Vertical reinforcement is used in masonry walls to resist tensile stresses that may arise from flexural and shear loads. Masonry columns and pilasters are also reinforced vertically to increase resistance to axial loads.
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Types & Sizes
Deformed reinforcing bars (rebar) should conform to ASTM A 615. Grade 60 reinforcement with a yield strength of 60,000 psi is most common; in some markets Grade 40 (yield strength of 40,000 psi) may also be available.
The Building Code Requirements and Specification for Masonry Structures reported by the Masonry Standards Joint Committee (MSJC) permits reinforcement sizes up to #11 (1 3/8” diameter) to be used in masonry construction, but it is rare to see bars larger than #8 (1” diameter). When masonry is designed using the Strength Design provisions (Building Code Requirements and Specification for Masonry Structures Chapter 3) the maximum bar size is limited to the lesser of #9 bars, 1/8 times the nominal wall thickness, or ¼ of the clear dimension of the cell, course, or collar joint being reinforced. The total reinforcement area placed in a cell may not exceed 4 percent of the cell area (8% at lap splice locations). For 8-inch masonry units, the cell area is approximately 32 square inches, into which a maximum of 1.3 square inches of reinforcement may be placed.
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Vertical reinforcement is typically placed by dropping the bar into empty cells after the wall is built. It is good construction practice to have vertical reinforcement in place before grouting. The Building Code Requirements and Specification for Masonry Structures requires reinforcement to be in place prior to grouting (See Building Code Requirements and Specification for Masonry Structures Section 3.2 E). The practice of stabbing bars into freshly placed grout is not permitted as it does not permit the inspector to verify proper reinforcement placement.
Another option is to use open-end “A” or “H” block units. These units have one or both end webs removed, and can be placed around vertical reinforcement projecting up from the foundation or the previous grout pour.
Keeping Cells Clean
Special care needs to be taken when building reinforced masonry walls to prevent excessive amounts of debris, mortar droppings, etc., from falling into the reinforced cell. Small amounts of mortar and debris are acceptable so long as grout bond is not severely inhibited. Mortar protruding more than ½” into the grouted cell must be removed before grouting (Building Code Requirements and Specification for Masonry Structures Spec. 3.3 B. c.) so it does not inhibit grout flow.
Lap splices are used to ensure reinforcement continuity up the height of the wall. A length of reinforcement is left protruding up from the top of every grout pour, to lap with the reinforcement in the next grout pour. Tensile stresses are transferred from one bar to the next through bond with the surrounding grout. It is not necessary for bars to be in contact with one another at lap splices and the Building Code Requirements and Specification for Masonry Structures permits adjacent bars to be separated by up to 8” for non-contact lap splices.
The length of overlap is designed by the engineer, but will vary depending on the masonry strength and bar diameter. Longer laps are also required for bars placed close to the wall face.
Bar positioners can take many forms but are often periodically embedded in mortar joints up the height of the wall. After the wall is built, reinforcement is fed down through the positioner to ensure the bar is held in the required location.
Older building codes required bars to be held in place by positioners during grouting. The current Building Code Requirements and Specification for Masonry Structures does not explicitly require bar positioners, but it is the contractor’s responsibility to “support and fasten reinforcement together to prevent displacement” during grouting (Section 3.4 B).
Reinforcement placement is critical for ensuring the wall has enough strength to resist design loads. Misplacing bars by as little as ½” can have a serious effect on the wall’s ability to resist loads.
Reinforcement placement tolerances are listed in the Building Code Requirements and Specification for Masonry Structures Section 3.4 B 7. Tight tolerances are required: bars must be placed within ±½” of the specified location for most designs (d distance 8” or less). Placement along the length of the wall is required to be within ±2” of the specified spacing.
Reinforcement placement needs to be designed to ensure there is enough space around the bar for proper grout flow. A minimum space of either ¼” (for fine grout) or ½” (for coarse grout) must be left between adjacent bars and any masonry surface. Bars are permitted to be in contact with one another at lap splices.
Masonry reinforcement is embedded deep in walls and protected from exterior weather by the masonry unit face shell and a layer of grout. No special corrosion protection is used for masonry construction. Epoxy-coated, galvanized, or stainless steel reinforcement may sometimes be used in severe environments such as seawalls, chemical plants, and some food processing facilities.