8.1 Masonry Units, Mortar Types (M/S/N/O), and Grout
Key Takeaways
- Mortar types M, S, N, O come from "MASON WORK"; minimum strengths are 2,500 / 1,800 / 750 / 350 psi respectively.
- More lime increases workability but decreases strength — Type O is the weakest, Type M the strongest.
- Nominal CMU 8x8x16 has actual dimensions 7-5/8 x 7-5/8 x 15-5/8 inches; the 3/8-inch difference is the mortar joint.
- Grout (ASTM C476) is high-slump (8-11 in) and flows around reinforcement; fine grout has no gravel, coarse grout adds 3/8-inch pea gravel.
- Never specify both the proportion and property methods of ASTM C270 for the same mortar.
Masonry Units, Mortar, and Grout
Masonry falls under CSI (Construction Specifications Institute) Division 04. On the NASCLA (National Association of State Contractors Licensing Agencies) open-book exam, expect direct lookups from the Carpentry/Masonry reference and the International Building Code (IBC) Chapter 21. Memorize where mortar-proportion tables live so you can flip to them fast.
Masonry units are governed by ASTM (American Society for Testing and Materials) standards. The key units and their standards are listed below.
Common Masonry Units and ASTM Standards
| Unit | ASTM Standard | Typical Use |
|---|---|---|
| Building brick (clay) | ASTM C62 | Backup, general |
| Facing brick | ASTM C216 | Exposed exterior face |
| Hollow load-bearing CMU | ASTM C90 | Structural walls |
| Concrete brick | ASTM C55 | Veneer, paving |
| Calcium silicate (sand-lime) | ASTM C73 | Specialty |
CMU (Concrete Masonry Unit) nominal size is 8 x 8 x 16 inches; actual size is 7-5/8 x 7-5/8 x 15-5/8 inches, with the missing 3/8 inch taken by the mortar joint. This 3/8-inch joint is the standard you assume unless plans say otherwise.
Mortar Types: M, S, N, O
Mortar is specified by ASTM C270. The four common types come from the mnemonic in MaSoN wOrK (M, S, N, O — the odd letters of "MASON WORK"). Each type balances compressive strength against workability and bond. Higher strength does not mean better — over-strong mortar can crack units.
| Type | Min. Compressive Strength (psi) | Typical Application |
|---|---|---|
| M | 2,500 | Below grade, foundations, retaining walls, severe loads |
| S | 1,800 | At/below grade, reinforced masonry, high lateral load |
| N | 750 | General above-grade exterior/interior, most common |
| O | 350 | Interior non-load-bearing, low-stress repointing |
Proportion Specification (ASTM C270, by volume)
The proportion specification sets cement : lime : sand ratios. Sand is always 2-1/4 to 3 times the combined cementitious volume.
| Type | Portland Cement | Hydrated Lime | Aggregate (sand) |
|---|---|---|---|
| M | 1 | 1/4 | 2-1/4 to 3 |
| S | 1 | 1/4 to 1/2 | 2-1/4 to 3 |
| N | 1 | 1/2 to 1-1/4 | 2-1/4 to 3 |
| O | 1 | 1-1/4 to 2-1/2 | 2-1/4 to 3 |
Exam trap: more lime equals more workability and LOWER strength. Type O has the most lime; Type M has the least. Do not confuse the two specification methods — proportion (by volume) and property (lab-tested strength). You never specify both for the same mortar.
Grout
Grout (ASTM C476) is a high-slump, flowable mix poured into CMU cells or cavity wythes to bond reinforcement. Unlike mortar, grout has high water content so it flows around rebar. Slump is typically 8 to 11 inches — far wetter than concrete (3 to 5 inch slump).
- Fine grout: cement, lime, sand only. Used in narrow spaces (joints under 2 inches).
- Coarse grout: adds pea gravel (3/8 inch). Used in wider cells/grout spaces.
- Minimum f'm (specified masonry compressive strength) per IBC is commonly 1,500 psi for clay/concrete masonry under the unit-strength method.
Lift height matters for OSHA and quality: grout placed in tall lifts can blow out forms or cause segregation. High-lift grouting requires cleanouts at the base of each cell.
A retaining wall below grade carrying heavy lateral soil pressure requires mortar. Which ASTM C270 type is most appropriate?
What is the actual dimension of a standard nominal 8 x 8 x 16 inch concrete masonry unit?
Mortar Types — Memorize "MaSoN wOrK"
The four ASTM C270 mortar types come from the word MASON WORK (M, S, N, O — every other letter). Compressive strength descends in that order:
| Type | ~28-day Strength | Use |
|---|---|---|
| M | 2,500 psi | Below-grade, foundations, retaining walls (highest strength) |
| S | 1,800 psi | At/below grade, high lateral load, structural |
| N | 750 psi | General above-grade exterior/interior (most common) |
| O | 350 psi | Interior, non-load-bearing, restoration |
More portland cement raises strength but lowers workability; more lime improves workability and bond.
Masonry Units and Grout
CMU (concrete masonry unit) nominal 8×8×16 in (actual 7-5/8 each way to allow a 3/8-in mortar joint). Brick modular nominal 4×2-2/3×8 in. Grade units for exposure (ASTM C90 load-bearing CMU; C216 facing brick). Grout (a fluid concrete with small aggregate and high slump 8–11 in) fills cells around reinforcement — it is NOT mortar. Grout is placed in lifts and consolidated to bond steel and block into a structural unit.
Mixing, Retempering, and Board Life
Mortar must be used within its board life — about 2.5 hours at normal temperatures; after that the cement has begun to set. Retempering (adding water to restore workability) is allowed once, within that window to replace evaporated water, but adding water after initial set ruins the mortar. Over-wet or aged mortar yields weak joints — a common cause of efflorescence and bond failure.
Common Exam Traps
- Trap: Type N is the strongest. Type M is strongest; N is general-purpose.
- Trap: Grout and mortar are interchangeable. Grout is high-slump and fills cells; mortar bonds units.
- Trap: More lime = more strength. More lime = more workability/bond, less strength.
- Trap: Retempering mortar hours later after it has set.
Which mortar type has the highest compressive strength and is specified for below-grade foundation walls?