I'm laying a 90-foot long, 8-foot tall CMU wall for a warehouse addition.

Spacing: Per TMS 402/602 (Building Code Requirements for Masonry Structures), maximum control joint spacing is 25 ft for walls with joint reinforcement (ladder/truss) and 20 ft without. For your 90-ft wall, place joints at 20–25 ft intervals (4–5 joints total). Locate at:

• Changes in wall height/thickness
• Within 10 ft of corners/returns
• At movement discontinuities (column interfaces, openings)

Construction (Running Bond):

1. Block Prep: Use sash units (open-end) or cut standard units to create a continuous vertical keyway. Do not use full blocks with mortar joints only—that cracks randomly.
2. Joint Width: 3/8" to 1/2" wide.
3. Bond Break: Insert polyethylene sheet or building paper (15# felt) in the keyway to prevent mortar bond across joint.
4. Reinforcement: Discontinue horizontal joint reinforcement (ladder wire) at the joint. Stop wires 2" short of each face. Do not bridge the joint.
5. Vertical Steel: If grouted cells with vertical rebar are required (structural), stop rebar 1" short of joint and place a greased dowel/sleeve across for shear transfer if engineering requires. Otherwise, omit vertical steel at joint.
6. Backer Rod & Sealant: After tooling mortar joints (concave), install closed-cell backer rod and polyurethane sealant (ASTM C920, Class 25) at both faces. Depth = width (3/8"–1/2").

Critical: Keep mortar out of the joint during laying. Clean keyway daily. Sealant is the weather barrier; mortar is not.

Code Ref: TMS 402 Sec. 7.12.2.1, 7.12.2.2; ASTM C920.

For a 90-foot-long, 8-foot-tall CMU (concrete masonry unit) wall in a warehouse addition, control joints are essential to manage shrinkage cracking due to moisture loss and temperature changes. Here’s the practical guidance per ACI 530/ASCE 5/TMS 402 (Building Code Requirements for Masonry Structures) and industry best practices:

Control Joint Spacing

• Maximum spacing: 20 to 24 feet on center for unreinforced or lightly reinforced CMU walls.
• For an 8-foot-tall wall (height-to-length ratio < 1:10), 24 feet is acceptable if the wall is not exposed to significant moisture or differential movement.
• Recommended for your 90-ft wall: Place control joints at 24-foot intervals → joints at 0’, 24’, 48’, 72’, and 90’ (end of wall). → This gives you 4 control joints (not counting ends, which may need isolation joints if abutting other structures).
• If the wall is reinforced with horizontal bond beams (e.g., at top and bottom) or vertical reinforcement, spacing can be increased to 30 ft, but 24 ft is safer and more conservative for warehouse walls subject to thermal cycling and minor settlement.

How to Build a Control Joint

1. Cut a continuous vertical groove in the mortar joint (not through the block) using a grinder or joint raker: - Depth: ¾ to 1 inch (minimum ½ the wall thickness; for 8” CMU, ¾” is standard). - Width: ⅜ to ½ inch (matching typical mortar joint width). - Run the groove full height of the wall (8 ft), stopping 6–8 inches above the footing and below the top bond beam to avoid weakening structural elements.

1. Install a compressible filler: - Insert closed-cell neoprene or polyethylene backer rod (½” diameter) into the groove. - Do not use wood, fiberglass, or absorbent materials — they can wick moisture or compress unevenly.

1. Apply sealant: - Tool a non-staining, elastomeric sealant (e.g., polyurethane or silicone) over the backer rod, flush with the wall surface. - Sealant must accommodate ±25% movement (check product specs for elongation capacity).

1. Do NOT reinforce across the joint: - Never place horizontal joint reinforcement (truss or ladder-type) or vertical rebar across the control joint — this defeats its purpose. - If vertical reinforcement is required for structural reasons (e.g., wind/seismic), use slip sleeves or debonded bars with grease or tape to allow movement.

Important Notes

• Control joints are not expansion joints — they relieve tensile stress from shrinkage, not accommodate expansion.
• If the wall abuts columns, foundations, or other walls, use isolation joints (full-depth gap with compressible material) at those interfaces.
• Always confirm with the project’s structural engineer — especially if the wall is