NCMA CCCM cheat sheet

Nominal vs. actual dimensions: a nominal 8 in. x 8 in. x 16 in. CMU measures 7-5/8 in. x 7-5/8 in. x 15-5/8 in.; the 3/8 in. difference is the mortar joint, so 8 in. module spacing is maintained.

ASTM C90 covers loadbearing concrete masonry units; ASTM C129 covers nonloadbearing units; ASTM C140 is the test method for sampling and testing units (compressive strength, absorption, density).

Standard hollow CMU face-shell thickness is typically 1-1/4 in. minimum; grout space must be at least 2 in. clear between face shells for grouted construction.

Masonry compressive strength f'm is derived from unit strength and mortar type via the unit strength method (TMS 602/ACI 530.1) or prism testing; Type M mortar yields the highest f'm, Type S next, then N, O, and K.

Mortar types: M (2500 psi, high strength, below grade), S (1800 psi, moderate-high, at grade), N (750 psi, general above grade), O (350 psi, interior nonloadbearing), K (low).

Grout must be 2000 psi minimum at 28 days; coarse grout has 1/2 in. maximum aggregate and is used where grout space is 2 in. or more; fine grout has 3/8 in. aggregate for narrower spaces.

Reinforcement: No. 4 bar is the minimum vertical bar size in typical reinforced walls; lap splice length is per bar size and grout/mortar, commonly 40 bar diameters or per TMS 602 tables.

Flashing must extend a minimum of 8 in. vertically above the finished grade and be installed with a drip edge; weeps are placed at 32 in. on center maximum and at 16 in. maximum above the flashing line.

Control joints in concrete masonry: space at 20 to 25 ft maximum, or a ratio of 1-1/2 to 2-1/2 times the wall height, whichever is less; place at openings, corners, and changes in wall height or thickness.

Paver base (aggregate base) depth: 4 in. minimum for pedestrian and residential driveway applications, 6 to 8 in. for vehicular or heavy-load areas, compacted in 2 to 4 in. lifts to 95% or greater density.

Bedding sand layer is 1 in. (25 mm) after compaction, installed with a screed; use ASTM C33 or C144 sand depending on the specification; do not exceed 1-1/2 in.

Interlocking concrete pavers: minimum 2-3/8 in. (60 mm) thick for pedestrian, 2-3/4 in. (70 mm) for residential driveways, 3-1/8 in. (80 mm) for commercial vehicular areas.

Edge restraint: use a rigid edge restraint (concrete curb or aluminum/steel edge) pinned at 8 to 12 in. on center; sand-set plastic edge is not acceptable for vehicular loads.

Joint sand: fill paver joints of 1/8 to 1/4 in. with ASTM C144 or C33 sand; polymeric sand is used for stabilization and must be installed dry and activated per manufacturer instructions.

Slope for drainage: minimum 1% (1/8 in. per foot) away from structures, typically 2% on paved surfaces; ensure positive drainage to a defined outfall.

Subgrade preparation: excavate to the required depth, proof-roll, and compact native soil to 95% standard Proctor; remove organic material and unstable soils.

Segmental retaining walls (SRW): geogrid reinforcement is required when wall height exceeds the leveling pad capacity; embed the base course a minimum of 6 in. below grade and 12 in. for walls over 4 ft.

Permeable interlocking concrete pavement (PICP): use open-graded aggregate base and joint aggregate, and provide a storage reservoir sized for the design storm; do not use dense-graded base or sand joints.