Concrete Masonry
What this domain covers
The Concrete Masonry domain tests whether you can link material specifications to field practice: which ASTM standard governs each component, how masonry is measured and coursed, and what construction requirements the masonry code (TMS 402/602, the MSJC documents) imposes on joints, reinforcement, flashing and control joints. Expect questions that mix a material spec with an installation condition.
Core rules
- Modular dimensions. An 8×8×16 unit is nominal; the actual unit is 7-5/8 × 7-5/8 × 15-5/8 in., because the 3/8 in. mortar joint supplies the missing 3/8 in. on each face. Coursing is 8 in. high; one unit covers 8 × 16 in. of wall face (0.889 sq ft), so roughly 1.125 units per square foot of wall.
- ASTM C90 is the specification for loadbearing concrete masonry units: dimensional tolerances, minimum face-shell and web thickness, absorption limits, and compressive strength reported on a net-area basis.
- ASTM C270 mortar types with 28-day strengths: M (2,500 psi), S (1,800), N (750), O (350), K (75). Type S is the usual CMU mortar; do not over-specify M thinking it is "better" - higher cement content reduces workability and water retention and can increase shrinkage cracking.
- ASTM C476 grout (fine or coarse) is placed at 8–11 in. slump and must reach 2,000 psi at 28 days. Prism testing follows ASTM C1314.
- TMS 602 field items: full bed and head joints, tooled when thumbprint-hard; horizontal joint reinforcement (9-gage wire) commonly every other course; flashing with weeps about 24 in. on center; control joints at changes of height, offsets and openings, typically 20–25 ft apart.
Worked example
Wall: 24 ft long × 9 ft high, 8×8×16 CMU. Face area = 216 sq ft. Units = 216 × 1.125 = 243; add 5% breakage = 255 units. Courses = 9 ft ÷ 0.667 ft = 13.5, so the wall lands on a half course - resolve it at the top with a bond beam course rather than by stretching joints, which would violate joint-thickness tolerances.
Traps that cost marks
Using nominal instead of actual dimensions; computing strength on gross area when the code is net-area; omitting the waste factor; and treating mortar type as a strength dial rather than a design choice matched to the specified f'm.