizes, by roughly 3/8 inch in each dimension. This is to accommodate the thickness of mortar joints between blocks.
Concrete block also comes in many shape configurations. The most common are:
- Stretcher unit—has flange extension on both ends. Used in the middle of walls where both ends are covered by adjoining blocks.
- Single corner unit—has one squared-off end. Used at the ends of walls where the end of the block is exposed.
- Double corner unit—has two squared-off ends. Often used when blocks are stacks to create vertical pillars.
- Sash units—have slots cut in flat ends; these are used around window and door openings to provide expansion space.
- Cap units—thin, solid concrete block used to cover the open cavities at the top of a concrete block wall.
Several other shapes are also available for specialty applications, such as bullnose blocks with one rounded end, jamb blocks which have an indentation where window and door jambs fit, partition blocks used to make interior partition walls, and lintel blocks used to make window and door headers.
There are also architectural masonry units, which have textured faces designed for decorative use.
Anatomy of a Concrete Block Wall
Virtually all walls built with CMUs share the same elements, although the application of those elements can vary considerably depending on the size, shape, and use of the wall.
- Foundation. All concrete block walls must rest on a sturdy foundation of poured concrete. The depth and size of the foundation will vary depending on the size of the concrete block wall and the weight it must support, but a typical freestanding wall requires a foundation that is about twice as wide as the wall itself and which extends about 1 foot down below the frost line.
- Concrete block. Block shapes and sizes are chosen to match the function of the wall and the configuration of the wall. Most cement block walls will use several different types of blocks, especially stretchers and corner units.
- Mortar joints. Each row of blocks is joined to the adjoining blocks with either type N (above grade) or type S (below grade) mortar. For greatest strength, most concrete block walls are assembled so the vertical joints are offset (staggered) from one course to the next.
- Reinforcement. Freestanding block walls can be subject to stresses that can crack joints and destroy walls, so both vertical and horizontal reinforcement is common. Vertical reinforcement is provided by lengths of steel rebar embedded in wet concrete that is poured into block cavities at prescribed intervals. Horizontal reinforcement is provided by metal reinforcement strips laid into the wet mortar after every third or fourth course of block.
Most concrete block walls are single-wythe walls, which means they are constructed from single-wide rows of block stacked one over the other. Where greater structural strength is required, you can construct double-wythe walls, in which two courses of blocks are laid together.
Tools and Supplies You Will Need
- Excavation tools (shovels, etc.)
- Foundation forms, if required
- Mason's string
- Line level
- Plumb bob
- Concrete blocks or cinder blocks
- Carpenter's level
- Work gloves
- Concrete mix
- Wheelbarrow or power cement mixer
- Masonry hoe
- Mason's trowel
- Jointing tool
- Masonry saw
- Masonry chisel
- Masonry hammer
The first step in building a concrete block wall is to lay out the foundation using stakes and masonry string. For a freestanding landscape wall, this involves creating a simple rectangular outline of the planned foundation. For building foundations, the layout requires a rectangular outline of the entire building, carefully adjusted to be perfectly square.
After leveling the layout strings with a line level, transfer the location of the foundation onto the ground before beginning excavation.
The next step is to excavate earth for the foundation. The work involved here can vary considerably, depending on the required size of the foundation and the circumstances. If you are building a small landscape wall in a warm-weather climate, this can involve simple hand-digging with a shovel. For a building foundation or in a cold-weather climate requiring deep frost footings, excavation can be a major undertaking requiring earth-moving equipment. Either way, the goal is to create a flat-bottom trench to pour a concrete foundation for supporting the cement block wall.
Make sure to consult local authorities on the required depth and size of the foundation for the cement block wall you are planning. Any wall higher than 2 feet requires a frost footing that extends 8 to 12 inches below the deepest winter frost level in your region. Generally, the foundation should be twice as wide as the wall itself.
Pour the Foundation
The concrete foundation needed to support a cement block wall is usually created by pouring concrete into a hollow form lining the sides of the excavation trench, but the concrete can also simply be poured into the trench—a common scenario for construction of a freestanding landscape wall. In this case, the top of the excavation is sometimes boxed with lumber to create a finished appearance. It is common for the top of the foundation to be kept slightly below grade, so that the foundation will be hidden when the wall is finished.
Concrete for the foundation can be mixed by hand in a power mixer or mortar box, or it can be ordered from ready-mix sources and delivered by truck.
The top of the poured foundation should be perfectly level, but it does not need to be floated and troweled to a perfectly smooth finish. Make sure the foundation is fully hardened and cured before beginning construction of the wall.
Lay the First Course of Block
After the poured foundation has fully cured and hardened, mark an outline for the cement block wall onto the surface of the foundation, using a chalk line.
Mix the appropriate mortar in a mortar box, then lay a 1-inch thick layer of mortar onto the foundation inside the outline.
Position the first course of cement block into the mortar and tap the blocks downward slightly to embed them in the foundation mortar. Begin the wall with a corner unit, then "butter" the flanges at the end of each subsequent block with mortar before joining it to the previous block.
Strive for 3/8-inch-wide joints between blocks, both horizontally and vertically. This provides an optimal amount of strength.
As you work down the first course, use a level to adjust the blocks so they are perfectly vertical, and use stakes and strings to ensure that the row of blocks remains perfectly straight. At the opposite end of the wall, end the course with another corner unit.
If you plan carefully, you may not need to cut concrete blocks, but if it is necessary, this best done with a power saw fitted with a masonry blade, along with a masonry chisel and hammer.
Score across the face of the block with a power saw, cutting a line about 1/4 inch deep. Then, use a masonry chisel and hammer to pound along the scored line until the block cleaves along the line. Turn the block over, and repeat the process on the opposite side.
Lay Subsequent Courses of Block
Begin the next course of blocks with a half block to ensure that vertical joints will be offset as you work down the course. Install the second course in the same fashion as the first—applying a bed of mortar along the top of the previous course, and buttering the ends of each block as you lay it into the mortar. Use the mason's string and level to frequently check each course of blocks for level and straight. Excess mortar can be lightly scraped off the face of the blocks with a trowel as you work.
For very tall walls, it's best to lay no more than six courses each day. This will allow the mortar to set up and harden completely and will reduce the chances that the wall will fail. Avoid placing too much weight on the joints before the mortar is fully hard.
As you work upward, add metal reinforcement, as needed. After every third or fourth course, the horizontal joint should be reinforced with metal reinforcement strips laid into the mortar before the next course of blocks is laid.
Vertical reinforcement is added by filling hollow cavities with concrete and driving lengths of metal rebar down into the hollows. This will increase the lateral strength between rows of block. Check with local building authorities on recommendations for vertical reinforcement of the wall.
Finish the Joints
Depending on the size of your project and the speed with which you work, you will periodically need to smooth and finish the joints between blocks. This needs to be done after the mortar has set up but before it is fully hardened. Use a finishing tool to lightly press into the mortar as you draw the tool along the joint. The tool should form a slight indentation in the mortar joint.
Lay Cap Blocks
The tops of cement block walls are typically capped by applying a layer of mortar, embedding metal reinforcement strips, then covering the wall with solid concrete cap blocks. Make sure joints between cap blocks are filled with mortar and smoothed with the finishing tool.
Below-Grade Cement Block Walls
Where a cement block wall will be below grade, such as when serving as a basement wall, it is important that the surface is waterproofed before backfilling with soil. This can be done with a variety of waterproofing membrane products, or with a rubberized brush-on waterproofing material.