The Do’s & Don’ts of CMUs

The following are a few of the typical Dos and Don’ts when it comes to installing concrete masonry units (CMUs)…

Do: Spread the mortar bed joints to a uniform thickness with fresh mortar

Do: Tool after mortar is partially set, but before fully set.

Don’t: Throw away mortar scrapings or slushing mortar into joints.

Do: Tool all exposed joints smooth, dense and slightly concave, so that no less than 5/8” coverage occurs at joint reinforcement.

Don’t: Excessively furrow bed joints.

Do: Lay masonry units in running bond when no bond pattern is indicated on the drawings.

Do: Bond corners and intersections.

Do: Cut joints flush in concealed areas and where the surface will be tiled.

Don’t: Allow joint dimensions to be less than ¼” nor more than ½”.

Do: Adhere to control heights shown on the drawings throughout the building.

Do: Lay masonry units plumb, true to line, and with accurately spaced level courses.

Don’t: allow vertical joints to fall on the centerline of the unit below.

Do: Use a masonry saw to cut units when required.

Don’t: Install broken or cracked units.

Do: Cut or block out required chases for other trades or as shown on the drawings.

Do: Saw out CMU to fit around frames, ducts, pipe chases and other construction, and fill the spaces between masonry and door frames or other built-in items solidly with grout or mortar.

Do: Lay up masonry units with full mortar coverage on horizontal and vertical face shells.

Do: Bed webs in mortar in starting course on the concrete floor or footing.

Don’t: Adjust any units after the mortar starts to set up. Remove any unit that has been disturbed after mortar has stiffened and re-laying using fresh mortar.

Don’t: Allow more than ¼” in 10 ft maximum variation from plumb in vertical lines and surfaces, nor more than 3/8” maximum variation in any story.

Do: Install bond beams where shown on the drawings using bond beam units.

Do: Reinforce bond beams as shown on the drawings, fill with grout, and lap reinforcement 30 diameters or 12 inches whichever is greater.

Do: Extend at least 40 diameters past the opening where the bond beams are used for lintels, except when indicated otherwise.

Don’t: Allow loose mortar and grout to remain on exposed surfaces of set masonry. Clean all surfaces prior to laying fresh masonry whenever starting or resuming work.

Do: Use rebar positioners to hold vertical reinforcing away from CMU cell walls.

Do: Maintain at least ½ inch clearance between reinforcement and the interior face of units.

Do: Place vertical reinforcement centrally within cores unless shown otherwise on the drawings.

Don’t: Allow excessive mortar droppings to fall into cores.

Do: Embed bolts, anchors and other inserts that attach adjoining construction within mortar joints and hole them in the proper position until the wall is grouted.

Do: Form horizontal construction joints by stopping the pour of grout no less than ½ inch below the top of the uppermost unit grouted whenever grouting is stopped for an hour or longer.

For more detailed installation information visit: www.ncma.org

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