10.3 Masonry Materials, Mortar Types & Structural Masonry Walls

Concrete Masonry Units (CMU) & Modular Coordination

Commercial masonry construction relies on the structural integration of masonry units, bonding mortar, steel reinforcement, and fluid grout. Masonry units function under modular coordination principles codified in architectural and structural design standards.

Modular Coordination & Actual vs. Nominal Dimensions

American commercial masonry follows a 4-inch modular grid. The dimensions of masonry units are specified in nominal terms that include the unit thickness plus the thickness of one standard mortar joint. In commercial construction, the standard mortar joint thickness is 3/8 inch (0.375") for both bed (horizontal) and head (vertical) joints:

$$\text{Actual Dimension} = \text{Nominal Dimension} - \frac{3}{8}\text{ inch}$$

+--------------------------------------------------------------------------------------------------+
|                       NOMINAL VS. ACTUAL DIMENSIONS OF STANDARD MASONRY UNITS                    |
+-----------------------+-----------------------------+--------------------------------------------+
| UNIT TYPE             | NOMINAL DIMENSIONS (W x H x L)| ACTUAL MANUFACTURED DIMENSIONS (W x H x L) |
+-----------------------+-----------------------------+--------------------------------------------+
| Standard 8" CMU       | 8" x 8" x 16"               | 7-5/8" x 7-5/8" x 15-5/8"                  |
| 4" CMU (Veneer/Backer)| 4" x 8" x 16"               | 3-5/8" x 7-5/8" x 15-5/8"                  |
| 6" CMU (Partition)    | 6" x 8" x 16"               | 5-5/8" x 7-5/8" x 15-5/8"                  |
| 10" CMU (Foundation)  | 10" x 8" x 16"              | 9-5/8" x 7-5/8" x 15-5/8"                  |
| 12" CMU (Retaining)   | 12" x 8" x 16"              | 11-5/8" x 7-5/8" x 15-5/8"                 |
| Standard Modular Brick| 4" x 2-2/3" x 8"            | 3-5/8" x 2-1/4" x 7-5/8" (3 courses = 8") |
+-----------------------+-----------------------------+--------------------------------------------+

ASTM C90 Standards for Loadbearing CMU

Loadbearing concrete masonry units are governed by ASTM C90, which establishes minimum compressive strength, dimensional tolerances, and moisture absorption parameters:

• Compressive Strength: Under the current ASTM C90, the average net-area compressive strength of three units must be at least 2,000 psi, and no individual unit may test below 1,800 psi. Older editions used 1,900 and 1,700 psi.
• Hollow vs. 100% Solid Units:
  • Hollow Units: The net cross-sectional area parallel to the bearing plane is less than 75% of the gross cross-sectional area (typical two-core blocks have net areas between 50% and 65%).
  • Solid Units: The net cross-sectional area is 75% or greater of the gross area. Completely solid concrete bricks are utilized where high concentrated point loads bear beneath steel joists or lintels.
• Weight Classifications:
  • Normal Weight: Unit density ≥ 125 lbs/cu ft (uses sand and crushed limestone/gravel; highest sound transmission loss and compressive strength).
  • Medium Weight: Unit density between 105 and 125 lbs/cu ft.
  • Lightweight: Unit density < 105 lbs/cu ft (uses expanded clay, shale, or slate; lighter to handle for masons, offers superior thermal R-values and higher fire resistance ratings per inch of thickness).

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Mortar Types and ASTM C270 Standards

Mortar is the bonding agent that cushions masonry units, accommodates dimensional tolerances, bonds individual blocks into a structural composite, and seals joints against wind-driven moisture penetration.

The "MaSoN wOrK" Mnemonic & Mortar Selection

ASTM C270 governs mortars for unit masonry. The standard designates four primary mortar types identified by the alternating letters in the phrase "MaSoN wOrK" (Types M, S, N, and O), arranged from highest to lowest compressive strength:

+--------------------------------------------------------------------------------------------------+
|                             ASTM C270 MORTAR CLASSIFICATIONS & CAPABILITIES                      |
+--------+---------------------+-------------------+-----------------------------------------------+
| TYPE   | MIN. 28-DAY STRENGTH| FLEXURAL BOND /   | PRIMARY STRUCTURAL APPLICATIONS               |
|        | (ASTM C270 PROPERTY)| WORKABILITY       |                                               |
+--------+---------------------+-------------------+-----------------------------------------------+
| **M**  | **2,500 psi**       | High compression; | Heavy compressive loads, below-grade          |
|        |                     | Moderate flexure; | foundation walls, earth retaining walls,      |
|        |                     | Low lime content  | manholes, vaults, and heavy masonry pavements.|
+--------+---------------------+-------------------+-----------------------------------------------+
| **S**  | **1,800 psi**       | **High flexural   | High lateral load resistance: exterior walls  |
|        |                     | bond strength**;  | subject to high coastal hurricane winds,      |
|        |                     | Excellent adhesion| seismic shear walls, below-grade walls, and   |
|        |                     |                   | reinforced structural masonry columns.        |
+--------+---------------------+-------------------+-----------------------------------------------+
| **N**  | **750 psi**         | Balanced strength,| General above-grade exterior walls, exterior  |
|        |                     | Superior weather  | brick veneers, interior loadbearing walls,    |
|        |                     | resistance & flow | and chimneys. Most widely specified mortar.   |
+--------+---------------------+-------------------+-----------------------------------------------+
| **O**  | **350 psi**         | Low strength;     | Interior non-loadbearing partition walls,     |
|        |                     | High lime content;| historic tuckpointing and restoration where   |
|        |                     | High flexibility  | soft historic brick requires flexible mortar. |
|        |                     |                   | **PROHIBITED** below grade or where frozen.   |
+--------+---------------------+-------------------+-----------------------------------------------+

Proportion Specification vs. Property Specification

ASTM C270 provides two distinct compliance methods. The project specifier must select one method; they cannot be combined:

1. Proportion Specification: Mortar is mixed according to prescribed volumetric ratios of Portland cement (ASTM C150), hydrated lime (ASTM C207 Type S), and clean masonry sand (ASTM C144). For example, Type N requires 1 part Portland cement, 1 part hydrated lime, and 4.5 to 6 parts damp loose sand.
2. Property Specification: The mix is formulated based on documented laboratory testing of compressive strength, water retention (minimum 75%), and air content (maximum 12% to 14%).

Retempering Mortar Standards

As mixed mortar sits on the mortar board, evaporation depletes mixing water, causing the mortar to stiffen and lose workability:

• Retempering Rule: Mortar that stiffens due to evaporation may be retempered by adding clean water and thoroughly remixing to restore its plastic workability, provided this occurs within 2.5 hours of initial mixing.
• The 2.5-Hour Hard Limit: Mortar remaining unused after 2.5 hours from initial batching must be discarded immediately. Beyond 2.5 hours, chemical hydration has progressed into early crystal formation; adding water breaks these hydrating bonds, permanently destroying compressive strength and flexural bond adhesion.

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Grout vs. Mortar: Structural Grouting (ASTM C476)

A common and critical construction exam error is confusing mortar with grout. While both contain cement, aggregate, and water, their functions and physical properties are completely different:

+--------------------------------------------------------------------------------------------------+
|                             STRUCTURAL COMPARISON: MORTAR VS. GROUT                              |
+------------------------------------+-------------------------------------------------------------+
| MASONRY MORTAR (ASTM C270)         | STRUCTURAL GROUT (ASTM C476)                                |
+------------------------------------+-------------------------------------------------------------+
| - **Function:** Bonds masonry      | - **Function:** Fills hollow CMU cores, embeds rebar,       |
|   units together into a structural |   and creates solid reinforced columns/bond beams           |
|   wall assembly                    | - **Slump:** Exceptionally fluid: **8 to 11 inches**        |
| - **Slump:** Stiff plastic mix:    | - **Aggregate:** Fine sand or coarse pea gravel (up to 3/8" |
|   typically **3 to 5 inches** to   |   or 1/2" aggregate per ASTM C404)                          |
|   support unit weight without      | - **Hydration Mechanics:** High fluidity allows grout to    |
|   sagging or squeezing out of joint|   flow around dense rebar; porous dry CMU blocks absorb     |
| - **Placement:** Hand troweled on  |   excess water, automatically lowering effective w/cm ratio |
|   face shells and cross webs       | - **Placement:** Poured or pumped directly into vertical    |
|                                    |   cores and horizontal bond beams                           |
+------------------------------------+-------------------------------------------------------------+

Low-Lift vs. High-Lift Grouting

• Low-Lift Grouting: The masonry wall is erected in vertical increments not exceeding 5 feet in height before grout is placed. Vertical rebar is placed in position, cores are filled with grout in lifts not exceeding 5 feet, and consolidated with a mechanical vibrator. Cleanout holes are not mandatory.
• High-Lift Grouting: The masonry wall is constructed to full story height (up to 24 feet) before any grout is pumped:
  • Mandatory Cleanout Openings: Cleanout openings measuring at least 3 × 3 inches must be cut into the bottom course of CMU at the base of every vertical grouted cell.
  • Debris Removal & Inspection: Before grouting, masons use compressed air or water to flush mortar droppings, loose debris, and dust out through the cleanouts. The structural engineer or municipal building inspector inspects rebar placement and cleanouts, after which the cleanouts are mortared shut.
  • Consolidation and Reconsolidation: Grout is pumped in continuous vertical lifts of not more than 5 feet. Each lift must be consolidated with an internal pencil vibrator. Crucially, because dry CMU absorbs water rapidly, the grout must be reconsolidated 5 to 30 minutes later (before initial set) to close plastic settlement shrinkage voids.

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Structural Masonry Reinforcement, Bond Beams & Installation Practices

Modern commercial CMU walls function as reinforced structural diaphragms capable of resisting extreme wind shear and seismic forces:

Reinforcement Detailing

1. Vertical Core Reinforcement: Deformed Grade 60 steel rebar placed in designated CMU cells. Rebar positioners (wire centering devices) must be installed at intervals not exceeding 200 bar diameters (or maximum 10 feet) to keep rebar centered in the core, ensuring minimum 1/2-inch grout cover between steel and CMU shell.
2. Horizontal Joint Reinforcement: Prefabricated longitudinal steel wires connected by cross wires, embedded completely in the 3/8-inch horizontal bed mortar joints:
   • Ladder Type: Cross wires run perpendicular to side rods; preferred for grouted walls because cross wires leave vertical CMU cores unobstructed for vertical rebar and grout flow.
   • Truss Type: Cross wires run diagonally in a zig-zag truss configuration; provides greater lateral stiffness in unreinforced hollow walls.
   • Wire Gauges & Spacing: Standard 9-gauge or heavy-duty 3/16-inch wire; typically spaced vertically every 16 inches on center (every second 8-inch CMU course).
3. Bond Beams: Continuous reinforced horizontal beams integrated around the building perimeter at floor and roof bearing levels. Constructed using special U-shaped lintel/bond beam blocks with knocked-out web slots, loaded with continuous horizontal rebar (typically two #5 or #6 bars with 48-diameter lap splices), and fully grouted solid. Bond beams distribute concentrated roof joist loads and tie adjacent walls together.

Masonry Installation & Tooling Practices

• Bond Patterns:
  • Running Bond: Units overlap the course below by at least 1/4 of unit length (standard half-lap bond). Provides superior structural shear interlock.
  • Stack Bond: Vertical head joints align in a continuous unbroken vertical line. Stack bond has no mechanical interlock between units, so TMS 402 requires minimum horizontal reinforcement for masonry not laid in running bond. The designer specifies the amount, usually bond beams or joint reinforcement.
• Mortar Bedding:
  • Face-Shell Bedding: Mortar is spread only along the outer longitudinal edges (face shells) of the CMU; cross webs are left bare. Standard practice for normal loadbearing hollow CMU.
  • Full Bedding: Mortar covers both face shells and all cross webs; required for the first course on footings, solid units, columns, piers, and around cleanouts.
• Joint Tooling Profiles: When mortar is "thumbprint hard," joints are compressed with a curved jointing tool:
  • Concave Joints & V-Joints: Compress mortar tightly against unit edges, creating a dense, weather-resistant barrier that sheds water rapidly. Required for all exterior weather-exposed masonry.
  • Raked, Struck, Flush, and Beaded Joints: Create flat ledges or scrape away surface paste; highly susceptible to water intrusion and freeze-thaw damage; strictly prohibited on exterior weather-exposed walls.

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Cavity Wall Construction, Flashing & Moisture Control

Exterior brick veneer over CMU backup is designed under the drainage wall principle. Brick masonry is porous; wind-driven rain inevitably penetrates the outer veneer through microscopic interface cracks. A properly designed cavity wall manages this water by directing it harmlessly back to the building exterior.

+--------------------------------------------------------------------------------------------------+
|                             ANATOMY OF A MASONRY CAVITY WALL ASSEMBLY                            |
+------------------------------------+-------------------------------------------------------------+
| COMPONENT                          | TECHNICAL REQUIREMENTS & PERFORMANCE FUNCTION               |
+------------------------------------+-------------------------------------------------------------+
| **1. Exterior Brick Veneer**       | High-density facing brick (ASTM C216 Grade SW).             |
+------------------------------------+-------------------------------------------------------------+
| **2. Continuous Clear Air Space**  | Minimum **1.0 inch clear**, preferably **2.0 inches**.      |
|                                    | Provides capillary break preventing water transfer inward.  |
+------------------------------------+-------------------------------------------------------------+
| **3. Mortar Collection Mesh**      | Trapezoidal open-weave polymer mesh installed at cavity     |
|    **(Mortar Net)**                | base to catch mortar droppings at staggered heights,        |
|                                    | preventing continuous bridging clogs over weep holes.       |
+------------------------------------+-------------------------------------------------------------+
| **4. Rigid Foam Insulation**       | Extruded polystyrene (XPS) or polyisocyanurate installed    |
|                                    | against backup CMU for continuous thermal envelope.         |
+------------------------------------+-------------------------------------------------------------+
| **5. Water-Resistive Barrier (WRB)**| Vapor-permeable fluid-applied membrane or self-adhering     |
|                                    | sheet on exterior face of backup CMU.                       |
+------------------------------------+-------------------------------------------------------------+
| **6. Through-Wall Flashing**       | Flexible membrane (rubberized asphalt, EPDM, copper)        |
|                                    | extending minimum **8 inches up backup wall**, bonded behind|
|                                    | WRB, across cavity, through brick bed joint, terminating in |
|                                    | a stainless steel drip edge bent 45 degrees outward.        |
+------------------------------------+-------------------------------------------------------------+
| **7. Weep Holes**                  | In head joints directly above flashing. Code (IBC/TMS 402): |
|                                    | **max 33 inches o.c.**, at least **3/16-inch** diameter.     |
|                                    | Common specs: open head joints / vents **24 in. o.c.**;      |
|                                    | cotton wicks **16 in. o.c.** (Brick Industry Association). |
+------------------------------------+-------------------------------------------------------------+

Masonry Control Joints (Shrinkage Control)

Concrete masonry shrinks as it cures and dries. To prevent random vertical cracking, continuous vertical control joints must be constructed through the CMU wall:

• Spacing Rule: Spaced at intervals not exceeding 20 to 25 feet on center, or at a length-to-height ratio of 1.5 to 1, whichever is less.
• Location Triggers: Mandatory at abrupt changes in wall height or thickness, at one or both sides of large window/door openings, and adjacent to building corners (typically within 4 to 8 feet).
• Detailing: Head joints are left open of mortar and fitted with a preformed neoprene or rubber cross gasket to transfer lateral wind loads, sealed with closed-cell backer rod and elastomeric polyurethane sealant.