14.3 Masonry: CMU, Mortar, Grout, Veneer & Wall Bracing

Concrete Masonry Units (CMU): Modular Standards & Properties

Reinforced concrete masonry construction combines the high compressive strength of factory-manufactured concrete masonry units (CMU) with field-placed reinforcing steel, structural mortar, and high-slump masonry grout. In commercial and industrial buildings across Nevada, reinforced CMU systems serve as fire-rated exterior envelope walls, retaining walls, and primary seismic shear walls.

Dimensional Modularity & Actual vs. Nominal Sizing

American masonry is designed around a 4-inch modular grid. Standard CMU units are manufactured undersized by exactly $3/8\ \text{inch}$ in all three dimensions to allow for a standard $3/8\ \text{inch}$ mortar joint:

                         STANDARD 8x8x16 CMU BLOCK
  ◄──────────────────────── 15-5/8" Actual Length ────────────────────────►
  ┌─────────────────────────┬─────────────────────────┐ ▲                 ▲
  │        Face Shell       │        Face Shell       │ │                 │
  │  ┌───────────────────┐  │  ┌───────────────────┐  │ │                 │
  │  │   Vertical Cell   │  │  │   Vertical Cell   │  │ │ 7-5/8" Actual   │ 8"
  │  │   (Grout Core)    │  │  │   (Grout Core)    │  │ │ Width           │ Nominal
  │  └───────────────────┘  │  └───────────────────┘  │ │                 │ Width
  │      Cross Web          │      Cross Web          │ │                 │
  └─────────────────────────┴─────────────────────────┘ ▼                 ▼
  ◄─── + 3/8" Mortar Joint = 16" Nominal Length ──────►
  (Vertical Height: 7-5/8" Actual + 3/8" Joint = 8" Nominal Height)

• Nominal Dimensions: $8 \times 8 \times 16\ \text{inches}$ (Width $\times$ Height $\times$ Length).
• Actual Manufactured Dimensions: $7\text{-}5/8 \times 7\text{-}5/8 \times 15\text{-}5/8\ \text{inches}$ (Tolerance: $\pm 1/8\ \text{inch}$ per ASTM C90).
• Modular Course Counting: Because one block plus one mortar joint equals exactly 8 inches of vertical rise, masonry heights are easily calculated: exactly 3 courses per 2 vertical feet (24 inches), or 1.5 vertical courses per foot.

---

CMU Classifications: Hollow vs. Solid & Unit Weights

Under ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units), units are categorized by net cross-sectional area and weight classification:

• Hollow Units: Net cross-sectional area is less than 75% of the gross area, so the cores make up 25% or more (typical units are roughly 45% to 50% hollow). Used for reinforced masonry walls where vertical cells accommodate rebar and grout.
• Solid Units: Net cross-sectional area in every plane parallel to the bearing surface equals 75% or more of the gross cross-sectional area. Used for foundation caps, lintel bearing points, and high-load piers.
• Weight Classifications (ASTM C90):
  • Lightweight CMU: Oven-dry concrete density $< 105\ \text{pcf}$ (uses expanded clay, shale, or slate aggregate). Offers better fire resistance for a given thickness, lower thermal conductivity, reduced seismic dead load, and better mason productivity because the units are lighter.
  • Medium-Weight CMU: Density of $105$ to $125\ \text{pcf}$ (uses blend of lightweight aggregate, pumice, and sand/gravel). Widely used for commercial walls.
  • Normal-Weight CMU: Density $\ge 125\ \text{pcf}$ (uses standard sand, gravel, and crushed stone). Higher strength, lower absorption, and better sound attenuation (STC), but the heaviest units for masons to handle.

---

Masonry Mortar & Grout Technology

General contractors must recognize that mortar and grout serve fundamentally different structural functions in masonry assemblies and cannot be substituted for one another.

ASTM C270: Mortar Classifications & The M-a-S-o-N-w-O-r-K Hierarchy

Mortar bonds masonry units together, seals joints against water and air infiltration, cushions irregular bearing surfaces, and transfers compressive and shear stresses between units. ASTM C270 establishes four modern structural mortar types, easily remembered using every second letter of the mnemonic phrase M-a-S-o-N-w-O-r-K:

                      ASTM C270 MORTAR CLASSIFICATIONS

  TYPE   MIN COMPR. STRENGTH  PRIMARY STRUCTURAL FUNCTION & FIELD APPLICATIONS
  ─────────────────────────────────────────────────────────────────────────────
   M          2,500 psi       Highest compressive strength; high lateral earth
                              pressure; foundation walls, retaining walls, sewers.
  ─────────────────────────────────────────────────────────────────────────────
   S          1,800 psi       HIGH FLEXURAL BOND STRENGTH;
                              usual choice for reinforced shear walls and
                              seismic or high-wind structural masonry.
  ─────────────────────────────────────────────────────────────────────────────
   N            750 psi       Medium strength; general exterior above-grade
                              non-loadbearing or light loadbearing walls, veneers.
  ─────────────────────────────────────────────────────────────────────────────
   O            350 psi       Low strength; interior non-loadbearing partitions;
                              historic preservation, repointing, and tuckpointing.
  ─────────────────────────────────────────────────────────────────────────────

• Type M (2,500 psi): Formulated with high Portland cement content. Designed for heavy compressive loads, below-grade foundation walls, deep retaining walls, and masonry in contact with earth where freeze-thaw durability and soil pressures are severe.
• Type S (1,800 psi): Balanced portland cement and hydrated lime. The usual structural mortar in Nevada: specified where high flexural bond and lateral resistance are needed. In Seismic Design Category D and higher, ACI 530/TMS 402 does not allow Type N mortar or masonry cement mortar in the seismic-force-resisting system, so reinforced shear walls there use Type S or M portland cement-lime mortar or mortar cement mortar.
• Type N (750 psi): General-purpose exterior above-grade mortar. Provides excellent workability, water retention, and weather resistance under moderate climate exposures.
• Type O (350 psi): Low-strength, high-lime mortar used for non-loadbearing interior partitions and historical restoration where soft vintage brick requires a flexible mortar to prevent brick spalling.
• Proportion vs. Property Specification: ASTM C270 allows mortar compliance via the Proportion Specification (exact measured volumes of Portland cement, hydrated lime, and sand) or the Property Specification (laboratory testing verifying minimum compressive strength, water retention $\ge 75\%$, and air content limits). The two specifications cannot be mixed.

---

ASTM C476: Masonry Grout & Fluid Slump Requirements

Masonry grout is a structural concrete mixture of cementitious materials, aggregates, and water formulated to flow into hollow CMU cores, completely encasing reinforcing bars and bonding them to the block shell.

• The 8-to-11 Inch Slump Mandate: While structural foundation concrete requires a stiff slump of 3 to 5 inches, ASTM C476 dictates that masonry grout must be placed at a fluid slump of $8$ to $11\ \text{inches}$. Engineering Rationale: Dry concrete masonry units are highly porous and immediately absorb substantial mixing water from the fresh grout upon contact. If grout were placed at a standard 4-inch slump, water suction by the block would immediately cause aggregate bridging, leaving massive structural voids and honeycombs around reinforcing rebar.
• Grout Classifications:
  • Fine Grout: Sand aggregate only (ASTM C404). Used where grout spaces are small.
  • Coarse Grout: Adds pea gravel (about $3/8\ \text{inch}$ maximum). Used in larger cores and bond-beam channels.
  • Grout space table: ACI 530.1/TMS 602 sets the minimum grout-space width and cell size for each grout type and each maximum pour height. Coarse grout always needs the larger space, and taller pours need larger cells.
• Compressive Strength: Minimum compressive strength at 28 days is $2,000\ \text{psi}$ (or matching specified design strength $f'_m$).

---

Grout Pours, Lifts & Cleanouts (ACI 530.1 / TMS 602)

The masonry specification separates two terms that candidates often mix up:

• Grout pour: the total height of masonry grouted before more masonry is laid on top.
• Grout lift: the height of grout placed in one continuous operation within a pour.

                 GROUT POUR VS. GROUT LIFT (TMS 602)

   POUR <= 5 ft 4 in ("low-lift")        POUR > 5 ft 4 in ("high-lift")
   ┌──────────────────────────────┐      ┌──────────────────────────────┐
   │ Lay up to 5'-4", then grout  │      │ Lay wall to full pour height │
   │ No cleanouts required        │      │ CLEANOUTS in bottom course   │
   │ Keep cells free of mortar    │      │ (min 3-in opening)           │
   │ droppings as you lay         │      │ Clean, inspect, then seal    │
   └──────────────────────────────┘      │ Grout in lifts <= 5'-4"      │
                                         │ (12'-8" lifts if conditions) │
                                         └──────────────────────────────┘

• Lift height: A grout lift may not exceed 5 feet 4 inches. It may go up to 12 feet 8 inches only if all three conditions are met: the masonry has cured for at least 4 hours, the grout slump is held between 10 and 11 inches, and there are no intermediate reinforced bond beams between the top and bottom of the pour.
• Cleanouts: When the grout pour height exceeds 5 feet 4 inches, provide cleanouts in the bottom course so each grout space can be cleaned and inspected. The minimum opening dimension is 3 inches. In solid-grouted walls, space cleanouts no more than 32 inches on center. Remove mortar fins and droppings, let the inspector check bar position and laps, then seal the cleanouts before grouting.
• Maximum pour height: Limited by grout type and grout-space size in the grout space table, up to 24 feet.
• Consolidation: Consolidate grout pours over 12 inches with a mechanical vibrator, then reconsolidate after initial water loss and settlement. This closes the voids and shrinkage gaps left as the block absorbs water. Pours of 12 inches or less may be puddled.
• Starting course: The bed joint of the starting course on the foundation must be at least 1/4 inch and no more than 3/4 inch thick. Larger elevation errors are corrected in the foundation, not by building a thick mortar joint.

---

Joint Tooling & Weather Resistance

Tooling compresses and shapes mortar in the joints as it becomes "thumbprint hard" (stiff enough to retain an indentation without sticking to fingers):

                         MASONRY JOINT PROFILES
   ┌───────────────────────┐                    ┌───────────────────────┐
   │   WEATHER RESISTANT   │                    │  WEATHER VULNERABLE   │
   │  (Compacted Surfaces) │                    │  (Water Catch Ledges) │
   ├───────────────────────┤                    ├───────────────────────┤
   │ 1. Concave Joint      │                    │ 3. Raked Joint        │
   │    )  (Best Shedding) │                    │    [  (Catches Water) │
   │                       │                    │                       │
   │ 2. V-Joint            │                    │ 4. Struck Joint       │
   │    >  (High Weather)  │                    │    /  (Pockets Water) │
   └───────────────────────┘                    └───────────────────────┘

• Concave Joints & V-Joints: Compact the mortar firmly against the top and bottom unit edges, producing a dense surface that sheds driving rain. These are the recommended profiles for exterior walls.
• Raked, Struck, Flush, and Beaded Joints: Leave ledges or uncompacted mortar where water, dust, and ice collect. Avoid them on exterior walls exposed to wind-driven rain or freezing.

---

Hot & Cold Weather Masonry

ACI 530.1/TMS 602 adds protection requirements at temperature extremes, and both apply in Nevada:

• Cold weather: applies when the ambient temperature falls below $40^\circ\text{F}$ or the masonry units are colder than $40^\circ\text{F}$. Do not lay frozen or ice-coated units, heat mortar materials as required, and cover and protect new work so the mortar and grout do not freeze while they gain strength. This is common in Elko and Reno winters.
• Hot weather: applies when the ambient temperature exceeds $100^\circ\text{F}$, or exceeds $90^\circ\text{F}$ with wind above 8 mph. Keep mortar cool and wet, limit how far mortar is spread ahead of the units, use mortar soon after mixing, and fog-spray or cover new work. Las Vegas summers regularly meet this trigger.

---

Anchored Masonry Veneer

Brick or stone veneer over wood or steel framing is a non-structural facing tied to the backup wall:

• Anchors: ACI 530 prescriptive rules require at least one 22-gauge corrugated sheet-metal anchor per 2.67 square feet of wall, or one per 3.5 square feet for most other anchor types, plus extra anchors around openings.
• Air space and drainage: Keep the cavity clear of mortar droppings. Install flashing at the base, at shelf angles and lintels, and over openings, with weep holes so water that enters the cavity drains out.
• Lintels: Steel angle lintels over openings carry the veneer above; their bearing length and size come from the drawings.

---

Masonry Wall Safety (OSHA 1926.706)

• Limited access zone: Establish it before a masonry wall is started, on the side of the wall that will not be scaffolded. It must be equal to the height of the wall plus 4 feet and run the entire length of the wall. Only employees actively building the wall may enter.
• Bracing: Walls over 8 feet high must be braced against overturning and collapse unless they are otherwise adequately supported. Bracing stays until the permanent supporting elements are in place. For walls over 8 feet, the limited access zone stays until the bracing requirement is met.
• Wind: Freshly laid, unbraced walls are vulnerable to Nevada's afternoon winds. Follow the bracing design's wind limits and clear the restricted zone when wind exceeds them.