CMU and Masonry Block in Georgia
Concrete block walls for retaining, enclosure and structure, built with the right mortar, grout, steel and drainage details.
Concrete masonry units, commonly called CMU or concrete block, are the standard material for durable walls: garage and utility enclosures, retaining and screen walls, commercial partitions and the foundation walls of many buildings. Jermaine Enterprises, a general contractor on Greenbriar Parkway SW in Atlanta, builds and repairs block walls for homeowners, businesses and property owners.
A block wall performs well when four things are specified: drainage and flashing, reinforcement and grout, the right mortar type and a layout that follows the block module. The sections below cover each.
Block sizes and layout
A nominal 8x8x16 concrete masonry unit has specified dimensions of 7.625 by 7.625 by 15.625 inches. The nominal size equals the specified size plus the mortar joint, which is 3/8 inch for standard units, so courses and wall lengths fall on a 4 or 8 inch module. ASTM standards allow actual overall dimensions to vary by plus or minus 1/8 inch. Laying out openings and corners on that module before the first course saves cutting and keeps the wall plumb.
Mortar: Types M, S, N and O
Masonry mortar types M, S, N and O are defined in ASTM C270, with minimum average 28-day compressive strengths of 2,500, 1,800, 750 and 350 psi under the property specification. Building codes typically call for Types M, S or N. For example, empirical design of foundation walls requires Type M or S. The right type depends on the wall and its loads, so it is named in the scope and not left to whatever is on the truck.
Grout and steel reinforcement
Reinforced block walls are strengthened with steel bars in the cells and filled with grout. Grout should comply with ASTM C476. Fine grout uses sand smaller than 3/8 inch as its only aggregate, and coarse grout adds pea gravel smaller than 1/2 inch. At placement the grout slump is required to be 8 to 11 inches, and ASTM C476 sets a minimum of 2,000 psi at 28 days under the strength method.
In a partially grouted wall, grout goes only into the cells that hold steel. In a solidly grouted wall every core is filled. In low-lift grouting the wall is built in segments of up to 5 feet and cleanouts are not required, while high-lift grouting uses cleanout openings of at least 3 inches at the base of the wall so debris can be removed before pouring. Which approach is used is a design decision recorded in the plans.
Keeping water out
The Concrete Masonry and Hardscapes Association (formerly NCMA) advises designers never to assume that any material makes a wall totally impervious to water, and not to rely on surface treatments alone. Walls are detailed with flashing where moisture could enter and free drainage is blocked, such as wall tops, openings, sills, shelf angles and the wall base, with weep holes to send trapped water back outside. North and central Georgia sees measurable rain on about 120 days a year according to the National Weather Service, so drainage details deserve early attention.
Cutting block safely
Cutting concrete block creates respirable crystalline silica dust. OSHA's silica standard lists continuous water on the blade as an engineering control for stationary masonry saws, and when wet methods are used its Table 1 does not require respiratory protection. Wet cutting reduces airborne silica dust at the source.
Terms you will hear on this job
- CMU (concrete masonry unit)
- nominal versus specified dimensions
- partial grouting and solid grouting
- low-lift and high-lift grouting
- cleanout
- fine grout and coarse grout
- grout slump
- mortar Types M, S, N and O (ASTM C270)
- TMS 402 and TMS 602
- flashing and weep holes
- joint reinforcement
- wet cutting (silica control)
How a job like this runs
Describe the wall
What it is for, how long and high it must be and what it must hold back or carry.
Scope and price in writing
Block size, mortar type, steel, grouting method and drainage details are written into the scope.
Build and grout
Courses are laid to the module, steel is placed and cells are grouted as designed, with inspections called where the permit requires.
Finish and check
Joints are tooled, flashing and weep holes are checked and the site is cleaned up.
Codes that cover masonry
Masonry design and construction are governed by referenced standards such as TMS 402 and TMS 602, and the building department and the project's designer confirm what applies. Georgia cities enforce the state minimum codes, and DCA's announcement and city notices give different effective dates for the 2024 editions, so confirm which edition applies at your address. Retaining walls and taller or loaded walls may need a permit and an engineer's design; the local building department decides.
Questions about cmu and masonry block
What size is a standard concrete block?
A nominal 8x8x16 block measures 7-5/8 by 7-5/8 by 15-5/8 inches. The nominal size includes a 3/8 inch mortar joint so courses and lengths land on a 4 or 8 inch module.
What is the difference between partially grouted and solid grouted block walls?
In a partially grouted wall, grout fills only the cells containing steel reinforcement. In a solidly grouted wall every core is filled. Solid grouting can be faster and more economical when vertical bars are closely spaced or there are many bond beams.
What mortar is used for block walls?
Codes generally reference ASTM C270 mortar types M, S and N. Which one is used depends on the wall type and design, for example Type M or S for empirically designed foundation walls.
How do block walls handle moisture?
Exterior masonry is expected to see some water entry, so walls are detailed with flashing and weep holes to catch moisture and send it back outside. Water repellents help but do not replace flashing.
Is cutting concrete block dangerous?
Cutting block can create respirable crystalline silica dust. OSHA's silica standard recognizes continuous water on the saw blade as an effective control, with equipment maintained as the manufacturer specifies.
Verify before you hire anyone. Check any contractor's state license on the Secretary of State contractor board and your city's permit requirements before work starts.
Where this work is quoted
CMU and Masonry Block is quoted in seven towns around Atlanta. Pick your town to see its permit office, neighborhoods and the building context that shapes a project there.
Related services: Framing and Drywall, Painting, Demolition, Office Interior.
Sources: [1] angelusblock.com [2] cmha.org [3] cmha.org [4] cmha.org [5] cmha.org [6] osha.gov [7] weather.gov [8] dca.georgia.gov [9] dca.georgia.gov
Plan your block wall
Describe the work and the address. We will follow up with the phone number you provide.