NOTE This standard governs concrete masonry units procured as a manufactured product: their specification, physical properties, factory fabrication, surface finish, and verification before they are incorporated into the work. (1.1)

NOTE CMU is the procurement layer beneath the masonry assembly. The block delivered to the site has fixed properties — strength, density, dimension, finish — set at the plant before any mortar is mixed. This standard fixes those properties so the units that arrive match the structural and architectural intent; the wall built from them is governed separately. (1.2)

NOTE Mortar, grout, joint reinforcement, wall ties, flashing, control-joint materials, and the construction and tolerances of the finished masonry wall are not covered here; they are governed by Unit Masonry. (1.7)

NOTE Structural design of the masonry — determination of specified compressive strength of masonry (f'm), reinforcement sizing, shear-wall design, and special-inspection requirements — is performed by the Engineer of Record under TMS 402/602 and the building code, and is outside this product standard. (1.8)

NOTE Grout placed in CMU cells is masonry grout to ASTM C476 and is specified with the assembly; it is not the non-shrink cementitious grout of Non Shrink Grout, which is a different material used under base plates. (1.9)

NOTE References to "ACI 530" or "ACI 530.1" are obsolete; the masonry code and specification have been published as the consolidated TMS 402/602 document since the 2013 cycle, and the current edition is coordinated with ASCE 7-22. (2.3)

NOTE ASTM C90 was retitled "Standard Specification for Dry-Cast Loadbearing Concrete Masonry Units" in its 2024A revision; earlier editions used the shorter "Loadbearing Concrete Masonry Units." The unit requirements are otherwise continuous across recent editions. (2.4)

NOTE Color consistency between production runs of pigmented or specialty-finish units varies; without an approved mock-up establishing the acceptable range, finish disputes at delivery are nearly unavoidable, which is why exposed work is gated on panel approval. (4.2.3)

NOTE Loadbearing versus non-loadbearing is the first selection and must follow the structural design intent. Loadbearing units (ASTM C90) carry vertical and lateral load and have the higher minimum strength; non-loadbearing units (ASTM C129) are limited to partitions and have a lower strength floor. Substituting C129 where C90 is required is a structural deficiency, not a cost saving. (5.1)

NOTE Concrete brick (ASTM C55) is a solid or cored unit, typically up to 4 in. wide, used for veneer coursing, sills, and accent work rather than as the primary structural wythe; specify it only where the Drawings call for a brick-scale concrete unit. (5.2)

NOTE A hollow unit is "solid-grouted" by filling its cells with masonry grout in the field; that is an assembly operation, not a property of the unit. Do not confuse a solid unit (a casting with little or no cell void) with a grouted hollow unit — they are specified differently and the grouting belongs to Unit Masonry. (5.4)

NOTE Concrete masonry units are classified by oven-dry density of the concrete, which drives structural capacity, thermal mass, sound transmission, handling weight, and floor loading. Density class and strength class are independent: a lightweight unit can be specified to meet the same 2,000 psi net strength as a normal-weight unit. Specifying "normal weight for structural" without also citing a strength conflates two unrelated properties. (6.1)

NOTE Weight classes per ASTM C90 are: normal weight at 125 pcf (2,000 kg/m³) or more; medium weight from 105 to less than 125 pcf; and lightweight at less than 105 pcf (1,680 kg/m³). Medium weight is the most common selection in commercial construction, balancing handling weight against thermal mass. (6.2)

NOTE CMU is specified by nominal dimension (width by height by length). Actual dimensions are 3/8 in. less than nominal in each direction, so the unit plus one mortar joint courses out to the nominal module. An 8x8x16 nominal unit measures 7-5/8 by 7-5/8 by 15-5/8 in. The 8x8x16 unit is the dominant module across virtually all producers and is the 80% default for structural walls. (7.1)

NOTE Minimum face-shell thickness in ASTM C90 increases with unit width — about 1-1/4 in. for an 8 in. unit and 1-1/2 in. for a 12 in. unit. Face-shell and web geometry also determine the clear cell space available for grout and reinforcement, which is why cell geometry must be checked against the grout type before the submittal is approved. (7.3)

NOTE Fine grout to ASTM C476 requires a smaller minimum clear cell dimension than coarse grout; selecting a coarse grout against a unit whose cells provide only fine-grout clearance produces rejected submittals or forced re-design, so verify the cell against the grout before specifying. (7.4.2)

NOTE A wall built only from stretcher units is not buildable. Corners, ends, openings, reinforcement, and bearing all require purpose-made shapes, and every shape needed must appear on the unit schedule. Specifying "CMU per ASTM C90" with no shape schedule is the single most common cause of RFIs for corner, lintel, bond-beam, and open-end units once the masons reach those conditions. (8.1)

NOTE Open-end (A-block / knockout) units have one cell open at the end so a vertical reinforcing bar can be set in place and the unit laid around it, instead of threading the bar down through closed cells. They are the standard way to build continuously reinforced cells and are a catalog item at most producers. (8.2)

NOTE Bond beam units have a reduced or knocked-out top web forming a continuous channel for horizontal reinforcing bars at lintels, bearing courses, and bond-beam courses. (8.3)

NOTE Lintel (channel / U-block) units form a trough to carry reinforced grout over openings where the masonry itself spans. (8.4)

NOTE For concealed structural masonry, standard gray plant-finish units are specified and finish is not a consideration. For exposed masonry, the unit finish is the architectural surface, achieved at the plant by molding, splitting, grinding, or polishing, and it must be selected, sampled, and approved before production. (9.1)

NOTE Available exposed finishes include: standard smooth (gray or pigmented); split-face, where the unit is molded double and fractured to expose a coarse aggregate texture; ground-face (honed/burnished), where the cured face is ground to expose and polish the aggregate; slump block, where deliberate overwatering yields an irregular coursing profile; and integrally colored variants of any of these using pigments batched into the mix. (9.2)

NOTE Ground-face and other specialty finishes may carry longer manufacturing lead times than standard units. (9.2.4)

NOTE The physical-property limits below are the acceptance thresholds for the unit as delivered, drawn from ASTM C90 and verified per ASTM C140. They are minimums and maximums on the procured product; they are not the specified compressive strength of masonry (f'm), which is an assembly property set by the Engineer of Record and is typically lower than the unit strength. (10.1)

NOTE Typical specified f'm for standard reinforced CMU walls falls between 1,500 and 2,000 psi, with 1,500 psi common for lightly loaded structures and 2,000 to 3,000 psi where special inspection and higher demand apply. The unit strength is selected to satisfy the f'm by the unit-strength method or the prism-test method of TMS 602. (10.3)

NOTE Lower absorption improves resistance to moisture penetration and efflorescence on exposed walls. (10.4.2)

NOTE Units near the shrinkage limit placed in hot, dry climates are prone to cracking at control joints and at the corners of openings. (10.5.3)

NOTE Units out of tolerance disrupt coursing and mortar-joint thickness. (10.6.3)

NOTE Fire-resistance rating of a CMU wall is a function of the equivalent thickness of the units (which depends on unit density and percent solid) and of the grout fill, established by IBC Table 722.3 (calculated method) or by a tested, listed assembly. The rating belongs to the assembly, but it constrains the unit selection — density class and percent solid determine the equivalent thickness the units contribute. (11.1)

NOTE Substituting "solid grout all cells" for a partially grouted listed assembly, or vice versa, voids the listing. (11.1.3)

NOTE Acceptance testing of CMU is performed per ASTM C140 on units sampled from the delivered lot. The same standard covers the sampling frequency, the net-area compressive-strength procedure, absorption, density, and the dimensional checks; manufacturer plant testing supplements but does not replace acceptance testing where the Drawings require it. (12.1)

NOTE Concrete masonry units are dimensionally and chemically sensitive to moisture before they are laid. Units stored in contact with the ground or left unprotected absorb water, which raises their moisture content at laying, increases later drying shrinkage in the wall, and promotes efflorescence on exposed faces. Damaged or chipped units from careless handling generate jobsite RFIs and rejected coursing. (13.1)

NOTE The recurring errors below cause rejected submittals, RFIs, and field rework. They are collected here as guidance, not as additional requirements. (14.1)

NOTE Do not specify "Grade N, Type I" CMU; those classifications were removed from ASTM C90 in 2000 and no longer exist, and citing them only generates contractor confusion and RFIs. (14.2)

NOTE Do not call out non-shrink grout to fill CMU cells; non-shrink (ASTM C1107) grout expands on cure and can split cell webs. Cell grout is masonry grout to ASTM C476, specified with the assembly in Unit Masonry. (14.3)

NOTE Do not place control joints at cells that are grouted and reinforced; a grouted, bonded cell cannot move, so control-joint locations must be coordinated early with the reinforcement layout on the structural Drawings and with the architectural elevations. (14.4)

NOTE Do not assume normal weight means stronger; density class and strength class are independent, and a lightweight unit can meet the same net strength. Cite the required strength explicitly rather than inferring it from weight. (14.5)

NOTE Do not omit the shape schedule, the mock-up for exposed finishes, or the shrinkage data on hot-dry-climate projects; each omission produces a predictable downstream failure documented in the sections above. (14.6)

NOTE The assembled masonry wall — mortar, grout, joint reinforcement, ties, control joints, flashing, and construction tolerances — is governed by Unit Masonry. (15.1)

NOTE Masonry veneer anchors and tie systems for CMU backup are governed by Masonry Anchorage And Veneer. (15.2)

NOTE Non-shrink cementitious grout for base plates and equipment bases (distinct from cell grout) is governed by Non Shrink Grout. (15.3)

NOTE Foundation bearing and frost-depth requirements supporting CMU loadbearing walls are governed by Shallow Foundations. (15.4)