CMU (concrete masonry unit)
CMU is a concrete masonry unit. Hollow cores can be filled with concrete and steel for structural capacity. Standard 200/300/400 series. Distinct from clay brick.
Ask Chalkline about this →A CMU (concrete masonry unit) is a concrete-cast block, typically rectangular with hollow cores, used as a structural or non-structural masonry unit in residential and small-commercial construction. The hollow cores can be left empty, filled with insulation, or filled with concrete and steel reinforcement to provide significant structural capacity. CMUs are made and supplied to AS/NZS 4455.1:2008 (masonry units) and designed under AS 3700:2018 (masonry structures). Locally “besser block” is a colloquial term derived from the historical CSR Besser machine that pressed them.
Standard CMU sizes in Australian residential:
| Designation | Nominal dimensions (length x height x width, mm) | Notes |
|---|---|---|
| 200 series (20.01, 20.02) | 390 x 190 x 90 mm | ”Half block”, non-structural infill, fences |
| 200 series (20.31, 20.32) | 390 x 190 x 190 mm | Standard structural block, retaining walls, party walls |
| 300 series | 390 x 190 x 290 mm | Heavy structural, basement retaining |
| 400 series | 390 x 190 x 390 mm | Mass retaining walls, civil applications |
Where CMU appears in residential:
- Retaining walls (most common residential use), often steel-reinforced and core-filled.
- Basement walls and below-ground walls.
- Party walls between attached dwellings (fire-rated, often reinforced or core-filled).
- Fence walls (lower-strength, often non-structural).
- Garage and shed walls as cheap alternative to clay brick.
- Pier columns in pier-and-beam systems where solid concrete pier is uneconomic.
CMU vs clay brick:
| Property | CMU | Clay brick |
|---|---|---|
| Material | Cast concrete (cement + aggregate + sand) | Fired clay |
| Density | Higher (concrete = ~1,800-2,200 kg/m³) | Lower (clay = ~1,800 kg/m³ but smaller unit) |
| Compressive strength | Higher (12 to 25+ MPa) | Moderate (15 to 30 MPa) |
| Hollow / solid | Hollow with cores | Solid (typically) or perforated |
| Mortar joint | 10 mm typical | 10 mm typical |
| Reinforcement options | Cores accept rebar + grout fill | Generally needs cavity construction for reinforcement |
| Cost | Lower per m² | Higher per m² |
| Aesthetic | Industrial / utilitarian (unless painted/rendered) | Architectural / domestic |
Reinforcement and core-fill:
For a structural CMU wall (retaining, basement, party), the design typically requires:
- Vertical reinforcement in the cores at specified centres (commonly N12 bar at 800 mm centres for low retaining, denser for taller).
- Horizontal reinforcement in bed-joint reinforcement or in horizontal bond beams cast in special “knock-out” blocks.
- Cores grouted with concrete (commonly N20 grout, smaller aggregate than slab concrete) once reinforcement is placed.
- Engineered design under AS 3700 by a structural engineer.
Common defects:
- Cores not filled where the design called for them (a “hollow” wall meant to be reinforced).
- Vertical reo missed at every other core, instead of every core as designed.
- Bond-beam knock-out blocks substituted with standard blocks, omitting the horizontal tie.
- DPC missed at the base course (a CMU wall on damp ground without DPC wicks moisture continuously upward).
Also known as: concrete block; besser block; concrete masonry; hollow block; CMU block.
Category: Materials.
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Last updated: 2026-05-16. Verified: 2026-05-16. Quarterly review for currency.