11.3 Masonry: Brick, Block & Mortar
Key Takeaways
- Masonry accounts for 10 of 100 items on the B-1/B-2 commercial trade exam.
- ASTM C270 defines four mortar types — M, S, N, O — rated by compressive strength; Type N is the general-purpose mortar for above-grade brick and block walls.
- Type S mortar (1,800 psi) is the standard for CMU walls below grade or resisting lateral loads, because it balances high strength with good bond.
- Efflorescence — the white salt deposit on masonry — is common in Arizona's dry climate and signals moisture moving through the wall; control it with proper drainage, venting, and low-alkali materials.
- Joint reinforcement (ladder or truss wire) in horizontal mortar beds and grouted, reinforced cells give CMU walls their flexural and shear capacity.
Brick and Concrete Masonry Units (CMU)
Masonry relies on units bonded by mortar. Two unit families dominate commercial masonry construction:
- Brick (clay masonry, ASTM C216 for facing brick) — modular dimensions, weather-resistant, fired clay. Nominal size is often 2-2/3 × 4 × 8 inches (modular), with mortar joints making courses 3 inches high.
- Concrete masonry units (CMU) (ASTM C90) — hollow concrete blocks, nominal 8 × 8 × 16 inches. The hollow cores accept vertical rebar and grout where reinforcement is required. CMU is the workhorse of Arizona commercial and residential foundation and retaining walls.
Mortar Types per ASTM C270
Mortar bonds units together and distributes loads. ASTM C270 classifies mortar by compressive strength into four types:
| Type | Compressive Strength (psi) | Common Use |
|---|---|---|
| M | 2,500 | High-strength: foundations, retaining walls, below-grade, heavy loads |
| S | 1,800 | Below-grade CMU walls, lateral-load-resisting walls, patios |
| N | 750 | General above-grade exterior walls (brick veneer, parapets, chimneys) |
| O | 350 | Interior, non-load-bearing walls, tuck-pointing historic masonry |
A useful mnemonic is "MaSoN wOrk" (M, S, N, O, from strongest to weakest). Note the trade-off: stronger mortar is not always better. Type N is more flexible and accommodates movement; Type M is brittle and can crack if the wall flexes. Match the mortar to the application, not just to the highest strength.
Quick Answer: Type N (750 psi) is the general-purpose mortar for above-grade brick and block; Type S (1,800 psi) is standard for reinforced CMU and below-grade walls.
Mortar Mixing
Mortar is mixed from cement, lime, sand, and water. Field proportioning by volume must match the ASTM C270 recipe. Lime adds workability and water retention; cement provides strength. Retempering — adding water to dry mortar — is permitted once within 2.5 hours of original mixing, but mortar that has set beyond that must be discarded. Over-retempering weakens the mortar.
Laying Techniques: Bond Patterns and Joints
Bond patterns distribute load and create the visible face. Common patterns:
- Running bond — the default for CMU and most brick. Each unit overlaps the one below by half its length. Simple, strong, and standard.
- Stack bond — units align vertically with continuous head joints. Aesthetically striking but structurally weak without joint reinforcement; the 2018 IBC and TMS (The Masonry Society) 402 require horizontal joint reinforcement at 24 inches on center for stack bond.
- Common bond (American bond) — brick walls with a course of headers every 5–6 courses to tie wythes together.
Mortar joints are typically 3/8 inch thick. Joint profile affects weather resistance:
- Concave (tooled) — the most weather-resistant; compresses mortar against the unit edge. Standard for exterior walls.
- V-joint — similar weather resistance, decorative.
- Raked — recessed; looks good but collects water, so it is only for interior or sheltered walls.
- Flush or struck — quick but offers little weather shedding.
Reinforcement: Joint Reinforcement and Grouted Cells
Masonry walls resist wind and seismic loads through a combination of units, mortar, and reinforcement:
- Joint reinforcement — galvanized or stainless steel wire (ladder or truss type) laid in horizontal mortar beds, typically every 16 inches for CMU. It controls shrinkage cracking and ties the wall together.
- Grouted cells — vertical CMU cores are filled with grout (a fluid concrete mix) around vertical rebar. The 2018 IBC, adopting TMS 402, specifies minimum vertical reinforcement for reinforced masonry walls — spacing and bar size depend on seismic design category and wall loads.
- Bond beams — horizontal courses of U-shaped CMU filled with grout and horizontal rebar, placed at the top of walls and at lintels to spread loads and tie the wall together.
Grout for reinforced masonry is specified by ASTM C476. Coarse grout (max aggregate 3/8 inch) is used for cells 2 inches or wider; fine grout is used for smaller cells. Minimum grout compressive strength is typically 2,000 psi. Grout is placed in lifts of up to 12 inches and consolidated by vibration or puddling to eliminate voids around the rebar.
Weather Resistance
Masonry is durable but not waterproof. Weather resistance depends on:
- Proper mortar type and fully filled joints (no voids).
- Tooled concave joints that shed water.
- Flashing and weep holes at the base of brick veneer to let trapped water escape. Weep holes are typically spaced 24–33 inches on center.
- Water-resistant coatings (silane/siloxane sealers) on porous units, where specified.
Arizona Considerations: Efflorescence and Corrosion
Efflorescence — the white, powdery salt deposit that appears on masonry surfaces — is common in Arizona. It forms when water-soluble salts in the mortar, units, or soil dissolve, migrate to the surface, and crystallize as the water evaporates. Arizona's dry, hot climate accelerates evaporation, driving salts to the surface quickly. Control efflorescence by:
- Limiting water entry — good flashing, drainage, and cap break details.
- Using low-alkali cement and clean, salt-free sand.
- Storing units off the ground and covering them before installation.
- Venting cavity walls so moisture escapes.
Corrosion of joint reinforcement and rebar is accelerated by chlorides (from soil or deicing salts) and by moisture. Use hot-dip galvanized or stainless steel joint reinforcement in exposed or corrosive environments, and maintain adequate grout cover around vertical rebar in CMU cells.
Which ASTM C270 mortar type is the general-purpose choice for above-grade exterior brick veneer and chimney walls?
What causes the white, powdery efflorescence commonly seen on Arizona masonry walls, and what is the primary control strategy?