Per the Specification for Masonry Structures, TMS 602-11/ACI 530.1-11/ASCE 6-11, the tolerance for mortar bed joint thickness is plus or minus 1/8 inch from the specified dimension. In standard modular construction, the default specified bed joint thickness is 3/8 inch. Therefore, a standard mortar bed joint is allowed to be placed with a minimum ¼ inch thickness.
Placing 3/16-inch joint reinforcement wire on the top surface of block units would, theoretically, leave 1/16 inch of top mortar coverage above the wire. However, no top mortar coverage may result when construction tolerances and code-required corrosion protection for the joint reinforcement are considered, as follows:
- Two (2) to three (3) mil thickness of galvanizing increases the wire diameter by four (4) to six (6) mils.
- The as-manufactured top and bottom surfaces of a concrete block can vary due to texture.
- The top surface of block courses is required by the Specification to be placed true-to-a-line, but the permitted tolerance for true-to-a-line is plus or minus 1/4 inch in 10 feet.
- The height of individual concrete blocks can vary by plus or minus 1/8 inch (per ASTM C90) from the specified height, resulting in potential offsets in the bottom of the block course above the joint reinforcement.
- A 10-foot length of horizontal joint reinforcement will have some curvature.
Although a 3/16-in. (4.8-mm) diameter wire can, theoretically, be placed in a 3/8-in. (9.5-mm) joint, limiting such heavy-duty joint reinforcement to construction in which the specified mortar bed joint thickness is 1/2-in. (12.7 mm) or more should be considered to accommodate the tolerances described above. According to the January 1995 Masonry Construction magazine article4 by Mario J. Catani titled“Selecting the Right Joint Reinforcement for the Job”:
“One compelling reason to use 9 gauge reinforcement is for fit and constructability. While the code allows joint reinforcement to have a diameter one half the mortar joint width, the tolerances allowed for units, joints and the wire itself can hinder the placement of large diameter reinforcement. Use it only when there is no other choice.”
- Specification for Masonry Structures, TMS 602-11/ACI 530.1-11/ASCE 6-11, Masonry Standards Joint Committee (MSJC), Part 3 - Execution
- Joint Reinforcement: Less Is More, Masonry Magazine, July 2015, pages 44 – 50
- Selecting Joint Reinforcement, The Construction Specifier Magazine, June 2014, pages 50 – 58
- Selecting the Right Joint Reinforcement for the Job, Masonry Construction Magazine, January 1995, pages 8 – 14
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