Concrete Masonry Unit (CMU) Walls - Material, Types, Sizes, Surface Finishes, and Construction

CMU stands for Concrete Masonry Unit, are also known as Concrete Blocks, are standard rectangular sized concrete blocks that are used as structural and non-structural elements in the construction of structures especially the building structures. Concrete Masonry Units (CMUs) are available in various sizes and shapes just like bricks. Mainly they are used for the the construction of walls.

1. Benefits of Concrete Masonry Unit (CMU)

Concrete Masonry Units may provide the following benefits;

1. Less Expensive then Masonry Bricks
2. Durable (longer life span)
3. Sound Absorbent
4. Easy to construct
5. Fire-resistant
6. Low maintenance cost
7. Decorative finishes
8. Pleasing Appearance
9. CMU are larger then bricks thus increasing the load bearing capacity of wall
10. Supports various wall facings i.e. stone, brick, tile facing.

2. Drawbacks of Concrete Masonry Unit (CMU)

1. Absorb moisture easily
2. Additional cost for water-proofing like painting, plastering, etc.
3. Occupy more space
4. Low thermal resistance requiring higher heating and cooling cost
5. Heavier then wood and metals

3. Applications of Concrete Masonry Unit (CMU)

1. Used in foundation and earth retaining walls like basement walls, garden walls, etc.
2. Used for decorative purposes.
3. Used in internal wall like space partitioning and also used as external walls in structures.

4. Nomenclature of Concrete Masonry Unit (CMU)

Concrete Masonry Unit is a generic name known all around the world. However, different countries have selected nomenclature for CMUs. Details of various names used for CMUs in different countries is given below, in addition to the reasons behind there nomenclature.

5. Composition of Concrete Masonry Unit (CMU)

Concrete Masonry Unit (CMU) is composed of cement (as the binding agent), sand (as the fine aggregate), gravel (as the coarse aggregate), and water (as the liquid medium for uniform mixing and hydration of cement).

Some plasticizer, super-plasticizer, additive or secondary raw materials may also be used in CMUs. Hence, composition of CMUs is much similar to conventional concrete, however, as they are cast in different smaller units and mostly they are hollow, hence, categorized differently as compared to conventional concrete.

6. Types of CMUs – Density and Material

Concrete Masonry Units may be classified according to the density of coarse aggregate being used. Rest of the material composition in CMUs is being same.

1. Low Density CMUs (Light Weight CMUs)
2. Medium Density CMUs (Medium Weight CMUs)
3. High Density CMUs (Normal Weight CMUs)
4. Recycled Aggregate CMUs (Light / Medium / Heavy CMUs)

• Light Density CMUs are lighter in weight, easy to carry, having better fire resistance, lower thermal conductivity, lower thermal storage capacity, and require less labor.
• Heavy Density CMUs are heavier in weight, difficult to carry, poor fire rating, and require more labors. They have better sound insulation, more compressive and tensile strength, better thermal conductivity and thermal storage capacity.
• Physical, chemical, thermal, electrical, mechanical prosperities of Recycled Aggregate CMUs may be dependent upon the type of recycled aggregate being used in the CMU. As the name suggest, Recycled Aggregate CMUs may be having industrial waster, ashes, post-consumer glass, slag, or recycled aggregate, etc.

6.1 Who decides what type of CMUs should be used?

Structural Engineer (specify the load requirements), Architect (specify the aesthetic requirements) and the Client (specify the budget requirements) may decide what type of concrete masonry unit need to be included in the structure.

7. Compressive Strength of CMU Blocks

Compressive strength of CMUs must be decided by the structural engineer if that is carrying some load i.e. wall is load bearing. If concrete masonry unit is used as infill walls i.e. non-load bearing the lower compressive strength of CMUs might be acceptable.

However, typical ranges of compressive strength of concrete masonry unit (CMU) is from 1,000 psi (8 MPa) to 5,000 psi (40 MPa) for normal density CMU. Higher strength CMUs can be manufactured by proper mix designing.

Compressive strength of CMU can be increased by using lower water/cement ratio, smaller sized coarse aggregates, higher quantity of cement, use of plasticizers and super-plasticizers, proper curing, etc.

8. Concrete Masonry Unit (CMU) Specifications

The following American Standards for Testing Materials (ASTM) should be used for getting the exact specifications regarding the material, ingredient quantity, mix design, size, shape, casting, curing, temperature, compressive strength, dimensional tolerance, moisture absorption, etc. Engineers in different countries might conform to their own regional or local standards as prescribed by the law of the state.

9. CMUs – Nominal vs. Actual Dimensions

Similar to brick sizes, concrete masonry units (CMUs) also have Nominal and Actual Dimensions.

9.1 Actual Dimension of CMU

Actual dimension of CMU is the physical size of the concrete block.

9.2 Nominal Dimension of CMU

Nominal Dimension of CMU is equal to Actual Dimension of CMU plus an extra width of the mortar joint from 3 dimensional side. Usually this extra mortar joint is 3/8″.

10. Sizes – Concrete Masonry Unit (CMU)

Concrete Masonry Units are available in various sizes. Sizes of CMUs are based upon the width of the wall required. For example, a 6″ CMU is nominally 6″ wide, while a 10″ CMU is nominally 10″ wide i.e. equal to the thickness of the wall required.

CMUs are available in full size blocks and half sized blocks. Contractor should structure the wall in such a manner that minimum CMU need to be break. Or half width of the CMUs should be used in corner and ends of the walls beings constructed.

Standard sizes of the walls are  4″, 6″, 8″, 10″, and 12″ based on which standard CMU sizes are being manufactured. However, some CMU industries may also produce non-standard sizes that are not enlisted below.

10.1 Full-sizes Concrete Masonry Units (CMUs)

Full-sizes of Concrete Masonry Units (CMUs) are given below;

10.2 Half-sizes Concrete Masonry Units (CMUs)

Half-sizes of Concrete Masonry Units (CMUs) are given below;

11. Shapes – Concrete Masonry Unit (CMU)

Concrete masonry units can be casted into infinite shapes by the manufacturer. Few shapes are standard industrial shapes available throughout the country where as many custom shapes can be manufactured on order.

12. Block Cores in Concrete Masonry Units (CMUs)

The number of block cores in most of the concrete masonry units may vary from 2 core units to 3 and rarely 4 core block units. Half CMU block may contain 1 hollow core. The greater the number of hollow core, the lower its compressive strength might be provided the width of the core remains same.

12.1 Benefit of Hollow Core of CMU

In order to increase the tensile strength of the CMUs, steel reinforcement may be used inside these hollow core units and embed with the help of cement grouting. Such reinforcement streegnth helps the CMU wall against the lateral loads, for instance, the earthquake loads, wind loads, and the blast loads, etc.

To control temperature difference, hollow cores of CMUs might be filled with expanded-polystyrene (EPS) foam insulation in order to fullfil the requirements set by the US national energy code for increasing the R-value of the CMU wall.

12.2 Concrete Block Structures (CBS)

If concrete masonry units (CMUs) are embed with steel reinforcement and further filled up grouting then the walls of such structures are load bearing and are known as Concrete Block Structures (CBS). Concrete Block Structures may be un-grouted, partially grouted or fully grouted. Partially grouted and fully grouted CMU that are embed with steel reinforcement enhances the lateral load carrying capacity of the concrete masonry unit wall.

Steel reinforcement may be provided in horizontal and vertical layers of CMU to enhance the structural performance of the wall. In the above figure, one can see the vertical reinforcement embed into the CMU blocks, which will later be filled with grout (thick paste of water, cement and sand).

Retaining walls, that helps in retaining earth mass or maybe liquid mass, may also be constructed of concrete block structures (CBS). However, one will need to make sure CMU is impervious to moisture. In the figure below you can see the use of CMU used as earth retaining wall.

13. Constructing a CMU Wall

CMU must be standing on a strong foundation and should be capable enough to bear the external loads that is being applied either from the vertical direction or from the horizontal direction. Construction of CMU wall includes several steps like clearing the site, leveling the site, excavation, foundation of CMU wall, construction the CMU wall, finishing, etc. These stages are discussed in summarized form below;

13.1 Clearing and Levelling the Site

The foremost step in construction of any wall includes the clearing of site. There maybe trees and bushes that needs to be removed. Remove them and grub (roots removal) the trees. Further, level the site so that foundation excavation is uniform through out the wall strip.

13.2 Excavation

Next step includes the excavation of site till the exact depth and width that is shown in engineering drawings. If the wall structure is not designed then you may excavate 3′ (ft.) deep below the ground level.

After excavation, do compact the soil with the help of surface vibrator or use of compactor. Sprinkling of water might help in further compaction.

Soling layer of coarse aggregate i.e. gravel and sand may be placed or plain cement concrete of 6” (inches) depth should be placed immediately above the compacted soil. By this way weak soil can be enhanced to create high bearing capacity layer.

13.3 Laying the Foundation CMU

The next step for the non-load carrying CMU walls includes the laying of the foundation that should be in stepped. Normally the depth of foundation should be twice the width of the wall. CMU blocks can be placed in steps, the lowest layer should have the greatest spread, while the next layer above should be one step shorter in width, till the plinth level is reached.

13.4 Super Structure of CMU

After constructing the foundation CMU, next stage is the construction of super-structure. Super-structure is the portion of structure that is above the plinth level. A damp proof course (D.P.C.) needs to be placed immediately above the sub-structure. Followed by the construction of super-structure of CMU.

Width of the CMU should be 1 Wythe conventionally or you may use 2 Wythe CMU, if larger thickness of wall is required.

Blocks of CMU should not create a continuous vertical joint. Joint between two stretcher should be just at the center of CMU block above it. Thickness of the mortar joint should be 10 mm, both in vertical and horizontal layers of CMU.

Cement to sand ratio for CMU mortar should be 1 part of cement, and 6 parts of sand. Reinforcement should be used for achieving higher strength CMUs.

14. Cost of Cinder Block per Square Foot

A cinder block costs about $1 to $5 on average. A cinder block wall costs between $15 and $30 per square foot, or $60 and $240 per linear foot, including installation. As a result, depending on the height and thickness of the wall, you can expect to pay between $1,500 and $6,000 for a 25-foot cinder block wall.

14.1 Cinder Block Wall Costs per Linear Foot

14.2 Labor Costs for Cinder Block Wall

Cinder block wall installation typically ranges from $10 to $17 per square foot, or $34 to $100 per hour.

However, the labor costs vary from city to city. If you live in a high-cost-of-living area, such as New York City, the you should expect to pay more per hour than someone who lives in a low-cost-of-living area.

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