7.5 Masonry Units, Mortar, Grout & Testing
Key Takeaways
- Standard concrete masonry units (CMU) have actual dimensions 3/8" smaller than nominal dimensions (actual 7-5/8" x 7-5/8" x 15-5/8" for a nominal 8x8x16 unit); ASTM C90 mandates a minimum net area compressive strength of 2,000 psi.
- Modular clay bricks have actual dimensions of 3-5/8" x 2-1/4" x 7-5/8" so that 3 vertical courses plus 3/8" mortar joints equal exactly 8 inches in height, matching standard CMU coursing.
- ASTM C270 classifies mortar types by the "MaSoNWorK" hierarchy: Type M (2,500 psi, high compressive/foundations), Type S (1,800 psi, high flexural bond/shear walls), Type N (750 psi, general above-grade), and Type O (350 psi, interior/restoration).
- Mortar retempering is strictly limited: water may be added to restore workability within 2.5 hours of initial mixing; any mortar not placed within 2.5 hours must be discarded.
- ASTM C476 structural grout requires a high slump of 8 to 11 inches to ensure fluid placement into porous CMU cores; high-lift grouting (> 5 ft up to 24 ft) mandates cleanout openings at the base of every reinforced cell.
7.5 Masonry Units, Mortar, Grout & Testing
Structural masonry combines factory-manufactured masonry units (concrete block or fired clay brick) bonded by Portland cement-lime mortar and consolidated with fluid cementitious grout encasing steel reinforcement. In commercial building construction across Georgia, masonry systems must resist vertical gravity loads, extreme lateral wind and seismic forces, and exterior water penetration. General contractors must master the geometric modular coordination of Concrete Masonry Units (CMU) and clay brick, understand weathering grades, specify the appropriate ASTM C270 mortar type using the MaSoNWorK hierarchy, enforce strict mortar retempering timeframes, and execute code-compliant low-lift and high-lift grouting procedures under TMS 402/602.
Concrete Masonry Units (CMU): Sizing, Weights & ASTM C90 Standards
Concrete masonry units are precast rectangular hollow or solid building blocks manufactured from zero-slump Portland cement, water, and graded aggregates consolidated in high-speed vibrating molds and steam-cured.
Modular Coordination & Dimensional Rules
Masonry architecture relies on a 4-inch modular grid. All standard masonry unit dimensions are specified nominally, which includes the thickness of a standard 3/8-inch (0.375") mortar joint:
- Nominal $8\times8\times16 CMU$: Actual dimensions are $7-5/8 in wide \times 7-5/8 in high \times 15-5/8 in long$ ($194 \times 194 \times 397 mm$).
- Nominal $12\times8\times16 CMU$: Actual dimensions are $11-5/8 in wide \times 7-5/8 in high \times 15-5/8 in long$.
- When laid with standard 3/8-inch bed (horizontal) and head (vertical) mortar joints, one standard CMU measures exactly 8 inches high by 16 inches long center-to-center of joints.
NOMINAL 8x8x16 CONCRETE MASONRY UNIT (CMU)
◄──────────────────── Actual 15-5/8" ────────────────────► ┌── 3/8" Head Joint
┌──────────────────────────────────────────────────────────┐│
│ ┌───────────────┐ Web ┌───────────────┐ │▼
│ │ │ │ │ Face │
│ │ Core │ │ Core │ Shell │ Actual
│ │ (Grouted) │ │ (Grouted) │ (1¼" │ 7-5/8"
│ │ │ │ │ min) │
│ └───────────────┘ └───────────────┘ │▲
└──────────────────────────────────────────────────────────┘│
▲ └── 3/8" Bed Joint
└────────────── Actual Width 7-5/8" ──────────────►
ASTM C90 Physical & Compressive Requirements
Under ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units), loadbearing units must meet strict density, water absorption, and compressive strength criteria:
| Unit Density Classification | Oven-Dry Concrete Density | Key Performance Attributes & Applications |
|---|---|---|
| Lightweight Units | Less than $105 pcf$ ($< 1,680 kg/m^3$) | Manufactured with expanded shale, slate, or slag. Superior thermal insulation (R-value), higher fire-resistance ratings (up to 4 hours), lighter handling weight, but higher moisture absorption. |
| Medium Weight Units | $105 pcf to 125 pcf$ ($1,680 - 2,000 kg/m^3$) | Balance of structural strength, thermal mass, fire resistance, and workability. Most common specification in commercial construction. |
| Normal Weight Units | Greater than $125 pcf$ ($> 2,000 kg/m^3$) | Manufactured with sand and crushed gravel. Highest compressive strength, maximum acoustic isolation (STC rating), lowest water absorption, but heavier physical weight. |
- Compressive Strength Mandate: ASTM C90 requires an average minimum compressive strength of 2,000 psi (13.8 MPa) for a 3-unit test sample (and no individual unit below 1,800 psi), calculated across the net cross-sectional area of the unit.
- Common CMU Configurations: Stretcher block (flanged ends for mortar), sash block (end slot for window frames), open-end / H-blocks (open end webs allowing placement around continuous vertical rebar without threading block over the top), and U-block / Knockout bond beam blocks (trough-shaped for horizontal rebar and grout).
Clay Facing Brick: Modular Dimensions & Weathering Grades
Clay bricks are solid or cored ceramic masonry units manufactured by shaping natural clay or shale, drying, and firing in high-temperature tunnel kilns at $1,800^ degF to 2,100^ degF$.
Modular Brick Dimensions & Coursing
Standard Modular Brick is designed to coordinate directly with the standard 4-inch modular grid and 8-inch CMU coursing:
- Actual Modular Brick Size: $3-5/8 in wide \times 2-1/4 in high \times 7-5/8 in long$ ($92 \times 57 \times 194 mm$).
- Nominal Modular Brick Size (with 3/8" joint): $4 in wide \times 2-2/3 in high \times 8 in long$.
- Vertical Coursing Rule: Because each nominal course is $2-2/3 inches$ high:
MODULAR BRICK TO CMU COURSING ALIGNMENT
┌────────────────────────────────────────────────────────┐ ───┐
│ MODULAR CLAY BRICK │ 2¼"│ Course 3
├────────────────────────────────────────────────────────┤ 3/8" Joint
│ MODULAR CLAY BRICK │ 2¼"│ Course 2 ──► TOTAL 8"
├────────────────────────────────────────────────────────┤ 3/8" Joint HEIGHT
│ MODULAR CLAY BRICK │ 2¼"│ Course 1
└────────────────────────────────────────────────────────┘ ───┴─── (Matches 1 CMU)
ASTM C216 Weathering Grades for Facing Brick
- Grade SW (Severe Weathering): High resistance to damage from cyclic freezing and thawing in saturated moisture conditions. Mandatory in North Georgia, exterior grade-level walls, parapets, foundation veneer, and retaining walls. Low water absorption ($le 17%$ 5-hour boil absorption) and high compressive strength ($ge 3,000 psi$ average).
- Grade MW (Moderate Weathering): Moderate resistance to freezing; permitted in above-grade exterior walls in regions with light frost where bricks are not saturated.
- Grade NW (Negligible Weathering): Intended only for interior applications or frost-free tropical environments.
ASTM C270 Mortar Types & Specifications
Mortar serves three essential functions: bonding individual masonry units together, sealing joints against wind and rain penetration, and cushioning units to distribute compressive loads evenly across irregular surfaces. Mortar is specified under ASTM C270 (Standard Specification for Mortar for Unit Masonry) by either the Proportion Specification (volumetric mix recipes) or the Property Specification (laboratory-tested 2-inch compressive cubes at 28 days).
The "MaSoNWorK" Mnemonic & Compressive Strength Hierarchy
To remember the relative strengths of ASTM C270 mortar types, industry professionals use the acronym MaSoNWorK, taking every other letter to establish the descending compressive strength hierarchy:
| Mortar Type | 28-Day Compressive Strength (Property Spec) | Volumetric Proportion Recipe (Cement : Lime : Sand) | Primary Commercial Structural Applications |
|---|---|---|---|
| Type M | 2,500 psi ($17.2 MPa$) | $1 Portland : (1) / (4) Lime : 3 to 3(3) / (4) Sand$ | High compressive strength. Heavy loadbearing columns, below-grade foundation walls, earth-retaining vaults, manholes, sewers, and deep masonry pavements. |
| Type S | 1,800 psi ($12.4 MPa$) | $1 Portland : (1) / (2) Lime : 4(1) / (2) Sand$ | High flexural tensile bond strength. Engineered reinforced masonry walls, exterior cavity walls subject to high seismic or hurricane wind loads, shear walls, and masonry at grade. |
| Type N | 750 psi ($5.2 MPa$) | $1 Portland : 1 Lime : 6 Sand$ | Medium strength / general purpose. Above-grade exterior non-loadbearing veneer, exterior chimneys, parapet walls, and commercial building envelopes. Excellent freeze-thaw durability and workability. |
| Type O | 350 psi ($2.4 MPa$) | $1 Portland : 2 Lime : 9 Sand$ | Low strength / high lime flexibility. Interior non-loadbearing partitions and historic preservation / restoration tuckpointing (cushions historic soft bricks). |
| (Type K) | 75 psi | Historic restoration only; no longer included in standard ASTM C270 tables. |
Core Rule of Mortar Selection: In masonry design, stronger is not always better. Specifying high-strength Type M mortar for above-grade veneer walls is a serious engineering mistake. High-strength mortar is brittle, has lower flexural bond strength than Type S, exhibits poor water retention, and can cause spalling of clay brick faces during thermal movement. Mortar strength should match—and never excessively exceed—the structural requirements of the assembly.
Mortar Ingredients & The 2.5-Hour Retempering Rule
- Portland Cement (ASTM C150): Provides compressive strength and initial setting.
- Hydrated Lime (ASTM C207 - Type S): Essential for plasticity, water retentivity (preventing porous units from sucking all water out of the mortar), workability, and autogenous self-healing of micro-cracks.
- Masonry Sand (ASTM C144): Clean, sharp, well-graded aggregate providing volume and bulk stability.
- The 2.5-Hour Retempering Rule: When freshly batched mortar sits on mortar boards, moisture evaporates into the air, causing the mortar to stiffen. Under ASTM C270, mortar may be retempered with water to restore plastic workability within 2.5 hours of initial mixing. Any mortar that has not been laid in the wall within 2.5 hours after batching must be completely discarded. Adding water to mortar after 2.5 hours severely weakens the Portland cement crystalline paste and drastically reduces flexural bond strength.
ASTM C476 Masonry Grout: Types, Slump & Placement
Grout is a high-slump, fluid mixture of Portland cement, fine aggregate, coarse aggregate (pea gravel), and water specified under ASTM C476 (Standard Specification for Grout for Masonry). Grout flows into hollow CMU cores, fills collar joints, and encases reinforcing steel to form a monolithic composite structural system.
Grout Types & Sizing Requirements
| Grout Type | Aggregate Grading & Sizing | Minimum Clear Core Grout Space |
|---|---|---|
| Fine Grout | Sand aggregate only (passes 3/8" sieve per ASTM C404). | For small grout spaces: clear openings less than 2 inches (50 mm) in narrow dimension. |
| Coarse Grout | Sand plus coarse pea gravel (aggregate size up to 3/8" or 1/2"). | For large grout spaces: clear openings 2 inches or greater in narrow dimension (standard 8" and 12" CMU cores). |
The 8 to 11 Inch Slump Requirement
While structural concrete requires stiff slumps (3 to 5 inches), ASTM C476 mandates a fluid slump of 8 to 11 inches (200 to 280 mm) for masonry grout (measured per ASTM C143). This high slump is necessary because dry, porous concrete masonry unit cells rapidly absorb water from the grout mix immediately upon placement. The fluid consistency prevents bridging, eliminates voids around vertical rebar, and ensures full mechanical bond across unit webs.
Low-Lift vs. High-Lift Grouting Procedures (TMS 602)
┌─────────────────────────────────────────────────────────────────────────┐
│ LOW-LIFT vs. HIGH-LIFT GROUTING PROCEDURES │
└─────────────────────────────────────────────────────────────────────────┘
│
├─► LOW-LIFT GROUTING (Standard ≤ 5-Foot Wall Heights)
│ • Wall erected to maximum height of 5 feet.
│ • Vertical rebar placed and inspected.
│ • Grout poured in continuous 5-foot lifts; no cleanout holes required.
│ • Consolidated immediately using pencil mechanical vibrator.
│
└─► HIGH-LIFT GROUTING (Full-Story Height up to 24 Feet)
• Wall erected to full story height (up to 24 feet).
• MANDATORY CLEANOUT OPENINGS (minimum 3"x3" or full face shell)
cut at the base of every vertical cell containing rebar.
• Mortar droppings, fins, and debris cleaned out from cell base.
• Cleanout covers sealed and braced prior to grouting.
• Grout placed in maximum 5-foot intermediate layers.
• Consolidated and reconsolidated after 5-15 min absorption.
Compressive Strength Verification ($f'_m$)
Specified compressive strength of masonry ($f'_m$) is verified via two methods under TMS 602:
- Unit Strength Method (TMS 602 Table 2): Conservative tabular correlation cross-referencing ASTM C90 unit strength and mortar type (e.g., 2,000 psi CMU + Type S mortar yields $f'_m = 1,500 psi$).
- Prism Test Method (ASTM C1314): Laboratory testing of fabricated masonry prisms (units + mortar + grout) tested in compression at 28 days.
Under ASTM C270, which mortar type provides high flexural tensile bond strength (approximately 1,800 psi compressive strength) and is the standard recommendation for reinforced masonry walls subject to high seismic and wind lateral loads?
According to ASTM C270 field mixing rules, within what maximum timeframe following initial batching may masonry mortar be retempered with water to restore workability before it must be discarded?
What is the specified slump range for structural masonry grout placed in concrete masonry unit (CMU) cores under ASTM C476, and why is this consistency required?