Masonry Materials

Masonry is one of humanity's most ancient construction methods — stone and mud brick structures have stood for thousands of years. Modern masonry uses manufactured units (brick, concrete masonry units, and stone) bonded with mortar to create walls, foundations, facades, and architectural features that are durable, fire-resistant, and require minimal maintenance. This lesson covers masonry materials, mortar types, and fundamental masonry construction principles.

Clay Brick

Clay brick is manufactured by forming clay into rectangular units and firing them in a kiln at temperatures of 1,800-2,400°F (980-1,315°C). The high firing temperature fuses the clay particles into a hard, durable, weather-resistant unit.

Standard modular brick dimensions: 3-5/8" wide × 2-1/4" tall × 7-5/8" long (nominal 4" × 2-2/3" × 8" with mortar joints).

Brick grades (ASTM C62):

• SW (Severe Weathering): For exterior use in freeze-thaw climates. Most durable, lowest absorption.
• MW (Moderate Weathering): For exterior use in mild climates or protected locations.
• NW (Negligible Weathering): For interior use or backup walls only.

Brick types:

• Face brick: For exposed surfaces. Uniform color, size, and texture.
• Building brick (common brick): For structural use where appearance is not critical.
• Firebrick: Refractory brick for fireplaces, kilns, and high-temperature applications.
• Paving brick: Heavy-duty brick for sidewalks, driveways, and plazas.

Concrete Masonry Units (CMU)

CMU (concrete block) is manufactured from Portland cement, aggregates, and water, molded under pressure and cured. CMU is the workhorse of masonry construction — used for foundation walls, structural walls, retaining walls, and backup walls behind brick veneer.

Standard CMU dimensions: 7-5/8" wide × 7-5/8" tall × 15-5/8" long (nominal 8" × 8" × 16" with mortar joints).

Common CMU widths: 4", 6", 8", 10", 12" (nominal). The 8" wide unit is the most common for structural walls.

CMU types:

• Hollow units: The standard CMU with two or three hollow cores. The cores can be grouted (filled with concrete) and reinforced with rebar for structural walls.
• Solid units: At least 75% solid. Used for cap blocks and special structural applications.
• Split-face units: One face is split to create a rough, textured appearance.
• Ground-face (honed) units: Smooth, polished surface for architectural applications.
• Insulated CMU: Cores filled with insulation inserts to improve thermal performance.

CMU weight classifications:

• Normal weight: 125-145 lb/ft³ aggregate density. Standard for structural applications.
• Medium weight: 105-125 lb/ft³. Lighter but still structural.
• Lightweight: Less than 105 lb/ft³. Uses lightweight aggregate (expanded shale, clay, or slag). Easier to handle, better insulating, but lower strength.

Natural Stone

Natural stone is classified by its geological origin:

• Ignite (formed by volcanic activity): Granite — extremely hard, durable, available in many colors. Used for countertops, flooring, cladding, and monuments.
• Sedimentary (formed by accumulated sediment): Limestone, sandstone — softer than granite, easier to cut. Used for building facades, trim, and walls. Indiana limestone is a classic American building stone.
• Metamorphic (formed by heat/pressure transformation): Marble, slate — marble is prized for its beauty (flooring, countertops); slate is used for roofing and flooring.

Stone construction today is primarily veneer (thin slabs attached to a structural backup wall) rather than solid structural stone, which is prohibitively expensive and time-consuming.

Mortar is the material that bonds masonry units together. It is composed of Portland cement, lime, sand, and water. Mortar accounts for roughly 7% of a masonry wall's surface area but has a disproportionate effect on the wall's strength, weather resistance, and appearance.

Mortar Types (ASTM C270)

Mortar types are designated by letters: M, S, N, O, and K (from the words MaSoN wOrK).

Key concept: Stronger is not always better. Mortar should be softer than the masonry units it bonds. If the mortar is harder than the brick, the brick will crack under stress instead of the mortar joint (which is easier and cheaper to repair). This is especially important with historic or soft brick.

Mortar Components

• Portland cement: Provides strength and durability. More cement = stronger mortar (but stiffer and less workable).
• Hydrated lime: Improves workability (makes the mortar "buttery" and easy to spread), self-healing ability (autogenous healing of hairline cracks), water retention, and bond to units. More lime = more workable (but lower strength).
• Sand: Provides volume and reduces shrinkage. Must be clean, well-graded mason's sand.
• Water: Activates cement hydration and provides workability.

Grout is a fluid mixture of Portland cement, sand, aggregates, and water used to fill the cores of CMU walls and the cavities between wythes (layers) of masonry. When reinforcing bars are placed in the cores, grout surrounds and bonds to the rebar, creating a reinforced masonry wall.

Grout types:

• Fine grout: Contains only sand (fine aggregate). Used for cores smaller than 2" × 3".
• Coarse grout: Contains sand and pea gravel (coarse aggregate). Used for larger cores and cavities.

Minimum compressive strength: 2,000 psi (typically much higher — 3,000-5,000 psi).

Modern structural masonry is typically reinforced — rebar is placed vertically in CMU cores (which are then grouted) and horizontally in bond beams (courses with U-shaped lintel blocks that form a continuous horizontal channel). This creates a reinforced masonry wall that can resist lateral forces (wind, seismic) and support significant loads.

• Efflorescence: White, powdery salt deposits on the surface of masonry. Caused by water dissolving salts within the masonry and carrying them to the surface as it evaporates. Primarily a cosmetic issue. Prevention: use low-alkali cement, keep units dry before installation, and provide proper drainage.
• Freeze-thaw damage: Water absorbed into masonry units expands when it freezes, causing spalling and cracking. Prevention: use SW-grade brick, proper flashing and drainage to minimize water entry.
• Cracking: Can result from settlement, thermal expansion/contraction, or moisture movement. Control joints are placed in CMU walls at regular intervals (typically every 20-25 feet and at openings) to accommodate movement.